ACL impingement prediction based on MRI scans of individual knees.

Although tibial external rotation and abduction do not load the ACL strongly in cadaver-based biomechanical studies, such knee positions are associated with ACL injuries in clinical practice. We hypothesized the ACL could be injured in such knee positions because of its impingement against the intercondylar notch. We developed a three-dimensional geometric ACL impingement model through segmentation of MR images of individual knees. We investigated impingement by determining the deformed geometry and elongation of the ligament as it wrapped around the notch surface during impingement. When impingement did not occur, the gap width separating the ligament and the notch surface was computed. Tibial external rotation/abduction could cause the ACL to impinge against the lateral notch wall and elongate as it wraps around the surface of the notch wall. The impingement occurred between the middle portion of the ligament (at 45% +/- 8% [mean +/- standard deviation] of the ligament length) and the convex surface of the lateral notch wall. Considering the multiband fiber architecture of the ligament, the anteromedial band of the ACL sustained greater elongation during impingement and showed a smaller gap width from the notch surface than the intermediate band or the posterolateral band.