Wear pattern of retrieved patellar implants.

The so-called "papillon" pattern of polyethylene wear of 17 patellar components from 5 Kinematic and 12 Total Condylar prostheses, retrieved after an average of 80 months in situ, was studied. The primary diagnosis was osteoarthritis in all cases. Four modes of damage were observed: polishing in 13 cases, delamination in 12, cold flow in 6 and scratching in 3. The median total area of polyethylene damage was 76.5% for polishing, 70.6% for delamination, 35.3% for cold flow and 17.6% for scratching. The importance of the conformity of the Kinematic patellar component in order to decrease contact stresses was confirmed. The average contact stresses on the nonconforming total Condylar patellar component (12.9 kgf/mm2), were significantly higher (p < 0.002) than the average contact stresses on the conforming Kinematic patellar component (2.9 kgf/mm2). The area of wear was smaller for the nonconforming Total Condylar (357.2 mm2) than for the conforming Kinematic patella (439.2 mm2). This difference, however, is not statistically significant. The average weight of the patients with a Kinematic Knee (74.5 kg) was higher than that of patients with a Total Condylar knee (66 kg), but the difference was not significant. The high incidence of significant wear of the patellar components indicates that a basic deficiency is present in the design of patellar implants, and calls for the improvement of two mechanical features: adequate thickness of the polyethylene implant and conforming articulating surfaces.