From Laura Inverarity, D.O.,
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About.com Health's Disease and Condition content is reviewed by Kate Grossman, MD

Definition: The “unhappy triad” refers a sprain injury that involves three structures of the knee. These are the medial collateral ligament, anterior cruciate ligament, and the medial meniscus. The mechanism for this injury occurs when a lateral force to the knee is received while the foot is fixed on the ground. This places an increased valgus, or abduction, and rotational stress on the knee that results in the stretching of these three structures.
A good example being during a tackle in football when the athlete is hit on the outside of his knee while in the stance position with the foot planted on the ground.