Recovery of preoperative absolute muscle strength in the quadriceps or hamstrings at time of return to sport did not affect rate of a second anterior cruciate ligament injury: a registry study

Recovery of preoperative absolute muscle strength in the quadriceps or hamstrings at time of return to sport did not affect rate of a second anterior cruciate ligament injury: a registry study | 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 Recovery of preoperative absolute muscle strength in the quadriceps or hamstrings at time of return to sport did not affect rate of a second anterior cruciate ligament injury: a registry study Jakob Lindskog, Daniel Broman, Ramana Piussi, Roland Thomeé, Eric Hamrin Senorski This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6971659/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 11 You are reading this latest preprint version Abstract Background Recovery of preoperative absolute muscle strength (PAMS) after anterior cruciate ligament (ACL) reconstuction remains a significant challenge. However, whether recovery of PAMS contributes to an affected rate of second ACL injury has yet to be explored. This study aimed to analyse the rate of a second ACL injury incidence after return to sports (RTS) between patients with ACL reconstruction dependant on the recovery of their PAMS for quadriceps and hamstrings, respectively and in combination, compared to patients who had not. Methods A retrospective cohort study with data from a rehabilitation specific registry were used. Patients who had undergone primary ACL reconstruction, were aged between 15 and 35 years, had performed complete preoperative isokinetic strength tests for quadriceps and hamstrings prior to primary ACL reconstruciton, who had a pre-injury Tegner Activty Scale ≥ 6, had returned to sport after ACL reconstruction and complete isokinetic strength tests for quadriceps and hamstrings at the same time of RTS were included. The main outcome was the rate of second ACL injury after RTS based on recovery of ≥ 90% PAMS for the quadriceps, and hamstrings indivudually, and in combination, or not. Results A total of 190 patients (mean age 22.2 ± 5.2 years, 56.3% women) were included. Thirty-one patients (16.3%) sustained a second ACL injury. There was no difference in rate of second ACL injury between patients who had recovered ≥ 90% of PAMS in their quadriceps, or hamstrings, in both limbs, respectively and in combination, compared to patients who had not. Conclusion Recovery of preoperative absolute muscle strength in the quadriceps and hamstrings respectively and in combination, at time of RTS did not affect the rate of second ACL injury. Anterior cruciate ligament reconstruction preoperative absolute muscle strength reinjury Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction Each year, approximately 68.6 and 80 anterior cruciate ligament (ACL) injuries per 100,000 individuals are reported in the United States and Sweden, respectively( 1 , 2 ). After an ACL injury, patients treated with or without ACL reconstruction will undergo rehabilitation, often guided by a physical therapist. In Sweden, roughly half of patients who have sustained an ACL injury choose to undergo for ACL reconstruction( 1 ), especially younger individuals that aim to return to demanding knee-intensive sports( 3 ). However, it is important to note that previous studies have reported that younger age and to return to sport (RTS) increased the risk of a subsequent ACL injury( 4 , 5 ), where up to 23% of patients under the age of 25 years who returned to sport after ACL reconstruction experienced a second ACL injury( 6 ). Recovery of lower extremity muscle strength has been suggested as a key factor for successful outcomes following ACL reconstruction( 7 ). Seated concentric isokinetic muscle strength tests for quadriceps and hamstrings are considered the ‘gold standard’ after ACL reconstruction( 8 , 9 ) and are commonly used to assess lower extremity muscle strength( 10 – 12 ). However, despite the widespread acceptance of these test batteries, the evidence is inconsistent with regard to the tests’ ability to identify patients at risk of a subsequent ACL injury( 13 – 17 ). Results from test batteries are commonly presented as side-to-side differences, calculated with the limb symmetry index (LSI): LSI = (involved limb result / uninvolved limb result) x 100( 18 – 20 ). Yet, the use of LSI has limitation, as it can overestimate muscle strength recovery; a 100% LSI might signify bilateral weakness if the uninvolved limb weakens during rehabilitation( 21 , 22 ). This limitation might contribute to the contradictory evidence concerning the role of results from test batteries to identify patients at risk of subsequent ACL injuries and suggests the need for an additional method to assess or interpret muscle strength. An alternative to LSI is to evaluate whether patients recover their preoperative absolute muscle strength (PAMS)( 11 , 22 ). Discrepancies have been reported between patients who achieve symmetrical quadriceps and hamstring strength (LSI ≥ 90%) and those who reach ≥ 90% of PAMS for quadriceps and hamstrings one year after ACL reconstruction( 11 ). Around one-third of patients who achieve ≥ 90% LSI do not reach ≥ 90% of PAMS, which indicates persistent weakness compared to their pre-reconstruction state, despite that the patients had recovered symmetry. Conversely, about one-third of patients who achieve ≥ 90% of PAMS do not attain ≥ 90% LSI, which indicates that the patients were as strong or stronger than pre-reconstruction, however, lacked symmetry. Lynch et al.( 7 ) stated in a consensus paper that muscle strength recovery before RTS should include achievement of symmetrical muscle strength. However, to solely rely on achievment of symmetrical muscle strength before RTS may lead to some patients RTS with symmetrical, but inadequate, muscle strength. Nevertheless, the significance of PAMS recovery with regard to the risk of a subsequent ACL injury remains unclear. Therefore, the aim of this study was to determine the rate of a second ACL injury after RTS in patients treated with ACL reconstruction dependant on the recovery of PAMS for quadriceps and hamstrings, respectively, and in combination. Methods Setting This observational retrospective cohort study was reported according to the “Strengthening the reporting of observational studies in epidemiology” (STROBE) statement( 23 ). The study was based on data from a rehabilitation outcome registry, Project ACL, which started in 2014, located in the south-west region of Gothenburg. Sweden, aims to improve the care of patients who have sustained an ACL injury, regardless of treatment choice, through a continuous standardized patient assessment. These assessments adhere to a predetermined schedule, at 10 weeks, 4, 8, 12, 18, 24, 60 months after baseline, that is injury or reconstruction, and after that every 5 years. The assessments are performed with both validated patient-reported outcomes (PROs) and muscle function tests. Patients scheduled for ACL reconstruction are also asked to perform a preoperative baseline test within 6 weeks prior to reconstruction. The project has ethical approval from the Swedish Ethical Review Authority (registration numbers: 2020–02501, 2024-08724-01), and the Regional Ethical Review Board in Gothenburg, Sweden (registration numbers: 265–13, T023–17). Participants Patients from Project ACL were eligible for inclusion in the study if they met the following criteria: Registered with a primary ACL injury treated with ACL reconstruction Aged between 15 and 35 years at the time of primary ACL reconstruction Performed complete preoperative isokinetic strength tests for both quadriceps and hamstrings prior to primary ACL reconstruction Had a pre-injury Tegner activity scale( 24 ) (Tegner) ≥ 6 i.e., participated in knee-strenuous sports, such as tennis, badminton or more demanding Had returned to sport after ACL reconstruction (reported Tegner ≥ 6) Performed complete postoperative isokinetic strength tests for both quadriceps and hamstrings at the same follow-up as to when they returned to sport Had at least one year follow-up after RTS The reason for the chosen inclusion criteria was to study a population of active patients with participation in knee-strenuous sport prior to A C L injury and to account for adequate risk exposure to a second ACL injury. Variables Isokinetic strength testing for quadriceps and hamstrings For isokinetic unilateral quadriceps and hamstrings muscle strength, a Biodex System 4 (Biodex Medical Systems, Shirley, New York) was used. The Biodex System has produced reliable test re-test measures in healthy active individuals (ICC = 0.95)( 25 ). All strength tests were performed in a lab setting in Project ACL’s facilities led by physical therapists. The test was first performed for the involved limb, followed by the uninvolved limb, at an angular velocity of 90°/second. Patients were in a seated position with 85° hip flexion and strapped across the trunk, pelvis, and thigh. During testing the patients’ arms were held crossed at chest level. Knee extension was performed between 90 − 0° of knee flexion and knee flexion was performed between 0–90° of knee flexion. Both movements were performed subsequently in each repetition. A standardized protocol was used during testing. The protocol consisted of 10 minutes warm up on a stationary bike, followed by familiarization in the Biodex machine with 10 repetitions at 50% of maximum effort, 10 repetitions at 75% of maximum effort followed by 1–2 repetitions of 90% of maximum effort. After familiarization, patients were asked to perform 3–4 maximum effort repetitions with 40 seconds of rest between each repetition. During the maximum effort repetitions patients were verbally encouraged by the test leader. The highest achieved peak torque (in newton meters) for quadriceps and hamstrings muscle strength (knee extension and knee flexion) for the respective limbs, was registered in Project ACL database and used for analysis. Data Data for patient demographics, results from preoperative and postoperative isokinetic muscle strength tests was extracted from Project ACL in October 2024. Return to sport In this study, patients were classified as having returned to sport when they reported that they had reached Tegner level ≥ 6 after ACL reconstruction. The Tegner is a scale used to determine how strenuous different activities are for the knees( 24 ). The Tegner is graded from 0–10, with 0 indicating the lowest level of strenuous knee-related activity (sick leave or disability pension) and 10 indicating the highest level of strenuous knee-related activity (e.g., competitive soccer at national or international elite level). In the present study a modified version of Tegner was used, which ranges from 1–10, with sports such as American football, rugby and floorball has been added to the scale. In addition, level 0, sick leave, was excluded. The modified version of Tegner has previously been described in detail( 26 ). Patients were assessed with Tegner at the follow-ups 4, 8, 12, 24 and 60 months after ACL reconstruction. Test-retest reliability of Tegner has been reported acceptable with an Intraclass Correlation Coefficient (ICC) = 0.80 for patients who have sustained an ACL injury( 27 ). Outcomes The primary outcome in the present study was rate of a second ACL injury after RTS (yes/no). At every follow-up after ACL reconstruction patients in Project ACL were asked to answer questions regarding any new potential ACL injury through an online questionnaire. In addition, patients are also encouraged to register any new potential ACL injuries online, outside of the follow-up schedule. Any potential new injury should be assessed and diagnosed by the responsible physical therapist or orthopedic surgeon. To minimize the risk of missing data regarding a new ACL injury, patients are also asked by the test leader, during the clinical follow-ups, about any new injuries. Both new ipsilateral and contralateral ACL injuries were considered as “second ACL injuries” in this study. Patients were evaluated for second ACL injuries up to date of data extraction or until they sustained a second ACL injury. The recovery of PAMS was calculated for both the involved and the uninvolved limb, respectively, with the following formula: peak torque at the same follow-up as the patient had returned to sport / highest achieved preoperative peak torque (irrespective of whether it originated from the involved or uninvolved limb) x 100. Patients were classified as having recovered PAMS if they score ≥ 90% for both the involved and the uninvolved limb( 11 ). In addition, results from muscle strength test were also presented and compared as a relative strength, i.e., peak torque for the involved limb / body mass in kilograms. Statistical analysis Demographic variables were presented stratified by patients who sustained a second ACL injury or not. The primary outcome - the difference in rate of second ACL injury – was analysed between patients who recovered ≥ 90% of PAMS or not. The rate of second ACL injury between groups was analysed using censored survival analysis. Hazard ratios (HR) with 95% confidence intervals (CI) were estimated with Cox proportional hazard regression models. A univariable model was applied, and a multivariable model was constructed based on baseline demographic variables with a significance level of p < 0.2. Additional repeated sensitivity analyses with the univariable cox proportional hazard regression model were carried out, in which 1) follow-up end point was set at one year after RTS, 2) follow-up end point was set at two years after RTS, 3) RTS was classified as reaching the same or higher pre-injury Tegner, and 4) PAMS was classified as achieving ≥ 100% instead for ≥ 90% of preoperative strength, respectively. To investigate the difference in survival rate between groups in the cox proportional hazard regression model, a log-rank test was employed, and was illustrated with Kaplan-Meier curves. To compare the follow-up time between groups, months from RTS before data extraction date, presented as mean with standard deveiation (SD), was added and analysed with the independent t-test. For patient demographics, mean value with SD for parametric data, median with interquartile range (IQR) for non-parametric data, and number (n) with proportion (%) for categorical data was presented. For analysis of patient demographics, the independent t-test for parametric data, the Mann-Whitney U-test for non-parametric data, and the Fisher’s Exact test for categorical data was used. A significance level of 5% was used. Statistical analyses were performed with the Statistical Package for the Social Sciences (IBM Corp. Released 2017. IBM SPSS Statistics for Windows, Version 27.0. Armonk, NY: IBM Corp.) Results A total of 190 patients with a mean age of 22.2 ± 5.2 years at primary ACL reconstruction, 56.3% women, were included in the present study. In total, 31 patients (16.3%) sustained a second ACL injury. Figure 1 presents the flow-chart of the inclusion process. Table 1 shows baseline patient demographics, stratified by patients who did or did not sustain a second ACL injury. There were no baseline differences between patients who sustained or did not sustain a second ACL injury. Table 1 Baseline demographics for patients with and without a second ACL injury. Variable Second ACL injury, n = 31 No second ACL injury, n = 159 p-value Age at primary reconstruction, [years] 20.9 ± 5.1 22.5 ± 5.2 0.125 Patient sex, [female] n (%) 16 (51.6) 91 (57.2) 0.693 Height, [m] 1.76 ± 0.10 1.74 ± 0.10 0.498 Body mass, [kg] 73.5 ± 13.2 70.8 ± 11.7 0.275 BMI, [kg/m 2 ] 23.6 ± 2.5 23.2 ± 2.7 0.419 Time from ACL injury to reconstruction [months] 3.7 ± 2.9 5.8 ± 12.4 0.358 Preoperative quadriceps strength (involved limb) (Nm/kg) 2.6 ± 0.4 2.6 ± 0.5 0.552 Preoperative quadriceps strength (uninvolved limb) (Nm/kg) 2.8 ± 0.4 2.8 ± 0.5 0.703 Preoperative hamstrings strength (involved limb) (Nm/kg) 1.5 ± 0.3 1.5 ± 0.3 0.909 Preoperative hamstrings strength (uninvolved limb) (Nm/kg) 1.6 ± 0.3 1.5 ± 0.3 0.229 Preoperative quadriceps LSI 93.6 ± 14.6 92.6 ± 13.7 0.706 Preoperative hamstrings LSI 96.0 ± 12.0 99.8 ± 12.5 0.119 Pre-injury Tegner score, median (IQR) 9 ( 2 ) 9 ( 1 ) 0.117 Graft type, n (%) Hamstring tendon 24 (77.4) 139 (87.4) 0.230 Patellar tendon 7 (22.6) 18 (11.3) Allograft - 2 (1.3) ACL = anterior cruciate ligament; LSI = Limb symmetry index; BMI = Body mass index; SD = Standard deviation; Nm/kg = Newton meters per kilogram bodymass; n = number of patients. There were no differences in postoperative outcomes at time of RTS between patients who sustained or did not sustain a second ACL injury. Complete postoperative outcomes are reported in Table 2 . Table 2 Post-operative outcomes at time of RTS for patients with and without a second ACL injury Variable Second ACL injury, n = 31 No second ACL injury, n = 159 p-value Time from ACL reconstruction to RTS [months] 10.1 ± 5.1 10.9 ± 4.4 0.401 Strength at time of RTS (Nm/kg) Quadriceps (involved limb) 2.9 ± 0.6 2.8 ± 0.6 0.719 Quadriceps (uninvolved limb) 3.0 ± 0.5 3.0 ± 0.5 0.797 Hamstrings (involved limb) 1.6 ± 0.3 1.5 ± 0.4 0.298 Hamstrings (uninvolved limb) 1.6 ± 0.3 1.6 ± 0.3 0.503 % Recovery of PAMS at time of RTS Quadriceps (involved limb) 98.7 ± 17.0 100.1 ± 16.4 0.667 Quadriceps (uninvolved limb) 105.0 ± 13.0 106.7 ± 13.2 0.507 Hamstrings (involved limb) 100.4 ± 16.4 98.2 ± 15.2 0.475 Hamstrings (uninvolved limb) 100.9 ± 14.1 101.3 ± 13.2 0.872 LSI for quadriceps at time of RTS 94.3 ± 14.3 94.1 ± 12.8 0.929 LSI for hamstrings at time of RTS 99.7 ± 9.3 97.3 ± 11.4 0.269 ACL = anterior cruciate ligament; PAMS = Preoperative absolute muscle strength; LSI = Limb symmetry index; SD = Standard deviation; RTS = Return to sports; Nm/kg = Newton meters per kilogram bodymass There was no difference in the proportion of patients who recovered ≥ 90% of PAMS for the quadriceps between patients who sustained or did not sustain a second ACL injury, which did not correspond to an affected hazard ratio of a second ACL injury (Table 3 ). There was no difference in mean follow-up time after RTS between patients who had recovered ≥ 90% of PAMS for the quadriceps, and hamstrings individually, or both muscle groups in combination (Appendix table 1). There was no difference in cumulative survival rate for patients who recovered ≥ 90% of PAMS in the quadriceps, the hamstrings, and in the quadriceps and hamstrings in combination (Fig. 2 – 4 ). Table 3 Proportion of patients who recovered 90% of PAMS and hazard ratios for a second ACL injury after RTS Recovered 90% of PAMS (involved + uninvolved limb), n/total patients (%) HR for a second ACL injury, recovering PAMS compared to not [95% CI] Muscle group(s) Second ACL injury , n = 31 No second ACL injury , n = 159 p-value Unadjusted p-value Adjusted* p-value Quadriceps 22/31 (71.0%) 117/159 (73.6%) 0.825 0.97 [0.40–1.89] 0.720 0.71 [0.31–1.62] 0.415 Hamstrings 22/31 (71.0%) 101/159 (63.5%) 0.539 1.36 [0.63–2.63] 0.433 1.26 [0.57–2.75] 0.567 Quadriceps + Hamstrings 17/31 (54.8%) 82/159 (51.6%) 0.845 1.13 [0.56–2.29] 0.739 0.98 [0.47–2.03] 0.948 PAMS = Preoperative absolute muscle strength; HR = Hazard ratio; CI = Confidence interval; n = number of patients; * = adjusted for age at primary ACL reconstruction, pre-injury Tegner Activity Scale values and pre-operative hamstring Limb Symmetry Index values. Sensitivity analyses In the sensitivity analyses, there was no effect on the rate of a second ACL injury for patients who recovered PAMS when 1) follow-up end point was set to one year after RTS, 2) follow-up end point was set to two years after RTS, 3) RTS was determined as returned to the same or higher pre-injury Tegner, or 4) the cut-off for recovery of PAMS was set to ≥ 100% in the quadriceps, the hamstrings, or both the quadriceps or hamstrings at time of RTS (Appendix table 2–5). Discussion The main findings of this study were that there was no effect on the rate of second ACL injury after RTS for patients who recovered ≥ 90% of PAMS for the quadriceps, the hamstring or both at time of RTS compared to patients who did not recover ≥ 90% of PAMS. In this study, 16.3% of included patients sustained a second ACL injury during follow-up, which averaged approximately 51 months after RTS, and the rate is considered consistent with findings from previous research.( 13 , 28 ) In previous studies, the LSI has been used to identify patients at risk for a subsequent ACL injury and reported contradictory results( 13 – 17 ), which could partly be affected by an overestimation of recovery due to the possible weakening of the uninvolved limb( 21 , 22 , 29 ). Overall, in this study, between 63.5% and 73.6% of patients recovered ≥ 90% of PAMS in the quadriceps and hamstrings, which could indicate that some patients present a deconditioned state of knee muscle strength compared to before ACL reconstruction at RTS. Despite the potentially deconditioned state of thigh muscles, there was no difference in rate of second ACL injury based on recovery of pre-operative absolute quadriceps or hamstrings strength in this study. Prior research suggest that the evaluation of PAMS could potentially serve as a valuable complement to LSI-based assessments throughout ACL reconstruction rehabilitation( 21 , 22 , 29 ), however, the rate reduction of a second ACL injury by recovery of ≥ 90% of PAMS for quadriceps or hamstrings was not supported by our study. In contrast, Wellsandt et al.( 22 ) previously reported that the recovery of PAMS for the quadriceps, coupled with the recovery of preoperative absolute hop function, 6 months after ACL reconstruction appeared more sensitive to identify patients at risk of a second ACL injury compared to the recovery of LSI for quadriceps strength and hop function. There are a few differences between the study by Wellsandt et al.( 22 ) and ours, namely: post-operative strength test evaluation was on average performed approximately 4 to 5 months earlier compared to our study. Additionally, patients in our study were on average more than 4 years younger (mean 22.2 years versus 26.6 years), and the included patients in Wellsandt et al.( 22 ) were not controlled for RTS as in our study, which collectively obstruct direct comparisons. Taken together, the findings in this study do not support the rate reduction effect for a second ACL injury for recovery of PAMS for quadriceps and hamstrings, meaning that clinicians should not solely rely on the recovery of PAMS of the quadriceps and hamstring to determine a RTS with minimized risk of a second ACL injury. The univariable cox proportional hazard regression model was chosen for this study to provide a powerful statistical analysis and despite that the number of second ACL injuries met the recommendation of at least 20 events( 30 ), the results from the primary analysis had wide CIs, which also encompassed 1 for the HRs, which suggest that larger groups are needed to reach statistical precision. Interestingly, in the sensitivity analysis which applied the ≥ 100% of PAMS threshold, between 32.3% and 25.8% of patients recovered ≥ 100% of PAMS for the quadriceps and hamstrings at time of RTS, which corresponded to narrower CIs for the rates for a second ACL injury compared to the ≥ 90% of PAMS cut-off. In addition, the rates for second ACL injury with the use of ≥ 100% cut-off were, despite not being statistically significant, skewed toward a decreased rate of a second ACL injury. Our study suggests that the use of a more challenging threshold for PAMS recovery, i.e., ≥ 100% compared to ≥ 90%, may be associated with a better ability to differentiate between patients who do and do not go on to sustain a second ACL injury, though further investigation is merited. Future studies, that include large populations to increase power, should further investigate the impact of different PAMS cut-off values for both the quadriceps and hamstrings at the time of RTS on the risk of second ACL injury. This study contributes to the ongoing discussion in sports medicine with regard to optimal criteria for RTS following ACL reconstruction. Contrary to earlier studies which suggests that recovery of PAMS may reduce the risk of second ACL injuries, our findings indicate no such protective effect for patients achieving ≥ 90% PAMS for the quadriceps or hamstrings. This aligns with previous work reporting inconsistent associations between LSIs and re-injury risk, possibly due to methodological limitations such as overestimation of recovery in the uninvolved limb. Our findings underscore that PAMS alone may not be a sufficient standalone metric for RTS decision-making. Ultimately, these results emphasize the need for multifactorial, individualized RTS criteria and support continued research with larger cohorts to refine strength-based thresholds and improve long-term outcomes after ACL reconstruction. Limitations A potential limitation in this study is the potential loss of muscle strength in the involved limb prior to surgery following the primary ACL injury( 31 ), which was addressed by the use of the highest preoperative torque as a reference for recovery, irrespective of whether it originated from the involved or uninvolved limb( 11 ). Second ACL injuries have multifactorial causes which include younger age, concomitant knee injury, returning to high knee-demanding sports, longer time between initial injury and ACL reconstruction, time to RTS, injury mechanism, and joint hypermobility( 4 , 5 , 10 , 32 – 34 ). In this study, no differences in baseline demographics were observed, however, data on concomitant knee injury, injury mechanism, joint mobility, and athletic exposure after RTS, e.g. volume, intensity and frequency, were not available and could not be accounted for. Additionally, both high and low psychological responses have been reported to impact the risk of second ACL injury( 35 , 36 ), which was not analysed in this study. The average age of patients in this cohort was approximately 22 years at the time of primary ACL reconstruction, with a nearly equal sex distribution (consistent with the general population of ACL injured patients in Sweden)( 1 ). Additionally, the patients included were actively involved in high knee-demanding sports before the primary ACL injury (median Tegner = 9, equivalent to participation in sports such as soccer, ice-hockey, and alpine skiing). However, it is important to exercise caution when generalizing our results to a younger or older population or patients engaged in non-pivoting sports. One final limitation is patients’ adherence to the preoperative follow-up in Projetct ACL. Of the 4,493 patients registered at the time of data extraction, 735 patients (16.4%) had performed preoperative muscle strength tests. It is unknown whether the cohort included in the study differed significantly in terms of patient characteristics from the patients not eligible for inclusion and therefore caution should be taken to generalize the results to the general population of patients with an ACL injury. Conclusion Recovery of preoperative absolute muscle strength in the quadriceps and hamstrings respectively and in combination, at time of RTS did not affect the rate of second ACL injury. Clinicians should not solely rely on recovery of PAMS for quadriceps or hamstrings for a decreased rate of second ACL injury. Declarations Ethics approval and consent to participate The principles of the Helsinki declaration have been adhered in this study. Ethical approval has been obtained from the Swedish Ethical Review Authority (registration numbers: 2020-02501, 2024-08724-01), and the Regional Ethical Review Board in Gothenburg, Sweden (registration numbers: 265–13, T023–17). Data for this study is based on a rehabilitation registry project (Project ACL), where all patients have received written information and have given their informed consent in the research project. Consent for publication Not applicable. Competing interests We have no competing interests to declare. Funding Partial financial support was granted from Research and development primary health care Gothenburg and Södra Bohuslän. Author contributions First (JL) author drafted the initial version of the manuscript. Authors DB, RP, and EHS contributed majorly during analysis of the data. 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Knee surgery, sports traumatology, arthroscopy: official journal of the ESSKA. 2018;26(7):1966–74. Nawasreh Z, Logerstedt D, Cummer K, Axe M, Risberg MA, Snyder-Mackler L. Functional performance 6 months after ACL reconstruction can predict return to participation in the same preinjury activity level 12 and 24 months after surgery. Br J Sports Med. 2018;52(6):375. Broman D, Piussi R, Thomeé R, Hamrin Senorski E. A clinician-friendly test battery with a passing rate similar to a 'gold standard' return-to-sport test battery 1 year after ACL reconstruction: Results from a rehabilitation outcome registry. Phys therapy sport: official J Association Chart Physiotherapists Sports Med. 2023;59:144–50. Thomeé R, Neeter C, Gustavsson A, Thomeé P, Augustsson J, Eriksson B et al. Variability in leg muscle power and hop performance after anterior cruciate ligament reconstruction. Knee surgery, sports traumatology, arthroscopy: official journal of the ESSKA. 2012;20(6):1143–51. Wellsandt E, Failla MJ, Snyder-Mackler L. Limb Symmetry Indexes Can Overestimate Knee Function After Anterior Cruciate Ligament Injury. J Orthop Sports Phys Ther. 2017;47(5):334–8. von Elm E, Altman DG, Egger M, Pocock SJ, Gøtzsche PC, Vandenbroucke JP. The Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) statement: guidelines for reporting observational studies. J Clin Epidemiol. 2008;61(4):344–9. Tegner Y, Lysholm J. Rating systems in the evaluation of knee ligament injuries. Clin Orthop Relat Res. 1985(198):43–9. Feiring DC, Ellenbecker TS, Derscheid GL. Test-retest reliability of the biodex isokinetic dynamometer. J Orthop Sports Phys Ther. 1990;11(7):298–300. Beischer S, Hamrin Senorski E, Thomeé P, Thomeé R. Validation of an 18-item version of the swedish knee self-efficacy scale for patients after ACL injury and ACL reconstruction. J experimental Orthop. 2021;8(1):96. Thomeé P, Währborg P, Börjesson M, Thomeé R, Eriksson BI, Karlsson J. Self-efficacy, symptoms and physical activity in patients with an anterior cruciate ligament injury: a prospective study. Scand J Med Sci Sports. 2007;17(3):238–45. Webster KE, Feller JA, Leigh WB, Richmond AK. Younger patients are at increased risk for graft rupture and contralateral injury after anterior cruciate ligament reconstruction. Am J Sports Med. 2014;42(3):641–7. Larsen JB, Farup J, Lind M, Dalgas U. Muscle strength and functional performance is markedly impaired at the recommended time point for sport return after anterior cruciate ligament reconstruction in recreational athletes. Hum Mov Sci. 2015;39:73–87. Concato J, Feinstein AR. Monte Carlo methods in clinical research: applications in multivariable analysis. J Invest medicine: official publication Am Federation Clin Res. 1997;45(6):394–400. Gardinier ES, Manal K, Buchanan TS, Snyder-Mackler L. Gait and neuromuscular asymmetries after acute anterior cruciate ligament rupture. Med Sci Sports Exerc. 2012;44(8):1490–6. Laxdal G, Kartus J, Ejerhed L, Sernert N, Magnusson L, Faxén E, et al. Outcome and risk factors after anterior cruciate ligament reconstruction: a follow-up study of 948 patients. Arthroscopy: J arthroscopic Relat Surg : official publication Arthrosc Association North Am Int Arthrosc Association. 2005;21(8):958–64. Svantesson E, Hamrin Senorski E, Alentorn-Geli E, Westin O, Sundemo D, Grassi A, et al. Increased risk of ACL revision with non-surgical treatment of a concomitant medial collateral ligament injury: a study on 19,457 patients from the Swedish National Knee Ligament Registry. Knee surgery, sports traumatology, arthroscopy. official J ESSKA. 2019;27(8):2450–9. Zsidai B, Piussi R, Thomeé R, Sundemo D, Musahl V, Samuelsson K et al. Generalised joint hypermobility leads to increased odds of sustaining a second ACL injury within 12 months of return to sport after ACL reconstruction. Br J Sports Med. 2023. Piussi R, Beischer S, Thomeé R, Thomeé C, Sansone M, Samuelsson K et al. Greater Psychological Readiness to Return to Sport, as Well as Greater Present and Future Knee-Related Self-Efficacy, Can Increase the Risk for an Anterior Cruciate Ligament Re-Rupture: A Matched Cohort Study. Arthroscopy. 2022;38(4):1267-76.e1. McPherson AL, Feller JA, Hewett TE, Webster KE. Psychological Readiness to Return to Sport Is Associated With Second Anterior Cruciate Ligament Injuries. Am J Sports Med. 2019;47(4):857–62. Additional Declarations No competing interests reported. Supplementary Files RECORDChecklist.pdf AppendixPAMS2ndACLInjury.docx Cite Share Download PDF Status: Under Review Version 1 posted Reviews received at journal 04 Oct, 2025 Reviews received at journal 18 Sep, 2025 Reviewers agreed at journal 12 Sep, 2025 Reviewers agreed at journal 12 Sep, 2025 Reviews received at journal 06 Aug, 2025 Reviewers agreed at journal 05 Aug, 2025 Reviewers invited by journal 11 Jul, 2025 Editor assigned by journal 03 Jul, 2025 Editor invited by journal 30 Jun, 2025 Submission checks completed at journal 28 Jun, 2025 First submitted to journal 28 Jun, 2025 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. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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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-6971659","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":483962810,"identity":"fa1d2404-3efd-4192-a368-5c75c372aeaf","order_by":0,"name":"Jakob Lindskog","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA8ElEQVRIie3QPQrCMBiA4a8U2iU6VxS8QkrAH5D2Ki2BdnFwdOxkl2LX9hYeIVDQRXQVuigBXRw6ijgY0Q4u0VEw75CQ4SFfAqBS/WBaZIAOMEAAnthh9DWxahJ8c8+TwIsUn4EeL30+AavTN0OOq+vWWTDzsJcOlgQFycRgw+RM/Hxe0gVDBMvfEs7aSBC8G+tFIykpZsiwpCQ91STkgmwEMU8XKcmC5Yt4hKILczCDnkwIcqQkw4Ksz8TOI+q1CkSkg9lpYPPJdOTiVcit6ua4zVV8qKQkeqz1D2kz8CNdOhZA9/14A/cDUKlUqj/sDqUoSB6m+uroAAAAAElFTkSuQmCC","orcid":"","institution":"University of Gothenburg","correspondingAuthor":true,"prefix":"","firstName":"Jakob","middleName":"","lastName":"Lindskog","suffix":""},{"id":483962811,"identity":"85a80fe8-58a7-4818-8ef7-c8af61fae6a1","order_by":1,"name":"Daniel Broman","email":"","orcid":"","institution":"University of Gothenburg","correspondingAuthor":false,"prefix":"","firstName":"Daniel","middleName":"","lastName":"Broman","suffix":""},{"id":483962812,"identity":"8c5e916c-4fe7-4e2e-916c-c6cf0cac3c77","order_by":2,"name":"Ramana Piussi","email":"","orcid":"","institution":"University of Gothenburg","correspondingAuthor":false,"prefix":"","firstName":"Ramana","middleName":"","lastName":"Piussi","suffix":""},{"id":483962813,"identity":"08d372d2-9a9c-45be-9fd8-ca7327159883","order_by":3,"name":"Roland Thomeé","email":"","orcid":"","institution":"University of Gothenburg","correspondingAuthor":false,"prefix":"","firstName":"Roland","middleName":"","lastName":"Thomeé","suffix":""},{"id":483962814,"identity":"c5373076-c8c8-4474-9936-23cd0db943a2","order_by":4,"name":"Eric Hamrin Senorski","email":"","orcid":"","institution":"University of Gothenburg","correspondingAuthor":false,"prefix":"","firstName":"Eric","middleName":"Hamrin","lastName":"Senorski","suffix":""}],"badges":[],"createdAt":"2025-06-25 07:24:07","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6971659/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6971659/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":86787126,"identity":"29475f67-3134-4b69-9631-9432c6143d83","added_by":"auto","created_at":"2025-07-15 14:15:54","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":27632,"visible":true,"origin":"","legend":"\u003cp\u003eFlowchart of inclusion and exclusion of the study population. ACL = anterior cruciate ligament; n = number of patients; RTS = return to sport; Tegner = Tegner activity scale. \u003csup\u003ea \u003c/sup\u003e= Patients may be not elgible for includsion due to multple critera.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-6971659/v1/f14dbb2b300f897bd7ee2576.png"},{"id":86787124,"identity":"13e0c53b-a016-4a25-a6e8-3f47332284eb","added_by":"auto","created_at":"2025-07-15 14:15:54","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":60454,"visible":true,"origin":"","legend":"\u003cp\u003eKaplan-Meier curve on second ACL injury after RTS in patients stratified by recovery of ≥90% of PAMS in the quadriceps or not. Log rank test: p=0.719. PAMS = Preoperative absolute muscle strength; RTS = Return to sport.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-6971659/v1/3a83664790611677b763547e.png"},{"id":86787127,"identity":"72351e56-1ec3-4312-9498-05ceabed18a3","added_by":"auto","created_at":"2025-07-15 14:15:54","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":63841,"visible":true,"origin":"","legend":"\u003cp\u003eKaplan-Meier curve on second ACL injury after RTS in patients stratified by recovery of ≥90% of PAMS in the hamstrings or not. Log rank test: p=0.431. PAMS = Preoperative absolute muscle strength; RTS = Return to sport.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-6971659/v1/fa7d12f842e5d143b4a7c263.png"},{"id":86787128,"identity":"03b86210-37de-4eee-ae2d-d7243f72e0ca","added_by":"auto","created_at":"2025-07-15 14:15:54","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":64608,"visible":true,"origin":"","legend":"\u003cp\u003eKaplan-Meier curve on second ACL injury after RTS in patients stratified by recovery of ≥90% of PAMS in the quadriceps and hamstrings or not. Log rank test: p=0.739. PAMS = Preoperative absolute muscle strength; RTS = Return to sport.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-6971659/v1/410d5ddf6ef33a3eed7ce563.png"},{"id":86788881,"identity":"e32c56f1-e640-4ab4-80a5-84f52b2db753","added_by":"auto","created_at":"2025-07-15 14:31:55","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1201247,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6971659/v1/12d6dbd2-1514-413e-9e61-a5d43816e040.pdf"},{"id":86785326,"identity":"c0064418-6a93-4a03-b431-fd41eb8fdb28","added_by":"auto","created_at":"2025-07-15 14:07:54","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":66762,"visible":true,"origin":"","legend":"","description":"","filename":"RECORDChecklist.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6971659/v1/f3ee2c224f2680486c10e7a4.pdf"},{"id":86785332,"identity":"b25e68a0-d206-4559-888f-9843a7cbae7a","added_by":"auto","created_at":"2025-07-15 14:07:54","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":41188,"visible":true,"origin":"","legend":"","description":"","filename":"AppendixPAMS2ndACLInjury.docx","url":"https://assets-eu.researchsquare.com/files/rs-6971659/v1/a7df28935d67cf9790493373.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Recovery of preoperative absolute muscle strength in the quadriceps or hamstrings at time of return to sport did not affect rate of a second anterior cruciate ligament injury: a registry study","fulltext":[{"header":"Introduction","content":"\u003cp\u003eEach year, approximately 68.6 and 80 anterior cruciate ligament (ACL) injuries per 100,000 individuals are reported in the United States and Sweden, respectively(\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e). After an ACL injury, patients treated with or without ACL reconstruction will undergo rehabilitation, often guided by a physical therapist. In Sweden, roughly half of patients who have sustained an ACL injury choose to undergo for ACL reconstruction(\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e), especially younger individuals that aim to return to demanding knee-intensive sports(\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e). However, it is important to note that previous studies have reported that younger age and to return to sport (RTS) increased the risk of a subsequent ACL injury(\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e), where up to 23% of patients under the age of 25 years who returned to sport after ACL reconstruction experienced a second ACL injury(\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eRecovery of lower extremity muscle strength has been suggested as a key factor for successful outcomes following ACL reconstruction(\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e). Seated concentric isokinetic muscle strength tests for quadriceps and hamstrings are considered the \u0026lsquo;gold standard\u0026rsquo; after ACL reconstruction(\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e) and are commonly used to assess lower extremity muscle strength(\u003cspan additionalcitationids=\"CR11\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e). However, despite the widespread acceptance of these test batteries, the evidence is inconsistent with regard to the tests\u0026rsquo; ability to identify patients at risk of a subsequent ACL injury(\u003cspan additionalcitationids=\"CR14 CR15 CR16\" citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e). Results from test batteries are commonly presented as side-to-side differences, calculated with the limb symmetry index (LSI): LSI = (involved limb result / uninvolved limb result) x 100(\u003cspan additionalcitationids=\"CR19\" citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e). Yet, the use of LSI has limitation, as it can overestimate muscle strength recovery; a 100% LSI might signify bilateral weakness if the uninvolved limb weakens during rehabilitation(\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e). This limitation might contribute to the contradictory evidence concerning the role of results from test batteries to identify patients at risk of subsequent ACL injuries and suggests the need for an additional method to assess or interpret muscle strength.\u003c/p\u003e\u003cp\u003eAn alternative to LSI is to evaluate whether patients recover their preoperative absolute muscle strength (PAMS)(\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e). Discrepancies have been reported between patients who achieve symmetrical quadriceps and hamstring strength (LSI\u0026thinsp;\u0026ge;\u0026thinsp;90%) and those who reach\u0026thinsp;\u0026ge;\u0026thinsp;90% of PAMS for quadriceps and hamstrings one year after ACL reconstruction(\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e). Around one-third of patients who achieve\u0026thinsp;\u0026ge;\u0026thinsp;90% LSI do not reach\u0026thinsp;\u0026ge;\u0026thinsp;90% of PAMS, which indicates persistent weakness compared to their pre-reconstruction state, despite that the patients had recovered symmetry. Conversely, about one-third of patients who achieve\u0026thinsp;\u0026ge;\u0026thinsp;90% of PAMS do not attain\u0026thinsp;\u0026ge;\u0026thinsp;90% LSI, which indicates that the patients were as strong or stronger than pre-reconstruction, however, lacked symmetry. Lynch et al.(\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e) stated in a consensus paper that muscle strength recovery before RTS should include achievement of symmetrical muscle strength. However, to solely rely on achievment of symmetrical muscle strength before RTS may lead to some patients RTS with symmetrical, but inadequate, muscle strength. Nevertheless, the significance of PAMS recovery with regard to the risk of a subsequent ACL injury remains unclear. Therefore, the aim of this study was to determine the rate of a second ACL injury after RTS in patients treated with ACL reconstruction dependant on the recovery of PAMS for quadriceps and hamstrings, respectively, and in combination.\u003c/p\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003eSetting\u003c/h2\u003e\u003cp\u003eThis observational retrospective cohort study was reported according to the \u0026ldquo;Strengthening the reporting of observational studies in epidemiology\u0026rdquo; (STROBE) statement(\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e). The study was based on data from a rehabilitation outcome registry, Project ACL, which started in 2014, located in the south-west region of Gothenburg. Sweden, aims to improve the care of patients who have sustained an ACL injury, regardless of treatment choice, through a continuous standardized patient assessment. These assessments adhere to a predetermined schedule, at 10 weeks, 4, 8, 12, 18, 24, 60 months after baseline, that is injury or reconstruction, and after that every 5 years. The assessments are performed with both validated patient-reported outcomes (PROs) and muscle function tests. Patients scheduled for ACL reconstruction are also asked to perform a preoperative baseline test within 6 weeks prior to reconstruction. The project has ethical approval from the Swedish Ethical Review Authority (registration numbers: 2020\u0026ndash;02501, 2024-08724-01), and the Regional Ethical Review Board in Gothenburg, Sweden (registration numbers: 265\u0026ndash;13, T023\u0026ndash;17).\u003c/p\u003e\u003cp\u003eParticipants\u003c/p\u003e\u003cp\u003ePatients from Project ACL were eligible for inclusion in the study if they met the following criteria:\u003c/p\u003e\u003cp\u003e\u003cul\u003e\u003cli\u003e\u003cp\u003eRegistered with a primary ACL injury treated with ACL reconstruction\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eAged between 15 and 35 years at the time of primary ACL reconstruction\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003ePerformed complete preoperative isokinetic strength tests for both quadriceps and hamstrings prior to primary ACL reconstruction\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eHad a pre-injury Tegner activity scale(\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e) (Tegner)\u0026thinsp;\u0026ge;\u0026thinsp;6 i.e., participated in knee-strenuous sports, such as tennis, badminton or more demanding\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eHad returned to sport after ACL reconstruction (reported Tegner\u0026thinsp;\u0026ge;\u0026thinsp;6)\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003ePerformed complete postoperative isokinetic strength tests for both quadriceps and hamstrings at the same follow-up as to when they returned to sport\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eHad at least one year follow-up after RTS\u003c/p\u003e\u003c/li\u003e\u003c/ul\u003e\u003c/p\u003e\u003cp\u003eThe reason for the chosen inclusion criteria was to study a population of active patients with participation in knee-strenuous sport prior to A\u003cem\u003eC\u003c/em\u003eL injury and to account for adequate risk exposure to a second ACL injury.\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eVariables\u003c/h3\u003e\n\u003cp\u003eIsokinetic strength testing for quadriceps and hamstrings\u003c/p\u003e\u003cp\u003eFor isokinetic unilateral quadriceps and hamstrings muscle strength, a Biodex System 4 (Biodex Medical Systems, Shirley, New York) was used. The Biodex System has produced reliable test re-test measures in healthy active individuals (ICC\u0026thinsp;=\u0026thinsp;0.95)(\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e). All strength tests were performed in a lab setting in Project ACL\u0026rsquo;s facilities led by physical therapists. The test was first performed for the involved limb, followed by the uninvolved limb, at an angular velocity of 90\u0026deg;/second. Patients were in a seated position with 85\u0026deg; hip flexion and strapped across the trunk, pelvis, and thigh. During testing the patients\u0026rsquo; arms were held crossed at chest level. Knee extension was performed between 90\u0026thinsp;\u0026minus;\u0026thinsp;0\u0026deg; of knee flexion and knee flexion was performed between 0\u0026ndash;90\u0026deg; of knee flexion. Both movements were performed subsequently in each repetition. A standardized protocol was used during testing. The protocol consisted of 10 minutes warm up on a stationary bike, followed by familiarization in the Biodex machine with 10 repetitions at 50% of maximum effort, 10 repetitions at 75% of maximum effort followed by 1\u0026ndash;2 repetitions of 90% of maximum effort. After familiarization, patients were asked to perform 3\u0026ndash;4 maximum effort repetitions with 40 seconds of rest between each repetition. During the maximum effort repetitions patients were verbally encouraged by the test leader. The highest achieved peak torque (in newton meters) for quadriceps and hamstrings muscle strength (knee extension and knee flexion) for the respective limbs, was registered in Project ACL database and used for analysis.\u003c/p\u003e\u003cp\u003eData\u003c/p\u003e\u003cp\u003eData for patient demographics, results from preoperative and postoperative isokinetic muscle strength tests was extracted from Project ACL in October 2024.\u003c/p\u003e\u003cp\u003eReturn to sport\u003c/p\u003e\u003cp\u003eIn this study, patients were classified as having returned to sport when they reported that they had reached Tegner level\u0026thinsp;\u0026ge;\u0026thinsp;6 after ACL reconstruction. The Tegner is a scale used to determine how strenuous different activities are for the knees(\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e). The Tegner is graded from 0\u0026ndash;10, with 0 indicating the lowest level of strenuous knee-related activity (sick leave or disability pension) and 10 indicating the highest level of strenuous knee-related activity (e.g., competitive soccer at national or international elite level). In the present study a modified version of Tegner was used, which ranges from 1\u0026ndash;10, with sports such as American football, rugby and floorball has been added to the scale. In addition, level 0, sick leave, was excluded. The modified version of Tegner has previously been described in detail(\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e). Patients were assessed with Tegner at the follow-ups 4, 8, 12, 24 and 60 months after ACL reconstruction. Test-retest reliability of Tegner has been reported acceptable with an Intraclass Correlation Coefficient (ICC)\u0026thinsp;=\u0026thinsp;0.80 for patients who have sustained an ACL injury(\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eOutcomes\u003c/p\u003e\u003cp\u003eThe primary outcome in the present study was rate of a second ACL injury after RTS (yes/no). At every follow-up after ACL reconstruction patients in Project ACL were asked to answer questions regarding any new potential ACL injury through an online questionnaire. In addition, patients are also encouraged to register any new potential ACL injuries online, outside of the follow-up schedule. Any potential new injury should be assessed and diagnosed by the responsible physical therapist or orthopedic surgeon. To minimize the risk of missing data regarding a new ACL injury, patients are also asked by the test leader, during the clinical follow-ups, about any new injuries. Both new ipsilateral and contralateral ACL injuries were considered as \u0026ldquo;second ACL injuries\u0026rdquo; in this study. Patients were evaluated for second ACL injuries up to date of data extraction or until they sustained a second ACL injury.\u003c/p\u003e\u003cp\u003eThe recovery of PAMS was calculated for both the involved and the uninvolved limb, respectively, with the following formula: peak torque at the same follow-up as the patient had returned to sport / highest achieved preoperative peak torque (irrespective of whether it originated from the involved or uninvolved limb) x 100. Patients were classified as having recovered PAMS if they score\u0026thinsp;\u0026ge;\u0026thinsp;90% for both the involved and the uninvolved limb(\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e). In addition, results from muscle strength test were also presented and compared as a relative strength, i.e., peak torque for the involved limb / body mass in kilograms.\u003c/p\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003eStatistical analysis\u003c/h2\u003e\u003cp\u003eDemographic variables were presented stratified by patients who sustained a second ACL injury or not. The primary outcome - the difference in rate of second ACL injury \u0026ndash; was analysed between patients who recovered\u0026thinsp;\u0026ge;\u0026thinsp;90% of PAMS or not. The rate of second ACL injury between groups was analysed using censored survival analysis. Hazard ratios (HR) with 95% confidence intervals (CI) were estimated with Cox proportional hazard regression models. A univariable model was applied, and a multivariable model was constructed based on baseline demographic variables with a significance level of p\u0026thinsp;\u0026lt;\u0026thinsp;0.2. Additional repeated sensitivity analyses with the univariable cox proportional hazard regression model were carried out, in which 1) follow-up end point was set at one year after RTS, 2) follow-up end point was set at two years after RTS, 3) RTS was classified as reaching the same or higher pre-injury Tegner, and 4) PAMS was classified as achieving\u0026thinsp;\u0026ge;\u0026thinsp;100% instead for \u0026ge;\u0026thinsp;90% of preoperative strength, respectively. To investigate the difference in survival rate between groups in the cox proportional hazard regression model, a log-rank test was employed, and was illustrated with Kaplan-Meier curves. To compare the follow-up time between groups, months from RTS before data extraction date, presented as mean with standard deveiation (SD), was added and analysed with the independent t-test. For patient demographics, mean value with SD for parametric data, median with interquartile range (IQR) for non-parametric data, and number (n) with proportion (%) for categorical data was presented. For analysis of patient demographics, the independent t-test for parametric data, the Mann-Whitney U-test for non-parametric data, and the Fisher\u0026rsquo;s Exact test for categorical data was used. A significance level of 5% was used. Statistical analyses were performed with the Statistical Package for the Social Sciences (IBM Corp. Released 2017. IBM SPSS Statistics for Windows, Version 27.0. Armonk, NY: IBM Corp.)\u003c/p\u003e\u003c/div\u003e"},{"header":"Results","content":"\u003cp\u003eA total of 190 patients with a mean age of 22.2\u0026thinsp;\u0026plusmn;\u0026thinsp;5.2 years at primary ACL reconstruction, 56.3% women, were included in the present study. In total, 31 patients (16.3%) sustained a second ACL injury. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e presents the flow-chart of the inclusion process. Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows baseline patient demographics, stratified by patients who did or did not sustain a second ACL injury. There were no baseline differences between patients who sustained or did not sustain a second ACL injury.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eBaseline demographics for patients with and without a second ACL injury.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"4\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eVariable\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eSecond ACL injury, n\u0026thinsp;=\u0026thinsp;31\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eNo second ACL injury, n\u0026thinsp;=\u0026thinsp;159\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003ep-value\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003eAge at primary reconstruction, [years]\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e20.9\u0026thinsp;\u0026plusmn;\u0026thinsp;5.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e22.5\u0026thinsp;\u0026plusmn;\u0026thinsp;5.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.125\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003ePatient sex, [female] n (%)\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e16 (51.6)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e91 (57.2)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.693\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003eHeight, [m]\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1.76\u0026thinsp;\u0026plusmn;\u0026thinsp;0.10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1.74\u0026thinsp;\u0026plusmn;\u0026thinsp;0.10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.498\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003eBody mass, [kg]\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e73.5\u0026thinsp;\u0026plusmn;\u0026thinsp;13.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e70.8\u0026thinsp;\u0026plusmn;\u0026thinsp;11.7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.275\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003eBMI, [kg/m\u003c/b\u003e\u003csup\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sup\u003e\u003cb\u003e]\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e23.6\u0026thinsp;\u0026plusmn;\u0026thinsp;2.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e23.2\u0026thinsp;\u0026plusmn;\u0026thinsp;2.7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.419\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003eTime from ACL injury to reconstruction [months]\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e3.7\u0026thinsp;\u0026plusmn;\u0026thinsp;2.9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e5.8\u0026thinsp;\u0026plusmn;\u0026thinsp;12.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.358\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003ePreoperative quadriceps strength (involved limb) (Nm/kg)\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e2.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e2.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.552\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003ePreoperative quadriceps strength (uninvolved limb) (Nm/kg)\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e2.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e2.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.703\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003ePreoperative hamstrings strength (involved limb) (Nm/kg)\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.909\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003ePreoperative hamstrings strength (uninvolved limb) (Nm/kg)\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.229\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003ePreoperative quadriceps LSI\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e93.6\u0026thinsp;\u0026plusmn;\u0026thinsp;14.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e92.6\u0026thinsp;\u0026plusmn;\u0026thinsp;13.7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.706\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003ePreoperative hamstrings LSI\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e96.0\u0026thinsp;\u0026plusmn;\u0026thinsp;12.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e99.8\u0026thinsp;\u0026plusmn;\u0026thinsp;12.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.119\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003ePre-injury Tegner score, median (IQR)\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e9 (\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e9 (\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.117\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003eGraft type, n (%)\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eHamstring tendon\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e24 (77.4)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e139 (87.4)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\" morerows=\"2\" rowspan=\"3\"\u003e\u003cp\u003e0.230\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePatellar tendon\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e7 (22.6)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e18 (11.3)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eAllograft\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e2 (1.3)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colspan=\"4\" nameend=\"c4\" namest=\"c1\"\u003e\u003cp\u003eACL\u0026thinsp;=\u0026thinsp;anterior cruciate ligament; LSI\u0026thinsp;=\u0026thinsp;Limb symmetry index; BMI\u0026thinsp;=\u0026thinsp;Body mass index; SD\u0026thinsp;=\u0026thinsp;Standard deviation; Nm/kg\u0026thinsp;=\u0026thinsp;Newton meters per kilogram bodymass; n\u0026thinsp;=\u0026thinsp;number of patients.\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eThere were no differences in postoperative outcomes at time of RTS between patients who sustained or did not sustain a second ACL injury. Complete postoperative outcomes are reported in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003ePost-operative outcomes at time of RTS for patients with and without a second ACL injury\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"4\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eVariable\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eSecond ACL injury, n\u0026thinsp;=\u0026thinsp;31\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eNo second ACL injury, n\u0026thinsp;=\u0026thinsp;159\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003ep-value\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003eTime from ACL reconstruction to RTS [months]\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e10.1\u0026thinsp;\u0026plusmn;\u0026thinsp;5.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e10.9\u0026thinsp;\u0026plusmn;\u0026thinsp;4.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.401\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003eStrength at time of RTS (Nm/kg)\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eQuadriceps (involved limb)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e2.9\u0026thinsp;\u0026plusmn;\u0026thinsp;0.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e2.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.719\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eQuadriceps (uninvolved limb)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e3.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e3.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.797\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eHamstrings (involved limb)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.298\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eHamstrings (uninvolved limb)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.503\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003e% Recovery of PAMS at time of RTS\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eQuadriceps (involved limb)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e98.7\u0026thinsp;\u0026plusmn;\u0026thinsp;17.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e100.1\u0026thinsp;\u0026plusmn;\u0026thinsp;16.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.667\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eQuadriceps (uninvolved limb)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e105.0\u0026thinsp;\u0026plusmn;\u0026thinsp;13.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e106.7\u0026thinsp;\u0026plusmn;\u0026thinsp;13.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.507\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eHamstrings (involved limb)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e100.4\u0026thinsp;\u0026plusmn;\u0026thinsp;16.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e98.2\u0026thinsp;\u0026plusmn;\u0026thinsp;15.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.475\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eHamstrings (uninvolved limb)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e100.9\u0026thinsp;\u0026plusmn;\u0026thinsp;14.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e101.3\u0026thinsp;\u0026plusmn;\u0026thinsp;13.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.872\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003eLSI for quadriceps at time of RTS\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e94.3\u0026thinsp;\u0026plusmn;\u0026thinsp;14.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e94.1\u0026thinsp;\u0026plusmn;\u0026thinsp;12.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.929\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003eLSI for hamstrings at time of RTS\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e99.7\u0026thinsp;\u0026plusmn;\u0026thinsp;9.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e97.3\u0026thinsp;\u0026plusmn;\u0026thinsp;11.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.269\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colspan=\"4\" nameend=\"c4\" namest=\"c1\"\u003e\u003cp\u003eACL\u0026thinsp;=\u0026thinsp;anterior cruciate ligament; PAMS\u0026thinsp;=\u0026thinsp;Preoperative absolute muscle strength; LSI\u0026thinsp;=\u0026thinsp;Limb symmetry index; SD\u0026thinsp;=\u0026thinsp;Standard deviation; RTS\u0026thinsp;=\u0026thinsp;Return to sports; Nm/kg\u0026thinsp;=\u0026thinsp;Newton meters per kilogram bodymass\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eThere was no difference in the proportion of patients who recovered\u0026thinsp;\u0026ge;\u0026thinsp;90% of PAMS for the quadriceps between patients who sustained or did not sustain a second ACL injury, which did not correspond to an affected hazard ratio of a second ACL injury (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). There was no difference in mean follow-up time after RTS between patients who had recovered\u0026thinsp;\u0026ge;\u0026thinsp;90% of PAMS for the quadriceps, and hamstrings individually, or both muscle groups in combination (Appendix table 1). There was no difference in cumulative survival rate for patients who recovered\u0026thinsp;\u0026ge;\u0026thinsp;90% of PAMS in the quadriceps, the hamstrings, and in the quadriceps and hamstrings in combination (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e\u0026ndash;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eProportion of patients who recovered 90% of PAMS and hazard ratios for a second ACL injury after RTS\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"8\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e\u003cp\u003eRecovered 90% of PAMS (involved\u0026thinsp;+\u0026thinsp;uninvolved limb),\u003c/p\u003e\u003cp\u003en/total patients (%)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"4\" nameend=\"c8\" namest=\"c5\"\u003e\u003cp\u003eHR for a second ACL injury, \u003c/p\u003e\u003cp\u003erecovering PAMS compared to not [95% CI]\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003eMuscle group(s)\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cb\u003eSecond ACL injury\u003c/b\u003e,\u003c/p\u003e\u003cp\u003en\u0026thinsp;=\u0026thinsp;31\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e\u003cb\u003eNo second ACL injury\u003c/b\u003e,\u003c/p\u003e\u003cp\u003en\u0026thinsp;=\u0026thinsp;159\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e\u003cb\u003ep-value\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e\u003cb\u003eUnadjusted\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e\u003cb\u003ep-value\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e\u003cb\u003eAdjusted*\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e\u003cb\u003ep-value\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003eQuadriceps\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e22/31 (71.0%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e117/159 (73.6%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.825\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.97 [0.40\u0026ndash;1.89]\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.720\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.71 [0.31\u0026ndash;1.62]\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.415\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003eHamstrings\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e22/31 (71.0%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e101/159 (63.5%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.539\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1.36 [0.63\u0026ndash;2.63]\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.433\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e1.26 [0.57\u0026ndash;2.75]\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.567\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003eQuadriceps\u0026thinsp;+\u0026thinsp;Hamstrings\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e17/31 (54.8%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e82/159 (51.6%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.845\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1.13 [0.56\u0026ndash;2.29]\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.739\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.98 [0.47\u0026ndash;2.03]\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.948\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colspan=\"8\" nameend=\"c8\" namest=\"c1\"\u003e\u003cp\u003ePAMS\u0026thinsp;=\u0026thinsp;Preoperative absolute muscle strength; HR\u0026thinsp;=\u0026thinsp;Hazard ratio; CI\u0026thinsp;=\u0026thinsp;Confidence interval; n\u0026thinsp;=\u0026thinsp;number of patients; * = adjusted for age at primary ACL reconstruction, pre-injury Tegner Activity Scale values and pre-operative hamstring Limb Symmetry Index values.\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eSensitivity analyses\u003c/p\u003e\u003cp\u003eIn the sensitivity analyses, there was no effect on the rate of a second ACL injury for patients who recovered PAMS when 1) follow-up end point was set to one year after RTS, 2) follow-up end point was set to two years after RTS, 3) RTS was determined as returned to the same or higher pre-injury Tegner, or 4) the cut-off for recovery of PAMS was set to \u0026ge;\u0026thinsp;100% in the quadriceps, the hamstrings, or both the quadriceps or hamstrings at time of RTS (Appendix table 2\u0026ndash;5).\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eThe main findings of this study were that there was no effect on the rate of second ACL injury after RTS for patients who recovered\u0026thinsp;\u0026ge;\u0026thinsp;90% of PAMS for the quadriceps, the hamstring or both at time of RTS compared to patients who did not recover\u0026thinsp;\u0026ge;\u0026thinsp;90% of PAMS.\u003c/p\u003e\u003cp\u003eIn this study, 16.3% of included patients sustained a second ACL injury during follow-up, which averaged approximately 51 months after RTS, and the rate is considered consistent with findings from previous research.(\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e) In previous studies, the LSI has been used to identify patients at risk for a subsequent ACL injury and reported contradictory results(\u003cspan additionalcitationids=\"CR14 CR15 CR16\" citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e), which could partly be affected by an overestimation of recovery due to the possible weakening of the uninvolved limb(\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e). Overall, in this study, between 63.5% and 73.6% of patients recovered\u0026thinsp;\u0026ge;\u0026thinsp;90% of PAMS in the quadriceps and hamstrings, which could indicate that some patients present a deconditioned state of knee muscle strength compared to before ACL reconstruction at RTS. Despite the potentially deconditioned state of thigh muscles, there was no difference in rate of second ACL injury based on recovery of pre-operative absolute quadriceps or hamstrings strength in this study. Prior research suggest that the evaluation of PAMS could potentially serve as a valuable complement to LSI-based assessments throughout ACL reconstruction rehabilitation(\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e), however, the rate reduction of a second ACL injury by recovery of \u0026ge;\u0026thinsp;90% of PAMS for quadriceps or hamstrings was not supported by our study. In contrast, Wellsandt et al.(\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e) previously reported that the recovery of PAMS for the quadriceps, coupled with the recovery of preoperative absolute hop function, 6 months after ACL reconstruction appeared more sensitive to identify patients at risk of a second ACL injury compared to the recovery of LSI for quadriceps strength and hop function. There are a few differences between the study by Wellsandt et al.(\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e) and ours, namely: post-operative strength test evaluation was on average performed approximately 4 to 5 months earlier compared to our study. Additionally, patients in our study were on average more than 4 years younger (mean 22.2 years versus 26.6 years), and the included patients in Wellsandt et al.(\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e) were not controlled for RTS as in our study, which collectively obstruct direct comparisons. Taken together, the findings in this study do not support the rate reduction effect for a second ACL injury for recovery of PAMS for quadriceps and hamstrings, meaning that clinicians should not solely rely on the recovery of PAMS of the quadriceps and hamstring to determine a RTS with minimized risk of a second ACL injury.\u003c/p\u003e\u003cp\u003eThe univariable cox proportional hazard regression model was chosen for this study to provide a powerful statistical analysis and despite that the number of second ACL injuries met the recommendation of at least 20 events(\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e), the results from the primary analysis had wide CIs, which also encompassed 1 for the HRs, which suggest that larger groups are needed to reach statistical precision. Interestingly, in the sensitivity analysis which applied the \u0026ge;\u0026thinsp;100% of PAMS threshold, between 32.3% and 25.8% of patients recovered\u0026thinsp;\u0026ge;\u0026thinsp;100% of PAMS for the quadriceps and hamstrings at time of RTS, which corresponded to narrower CIs for the rates for a second ACL injury compared to the \u0026ge;\u0026thinsp;90% of PAMS cut-off. In addition, the rates for second ACL injury with the use of \u0026ge;\u0026thinsp;100% cut-off were, despite not being statistically significant, skewed toward a decreased rate of a second ACL injury. Our study suggests that the use of a more challenging threshold for PAMS recovery, i.e., \u0026ge;\u0026thinsp;100% compared to \u0026ge;\u0026thinsp;90%, may be associated with a better ability to differentiate between patients who do and do not go on to sustain a second ACL injury, though further investigation is merited. Future studies, that include large populations to increase power, should further investigate the impact of different PAMS cut-off values for both the quadriceps and hamstrings at the time of RTS on the risk of second ACL injury.\u003c/p\u003e\u003cp\u003eThis study contributes to the ongoing discussion in sports medicine with regard to optimal criteria for RTS following ACL reconstruction. Contrary to earlier studies which suggests that recovery of PAMS may reduce the risk of second ACL injuries, our findings indicate no such protective effect for patients achieving\u0026thinsp;\u0026ge;\u0026thinsp;90% PAMS for the quadriceps or hamstrings. This aligns with previous work reporting inconsistent associations between LSIs and re-injury risk, possibly due to methodological limitations such as overestimation of recovery in the uninvolved limb. Our findings underscore that PAMS alone may not be a sufficient standalone metric for RTS decision-making. Ultimately, these results emphasize the need for multifactorial, individualized RTS criteria and support continued research with larger cohorts to refine strength-based thresholds and improve long-term outcomes after ACL reconstruction.\u003c/p\u003e\u003cp\u003eLimitations\u003c/p\u003e\u003cp\u003eA potential limitation in this study is the potential loss of muscle strength in the involved limb prior to surgery following the primary ACL injury(\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e), which was addressed by the use of the highest preoperative torque as a reference for recovery, irrespective of whether it originated from the involved or uninvolved limb(\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eSecond ACL injuries have multifactorial causes which include younger age, concomitant knee injury, returning to high knee-demanding sports, longer time between initial injury and ACL reconstruction, time to RTS, injury mechanism, and joint hypermobility(\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan additionalcitationids=\"CR33\" citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e). In this study, no differences in baseline demographics were observed, however, data on concomitant knee injury, injury mechanism, joint mobility, and athletic exposure after RTS, e.g. volume, intensity and frequency, were not available and could not be accounted for. Additionally, both high and low psychological responses have been reported to impact the risk of second ACL injury(\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e, \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e), which was not analysed in this study.\u003c/p\u003e\u003cp\u003eThe average age of patients in this cohort was approximately 22 years at the time of primary ACL reconstruction, with a nearly equal sex distribution (consistent with the general population of ACL injured patients in Sweden)(\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e). Additionally, the patients included were actively involved in high knee-demanding sports before the primary ACL injury (median Tegner\u0026thinsp;=\u0026thinsp;9, equivalent to participation in sports such as soccer, ice-hockey, and alpine skiing). However, it is important to exercise caution when generalizing our results to a younger or older population or patients engaged in non-pivoting sports.\u003c/p\u003e\u003cp\u003eOne final limitation is patients\u0026rsquo; adherence to the preoperative follow-up in Projetct ACL. Of the 4,493 patients registered at the time of data extraction, 735 patients (16.4%) had performed preoperative muscle strength tests. It is unknown whether the cohort included in the study differed significantly in terms of patient characteristics from the patients not eligible for inclusion and therefore caution should be taken to generalize the results to the general population of patients with an ACL injury.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eRecovery of preoperative absolute muscle strength in the quadriceps and hamstrings respectively and in combination, at time of RTS did not affect the rate of second ACL injury. Clinicians should not solely rely on recovery of PAMS for quadriceps or hamstrings for a decreased rate of second ACL injury.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eEthics approval and consent to participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe principles of the Helsinki declaration have been adhered in this study.\u0026nbsp;Ethical approval has been obtained from the Swedish Ethical Review Authority (registration numbers: 2020-02501, 2024-08724-01), and the Regional Ethical Review Board in Gothenburg, Sweden (registration numbers: 265\u0026ndash;13, T023\u0026ndash;17).\u003c/p\u003e\n\u003cp\u003eData for this study is based on a rehabilitation registry project (Project ACL), where all patients have received written information and have given their\u0026nbsp;informed consent in the research project.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe have no competing interests to declare.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003ePartial financial support was granted from Research and development primary health care Gothenburg and S\u0026ouml;dra Bohusl\u0026auml;n.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFirst (JL) author drafted the initial version of the manuscript. Authors DB, RP, and EHS contributed majorly during analysis of the data. All authors were responsible for preparing the manuscript and critically revising the work for important content. Author RT made large contributions to the interpretation of data, revision and final design of the work. Author EHS is responsible for the design concept.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors would also like to thank Eric Musslinder and Susanne Beischer, who co-authored a prior article on recovery of PAMS. This work inspired the idea for our current study.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eForssblad M. Svenska Korsbandsregister \u0026aring;rsrapport. 2019.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eSanders TL, Maradit Kremers H, Bryan AJ, Larson DR, Dahm DL, Levy BA, et al. 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Younger patients are at increased risk for graft rupture and contralateral injury after anterior cruciate ligament reconstruction. Am J Sports Med. 2014;42(3):641\u0026ndash;7.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLarsen JB, Farup J, Lind M, Dalgas U. Muscle strength and functional performance is markedly impaired at the recommended time point for sport return after anterior cruciate ligament reconstruction in recreational athletes. Hum Mov Sci. 2015;39:73\u0026ndash;87.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eConcato J, Feinstein AR. Monte Carlo methods in clinical research: applications in multivariable analysis. J Invest medicine: official publication Am Federation Clin Res. 1997;45(6):394\u0026ndash;400.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eGardinier ES, Manal K, Buchanan TS, Snyder-Mackler L. Gait and neuromuscular asymmetries after acute anterior cruciate ligament rupture. 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Knee surgery, sports traumatology, arthroscopy. official J ESSKA. 2019;27(8):2450\u0026ndash;9.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZsidai B, Piussi R, Thome\u0026eacute; R, Sundemo D, Musahl V, Samuelsson K et al. Generalised joint hypermobility leads to increased odds of sustaining a second ACL injury within 12 months of return to sport after ACL reconstruction. Br J Sports Med. 2023.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003ePiussi R, Beischer S, Thome\u0026eacute; R, Thome\u0026eacute; C, Sansone M, Samuelsson K et al. Greater Psychological Readiness to Return to Sport, as Well as Greater Present and Future Knee-Related Self-Efficacy, Can Increase the Risk for an Anterior Cruciate Ligament Re-Rupture: A Matched Cohort Study. Arthroscopy. 2022;38(4):1267-76.e1.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eMcPherson AL, Feller JA, Hewett TE, Webster KE. Psychological Readiness to Return to Sport Is Associated With Second Anterior Cruciate Ligament Injuries. Am J Sports Med. 2019;47(4):857\u0026ndash;62.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"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":"bmc-sports-science-medicine-and-rehabilitation","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"ssmr","sideBox":"Learn more about [BMC Sports Science, Medicine and Rehabilitation](http://bmcsportsscimedrehabil.biomedcentral.com/)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/ssmr/default.aspx","title":"BMC Sports Science, Medicine and Rehabilitation","twitterHandle":"BMC_series","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"em","reportingPortfolio":"BMC Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Anterior cruciate ligament reconstruction, preoperative absolute muscle strength, reinjury","lastPublishedDoi":"10.21203/rs.3.rs-6971659/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6971659/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003ch2\u003eBackground\u003c/h2\u003e\u003cp\u003eRecovery of preoperative absolute muscle strength (PAMS) after anterior cruciate ligament (ACL) reconstuction remains a significant challenge. However, whether recovery of PAMS contributes to an affected rate of second ACL injury has yet to be explored. This study aimed to analyse the rate of a second ACL injury incidence after return to sports (RTS) between patients with ACL reconstruction dependant on the recovery of their PAMS for quadriceps and hamstrings, respectively and in combination, compared to patients who had not.\u003c/p\u003e\u003ch2\u003eMethods\u003c/h2\u003e\u003cp\u003eA retrospective cohort study with data from a rehabilitation specific registry were used. Patients who had undergone primary ACL reconstruction, were aged between 15 and 35 years, had performed complete preoperative isokinetic strength tests for quadriceps and hamstrings prior to primary ACL reconstruciton, who had a pre-injury Tegner Activty Scale\u0026thinsp;\u0026ge;\u0026thinsp;6, had returned to sport after ACL reconstruction and complete isokinetic strength tests for quadriceps and hamstrings at the same time of RTS were included. The main outcome was the rate of second ACL injury after RTS based on recovery of \u0026ge;\u0026thinsp;90% PAMS for the quadriceps, and hamstrings indivudually, and in combination, or not.\u003c/p\u003e\u003ch2\u003eResults\u003c/h2\u003e\u003cp\u003eA total of 190 patients (mean age 22.2\u0026thinsp;\u0026plusmn;\u0026thinsp;5.2 years, 56.3% women) were included. Thirty-one patients (16.3%) sustained a second ACL injury. There was no difference in rate of second ACL injury between patients who had recovered\u0026thinsp;\u0026ge;\u0026thinsp;90% of PAMS in their quadriceps, or hamstrings, in both limbs, respectively and in combination, compared to patients who had not.\u003c/p\u003e\u003ch2\u003eConclusion\u003c/h2\u003e\u003cp\u003eRecovery of preoperative absolute muscle strength in the quadriceps and hamstrings respectively and in combination, at time of RTS did not affect the rate of second ACL injury.\u003c/p\u003e","manuscriptTitle":"Recovery of preoperative absolute muscle strength in the quadriceps or hamstrings at time of return to sport did not affect rate of a second anterior cruciate ligament injury: a registry study","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-07-15 14:07:50","doi":"10.21203/rs.3.rs-6971659/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"editorInvitedReview","content":"","date":"2025-10-04T20:41:16+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-09-18T09:07:06+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"151821903987861014513572075237177359694","date":"2025-09-13T01:31:15+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"298165955702693028961540773209637529807","date":"2025-09-12T10:47:07+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-08-06T19:19:52+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"104292809151359363101199242202663768841","date":"2025-08-05T04:55:12+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-07-11T09:22:26+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-07-03T08:43:25+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2025-06-30T14:23:21+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-06-28T11:14:28+00:00","index":"","fulltext":""},{"type":"submitted","content":"BMC Sports Science, Medicine and Rehabilitation","date":"2025-06-28T11:11:20+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"bmc-sports-science-medicine-and-rehabilitation","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"ssmr","sideBox":"Learn more about [BMC Sports Science, Medicine and Rehabilitation](http://bmcsportsscimedrehabil.biomedcentral.com/)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/ssmr/default.aspx","title":"BMC Sports Science, Medicine and Rehabilitation","twitterHandle":"BMC_series","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"em","reportingPortfolio":"BMC Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"e1b4a6c6-491c-4216-af7b-5ae594aaf818","owner":[],"postedDate":"July 15th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2025-07-15T14:07:50+00:00","versionOfRecord":[],"versionCreatedAt":"2025-07-15 14:07:50","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6971659","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6971659","identity":"rs-6971659","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}