Abstract
Sub-micron polyethylene wear particles have been identified as a cause of osteolysis frequently found in the bone surrounding total hip replacements (THR). However, the wear of the hard femoral components is much less understood and is often assumed to be negligible; yet, metal particulate and ionic debris are of rising clinical concern. This study investigates not only the wear rates of ultra high molecular weight polyethylene (UHMWPE) acetabular liners, but also the wear rates of metallic femoral heads in several THR designs and sizes, which until now have usually been ignored in this type of wear study.
Conventional UHMWPE liners (three 40mm, three 44mm I.D.), highly cross-linked (HXL) UHMWPE liners (three 40mm, three 44mm I.D.), and HXL UHMWPE liners with vitamin E blended (four 36mm and six 40mm I.D.) were tested against CoCrMo femoral heads, appropriately sized and matched to the particular THR design, on a 12 station hip simulator (AMTI, Boston). The specimens were mounted in a physiologically correct manner on custom made fixtures, lubricated with bovine serum (20g/L protein, 37°C) and subjected to the walking cycle specified in ISO-14242-1 at 1Hz for 5 million cycles (Mc). The femoral heads and acetabular liners were carefully cleaned and gravimetrically weighed at standard intervals, and the wear was corrected with the weight gain of active load soak control heads and liners, and calibration weights.
The conventional UHMWPE liners showed the highest wear (40mm: 55.7±3.00mg/Mc, 44mm: 72.0±2.81mg/Mc) while HXL liners displayed much lower wear (40mm: 2.58±0.97mg/Mc, 44mm: 14.2±3.57mg/Mc) as expected. Vitamin E liners also showed very low wear (36mm: 20.1±2.00mg/Mc, 40mm: 5.97±0.50mg/Mc). Interestingly however, the CoCr femoral heads also showed measurable wear for all liner types and designs (Conv. 40mm: 0.28±0.16mm3/Mc, 44mm: 0.22±0.014mm3/Mc, HXL 40mm: 0.041±0.0060mm3/Mc, 44mm: 0.21±0.0024mm3/Mc, Vit-E 36mm: 0.029±0.0097mm3/Mc, 40mm: 0.064±0.019mm3/Mc). Heads in a previously reported 44mm metal-on-metal test [1] showed burnishing and scratching (0.22±0.022 mm3/Mc, liners: 0.16±0.013 mm3/Mc). The burnishing of the metal femoral heads from all tests (including the MOM test) can be seen in Fig. 1 [Fig. 1 here]. An example showing the circular scratching patterns seen on nearly all femoral heads is shown in Fig. 2, of a 40mm femoral head that was paired with a HXL vitamin E liner [Fig. 2 here].
Our simulator results confirm low wear for HXL UHMWPE acetabular liners both with and without vitamin E. Wear of metal femoral heads, although much less in weight than liner wear, was still clearly detectable and measurable for CoCr heads articulating against all types of UHMWPE liners. Therefore, in wear studies focusing on hard-on-soft material couples such as MOP, the metal head wear should not be ignored.
