Computer Modeling of the Patellofemoral Joint Instability: Patella Inherent Geometry Influence on Stress and Loading

Despite a large range of surgical options, treatment of patellofemoral instability continues to be a challenging clinical problem. Currently, there have been a number of computer models developed to try and better define the stabilizing forces about the patellofemoral joint. However, most of these studies have focused on patellofemoral contact pressures, omitting evaluation of patellar stability in the first 30° of knee flexion. The purpose of this study was to reconstruct a pathologic and normative patellar shape within a computer model based on previously described morphologic ratios. We then applied physiologic force vectors to each patellar shape and calculated the stress and moment about each. We hypothesized that individuals with a pathologic patellar shape would have internal stress and moments about the patella which could predispose them to instability. We found that there were significant differences in patterns of stress, strain, and internal displacement about the control and study patellar shapes. This study suggests that a change of patellar shape could have significant effects on the stresses and moments about the patella. Future studies in which calculations of moments could be performed on a series of control and study patellar groups could further define this difference. Dynamic simulations of the control and study patellar shapes could also visually demonstrate the difference in patellar translation. In the future, the results of patellofemoral modeling could be applied to individual patients to more accurately detail an individual treatment plan.