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Anisometry of Medial Patellofemoral Ligament Reconstruction in the Setting of Patella Alta and Increased Tibial Tubercle-Trochlear Groove (TT-TG) Distance

      Introduction

      To assess the effect of increased lateralization and proximalization of the tibial tubercle (TT) on isometry of the reconstructed medial patellofemoral ligament.

      Methods

      Ten fresh-frozen cadaveric knees were placed on a custom testing fixture. A tunnel was drilled under fluoroscopic guidance from Schottle’s point through the lateral femoral cortex. A suture anchor was placed at the upper 41% of the medial border of the patella and the sutures were shuttled through to the lateral side and attached to a pulley with a 1N weight. Retroreflective markers were attached to the suture and MPFL length change, as measured by suture marker motion, was assessed using a 3D motion capture system through a range of motion between 0deg and 110deg with the native TT anatomy. Recordings were repeated after a flat TT osteotomy and transfer to TT-TGs of 20mm and 25mm and Caton Deschamps (C/D) ratios of 1.2 and 1.4, including all combinations. Generalized estimating equation (GEE) modeling technique was used to analyze and control for the clustered nature of the data. SAS version 9.3 was used for all data analyses.

      Results

      Analysis was performed on 9 specimens secondary to significant deviations in the baseline normative data. Intact knees showed MPFL isometry through 20-70 degrees range of motion. Tibial tubercle lateralization significantly altered MPFL isometry with a threshold TT-TG of 25mm (p=0.045). Patella alta significantly altered MPFL isometry with a threshold C/D of 1.4 (p=0.025). The effect of TT lateralization combined with patella alta compounded the anisometry, lowering the threshold for patella alta to a C/D of 1.2 when combined with a TT-TG of 25mm (P<0.001).

      Conclusion

      An isolated MPFL reconstruction may be prone to failure in the setting of patella alta and/or elevated TT-TG given the anisometry demonstrated. Consider a tibial tubercle transfer in these patients.