The Innovative Concept of Craniotomy in suboccipital retrosigmoid keyhole approach: anatomical study, technique nuances, and clinical application

The Innovative Concept of Craniotomy in suboccipital retrosigmoid keyhole approach: anatomical study, technique nuances, and clinical application | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article The Innovative Concept of Craniotomy in suboccipital retrosigmoid keyhole approach: anatomical study, technique nuances, and clinical application Chang-chun Liao, Xun-jie Ma, Bai-cha Tang, Ling Zhou, Kai-hua Wu, and 8 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7557430/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Background : The suboccipital retrosigmoid approach is a classic approach for surgical access to cerebellopontine angle diseases. Previously, we identified a novel method for precise localization of keypoint, known as the "one point, two lines, two distances" theory. However, a comprehensive, safe method for craniotomy remains lacking. Objective : Based on the new landmark method of localization , this study aims to further describe the anatomical basis, surgical technique, and outcomes of the suboccipital retrosigmoid keyhole approach for craniotomy. Methods : Twelve adult specimens of skull were used for the study. The anatomical relationships between the keypoint were analyzed to establish a precise, rapid, and safe method for suboccipital retrosigmoid keyhole craniotomy. This method was then validated through cadaveric dissection. Furthermore, a retrospective analysis of surgical outcomes was performed on 122 clinical patients, assessing accuracy, safety, and exposure results. Results : Measurements from specimens of skull revealed that the up point of digastatic sulci roughly corresponds to the margin of the sigmoid sinus. In craniotomy simulations performed on 12 cadaveric specimens, we freed the bone flap in a "rear-down-front" direction after drilling at the key point. This process passed through the up point of digastatic sulci, with bone around the mastoid emissary vein being removed using a grinding drill, followed by resection of the residual bone along the margin of the sigmoid sinus. In all cadaver specimens, the formation of bone flap resulted in a well-exposed window. In the clinical cohort of 122 patients, keypoints were accurately located, and suitable bone windows were created with good exposure and without damage to important structures. Conclusion : The up point of digastatic sulci serves as a crucial reference point for keypoint localization. Craniotomy techniques based on this reference point provide a precise and safe approach for suboccipital retrosigmoid craniotomy. suboccipital retrosigmoid approach transverse sinus sigmoid sinus keypoint digastric sulci Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 11 Introduction The suboccipital retrosigmoid approach to the sigmoid sinus is a classic surgical route for treating lesions in the cerebellopontine angle [1-4] . With the advancement of minimally invasive techniques, the suboccipital retrosigmoid keyhole approach, involving small incisions, micro bone windows, and bone flap repositioning, has been increasingly recognized by scholars [5-7] . The ideal skin incision for this approach is approximately 4 cm, with the bone window ranging from 2 to 2.5 cm in size. The upper edge of the bone window exposes the inferior border of the transverse sinus, while the anterior edge exposes the posterior margin of the sigmoid sinus, effectively avoiding damage to the transverse and sigmoid sinuses. Moreover, good visibility during surgery is critical, demanding high precision from neurosurgeons. To minimize unnecessary exposure of skin and bone, and to prevent injury to the transverse and sigmoid sinuses, it is essential to accurately locate the bony landmark corresponding to the junction of the transverse and sigmoid sinuses and the key point of the craniotomy. Currently, various localization methods of craniotomy have been proposed in the literature [8-13] , utilizing cranial surface anatomical landmarks and imaging technologies. However, these methods often suffer from inaccuracies in localization and operational complexity, limiting their clinical applicability. During the craniotomy process of the suboccipital retrosigmoid keyhole approach, the formation of bone flap is achieved using a grinding drill and a milling cutter. The bone flap is repositioned and fixed at the end of the procedure to avoid skull defects. The procedure requires careful navigation around important structures such as the transverse sinus, sigmoid sinus, and mastoid emissary vein in a confined space. It remains a key focus for clinicians that ensuring precise and safe bone cutting, bone flap formation, and exposure of intracranial structures after dural opening while avoiding injury to venous sinuses and ensuring proper exposure of neural and vascular structures [1-3, 5, 14] . In previous studies, we have demonstrated that the up point of digastatic sulci is an important landmark for locating key anatomical points [15-17] . There is a fixed relationship between this landmark and others. Using the Frankfurt horizontal plane projection line as a reference, the key point is located approximately 16.97 ± 2.50 mm vertically above the up point of digastatic sulci and 4.80 ± 1.87 mm horizontally posterior to it. We refer to this landmark localization technique as the "one point, two lines, two distances" theory [15-17] . However, how to precisely and safely mill the cranium in the confined space using a grinding drill and milling cutter, while avoiding injury to the transverse and sigmoid sinuses and ensuring adequate exposure of intracranial structures, remains a critical issue. This study, conducted from January 2020 to December 2024, utilized adult dry skull specimens to analyze the relationships between cranial surface anatomical landmarks. The research aimed to explore precise localization of craniotomy and the safe, rapid bone flap formation technique for the suboccipital retrosigmoid keyhole approach. Feasibility was verified through cadaveric dissections, followed by clinical application to evaluate the effectiveness of this safe, rapid bone flap formation method. The findings aim to provide clinical evidence for achieving safe, precise, and rapid craniotomy. Materials and Methods 1.1 Specimens and Instruments Twelve adult specimens of dry skulls and twelve wet cadaveric specimens were selected for research. All skulls had no significant pathological deformities in the posterior fossa, temporal bone petrous part, mastoid area, or occipital bone. The following instruments were used: microscope (OPMI PROergo, Carl Zeiss AG, Germany), craniotomy power system (MicrospeeduniGD670, Aesculap, Germany), steel ruler (DL8050, Deli Group Co., Ltd., China), digital caliper (AS0020151, Shanghai Tool Factory, China), digital angle ruler (Casio 0-200mm, Henan Bant Tool Co., Ltd., China), custom head frame, and micro instruments. The software used for analysis was 3D-Slicer 4.11.20210226. 1.2 Patient cohort This study retrospectively analyzed 122 patients who underwent suboccipital retrosigmoid craniotomy via the sigmoid sinus approach in the Neurosurgery Department from January 2020 to December 2024. The study was approved by the Medical Ethics Committee of this hospital. Written informed consent was obtained from each subject. All craniotomies followed the technique proposed in this study. The surgeries were performed under a ZEISS KINEVO 900 microscope(Carl Zeiss AG, Germany) and intraoperative electrophysiological monitoring (Nicolet PROTEKTOR 32, Thermo Nicolet Corporation, America). Among the 122 patients, 31 had trigeminal neuralgia, 67 had hemifacial spasm, 17 had acoustic neuroma, 5 had meningiomas, and 2 had cholesteatomas. There were 54 males and 68 females, aged 26 to 72 years, with a mean age of 51.6 ± 10.4 years. There were 36 left-sided lesions and 49 right-sided lesions. (Table 1) 1.3 Experimental Methods 1.3.1 Innovative Concept of Craniotomy on the Dry Skulls 1.3.1.1 Localization of Key Points and Key Hole Points on Dry Skulls Based on our previous research, a coordinate system was established using the Frankfurt horizontal plane (FHP) as the X-axis and a vertical line through the up point of digastatic sulci (UPDS) as the Y-axis, with UPDS as the origin (Figure 1A, B). The vertical and horizontal average distances between the key point(the Junction Point of the Transverse and Sigmoid sinuses, JPTS) and UPDS were 16.97 ± 2.50 mm and 4.80 ± 1.87 mm, respectively. The vertical and horizontal average distances between the key hole point(KP) and UPDS were 14.58 ± 2.58 mm and 6.60 ± 1.72 mm, respectively (Figure 1C, D). This method is referred to as the “One Point, Two Lines, Two Distances” technique: "One Point" is the up point of digastatic sulci (UPDS), "Two Lines" are the FHP line (X-axis) and the vertical line through UPDS (Y-axis), and "Two Distances" refer to the vertical and horizontal distances [15-17] . 1.3.1.2 Relationship Between the UPDS and the Posterior Margin of the Sigmoid Sinus In six cadaveric skulls, a hole of approximately 6 mm in diameter was drilled at the UPDS into the outer surface, and the relationship between the hole and the sigmoid sinus was examined from the inner surface (Figure 1E,F). 1.3.1.3 Relationship Between Key Points, the Key Hole Point, the UPDS, and the Posterior Margin of the Sigmoid Sinus in Dry Skull After creating a bone window along the line connecting the key hole point and UPDS, there remained a distance from the anterior edge of bone window to the sigmoid sinus margin. In order to just expose the posterior margin of the sigmoid sinus, the bone surrounding the mastoid part of the sigmoid sinus needed to be removed. We first identified and marked the Inferior margin of transverse sinus (IPS), the posterior margin of sigmoid sinus (PSS), and key point (the Junction Point of the Transverse and Sigmoid sinuses,JPTS) on the internal surface of the skull (Figure 2A). At the key point (JPTS), a 6 mm hole was drilled vertically into the skull (Figure 2B). This hole’s center point is the key hole point (KP) (Figure 2C). On the external surface, the UPDS, mastoid tip (TM), star point (SP), key point (JPTS), and mastoid emissary vein fossa (EF) were marked (Figure 2C), followed by drilling a 6 mm hole from UPDS toward the internal surface (Figure 2D). The characteristics of the bone of the posterior border of the sigmoid sinus to be removed were drawn 1:1 with pape, and the length (the distance between UPDS and KP), width (the maximum vertical distance between the posterior margin of the sigmoid sinus and the connection between UPDS and KP), and area (the area formed by the connection between UPDS and KP and PSS) were measured(Figures 2E, F). 1.3.1.4 Innovative Concept of suboccipital retrosigmoid keyhole approach on the dry skull (1) Localization of the key hole point: Using the “One Point, Two Lines, Two Distances” method, the UPDS was located, and the KP was determined (Figure 3A–C). (2) Formation of bone flaps: Ⅰ- According to the original concept of craniotomy, the hole was drilled at the key hole point (6-10 mm diameter) in sequence (Figure 3B–C). Ⅱ- A milling cutter was used to mill the bone from the key hole toward the posterior, downward, and forward directions in turn. Once the UPDS was reached, the cutter was moved upward to the level of the emissary vein, and the cutter was withdrawn (Figure 3D). Ⅲ- The same method was applied for the mastoid bone from the other direction (Figure 3E). Ⅳ- The original bone window was formed after removing the bone flap (Figure 3F). Ⅴ- Residual bone near the sigmoid sinus was removed using a grinding drill referring to the results(the length, width and area measured above), exposing the bone window of approximately 2.5 cm in diameter (Figure 3G–I). 1.3.2 Validation On Cadaveric Specimens Based on the dry skull results, the “One Point, Two Lines, Two Distances” method and the innovative concept of craniotomy were validated on cadaveric specimens(video 1). The following steps were performed: (1) Marking anatomical landmarks on the skin : TM, UPDS, the Frankfort horizontal plane (FHP).(Figure 4A) (2) Incision placement : A straight vertical incision measuring approximately 4 cm was made 1.0 cm posterior to the UPDS. Approximately one-quarter of the incision was made above the FHP. The inferior border of the incision reached the level of the mastoidale.(Figure 4A) (3) Exposure of the skull: Incise the scalp, expose the skull, and determine the UPDS. (4) Locate key holes: According to the "one point, two lines, two distances" method, the key hole points (KP) were determined (Figure 4C). (5) Formation of bone flaps: ① According to the original concept of craniotomy, drill holes at key hole points, with a diameter of 6mm (Figure 4C); ② Mill the skull with the milling cutter from the key hole toward the posterior, downward, and forward directions in turn. After reaching the UPDS, the milling cutter moves upward, terminates at the level of the EF, and then the milling cutter was withdrawn(Figure 4D); ③ Milling the mastoid bone with a milling cutter from the key hole to the UPDS, terminating at the level of the EF, and withdrawn the milling cutter (Figure 4E); ④ Milling most of the bone of mastoid emissary vein catheter. ⑤ Original bone flap: Pry off the flap, and the original bone window formed. (6) Milling of bone near the sigmoid sinus to create the final bone window : The residual bone in the mastoid of the posterior margin of the sigmoid sinus was removed with a drill. The bone window margin was investigated and determined to be located at the posterior margin of the sigmoid sinus, forming a bone window with a diameter of approximately 2.5cm (Figure 4F). (7) Exposure of intracranial structures: The dural was cut in a "Y" shape (Figure 4G), and then Pullegd aside the cerebellum and observed various structures such as nerves and blood vessels in the cerebellopontine Angle region (Figure 4H-O). 1.3.3 Clinical Application of the Innovative Concept of craniotomy In clinical cases, the “One Point, Two Lines, Two Distances” localization method and the innovative concept of craniotomy were applied in 122 patients(video2). The following steps were performed: (1)Preoperative CT and MR were obtained with 3D reconstructions for surgical planning.(Figure 5A-C). (2) All surgical procedures were performed by senior neurosurgeon, and lateral decubitus position was adopted after full and satisfactory anesthesia for tracheal intubation. (3) The craniotomy was performed with a 4-5 cm incision (Figure 5D). (4) Formation of bone flaps: Made the final bone window according to the innovative concept of craniotomy mentioned above (Figure 5E-L). (5)Further operation: Y-shaped incision of the dural membrane (Figure 5M), then intracranial operation according to the patient's disease type (Figure N- O). (6) Dural suture and bone flap reduction: During intracranial closure, the dural membrane was repaired with close suture (Figure 5P), and bone flap reduction was performed (Figure 5Q, R). (7) Thin-slice CT scan, MR Scan and three-dimensional reconstruction of the nerves, vessels, brain stem and skull were performed again after surgery (Figure 5T, U) to verify the accuracy of the positioning method. 1.3.4 Evaluation Criterion for Cadaveric Validation and Clinical application. 1) Evaluation Criterion: ① Accuracy: Whether the localization and exposure of local anatomical structures are precise and sufficient.② Safety: Whether there is any damage to local major anatomical structures related to accurate craniotomy.③ Speed: Measurement of time of craniotomy, specifically the time taken from skin incision to bone window exposure. ④ Minimally Invasive Nature: The size of the bone flap and bone window, and whether the bone flap is repositioned.⑤ Treatment effect: Whether the patient's preoperative symptoms were relieved after surgery, and whether there were postoperative complications such as death, coma, hemiplegia, facial paralysis, facial numbness, leakage of cerebrospinal fluid, tinnitus and so on 2) Follow-Up: All patients underwent telephone or outpatient follow-up 1–6 months post-surgery to observe whether the patients have recurrence or other complications after the operation. 1.4 Statistical Methods The distances between relevant structures were measured using digital calipers, with results expressed as mean ± standard deviation (SD). Data analysis was performed using SPSS 21 software, with group comparisons conducted via t-tests, with significance set at p-value<0.05. Results 2.1 Innovative Concept of Craniotomy on the Dry Skulls 2.1.1 Relationship Between the UPDS and the Posterior Margin of the Sigmoid Sinus In six cadaveric skulls, a hole approximately 6 mm in diameter was drilled vertically at the UPDS on the external surface of the skull. The position of the hole was then examined on the internal surface of the skulls. The results showed that in all six specimens, the drilled hole corresponded exactly to the edge of the sigmoid sinus, located at the posterior margin near the horizontal and vertical segments of the sigmoid sinus (Figure 1E,F). No hole was located directly on the sigmoid sinus. This result is consistent with the results of our previous study [15-17] . 2.1.2 Relationship Between Key Points, the Key Hole Point, the UPDS, and the Posterior Margin of the Sigmoid Sinus in Dry Skull To further study the relationship between the key point, the Key Hole Point, UPDS, and the posterior margin of the sigmoid sinus in dry skulls, and to determine the extent of bone removal required at the posterior margin of the sigmoid sinus, data were collected from 12 dry skulls, including measurements from both left and right sides (Figures 6A, B, C). The analysis showed that the length (height) of bone that needed to be removed to reach the sigmoid sinus boundary was 14.71 ± 2.19 mm (left side), 14.29 ± 2.01 mm (right side), with no significant statistical difference between the sides(P>0.05), and an average of 14.50 ± 2.10 mm. The maximum depth (width) of bone removal to reach the sigmoid sinus boundary was 3.26 ± 0.55 mm (left side), 3.64 ± 0.72 mm (right side), with no significant statistical difference between sides(P>0.05), and an average of 3.45 ± 0.64 mm. The area of bone removal to reach the sigmoid sinus boundary was 35.93 ± 8.30 mm² (left side), 36.11 ± 7.38 mm² (right side), with no significant statistical difference between the sides(P>0.05), and an average of 36.02 ± 7.84 mm²(Table 2). 2.2 Validation On Cadaveric Specimens Based on the results from dry skulls and cadaveric specimens, the “One Point, Two Lines, Two Distances” method for key hole localization and the original concept of craniotomy were tested in fresh cadaveric specimens. This study preliminarily validated the accuracy, safety, and minimally invasive nature of our localization and craniotomy methods. The results were as follows: ① Accuracy : The localization of the key hole point was generally accurate, and the sigmoid sinus margin, as well as other important intracranial neurovascular structures, were well exposed.② Safety : No damage to the venous sinuses was observed during drilling or milling in the cadaveric dissection.③ Speed : Due to the use of cadaveric specimens, the dissection process and clinical surgeries differ, and thus time could not be measured. However, no particularly time-consuming or difficult situations were encountered during the dissection process.④ Minimally Invasive Nature : The incision length was approximately 4–5 cm, with an average bone flap size of 1.8–2.5 cm, and an average bone window size of 2.1–3.0 cm. 2.3 Clinical Application Results and Evaluation In 122 clinical cases, the novel method of key hole localization and craniotomy was further validated. The specific results are as follows:① Accuracy: Among the 122 cases, 99 had sufficient exposure of the transverse and sigmoid sinuses, and 23 cases achieved adequate exposure after appropriate bone removal. After dural incision, the local microscopic anatomical structures of the intracranial surgical area were well exposed.②Safety: In all 122 cases, the craniotomy was performed using the anatomical localization method developed in this study, with no venous sinus damage caused by the drilling. No injuries to important neurovascular structures occurred during the clinical procedure.③ Speed: The craniotomy time ranged from 21 to 35 minutes, with an average craniotomy time of 26.8 ± 3.6 minutes.④ Minimally Invasive Nature: The average incision length was 4.0 cm, with the longest incision not exceeding 5 cm. The bone flap size was 1.8 cm × 2.0 cm, and the bone window diameter ranged from 2.0 to 2.5 cm. The bone flap was repositioned during the procedure, fully reflecting the minimally invasive concept. Regarding clinical outcomes, among the 122 patients who underwent Craniotomy using the suboccipital retrosigmoid keyhole approach, 24 had tumor-related diseases. During surgery, the operative field, tumor, and important structures were clearly exposed, with no serious complications postoperatively, such as death, coma, hemiplegia, leakage of cerebrospinal fluid. Despite adequate surgical field exposure, 9 patients with acoustic neuroma still experienced varying degrees of facial paralysis and facial numbness postoperatively. These complications were not related to the exposure of the surgical area but were primarily associated with the growth characteristics of the tumor. During postoperative follow-up, these symptoms showed varying degrees of improvement. Postoperative MR confirmed total resection of the tumor. Of the 98 patients with functional disorders, the responsible vessels and nerves were clearly exposed during surgery. There were no postoperative cerebrospinal fluid leaks or other serious complications. Among these, 92 patients had complete relief from trigeminal neuralgia and hemifacial spasm, and 6 (4 with trigeminal neuralgia and 2 with hemifacial spasm) had partial relief. Two patients with trigeminal neuralgia developed tinnitus and dizziness postoperatively, but no other neurological dysfunctions were observed. These cases with complications all had good visual field exposure during the operation, and there was no direct relationship with the innovative Concept of Craniotomy. No recurrence of symptoms or other major complications was noted within half a year, and the symptoms of the two patients with tinnitus were relieved half a month after the operation after rehabilitation treatment. 2.4 Typical Surgical Cases Typical Case 1: Patient with Acoustic Neuroma A 58-year-old male patient was admitted with "progressive hearing loss in the left ear for 2 years, accompanied by facial numbness for 3 months." A CPA tumor was diagnosed and acoustic neuroma was considered. During surgery, we performed craniotomy based on the "one point, two lines, and two distances" theory and the innovative Concept of craniotomy. The key point was accurately located, avoiding any injury to the venous sinus. The surgical field was well exposed. Total tumor resection was achieved intraoperatively. Postoperatively, the patient's facial and auditory nerve functions were well preserved without facial paralysis or other surgical complications. Postoperative brain MRI confirmed no residual tumor.The general surgical process is shown in Figure 7. Typical Case 2: Patient with Trigeminal Neuralgia A 45-year-old female patient with left trigeminal neuralgia for 4 years. Preoperative magnetic resonance imaging revealed a close relationship between the left superior cerebellar artery and the trigeminal nerve. During the microvascular decompression surgery, we performed craniotomy based on the "one point, two lines, and two distances" theory and the innovative Concept of craniotomy. The key point was accurately located, avoiding any injury to the venous sinus. The surgical field was well exposed. The operation procedure was completed smoothly, and the patient’s facial pain symptoms completely disappeared after[1]surgery. The general operation process is shown in Figure 8. Discussion 3.1 Relationship between the UPDS and the Sigmoid Sinus In six cranial specimens, we drilled a 6 mm diameter hole perpendicular to the outer surface of the skull at the UPDS and then observed the corresponding location on the inner surface of the skull. The results showed that in all six specimens, the drilled holes were located precisely at the trailing edge of the sigmoid sinus, approximately at the posterior margin near the junction of the horizontal and vertical segments of the sigmoid sinus. No holes were located on the sigmoid sinus itself. Furthermore, we observed that the vertical segment of the sigmoid sinus typically courses with an anteriorly convex curve. Therefore, we speculate that during craniotomy with a milling cutter, creating a bone flap along the straight-line distance between the Key Point and the UPDS is generally safe and will not injure the sigmoid sinus. This finding is of great importance as it provides crucial guidance for safely and effectively creating the bone window in the next step. 3.2 How to Efficiently and Safely make the Bone Window? During craniotomy, after locating the Key Point, the next step is to create the bone window. In our surgical philosophy, we believe that the bone flap should be routinely fixed back. Therefore, we need to investigate how to safely and accurately create the surgical bone window to minimize bone damage. We have not found any relevant authoritative literature on this aspect. We encounter many challenges during the process of bone flap creation. Therefore, we designed a safe craniotomy protocol. After drilling the burr hole at the Key Hole Point, we first use a milling cutter to cut the skull from the Key Point towards the occiput in a "posterior-inferior-anterior" sequence, finally dealing with the most dangerous area, the edge of the sigmoid sinus. During this process, we need to use the UPDS as a reference, ensuring the milling cutter passes just over the UPDS to get as close as possible to the sigmoid sinus without injuring it. In addition, in the retromastoid region, there is generally an emissary vein, the mastoid emissary vein. If the milling cutter passes directly through this area, it is likely to cause a tear in the emissary vein. Rupture of the emissary vein is often a cause of significant bleeding during craniotomy. The mastoid emissary vein is a intracranial-extracranial communicating vein located in the mastoid region. Its internal opening is generally in the middle of the vertical segment of the sigmoid sinus, and its external opening is located at the EF on the outer surface of the skull. The mastoid emissary vein foramen may have multiple variations. Among them, 46% of the left mastoid emissary vein foramina are single, 7% are double, 19% are multiple, and 28% are absent; on the right side, 55% are single, 13% are double, 29% are multiple, and 2% are absent [18-20] . Therefore, we recommend using a milling cutter to cut the skull from the Key Point towards the occiput in a "posterior, inferior, anterior" sequence, stopping near the UPDS. The next step is to use a grinding drill to grind a groove between the Key Point and the UPDS, rather than directly passing the milling cutter. Finally, an osteotome is used to pry open the bone flap, forming the initial bone window. After forming the initial bone window as described above, the edge of the bone window should be a short distance from the edge of the sigmoid sinus. The presence of this remaining bone may obstruct the surgical view during the operation. Therefore, to better expose the surgical field and meet the requirements of the surgical procedure, we should continue to expose the bone window towards the sigmoid sinus by carefully grinding away the bone with a grinding drill. According to our study results, after forming the bone flap, the maximum depth of bone that needs to be further removed to reach the sigmoid sinus border is 3.26 ± 0.55 mm (left side) and 3.64 ± 0.72 mm (right side), with an average of 3.45 ± 0.64 mm. The area of bone that needs to be further removed to reach the sigmoid sinus border is 35.93 ± 8.30 mm 2 (left side) and 36.11 ± 7.38 mm 2 (right side), with an average of 36.02 ± 7.84 mm 2 . Continuing to expose the bone window towards the posterior edge of the sigmoid sinus is crucial because the space posterior to the sigmoid sinus is very narrow. The posterior margin of the sigmoid sinus is still a considerable distance from the pontobulbar cistern. If the exposure is insufficient, the distance to the pontomedullary cistern will be even greater, making it more difficult to release cerebrospinal fluid and create operative space, which will cause significant difficulty for the surgery. 3.3 Precautions During Craniotomy Based on previous surgical experience, the key to safely access the surgical field lies in the release of cerebrospinal fluid (CSF) to reduce intracranial pressure, allow cerebellar relaxation, and provide adequate operating space. We advocate for a minimally invasive keyhole approach, which involves a small bone window. This can sometimes make CSF release challenging. Therefore, we typically place a lumbar drain preoperatively to allow for the controlled release of CSF intraoperatively, thereby reducing intracranial pressure. Furthermore, when creating the bone window, we recommend further extending the bone removal anteroinferiorly towards the UPDS with a diamond burr. This facilitates access to the cerebellomedullary cistern, promoting CSF release. It is crucial to avoid directly entering the cerebellopontine angle (CPA) from the lateral aspect of the cerebellum. This approach not only hinders CSF release but also significantly increases the risk of cerebellar contusion, potentially making the surgery difficult or even leading to failure. The exact extent of this additional bone removal has not yet been precisely measured, and the surgeon can determine the appropriate amount based on the specific intraoperative situation. We believe these are very practical tips during retrosigmoid small keyhole craniotomy. 3.4 Limitations Despite demonstrating the importance of the UPDS in locating the Key Point and its role in subsequent surgery through numerous anatomical and clinical studies, our study has several limitations: 1) The limited availability of skull and cadaveric specimens resulted in a small sample size, potentially reducing the reliability of our data. 2) Currently, various navigation and endoscopic techniques are well-established, but we did not perform comparative data analysis with these techniques. 3) We only studied patient data from certain regions of China. Given the significant anatomical variations among different ethnicities worldwide, anatomical data is likely to differ across regions, meaning our findings may not be applicable to other countries. However, with the ongoing development of various technologies, we may be able to obtain more data through other methods, even including skull data from various ethnicities. For example, 3D printing technology could be used to print patient-specific skulls of the same size, allowing for experimentation and measurement on these printed skulls to acquire a large amount of data. Researchers in other countries could also conduct corresponding studies based on this concept to further improve the research results. Conclusion The Innovative Concept of craniotomy based on the "one point, two lines, two distances" theory is safe and accurate. After forming the Key Hole according to the aforementioned localization method, a high-speed milling cutter is used to cut the skull from the Key Hole towards the occiput in a posterior-inferior-anterior sequence in turn. Upon reaching the UPDS, the milling cutter is moved superiorly, stopping at the level of the mastoid emissary vein canal, and then the cutter is withdrawn. Secondly, the milling cutter is used to cut the mastoid bone from the Key Hole downwards the UPDS, stopping at the level of the mastoid emissary vein canal, and then the cutter is withdrawn. Thirdly, the remaining bone near the posterior edge of the sigmoid sinus adjacent to the mastoid emissary vein canal is removed with a grinding drill, and the bone flap is removed. Fourthly, the mastoid bone at the posterior edge of the sigmoid sinus is quantitatively removed with a grinding drill to ensure that the anterior edge of the bone window is located at the posterior edge of the sigmoid sinus, forming a bone window approximately 2.5 cm in diameter. Finally, the mastoid bone at the posterior lower edge of the sigmoid sinus near the UPDS is removed for better release of cerebrospinal fluid. Our proposed theory and method for Key Point localization and minimally invasive craniotomy have been validated in both cadaveric specimens and clinical surgeries. we plan to increase the number of samples and further validate the accuracy of this method in combination with neuronavigation in future surgeries. Abbreviations FHP: the Frankfort horizontal plane TM: tip of the mastoid UPDS ： the up point of digastatic sulci JPTS: key point,the Junction Point of the Transverse and Sigmoid EF: mastoid emissary vein fossa SP: star point KP: key hole point IPS: the Inferior margin of transverse sinus PSS: the posterior margin of sigmoid sinus SNL: superior nuchae line AICA: anterior inferior cerebellar artery SCA: superior cerebellar artery VA: vertebral artery BA: basilar artery MRI: magnetic resonance imaging Declarations Conflicts of Interest : The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Author Contributions Chang-chun Liao: data collection, data analysis and interpretation, writing and revising the manuscript. Xunjie-Ma, DeZhu An, Bai-cha Tang, Ling Zhou,Kai-hua Wu, Jia-yan Li, Xin-feng-Yi, Zhi-heng Jian, Zhi-jian Weng, Min-Feng Sheng,De-xiang Zhou: data collection, writing and revising the manuscript. Gang Chen: drafting of the study, writing and revising the manuscript. All authorsread and approved the final manuscript. Funding This work was supported by the following: Guangzhou City and Jinan University Associated Project Foundation (grant nos.SL2023A03J00347) and Zhuhai People’s Hospital scientific initiation project (grant nos.2021KYQD-02) to Gang Chen. Acknowledgments The authors thank the participants and all who were involved in our study. Disclosure None Ethic statement This study is a anatomical study and did not involved any human participants.this studies were reviewed and approved by [ethics committee of Zhuhai people’s hospital]. References Basma J, Anagnostopoulos C, Tudose A, et al. History, variations, and extensions of the retrosigmoid approach: anatomical and literature review[J]. Journal of Neurological Surgery Part B: Skull Base, 2022, 83(S 02): e324-e335. Gulec S, Spedicato F, Senjaya F, et al. Lateral Suboccipital Approach (Retrosigmoid)[J]. Neurovascular Surgery: Surgical Approaches for Neurovascular Diseases, 2019: 43-48. Matsushima T, Matsushima T. The Retrosigmoid Lateral Suboccipital Approach: Basic Approach and Variations[J]. Microsurgical Anatomy and Surgery of the Posterior Cranial Fossa: Surgical Approaches and Procedures Based on Anatomical Study, 2015: 109-126. Lee S, Park S K, Joo B E, et al. A surgical strategy to prevent delayed epidural hematoma after posterior fossa surgery using lateral suboccipital retrosigmoid approach[J]. Journal of Clinical Neuroscience, 2018, 52: 156-158. Wong A K, Wong R H. Keyhole retrosigmoid approach without watertight dural closure–Evaluation of a minimalistic approach[J]. Clinical Neurology and Neurosurgery, 2023, 227: 107625. ZHENG C S Y B, Wan L F U Z Y. Microanatomy of suboccipital retrosigmoid keyhole approach via petrosal fissure and cerebello-pontine fissure[J]. Acta Anatomica Sinica, 2018, 49(5): 646. Dabecco R, Maniakhina L, Borghei-Razavi H. Keyhole retrosigmoid approach for resection of a petro-tentorial meningioma causing trigeminal neuralgia[J]. Clinical Neurology and Neurosurgery, 2023, 229: 107723. Mwachaka P M, Hassanali J, Odula P O. Anatomic position of the asterion in Kenyans for posterolateral surgical approaches to cranial cavity[J]. Clinical Anatomy: The Official Journal of the American Association of Clinical Anatomists and the British Association of Clinical Anatomists, 2010, 23(1): 30-33. Avci E, Kocaogullar Y, Fossett D, et al. Lateral posterior fossa venous sinus relationships to surface landmarks[J]. Surgical neurology, 2003, 59(5): 392-397. Day J D, Kellogg J X, Tschabitscher M, et al. Surface and superficial surgical anatomy of the posterolateral cranial base: significance for surgical planning and approach[J]. Neurosurgery, 1996, 38(6): 1079-1084. Kubo M, Mizutani T, Shimizu K, et al. New methods for determination of the keyhole position in the lateral suboccipital approach to avoid transverse-sigmoid sinus injury: proposition of the groove line as a new surgical landmark[J]. Neurochirurgie, 2021, 67(4): 325-329. Tao F U, Jie S, HUANG J, et al. Localization of key holes in adult retrosigmoid sinus approach based on skull surface anatomic markers[J]. Chinese Journal of Contemporary Neurology & Neurosurgery, 2022, 22(12): 1079. Raso J L, Gusmão S N S. A new landmark for finding the sigmoid sinus in suboccipital craniotomies[J]. Operative Neurosurgery, 2011, 68: ons1-ons6. Kurucz P, Baksa G, Patonay L, et al. Endoscopic approach-routes in the posterior fossa cisterns through the retrosigmoid keyhole craniotomy: an anatomical study[J]. Neurosurgical Review, 2017, 40: 427-448. Jian Z H , Sheng M F , Li J Y ,et al.Developing a Method to Precisely Locate the Keypoint During Craniotomy Using the Retrosigmoid Keyhole Approach: Surgical Anatomy and Technical Nuances[J].Frontiers in surgery, 2021, 8:700777.DOI:10.3389/fsurg.2021.700777. Jian Z H, Sheng M F, Li J Y, et al. Precise localization in craniotomy with a retrosigmoid keyhole approach: microsurgical anatomy and clinical study[J]. Frontiers in Surgery, 2022, 9: 809098. Jian Z H, Sheng M F, Liao C C, et al. A novel theory for rapid localization of the transverse-sigmoid sinus junction and “keyhole” in the retrosigmoid keyhole approach: micro-anatomical study, technique nuances, and clinical application[J]. Neurosurgical Review, 2024, 47(1): 331. Kim L K P, Ahn C S, Fernandes A E L. Mastoid emissary vein: anatomy and clinical relevance in plastic & reconstructive surgery[J]. Journal of Plastic, Reconstructive & Aesthetic Surgery, 2014, 67(6): 775-780. Jr L R, Loukas M, Wartmann C T, et al. Clinical anatomy of the mastoid and occipital emissary veins in a large series.[J]. Surgical and Radiologic Anatomy, 2009, 31(2):139-144. Murlimanju B V, Chettiar G K, Prameela M D, et al. Mastoid emissary foramina: an anatomical morphological study with discussion on their evolutionary and clinical implications[J]. Anatomy & cell biology, 2014, 47(3): 202-206. Tables Table 1. Summary table of patient demographics (n=122) Characteristic Trigeminal neuralgia hemifacial spasm Acoustic neuroma Meningioma jugular foramen neurinoma Total NO 31 67 17 5 2 122 Sex, n Male 9 31 6 2 0 48 Female 22 36 11 3 2 74 Age(years),mean (range) 54.7(39-72) 51.2(26-69) 49.2(29-69) 48.8(32-64) 44.0(37-51) 51.6(26-72) Side affected, n Left 14 29 7 3 1 54 Right 17 38 10 2 1 68 Table 2. The relationship between KP 、 UPDS and PSS Length（mm） width（mm） area(mm 2 ) L 14.71±2.19 3.26±0.55 35.93±8.30 R 14.29±2.01 3.64±0.72 36.11±7.38 t 0.440 -1.324 0.052 P 0.517 0.710 0.531 Length: the straight-line distance between KP and UPDS; width: the maximum perpendicular distance between Length and PSS; area: the area between between Length and PSS; L：left，R：right. Additional Declarations No competing interests reported. Supplementary Files video.doc Video1：A video about the Innovative concept of craniotomy in the retrosigmoid keyhole approach in a cadaveric head specimen. Video2：A video about the clinical application of the Innovative concept of craniotomy in the retrosigmoid keyhole approach. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7557430","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":518052232,"identity":"96465b2f-f16a-4cab-a273-bcbdb5ba2444","order_by":0,"name":"Chang-chun 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Chen","email":"","orcid":"","institution":"Zhuhai People's Hospital(The Affiliated Hospital of Beijing Institute of Technology, Zhuhai Clinical Medical College of Jinan University)","correspondingAuthor":false,"prefix":"","firstName":"Gang","middleName":"","lastName":"Chen","suffix":""}],"badges":[],"createdAt":"2025-09-07 15:53:22","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7557430/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7557430/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":92542554,"identity":"0c080eba-09ce-4595-ac35-7da17e3fd023","added_by":"auto","created_at":"2025-09-30 19:24:34","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":6786778,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eLocalization of Key Points and Key Hole Points.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e(1A) Surface view of structures in a normal skull. (1B) The relationship between UPDS and JPTS. (1C) The relationship between UPDS and KP. (1D) Lateral view of the skull and FHP projection. (1E) Vertical drilling at UPDS. (1F) Internal view of the skull showing the drilled hole at UPDS, corresponding to the margin of the sigmoid sinus. FHP:the Frankfort horizontal plane;TM:tip of the mastoid; UPDS：the up point of digastatic sulci; JPTS:key point,the Junction Point of the Transverse and Sigmoid; EF:mastoid emissary vein fossa; SP:star point; KP:key hole point.\u003c/p\u003e","description":"","filename":"Fig1image.png","url":"https://assets-eu.researchsquare.com/files/rs-7557430/v1/1907a65d43f8f760651ceb2c.png"},{"id":92542557,"identity":"90de6546-9e07-424f-9d37-cabfb6d0b2b8","added_by":"auto","created_at":"2025-09-30 19:24:34","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":7238162,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eRelationship Between Dry Skull KP, UPDS, and PSS.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e(2A) The KP, SP and transverse sigmoid sinuses were identified on the inner surface of the skull. (2B) A 6 mm hole drilled from the key point (JPTS). (2C) External surface marking of various anatomical points. (2D) Internal surface marking of various anatomical points. (2E) Yellow curves indicate the PSS, and AB represents the required length for bone removal, CD represents the Depth. (2F) Blue areas indicate further bone removal. \u0026nbsp;IPS:the Inferior margin of transverse sinus; PSS:the posterior margin of sigmoid sinus.\u003c/p\u003e","description":"","filename":"Fig2image.png","url":"https://assets-eu.researchsquare.com/files/rs-7557430/v1/cf18cbb088149802671569b1.png"},{"id":92542643,"identity":"c607701f-61d6-4c21-bbcc-93fbc2eb54c9","added_by":"auto","created_at":"2025-09-30 19:27:11","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":5171358,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eDemonstration of Innovative suboccipital retrosigmoid keyhole approach on Dry Skulls.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e(3A) Surface view of the sigmoid sinus posterior region. (3B) Landmarks were observed from the outer surface. (3C) The solid green area represents the body surface projection of the transverse sinus and sigmoid sinus groove, and the red area represents the direction of the EF. (3D) Bone milling in the posterior direction follow the direction of the blue arrow in the picture . (3E) Bone milling in the other direction downward to the level of the EF. (3F)The blue area represents the original extent of the bone flap. (3G)Yellow curves indicate the PSS, and AB represents the required length for bone removal, CD represents the Depth. (3H) The blue areas represent the areas of bone that need to be further removed. (3I) Final range of bone window viewed from the intracranial side. SNL:superior nuchae line.\u003c/p\u003e","description":"","filename":"Fig3image.png","url":"https://assets-eu.researchsquare.com/files/rs-7557430/v1/f510e620eeef01775b47e02b.png"},{"id":92542555,"identity":"045053d9-a448-4f56-9c6b-69071cf16b1f","added_by":"auto","created_at":"2025-09-30 19:24:34","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":3791953,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eValidation Using Cadaveric Heads.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e(4A) Mark TM, UPDS, FHP and the incision line on the skin; (4B) Mark he UPDS. (4C) Determine the KP and drilled; (4D) Milling the skull with the milling cutter from the key hole toward the posterior, downward, and forward directions; (4E) Milling the mastoid bone from the key hole downward to the UPDS; (4F) Remove the residual bone in the mastoid to form the final bone window. (4G) \"Y\" shape cut the dural. (4H) The outer lower part of the cerebellum was retracted to observe and reveal the glossopharynx, vagus and accessory nerves. (4I) The outer middle part of the cerebellum was retracted to observe and reveal the facio-auditory cranial nerve and anterior inferior cerebellar artery. (4J) The outer upper part of the cerebellum was retracted to reveal the trigeminal nerve and superior cerebellar artery. (4K, L, M, N) further retract the outer lower part of the cerebellum, observe and reveal the glossopharynx, vagus, accessory, hypoglossal nerve, vertebral artery, and posterior inferior cerebellar artery. (4O) Retract the deep outer middle part of the cerebellum, observe and reveal the abducens nerve, vertebral artery and the initial segment of the anterior inferior cerebellar artery. AICA:anterior inferior cerebellar artery; SCA:superior cerebellar artery; VA: vertebral artery; BA: basilar artery; Ⅶ: facial nerve, Ⅷ: auditory nerve, Ⅸ: glossopharyngeal nerve, Ⅹ: vagus nerve, Ⅺ: accessory nerve, Ⅻ: hypoglossal nerve.\u003c/p\u003e","description":"","filename":"Fig4image.png","url":"https://assets-eu.researchsquare.com/files/rs-7557430/v1/e3b7bc3d462c5606afd96033.png"},{"id":92542592,"identity":"1feb15ba-c6ec-4a69-9416-c9f1ca96c859","added_by":"auto","created_at":"2025-09-30 19:25:30","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":4284236,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eProcedure of microvascular decompression in a patient with left facial spasm.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e(5A) Preoperative MRI showed that the left vertebral artery was closely related to the left facial nerve. (5B) Three-dimensional reconstruction of the skull. (5C) Preoperative multimodal image fusion. (5D) Mark TM, UPDS, FHP and the incision line on the skin; (5E) Incision of the scalp, exposure of the skull and exposure of the UPDS. (5F) Mark a distance of 14.6mm above the UPDS; (5G) Mark a distance of 6.6mm behind. (5H) The drilling point, KP. (5I) Drill a hole at KP about 6mm. (5J) Milling the skull with a milling cutter from the key hole to the back of the occipital in a posterior-inferior-anterior sequence. (5K) Remove the residual bone in the mastoid of the posterior margin of the sigmoid sinus and release the bone flap. (5L) Continued to remove residual bone toward the edge of the sigmoid sinus. The green line in the figure represents the extent of bone removal. (5M) \"Y\" cut the dural. (5N) Left VA and PICA entrapment were found at the root of the facial nerve. (5O) After arachnoid separation, telflon was used to place between the nerve and vessels. (5P) Close suture of the dural membrane. (5Q) The bone flap was fixed with a titanium connector. (5R) bone flap size was about 1.5*1.8cm, (5S) actual incision length was about 4.2cm. (5T) Postoperative craniocerebral MRI. (5U) Postoperative multimodal image fusion. (5V) Postoperative 3D reconstruction of the skull. MRI:magnetic resonance imaging; Ⅴ: trigeminal nerve; Ⅶ: facial nerve, Ⅷ: auditory nerve.\u003c/p\u003e","description":"","filename":"Fig5image.png","url":"https://assets-eu.researchsquare.com/files/rs-7557430/v1/c481210646698aec9ab3a2c5.png"},{"id":92542595,"identity":"1cd5149c-59b2-4201-be32-9d842f6f1d5a","added_by":"auto","created_at":"2025-09-30 19:25:30","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":1311287,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eA, B, and C\u003c/strong\u003e respectively show the shape and size of the bone that needs to be further removed toward the sigmoid sinus edge after forming an initial bone flap in specimens 1-12.\u003c/p\u003e","description":"","filename":"Fig6image.png","url":"https://assets-eu.researchsquare.com/files/rs-7557430/v1/a413191ac3914e824035202f.png"},{"id":92542594,"identity":"002dba32-f2a9-41af-b67e-6e362e9ff2e9","added_by":"auto","created_at":"2025-09-30 19:25:30","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":4141442,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSurgical procedure in a patient with acoustic neuroma.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e(7A-C) Preoperative MRI enhancement suggested a tumor in the right CPA region. (7D-F) Preoperative multimodal image fusion imaging. (7G) Three-dimensional reconstruction of the skull. (7H) Mark TM, UPDS, FHP and the incision line on the skin. (7I) Incision of the scalp, exposure of the skull and the UPDS. (7J) Mark a distance of 14.5mm above the UPDS; (7K) Mark a distance of 6.6mm behind. (7L) The drilling point (KP). (7M) Drill a hole at KP about 6mm. (7N) Milling the skull with a milling cutter from the key hole to the back of the occipital in a posterior-inferior-anterior sequence. (7O) Remove the residual bone in the mastoid of the posterior margin of the sigmoid sinus and release the bone flap. (7P) The final bone window. (7Q) The middle part of the cerebellum was retracted and the tumor was explored. (7R) The tumor was removed piecemeal under electrophysiological monitoring. (7S). Resection of the tumor in the internal auditory canal. (7T).The tumor was totally removed and the internal auditory canal is filled with muscles. (7U-W) Postoperative MRI enhancement Showed that the tumor was totally removed. (7X) Postoperative 3D reconstruction.\u003c/p\u003e","description":"","filename":"Fig7image.png","url":"https://assets-eu.researchsquare.com/files/rs-7557430/v1/0e21cb81e8d8107c33d1e635.png"},{"id":92542593,"identity":"f9999725-7f7d-4253-b383-0ae2da51a36b","added_by":"auto","created_at":"2025-09-30 19:25:30","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":4475231,"visible":true,"origin":"","legend":"\u003cp\u003eProcedure of microvascular decompression in a patient with left trigeminal neuralgia. (8A) Preoperative MRI showed that the left SCA was closely related to the left trigeminus nerve. (8B) Preoperative multimodal image fusion imaging. (8C) Three-dimensional reconstruction of the skull. (8D) Mark TM, UPDS, FHP and the incision line on the skin. (8E) Mark a distance of 14.5mm above the UPDS; (8F) Mark a distance of 6.6mm behind. (8G) The drilling point (KP). (8H) Drill a hole at KP about 6mm. (8I)Milling the skull with a milling cutter from the key hole to the back of the occipital in a posterior-inferior-anterior sequence. (8J) Remove the residual bone in the mastoid of the posterior margin of the sigmoid sinus and release the bone flap. (8K) The original bone window. (8L) Continued to remove residual bone toward the edge of the sigmoid sinus. (8M) The final bone window. (8N) \"Y\" cut the dural. (8O) Left SCA were found at the root of the trigeminus nerve. (8P) After arachnoid separation, telflon was used to place between the nerve vessels. (8Q) Postoperative craniocerebral MR Showed that the SCA was far away from the right trigeminus nerve. (8R) Postoperative multimodal image fusion. (8S) Postoperative 3D reconstruction of the skull.\u003c/p\u003e","description":"","filename":"Fig8image.png","url":"https://assets-eu.researchsquare.com/files/rs-7557430/v1/b63f2402144c29e1477a58c5.png"},{"id":92542765,"identity":"84cb002c-bb5e-4e5c-8391-4fac2ac8de27","added_by":"auto","created_at":"2025-09-30 19:32:36","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":5171358,"visible":true,"origin":"","legend":"\u003cp\u003e3\u003c/p\u003e","description":"","filename":"Fig3image.png","url":"https://assets-eu.researchsquare.com/files/rs-7557430/v1/7f308e6599f33ff4cbc4f93d.png"},{"id":99798775,"identity":"74e85530-1883-4415-b51a-fa3d1181952a","added_by":"auto","created_at":"2026-01-08 13:48:54","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":52855433,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7557430/v1/171a3921-d696-4da6-aa88-95924322108c.pdf"},{"id":92012034,"identity":"7ab9e0a8-a5b2-48b1-bb58-245a023dba84","added_by":"auto","created_at":"2025-09-23 15:51:06","extension":"doc","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":11776,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eVideo1：\u003c/strong\u003eA video about the Innovative concept of craniotomy in the retrosigmoid keyhole approach in a cadaveric head specimen.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eVideo2：\u003c/strong\u003eA video about the clinical application of the Innovative concept of craniotomy in the retrosigmoid keyhole approach.\u003c/p\u003e","description":"","filename":"video.doc","url":"https://assets-eu.researchsquare.com/files/rs-7557430/v1/3ebec44b01ec05d6ab2da083.doc"}],"financialInterests":"No competing interests reported.","formattedTitle":"The Innovative Concept of Craniotomy in suboccipital retrosigmoid keyhole approach: anatomical study, technique nuances, and clinical application","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe suboccipital retrosigmoid approach to the sigmoid sinus is a classic surgical route for treating lesions in the cerebellopontine angle \u003csup\u003e[1-4]\u003c/sup\u003e. With the advancement of minimally invasive techniques, the suboccipital retrosigmoid keyhole approach, involving small incisions, micro bone windows, and bone flap repositioning, has been increasingly recognized by scholars\u003csup\u003e\u0026nbsp;[5-7]\u003c/sup\u003e. The ideal skin incision for this approach is approximately 4 cm, with the bone window ranging from 2 to 2.5 cm in size. The upper edge of the bone window exposes the inferior border of the transverse sinus, while the anterior edge exposes the posterior margin of the sigmoid sinus, effectively avoiding damage to the transverse and sigmoid sinuses. Moreover, good visibility during surgery is critical, demanding high precision from neurosurgeons. To minimize unnecessary exposure of skin and bone, and to prevent injury to the transverse and sigmoid sinuses, it is essential to accurately locate the bony landmark corresponding to the junction of the transverse and sigmoid sinuses and the key point of the craniotomy.\u003c/p\u003e\n\u003cp\u003eCurrently, various localization methods of craniotomy have been proposed in the literature \u003csup\u003e[8-13]\u003c/sup\u003e, utilizing cranial surface anatomical landmarks and imaging technologies. However, these methods often suffer from inaccuracies in localization and operational complexity, limiting their clinical applicability.\u003c/p\u003e\n\u003cp\u003eDuring the craniotomy process of the suboccipital retrosigmoid keyhole approach, the formation of bone flap is achieved using a grinding drill and a milling cutter. The bone flap is repositioned and fixed at the end of the procedure to avoid skull defects. The procedure requires careful navigation around important structures such as the transverse sinus, sigmoid sinus, and mastoid emissary vein in a confined space. It remains a key focus for clinicians that ensuring precise and safe bone cutting, bone flap formation, and exposure of intracranial structures after dural opening while avoiding injury to venous sinuses and ensuring proper exposure of neural and vascular structures\u003csup\u003e\u0026nbsp;[1-3, 5, 14]\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eIn previous studies, we have demonstrated that the up point of digastatic sulci is an important landmark for locating key anatomical points \u003csup\u003e[15-17]\u003c/sup\u003e. There is a fixed relationship between this landmark and others. Using the Frankfurt horizontal plane projection line as a reference, the key point is located approximately 16.97 ± 2.50 mm vertically above the up point of digastatic sulci and 4.80 ± 1.87 mm horizontally posterior to it. We refer to this landmark localization technique as the \"one point, two lines, two distances\" theory \u003csup\u003e[15-17]\u003c/sup\u003e. However, how to precisely and safely mill the cranium in the confined space using a grinding drill and milling cutter, while avoiding injury to the transverse and sigmoid sinuses and ensuring adequate exposure of intracranial structures, remains a critical issue.\u003c/p\u003e\n\u003cp\u003eThis study, conducted from January 2020 to December 2024, utilized adult dry skull specimens to analyze the relationships between cranial surface anatomical landmarks. The research aimed to explore precise localization of craniotomy and the safe, rapid bone flap formation technique for the suboccipital retrosigmoid keyhole approach. Feasibility was verified through cadaveric dissections, followed by clinical application to evaluate the effectiveness of this safe, rapid bone flap formation method. The findings aim to provide clinical evidence for achieving safe, precise, and rapid craniotomy.\u003c/p\u003e"},{"header":"Materials and Methods","content":"\u003cp\u003e\u003cstrong\u003e1.1 Specimens and Instruments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTwelve adult specimens of dry skulls and twelve wet cadaveric specimens were selected for research. All skulls had no significant pathological deformities in the posterior fossa, temporal bone petrous part, mastoid area, or occipital bone. The following instruments were used: microscope (OPMI PROergo, Carl Zeiss AG, Germany), craniotomy power system (MicrospeeduniGD670, Aesculap, Germany), steel ruler (DL8050, Deli Group Co., Ltd., China), digital caliper (AS0020151, Shanghai Tool Factory, China), digital angle ruler (Casio 0-200mm, Henan Bant Tool Co., Ltd., China), custom head frame, and micro instruments. The software used for analysis was 3D-Slicer 4.11.20210226.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e1.2 Patient cohort\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study retrospectively analyzed 122 patients who underwent suboccipital retrosigmoid craniotomy via the sigmoid sinus approach in the Neurosurgery Department from January 2020 to December 2024. The study was approved by the Medical Ethics Committee of this hospital. Written informed consent was obtained from each subject. All craniotomies followed the technique proposed in this study. The surgeries were performed under a ZEISS KINEVO 900 microscope(Carl Zeiss AG, Germany) and intraoperative electrophysiological monitoring (Nicolet PROTEKTOR 32, Thermo Nicolet Corporation, America).\u003c/p\u003e\n\u003cp\u003eAmong the 122 patients, 31 had trigeminal neuralgia, 67 had hemifacial spasm, 17 had acoustic neuroma, 5 had meningiomas, and 2 had cholesteatomas. There were 54 males and 68 females, aged 26 to 72 years, with a mean age of 51.6 ± 10.4 years. There were 36 left-sided lesions and 49 right-sided lesions. (Table 1)\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e1.3 Experimental Methods\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e1.3.1 Innovative Concept of Craniotomy on the Dry Skulls\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e1.3.1.1 Localization of Key Points and Key Hole Points on Dry Skulls\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eBased on our previous research, a coordinate system was established using the Frankfurt horizontal plane (FHP) as the X-axis and a vertical line through the up point of digastatic sulci (UPDS) as the Y-axis, with UPDS as the origin (Figure 1A, B). The vertical and horizontal average distances between the key point(the Junction Point of the Transverse and Sigmoid sinuses, JPTS) and UPDS were 16.97 ± 2.50 mm and 4.80 ± 1.87 mm, respectively. The vertical and horizontal average distances between the key hole point(KP) and UPDS were 14.58 ± 2.58 mm and 6.60 ± 1.72 mm, respectively (Figure 1C, D). This method is referred to as the “One Point, Two Lines, Two Distances” technique: \"One Point\" is the up point of digastatic sulci (UPDS), \"Two Lines\" are the FHP line (X-axis) and the vertical line through UPDS (Y-axis), and \"Two Distances\" refer to the vertical and horizontal distances \u003csup\u003e[15-17]\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e1.3.1.2 Relationship Between the UPDS and the Posterior Margin of the Sigmoid Sinus\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eIn six cadaveric skulls, a hole of approximately 6 mm in diameter was drilled at the UPDS into the outer surface, and the relationship between the hole and the sigmoid sinus was examined from the inner surface (Figure 1E,F).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e1.3.1.3 Relationship Between Key Points, the Key Hole Point, the UPDS, and the Posterior Margin of the Sigmoid Sinus in Dry Skull\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAfter creating a bone window along the line connecting the key hole point and UPDS, there remained a distance from the anterior edge of bone window to the sigmoid sinus margin. In order to just expose the posterior margin of the sigmoid sinus, the bone surrounding the mastoid part of the sigmoid sinus needed to be removed. We first identified and marked the Inferior margin of transverse sinus (IPS), the posterior margin of sigmoid sinus (PSS), and key point (the Junction Point of the Transverse and Sigmoid sinuses,JPTS) on the internal surface of the skull (Figure 2A). At the key point (JPTS), a 6 mm hole was drilled vertically into the skull (Figure 2B). This hole’s center point is the key hole point (KP) (Figure 2C). On the external surface, the UPDS, mastoid tip (TM), star point (SP), key point (JPTS), and mastoid emissary vein fossa (EF) were marked (Figure 2C), followed by drilling a 6 mm hole from UPDS toward the internal surface (Figure 2D).\u003c/p\u003e\n\u003cp\u003eThe characteristics of the bone of the posterior border of the sigmoid sinus to be removed were drawn 1:1 with pape, and the length (the distance between UPDS and KP), width (the maximum vertical distance between the posterior margin of the sigmoid sinus and the connection between UPDS and KP), and area (the area formed by the connection between UPDS and KP and PSS) were measured(Figures 2E, F).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e1.3.1.4 Innovative Concept of suboccipital retrosigmoid keyhole approach on the dry skull\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e(1) Localization of the key hole point: Using the “One Point, Two Lines, Two Distances” method, the UPDS was located, and the KP was determined (Figure 3A–C).\u003c/p\u003e\n\u003cp\u003e(2) Formation of bone flaps:\u003c/p\u003e\n\u003cp\u003eⅠ- According to the original concept of craniotomy, the hole was drilled at the key hole point (6-10 mm diameter) in sequence (Figure 3B–C).\u003c/p\u003e\n\u003cp\u003eⅡ- A milling cutter was used to mill the bone from the key hole toward the posterior, downward, and forward directions in turn. Once the UPDS was reached, the cutter was moved upward to the level of the emissary vein, and the cutter was withdrawn (Figure 3D).\u003c/p\u003e\n\u003cp\u003eⅢ-\u0026nbsp;The same method was applied for the mastoid bone from the other direction (Figure 3E).\u003c/p\u003e\n\u003cp\u003eⅣ-\u0026nbsp;The original bone window was formed after removing the bone flap (Figure 3F).\u003c/p\u003e\n\u003cp\u003eⅤ- Residual bone near the sigmoid sinus was removed using a grinding drill referring to the results(the length, width and area measured above), exposing the bone window of approximately 2.5 cm in diameter (Figure 3G–I).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e1.3.2 Validation On Cadaveric Specimens\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eBased on the dry skull results, the “One Point, Two Lines, Two Distances” method and the innovative concept of craniotomy were validated on cadaveric specimens(video 1). The following steps were performed:\u003c/p\u003e\n\u003cp\u003e(1)\u0026nbsp;\u003cstrong\u003eMarking anatomical landmarks on the skin\u003c/strong\u003e: TM, UPDS, the Frankfort horizontal plane (FHP).(Figure 4A)\u003c/p\u003e\n\u003cp\u003e(2)\u0026nbsp;\u003cstrong\u003eIncision placement\u0026nbsp;\u003c/strong\u003e: A straight vertical incision measuring approximately 4 cm was made 1.0 cm posterior to the UPDS. Approximately one-quarter of the incision was made above the FHP. The inferior border of the incision reached the level of the mastoidale.(Figure 4A)\u003c/p\u003e\n\u003cp\u003e(3)\u0026nbsp;\u003cstrong\u003eExposure of the skull:\u003c/strong\u003e Incise the scalp, expose the skull, and determine the UPDS.\u003c/p\u003e\n\u003cp\u003e(4)\u0026nbsp;\u003cstrong\u003eLocate key holes:\u003c/strong\u003e According to the \"one point, two lines, two distances\" method, the key hole points (KP) were determined (Figure 4C).\u003c/p\u003e\n\u003cp\u003e(5)\u0026nbsp;\u003cstrong\u003eFormation of bone flaps:\u003c/strong\u003e ① According to the original concept of craniotomy, drill holes at key hole points, with a diameter of 6mm (Figure 4C); ② Mill the skull with the milling cutter from the key hole toward the posterior, downward, and forward directions in turn. After reaching the UPDS, the milling cutter moves upward, terminates at the level of the EF, and then the milling cutter was withdrawn(Figure 4D); ③ Milling the mastoid bone with a milling cutter from the key hole to the UPDS, terminating at the level of the EF, and withdrawn the milling cutter (Figure 4E); ④ Milling most of the bone of mastoid emissary vein catheter. ⑤ Original bone flap: Pry off the flap, and the original bone window formed.\u003c/p\u003e\n\u003cp\u003e(6)\u003cstrong\u003e\u0026nbsp;Milling of bone near the sigmoid sinus to create the final bone window\u003c/strong\u003e: The residual bone in the mastoid of the posterior margin of the sigmoid sinus was removed with a drill. The bone window margin was investigated and determined to be located at the posterior margin of the sigmoid sinus, forming a bone window with a diameter of approximately 2.5cm (Figure 4F).\u003c/p\u003e\n\u003cp\u003e(7)\u0026nbsp;\u003cstrong\u003eExposure of intracranial structures:\u003c/strong\u003e The dural was cut in a \"Y\" shape (Figure 4G), and then Pullegd aside the cerebellum and observed various structures such as nerves and blood vessels in the cerebellopontine Angle region (Figure 4H-O).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e1.3.3 Clinical Application of the Innovative Concept of craniotomy\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eIn clinical cases, the “One Point, Two Lines, Two Distances” localization method and the innovative concept of craniotomy were applied in 122 patients(video2). \u0026nbsp;The following steps were performed:\u003c/p\u003e\n\u003cp\u003e(1)Preoperative CT and MR were obtained with 3D reconstructions for surgical planning.(Figure 5A-C).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e(2) All surgical procedures were performed by senior neurosurgeon, and lateral decubitus position was adopted after full and satisfactory anesthesia for tracheal intubation.\u003c/p\u003e\n\u003cp\u003e(3) The craniotomy was performed with a 4-5 cm incision (Figure 5D).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e(4) Formation of bone flaps: Made the final bone window according to the innovative concept of craniotomy mentioned above (Figure 5E-L).\u003c/p\u003e\n\u003cp\u003e(5)Further operation: Y-shaped incision of the dural membrane (Figure 5M), then intracranial operation according to the patient's disease type (Figure N- O).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e(6) Dural suture and bone flap reduction: During intracranial closure, the dural membrane was repaired with close suture (Figure 5P), and bone flap reduction was performed (Figure 5Q, R).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e(7) Thin-slice CT scan, MR Scan and three-dimensional reconstruction of the nerves, vessels, brain stem and skull were performed again after surgery (Figure 5T, U) to verify the accuracy of the positioning method.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e1.3.4 Evaluation Criterion for Cadaveric Validation and Clinical application.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e1) Evaluation Criterion: ① Accuracy: Whether the localization and exposure of local anatomical structures are precise and sufficient.② Safety: Whether there is any damage to local major anatomical structures related to accurate craniotomy.③ Speed: Measurement of time of craniotomy, specifically the time taken from skin incision to bone window exposure. ④ Minimally Invasive Nature: The size of the bone flap and bone window, and whether the bone flap is repositioned.⑤ Treatment effect: Whether the patient's preoperative symptoms were relieved after surgery, and whether there were postoperative complications such as death, coma, hemiplegia, facial paralysis, facial numbness, leakage of cerebrospinal fluid, tinnitus and so on\u003c/p\u003e\n\u003cp\u003e2) Follow-Up:\u003c/p\u003e\n\u003cp\u003eAll patients underwent telephone or outpatient follow-up 1–6 months post-surgery to observe whether the patients have recurrence or other complications after the operation.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e1.4 Statistical Methods\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe distances between relevant structures were measured using digital calipers, with results expressed as mean ± standard deviation (SD). Data analysis was performed using SPSS 21 software, with group comparisons conducted via t-tests, with significance set at p-value\u0026lt;0.05.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003e\u003cstrong\u003e2.1 Innovative Concept of Craniotomy on the Dry Skulls\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.1.1 Relationship Between the UPDS and the Posterior Margin of the Sigmoid Sinus\u0026nbsp;\u003c/strong\u003e\u003cbr\u003eIn six cadaveric skulls, a hole approximately 6 mm in diameter was drilled vertically at the UPDS on the external surface of the skull. The position of the hole was then examined on the internal surface of the skulls. The results showed that in all six specimens, the drilled hole corresponded exactly to the edge of the sigmoid sinus, located at the posterior margin near the horizontal and vertical segments of the sigmoid sinus (Figure 1E,F). No hole was located directly on the sigmoid sinus. This result is consistent with the results of our previous study\u003csup\u003e\u0026nbsp;[15-17]\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.1.2 Relationship Between Key Points, the Key Hole Point, the UPDS, and the Posterior Margin of the Sigmoid Sinus in Dry Skull\u003c/strong\u003e\u003cbr\u003e\u0026nbsp;To further study the relationship between the key point, the Key Hole Point, UPDS, and the posterior margin of the sigmoid sinus in dry skulls, and to determine the extent of bone removal required at the posterior margin of the sigmoid sinus, data were collected from 12 dry skulls, including measurements from both left and right sides (Figures 6A, B, C). The analysis showed that the length (height) of bone that needed to be removed to reach the sigmoid sinus boundary was 14.71 ± 2.19 mm (left side), 14.29 ± 2.01 mm (right side), with no significant statistical difference between the sides(P\u0026gt;0.05), and an average of 14.50 ± 2.10 mm. The maximum depth (width) of bone removal to reach the sigmoid sinus boundary was 3.26 ± 0.55 mm (left side), 3.64 ± 0.72 mm (right side), with no significant statistical difference between sides(P\u0026gt;0.05), and an average of 3.45 ± 0.64 mm. The area of bone removal to reach the sigmoid sinus boundary was 35.93 ± 8.30 mm² (left side), 36.11 ± 7.38 mm² (right side), with no significant statistical difference between the sides(P\u0026gt;0.05), and an average of 36.02 ± 7.84 mm²(Table 2).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.2 Validation On Cadaveric Specimens\u003c/strong\u003e\u003cbr\u003eBased on the results from dry skulls and cadaveric specimens, the “One Point, Two Lines, Two Distances” method for key hole localization and the original concept of craniotomy were tested in fresh cadaveric specimens. This study preliminarily validated the accuracy, safety, and minimally invasive nature of our localization and craniotomy methods. The results were as follows:\u003cstrong\u003e①\u003c/strong\u003e \u003cstrong\u003eAccuracy\u003c/strong\u003e: The localization of the key hole point was generally accurate, and the sigmoid sinus margin, as well as other important intracranial neurovascular structures, were well exposed.②\u003cstrong\u003eSafety\u003c/strong\u003e: No damage to the venous sinuses was observed during drilling or milling in the cadaveric dissection.③ \u003cstrong\u003eSpeed\u003c/strong\u003e: Due to the use of cadaveric specimens, the dissection process and clinical surgeries differ, and thus time could not be measured. However, no particularly time-consuming or difficult situations were encountered during the dissection process.④\u003cstrong\u003eMinimally Invasive Nature\u003c/strong\u003e: The incision length was approximately 4–5 cm, with an average bone flap size of 1.8–2.5 cm, and an average bone window size of 2.1–3.0 cm.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.3 Clinical Application Results and Evaluation\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eIn 122 clinical cases, the novel method of key hole localization and craniotomy was further validated. The specific results are as follows:① Accuracy: Among the 122 cases, 99 had sufficient exposure of the transverse and sigmoid sinuses, and 23 cases achieved adequate exposure after appropriate bone removal. After dural incision, the local microscopic anatomical structures of the intracranial surgical area were well exposed.②Safety: In all 122 cases, the craniotomy was performed using the anatomical localization method developed in this study, with no venous sinus damage caused by the drilling. No injuries to important neurovascular structures occurred during the clinical procedure.③ Speed: The craniotomy time ranged from 21 to 35 minutes, with an average craniotomy time of 26.8 ± 3.6 minutes.④ Minimally Invasive Nature: The average incision length was 4.0 cm, with the longest incision not exceeding 5 cm. The bone flap size was 1.8 cm × 2.0 cm, and the bone window diameter ranged from 2.0 to 2.5 cm. The bone flap was repositioned during the procedure, fully reflecting the minimally invasive concept.\u003c/p\u003e\n\u003cp\u003eRegarding clinical outcomes, among the 122 patients who underwent Craniotomy using the suboccipital retrosigmoid keyhole approach, 24 had tumor-related diseases. During surgery, the operative field, tumor, and important structures were clearly exposed, with no serious complications postoperatively, such as death, coma, hemiplegia, leakage of cerebrospinal fluid. Despite adequate surgical field exposure, 9 patients with acoustic neuroma still experienced varying degrees of facial paralysis and facial numbness postoperatively. These complications were not related to the exposure of the surgical area but were primarily associated with the growth characteristics of the tumor. During postoperative follow-up, these symptoms showed varying degrees of improvement. Postoperative MR confirmed total resection of the tumor. Of the 98 patients with functional disorders, the responsible vessels and nerves were clearly exposed during surgery. There were no postoperative cerebrospinal fluid leaks or other serious complications. Among these, 92 patients had complete relief from trigeminal neuralgia and hemifacial spasm, and 6 (4 with trigeminal neuralgia and 2 with hemifacial spasm) had partial relief. Two patients with trigeminal neuralgia developed tinnitus and dizziness postoperatively, but no other neurological dysfunctions were observed. These cases with complications all had good visual field exposure during the operation, and there was no direct relationship with the innovative Concept of Craniotomy. No recurrence of symptoms or other major complications was noted within half a year, and the symptoms of the two patients with tinnitus were relieved half a month after the operation after rehabilitation treatment.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.4 Typical Surgical Cases\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTypical Case 1: Patient with Acoustic Neuroma\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eA 58-year-old male patient was admitted with \"progressive hearing loss in the left ear for 2 years, accompanied by facial numbness for 3 months.\" A CPA tumor was diagnosed and acoustic neuroma was considered. During surgery, we performed craniotomy based on the \"one point, two lines, and two distances\" theory and the innovative Concept of craniotomy. The key point was accurately located, avoiding any injury to the venous sinus. The surgical field was well exposed. Total tumor resection was achieved intraoperatively. Postoperatively, the patient's facial and auditory nerve functions were well preserved without facial paralysis or other surgical complications. Postoperative brain MRI confirmed no residual tumor.The general surgical process is shown in Figure 7.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTypical Case 2: Patient with Trigeminal Neuralgia\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eA 45-year-old female patient with left trigeminal neuralgia for 4 years. Preoperative magnetic resonance imaging revealed a close relationship between the left superior cerebellar artery and the trigeminal nerve. During the microvascular decompression surgery, we performed craniotomy based on the \"one point, two lines, and two distances\" theory and the innovative Concept of craniotomy. The key point was accurately located, avoiding any injury to the venous sinus. The surgical field was well exposed. The operation procedure was completed smoothly, and the patient’s facial pain symptoms completely disappeared after[1]surgery. The general operation process is shown in Figure 8.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003e\u003cstrong\u003e3.1 Relationship between the UPDS and the Sigmoid Sinus\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eIn six cranial specimens, we drilled a 6 mm diameter hole perpendicular to the outer surface of the skull at the UPDS and then observed the corresponding location on the inner surface of the skull. The results showed that in all six specimens, the drilled holes were located precisely at the trailing edge of the sigmoid sinus, approximately at the posterior margin near the junction of the horizontal and vertical segments of the sigmoid sinus. No holes were located on the sigmoid sinus itself. Furthermore, we observed that the vertical segment of the sigmoid sinus typically courses with an anteriorly convex curve. Therefore, we speculate that during craniotomy with a milling cutter, creating a bone flap along the straight-line distance between the Key Point and the UPDS is generally safe and will not injure the sigmoid sinus. This finding is of great importance as it provides crucial guidance for safely and effectively creating the bone window in the next step.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.2 How to Efficiently and Safely make the Bone Window?\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eDuring craniotomy, after locating the Key Point, the next step is to create the bone window. In our surgical philosophy, we believe that the bone flap should be routinely fixed back. Therefore, we need to investigate how to safely and accurately create the surgical bone window to minimize bone damage. We have not found any relevant authoritative literature on this aspect. We encounter many challenges during the process of bone flap creation. Therefore, we designed a safe craniotomy protocol. After drilling the burr hole at the Key Hole Point, we first use a milling cutter to cut the skull from the Key Point towards the occiput in a \"posterior-inferior-anterior\" sequence, finally dealing with the most dangerous area, the edge of the sigmoid sinus. During this process, we need to use the UPDS as a reference, ensuring the milling cutter passes just over the UPDS to get as close as possible to the sigmoid sinus without injuring it. In addition, in the retromastoid region, there is generally an emissary vein, the mastoid emissary vein. If the milling cutter passes directly through this area, it is likely to cause a tear in the emissary vein. Rupture of the emissary vein is often a cause of significant bleeding during craniotomy. The mastoid emissary vein is a intracranial-extracranial communicating vein located in the mastoid region. Its internal opening is generally in the middle of the vertical segment of the sigmoid sinus, and its external opening is located at the EF on the outer surface of the skull. The mastoid emissary vein foramen may have multiple variations. Among them, 46% of the left mastoid emissary vein foramina are single, 7% are double, 19% are multiple, and 28% are absent; on the right side, 55% are single, 13% are double, 29% are multiple, and 2% are absent\u003csup\u003e[18-20]\u003c/sup\u003e. Therefore, we recommend using a milling cutter to cut the skull from the Key Point towards the occiput in a \"posterior, inferior, anterior\" sequence, stopping near the UPDS. The next step is to use a grinding drill to grind a groove between the Key Point and the UPDS, rather than directly passing the milling cutter. Finally, an osteotome is used to pry open the bone flap, forming the initial bone window.\u003c/p\u003e\n\u003cp\u003eAfter forming the initial bone window as described above, the edge of the bone window should be a short distance from the edge of the sigmoid sinus. The presence of this remaining bone may obstruct the surgical view during the operation. Therefore, to better expose the surgical field and meet the requirements of the surgical procedure, we should continue to expose the bone window towards the sigmoid sinus by carefully grinding away the bone with a grinding drill. According to our study results, after forming the bone flap, the maximum depth of bone that needs to be further removed to reach the sigmoid sinus border is 3.26 ± 0.55 mm (left side) and 3.64 ± 0.72 mm (right side), with an average of 3.45 ± 0.64 mm. The area of bone that needs to be further removed to reach the sigmoid sinus border is 35.93 ± 8.30 mm\u003csup\u003e2\u003c/sup\u003e (left side) and 36.11 ± 7.38 mm\u003csup\u003e2\u003c/sup\u003e (right side), with an average of 36.02 ± 7.84 mm\u003csup\u003e2\u003c/sup\u003e. Continuing to expose the bone window towards the posterior edge of the sigmoid sinus is crucial because the space posterior to the sigmoid sinus is very narrow. The posterior margin of the sigmoid sinus is still a considerable distance from the pontobulbar cistern. If the exposure is insufficient, the distance to the pontomedullary cistern will be even greater, making it more difficult to release cerebrospinal fluid and create operative space, which will cause significant difficulty for the surgery.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.3 Precautions During Craniotomy\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eBased on previous surgical experience, the key to safely access the surgical field lies in the release of cerebrospinal fluid (CSF) to reduce intracranial pressure, allow cerebellar relaxation, and provide adequate operating space. We advocate for a minimally invasive keyhole approach, which involves a small bone window. This can sometimes make CSF release challenging. Therefore, we typically place a lumbar drain preoperatively to allow for the controlled release of CSF intraoperatively, thereby reducing intracranial pressure. Furthermore, when creating the bone window, we recommend further extending the bone removal anteroinferiorly towards the UPDS with a diamond burr. This facilitates access to the cerebellomedullary cistern, promoting CSF release. It is crucial to avoid directly entering the cerebellopontine angle (CPA) from the lateral aspect of the cerebellum. This approach not only hinders CSF release but also significantly increases the risk of cerebellar contusion, potentially making the surgery difficult or even leading to failure. The exact extent of this additional bone removal has not yet been precisely measured, and the surgeon can determine the appropriate amount based on the specific intraoperative situation. We believe these are very practical tips during retrosigmoid small keyhole craniotomy.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.4 Limitations\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eDespite demonstrating the importance of the UPDS in locating the Key Point and its role in subsequent surgery through numerous anatomical and clinical studies, our study has several limitations: 1) The limited availability of skull and cadaveric specimens resulted in a small sample size, potentially reducing the reliability of our data. 2) Currently, various navigation and endoscopic techniques are well-established, but we did not perform comparative data analysis with these techniques. 3) We only studied patient data from certain regions of China. Given the significant anatomical variations among different ethnicities worldwide, anatomical data is likely to differ across regions, meaning our findings may not be applicable to other countries. However, with the ongoing development of various technologies, we may be able to obtain more data through other methods, even including skull data from various ethnicities. For example, 3D printing technology could be used to print patient-specific skulls of the same size, allowing for experimentation and measurement on these printed skulls to acquire a large amount of data. Researchers in other countries could also conduct corresponding studies based on this concept to further improve the research results.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThe Innovative Concept of craniotomy based on the \"one point, two lines, two distances\" theory is safe and accurate. After forming the Key Hole according to the aforementioned localization method, a high-speed milling cutter is used to cut the skull from the Key Hole towards the occiput in a posterior-inferior-anterior sequence in turn. Upon reaching the UPDS, the milling cutter is moved superiorly, stopping at the level of the mastoid emissary vein canal, and then the cutter is withdrawn. Secondly, the milling cutter is used to cut the mastoid bone from the Key Hole downwards the UPDS, stopping at the level of the mastoid emissary vein canal, and then the cutter is withdrawn. Thirdly, the remaining bone near the posterior edge of the sigmoid sinus adjacent to the mastoid emissary vein canal is removed with a grinding drill, and the bone flap is removed. Fourthly, the mastoid bone at the posterior edge of the sigmoid sinus is quantitatively removed with a grinding drill to ensure that the anterior edge of the bone window is located at the posterior edge of the sigmoid sinus, forming a bone window approximately 2.5 cm in diameter. Finally, the mastoid bone at the posterior lower edge of the sigmoid sinus near the UPDS is removed for better release of cerebrospinal fluid. Our proposed theory and method for Key Point localization and minimally invasive craniotomy have been validated in both cadaveric specimens and clinical surgeries. we plan to increase the number of samples and further validate the accuracy of this method in combination with neuronavigation in future surgeries.\u003c/p\u003e\n"},{"header":"Abbreviations","content":"\u003cp\u003e\u003cstrong\u003eFHP:\u003c/strong\u003ethe Frankfort horizontal plane\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTM:\u003c/strong\u003etip of the mastoid\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eUPDS\u003c/strong\u003e\u003cstrong\u003e：\u003c/strong\u003ethe up point of digastatic sulci\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eJPTS:\u003c/strong\u003ekey point,the Junction Point of the Transverse and Sigmoid\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEF:\u003c/strong\u003emastoid emissary vein fossa\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSP:\u003c/strong\u003estar point\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eKP:\u003c/strong\u003ekey hole point\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eIPS:\u003c/strong\u003ethe Inferior margin of transverse sinus\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePSS:\u003c/strong\u003ethe posterior margin of sigmoid sinus\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSNL:\u003c/strong\u003esuperior nuchae line\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAICA:\u003c/strong\u003eanterior inferior cerebellar artery\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSCA:\u003c/strong\u003esuperior cerebellar artery\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eVA:\u003c/strong\u003e vertebral artery\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eBA:\u003c/strong\u003e basilar artery\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMRI:\u003c/strong\u003emagnetic resonance imaging\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eConflicts of Interest\u003c/strong\u003e:\u003c/p\u003e\n\u003cp\u003eThe authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eChang-chun Liao: data collection, data analysis and interpretation, writing and revising the manuscript. Xunjie-Ma, DeZhu An, Bai-cha Tang, Ling Zhou,Kai-hua Wu, Jia-yan Li, Xin-feng-Yi, Zhi-heng Jian, Zhi-jian Weng, Min-Feng Sheng,De-xiang Zhou: data collection, writing and revising the manuscript. Gang Chen: drafting of the study, writing and revising the manuscript. All authorsread and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by the following: Guangzhou City and Jinan University Associated Project Foundation (grant nos.SL2023A03J00347) and Zhuhai People’s Hospital scientific initiation project (grant nos.2021KYQD-02) to Gang Chen.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors thank the participants and all who were involved in our study.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDisclosure\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNone\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthic statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study is a anatomical study and did not involved any human participants.this studies were reviewed and approved by [ethics committee of Zhuhai people’s hospital].\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eBasma J, Anagnostopoulos C, Tudose A, et al. 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Clinical Anatomy: The Official Journal of the American Association of Clinical Anatomists and the British Association of Clinical Anatomists, 2010, 23(1): 30-33.\u003c/li\u003e\n \u003cli\u003eAvci E, Kocaogullar Y, Fossett D, et al. Lateral posterior fossa venous sinus relationships to surface landmarks[J]. Surgical neurology, 2003, 59(5): 392-397.\u003c/li\u003e\n \u003cli\u003eDay J D, Kellogg J X, Tschabitscher M, et al. Surface and superficial surgical anatomy of the posterolateral cranial base: significance for surgical planning and approach[J]. Neurosurgery, 1996, 38(6): 1079-1084.\u003c/li\u003e\n \u003cli\u003eKubo M, Mizutani T, Shimizu K, et al. New methods for determination of the keyhole position in the lateral suboccipital approach to avoid transverse-sigmoid sinus injury: proposition of the groove line as a new surgical landmark[J]. Neurochirurgie, 2021, 67(4): 325-329.\u003c/li\u003e\n \u003cli\u003eTao F U, Jie S, HUANG J, et al. 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Precise localization in craniotomy with a retrosigmoid keyhole approach: microsurgical anatomy and clinical study[J]. Frontiers in Surgery, 2022, 9: 809098.\u003c/li\u003e\n \u003cli\u003eJian Z H, Sheng M F, Liao C C, et al. A novel theory for rapid localization of the transverse-sigmoid sinus junction and \u0026ldquo;keyhole\u0026rdquo; in the retrosigmoid keyhole approach: micro-anatomical study, technique nuances, and clinical application[J]. Neurosurgical Review, 2024, 47(1): 331.\u003c/li\u003e\n \u003cli\u003eKim L K P, Ahn C S, Fernandes A E L. Mastoid emissary vein: anatomy and clinical relevance in plastic \u0026amp; reconstructive surgery[J]. Journal of Plastic, Reconstructive \u0026amp; Aesthetic Surgery, 2014, 67(6): 775-780.\u003c/li\u003e\n \u003cli\u003eJr L R, Loukas M, Wartmann C T, et al. Clinical anatomy of the mastoid and occipital emissary veins in a large series.[J]. Surgical and Radiologic Anatomy, 2009, 31(2):139-144.\u003c/li\u003e\n \u003cli\u003eMurlimanju B V, Chettiar G K, Prameela M D, et al. Mastoid emissary foramina: an anatomical morphological study with discussion on their evolutionary and clinical implications[J]. Anatomy \u0026amp; cell biology, 2014, 47(3): 202-206.\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Tables","content":"\u003cp\u003e\u003cstrong\u003eTable 1. Summary table of patient demographics (n=122)\u003c/strong\u003e\u003c/p\u003e\n\u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\" width=\"587\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eCharacteristic\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eTrigeminal neuralgia\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\"\u003e\n \u003cp\u003ehemifacial\u0026nbsp;\u003c/p\u003e\n \u003cp\u003espasm\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eAcoustic neuroma\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eMeningioma\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003ejugular foramen neurinoma\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eTotal\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eNO\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e31\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e67\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\"\u003e\n \u003cp\u003e17\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e122\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eSex, n\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd colspan=\"2\"\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd\u003e\u003cbr\u003e\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eMale\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e31\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e48\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eFemale\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e22\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e36\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\"\u003e\n \u003cp\u003e11\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e74\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eAge(years),mean (range)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e54.7(39-72)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e51.2(26-69)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\"\u003e\n \u003cp\u003e49.2(29-69)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e48.8(32-64)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e44.0(37-51)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e51.6(26-72)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eSide affected, n\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd colspan=\"2\"\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd\u003e\u003cbr\u003e\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eLeft\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e14\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e29\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\"\u003e\n \u003cp\u003e7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e54\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eRight\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e17\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e38\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\"\u003e\n \u003cp\u003e10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e68\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 2. The relationship between KP\u003c/strong\u003e\u003cstrong\u003e、\u003c/strong\u003e\u003cstrong\u003eUPDS and PSS\u003c/strong\u003e\u003c/p\u003e\n \u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\" width=\"94%\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eLength（mm）\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003ewidth（mm）\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003earea(mm\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eL\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e14.71\u0026plusmn;2.19\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e3.26\u0026plusmn;0.55\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e35.93\u0026plusmn;8.30\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eR\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e14.29\u0026plusmn;2.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e3.64\u0026plusmn;0.72\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e36.11\u0026plusmn;7.38\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003et\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e0.440\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e-1.324\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e0.052\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e0.517\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e0.710\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e0.531\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003eLength: the straight-line distance between KP and UPDS; width: the maximum perpendicular distance between Length and PSS; area: the area between between\u0026nbsp;Length\u0026nbsp;and PSS; L：left，R：right.\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"suboccipital retrosigmoid approach, transverse sinus, sigmoid sinus, keypoint, digastric sulci","lastPublishedDoi":"10.21203/rs.3.rs-7557430/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7557430/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003e\u003cstrong\u003eBackground\u003c/strong\u003e: The suboccipital retrosigmoid approach is a classic approach for surgical access to cerebellopontine angle diseases. Previously, we identified a novel method for precise localization of keypoint, known as the \"one point, two lines, two distances\" theory. However, a comprehensive, safe method for craniotomy remains lacking.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eObjective\u003c/strong\u003e: Based on the new landmark method of localization , this study aims to further describe the anatomical basis, surgical technique, and outcomes of the suboccipital retrosigmoid keyhole approach for craniotomy.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMethods\u003c/strong\u003e: Twelve adult specimens of skull were used for the study. The anatomical relationships between the keypoint were analyzed to establish a precise, rapid, and safe method for suboccipital retrosigmoid keyhole craniotomy. This method was then validated through cadaveric dissection. Furthermore, a retrospective analysis of surgical outcomes was performed on 122 clinical patients, assessing accuracy, safety, and exposure results.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eResults\u003c/strong\u003e: Measurements from specimens of skull revealed that the up point of digastatic sulci roughly corresponds to the margin of the sigmoid sinus. In craniotomy simulations performed on 12 cadaveric specimens, we freed the bone flap in a \"rear-down-front\" direction after drilling at the key point. This process passed through the up point of digastatic sulci, with bone around the mastoid emissary vein being removed using a grinding drill, followed by resection of the residual bone along the margin of the sigmoid sinus. In all cadaver specimens, the formation of bone flap resulted in a well-exposed window. In the clinical cohort of 122 patients, keypoints were accurately located, and suitable bone windows were created with good exposure and without damage to important structures.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConclusion\u003c/strong\u003e: The up point of digastatic sulci serves as a crucial reference point for keypoint localization. Craniotomy techniques based on this reference point provide a precise and safe approach for suboccipital retrosigmoid craniotomy.\u003c/p\u003e","manuscriptTitle":"The Innovative Concept of Craniotomy in suboccipital retrosigmoid keyhole approach: anatomical study, technique nuances, and clinical application","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-09-23 15:51:01","doi":"10.21203/rs.3.rs-7557430/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"b319f951-1db7-4ce9-ac94-5d7e84c0d5d0","owner":[],"postedDate":"September 23rd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-01-07T21:23:34+00:00","versionOfRecord":[],"versionCreatedAt":"2025-09-23 15:51:01","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7557430","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7557430","identity":"rs-7557430","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}