Patellar prosthesis positioning in total knee arthroplasty. A roentgenographic study.

Patellar prosthesis positioning in 40 primary total knee replacements was evaluated with regard to (1) patellar tilt, (2) angle between the patellar component and patellar bony remnant, (3) lateral versus medial placement, (4) patellar height, and (5) size of the patellar component versus patellar length. Two different designs were evaluated. Twenty knees that were resurfaced with a patellar button prosthesis and implanted with conventional surgical technique constituted Group A. Twenty knees that were resurfaced with a new biconvex prosthesis and implanted with specially designed instrumentation constituted Group B. For Group A, the patellar tilt averaged 4.25 degrees preoperative and 8.35 degrees postoperative. For Group B the patellar tilt averaged 4.60 degrees preoperative and 1.83 degrees postoperative. In Group B, there was significant improvement in patellar position, with only two patients showing values exceeding the neutral range (p = 0.0409). The angle between the patellar component and the bony remnant in Group A averaged 5.975 degrees, and in Group B, 1.447 degrees. Two patients in Group A and 18 patients in Group B had values in the normal range for patellar tilt and the angle between the prosthesis and the bone. The prosthesis was inserted medial to the patellar center in six patients in Group A and one patient in Group B. A significant decrease in patellar height was noted for both groups, but values were still within the normal range. Both groups showed significant increase in the postoperative articular length, indicating that oversized prostheses were inserted and excess bone removed.