Abstract
Purpose of Review
In the setting of ever improving outcomes following anterior cruciate ligament (ACL) reconstruction, both objectively and subjectively, there remains continued interest in better understanding the differences in outcomes between male and female patients. The current review investigates the recent literature surrounding the roles of biological sex in adult ACL reconstruction outcomes.
Recent Findings
Within the past five years, the most prominent and recurrent difference between male and female patient cohorts is the subjective report of pain and function. Female patients have been consistently shown to have worse self-reported outcomes following ACL reconstruction. Specific and quantifiable subjective outcomes that are highlighted include the International Knee Documentation Committee (IKDC) subjective score, the Knee injury and Osteoarthritis Outcome Score (KOOS), and patient-acceptable symptom state (PASS).
Summary
Current research on the topic of biological sex–based differences in ACL reconstruction outcomes can be divided into four main categories: patient-reported subjective outcomes; physical assessment and gait analysis; kinematics; and rates of reinjury, revision, and contralateral reconstruction. This chapter provides a summary of recent literature focused on these four main areas of interest related to ACL reconstruction outcomes in males and females.
Keywords: ACL reconstruction, Biological sex, Differential outcomes
Introduction
Anterior cruciate ligament (ACL) tears are among the most common orthopedic injuries, with annual incidence of isolated tears recently estimated to be approximately 70 cases per 100,000 person-years when adjusting for age and biological sex [1]. Female sex is a known risk factor for ACL injuries, though there is a higher incidence among male patients with a higher annual incidence at 81.7 vs 55.3 cases per 100,000 [1, 2]. ACL reconstruction (ACLR) for ACL tears remains the standard of care for patients participating in high-demand activities, with the literature showing excellent postoperative outcomes in the vast majority of patients, particularly with the advancement of arthroscopic techniques [3]. In an effort to better understand outcomes following ACLR, there has been a continued interest in the differences between male patients and female patients. ACLR outcomes have been evaluated in a variety of different ways within recent literature. Research has been conducted in a variety of study designs including prospective cohort studies, case-control studies, retrospective cohort studies, randomized control trials, and systematic reviews. For the purpose of this chapter, the studies were analyzed based on subcategories of outcomes measured. These subcategories include functional outcomes, subjective outcomes, kinematics, and rates of reinjury or revision. The current review investigates the recent literature surrounding the role of biological sex in primary adult ACLR outcomes based on these categories.
Patient-Reported/Subjective Outcomes
Throughout the literature, the most common subcategory of outcomes analysis involves the patient’s subjective perception of their postoperative status via the use of patient-reported outcomes (PROs). Many validated scales are utilized to capture PROs for analysis. Self-reported outcomes vary based on many factors and specific to ACLR; female patients tend to have lower satisfaction and function than their male counterparts. The following sections will highlight some of literature describing ACLR PROs as a function of biological sex.
The Multicenter Orthopaedic Outcomes Network (MOON) longitudinal prospective cohort study published in 2018 utilized the International Knee Documentation Committee (IKDC) subjective score, the Knee injury and Osteoarthritis Outcome Score (KOOS), and the Marx activity rating scale preoperatively and at 2 years, 6 years, and 10 years postoperatively with a multivariate regression model to analyze outcomes following ACLR [4]. Among many findings, the authors found that female sex was among several significant predictors for worse outcomes 10 years after ACLR when measured with IKDC, KOOS sports and recreation, and Marx activity rating scale [4].
Nwachukwu et al. assessed factors associated with return to play (RTP), sports performance, and patient satisfaction with a minimum 2-year follow-up for active athletes after ACLR utilizing similar outcomes as the previously described MOON study [5]. Interestingly, female sex was not found to be a statistically significant variable in RTP outcomes.
Cristiani et al. also looked at KOOS subscales and whether or not patients achieved a patient-acceptable symptom state (PASS) following ACLR [6]. These authors found at 2 years after ACLR, female sex was associated with reduced odds of achieving a PASS on the KOOS Pain (OR 0.76; 95% CI 0.62–0.94; P = 0.01), activities of daily living (ADL) (OR 0.79; 95% CI 0.64–0.97; P = 0.02), and sport and recreation (OR 0.72; 95% CI 0.58–0.89; P = 0.003) subscales. Cristiani et al. did, however, discuss that the observed subjective differences were not matched by objective parameters in their study. A recent systematic review and meta-analysis looking at more than 120,000 patients found female patients report poorer subjective outcomes than male patients after ACLR, but no difference in objective parameters were noted [6, 7].
In the 2018 Anterior Cruciate Ligament–Specialized Post-Operative Return to Sports (ACL-SPORTS) randomized control trial, differences between male and female self-reported function and outcomes following ACLR were found [8]. In the first year after surgery, this study found that female patients tended to have lower IKDC scores and were less likely to have returned to sport when compared with male patients [8].
Webster and Keller in 2017 [2] reported higher IKDC scores in male patients than in female patients following ACLR at 1-year follow-up (males 84 ± 12, females 82 ± 12, mean difference 2 [1–3], p < 0.001) [2]. This study also incorporated the Single Assessment Numeric Evaluation (SANE) which demonstrated similar scores between men and women (men 86 ± 12; women 85 ± 11; mean difference 1 point [−0.5 to 1], p = 0.3) [2].
An additional aspect to consider following ACLR is psychological readiness. Webster et al. [9] recently published on the factors that were associated with psychological readiness to RTP after ACLR surgery. Patients completed the Anterior Cruciate Ligament–Return to Sport After Injury (ACL-RSI) scale (Fig. 1) [10] assessing emotions, confidence, and risk appraisal, which was designed to measure psychological readiness to return to sports after ACL injury or reconstruction surgery. Female sex was found to be a significant indicator for lower ACL-RSI scores and the authors concluded that female athletes may benefit from structured interventions designed to ease the transition back to sport [9].
Fig. 1.
ACL-RSI Scale [10]
In summary, the majority of recent literature supports that female patients report worse subjective functional status and overall outcomes than their male counterpart after ACLR.
Postoperative Gait Analysis and Physical Assessment
In addition to subjective outcomes as reported by patients, another way to measure outcomes following ACLR includes postoperative functional performance, which can be assessed via functional testing such as gait analysis, performance assessment, and other objective measurements. The following sections will highlight some of literature describing functional outcomes following ACLR as a function of biological sex.
Asaeda et al. [11] conducted a gait analysis study which aimed to evaluate the effects of sex on recovery of knee joint biomechanics over the stance phase of gait after ACLR. When functioning properly, the ACL limits rotational and anteroposterior translation movement of the tibia in relation to the femur. This therefore indicates that, in the absence of a competent ACL, instability of the knee would occur in these same axes. This study analyzed pre-injury gait differences between male and female patients including lower peak knee flexion angles, higher peak abduction angles, and a greater total range of internal-external rotation of the tibia during a range of non-weight bearing to partial weight bearing in females compared to males [11]. The study found biological sex–based differences in tibial rotation influenced the knee joint kinematics pre-injury, preoperatively and after ACLR. The authors suggested that these differences could lead to excessive joint contact forces in female patients which could, in part, explain the higher incidence of development of knee osteoarthritis after ACL injury and ACLR compared to male patients [11].
A recent review by Tan et al. [7] objectively assessed the importance of patient sex in the outcomes of ACLR. Neuromuscular testing was reported in numerous studies included in this systematic review, but there were no significant sex-related differences in the pooled timed single-legged hop, single-legged hop, isokinetic quadriceps testing, and isokinetic hamstrings testing. The pooled relative risk estimates of extension and flexion loss in females compared with males were also not significant [7]. Overall, this meta-analysis found no statistically significant difference between male and female patients.
Webster et al. [2] evaluated knee range of motion (ROM), instrumented knee laxity, and single and triple hop for distance limb symmetry index (LSI) following ACLR. The authors found that male patients had less knee laxity after ACLR (males: 1.1 ± 2.2 mm, females: 1.3 ± 2.4 mm; mean difference 0.2 mm [0.1–0.4], p < 0.001) and greater limb symmetry (single limb hop males: 94% ± 12%, females 91% ± 13%, mean difference 3% [2–4%], p < 0.001) when compared with female patients. Notably, there were no significant differences between males and females with respect to postoperative ROM [2].
The ACL-SPORTS randomized control trial analyzed objective outcome data in addition to the PROs as described in the previous section [8]. Female patients were found to have decreased quadriceps strength limb symmetry (QI) when compared with male counterparts [8], which may have implications in secondary injury risk. In this study, the authors evaluated rehabilitation programs of progressive strengthening, agility training, plyometrics (SAP), and other components of current primary prevention protocols which were trialed with and without perturbation training. The authors suggested that women may require further specific quadriceps strengthening to maintain and improve QI in order to counter the known relationship between reduced QI and risk for reinjury. In the discussion of QI, the authors also cited a study in which knee strength was assessed using isokinetic parameters that demonstrated extensor muscle deficits in the female patient cohort when compared with male patients who underwent ACLR [12].
Overall, recent literature has demonstrated sex-related differences in quadriceps symmetry and increased tibial rotation in regard to objective outcomes.
Kinematics and Graft Selection in Male Versus Female Patients
Proper knee kinematic function is essential for successful outcomes following ACLR, especially for female patients who have a predilection for biomechanical imbalance leading to increased risk for ACL tears. Kinematic function, particularly as it relates to graft selection for ACLR, and their association with ACLR outcomes have been studied in patient populations broadly, and specifically with female patients.
Postoperative kinematic testing for laxity in patients following ACLR can be assessed by instrumented laxity testing, pivot shift, and Lachman testing. A meta-analysis by Samuelsen et al. [13] recently analyzed 47,613 ACLR patients (39,768 bone-tendon-bone grafts (63% male) and 7845 hamstring grafts (57% male)) from 14 RCTs, 10 prospective comparative studies, and 1 national registry study. In this study, the authors found that hamstring autografts failed at a higher rate than BTB grafts, though overall failure rates in both groups were low (less than 3%). The authors found no significant difference between grafts with respect to postoperative laxity in the Scandinavia registry study, but among the RCTs, laxity with KT1000/2000 instrumented testing showed a significant difference favoring increased postoperative laxity with hamstring autograft (p = 0.006) [13]. Unfortunately, their study did not perform an analysis based on patient biologic sex. In a separate cohort study assessing 166 patients undergoing ACLR (100 BTB graft, 66 hamstring autograft; 52% female overall), Lesevic et al. [14] found female patients with hamstring grafts had a significantly lower mass-normalized knee flexor torque compared with female patients with BTB grafts at 6 months following ACLR. The authors of this study concluded that female patients reconstructed with hamstring autografts demonstrated weaker postoperative hamstring strength compared with female patients with BTB autografts [14].
In a recent systematic review and meta-analysis, Mouarbes et al. [15] looked at ACLR outcomes with quadriceps tendon (QT) autograft, BTB autograft, and hamstring tendon autograft with a minimum 12-month follow-up. In both males and females, the authors found that QT autograft had comparable clinical outcomes, and graft survival rates compared with BTB and hamstring autografts, but that they had shown significantly less harvest site pain compared with BTB autograft and better functional outcome scores compared with hamstring autograft [15]. However, the authors noted there while grafts are comparable, there are studies with reports of objectively increased laxity in female patients with hamstring autografts [15].
In 2019, Salem et al. [16] performed a study comparing ACLR outcomes with BTB versus hamstring autografts in female patients (ages 15 to 20 years). At 24 months following ACLR, the authors found that BTB autograft leads to fewer graft ruptures than hamstring autograft in female patients [16]. Within this subpopulation female athletes age 15–20, they are 2–8 times more likely to suffer a primary ACL tear than males playing similar sports and those who underwent ACL reconstruction the graft failure rate of BTB was 6.4% which was significantly lower than hamstring at 17.5% (p = 0.02) [16], while there has been no statistically significant graft-specific failure rate in the general population [16]. Additionally, younger female patients are more likely to have smaller hamstring grafts (8.22 mm (7–10.5)) which has been linked to increased risk of graft failure [16]. This increased rate of graft failure was also demonstrated in the 2020 MOON group study which showed the incidence of ACL graft revision at 6 years after index surgery was 2.1 times higher with hamstring autograft compared to BTB autograft in a cohort of high school– and college-aged male and female athletes [17].
In summary, postoperative graft laxity and graft re-rupture rates tend to differ in female patients undergoing ACLR, with several studies showing favorable outcomes with BTB or QT over hamstring autograft. Graft selection and patient counseling are critical to operative planning and postoperative rehabilitation success.
Rates of Reinjury, Revision Surgery, and Contralateral Injury
Perhaps the most critical outcome measurement following ACLR is durability of the graft. In this section, comparative rates of reinjury, revision surgery, and contralateral injury will be described based on biological sex–based differences.
In 2017, Paterno et al. [18] evaluated clinical factors that predicted a second ACL injury after ACLR and return to sport. The authors found that approximately one-fifth of the cohort of athletes who returned to play after ACLR suffered a second ACL injury. They divided participants up into low- and high-risk profiles, where a component of the high-risk profiles included female biological sex as a significant risk factor. Overall, the high-risk group was found to be 5 times more likely to suffer a second ACL injury when compared to the low-risk group and 30% of the high-risk females suffered a second ACL injury [18]. This is the only recent study found upon literature review limited to isolated ACLR outcomes that came to the conclusion of female biological sex being a risk factor for re-injury after ACLR. When considering ACLR with anterolateral ligament reconstruction (ALLR) in professional athletes from the Scientific Anterior Cruciate Ligament Network International (SANTI) Study Group database in a retrospective analysis, however, Rosenstiel et al. found that the risk of graft rupture was significantly higher in female patients (13.6% vs 2.1% in male patients; P = .048) [19].
In contrast, Pierce et al. analyzed risk factors associated with requiring revision ACLR surgery, and showed that there was no significant difference related to biological sex between the groups that did and did not require revision surgery [20]. An additional, newer retrospective cohort study out of Copenhagen evaluating graft failure, revision ACLR, and reoperation rates after ACLR found that there was no difference, with biological females not having a higher rate of revision [21].
A high-powered retrospective cohort study evaluated the risk for contralateral ACLR after index surgery [22]. Three percent of over 17,000 patients included in the original study population underwent contralateral ACLR. The risk for undergoing contralateral ACLR surgery in female patients was found to be 33.7% higher than male patients in their cohort [22]. A recent systematic review with meta-analysis of risk factors for contralateral ACL injury by Cronström et al. found that female sex, along with young age (≤ 18 years) and family history of ACL injury, early reconstruction, and returning to a high activity level increased the risk of contralateral ACL injury [23].
Tan et al. [7] performed a meta-analysis evaluating the association between patient sex and outcomes after ACL reconstruction surgery [7]. The authors evaluated 135 studies and while no significant difference was found with respect to graft failure or rupture between males and females, females had comparable or inferior results on all outcomes assessed [7]. Specifically, the authors found that female patients had inferior outcomes in instrumented laxity (standardized mean difference [SMD], 0.24; 95% CI, 0.11–0.37), revision rate (relative risk [RR], 1.15; 95% CI, 1.02–1.28), Lysholm score (SMD, −0.33; 95% CI, −0.55 to −0.11), Tegner activity scale (SMD, −0.37; 95% CI, −0.49 to −0.24), and incidence of not returning to sports (RR, 1.12; 95% CI, 1.04–1.21), all of which were statistically significant [7]. In a separate study, Asaeda et al. [11] reported that female athletes were found to be four times more likely to sustain a second ACL injury and six times more likely to sustain a contralateral injury than male athletes following ACL reconstruction [11].
In summary, some studies in recent literature have found biological sex–based differences in regard to risk of reinjury and revision surgery.
Conclusion
The purpose of this chapter was to review the evidence in recent literature of whether biological sex–based differences exist in surgical outcomes after primary ACL reconstruction. Current research on this topic was able to be divided into four main categories: patient-reported outcomes; physical assessment and gait analysis; kinematics and graft selection; and rates of reinjury, revision, and contralateral reconstruction. Based on the best-available evidence, the most consistent difference between male and female patients following ACLR is the subjective report of pain and function. Overall, female patients have been shown to have worse self-reported outcomes, increased risk of contralateral injury, and worse outcomes with hamstring grafts compared to BTB and quad tendon autograft. Future research to evaluate possible root causes to these differences, as well as create effective interventions, is required as we continue to improve ACLR surgical outcomes for all patients.
Declarations
Conflict of Interest
Katherine Branche, Hannah Bradsell, Alex Lencioni, and Rachel Frank declare that they have no conflict of interest.
Human and Animal Rights and Informed Consent
This article does not contain any studies with human or animal subjects performed by any of the authors.
Footnotes
This article is part of the Topical Collection on Gender and Racial Disparities
Publisher’s note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Contributor Information
Katherine Branche, Email: kcb7tr@umsystem.edu.
Hannah L. Bradsell, Email: hannah.bradsell@cuanschutz.edu
Alex Lencioni, Email: alex.lencioni@cuanschutz.edu.
Rachel M. Frank, Email: rachel.frank@cuanschutz.edu
References
- 1.Sanders TL, Maradit Kremers H, Bryan AJ, Larson DR, Dahm DL, Levy BA, Stuart MJ, Krych AJ. Incidence of anterior cruciate ligament tears and reconstruction: a 21-year population-based study. Am J Sports Med. 2016;44(6):1502–1507. doi: 10.1177/0363546516629944. [DOI] [PubMed] [Google Scholar]
- 2.Webster KE, Feller JA. Younger patients and men achieve higher outcome scores than older patients and women after anterior cruciate ligament reconstruction. Clin Orthop Relat Res. 2017;475(10):2472–2480. doi: 10.1007/s11999-017-5418-2. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 3.Nwachukwu BU, Patel BH, Lu Y, Allen AA, Williams RJ., 3rd Anterior cruciate ligament repair outcomes: an updated systematic review of recent literature. Arthroscopy. 2019;35(7):2233–2247. doi: 10.1016/j.arthro.2019.04.005. [DOI] [PubMed] [Google Scholar]
- 4.Group MK, Spindler KP, Huston LJ, Chagin KM, Kattan MW, Reinke EK, et al. Ten-year outcomes and risk factors after anterior cruciate ligament reconstruction: a MOON longitudinal prospective cohort study. Am J Sports Med. 2018;46(4):815–825. doi: 10.1177/0363546517749850. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 5.Nwachukwu BU, Voleti PB, Berkanish P, Chang B, Cohn MR, Williams RJ, 3rd, et al. Return to play and patient satisfaction after ACL reconstruction: study with minimum 2-year follow-up. J Bone Joint Surg Am. 2017;99(9):720–725. doi: 10.2106/JBJS.16.00958. [DOI] [PubMed] [Google Scholar]
- 6.Cristiani R, Mikkelsen C, Edman G, Forssblad M, Engstrom B, Stalman A. Age, gender, quadriceps strength and hop test performance are the most important factors affecting the achievement of a patient-acceptable symptom state after ACL reconstruction. Knee Surg Sports Traumatol Arthrosc. 2020;28(2):369–380. doi: 10.1007/s00167-019-05576-2. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 7.Tan SH, Lau BP, Khin LW, Lingaraj K. The importance of patient sex in the outcomes of anterior cruciate ligament reconstructions: a systematic review and meta-analysis. Am J Sports Med. 2016;44(1):242–254. doi: 10.1177/0363546515573008. [DOI] [PubMed] [Google Scholar]
- 8.Arundale AJH, Capin JJ, Zarzycki R, Smith A, Snyder-Mackler L. Functional and patient-reported outcomes improve over the course of rehabilitation: a secondary analysis of the ACL-SPORTS Trial. Sports Health. 2018;10(5):441–452. doi: 10.1177/1941738118779023. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 9.Webster KE, Nagelli CV, Hewett TE, Feller JA. Factors associated with psychological readiness to return to sport after anterior cruciate ligament reconstruction surgery. Am J Sports Med. 2018;46(7):1545–1550. doi: 10.1177/0363546518773757. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 10.Webster KE, Feller JA, Lambros C. Development and preliminary validation of a scale to measure the psychological impact of returning to sport following anterior cruciate ligament reconstruction surgery. Phys Ther Sport. 2008;9(1):9–15. doi: 10.1016/j.ptsp.2007.09.003. [DOI] [PubMed] [Google Scholar]
- 11.Asaeda M, Deie M, Fujita N, Kono Y, Terai C, Kuwahara W, Watanabe H, Kimura H, Adachi N, Sunagawa T, Ochi M. Gender differences in the restoration of knee joint biomechanics during gait after anterior cruciate ligament reconstruction. Knee. 2017;24(2):280–288. doi: 10.1016/j.knee.2017.01.001. [DOI] [PubMed] [Google Scholar]
- 12.Kim DK, Park WH. Sex differences in knee strength deficit 1 year after anterior cruciate ligament reconstruction. J Phys Ther Sci. 2015;27(12):3847–3849. doi: 10.1589/jpts.27.3847. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 13.Samuelsen BT, Webster KE, Johnson NR, Hewett TE, Krych AJ. Hamstring autograft versus patellar tendon autograft for acl reconstruction: is there a difference in graft failure rate? A meta-analysis of 47,613 patients. Clin Orthop Relat Res. 2017;475(10):2459–2468. doi: 10.1007/s11999-017-5278-9. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 14.Lesevic M, Kew ME, Bodkin SG, Diduch DR, Brockmeier SF, Miller MD, Gwathmey FW, Werner BC, Hart JM. The affect of patient sex and graft type on postoperative functional outcomes after primary ACL reconstruction. Orthop J Sports Med. 2020;8(6):2325967120926052. doi: 10.1177/2325967120926052. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 15.Mouarbes D, Menetrey J, Marot V, Courtot L, Berard E, Cavaignac E. Anterior cruciate ligament reconstruction: a systematic review and meta-analysis of outcomes for quadriceps tendon autograft versus bone-patellar tendon-bone and hamstring-tendon autografts. Am J Sports Med. 2019;47(14):3531–3540. doi: 10.1177/0363546518825340. [DOI] [PubMed] [Google Scholar]
- 16.Salem HS, Varzhapetyan V, Patel N, Dodson CC, Tjoumakaris FP, Freedman KB. Anterior cruciate ligament reconstruction in young female athletes: patellar versus hamstring tendon autografts. Am J Sports Med. 2019;47(9):2086–2092. doi: 10.1177/0363546519854762. [DOI] [PubMed] [Google Scholar]
- 17.Group MK, Spindler KP, Huston LJ, Zajichek A, Reinke EK, Amendola A, et al. Anterior cruciate ligament reconstruction in high school and college-aged athletes: does autograft choice influence anterior cruciate ligament revision rates? Am J Sports Med. 2020;48(2):298–309. doi: 10.1177/0363546519892991. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 18.Paterno MV, Huang B, Thomas S, Hewett TE, Schmitt LC. Clinical factors that predict a second ACL injury after ACL reconstruction and return to sport: preliminary development of a clinical decision algorithm. Orthop J Sports Med. 2017;5(12):2325967117745279. doi: 10.1177/2325967117745279. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 19.Rosenstiel N, Praz C, Ouanezar H, Saithna A, Fournier Y, Hager JP, Thaunat M, Sonnery-Cottet B. Combined anterior cruciate and anterolateral ligament reconstruction in the professional athlete: clinical outcomes from the Scientific Anterior Cruciate Ligament Network International Study Group in a series of 70 patients with a minimum follow-up of 2 years. Arthroscopy. 2019;35(3):885–892. doi: 10.1016/j.arthro.2018.09.020. [DOI] [PubMed] [Google Scholar]
- 20.Pierce TP, Kurowicki J, Kelly JJ, Issa K, Festa A, McInerney VK, et al. Risk factors for requiring a revision anterior cruciate ligament reconstruction: a case-control study. J Knee Surg. 2019;34:859–863. doi: 10.1055/s-0039-3402075. [DOI] [PubMed] [Google Scholar]
- 21.Schmucker M, Haraszuk J, Holmich P, Barfod KW. Graft failure, revision ACLR, and reoperation rates after ACLR with quadriceps tendon versus hamstring tendon autografts: a registry study with review of 475 patients. Am J Sports Med. 2021;49(8):2136–2143. doi: 10.1177/03635465211015172. [DOI] [PubMed] [Google Scholar]
- 22.Snaebjornsson T, Hamrin Senorski E, Sundemo D, Svantesson E, Westin O, Musahl V, et al. Adolescents and female patients are at increased risk for contralateral anterior cruciate ligament reconstruction: a cohort study from the Swedish National Knee Ligament Register based on 17,682 patients. Knee Surg Sports Traumatol Arthrosc. 2017;25(12):3938–3944. doi: 10.1007/s00167-017-4517-7. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 23.Cronstrom A, Tengman E, Hager CK. Risk factors for contra-lateral secondary anterior cruciate ligament injury: a systematic review with meta-analysis. Sports Med. 2021;51(7):1419–1438. doi: 10.1007/s40279-020-01424-3. [DOI] [PMC free article] [PubMed] [Google Scholar]