MR appearances of the locked knee.

This review illustrates the MR appearances of commonly encountered problems that can present as a "locked knee", as well as several unusual causes. Internal derangement of menisci, particularly bucket handle tears, predominate. Loose bodies as a result of trauma/degeneration and lesions such as cysts of the cruciate ligaments and focal pigmented villonodular synovitis are also illustrated. While meniscal tears are the major cause of "locked knee" in clinical practice, interference with normal knee kinematics is non-specific with regard to the diagnosis. Emphasis is therefore given to less frequently seen abnormalities that lead to a mechanical block of knee extension.