The knee is the largest and most complex joint in the body, combining the tibiofemoral and patellofemoral articulations within one synovial capsule. Its articular surfaces are inherently incongruent, so stability depends mainly on the menisci, the cruciate and collateral ligaments and the surrounding muscles rather than on bony shape. This makes it the most frequently injured joint in sport, and total knee replacement for end-stage osteoarthritis is among the most common elective operations.
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The knee is the most commonly injured joint in sport, and its large synovial volume produces significant effusion after injury. A tense haemarthrosis within hours of trauma signals a significant intra-articular injury, most often an ACL tear. The classic unhappy triad (O'Donoghue) of ACL, MCL and medial meniscus injury follows a valgus contact force, although lateral meniscal tears are actually more common with acute ACL rupture. Medial compartment osteoarthritis predominates in older adults.
Combined compression-rotation tears the more fixed medial meniscus, producing joint line pain, catching, and positive McMurray test.
Progressive medial compartment cartilage loss with pain, morning stiffness, crepitus, and eventual varus deformity.
Combined ACL, MCL and meniscal injury from a valgus contact force, producing acute haemarthrosis and multidirectional instability.
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