Occult fracture patterns of the knee associated with anterior cruciate ligament tears: assessment with MR imaging.

One hundred consecutive magnetic resonance (MR) images of the knee in patients with acute complete anterior cruciate ligament (ACL) tears were reviewed to evaluate the prevalence and patterns of associated occult fractures. Eighty-nine occult fractures were identified in 56 knees. All occult fractures were in the posterior aspect of the lateral tibial plateau. Of these, occult fractures were isolated in 24 cases (43%) and were in combination with fractures in the middle aspect of the lateral femoral condyle in 26 (46%), with fractures in the posterior aspect of the medial tibial plateau in four (7%), and with fractures involving all three areas in one (2%). Disruption of the ACL under valgus stress leads to anterior translation of the tibia and relative external rotation of the femur. This allows impaction of the posterior portion of the lateral tibial plateau against the middle of the lateral femoral condyle and accounts for the unique pattern of occult fractures associated with ACL tears. An occult fracture of the posterior lateral tibial plateau with or without an associated fracture in the lateral femoral condyle ("kissing contusion") is a relatively frequent finding in acute ACL tears and, when present, is highly suggestive of such an associated tear.