The term unhappy triad was first used by O'Donoghue to identify an injury affecting the anterior cruciate ligament, medial collateral ligament, and medial meniscus. The unhappy triad occurs in approximately 25% of patients with an acute athletic knee injury. This video describes the surgical findings of the lateral compartment in patients with the unhappy triad.

Lateral compartment injuries were observed in all patients with an unhappy triad. Given the contrasting mechanisms of pivot-shift injury and valgus extension rotation, we attempted to explain the prevalence of lateral compartment injury as a result of forced valgus external rotation of the tibia. Anterior cruciate ligament tears occur in the first milliseconds when the knee is forced into valgus, but the tibia is put into internal rotation before the knee eventually collapses into external rotation. Therefore, as postulated by Muller, the unhappy triad is a tetrad of sequential injury to the anterior cruciate ligament, lateral compartment, lateral or medial meniscus, and medial collateral ligament.