Background

Patellar dislocation is one of the most common knee injuries in children and young adults. Recurrent patellofemoral instability often necessitates surgical management, of which there have been over 100 different techniques featuring modification of bony architecture, realignment, and soft tissue balancing that have been described. Improved biomechanical and anatomical understanding has led to the emergence of medial patellofemoral ligament (MPFL) reconstruction as a surgical treatment for recurrent patellar instability, especially in the skeletally immature population. In addition to MPFL reconstruction, release of the lateral retinaculum is a common practice in the management of recurrent patellar instability. Lateral retinacular release is a simple procedure founded on the premise that a tight lateral retinaculum places unnecessarily high strain on the patella.

AIMS
To develop and validate a finite element (FE) model of the patellofemoral joint to analyze the biomechanics of lateral retinacular release after MPFL reconstruction in patellar malalignment (increased TT-TG). We hypothesized that lateral retinacular release is not appropriate in patellar instability addressed by MPFL reconstruction due to decreased lateral stability and inappropriate adjustment in patellofemoral contact pressures.

Methods

A FE in-silico model of the patellofemoral joint was developed and validated. The model was obtained from the Open Knee public domain repository (Simtk.org) from a Magnetic Resonance (MR) study of a cadaveric knee from a 70-year-old female donor (right femur, 167 cm height and, 77 kg mass). The SDLPRT file was converted into an STL file using SolidWorks for analysis in Abaqus/Standard (SIMULIA, Providence, RI). The model was used to analyze the effect of lateral retinacular release in association with MPFL reconstruction on patellofemoral contact pressures, contact area, and lateral patellar displacement during knee flexion.

Results

MPFL reconstruction alone results in restoration of patellofemoral contact pressures throughout the entire range of motion (0 – 90?), mimicking the results from healthy condition. The addition of the lateral retinacular release to the MPFL reconstruction resulted in significant reductions in both patellofemoral contact pressure and contact area. Lateral retinacular release resulted in more lateral patellar displacement during the mid-flexion knee range of motion.

Discussion

The FE model described is optimal to assess the role of lateral release with concomitant MPFL reconstruction under various degrees of knee flexion with respect to patellofemoral contact pressures and patellar tracking kinematics. These biomechanical measurements may predict the effectiveness of this surgical procedure in restoring patellofemoral stability and the potential to develop patellofemoral arthritis due to focal patella overload. The results of this study demonstrate that MPFL reconstruction alone is effective in restoring patellofemoral contact pressures to resemble the healthy knee. Adjunct lateral retinacular release decreases patellofemoral contact pressures and contact areas beyond that of the healthy knee, and allows for more lateral displacement during mid-flexion of the knee. Combination of lateral retinacular release with MPFL reconstruction in patients with increased TT-TG is not recommended as MPFL reconstruction alone for first-line management of recurrent patellar instability offers a greater biomechanical advantage and restoration of contact forces to resemble that of the healthy knee.