An analysis of the pivot shift phenomenon

The description of the pivot shift test and its modifica tions is for the most part based on clinical observations. We wished to precisely determine the knee motions and medial-lateral tibiofemoral compartment subluxa tions that examiners induce in the knee joint to produce the pivot shift phenomenon. Eleven skilled knee sur geons performed the pivot shift test on an instrumented cadaveric lower limb. The anterior cruciate and super ficial medial collateral ligaments (long fibers) of one limb were sectioned to produce an abnormal state. An in strumented spatial linkage allowed all six degrees of freedom motions to be measured. Before and after ligament sectioning we determined the limits of knee motion under defined loading conditions. The tibial and femoral bony landmarks were digitized to determine the positions of the medial and lateral tibial plateaus in reference to the femoral condyles during the pivot shift tests. Each examiner performed his pivot shift test. The analysis of the data showed that examiners typically induced a coupled anterior translation and internal tibial rotation to produce an anterior tibial subluxation, and a coupled posterior translation and external tibial rotation to induce the reduction event. The magnitude of ante rior subluxation of each plateau depended upon the examiner's technique. The maximal anterior subluxa tion of the lateral tibial plateau varied from 14 to 19.8 mm (mean, 17.2 ± 2.0 mm), whereas anterior sublux ation of the medial tibial plateau ranged from 6 to 16.9 mm (mean, 11.2 ± 3.3 mm). This variation was due to the different amounts of internal tibial rotation that an examiner induced in the knee. Increased internal rota tion significantly limited the amount of anterior sublux ation of the medial tibial plateau (P < 0.01). Conversely, increased external tibial rotation (after the reduction event) induced anterior subluxation of the medial tibial plateau. We concluded that more diagnostic information on the magnitude of the anterior knee subluxation in the anterior cruciate deficient knee is available by perform ing the pivot shift test starting with the tibia in external rotation and using a technique that enhances anterior tibial translation and avoids excessive internal tibial rotation. We also concluded that grading of the pivot shift test would vary considerably because of the dif ferences we measured between examiners. We believe there is an unmet need for reliable measurement of medial and lateral tibiofemoral rotational subluxations under specified loading conditions.