Anterior Cruciate Ligament
The anterior cruciate ligament (ACL) is one of the most commonly injured ligaments of the knee. Each year, approximately 100,000 people sustain ACL injuries [ 10 ] with basketball, soccer, skiing, and gymnastics being the sports with the highest incidence [ 17 ], [ 28 ]. ACL injury rates are estimated to be two to eight times higher in females than males participating in the same sports [ 15, 16, 28, 38 ]. Numerous studies exploring why females are at a higher risk cite both intrinsic and extrinsic differences between genders. Intrinsic factors include age, gender, phase of menstrual cycle, ligamentous laxity, previous injury, aerobic fitness, body size and limb girth, limb dominance, flexibility, muscle strength and imbalance, reaction time and postural stability, anatomic alignment and foot morphology. Extrinsic factors include level of competition, skill level, shoes and orthotic equipment, and playing surface [ 10, 15 ].
What main factors contribute to female susceptibility to ACL injuries?
The cause of increased female susceptibility to ACL injury is unclear, but is likely due to a multitude of complex variables. Research has suggested three main views: anatomical and biomechanical considerations, neuromuscular imbalances, and hormonal influences.
Anatomical and Biomechanics
- Smaller notch width index. A smaller notch width index has been found to predispose females to ACL injuries. The smaller notch likely causes a shearing effect on the ACL by the femur. Although this smaller, A-shaped notch has been shown to be related to ACL injuries, there is no evidence that the relationship is causal [ 15 ].
- Increased ligamentous laxity. Females in general have ligaments that are more lax than males, which increases the risk of ACL rupture [ 15, 17 ]. In addition, it has been suggested that within the first year after surgery when “ligamentization” of the tendon occurs, females undergo a different remodeling response than males [ 32 ].
- Increased Q angle. The female pelvis is wider than the male pelvis, which increases the Q angle of the knee [ 10 ]. This leads to increase stresses at the knee, and causes other compensatory deviations in the surrounding joints. Other changes that occur include femoral anteversion, tibial external torsion, and subtalar pronation [ 10 ].
Research has shown disparity among females and males in knee proprioception and neuromuscular control [ 10, 16, 38 ].
- Quadriceps dominance pattern. Females demonstrate strength imbalance between quadriceps and hamstrings. Female athletes tend to rely on their quadriceps and gastrocnemius and less on their hamstrings when compared to males. In addition, females exhibited a delayed firing pattern of the hamstrings [ 15 ].
- Landing strategies. Females use different strategies when running, landing, or jumping than males and tend to land with an increased valgus moment. This may cause significant differences between their dominant and non-dominant knee [ 10, 16 ]. However, both knees may potentially be at an increased risk for ACL rupture. The dominant knee works to limit gravitational forces, while the non-dominant knee may be too weak to withstand such forces.
Female hormones have been suggested as a possible risk factor for ACL rupture. It has been hypothesized that these hormones increase ligament laxity and decrease ligament strength during the weeks prior to and immediately following the menstrual cycle [ 24 ]. Most research on this topic has several limiting factors, including the reliance on subject self-report of menstrual cycle phase or the confounding factor of oral contraceptives [ 10 ]. Therefore, further research is needed to confirm or deny the role of female sex hormones in ACL injury.