Tibial Tuberosity Realignment Alters in Vivo Patellar Tracking

OBJECTIVES: For patients with recurrent patellofemoral instability, tibial tuberosity medialization is commonly performed to reduce the lateral force applied to the patella by the patellar tendon. Medialization also alters the force acting on the tibia, which could alter tibiofemoral kinematics. Several other procedures are advocated to treat patellar instability, some of which are combined with medialization. The current study uses computational reconstruction of in vivo function to evaluate the influence of tibial tuberosity medialization on patellofemoral and tibiofemoral motion. METHODS: Six patients preparing to undergo tuberosity medialization for recurrent instability were evaluated pre-operatively and one year post-operatively. Procedures performed in combination with medialization included tuberosity anteriorization in five patients, medial patellofemoral ligament (MPFL) reconstruction in five patients, tuberosity distalization in two patients, and lateral retinacular release in two patients. Each patient performed a knee extension exercise without external resistance while lying on the bed of a dynamic CT scanner (Aquilion ONE scanner, Toshiba Medical Systems). Models of the femur, tibia and patella were reconstructed from 5 or 6 volumes of 320 axial images separated by 0.5 mm spanning the extension range. Local coordinate systems were created for each pre-operative bone in the most extended position based on anatomical landmarks. Every other reconstruction of the knee was aligned to the most extended knee by shape-matching the distal femurs. The patella and tibia with the embedded reference axes were copied and individually shape-matched to replace all other pre-operative and post-operative bones. Tibiofemoral and patellofemoral kinematics were quantified at each position of knee flexion. The patellar lateral shift and tilt and the external rotation of the tibia were interpolated to 5°, 10°, 20°, 30°, and 40° of flexion and compared between the pre-operative and post-operative conditions at each flexion angle with a paired t-test. RESULTS: Surgical realignment altered patellofemoral (Fig. 1) and tibiofemoral kinematics. Realignment decreased the average patellar lateral shift and tilt by more than 3 mm and 3°, respectively, at all flexion angles, with the change greatest at the lowest flexion angles (Table 1). Both changes were significant at 5° and 10° of flexion. The average tibial external rotation increased by approximately 2° following surgery, with the change most consistent with the knee flexed. The change was significant at 30° and 40° of flexion. CONCLUSION: The study indicates that tuberosity medialization reduces patellar lateral shift and tilt in patients with recurrent instability. The change is greatest when the patella is superior to the trochlear groove near full extension. The altered kinematics should reduce the risk of instability. The procedures performed in addition to medialization complicate the analysis. Simultaneous MPFL reconstruction performed for five knees could have contributed to altered tracking, although the MPFL was reconstructed to act as only a checkrein against dislocation. Increased tibial external rotation due to the change in orientation of the patellar tendon is an unintended consequence of tuberosity medialization. The long term influence of this change on the tibiofemoral joint is currently unknown.