An Association of Lateral Knee Sagittal Anatomic Factors with Non-Contact ACL Injury: Sex or Geometry?

Wahl CJ, Westermann RW, Blaisdell GY, Cizik AM. J Bone Joint Surg Am. 2012 Feb 1;94(3):217-26.

A risk factor is an exposure (variable) that is related to an outcome (e.g., knee injury). For example, being female is associated with an increased risk of having an anterior cruciate ligament (ACL) tear but as we all know some women never tear their ACL and some men do. Despite this, a lot of research has focused on comparisons between men and women which can be very informative but has potential limitations when discussing what variables contribute to an ACL tear. Wahl et al conducted an interesting retrospective chart review study to evaluate the lateral tibiofemoral articular geometry among males and females that sustained an ACL injury with activity-level matched athletes who had never sustained an ACL injury but were being evaluated for another type of knee injury (25 females, 36 males). Two groups of patients with acute ACL injuries were evaluated: those having an acute ACL injury with no history of previous ACL injuries in either knee (26 females, 43 males) and patients with an acute ACL injury but a history of an ACL injury in the opposite knee (21 females, 22 males). All of the patients were 16 to 45 years of age, had no evidence of knee osteoarthritis, and had magnetic resonance (MR) images available for review. The authors measured the MR images to determine the maximal anteroposterior length of the lateral femoral condyle, maximal anteroposterior length of the tibia, curvature of the lateral aspect of the tibial plateau, and curvature of the distal aspect of the femur (see article for more details). To account for differences between males and females the anteroposterior lengths as well as curvatures were also assessed as ratio (e.g., femur length to tibia length). Despite these adjustments, the femur to tibia length ratio was greater in females than males (curvature ratio was similar between males and females). Similarly, ACL injured knees, regardless of sex, had greater femur to tibia length than uninjured knees. However, among females, there were no structural differences between knees with and without an ACL injury. In contrast, among males, ACL-injured knees had greater femur to tibia length and curvature ratios than knees without an ACL injury. Furthermore, between males and females with ACL injuries there were no differences in femur to tibia length or curvature ratios. In summary, females (with or without ACL injury) and ACL-injured males had similar lateral knee shape defined by smaller tibial plateau length relative to the femur length and possibly a more convex articulating surfaces, which the authors hypothesize leads to greater knee instability compared to flatter (less convex) articulating surfaces.

This study is important because it highlights the complexity of comparing males and females to make inferences about risk factors for ACL injuries. Similar to previous research, this study found differences between males and females but this study also noted that males who tore their ACL shared knee shape characteristics with females who tore their ACL. The authors suggest that this might explain why there is not a clear difference in reinjury rates between males and females. In other words, ACL-injured patients, regardless of sex, share common risk factors (e.g., joint shape, neuromuscular control) that may not be corrected easily by rehabilitation or surgery and predispose them to reinjury. This also raises the challenging question with structural risk factors: are we going to spend the money to image our athletes? Almost definitely not, but once we verify a structural risk factor perhaps we can find a cheap and efficient clinical measurement to estimate the more expensive measures (e.g., knee girth). These studies may sometimes seem far removed from clinical practice but they may inspire new techniques to screen and treat our patients. Do you think screening athletes for ACL-injury risk factors will help us prevent injuries?

Written by: Jeffrey Driban

Wahl CJ, Westermann RW, Blaisdell GY, & Cizik AM (2012). An Association of Lateral Knee Sagittal Anatomic Factors with Non-Contact ACL Injury: Sex or Geometry? The Journal of Bone and Joint Surgery. American volume, 94 (3), 217-26 PMID: 22298053