On the biomechanics of the patellofemoral joint and patellar resurfacing in total knee arthroplasty

Abstract

The physiologic forces are essential for proper functioning and longevity of the patellofemoral joint. However, the abnormal forces in the patellofemoral joint are thought to have a strong correlation with patellar disorders in both the intact knees and the knees with total knee arthroplasty. Aims of this thesis were to develop biomechanical testing methods and devices to quantitatively assess the structural integrity of the patellofemoral joint as well as specific parameters for patellar resurfacing in total knee arthroplasty. To quantify the patellofemoral joint kinematics, contact areas, and contact pressures, custom patellofemoral joint testing jigs, a continuous video digitizing system and a three dimensional magnetic tracking system and a custom software for the Fuji pressure sensitive film were developed. Six biomechanical studies were performed. The first study showed that the increase in degree of fixed femur rotation resulted in a nonlinear increase in patellofemoral contact pressures on the contra-lateral facets of the patella in seven human cadaver knees. For total knee arthroplasty studies, anatomically based patellar resection criteria was first determined. This patellar resection criteria yielded a consistent and ideal resection for dome shaped patellar prosthesis in 36 patellae. Thereafter, using ten human cadaver knees, the effects of total knee arthroplasty was quantified. The results showed a significant decrease in the patellofemoral joint contact areas while the patellofemoral joint contact pressures increased well beyond the yield strength of ultra high molecular weight polyethylene following total knee arthroplasty. No statistically significant differences between preoperative and postoperative specimens were observed with respect to the patellofemoral joint kinematics. The effects of patellar component positioning were then determined in five human cadaver knees with total knee arthroplasty. The findings showed the central positioning of the patellar component resulted in the most optimal patellofemoral mechanics for dome shaped patellar prosthesis. The patellofemoral joint testing jig was then improved to simulate individual muscles of the quadriceps. We then determined the effects of anatomically based loading of the patellofemoral joint versus the traditional axial loading approach where a subtle yet significant differences in patellofemoral joint mechanics were quantified. Using this model, we demonstrated excessive edge loading of patellar components at higher knee flexion angles in two contemporary total knee arthroplasty systems with anatomic patellofemoral joints.Abnormal distribution of stresses resulting from improper kinematics of the patellofemoral joint has a strong correlation with patellar disorders in both the intact knees and the knees with total knee arthroplasty. Therefore, the greatest deficit in current knowledge of the patellofemoral joint biomechanics is still the understanding of the structure, function and the forces involved in the patellofemoral joint and its interactions with the extensor mechanism as well as the intra-articular patellofemoral joint contact pressures.