Anterior Cruciate Ligament Injuries: Sex-Based Differences

Introduction and Epidemiology

In spite of years of research and evolving techniques, rupture of the anterior cruciate ligament (ACL) continues to represent one of the most common traumatic injuries in competitive sports. Female athletes have been noted to be at particular risk for ACL tears, with injury rates as high as two to eight times as those experienced by males, especially in sports requiring quick or repetitive lateral movement. In addition to being at increased risk for primary rupture, females are more likely to sustain a contralateral injury or undergo revision surgery, experiencing rates of secondary ACL sequela as high as 34%.

Interestingly, several well-designed studies have failed to identify female gender as an independent risk factor for ACL injury on multivariate regression analysis. This has led to the suggestion that the observed increase in risk for female ACL injury is a mere reflection of increasing sports participation. However, this theory has been largely discredited by multiple surveillance studies, which failed to find an increase in ACL injury risk when expressed as a percentage of active female athletes. In one such study, performed by the National Collegiate Athletic Association, no significant change in the rate of ACL injury was noted over a 15-year period, in spite of increased female sports participation rates.

Given these findings, the increased propensity for female ACL injury is likely multifactorial, attributed to factors such as variations in anatomic size and morphology, hormonal levels, and alterations in neuromuscular control. Accordingly, this chapter reviews the current understanding of the female ACL, summarizing potential contributing causes for increased risk, while also discussing strategies for injury prevention, surgical reconstruction, and rehabilitation.

Anterior Cruciate Ligament Anatomy and Function

The ACL is the primary contributor of anteroposterior and rotatory stability to the knee, especially in lower flexion angles. It is composed of two distinct segments, the anteromedial and posterolateral bundles, which are named based on their respective insertion sites on the tibia. The native ACL is covered by synovial tissue, with the deeper areas being largely avascular, minimizing healing potential. Limited vascularity is supplied to the femoral insertion via the posterior soft tissues, with the tibial insertion site receiving a small vascular contribution from the anterior horn of lateral meniscus. The ACL is also rich in mechanoreceptors, functioning to provide proprioceptive feedback while initiating protective muscular reflexes.

The ACL's femoral insertion site is oval in shape and smaller in size than the ligament's broad tibial insertion. The isthmus of ACL exists at mid-substance, with a cross-sectional area of less than half of the tibial or femoral insertions. The intra-articular portion of the ACL is quite dynamic, demonstrating its shortest length at 90 degrees of knee flexion and increasing to its maximal size in full extension.

When viewed arthroscopically, the ACL can be readily appreciated originating from the posterior aspect of the lateral femoral condyle, coursing in an anteromedial direction to insert at the interspinous area of the tibia. This intra-articular course is located within the ‘femoral notch,’ composed of the area between the medial and lateral condyles of the femur. Femoral notch size and the shape often play an important role in ACL injury, with several gender-specific differences noted. Notch size can also affect intraoperative visualization, complicating the creation of anatomic tunnels, one of the most impactful metric on return-to-play outcomes.

Gender-Based Anatomic Differences

The size of the ACL itself has been postulated to contribute to gender-based differences in ACL injury patterns. When compared with their male counterparts, ACLs in females tend to be smaller in overall volume, while also possessing less tensile resistance, demonstrating decreased elongation distances to failure. Such mechanical differences have also been noted clinically, with females' knees exhibiting less resistance to rotation and anterior translation as measured on KT-1000.

Females also possess a larger quadriceps angle or ‘Q-angle’, resulting from a wider and shorter pelvis. The Q-angle is formed via the composite of two lines, originating from the anterior superior iliac spine and tibial tubercle, that course through the center of the patella. Increases in the Q-angle result in a more laterally based quadriceps force vector, placing the knee in increased valgus, thus predisposing to ACL rupture with athletic activity. ^,

In a 2020 systematic review, Bayer et al. examined the morphologic risk factors for ACL injury. Of all the factors considered, an A-shaped notch, sometimes referred to as ‘notch stenosis,’ was the most commonly cited cause for increased risk of ACL injury. This finding seems to correlate with increased injury risk, ^, as a review of patients undergoing ACL reconstruction noted a greater percentage of A-shaped notches in females than males. Other authors have reported similar findings, noting a decreased notch size in females.

Several morphologic factors have also been demonstrated to contribute to increased risk for ACL injury based on condylar anatomy. In one study, ACL-injured females were noted to demonstrate an increased condylar offset ratio, defined as the difference between the anatomic and transcondylar axes of the femur. A second study reported a lateral femoral condylar offset of >63% to similarly correlate with increased ACL injury. With regard to tibial morphology, multiple authors have reported an increased tibial slope to correlate with increased injury risk. ^, In one such study, small variations seemed predictive, with a mean lateral tibial slope of 6.3 degrees noted in individuals experiencing ACL injury, compared to just 4.1 degrees in their uninjured counterparts.

In an examination of risk factors for subsequent injury to the contralateral anterior cruciate ligament (CACL), Davey et al. noted a rate of 20% for CACL injury following an index ACL tear. In this cohort of 61 female athletes, at a mean follow-up of 45 months, younger females with increased hip anteversion and increased contralateral knee laxity were noted to be at the greatest risk for CACL rupture. Interestingly, while younger patients were at higher risk of CACL pathology, increased sports participation was somewhat protective, with prior competitive play associated with decreased overall injury risk.