Results of nonmetal-backed, high-density polyethylene, biconvex patellar prostheses. A 5-7-year follow-up evaluation.

This study evaluates the clinical and radiographic results of an all-polyethylene, biconvex, dome-shaped patellar prosthesis with precise instrumentation implanted in 53 knees with a minimum follow-up period of 5 years (average, 6.3 years; range, 5.0-7.1 years). The mean patient age was 70.9 years (range, 18.0-89.0 years). The mean Hospital for Special Surgery knee rating score was 63.6 before surgery and 83.4 after surgery. There was no fracture of the patella, no implant failure, or radiographic loosening of the prosthesis. Patellar complications consisted of two dislocations secondary to trauma and one case of patellar subluxation. Several radiographic parameters were measured. Means and SDs were computed for: (1) patellar tilt, as measured from a line between the anterior limits of the femoral condyles and the patella, which showed no significant difference after surgery (3.01 degrees +/- 5.12 degrees) compared to before surgery (3.73 degrees +/- 5.44 degrees); (2) the angle between the patellar component and the residual bone was -0.04 degrees +/- 2.04 degrees, with every case in the normal range (+/- 5 degrees); (3) there was no significant difference between pre- and postoperative patellar length, patellar thickness, or articular length of the patella; (4) the patellar height showed a small but statistically significant difference after surgery (2.69 +/- .64 cm) and before surgery (2.94 +/- .72 cm); (5) the distance from the tibial tubercle to the joint line did not differ significantly between preoperative (2.73 +/- 0.34 cm) and postoperative (3.06 +/- 0.36 cm) measurements; and (6) the distance from the center of the tibial plateau to the center line of the tibial prosthesis was 1.34 +/- 0.32 cm. These results are superior to previously reported series.