Magnetic resonance imaging of the extensor mechanism.

Trauma involving the extensor mechanism of the knee is a fairly common clinical problem, and can be addressed easily with a basic understanding of the anatomy of the involved structures. Injury to the muscles and tendons of this group is categorized broadly into complete versus partial tears based on the respective absence or presence of intact fiber bundles. Acute injuries are associated with edema, hemorrhage, and fluid collections; chronic injuries often demonstrate redundancy, atrophy, and retraction. Fracture of the patella can be related to direct or indirect trauma, and is functionally equivalent to a complete tear of the musculotendinous elements. Tendinitis is a more chronic condition seen with repetitive trauma, and usually affects the proximal aspect of the patellar tendon. Swelling and abnormal intrasubstance signal are commonly associated with this entity.