Abstract
Sub-micron polyethylene particles produced by the wear of metal on ultra-high molecular weight polyethylene (UHMWPE) in artificial joints have been identified as a principle culprit in the osteolysis frequently found in the bone surrounding these implants. To eliminate UHMWPE debris, highly crosslinked (HXL) UHMWPE and hardon-hard bearing surfaces have been developed. This study compares the wear rates of 14 designs and/or material combinations (total of 48 specimens) tested on a hip simulator in the biomechanics lab at the University of Nebraska Medical Center.
Twelve ceramic-on-metal (COM) (six 36mm and six 28mm, of high and low clearance (HC, LC)), twelve metalon-metal (MOM) (44mm, 3 TiN coated, 3 uncoated standard, and 6 resurfacing components), eighteen metal-on-UHMWPE (MOP) (36mm: six with CoCr-coated heads and six uncoated standard heads with conventional UHMWPE; 44mm: 3 conventional UHMWPE and 3 HXL), and six ceramic-on-UHMWPE (COP) (three 44mm and three 32mm all with conventional UHMWPE) were tested on a multi-station hip simulator (AMTI, Boston). The specimens were lubricated with bovine serum diluted to 20g/l protein concentration at 37°C and were subjected to the loading and rotations of the walking cycle as specified in ISO-14242-1 at 1Hz (for 5 million cycles (Mc) except where specified otherwise). The liners (and heads where specified) were cleaned and weighed at 0, 0.25, 0.5, and every 0.5Mc afterwards.
For 36mm COM liners the wear rates of HC and LC were the lowest observed (−0.019±0.118mg/Mc and −0.061±0.044mg/Mc, respectively). All three 28mm COM HC and one LC liner exhibited “break-away” wear in that they would lose several milligrams (HC: 5.99mg, 6.37mg, 8.50mg, LC: 10.22mg) after showing nearly no measurable wear (HC: 0.905±0.467mg/Mc, 28mm LC: 0.422±0.982mg/Mc). (Note that COM heads weighs were not quoted here but none of them lost weight). TiN-coated MOM THRs (heads and liners) showed higher wear than the uncoated MOM THRs (8.53±4.07mg/Mc, 3.19±0.281mg/Mc, respectively) as the TiN wore away from all three coated heads and liners. The MOM resurfacing components showed wear rates of 2.77±1.27mg/Mc over 2Mc. The 36mm MOP liners (CoCr-coated and uncoated heads) showed wear rates of 55.6±4.26mg/Mc and 44.5±4.46mg/Mc, respectively, as the coating wore away from the metal heads. Wear rates of the 44mm MOP conventional and HXL liners were 72.0±2.81mg/Mc and 14.2±3.57mg/Mc respectively. For COP, the larger size wore at a higher rate than the smaller size (44mm: 97.4±3.08mg/Mc, 32mm: 51.3±12.2mg/Mc) over 2Mc. The 44mm COP THR displayed the highest observed wear rate.
Our simulator results confirm low wear for hard-on-hard bearing couples (MOM, COM) except where coating failure had occurred. Size-36mm LC COM bearings faired the best of the four COM types tested (showing no measurable wear and no “break-away” wear). MOP THRs showed better wear performance when HXL UHMWPE was used, and also showed a sensitivity to femoral head coating removal. COP THRs showed high wear in the large 44mm size, and less in the smaller size. Simulator wear testing was able to successfully discriminate and characterize wear rates of different material bearing couples and different sizes/designs.
Correspondence should be addressed to Diane Przepiorski at ISTA, PO Box 6564, Auburn, CA 95604, USA. Phone: +1 916-454-9884; Fax: +1 916-454-9882; E-mail: ista@pacbell.net
