Unhappy triad

An unhappy triad (or terrible triad, or O'Donoghue's triad ) is an injury to the knee. It commonly occurs in contact sports (such as American football). The mechanism for this injury occurs when a lateral (outside) force to the knee is received while the foot is fixed on the ground.

Structures in triad
This scenario causes an injury to three knee structures:


 * the anterior cruciate ligament
 * the medial collateral ligament (or "tibial collateral ligament")
 * the lateral meniscus

The inclusion of the lateral meniscus in the triad has been recently ascertained as it previously had been incorrectly postulated that the medial meniscus was the third component.

Terminology
The term "unhappy triad" was coined by O'Donoghue in 1950. However, since then, this term and the term "terrible triad" have also been used to describe several other combinations of joint injuries, including those of the elbow and shoulder.

The term "terrible triad" is also sometimes used in the popular press to describe conditions relating to pain, or even to refer to the MacDonald triad.