The unhappy triad, also known as the terrible triad, or o'donoghues triad is a severe knee injury. It involves full or partial tears of the Anterior Cruciate Ligament, the Medial Collateral Ligament and a tear of the Medial Meniscus .
The Lachman test is the most sensitive test for acute ACL rupture. It is performed with the knee in 30° of flexion, with the patient lying supine. The amount of displacement (in mm) and the quality of endpoint are assessed (eg, firm, marginal, soft). Asymmetry in side-to-side laxity or a soft endpoint is indicative of an ACL tear. Although difficult to measure, a side-to-side difference of greater than 3 mm is considered abnormal.