Introduction
Anterior Cruciate Ligament (ACL) injuries are common orthopaedic injuries with preinjury risk, reinjury risk, and functional postoperative recovery potential all varying significantly by patient sex. A recent New York Times article highlighted the higher rates of ACL tears as a crisis among young female athletes.1 In our article, “male” and “female” refer to assigned sex at birth, recognizing that gender identity is a separate concept not captured by biologic sex.
Females face two to eight times higher ACL injury rates than males.2,3 Previous meta-analyses have identified the greatest incidence rates (IR) of ACL injury among adolescent female athletes competing in soccer (IR=0.149-0.166), basketball (IR=0.110), gymnastics (IR=0.114), and lacrosse (IR=0.112).4,5 While there has been no consistent difference found between males and females in overall reinjury rates,6 studies show four times greater second ACL injury rates and six times greater contralateral ACL injury risk among females.7 Females also demonstrate lower return-to-sport rates and shorter post-injury careers.6,8
Both nonmodifiable and modifiable factors influence these sex-based differences. Understanding these factors enables sports medicine practitioners to identify opportunities for early intervention to minimize injury risk and optimize recovery.
Nonmodifiable Factors
Anatomic Factors
Several anatomic differences increase ACL injury risk in females. Females typically have a smaller ACL cross-sectional area9 and narrower intercondylar notches, both associated with increased injury risk.10,11 Posterior tibial slope >12° increases ACL tear risk12, though studies have conflicting findings on whether females have greater mean tibial slope than males.13,14 Females have a greater quadriceps (Q) angle, which creates greater lateralizing force on the patella which can increase the valgus moment acting on the knee.15 Combined with a higher quadriceps-to-hamstring muscle mass ratio, this leads to disproportionate quadriceps use during landing and cutting, causing anterior displacement and higher valgus forces on the ACL,16 increasing the risk of injury. Peak ACL injury incidence occurs at younger ages in females compared to males,17 potentially related to smaller ACL cross-sectional area combined with the demands of pivoting and cutting force loads.
Hormonal Factors
Cyclic hormonal variation influences ACL injury risk in female athletes. Elevated estrogen levels increase anterior knee laxity, with peak estrogen during the late follicular/ovulatory menstruation phase correlating with higher ACL injury risk.18,19 Relaxin also contributes to injury risk, with relaxin receptors present on female but not male ACLs.20,21 This hormone increases matrix metalloprotease while decreasing alpha smooth muscle actin (αSMA) and type I/III collagen expression, potentially altering ACL structural integrity.22 Females with ACL tears demonstrate higher serum relaxin concentrations (SRC), associated with a greater than four-fold injury risk.23
Psychosocial Factors
Psychosocial factors also modulate ACL injury risk, particularly after ACL reconstruction (ACLR). Social determinants of health—including income, race and ethnicity, education level, insurance status, and employment—are nonmodifiable risk factors for worse ACLR outcomes. Black race, Hispanic ethnicity, public insurance, and lower socioeconomic status are associated with delayed access to care after ACL injury, greater severity of concomitant injuries at the time of ACLR, and inferior postoperative outcomes.24 These patients also receive fewer physical therapy sessions, correlating with greater knee extensor strength deficits during rehabilitation.25 Among adolescents, delayed surgery has been correlated with increased irreparable medial meniscal tears and lateral compartment chondral injuries at the time of ACLR.26
Modifiable Factors
In addition to the above, several factors impact the risk of ACL injury among female patients that are considered modifiable.
Hormonal Factors
While some hormonal factors are intrinsic to the natural hormonal cycle, this cycle is responsive to extrinsic influence in the form of hormonal therapy. Oral contraceptives, for example, have been observed to have a relationship with the risk of ACL injury. Population-based case-control studies in the United States and Denmark have observed up to a 20% decreased risk of ACL injury among females taking oral contraceptives compared to those who do not.27 However, no prospective randomized trials have evaluated this as a potential intervention.
Psychosocial Factors
Kinesiophobia, defined as an excessive, irrational, and debilitating fear of movement or physical activity, is an important consideration during rehabilitation following ACLR. Females have been found to be at greater risk of developing kinesiophobia, reporting more concern about environment-related factors influencing reinjury risk.28 Females have also reported less activity and psychological readiness for return to sport at 12 and 24 months postoperatively than males.29 Social, caregiver, and provider support is a critical factor in optimizing recovery, regardless of gender. Measures of social support have been found to positively predict knee symptoms and rehabilitation compliance after ACLR,30 and positive interpersonal dynamics among patients, parents, and physical therapists are felt to strongly influence emotional response and intrinsic motivation during rehabilitation.31
Biomechanical Factors
Several modifiable biomechanical factors contribute to increased ACL injury risk in females. Females tend to land in a more erect position with extended hips and knees,32 placing higher forces on the lower extremity and leading to greater knee valgus positioning, which increases shear forces on the ACL.33 This contributes to the higher rate of noncontact ACL injuries observed in females.34,35
Prevention
Providing literature-informed, individualized care to females at risk of ACL injury begins well before they take the field. Multiple studies have found that female athletes are both less likely to have a required strength and conditioning program as a component of sport participation and less likely to have a certified strength coach overseeing their training.36 This disparity, compared to male athletes, represents a missed opportunity to modify problematic biomechanical habits and mitigate injury risk. Injury prevention programs such as the FIFA 11+ program37,38 (pictured below) have been demonstrated to reduce the incidence of noncontact lower extremity injuries among female athletes.39 These programs are designed to be accessible to a lay audience and straightforward to incorporate into existing pre-practice and pregame routines.
Rehabilitation
When designing rehabilitation programs for females who have sustained an ACL injury, it is important to recognize potential areas of vulnerability. Females tend to demonstrate both greater knee laxity and lower patient-reported knee function at follow-up after ACL reconstruction,40 thus physical therapists and other rehabilitation providers may choose to emphasize stability training when treating female patients. Closely monitored and supervised rehabilitation is particularly important for these patients, as it has been shown to yield higher levels of functional and sports activity and better quality of life at 12 months postoperatively compared with unsupervised rehabilitation.41,42
Differences in Surgical Recommendations
Decisions regarding procedure and graft choice also carry important implications for recovery potential among female patients. Previous investigations suggest that female patients are indicated for hamstring autografts more often than males,43 even though this graft is known to carry a higher rate of graft failure44 and its lower initial stiffness can contribute to greater postoperative laxity compared to bone-patellar tendon-bone (BPTB) grafts.45 The use of large and potentially supraphysiologic ACL grafts must be weighed against the potential for increased arthrofibrosis, particularly with larger graft options such as quadriceps tendon autografts.46 While female sex alone has not achieved consensus as an independent indication for lateral extraarticular tenodesis (LET),47 many of the established indications — particularly hamstring autograft use, younger age, smaller graft diameter, generalized ligamentous laxity, and return to pivoting sports—disproportionately apply to female patients.3,48, Therefore, addition of an LET with ACLR in these at-risk patients may help to decrease reinjury rates.
A Call to Action
As sports medicine practitioners providing care to female athletes, it is imperative to keep these risk factors in mind when treating these patients. We must counsel female athletes on nonmodifiable factors and intervene early on modifiable factors, as those represent the areas where we may have the most impact on patient health and rehabilitation potential.
We must recognize the age groups considered at increased risk of injury. This includes females in their reproductive age as outlined above, but also younger patients and pregnant individuals. In a New York Post article, a pregnant patient describes the challenges of navigating ACL care during pregnancy and highlights the lack of consensus of clinical care recommendations.49 While relaxin promotes ligamentous laxity to support natural fetal delivery, relaxin levels remain elevated for days after birth and can remain detectable in breast milk for at least six weeks postpartum.50 It stands to reason, therefore, that pregnant and recently postpartum individuals should be treated similarly to hypermobile patients or those with increased laxity. Emphasis should be placed on increasing stability, particularly in patients competing in high-risk cutting sports like soccer, and patients should be counseled that increased laxity may persist well past delivery. Current ACOG guidelines recommend delaying elective surgery until after delivery when possible,51 underscoring the importance of ACL injury prevention in this population to avoid delays in definitive surgical management. However, surgeons should also recognize that severity of concurrent injuries—for example, bucket handle meniscus tears—may necessitate urgent surgical intervention and require consultation with obstetric providers as well as a discussion of appropriate DVT screening and possible prophylaxis. Finally, every encounter should include consideration of how a patient's social determinants of health modulate their injury risk and impact their potential recovery course.
We must serve as champions for our patients. Sports medicine healthcare professionals have the power to promote understanding of the clinical scenario and foster engagement in the recovery process, which in turn helps patients maintain a positive mindset and actively participate in their recovery. Validated screening tools such as the PROMIS Global52 or ACL-specific tools such as the Anterior Cruciate Ligament–Return to Sport after Injury (ACL-RSI) scale53 should be administered to assess psychological readiness for return to sport and identify potential areas for intervention.
We must also recognize inherent bias present in our surgical recommendations and decision-making. The benefits of utilizing larger grafts compared to the native ACL architecture should be weighed against other factors, such as the risk of postoperative laxity or arthrofibrosis. As females tend to have narrower intercondylar notches, particular attention should be paid to graft size and precise tunnel placement to avoid ACL graft impingement. The use of notchplasty may be considered as an option when impingement does occur in narrow based notches. Although no studies have directly compared graft failure rates between females and males treated with ACLR with LET, the addition of LET has been shown to reduce overall graft failure rates.54 This procedure should therefore be considered as an adjunct, particularly in females who participate in high-risk cutting and pivoting sports or who have risk factors for a repeat ACL injury.
While sports medicine physicians face the difficult situation of often making first contact with a patient after injury has occurred and after the chance to intervene early has passed, athletic trainers, strength and conditioning personnel, and physical therapists have the powerful opportunity to incorporate standardized exercise regimens such as the FIFA 11+ into training programs, enhancing neuromuscular training and reducing the risk of ACL injuries from occurring in the first place. By standing together as a community for these patients, we can work to recognize risks early, approach intervention objectively, and remain available to patients as they work to return to sport.
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