Comparison of the lateral release rates in the press fit condylar prosthesis and the PFC Sigma prosthesis

There are many different total knee replacements on the market. Most have undergone design changes since they were introduced. The manufacturers claim these changes are necessary to improve outcomes of their products. Johnson and Johnson changed their press fit condylar (PFC) knee to PFC Sigma. One of the reasons for the change was to improve patella tracking by combining the deeply radiused trochlear groove of the PFC cruciate sacrificing knee with the natural asymmetrical valgus angle of the PFC cruciate retaining knee. In 1995 we started a prospective audit of our knee replacements. We analysed our early outcomes to test this theory. Group 1 consisted of 468 patients who underwent 543 PFC replacements. Group 2 consisted of 359 patients who underwent 423 Sigma knee replacements. An audit nurse saw all the patients preoperatively and an American Knee Society Score was done. Operative details were recorded. Patients were followed up by an audit nurse at a dedicated knee clinic. The same surgeons using the same instrumentation carried out the operations. The change in design reduced the lateral release rate by 50% from 32 to 16%. The complications were similar in both groups and there was no difference in the outcomes at 6 months. The change in design from the PFC to the Sigma knee has had an effect in reducing the lateral release rate, without affecting the early outcomes. We plan to continue this audit to see if the change leads to less patellar problems in the future.