A tribological study of UHMWPE acetabular cups and polyurethane compliant layer acetabular cups

Abstract

A novel design of polyurethane compliant layer acetabular cup has been developed through a series of friction, creep and wear tests. Friction tests were initially conducted on ABG standard form, polyurethane acetabular cups and an ABG standard form, UHMWPE acetabular cup for comparison. The polyurethane cups showed lower friction than the UHMWPE cup with maximum friction factors between 0.008 and 0.02 compared with 0.035 for the UHMWPE cup. This indicated that, in the polyurethane cups, more of the load across the joint was carried by the fluid entrapped in the joint space rather than with asperity contact, compared with the UHMWPE cup. The inherent compliance of the polyurethane is used to promote elasto-hydrodynamic lubrication. However, this compliance raised concerns over excessive creep, which may in turn adversely affect tribological performance. Therefore, creep tests were undertaken on the ABG standard form, polyurethane acetabular cups followed by further friction tests. Small amounts of creep occurred in the polyurethane cups at ambient temperature, which reduced the friction slightly (maximum friction factors of 0.009) due to increased conformity between the head and the cup. However, at 37°C, greater creep occurred causing pinching of the femoral head by the acetabular cup resulting in lubricant starvation and higher friction (maximum friction factors of 0.035). The design of the polyurethane cups was subsequently modified to incorporate a flared rim to eliminate the possibility of fluid starvation through pinching. Creep in polyurethane acetabular cups is also affected by the method of fixation of the cups, due to the conformity with and the stiffness of the cup backing. Hence, a one-million-cycle wear test was performed on five ABG flared form, polyurethane acetabular cups on the Mk. I Durham Hip Joint Wear Simulator to evaluate the best method of fixation for the polyurethane cups. The smallest amount of penetration, due to creep and wear, was found with cement fixation (0.30 mm penetration with cement fixation, 0.44 mm with polyethylene holder mounting, and 0.52 mm with metal shell mounting). A 4.25-million-cycle wear test was then conducted on a further five ABG flared form, polyurethane acetabular cups with cement fixation. Five ABG standard form, UHMWPE acetabular cups were also wear-tested to 5.0-million cycles. The mean and standard error of the wear rate for the polyurethane cups were 14.1 ± 4.3 mg/106 (12.0 ± 3.6 mm3/106), cycles compared with 44.8 ± 3.4 mg/106 (48.2 ± 3.7 mm3/106), cycles for the UHMWPE cups. This study showed that the novel polyurethane-compliant layer acetabular cup with cement fixation was tribologically superior to the ABG standard form UHMWPE design currently being used clinically. © 2000 John Wiley & Sons, Inc. Biomed Mater Res (Appl Biomater) 53: 710–716, 2000