Mnemonic

Knee Unhappy Triad

A memory aid for the three structures torn in the unhappy triad of the knee.

Expansion

Anterior cruciate ligament, medial collateral ligament and medial meniscus

Expansion

The unhappy (O’Donoghue) triad, from a force to the outside of a planted knee:

  • Anterior cruciate ligament (ACL)
  • Medial collateral ligament (MCL)
  • Medial meniscus (classically; some studies find the lateral meniscus is more often involved)

The medial collateral ligament is attached to the medial meniscus, which is why they tend to tear together.