What is "unhappy triad"

n. An injury to the anterior cruciate ligament, medial collateral ligament, and meniscus.

An unhappy triad (or terrible triad, "horrible triangle", O'Donoghue's triad or a "blown knee") is an injury to the anterior cruciate ligament, medial collateral ligament, and the medial meniscus. The triad refers to a complete or partial tear of the anterior cruciate ligament, medial collateral ligament, and the lateral meniscus. Originally the "unhappy triad" included the medial meniscus and not the lateral meniscus. However, during the 1990s, analysis indicated that the 'classic' O'Donoghue triad is actually an unusual clinical entity among athletes with knee injuries. Some authors mistakenly believe that in this type of injury, acute tears of the medial meniscus always present with a concomitant lateral meniscus injury. However, the lateral meniscus tears are more common than medial meniscus tears in conjunction with sprains of the ACL according to the 1990s study.