Summary

The current study was performed to characterize MPFL graft isometry and evaluate anatomical factors that contribute to graft loading for symptomatic knees during function.

Abstract

Numerous biomechanical studies have evaluated graft isometry and loading following MPFL reconstruction for patellar instability. These studies have generally been performed with normal knees and passive knee motion. The current study was performed to characterize MPFL graft isometry and evaluate anatomical factors that contribute to graft loading for symptomatic knees during function. Dynamic simulation of knee function was performed with 12 models representing knees being treated for recurrent patellar instability. The models were individually validated for patellar tracking vs. motions performed by the patients within a diagnostic scanner. Each model was analyzed during a squatting motion with no MPFL graft and a graft attached at the Schoëttle point on the femur and the medial edge of the patella, with 1 and 0.5 quadrants of patellar lateral translation allowed during graft fixation. Applied quadriceps forces ramped to a 300 N at 90º of flexion. Measures of anatomy (Caton-Deschamps index, lateral tibial tuberosity to posterior cruciate ligament attachment distance, lateral trochlear inclination) and patellar tracking (bisect offset index) were quantified during squatting. Linear regression was used to relate pre-operative anatomy to maximum graft force. Graft forces were greatest at low flexion angles, approaching 0 N by 30° of flexion. The primary parameter correlated with maximum graft force was the maximum lateral tibial tuberosity to posterior cruciate ligament distance (p < 0.001, r2 > 0.79 for both graft tensioning protocols, Fig. 1). Based on the current analysis, MPFL grafts tend to carry minimal load beyond 30° of knee flexion during functional activities. The primary pre-operative anatomical parameter that determines the maximum force carried by the graft is the most lateral position of the tibial tuberosity during function. These findings suggest that tibial tuberosity medialization should be considered in patients with an elevated TT-PCL distance to decrease the risk of excessive MPFL graft tension.