Medial Patellofemoral Ligament Isometry in the setting of Patella Alta

OBJECTIVES: Patella alta increases the complexity of decision making in regards to surgical intervention for MPFL insufficiency and resultant patellar instability(1). Our objective was to develop a model to evaluate MPFL isometry(2) at multiple candidate femoral attachment sites in the setting of patella alta. We hypothesized that a femoral attachment location proximal to that previously described by Schoettle et al would produce a more anatomometric MPFL in the setting of patella alta. METHODS: Ten cadaveric knees were stripped of soft tissue leaving the patellar tendon, extensor mechanism, and collateral ligaments. A suture anchor was placed at the upper 41%(3) of the medial border of the patella for MPFL reconstruction. The suture emanating from the anchor was utilized as a surrogate MPFL graft. Four separate locations about the medial aspect of the femur were evaluated as candidate attachment sites; that which was described by Schoettle (p00), 5 and 10 mm proximal (p05 & p10) as well as 5 mm distal (d05) to this location (Fig 1a). Change in length of the graft, or anisometry, was assessed by attaching retrorefletive markers on the suture, femur, patella and tibia. A 3D motion capture system was used to track the markers’ motion as the knee was cycled from 0 - 110 degrees of flexion. Three separate degrees of patella alta were then created via tibial tubercle osteotomy, Caton-Deschamps (C-D) 1.3, 1.4 and 1.5. Graft isometry at each candidate attachment site was assessed with tubercle in it’s native position and at each level of C-D. RESULTS: MPFL isometery was defined as an observed change in length less than 2 mm. The p00 location produced an isometric MPFL throughout the initial 70 degrees of flexion when the tubercle was located in the native position only (Fig 1b). With the tibial tubercle in the native location the p00 location was more isometric than the other candidate attachment locations tested (Fig 1c, upper left). In the setting of patella alta with a C-D of 1.3, p05 exhibited more isometry than p00 (Fig 1c, upper right); however no significant differences were found in the average length change between these two location (Table 1, left). With patella alta with a C-D of 1.4, p05 continued to exhibited more isometry than p00 (Fig 1c, lower left) and significant differences were found in the mean change in length at both 40 and 50 degrees, p = 0.007 and 0.034 respectively (Table 1, middle). In the setting of patella alta with a C-D of 1.5, p10 began to exhibit more isometry than p05 (Fig 1c, lower right) and significant differences were found in the mean change in length at 40 degrees, p = 0.023 (Table 1, right). CONCLUSION: We successfully developed a cadaveric biomechanical model for evaluation of MPFL anatomometricity with the ability to investigate multiple femoral attachment locations and varying degrees of patella alta. With the tibial tubercle in the native location, C-D ~ 1, the location defined by Schoettle, p00, demonstrated the greatest degree of isometry. With increasing degrees of patella alta, more proximal candidate femoral attachment sites demonstrate decreased change in length compared to p00. None of the varied femoral attachment produced isometricity over the entirety of the flexion range from 0-70 degrees, suggesting that in cases of significant patella alta, a tibial tubercle distalization may be necessary in order to achieve an anatomometric MPFL reconstruction.