DIFFERENCES IN PATELLAR TRACKING AND KNEE KINEMATICS AMONG 3 DIFFERENT TOTAL KNEE DESIGNS

Patellar complications are the primary reason for reoperation of the c urrent condylar type designs. The aim of this study was to compare pat ellar tracking of various knee implant designs: Genesis II NexGen, and the P.F.C. Sigma Modular Knee System regarding trochlear groove cente r curvature. Nine unembalmed whole cadaveric lower extremities were us ed. The quadriceps and hamstrings were dissected into their individual muscles and loads were applied onto the muscles proximally based on t he cross sectional area of the muscles. The three-dimensional kinemati cs of the patellofemoral and tibiofemoral joint of the intact knee wer e measured using a 3Space tracking system. Three implants (one from ea ch company) were implanted onto the same cadaveric knee in random orde r consecutively. This was done to ensure consistency of the soft tissu e constraints in influencing the amount of patellar tracking. Patellar rotation, patellar tilting, patellar lateral shift and patellar displ acement in relation to groove center were measured. There was no signi ficant difference between the intact knee and the various implants reg arding patellar rotation and lateral shift. However, all three prosthe tic designs showed a significant degree of lateral tilting when compar ed with the intact knee. At 60 degrees knee flexion, the normal patell a was tilted laterally to 0.44 degrees +/- 2.15 degrees as compared wi th the Genesis II patella at 4.75 degrees +/- 4.81 degrees, the NexGen patella at 4.85 degrees +/- 4.81 degrees, and the P.F.C. Sigma patell a at 4.89 degrees +/- 3.79 degrees lateral tilt. There was no differen ce between the intact knee compared with the resurfaced patella in pat ellar displacement in relation to the groove center. This study sugges ts the relatively similar kinematic behavior between the implant desig ns as compared with the intact knees. However, additional modification of implant geometry may be required to help decrease the amount of pa tellar tilt.