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We present a case of the fracture of the posterior margin of the lateral tibial plateau, probably by pivot shift mechanism in a chronic anterior cruciate ligament (ACL) insufficient knee. Multitudes of direct and indirect signs of ACL injury were evident on MRI, including anterior translation of the lateral tibial plateau, buckling of the posterior cruciate ligament, deep sulcus sign, and the fracture of the posterior aspect of lateral tibial plateau. With careful interpretation of the history and arthroscopic findings, the main insult of the current case was identified as the fracture of the posterior margin of the lateral tibial plateau with antecedent chronic ACL insufficiency, rather than as the acute or subacute ACL rupture. This type of injury is worth due attention as a late complication of chronic ACL insufficiency, and the clinicians should be careful in constructing the clinical scenario for the temporal relationship of injury. To the best of our knowledge, this type of injury as a late complication of chronic ACL insufficiency has never been reported in the literature.