This 52-year-old male presents with knee pain following an injury one week ago.
Q1 – What is the most likely mechanism of injury in this individual with an ACL tear?
Q2 – What are the potential internal derangements associated with this knee contusion pattern?
A1 – Image 7 demonstrates an acute ACL tear (blue arrow). In addition, osseous contusions are identified in the posterior aspect of the lateral compartment in both the lateral femoral condyle (red arrow) and lateral tibial plateau (yellow arrow). Image 8 (arrow) also demonstrates an osseous contusion in the anterior medial tibial plateau (anterior medial tibial rim sign). Radiologists are most familiar with pivot-shift contusions in the lateral compartment. However, medial compartment osseous contusions also occur, and are associated with higher-energy injuries. The most commonly reported mechanism with an anterior medial tibial rim sign is axial loading. Axial loading occurred in this 52-year-old male following a fall from a ladder. An anterior medial rim sign is identified in approximately 30% of individuals with acute ACL tears.
A2 – The rim sign is associated with a significant increased risk of contusions and fractures as well as posterolateral corner injuries and tears of the posterior horn lateral meniscus. Image 9 (arrow) indicates a torn popliteofibular ligament. Images 10 and 11 (arrows) indicate a complex tear of the posterior horn / root attachment of the lateral meniscus. Image 12 (arrows) demonstrates an impaction fracture of the posterolateral tibial plateau. The proposed etiology for the anterior medial rim sign is a contusion between the inferior pole of the patella and anterior medial rim. Patellar edema is less common than the edema in the anterior medial tibia, likely related to the relative hardness of the sesamoid bone relative to the tibia. The identification of edema in both the patella and anterior medial rim is uncommon, occurring in approximately 4% of all acute ACL tears.
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