Effect of Limited Femoral Internal Rotation and Sex on the Number of Loading Cycles to ACL Failure during Repeated In Vitro Pivot Landings

Introduction: Until the mechanics of anterior cruciate ligament (ACL) injuries and contributing factors are better elucidated, effective preventive strategies will continue to elude us. Current dogma is that ACL injury results from a single loading cycle, but recent evidence supports tissue fatigue as a possible failure mechanism [1]. In a cadaveric pivot landing model, ACL rupture was found to occur during cyclic loading, hence suggesting cumulative trauma leading to ACL tissue fatigue. Similar failures due to tissue fatigue have been reported in rabbit medial collateral ligament [2]. Limited hip motion in internal rotation, as exhibited by individuals with femoroacetabular impingement (FAI) [3], has been shown to be a contributing factor to ACL injury risk [4]. Could FAI increase ACL loading and thereby reduce ACL fatigue life under repeated loading this also reduce the ACL’s fatigue life. We tested the primary hypothesis that the number of loading cycles required to injury the ACL would be significantly greater in an in vitro model with adequate femoral internal rotation than in a model with limited rotation. Given that women are at a greater risk of sustaining an ACL injury [5], we also tested the secondary hypothesis that the female knee specimens would require a fewer number of cycles to ACL fatigue failure than the male specimens.