Bone contusions of the posterior lip of the medial tibial plateau (contrecoup injury) and associated internal derangements of the knee at MR imaging.

PURPOSE: To determine if there are any predictable patterns of internal derangement associated with a bone contusion of the posterior lip of the medial tibial plateau at magnetic resonance (MR) imaging and to offer a biomechanical explanation for the findings. MATERIALS AND METHODS: A retrospective review of 215 consecutive MR examinations for knee trauma was conducted to identify contusions of the posterior lip of the medial tibial plateau. Any additional contusions and internal derangements were documented in the cases with these contusions. Medical charts and arthroscopic results, when available, were reviewed for mechanisms of injury. RESULTS: The specific medial tibial contusion was demonstrated in 25 of 215 (12%) knee MR examinations. Associated anterior cruciate ligament (ACL) tears were found in 25 of the 25 (100%) examinations. Injury to the meniscocapsular junction (14 of 25) or a peripheral tear of the posterior horn of the medial meniscus (10 of 25) occurred in a combined 96% of the cases. Lateral compartment contusions were noted in 24 (96%) cases. Pivot, twisting, or valgus forces were reported mechanisms of injury. CONCLUSION: Contusions involving the posterior lip of the medial tibial plateau may result from a contrecoup impaction injury directly following an ACL tear, as the knee reduces. These contusions are almost always associated with a far peripheral meniscal tear or with a meniscocapsular junction injury affecting the posterior horn of the medial meniscus.