Results of total knee arthroplasty with a metal-backed patellar component. A 6-year follow-up study

This study reviews the clinical outcome of knee arthroplasty (Microloc, Johnson & Johnson Orthopedics, Norderstedt, Germany) with a resurfacing metal-backed patellar component. Thirty patients were available for clinical and radiographic reexamination with follow-up periods of 36 and 72 months. There were 21 women and 9 men, with an average age of 71.1 years. At the follow-up examination, seven patients demonstrated a metallic friction noise, and three had only mild crepitation at the patellofemoral joint level. In the group complaining of knee pain, the average knee score dropped from 76.9 +/- 9.9 at 36 months to 44.7 +/- 15.3 (P < .05) prior to revision surgery on average at 66 months. The function score dropped during the same time from 72.0 +/- 19.7 to 53.5 +/- 14.9. In contrast, the pain-free group did not demonstrate any significant changes in either knee or function score. Radiographic evaluation showed a patellar tilt of 8.1 degrees +/- 4.0 degrees in the group complaining of pain, compared with 2.5 degrees +/- 2.3 degrees in the pain-free patient collective. All 10 patients complaining of pain and increasing disability underwent revision surgery. At the time of surgery, an obvious polyethylene particle-induced synovitis was found, and in 7 patients it was associated with a metallosis. One of the main reasons for failure of metal-backed polyethylene components is thought to be the undue high stresses forced onto the insert. High loading of only a small fraction of the surface will lead to increased polyethylene creep and particle formation, resulting in synovitis and finally metallosis. Early revision seems to be the best solution to prevent progressive destruction of the entire joint. Previously reported results on metal-backed patellar components show a failure rate of 8.4% after 12 to 24 months. In this study, this rate had already increased to 33.3% after 6 years. One might therefore speculate that at 10 years, revision surgery might become necessary in more than 50% of the surviving patients with this type of implant.