Wear of polyethylene acetabular components is an important issue in total hip arthroplasty. This study was designed to evaluate differences in polyethylene wear rates between machined and direct compression molded acetabular cups.

Two hundred thirty-nine prostheses underwent radiographic evaluation using the technique of Chevrot-Kerboull. One hundred thirty-one were all-polyethylene cups machined from extruded bar stock, and one hundred eight were all-polyethylene cups direct compression molded. Both groups, all operated on in 1988, were similar in the acetabular and femoral components, were all cemented, and the acetabular components were all-polyethylene, non metal backed. The femoral components were all Charnley-Kerboull MK III type. Patient weight, average weight, the post-operative PMA score and the duration of the follow-up were similar for the two groups (mean 7.4 years).

Results showed a mean linear wear rate of 0.06 mm per year for compression-molded polyethylene and 0.08 mm per year for machined polyethylene. Results were not significantly different. The number of acetabular radiolucencies in zone I (5.5 versus 5.3 p. 100) and the amount of lysis of the proximal part of the femur did not differ between the two groups. The number of excessive wears did not differ either.

Although polyethylene wear was lower with compression molded acetabular cups than with ram-extruded acetabular cups, results of this study suggest a non significant difference between the two types of components. A more controlled experiment would have to be performed to attribute a difference between two different types of UHMPE processing.

Les auteurs comparent l'usure du polyéthylène dans deux séries de malades opérés en 1988 par prothèse totale avec 131 fois une cupule usinée et 108 fois une cupule moulée. Les prothèses étaient toutes du type Charnley-Kerboull avec couple acier inoxydable - polyéthylène et tête de 22,2 mm. La technique de stérilisation consistait en une irradiation de 2,5 méga-rads, par rayonnement bêta sous air. La durée d'observation moyenne a été de 7,3 ans. La mesure de l'usure a été faite de façon graphique en comparant la position du centre de la tête fémorale et du centre initial de la cupule. Les résultats montrent une vitesse d'usure linéaire de 0,06 mm par an pour le polyéthylène moulé, et 0,08 mm par an pour le polyéthylène usiné. La différence n'était pas statistiquement significative. Dans les deux groupes, il existait des usures anormales suggérant d'autres facteurs que les modalités de fabrication des cupules. La tendance à la moindre usure des implants en polyéthylène moulé demande à être confirmée par une étude plus large avec un délai d'observation plus long.