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Unhappy Triad

By , About.com Guide

Updated July 31, 2003

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Definition: The "Unhappy Triad" is an injury to the knee. The unhappy triad injury commonly occurs in contact sports such as football when the knee is hit from the outside. This causes an injury to three knee structures: the anterior cruciate ligament (ACL), medial collateral ligament (MCL), and the medial meniscus.

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