Aging-related morphological changes in the obturator internus and levator ani

Aging-related morphological changes in the obturator internus and levator ani | 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 Aging-related morphological changes in the obturator internus and levator ani Satoru Muro, Junichi Tsuchiya, Takuya Ibara, Akimoto Nimura, Keiichi Akita This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7449052/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 Pelvic floor dysfunction is a common condition in adult women that substantially impacts their quality of life. Most studies have focused on the levator ani muscle, but recent anatomical insights suggest that the surrounding muscles, such as the obturator internus, also play a key structural role. However, the age-related morphological changes in these muscles remain poorly understood. Objective To quantitatively characterize age-related morphological changes in the obturator internus and levator ani muscles using geometric morphometric techniques. Study Design: This cross-sectional study included 27 adult women aged 25–83 years with gynecological conditions but without pelvic floor disorders, as evidenced by clinical or imaging parameters. Standardized pelvic magnetic resonance imaging scans were obtained between April 2024 and July 2024 at a single tertiary care center. Elliptic Fourier analysis was applied to the segmented contours of the obturator internus, and a landmark-based method was used to evaluate the combined shape of the obturator internus and levator ani. Principal component analysis and linear regression were performed to assess the associations with age. Procrustes analysis of variance was used to evaluate the overall shape-age relationships. Results Elliptic Fourier analysis revealed that increasing age was significantly associated with decreased thickness, downward displacement of the medial surface groove, and eminence of the obturator internus (p = .008, R² = .25). Landmark-based analysis revealed similar age-related shape changes, including inferior displacement of the groove, verticalization of the levator ani, and widening of the levator hiatus (p = .001, R² = .343). Procrustes analysis revealed a significant relationship between age and overall shape variation (p = .003). Conclusions Aging was associated with coordinated structural remodeling of the obturator internus and levator ani. These findings suggested that, in addition to traditional pelvic floor–focused approaches, strategies targeting hip joint function may offer novel opportunities for the assessment and rehabilitation of patients with pelvic floor dysfunction. Obstetrics & Gynecology Aging Analysis of Variance Cross-Sectional Studies Magnetic Resonance Imaging Pelvic Floor Pelvic Floor Disorders Skeletal Muscle Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 AJOG at a glance Why was the study conducted? To investigate how aging affects the shape of the obturator internus and levator ani muscles using geometric morphometric analysis. What are the key findings? The obturator internus thins and its medial groove descended with age, in parallel with structural changes in the levator ani. What does this study add to what is already known? This study suggested that morphological alterations in the obturator internus may constitute a fundamental component of pelvic floor aging. INTRODUCTION Pelvic floor dysfunction is a prevalent condition associated with aging, childbirth, and postoperative changes following pelvic surgery and is known to markedly impair quality of life. In women, pelvic organ prolapse (POP) and urinary incontinence are prominent manifestations, and their prevention and treatment constitute urgent issues from both clinical and societal perspectives. 1 – 3 Traditionally, research on the pathophysiology of pelvic floor dysfunction has primarily focused on morphological changes in the levator ani (LA) muscle, including the puborectalis muscle. However, recent studies have focused on the potential role of the hip joint muscles, particularly the obturator internus (OI) muscle. 4 , 5 The OI has been increasingly recognized for its functional relevance, as it interfaces with the LA through a broad fascial contact, contributing to the structural support of the pelvic floor. 6 Accordingly, a comprehensive understanding of age-related changes in pelvic floor function requires a morphological evaluation that includes not only the LA but also the OI. Previous studies on age-related morphological changes of the pelvic floor muscles (PFMs) have primarily utilized linear or area-based metrics, such as muscle volume, cross-sectional area, and positional measurements. Age-related changes in the LA and OI have been reported to reduce the physiological cross-sectional area (PCSA) and muscle volume, based on magnetic resonance imaging (MRI) analyses. 7 – 9 However, these conventional approaches have inherent limitations in capturing global morphological changes in muscle architecture. Recently, it has been reported that the OI is not a homogenous, disc-shaped muscle but is composed of two regions—superior and inferior—with distinct fiber orientations, forming a characteristic medial "groove" structure on its medial surface. 10 This groove corresponds approximately to the anatomical origin of the LA and represents a functionally relevant region. Although the vertical position of the groove demonstrated a weak negative correlation with age, this association was not statistically significant. 10 This lack of significance may be attributed to the inadequacy of single linear parameters, such as the groove superoinferior position, which represents the overall structural variation required to assess age-related morphological changes. We aimed to quantitatively analyze age-related morphological changes in the OI and LA muscles. To overcome the methodological limitations of previous studies that resulted in age-related morphological changes in the OI and LA remaining poorly understood, we used geometric morphometrics (GM), a multivariate analytical technique that captures shape configurations in their entirety. This technique has recently gained prominence in human anatomical research because of its ability to quantify subtle morphological variations. 11 We assumed that applying GM to the OI and LA offers a more robust and statistically valid framework for elucidating age-associated structural transformations in these muscles. This study is expected to provide novel insights into age-related morphological changes in PFMs and contribute foundational knowledge for the prevention, diagnosis, and treatment of pelvic floor dysfunction. MATERIALS AND METHODS Participants In this retrospective cross-sectional study, we utilized pelvic MRI images acquired between April 2024 and July 2024 at the Tokyo Medical and Dental University Hospital—renamed the Institute of Science Tokyo on October 1, 2024. The participants were women with gynecological conditions ranging in age from 25 to 83 years. Patients with considerable motion artifacts, adhesions, or tumor invasion involving the OI or LA muscles were excluded. In addition, patients with pelvic floor dysfunction such as POP were excluded, and only those with gynecological diseases unrelated to pelvic floor dysfunction were included. Finally, the MRI data from 27 consecutive cases were analyzed. This dataset is the same as that used in a previous study. 10 This study was approved by the Institutional Review Board (IRB) of our institution (approval number: M2024-091). All procedures involving human participants were conducted in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments. Given the retrospective study design, the IRB waived the requirement for individual informed consent, and an opt-out approach was employed via public notification. All patient data were anonymized prior to analysis, and the privacy rights of the participants have been fully protected. MRI Acquisition and Image Standardization Pelvic MRI was performed with patients in supine using coronal T2-weighted sequences (echo time, 85–118 ms; repetition time, 3,000–5,500 ms). Imaging was conducted using 1.5T (MAGNETOM Sola; Siemens Healthineers, Erlangen, Germany) and 3.0T (Signa Pioneer; GE Medical Systems, Milwaukee, WI, USA) MRI scanners. The slice thickness was 4 mm, with an inter-slice gap of 0.4 mm. The field of view (FOV) was set to 25 cm, and the matrix size ranged from 384 to 432 × 268 to 300. Anatomical landmarks, specifically bilateral pubic tubercles and anterior superior iliac spines, were used as reference points to obtain standardized coronal sections of the pelvis. Fiducial markers were placed at these four landmarks using a 3D slicer (version 5.6.2 r32448). The coronal planes passing through all four points were reconstructed using the Reformat tool (Registration > Specialized > Reformat module). This procedure ensured anatomically consistent sectioning across all participants, thereby maintaining the reliability of the morphological analysis. Elliptic Fourier Analysis of the Obturator Internus For shape analysis of the OI alone, standardized MRI coronal sections on the right side were used (n = 27). OI contours were extracted using Seg and Ref, a semiautomatic segmentation tool developed by our group (Fig. 1 A). 12 The extracted contours were subjected to shape analysis using elliptic Fourier descriptors (EFDs). We employed the SHAPE software (Version 1.3; Hiroyoshi Iwata) for the analysis. 13 This software automatically performs the following steps: (1) contour extraction from image files, (2) computation of EFDs, (3) standardization of contour size and orientation, (4) principal component analysis (PCA) based on EFD coefficients, and (5) shape reconstruction corresponding to each principal component (PC). PCA was performed on the contours of OI extracted from the MRI images using EFDs. Landmark-Based Morphometric Analysis of the Obturator Internus and Levator Ani The full shape of the bilateral OI and LA visualized on MRI was used for the combined morphological analysis of the OI and LA. Landmark placement was performed using the software tpsUtil and tpsDig2 (F. J. Rohlf, SUNY Stony Brook). Landmarks were defined based on the following anatomical criteria: (1) superior margin of the OI; (2) inferior margin of the OI; (3) characteristic “groove” on the medial surface of the OI; (4) superior margin of the LA; and (5) inferior margin of the LA, which was estimated as the boundary of the external anal sphincter. The landmarks were evenly distributed between these structural features as follows: 15 points between the superior and inferior margins of the OI, 7 points between the superior margin and groove, 7 points between the inferior margin and groove, and 15 points between the superior and inferior margins of the LA, resulting in 49 landmarks on each side and 98 landmarks bilaterally (Fig. 1 B). Shape analysis was conducted using the Geomorph package (version 4.0.10) in RStudio (Version 4.0.10). Generalized procrust analysis (GPA) was first applied to remove variations due to translation, scale, and rotation. PCA was performed on aligned landmark configurations. Statistical Analysis All statistical analyses were performed using RStudio software (version 4.0.10). PCA was applied to the shape data of the OI and LA, and the resulting PC scores were used as quantitative indicators of morphological variation. To evaluate the age-related shape changes, simple linear regression analyses were conducted with age as the explanatory (independent) variable and each PC score as the response (dependent) variable. Regression coefficients, p-values, coefficients of determination (R²), residual standard errors, and F-statistics were calculated to assess the statistical significance. Additionally, Procrustes ANOVA was performed, using the procD.lm function, with statistical significance assessed based on 1,000 permutations, on the GPA-aligned landmark coordinates to examine the association between age and overall shape variation. The effect size (f²) for each regression model was calculated based on a sample size of 27 and an alpha level of 0.05. Statistical significance was set at p < .05. RESULTS Shape Variation of the Obturator Internus: Elliptic Fourier Analysis The cumulative contribution of PC1 to PC3 was 79.19%, accounting for the majority of shape variation, with 53.24%, 15.69%, and 10.26% individual contribution rates of PC1, PC2, and PC3, respectively (Fig. 2 ). PC1 primarily reflected the overall scale and vertical shape characteristics, including the cross-sectional area of the OI, thickness of the superior portion, and vertical positioning of the medial groove and inferior medial eminence. In contrast, PC2 and PC3 captured variations related to the contour of the medial surface, particularly the depth of the groove and prominence of the medial eminence. Regarding age-related changes, PC1 showed a significant negative correlation with age (estimate = -0.00193, p = 0.008, R² = 0.25, f² = 0.33) (Fig. 3 ). In contrast, no significant association was found between age and PC2 or PC3 scores (PC2, p = 0.46, R² = 0.022; PC3, p = 0.068, R² = 0.127). However, the PC3 score demonstrated a trend toward a negative correlation with age. Morphological Changes of the Obturator Internus and Levator Ani: Landmark-Based Analysis The cumulative contribution of the first two PCs was 62.8%, with PC1 accounting for 40.16% and PC2 accounting for 22.61%. PC1 primarily reflected the vertical positions of the groove and medial eminence on the medial surface of the OI, inclination of the LA, vertical position of its inferior margin, and medial distance between the bilateral LA (i.e., width of the levator hiatus). PC2 was associated with the curvature of the medial eminence of the OI, vertical position of the superior margin of the LA, and width of the levator hiatus (Fig. 4 ). Although PC3 (14.35% of the variance) also contributed to the shape variation, it is omitted from the graphical presentation for clarity. Linear regression analysis revealed a statistically significant negative association between PC1 score and age (estimate = -0.00283, p = 0.001, R² = 0.343, f 2 = 0.522; Fig. 5 ). Furthermore, Procrustes ANOVA conducted using the GPA-aligned landmark coordinates showed that age accounted for approximately 15.1% (R² = 0.151) of the total shape variation (F = 4.44, p = 0.003, Z = 2.82). COMMENT Principal findings Elliptic Fourier analysis revealed that the coronal sectional shape of the OI changed with age and was particularly characterized by a reduction in the thickness of the superior part and a downward displacement of both the medial groove and medial eminence located inferior to it (Fig. 6 A). Landmark-based analysis corroborated these findings by demonstrating a similar downward shift in the groove and eminence, along with concurrent structural changes such as verticalization of the LA and widening of the levator hiatus (Fig. 6 B). To the best of our knowledge, this is the first study to quantitatively evaluate age-related shape changes of this muscle. Results in the Context of What is Known Previous studies on age-related changes in the PFMs have predominantly focused on alterations in the position or inclination of the LA. Even in studies that included the OI, the evaluations were largely limited to linear or areal parameters, such as muscle volume and physiological cross-sectional area. In contrast, the present study provides a novel structural and quantitative assessment of age-related shape alterations of the medial surface of the OI, a considerable advancement in the understanding of pelvic floor muscle aging, offering a new perspective and methodology for morphological analyses in this anatomical domain. Our finding that the groove and medial eminence on the medial surface of the OI shifted inferiorly with aging supports recent reports suggesting that the OI contributes to pelvic floor support and plays a role in defecation and urinary continence. 6 Although prior research has extensively examined age-related morphological changes in the pelvic floor musculature, particularly in the LA muscle, focusing on position, inclination, and overall shape changes using MRI and three-dimensional modeling, 14 , 15 studies on the OI have been largely limited to volumetric or cross-sectional indices. These studies reported age-related decreases in the PCSA and muscle thickness 7 – 9 , 16 but did not address morphological shape changes. We previously identified a characteristic groove on the medial surface of the OI, 10 and we posited that such localized features require multivariate and geometric approaches for accurate characterization. Therefore, we employed two GM methods —elliptic Fourier analysis and landmark-based analysis—to demonstrate that age-related shape changes in the OI occur in parallel with the verticalization of the LA and widening of the levator hiatus. In particular, the inferior displacement of localized structures, such as the groove and medial eminence, revealed age-related morphological alterations that could not be captured by traditional linear indices. Increasing evidence suggests that structural changes in the OI affect pelvic floor function. For instance, in men undergoing prostatectomy, greater preoperative OI thickness was significantly associated with earlier postoperative recovery of urinary continence. 17 In women with POP, the cross-sectional area of the OI was significantly smaller than that in controls, implying that OI atrophy may be a risk factor for POP development. 18 Additionally, patients with POP were demonstrated to exhibit an increased anterior pelvic cross-sectional area. 19 Although this does not directly measure the OI, it may reflect structural integrity reduction. Furthermore, studies linking hip joint function to pelvic floor performance have attracted increasing attention. Several reports have demonstrated improvements in urinary incontinence among women after undergoing total hip arthroplasty (THA). 5 , 20 Posterior approaches to THA can damage the OI, and muscle atrophy in this region has been associated with worsening urinary symptoms. 21 These observations suggest that among the hip muscles, the OI may play a particularly important structural role in supporting pelvic floor function. Anatomically, this functional linkage is underscored by the fascial interface between the medial surface of the OI and origin of the LA muscle. Previously, we demonstrated that these two muscles are broadly connected via the obturator fascia and that this interface constitutes the origin of the LA, 6 implying that the structural and mechanical properties of the OI mediate stability of the LA origin. In the present study, we demonstrated that this groove progressively shifts downward with age. Such displacement may compromise the positional stability of the LA origin and consequently contribute to age-related deterioration of pelvic floor function. Clinical Implications Our findings highlighted the importance of hip joint integrity as a necessary but insufficient condition for maintaining optimal pelvic floor function. Evidence supporting this view includes a study by Tuttle et al. that demonstrates that targeted strengthening of the OI led to increased peak contraction pressure of the PFMs, suggesting a functional contribution of the hip musculature to pelvic floor performance. 4 Conversely, other interventions combining hip adduction and abduction exercises failed to yield changes in vaginal pressure, 22 indicating that not all hip movements effectively enhance pelvic floor function. Hwang et al. reported no significant correlation between hip muscle strength and PFM function in women with stress urinary incontinence, 23 further suggesting that hip function alone cannot fully compensate for PFM deficits in the pelvic floor. These findings collectively imply that although adequate hip function is a prerequisite, it does not suffice on its own. Compromised PFMs, such as the LA, cannot be functionally restored through hip rehabilitation alone. Therefore, an integrative approach targeting both PFMs and the hip musculature is essential for restoring pelvic floor function. Given its anatomical location and fascial connectivity, the OI likely plays a central mediating role in this functional linkage. 6 In particular, the OI may provide a biomechanical environment that facilitates optimal activation and stability of the LA. In this context, the OI may serve not merely as a passive support structure but also as a dynamic substrate that enables LA to exert its contractile function efficiently. Emerging evidence points to the vulnerability of the OI to obstetric factors. Senkaya et al. reported a significant negative correlation between fetal weight and OI cross-sectional area, 24 suggesting that the OI may undergo structural remodeling in response to childbirth. These findings indicate that the functional status of the OI is influenced not only by aging, but also by reproductive history. Research Implications The present study’s findings advance the anatomical and physiological understanding of pelvic floor aging. Furthermore, the significant correlations observed between these structural changes and chronological age suggest their potential applications in imaging-based diagnosis, preventive strategies, and therapeutic interventions. Our findings suggest that future diagnostic and therapeutic frameworks for pelvic floor dysfunction should incorporate morphological and functional evaluations of OI. Particularly, in rehabilitation settings, comprehensive interventions that extend beyond LA-focused therapies to include the hip joint, specifically the OI, may prove beneficial. Such integrative perspectives could inform revisions to the current guidelines for the prevention, treatment, and surgical reconstruction of pelvic floor disorders. Nevertheless, several important questions remain unresolved. For example, the causal mechanisms linking age-related structural changes in the OI to functional decline of the pelvic floor have yet to be elucidated. In addition, whether these morphological alterations differ by sex, parity, or hormonal status remains unclear. Furthermore, longitudinal evidence is lacking to determine whether OI-targeted rehabilitation strategies can effectively prevent or reverse pelvic floor dysfunction. Future studies should therefore combine imaging-based morphometric analyses with biomechanical and functional assessments, as well as interventional trials, to clarify these relationships. Multicenter investigations with larger and more diverse populations would also be valuable to generalize our findings and to establish evidence-based guidelines that integrate both LA- and OI-focused approaches. Strengths and Limitations This study had several limitations. First, the sample was limited to adult women without pelvic floor disorders and no comparative analyses were conducted with populations with a history of childbirth or pelvic floor disorders. Second, the evaluation was based solely on static MRI images, which do not capture the dynamic functional interplay between muscle contraction and movement. Third, as this was a cross-sectional study, we did not observe age-related morphological changes over time. Despite these limitations, the significant association between age and morphological variation can be considered highly reliable. Conclusions We found that the OI and LA muscles undergo age-related morphological changes. The downward displacement of the groove and eminence on the medial surface of the OI occurred in parallel with the vertical alignment of the LA and widening of the levator hiatus, representing a novel and multifaceted insight into age-related changes in pelvic floor musculature. These shape alterations may influence the pelvic floor support, continence function, and rehabilitation strategies. Declarations Acknowledgement The authors sincerely thank those who donated their bodies to science so that anatomical research could be performed. The results of this research can potentially increase the overall knowledge of humankind and improve patient care. Therefore, the donors and their families deserve great gratitude. Conflict of Interest The authors have no relevant financial relationships to disclose. Data Availability Statement Data supporting this study’s findings are available from the corresponding author upon request. The code for the semiautomatic segmentation tool (Seg and Ref) developed by our group to extract OI contours is publicly available on GitHub (https://github.com/SatoruMuro/SAM2GUIfor3Drecon). Funding This study was supported byJSPS KAKENHI (grant number 25K18696). Declaration of generative AI and AI-assisted technologies in the writing process During the preparation of this study, ChatGPT was used to improve the clarity and grammatical usage of English. After using these services, the authors reviewed and edited the content as required and take full responsibility for the content of the publication. 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Int J Urol 21:729–731. 10.1111/iju.12404 Baba T, Homma Y, Takazawa N et al (2014) Is urinary incontinence the hidden secret complications after total hip arthroplasty? Eur J Orthop Surg Traumatol 24(8):1455–1460. 10.1007/s00590-014-1413-4 Amorim AC, Cacciari LP, Passaro AC et al (2017) Effect of combined actions of hip adduction/abduction on the force generation and maintenance of pelvic floor muscles in healthy women. PLoS ONE 12:e0177575. 10.1371/journal.pone.0177575 Hwang UJ, Lee MS, Jung SH, Ahn SH, Kwon OY (2021) Relationship between sexual function and pelvic floor and hip muscle strength in women with stress urinary incontinence. Sex Med 9:100325. 10.1016/j.esxm.2021.100325 Senkaya AR, Ismailoglu E, Ari SA, Karaca I (2023) How does the type of delivery affect pelvic floor structure? MRI parameter-based anatomical study. Ginekol Pol 94:57–63. 10.5603/GP.a2022.0140 Additional Declarations The authors declare no competing interests. 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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-7449052","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":504976707,"identity":"db2b9b28-88b3-4e84-bcad-43cf25a8f10d","order_by":0,"name":"Satoru Muro","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAzUlEQVRIiWNgGAWjYNACGyBmb2Bg4AHzDhCjJQ2IeQ6TrEUiGaaFANCdkfvwAUOCXR7/zPdHN7xhsJNnYDyL3xqzG+nGBgwJycUSt5PZbs5hSDZsYDiXQEBLGpsE4w/mxAaglts8DMxA5WcMCGthSKhPnH/zMEhLPdFaDiduuMEM0nKYCC1nnjEbJCQcT9x4Jtns5hyD44ZtBP1yPI3xwYeE6sR5xw8+u/GmolqeX4JAiIEBwlSgk9gkzhDWgQb4e0jWMgpGwSgYBcMbAABod0WBISj6QgAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0000-0002-4709-6359","institution":"Institute of Science Tokyo","correspondingAuthor":true,"prefix":"","firstName":"Satoru","middleName":"","lastName":"Muro","suffix":""},{"id":504976708,"identity":"4ac2f426-3d3a-4c89-9897-da22fa1e89af","order_by":1,"name":"Junichi Tsuchiya","email":"","orcid":"","institution":"Institute of Science Tokyo","correspondingAuthor":false,"prefix":"","firstName":"Junichi","middleName":"","lastName":"Tsuchiya","suffix":""},{"id":504976709,"identity":"a65423a3-9662-43a7-89e0-785a171284c4","order_by":2,"name":"Takuya Ibara","email":"","orcid":"","institution":"Institute of Science Tokyo","correspondingAuthor":false,"prefix":"","firstName":"Takuya","middleName":"","lastName":"Ibara","suffix":""},{"id":504976710,"identity":"1d0dd226-bbb3-43a3-add1-2eed302c27ae","order_by":3,"name":"Akimoto Nimura","email":"","orcid":"","institution":"Institute of Science Tokyo","correspondingAuthor":false,"prefix":"","firstName":"Akimoto","middleName":"","lastName":"Nimura","suffix":""},{"id":504976711,"identity":"b22e0a02-b1d8-4b33-ba4d-918c38050e4b","order_by":4,"name":"Keiichi Akita","email":"","orcid":"","institution":"Institute of Science Tokyo","correspondingAuthor":false,"prefix":"","firstName":"Keiichi","middleName":"","lastName":"Akita","suffix":""}],"badges":[],"createdAt":"2025-08-25 02:45:00","currentVersionCode":1,"declarations":{"humanSubjects":true,"vertebrateSubjects":false,"conflictsOfInterestStatement":false,"humanSubjectEthicalGuidelines":true,"humanSubjectConsent":true,"humanSubjectClinicalTrial":false,"humanSubjectCaseReport":false,"vertebrateSubjectEthicalGuidelines":false},"doi":"10.21203/rs.3.rs-7449052/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7449052/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":89994347,"identity":"23df3709-65f6-474b-8948-4391f359e612","added_by":"auto","created_at":"2025-08-27 07:50:47","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":527293,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMethods of shape analysis applied to the obturator internus and levator ani muscles.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e(A) Semi-automatic segmentation of the obturator internus (OI) on standardized coronal magnetic resonance imaging (MRI) images using the Seg and Ref tool. The extracted contour was used for elliptic Fourier analysis.\u003c/p\u003e\n\u003cp\u003e(B) Landmark configuration for the geometric morphometric analysis of the OI and levator ani (LA) muscles. A total of 98 landmarks were placed bilaterally according to defined anatomical features of muscles.\u003c/p\u003e\n\u003cp\u003eHB, hip bone.\u003c/p\u003e","description":"","filename":"figure1.png","url":"https://assets-eu.researchsquare.com/files/rs-7449052/v1/632f534b934f4f5699dbe8ad.png"},{"id":89994343,"identity":"3b12c0ce-a619-4d00-9015-970456eb2c34","added_by":"auto","created_at":"2025-08-27 07:50:47","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":142238,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ePrincipal component analysis (PCA) of the obturator internus based on elliptic Fourier descriptors.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe cumulative contribution of PC1–PC3 accounted for 79.19% of the total shape variation. PC1 captured the overall scale and vertical features, including the muscle thickness and groove position, whereas PC2 and PC3 reflected the contour variations on the medial surface.\u003c/p\u003e","description":"","filename":"figure2.png","url":"https://assets-eu.researchsquare.com/files/rs-7449052/v1/ad49d69bab4b671bc5d56b31.png"},{"id":89994344,"identity":"b29173d6-d4b9-418a-8ab2-bf78ead77441","added_by":"auto","created_at":"2025-08-27 07:50:47","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":116058,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eAssociation between PC1 score and age in the elliptic Fourier analysis.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eLinear regression revealed a significant negative correlation between age and PC1 score (p = 0.008, R² = 0.25), indicating an age-related downward shift in the medial groove and thinning of the superior portion of the OI.\u003c/p\u003e\n\u003cp\u003eOI, obturator internus\u003c/p\u003e","description":"","filename":"figure3.png","url":"https://assets-eu.researchsquare.com/files/rs-7449052/v1/893fe4e8cd4de4471cf0817c.png"},{"id":89995724,"identity":"7273e629-f2b2-4186-a117-418ac5caebab","added_by":"auto","created_at":"2025-08-27 07:58:47","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":183231,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ePrincipal component analysis (PCA) of the obturator internus and levator ani based on landmark-based morphometrics.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003ePC1 and PC2 cumulatively accounted for 62.8% of the shape variation. PC1 reflects the vertical displacement of the obturator internus (OI) groove, medial eminence, levator ani (LA) inclination, and width of the levator hiatus. PC2 captures the variations in the curvature and relative positions of the LA and OI.\u003c/p\u003e","description":"","filename":"figure4.png","url":"https://assets-eu.researchsquare.com/files/rs-7449052/v1/65c7c51a66ff8d5da6c41c4c.png"},{"id":89994353,"identity":"83214c98-2182-4501-8aba-b21117e6cecb","added_by":"auto","created_at":"2025-08-27 07:50:48","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":109195,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eAssociation between PC1 score and age in the landmark-based analysis.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eLinear regression analysis showed a significant negative correlation between age and PC1 score (p = 0.001, R² = 0.343), suggesting coordinated age-related structural changes in the obturator internus (OI) and levator ani (LA), including vertical displacement of the OI groove, and medial eminence and downward displacement of the LA and widening of the levator hiatus.\u003c/p\u003e","description":"","filename":"figure5.png","url":"https://assets-eu.researchsquare.com/files/rs-7449052/v1/2dd0f3ac41dde69e73fdc82f.png"},{"id":89995725,"identity":"5ed27df4-e733-4335-a5ed-3849bdab2823","added_by":"auto","created_at":"2025-08-27 07:58:47","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":184980,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eRepresentative shape changes associated with age.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e(A) Reconstructed shapes from the elliptic Fourier analysis illustrate age-related morphological trends in the obturator internus (OI). With aging, the medial groove and the inferior eminence of the OI shift inferolaterally. The superior portion of the OI becomes thinner, whereas the inferior portion shows a reduction in volume and vertical extent.\u003c/p\u003e\n\u003cp\u003e(B) Landmark-based reconstructions show coordinated shape changes in the obturator internus (OI) and levator ani (LA) with age. The groove and eminence of the OI descend inferiorly, accompanied by a downward displacement of both the superior and inferior margins of the LA. The entire LA muscle shifts downward, becomes more vertically oriented, and the levator hiatus becomes widened.\u003c/p\u003e","description":"","filename":"figure6.png","url":"https://assets-eu.researchsquare.com/files/rs-7449052/v1/269810334aae795acb9fe814.png"},{"id":89996116,"identity":"9a542148-3ce7-432e-9c13-61ef033b129f","added_by":"auto","created_at":"2025-08-27 08:06:53","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2262588,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7449052/v1/0a637f2d-4cb4-4eeb-8258-9742797c9cbd.pdf"}],"financialInterests":"The authors declare no competing interests.","formattedTitle":"\u003cp\u003eAging-related morphological changes in the obturator internus and levator ani\u003c/p\u003e","fulltext":[{"header":"AJOG at a glance","content":"\u003cul\u003e\n \u003cli\u003e\u003cstrong\u003eWhy was the study conducted?\u0026nbsp;\u003c/strong\u003e\u003c/li\u003e\n\u003c/ul\u003e\n\u003cp\u003eTo investigate how aging affects the shape of the obturator internus and levator ani muscles using geometric morphometric analysis.\u003c/p\u003e\n\u003cul\u003e\n \u003cli\u003e\u003cstrong\u003eWhat are the key findings?\u0026nbsp;\u003c/strong\u003e\u003c/li\u003e\n\u003c/ul\u003e\n\u003cp\u003eThe obturator internus thins and its medial groove descended with age, in parallel with structural changes in the levator ani.\u003c/p\u003e\n\u003cul\u003e\n \u003cli\u003e\u003cstrong\u003eWhat does this study add to what is already known?\u0026nbsp;\u003c/strong\u003e\u003c/li\u003e\n\u003c/ul\u003e\n\u003cp\u003eThis study suggested that morphological alterations in the obturator internus may constitute a fundamental component of pelvic floor aging.\u003c/p\u003e"},{"header":"INTRODUCTION","content":"\u003cp\u003ePelvic floor dysfunction is a prevalent condition associated with aging, childbirth, and postoperative changes following pelvic surgery and is known to markedly impair quality of life. In women, pelvic organ prolapse (POP) and urinary incontinence are prominent manifestations, and their prevention and treatment constitute urgent issues from both clinical and societal perspectives.\u003csup\u003e\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e Traditionally, research on the pathophysiology of pelvic floor dysfunction has primarily focused on morphological changes in the levator ani (LA) muscle, including the puborectalis muscle. However, recent studies have focused on the potential role of the hip joint muscles, particularly the obturator internus (OI) muscle.\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e,\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e The OI has been increasingly recognized for its functional relevance, as it interfaces with the LA through a broad fascial contact, contributing to the structural support of the pelvic floor.\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e Accordingly, a comprehensive understanding of age-related changes in pelvic floor function requires a morphological evaluation that includes not only the LA but also the OI.\u003c/p\u003e\u003cp\u003ePrevious studies on age-related morphological changes of the pelvic floor muscles (PFMs) have primarily utilized linear or area-based metrics, such as muscle volume, cross-sectional area, and positional measurements. Age-related changes in the LA and OI have been reported to reduce the physiological cross-sectional area (PCSA) and muscle volume, based on magnetic resonance imaging (MRI) analyses.\u003csup\u003e\u003cspan additionalcitationids=\"CR8\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e However, these conventional approaches have inherent limitations in capturing global morphological changes in muscle architecture. Recently, it has been reported that the OI is not a homogenous, disc-shaped muscle but is composed of two regions\u0026mdash;superior and inferior\u0026mdash;with distinct fiber orientations, forming a characteristic medial \"groove\" structure on its medial surface.\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e This groove corresponds approximately to the anatomical origin of the LA and represents a functionally relevant region. Although the vertical position of the groove demonstrated a weak negative correlation with age, this association was not statistically significant.\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e This lack of significance may be attributed to the inadequacy of single linear parameters, such as the groove superoinferior position, which represents the overall structural variation required to assess age-related morphological changes.\u003c/p\u003e\u003cp\u003eWe aimed to quantitatively analyze age-related morphological changes in the OI and LA muscles. To overcome the methodological limitations of previous studies that resulted in age-related morphological changes in the OI and LA remaining poorly understood, we used geometric morphometrics (GM), a multivariate analytical technique that captures shape configurations in their entirety. This technique has recently gained prominence in human anatomical research because of its ability to quantify subtle morphological variations.\u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e We assumed that applying GM to the OI and LA offers a more robust and statistically valid framework for elucidating age-associated structural transformations in these muscles. This study is expected to provide novel insights into age-related morphological changes in PFMs and contribute foundational knowledge for the prevention, diagnosis, and treatment of pelvic floor dysfunction.\u003c/p\u003e"},{"header":"MATERIALS AND METHODS","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003eParticipants\u003c/h2\u003e\u003cp\u003eIn this retrospective cross-sectional study, we utilized pelvic MRI images acquired between April 2024 and July 2024 at the Tokyo Medical and Dental University Hospital\u0026mdash;renamed the Institute of Science Tokyo on October 1, 2024. The participants were women with gynecological conditions ranging in age from 25 to 83 years. Patients with considerable motion artifacts, adhesions, or tumor invasion involving the OI or LA muscles were excluded. In addition, patients with pelvic floor dysfunction such as POP were excluded, and only those with gynecological diseases unrelated to pelvic floor dysfunction were included. Finally, the MRI data from 27 consecutive cases were analyzed. This dataset is the same as that used in a previous study.\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003cp\u003e This study was approved by the Institutional Review Board (IRB) of our institution (approval number: M2024-091). All procedures involving human participants were conducted in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments. Given the retrospective study design, the IRB waived the requirement for individual informed consent, and an opt-out approach was employed via public notification. All patient data were anonymized prior to analysis, and the privacy rights of the participants have been fully protected.\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eMRI Acquisition and Image Standardization\u003c/h3\u003e\n\u003cp\u003ePelvic MRI was performed with patients in supine using coronal T2-weighted sequences (echo time, 85\u0026ndash;118 ms; repetition time, 3,000\u0026ndash;5,500 ms). Imaging was conducted using 1.5T (MAGNETOM Sola; Siemens Healthineers, Erlangen, Germany) and 3.0T (Signa Pioneer; GE Medical Systems, Milwaukee, WI, USA) MRI scanners. The slice thickness was 4 mm, with an inter-slice gap of 0.4 mm. The field of view (FOV) was set to 25 cm, and the matrix size ranged from 384 to 432 \u0026times; 268 to 300.\u003c/p\u003e\u003cp\u003eAnatomical landmarks, specifically bilateral pubic tubercles and anterior superior iliac spines, were used as reference points to obtain standardized coronal sections of the pelvis. Fiducial markers were placed at these four landmarks using a 3D slicer (version 5.6.2 r32448). The coronal planes passing through all four points were reconstructed using the Reformat tool (Registration\u0026thinsp;\u0026gt;\u0026thinsp;Specialized\u0026thinsp;\u0026gt;\u0026thinsp;Reformat module). This procedure ensured anatomically consistent sectioning across all participants, thereby maintaining the reliability of the morphological analysis.\u003c/p\u003e\n\u003ch3\u003eElliptic Fourier Analysis of the Obturator Internus\u003c/h3\u003e\n\u003cp\u003eFor shape analysis of the OI alone, standardized MRI coronal sections on the right side were used (n\u0026thinsp;=\u0026thinsp;27). OI contours were extracted using Seg and Ref, a semiautomatic segmentation tool developed by our group (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eA).\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e The extracted contours were subjected to shape analysis using elliptic Fourier descriptors (EFDs). We employed the SHAPE software (Version 1.3; Hiroyoshi Iwata) for the analysis.\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e This software automatically performs the following steps: (1) contour extraction from image files, (2) computation of EFDs, (3) standardization of contour size and orientation, (4) principal component analysis (PCA) based on EFD coefficients, and (5) shape reconstruction corresponding to each principal component (PC). PCA was performed on the contours of OI extracted from the MRI images using EFDs.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\n\u003ch3\u003eLandmark-Based Morphometric Analysis of the Obturator Internus and Levator Ani\u003c/h3\u003e\n\u003cp\u003eThe full shape of the bilateral OI and LA visualized on MRI was used for the combined morphological analysis of the OI and LA. Landmark placement was performed using the software tpsUtil and tpsDig2 (F. J. Rohlf, SUNY Stony Brook). Landmarks were defined based on the following anatomical criteria: (1) superior margin of the OI; (2) inferior margin of the OI; (3) characteristic \u0026ldquo;groove\u0026rdquo; on the medial surface of the OI; (4) superior margin of the LA; and (5) inferior margin of the LA, which was estimated as the boundary of the external anal sphincter. The landmarks were evenly distributed between these structural features as follows: 15 points between the superior and inferior margins of the OI, 7 points between the superior margin and groove, 7 points between the inferior margin and groove, and 15 points between the superior and inferior margins of the LA, resulting in 49 landmarks on each side and 98 landmarks bilaterally (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eB).\u003c/p\u003e\u003cp\u003eShape analysis was conducted using the Geomorph package (version 4.0.10) in RStudio (Version 4.0.10). Generalized procrust analysis (GPA) was first applied to remove variations due to translation, scale, and rotation. PCA was performed on aligned landmark configurations.\u003c/p\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003eStatistical Analysis\u003c/h2\u003e\u003cp\u003eAll statistical analyses were performed using RStudio software (version 4.0.10). PCA was applied to the shape data of the OI and LA, and the resulting PC scores were used as quantitative indicators of morphological variation. To evaluate the age-related shape changes, simple linear regression analyses were conducted with age as the explanatory (independent) variable and each PC score as the response (dependent) variable. Regression coefficients, p-values, coefficients of determination (R\u0026sup2;), residual standard errors, and F-statistics were calculated to assess the statistical significance.\u003c/p\u003e\u003cp\u003eAdditionally, Procrustes ANOVA was performed, using the procD.lm function, with statistical significance assessed based on 1,000 permutations, on the GPA-aligned landmark coordinates to examine the association between age and overall shape variation. The effect size (f\u0026sup2;) for each regression model was calculated based on a sample size of 27 and an alpha level of 0.05. Statistical significance was set at p\u0026thinsp;\u0026lt;\u0026thinsp;.05.\u003c/p\u003e\u003c/div\u003e"},{"header":"RESULTS","content":"\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003eShape Variation of the Obturator Internus: Elliptic Fourier Analysis\u003c/h2\u003e\u003cp\u003eThe cumulative contribution of PC1 to PC3 was 79.19%, accounting for the majority of shape variation, with 53.24%, 15.69%, and 10.26% individual contribution rates of PC1, PC2, and PC3, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). PC1 primarily reflected the overall scale and vertical shape characteristics, including the cross-sectional area of the OI, thickness of the superior portion, and vertical positioning of the medial groove and inferior medial eminence. In contrast, PC2 and PC3 captured variations related to the contour of the medial surface, particularly the depth of the groove and prominence of the medial eminence.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eRegarding age-related changes, PC1 showed a significant negative correlation with age (estimate = -0.00193, p\u0026thinsp;=\u0026thinsp;0.008, R\u0026sup2; = 0.25, f\u0026sup2; = 0.33) (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). In contrast, no significant association was found between age and PC2 or PC3 scores (PC2, p\u0026thinsp;=\u0026thinsp;0.46, R\u0026sup2; = 0.022; PC3, p\u0026thinsp;=\u0026thinsp;0.068, R\u0026sup2; = 0.127). However, the PC3 score demonstrated a trend toward a negative correlation with age.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eMorphological Changes of the Obturator Internus and Levator Ani: Landmark-Based Analysis\u003c/h3\u003e\n\u003cp\u003eThe cumulative contribution of the first two PCs was 62.8%, with PC1 accounting for 40.16% and PC2 accounting for 22.61%. PC1 primarily reflected the vertical positions of the groove and medial eminence on the medial surface of the OI, inclination of the LA, vertical position of its inferior margin, and medial distance between the bilateral LA (i.e., width of the levator hiatus). PC2 was associated with the curvature of the medial eminence of the OI, vertical position of the superior margin of the LA, and width of the levator hiatus (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). Although PC3 (14.35% of the variance) also contributed to the shape variation, it is omitted from the graphical presentation for clarity.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eLinear regression analysis revealed a statistically significant negative association between PC1 score and age (estimate = -0.00283, p\u0026thinsp;=\u0026thinsp;0.001, R\u0026sup2; = 0.343, f\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.522; Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). Furthermore, Procrustes ANOVA conducted using the GPA-aligned landmark coordinates showed that age accounted for approximately 15.1% (R\u0026sup2; = 0.151) of the total shape variation (F\u0026thinsp;=\u0026thinsp;4.44, p\u0026thinsp;=\u0026thinsp;0.003, Z\u0026thinsp;=\u0026thinsp;2.82).\u003c/p\u003e"},{"header":"COMMENT","content":"\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003cdiv id=\"Sec12\" class=\"Section3\"\u003e\u003ch2\u003ePrincipal findings\u003c/h2\u003e\u003cp\u003eElliptic Fourier analysis revealed that the coronal sectional shape of the OI changed with age and was particularly characterized by a reduction in the thickness of the superior part and a downward displacement of both the medial groove and medial eminence located inferior to it (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eA). Landmark-based analysis corroborated these findings by demonstrating a similar downward shift in the groove and eminence, along with concurrent structural changes such as verticalization of the LA and widening of the levator hiatus (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eB). To the best of our knowledge, this is the first study to quantitatively evaluate age-related shape changes of this muscle.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\u003ch2\u003eResults in the Context of What is Known\u003c/h2\u003e\u003cp\u003ePrevious studies on age-related changes in the PFMs have predominantly focused on alterations in the position or inclination of the LA. Even in studies that included the OI, the evaluations were largely limited to linear or areal parameters, such as muscle volume and physiological cross-sectional area. In contrast, the present study provides a novel structural and quantitative assessment of age-related shape alterations of the medial surface of the OI, a considerable advancement in the understanding of pelvic floor muscle aging, offering a new perspective and methodology for morphological analyses in this anatomical domain.\u003c/p\u003e\u003cp\u003eOur finding that the groove and medial eminence on the medial surface of the OI shifted inferiorly with aging supports recent reports suggesting that the OI contributes to pelvic floor support and plays a role in defecation and urinary continence.\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e Although prior research has extensively examined age-related morphological changes in the pelvic floor musculature, particularly in the LA muscle, focusing on position, inclination, and overall shape changes using MRI and three-dimensional modeling,\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e,\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e studies on the OI have been largely limited to volumetric or cross-sectional indices. These studies reported age-related decreases in the PCSA and muscle thickness\u003csup\u003e\u003cspan additionalcitationids=\"CR8\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e,\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e but did not address morphological shape changes. We previously identified a characteristic groove on the medial surface of the OI,\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e and we posited that such localized features require multivariate and geometric approaches for accurate characterization. Therefore, we employed two GM methods \u0026mdash;elliptic Fourier analysis and landmark-based analysis\u0026mdash;to demonstrate that age-related shape changes in the OI occur in parallel with the verticalization of the LA and widening of the levator hiatus. In particular, the inferior displacement of localized structures, such as the groove and medial eminence, revealed age-related morphological alterations that could not be captured by traditional linear indices.\u003c/p\u003e\u003cp\u003eIncreasing evidence suggests that structural changes in the OI affect pelvic floor function. For instance, in men undergoing prostatectomy, greater preoperative OI thickness was significantly associated with earlier postoperative recovery of urinary continence.\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e In women with POP, the cross-sectional area of the OI was significantly smaller than that in controls, implying that OI atrophy may be a risk factor for POP development.\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e Additionally, patients with POP were demonstrated to exhibit an increased anterior pelvic cross-sectional area.\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e Although this does not directly measure the OI, it may reflect structural integrity reduction. Furthermore, studies linking hip joint function to pelvic floor performance have attracted increasing attention. Several reports have demonstrated improvements in urinary incontinence among women after undergoing total hip arthroplasty (THA).\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e,\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e Posterior approaches to THA can damage the OI, and muscle atrophy in this region has been associated with worsening urinary symptoms.\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eThese observations suggest that among the hip muscles, the OI may play a particularly important structural role in supporting pelvic floor function. Anatomically, this functional linkage is underscored by the fascial interface between the medial surface of the OI and origin of the LA muscle. Previously, we demonstrated that these two muscles are broadly connected via the obturator fascia and that this interface constitutes the origin of the LA,\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e implying that the structural and mechanical properties of the OI mediate stability of the LA origin. In the present study, we demonstrated that this groove progressively shifts downward with age. Such displacement may compromise the positional stability of the LA origin and consequently contribute to age-related deterioration of pelvic floor function.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e\u003ch2\u003eClinical Implications\u003c/h2\u003e\u003cp\u003eOur findings highlighted the importance of hip joint integrity as a necessary but insufficient condition for maintaining optimal pelvic floor function. Evidence supporting this view includes a study by Tuttle et al. that demonstrates that targeted strengthening of the OI led to increased peak contraction pressure of the PFMs, suggesting a functional contribution of the hip musculature to pelvic floor performance.\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e Conversely, other interventions combining hip adduction and abduction exercises failed to yield changes in vaginal pressure,\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e indicating that not all hip movements effectively enhance pelvic floor function. Hwang et al. reported no significant correlation between hip muscle strength and PFM function in women with stress urinary incontinence,\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e further suggesting that hip function alone cannot fully compensate for PFM deficits in the pelvic floor. These findings collectively imply that although adequate hip function is a prerequisite, it does not suffice on its own. Compromised PFMs, such as the LA, cannot be functionally restored through hip rehabilitation alone. Therefore, an integrative approach targeting both PFMs and the hip musculature is essential for restoring pelvic floor function. Given its anatomical location and fascial connectivity, the OI likely plays a central mediating role in this functional linkage.\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e In particular, the OI may provide a biomechanical environment that facilitates optimal activation and stability of the LA. In this context, the OI may serve not merely as a passive support structure but also as a dynamic substrate that enables LA to exert its contractile function efficiently. Emerging evidence points to the vulnerability of the OI to obstetric factors. Senkaya et al. reported a significant negative correlation between fetal weight and OI cross-sectional area,\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e suggesting that the OI may undergo structural remodeling in response to childbirth. These findings indicate that the functional status of the OI is influenced not only by aging, but also by reproductive history.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec15\" class=\"Section2\"\u003e\u003ch2\u003eResearch Implications\u003c/h2\u003e\u003cp\u003eThe present study\u0026rsquo;s findings advance the anatomical and physiological understanding of pelvic floor aging. Furthermore, the significant correlations observed between these structural changes and chronological age suggest their potential applications in imaging-based diagnosis, preventive strategies, and therapeutic interventions. Our findings suggest that future diagnostic and therapeutic frameworks for pelvic floor dysfunction should incorporate morphological and functional evaluations of OI. Particularly, in rehabilitation settings, comprehensive interventions that extend beyond LA-focused therapies to include the hip joint, specifically the OI, may prove beneficial. Such integrative perspectives could inform revisions to the current guidelines for the prevention, treatment, and surgical reconstruction of pelvic floor disorders.\u003c/p\u003e\u003cp\u003eNevertheless, several important questions remain unresolved. For example, the causal mechanisms linking age-related structural changes in the OI to functional decline of the pelvic floor have yet to be elucidated. In addition, whether these morphological alterations differ by sex, parity, or hormonal status remains unclear. Furthermore, longitudinal evidence is lacking to determine whether OI-targeted rehabilitation strategies can effectively prevent or reverse pelvic floor dysfunction.\u003c/p\u003e\u003cp\u003eFuture studies should therefore combine imaging-based morphometric analyses with biomechanical and functional assessments, as well as interventional trials, to clarify these relationships. Multicenter investigations with larger and more diverse populations would also be valuable to generalize our findings and to establish evidence-based guidelines that integrate both LA- and OI-focused approaches.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec16\" class=\"Section2\"\u003e\u003ch2\u003eStrengths and Limitations\u003c/h2\u003e\u003cp\u003eThis study had several limitations. First, the sample was limited to adult women without pelvic floor disorders and no comparative analyses were conducted with populations with a history of childbirth or pelvic floor disorders. Second, the evaluation was based solely on static MRI images, which do not capture the dynamic functional interplay between muscle contraction and movement. Third, as this was a cross-sectional study, we did not observe age-related morphological changes over time. Despite these limitations, the significant association between age and morphological variation can be considered highly reliable.\u003c/p\u003e\u003c/div\u003e"},{"header":"Conclusions","content":"\u003cp\u003eWe found that the OI and LA muscles undergo age-related morphological changes. The downward displacement of the groove and eminence on the medial surface of the OI occurred in parallel with the vertical alignment of the LA and widening of the levator hiatus, representing a novel and multifaceted insight into age-related changes in pelvic floor musculature. These shape alterations may influence the pelvic floor support, continence function, and rehabilitation strategies.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors sincerely thank those who donated their bodies to science so that anatomical research could be performed. The results of this research can potentially increase the overall knowledge of humankind and improve patient care. Therefore, the donors and their families deserve great gratitude.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflict of Interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors have no relevant financial relationships to disclose.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability Statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eData supporting this study\u0026rsquo;s findings are available from the corresponding author upon request.\u0026nbsp;The code for the semiautomatic segmentation tool (Seg and Ref) developed by our group to extract OI contours is publicly available on GitHub (https://github.com/SatoruMuro/SAM2GUIfor3Drecon).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study was supported byJSPS KAKENHI (grant number 25K18696).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDeclaration of generative AI and AI-assisted technologies in the writing process\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eDuring the preparation of this study, ChatGPT was used to improve the clarity and grammatical usage of English. After using these services, the authors reviewed and edited the content as required and take full responsibility for the content of the publication.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTweetable statement:\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eConcurrent age-related changes in the obturator internus and levator ani suggest that medial groove descent and thinning of the obturator internus may be core structural features of pelvic floor aging.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eMilsom I, Gyhagen M (2019) The prevalence of urinary incontinence. 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Ginekol Pol 94:57\u0026ndash;63. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.5603/GP.a2022.0140\u003c/span\u003e\u003cspan address=\"10.5603/GP.a2022.0140\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[{"identity":"42352855-a27b-4565-9928-b2ba10aed29b","identifier":"10.13039/501100001691","name":"Japan Society for the Promotion of Science","awardNumber":"25K18696","order_by":0}],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"Institute of Science Tokyo","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":"Aging, Analysis of Variance, Cross-Sectional Studies, Magnetic Resonance Imaging, Pelvic Floor, Pelvic Floor Disorders, Skeletal Muscle ","lastPublishedDoi":"10.21203/rs.3.rs-7449052/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7449052/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003e\u003cb\u003eBackground\u003c/b\u003e\u003c/p\u003e\u003cp\u003ePelvic floor dysfunction is a common condition in adult women that substantially impacts their quality of life. Most studies have focused on the levator ani muscle, but recent anatomical insights suggest that the surrounding muscles, such as the obturator internus, also play a key structural role. However, the age-related morphological changes in these muscles remain poorly understood.\u003c/p\u003e\u003cp\u003e\u003cb\u003eObjective\u003c/b\u003e\u003c/p\u003e\u003cp\u003eTo quantitatively characterize age-related morphological changes in the obturator internus and levator ani muscles using geometric morphometric techniques.\u003c/p\u003e\u003cp\u003e\u003cb\u003eStudy Design:\u003c/b\u003e\u003c/p\u003e\u003cp\u003eThis cross-sectional study included 27 adult women aged 25\u0026ndash;83 years with gynecological conditions but without pelvic floor disorders, as evidenced by clinical or imaging parameters. Standardized pelvic magnetic resonance imaging scans were obtained between April 2024 and July 2024 at a single tertiary care center. Elliptic Fourier analysis was applied to the segmented contours of the obturator internus, and a landmark-based method was used to evaluate the combined shape of the obturator internus and levator ani. Principal component analysis and linear regression were performed to assess the associations with age. Procrustes analysis of variance was used to evaluate the overall shape-age relationships.\u003c/p\u003e\u003cp\u003e\u003cb\u003eResults\u003c/b\u003e\u003c/p\u003e\u003cp\u003eElliptic Fourier analysis revealed that increasing age was significantly associated with decreased thickness, downward displacement of the medial surface groove, and eminence of the obturator internus (p\u0026thinsp;=\u0026thinsp;.008, R\u0026sup2; = .25). Landmark-based analysis revealed similar age-related shape changes, including inferior displacement of the groove, verticalization of the levator ani, and widening of the levator hiatus (p\u0026thinsp;=\u0026thinsp;.001, R\u0026sup2; = .343). Procrustes analysis revealed a significant relationship between age and overall shape variation (p\u0026thinsp;=\u0026thinsp;.003).\u003c/p\u003e\u003cp\u003e\u003cb\u003eConclusions\u003c/b\u003e\u003c/p\u003e\u003cp\u003eAging was associated with coordinated structural remodeling of the obturator internus and levator ani. These findings suggested that, in addition to traditional pelvic floor\u0026ndash;focused approaches, strategies targeting hip joint function may offer novel opportunities for the assessment and rehabilitation of patients with pelvic floor dysfunction.\u003c/p\u003e","manuscriptTitle":"Aging-related morphological changes in the obturator internus and levator ani","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-08-27 07:50:43","doi":"10.21203/rs.3.rs-7449052/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":"a6b2f1ad-5d20-417a-8e8a-cf1a47eeb553","owner":[],"postedDate":"August 27th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":53634503,"name":"Obstetrics \u0026 Gynecology"}],"tags":[],"updatedAt":"2025-08-27T07:50:43+00:00","versionOfRecord":[],"versionCreatedAt":"2025-08-27 07:50:43","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7449052","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7449052","identity":"rs-7449052","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}