Extension block secondary to partial anterior cruciate ligament tear on the femoral attachment of the posterolateral bundle.

PURPOSE: The purpose of this investigation was to prove that a partial tear of the anterior cruciate ligament (ACL) at the femoral attachment of the posterolateral bundle can result in mechanical knee locking and trigger the injury mechanism of an isolated ACL injury. TYPE OF STUDY: Case series.
METHODS: From February 1993 through June 1999, 19 cases of knee locking with observation of a torn ACL, confirmed under arthroscopy, were investigated. Patients' ages ranged from 22 to 54 years. The causes of injury were sporting activities in 14 patients (8 skiing, 4 soccer, and 2 badminton); slipping and falling in 2 cases; a pedestrian car accident in 2 cases; and an unknown low-velocity injury in 1 case.
RESULTS: On physical examination, each patient indicated that the chief complaint was knee pain; all had a locked knee at 5 degree to 20 degree of full extension and 4 cases showed a locked knee at full flexion. Of the 19 cases, Lachman testing was positive in 3. With respect to intra-articular injuries, 3 patients had a tear in the medial meniscus and 1 had a tear in the lateral meniscus. The average time span between arthroscopy examinations was 3 months with a range of 1 to 10 months. Arthroscopic diagnosis and treatment were performed and meniscal tear was ruled out as the cause of locking in all patients.
CONCLUSIONS: The mechanisms of injury for isolated ACL were knee hyperextension and internal rotation of lower extremity brought on especially by a low-velocity injury. The partial ACL tears were found on the femoral attachment site of the posterolateral bundle. The torn segment of the ACL was interposed between the lateral femoral condyle and the lateral tibial condyle and acted as a mechanical obstruction, giving rise to the locking symptom. In all 19 cases, the torn portion of the ACL was excised under arthroscopy and the locked knees were treated properly.