Anatomical and Postural Contributions to Knee Injury
Abstract
Abstract
In 1996, Huston and Wojtys examined sex differences in neuromuscular function in terms of the greater risk for knee injury in females.1 Since that time, sex differences in neuromuscular and biomechanical function about the knee have been widely studied, with females often reported to land and cut with greater2,3 and earlier1,4 quadriceps activation relative to the hamstring muscles, less hip and knee flexion,2,5–7 greater functional knee valgus,6,8–10 and greater anterior knee shear forces,11,12 etc. As such, sex differences in neuromuscular function are considered to be the most important factors in the greater risk for anterior cruciate ligament (ACL) injury in females,13 and this has led to the development of injury prevention programs aimed at reducing these potential neuromuscular risk factors.14 However, while these prevention programs have achieved some success, there is much we still do not understand: what facet(s) of these programs are most effective, the mechanisms through which the risk factors are modified, the short- versus long-term effects of these interventions, and who we should specifically target and when.14 To that end, we still need to understand the underlying factors that contribute to those neuromuscular and biomechanical patterns that are more often observed in females. To what extent do anatomical and postural factors contribute to these sex differences and/or the higher rates of ACL injuries in females?
Sex Differences in Anatomy and Posture
Before puberty, there are few anatomical differences in males and females in terms of body composition, strength, structural anatomy, joint laxity, and lower extremity postural alignment.15–22 However, upon maturation females secrete more estrogen while males secrete more testosterone. This leads to body compositional differences, where females develop a lower proportion of lean muscle mass and a greater proportion of body fat compared withmales, which accounts for many of the sex differences in strength. Furthermore, during this time of maturation, alignment and postural differences begin to emerge, where females develop and maintain a more inwardly rotated and valgus knee posture, characterized by greater degrees of hip anteversion and tibiofemoral and quadriceps angles compared with males upon full maturation.23 Beyond these maturational changes, adult females have also been reported to stand with greater anterior pelvic tilt24,25 and have greater magnitudes of joint laxity (as measured by anterior knee laxity, genu recurvatum, and general joint laxity) compared with males.25–33 However, given all these maturational changes and the sex differences that ultimately emerge in anatomy and posture, we still know little about their potential influence on ACL injury risk.
The Association Between Lower-extremity Posture and Anterior Cruciate Ligament Injury
The current consensus from the literature suggests that there is no conclusive evidence to reliably associate any one anatomical factor with an increased rate of injury.14 We believe this lack of agreement is because we have yet to take a comprehensive approach to the assessment of anatomical and postural factors as they relate to ACL injury risk. The few studies that have examined the potential link between anatomy and posture with ACL injury24,29,32,34–38 are largely retrospective study designs that have typically studied fewer than 20 ACL-injured participants, with the one prospective study32 limited to only eight ACL-injured females (of a total of 24 non-contact ACL injuries). While these studies varied widely in the type and number of anatomical risk factors examined, the anatomical factor most consistently linked to ACL injury has been foot pronation; however, pronation alone cannot explain the greater risk in females, as multiple studies have consistently reported that males and females do not differ in their magnitude of foot pronation. Moreover, pronation has been found to be a stronger predictor of ACL injury when present in combination with greater anterior pelvic tilt,24 genu recurvatum,36 and knee laxity;38 anatomical factors that do differ between the sexes.25,26,30
These findings suggest that the effect of one alignment factor may be dependent on other anatomical factors, and therefore support the need to consider the collective influence of lower-extremity postural characteristics when examining injury risk. This is supported by work that has identified different neuromuscular activation patterns when excessive navicular drop (as a measure of foot pronation) and quadriceps angles are present in isolation versus together. 39 Furthermore, we have identified distinct lower- extremity postures based on relationships between individual alignment factors along the lower-extremity chain.40 Therefore, rather than focusing on individual anatomical risk factors, we need to consider anatomy and posture as a whole if we are to understand their influence on lower-extremity function and ACL injury risk. The same may be said for other lower extremity injuries such as patella femoral pain syndrome and stress fractures. Ultimately, our injury prediction models are only as good as the variables we account for, and we need to be more complete in this regard.
Specifically, we need to understand how these factors contribute to dynamic hip and knee function and, consequently, injury risk (see Figure 1 ). For example, does the greater static valgus posture in females (greater hip anteversion, tibiofemoral angle, and quadriceps angle) in part contribute to
the greater dynamic valgus collapse that has often been observed in females 6,8–10 and found to be predictive of ACL injury risk? 10 To what extent do differences in postural alignments influence the load distribution of the knee during weight-bearing and contribute to excessive rotatory stress that may strain the ACL? For example, in vivo work has shown that internal rotation torques when applied in combination with knee hyperextension 41,42 or knee valgus 43 resulted in ACL loads that were substantially higher than when valgus or internal rotation torques were applied alone. These findings would suggest that lower-extremity alignments that promote a combination of excessive knee valgus, knee hyperextension, and tibia-on-femur internal rotation during weight-bearing may tension and stretch the ACL. Only by examining the entire lower-extremity posture in future studies will we begin to understand the potential impact of lower-extremity alignment on weightbearing knee-joint function and resulting ACL strain and injury.
