What is the terrible triad of knee injury?
An ACL tear, complete medial collateral ligament tear, accompanied by a meniscal tear. This combination is almost always a result of sporting activities, particularly a valgus impact to the knee with the foot firmly planted. The knee in these injuries exhibits a markedly positive anterior drawer test, a positive Lachman test and pivot shift test, and marked valgus angulation with applied stress in full extension. The knee will be stable to varus stress testing because the lateral collateral ligament and posterior cruciate ligament remain intact. Effusion from hemarthrosis may be mild secondary to medial capsular tearing, which allows the traumatic bleeding to exit the knee joint. Suspicion of an injury of this magnitude should lead to the prompt referral to an orthopedic surgeon.