The effect of the design of the femoral component on the conformity of the patellofemoral joint in total knee replacement.

One of the most controversial issues in total knee replacement is whether or not to resurface the patella. In order to determine the effects of different designs of femoral component on the conformity of the patellofemoral joint, five different knee prostheses were investigated. These were Low Contact Stress, the Miller-Galante II, the NexGen, the Porous-Coated Anatomic, and the Total Condylar prostheses. Three-dimensional models of the prostheses and a native patella were developed and assessed by computer. The conformity of the curvature of the five different prosthetic femoral components to their corresponding patellar implants and to the native patella at different angles of flexion was assessed by measuring the angles of intersection of tangential lines. The Total Condylar prosthesis had the lowest conformity with the native patella (mean 8.58 degrees ; 0.14 degrees to 29.9 degrees ) and with its own patellar component (mean 11.36 degrees ; 0.55 degrees to 39.19 degrees ). In the other four prostheses, the conformity was better (mean 2.25 degrees ; 0.02 degrees to 10.52 degrees ) when articulated with the corresponding patellar component. The Porous-Coated Anatomic femoral component showed better conformity (mean 6.51 degrees ; 0.07 degrees to 9.89 degrees ) than the Miller-Galante II prosthesis (mean 11.20 degrees ; 5.80 degrees to 16.72 degrees ) when tested with the native patella. Although the Nexgen prosthesis had less conformity with the native patella at a low angle of flexion, this improved at mid (mean 3.57 degrees ; 1.40 degrees to 4.56 degrees ) or high angles of flexion (mean 4.54 degrees ; 0.91 degrees to 9.39 degrees ), respectively. The Low Contact Stress femoral component had the best conformity with the native patella (mean 2.39 degrees ; 0.04 degrees to 4.56 degrees ). There was no significant difference (p > 0.208) between the conformity when tested with the native patella or its own patellar component at any angle of flexion. The geometry of the anterior flange of a femoral component affects the conformity of the patellofemoral joint when articulating with the native patella. A more anatomical design of femoral component is preferable if the surgeon decides not to resurface the patella at the time of operation.