An infrequently considered cause of knee locking is incomplete tear of the anterior cruciate ligament. In this series, 19 locked knees were diagnosed at the time of arthrotomy as isolated incomplete tears of the anterior cruciate ligament. No pathological findings associated with the menisci, patella, or collateral ligaments were evident. In each case, the torn portion of the anterior cruciate ligament was demonstrated to be the mechanism for locking, and locking resolved with excision or repair of the torn portion.
Monaco BR, Noble HB, Bachman DC. Incomplete Tears of the Anterior Cruciate Ligament and Knee Locking. JAMA. 1982;247(11):1582-1584. doi:10.1001/jama.1982.03320360032026