Pathomechanics of the pivot shift maneuver

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Abstract
This paper analyzes changes in instant centers of motion and joint surface velocity vectors in knees demonstrating a positive pivot shift test. Eight patients were studied on the basis of a sus pected tear of the anterior cruciate ligament. Clinical evaluation consisted of a history and examination with special attention to the anterior drawer test, Lach mann's test, and the pivot shift test. Control knees chosen had a negative history of injury and a normal examination. Serial roentgenograms were obtained on all knees through a range of motion from 0 to 90° of flexion, at increments of 7 to 10°. Internal rotation and valgus stress were applied to duplicate the pivot shift maneu ver during the filming. These serial studies were used to determine instant centers of motion, modifying the method of Rouleaux. Arthrography and arthoscopy were performed to document the pathologic anatomy. Alterations in instant center progression were noted in all patients with a clinically positive pivot shift as compared with the control group and a general pattern of progression was observed. From 0 to 20° of flexion the instant centers were located too near the joint line and displaced anteriorly. During the shift phase (from 20 to 45°) the instant centers were located posteriorly and proximally on the femur. These findings indicate that from 0 to 20° rolling predominated followed by a sudden change to gliding as the shift occurred. In a few cases the initial instant centers were actually located below the joint line, indicating a backwards glide phenomenon. From 50 to 90° the instant center progression was near normal. Surface velocity vectors were likewise abnormal in patients with pivot shift. During the initial phase of flexion the velocity vector was not tangential to the joint surface, indicating excessive friction and com pressive forces were present during knee motion. These findings demonstrate a possible mechanism for the development of posttraumatic arthritis of the tibial- femoral joint in patients with a positive pivot shift test. A quantitative analysis.of the pivot shift maneuver is shown which aids in understanding the pathomechan ics of the pivot shift maneuver. The method is also suggested for long-term evalu ation of anterior cruciate reconstructive procedures.

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