The unhappy triad: quest for a conclusive definition

The unhappy triad of the knee is classically defined as an acute injury to the anterior cruciate ligament (ACL) with concomitant damage to the medial collateral ligament and the medial meniscus. With the advent of advanced clinical and arthroscopic diagnosis, the classical definition may no longer be tenable. Consequently, a comprehensive review of the literature was performed to better define our understanding of the structures involved with acute injury to the ACL and chronic ACL deficiency. A search produced 32 peer‐reviewed references consistent with the review's aims. In a total of 598 knees with acute ACL and medial collateral ligament ruptures, there were 255 (43%) lateral meniscus tears, 233 (39%) medial meniscus tears, and 28 (4%) bilateral meniscus tears. In the setting of chronic ACL deficiency, with 1,780 ACL deficient knees examined, when combined with a rupture of the medial collateral ligament, there were 843 (47%) medial meniscus tears, 789 (44%) lateral meniscus tears and 52 (3%) bilateral tears. Our literature review suggests that the third member of the unhappy triad is the lateral meniscus, whereas the medial meniscus is more commonly injured in chronic ACL deficiency. These differences in injury patterns will aid in clinical diagnosis and management of patients with ACL injuries.