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

A novel design of polyurethane compliant layer acetabular cup has been deve loped through a series of friction, creep and wear tests. Friction tests we re initially conducted on ABG standard form, polyurethane acetabular cups a nd an ABG standard form, UHMWPE acetabular cup for comparison. The polyuret hane cups showed lower friction than the UHMWPE cup with maximum friction f actors 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 asp erity 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 adv ersely affect tribological performance. Therefore, creep tests were underta ken on the ABG standard form, polyurethane acetabular cups followed by furt her friction tests. Small amounts of creep occurred in the polyurethane cup s at ambient temperature, which reduced the friction slightly (maximum fric tion factors of 0.009) due to increased conformity between the head and the cup. However, at 37 degreesC, greater creep occurred causing pinching of t he femoral head by the acetabular cup resulting in lubricant starvation and higher friction (maximum friction factors of 0.035). The design of the pol yurethane cups was subsequently modified to incorporate a Bared rim to elim inate the possibility of fluid starvation through pinching. Creep in polyur ethane acetabular cups is also affected by the method of fixation of the cu ps, due to the conformity with and the stiffness of the cup backing. Hence, a one-million-cycle wear test was performed on five ABG hared form, polyur ethane acetabular cups on the Mk. I Durham Hip Joint Wear Simulator to eval uate the best method of fixation for the polyurethane cups. The smallest am ount 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 wea r test was then conducted on a further five ABG flared form, polyurethane a cetabular cups with cement fixation. Five ABG standard form, UHMWPE acetabu lar 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/10(6 ) (12.0 +/- 3.6 mm(3)/10(6)), cycles compared with 44.8 +/- 3.4 mg/10(6) (4 8.2 +/- 3.7 mm(3)/10(6)), cycles for the UHMWPE cups. This study showed tha t the novel polyurethane-compliant layer acetabular cup with cement fixatio n was tribologically superior to the ABG standard form UHMWPE design curren tly being used clinically. (C) 2000 John Wiley & Sons, Inc.