Offloading Patellofemoral Contact Pressures: A Biomechanical Comparison Between Tibial Tubercle Osteotomy and a Retropatellar Tendon Implant.

Background: A novel retropatellar tendon implant (LIFT) that is fixed deep to the patellar tendon proximal to the tibial tubercle may provide patellofemoral offloading benefits. Purpose/Hypothesis: The purpose of this study was to analyze the patellofemoral contact pressures from 0° to 60° of knee flexion in 3 states: (1) intact knee, (2) knee with the implantation of a retropatellar tendon implant, and (3) knee with a 45° anteromedialization tibial tubercle osteotomy (TTO). It was hypothesized that the implant would significantly reduce the contact pressure between the patella and trochlea compared with the intact state and produce a similar offloading effect as an anteromedializing TTO. Study Design: Controlled laboratory study. Methods: Trochlear pressure sensors were placed in 7 cadaveric knees (6 donors). Patellofemoral contact area, contact pressure, peak force, and center of force were measured using a model for native quadriceps force on the patella. On each specimen, measurements were taken in an intact state, after implantation of a novel retropatellar tendon implant, and after a 45° anteromedializing TTO. Force data were collected from 0° to 60° of flexion in 15° increments and normalized to each specimen's intact state. Results: Compared with the intact and implant groups, the TTO reduced the contact area at 30°, 45°, and 60° of flexion. TTO reduced contact pressure compared with intact from 15° to 60° of flexion, and reduced contact pressure at 15° when compared with the implant. The implant reduced the contact pressure compared with the intact state at 30° and 45° of flexion. For peak force, TTO reduced the force seen in 15° to 60° of flexion compared with the intact condition, while outperforming the implant at 15° of flexion. The implant reduced the peak force compared with the intact condition at 30° of flexion. Center of force analysis revealed that the implant medialized the center of force at 45° and 60° of flexion. Conclusion: The main finding from our biomechanical cadaveric study was that the novel retropatellar tendon implant did not reduce patellofemoral contact area, contact pressure, and peak force as much as a 45° anteromedializing TTO. The implant does reduce peak and contact pressures in early degrees of knee flexion compared with native states. Clinical Relevance: An implant that allows for offloading patellofemoral contact pressures without the morbidity, complication profile, or postoperative rehabilitation associated with a TTO may aid in the treatment of patellofemoral chondral lesions and retropatellar knee pain.