Abstract
BACKGROUND: Improved patient outcomes and decreased patellar instability have been reported after medial patellofemoral ligament (MPFL) reconstruction for recurrent lateral patellar dislocation; however, there is a lack of comparative evidence on functional outcomes associated with different femoral attachment sites for the MPFL graft. PURPOSE: To identify differences in MPFL reconstruction graft isometry with femoral tunnel malpositioning, specifically evaluating isometric differences as the femoral position is moved anterior, posterior, proximal, and distal relative to the Schöttle point, the femoral radiographic landmark of the MPFL. STUDY DESIGN: Descriptive laboratory study. METHODS: A biomechanical study evaluating 11 fresh-frozen cadaveric knees was conducted. Nonelastic suture, used as an analog to the MPFL graft, was anchored with the knee at 30° flexion at the Schöttle point and at 5 and 10 mm anterior, posterior, superior, and distal to the Schöttle point. A draw wire displacement sensor was used to evaluate length changes of the MPFL graft analog through 0° to 120° knee flexion. Knee flexion position was continuously measured using a motion tracking system. Pairwise t tests with Bonferroni correction were used to compare isometry between the Schöttle point and the nonanatomic femoral insertion points. RESULTS: Grafts placed at the Schöttle point proved mildly anisometric, with tightening in extension and loosening in flexion. Similarly, grafts placed distally and posteriorly also demonstrated tightening in extension and loosening in flexion. Grafts placed anteriorly and proximally demonstrated tightening in flexion. Pairwise comparisons relative to the Schöttle point found that grafts placed proximally or distally demonstrated significant differences in total MPFL excursion magnitude (10 mm proximal: 0.36 [P = .03], 5 mm distal: 0.14 [P = .01], 10 mm distal: 0.22 [P < .001]). CONCLUSION: When deviating from the Schöttle point, posterior and distal femoral tunnel positionings minimized the risk of MPFL graft tightening during knee flexion. Errant anterior and proximal positioning were concerning for MPFL overconstraint, and proximal tunnel placement was most at-risk. CLINICAL RELEVANCE: An understanding of the effects that femoral tunnel malpositioning has on graft isometry is crucial to minimizing instability or overconstraint, which leads to anterior knee pain, increased patellofemoral contact pressures, or graft failure.