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

OBJECTIVES: Medial patellofemoral ligament (MPFL) reconstruction is a common procedure to treat recurrent patellofemoral instability. However, the effects of an elevated tibial tubercle-trochlear groove (TT-TG) distance and patella alta, as measured by the Caton-Deschamps (C/D) ratio, on MPFL isometry remain unclear. We hypothesized that increased lateralization and proximalization of the tibial tubercle (TT) will have increasingly adverse effects on the isometry of the MPFL. METHODS: Ten fresh-frozen cadaveric knees were placed on a custom testing fixture, with a fixed femur and tibia mobile through 120 degrees of flexion. The quadriceps tendon was loaded with 10.8 N in an anatomic direction using a weighted pulley system. A 0.2 N patellar lateral displacement load was used to simulate an intact lateral retinaculum to avoid over-medializing the patella. A tunnel was drilled under fluoroscopic guidance from Schottle’s point on the medial distal femur through the lateral cortex. A suture anchor was placed at the upper 66% of the medial border of the patella and the sutures were shuttled through to the lateral side and attached to a pulley with a 1 N weight. Retroreflective markers were attached to the femur, tibia, patella, and suture. MPFL length change, as measured by suture marker motion, was assessed using a 3D motion capture system through a range of motion between 0 deg and 110 deg with the native TT anatomy. Recordings were repeated after a flat TT osteotomy and transfer to a TT-TG of 20 mm and 25 mm and a C/D ratio 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 (SAS Inc., Cary, NC) 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 25 mm (p=0.045) (Figure 1). Patella alta significantly altered MPFL isometry with a threshold C/D of 1.4 (p=0.025) (Figure 2). 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 25 mm (P<0.001) (Figure 3). CONCLUSION: Increased TT lateralization and proximalization significantly alter MPFL isometry. Tibial tubercle transfer should be considered when performing an MPFL reconstruction for recurrent patellofemoral instability in the setting of significant patella alta, elevated TT-TG and especially when both are present as an isolated MPFL reconstruction may be prone to failure given the anisometry demonstrated.