Study: MRI shows geometry, not gender, may explain ACL injury risk

An analysis of MR scans of male and female athletes with non-contact anterior cruciate ligament (ACL) injuries and similar athletes without a history of ligament injury found that injured athletes had shorter, more highly convex articulating surfaces than non-injured athletes, according to a study published Feb. 1 in The Journal of Bone and Joint Surgery.

Women have an ACL injury rate two to five times greater than men. Cumulative annual costs of ACL ruptures in the U.S. are estimated at $1 billion, and a great deal of research has attempted to uncover the cause of female athletes’ predisposition to ACL injury.

Christopher J. Wahl, MD, of the department of orthopedics and sports medicine at University of Washington in Seattle, and colleagues hypothesized that articular surfaces of the lateral aspect of the tibial plateau and/or the distal aspect of the femur would be more highly convex in patients with ACL injuries. They also suspected that the convex geometry would be more prevalent among female athletes.

Three observers, blinded to patient information, analyzed knee MRI scans of 112 athletes with a non-contact ACL injury and 61 activity-matched athletes without an ACL injury. They measured femoral anteroposterior length (FAP), tibial plateau anteroposterior length (TPAP), tibial plateau radius of curvature (TPr) and fibonacci femoral radius of curvature (Fr).

Wahl et al reported that the mean TPr, FR and TPAP values were significantly smaller in ACL-injured patients than in non-injured patients. Although the mean FAP value did not differ significantly between ACL-injured and uninjured patients, the difference in mean FAP:TPAP was significant.

When the researchers analyzed data for male patients only, they found that mean TPr, FR and TPAP values were also significantly smaller in ACL-injured patients than in uninjured patients, and noted a similar significant difference in mean FAP:TPAP between ACL-injured patients and uninjured patients. However, these mean values showed no significant differences in the analysis of female patients.

“The male geometry is more variable than the females in this respect,” Wahl said in a statement.

Wahl and colleagues concluded that all females, as well as ACL-injured males, share common knee geometry. “Our results suggested that there exists an ‘at-risk’ geometric phenotype of the lateral tibiofemoral articular surface, characterized by a shorter and more convex tibial plateau relative to the femur and by a smaller and more convex distal femoral surface, which may render the knee more susceptible to ACL injury.”

These findings also may help explain why women have a higher rate of ACL injuries, and yet the rate of re-injury is the same among men and women.