Abstract
Introduction
Recent concerns over adverse effects of metal ion release, have led to the development of alternative hip joint replacements. This study reports the performance of new hemispherical MOTIS® (milled pitch-carbon fibre reinforced polyetheretherketone) acetabular cups articulating against Biolox Delta® femoral heads with the aim of producing lower wear and more biologically compatible bearings.
Materials and Methods
The wear performance of 40mm hemispherical MOTIS® cups articulating against Biolox Delta® heads has been investigated. The diametral clearance was 322±15.3nm (mean ± standard deviation). Wear tests were carried out on the Simplified Mark II Durham Hip Wear Simulator to 8 million cycles. New born bovine calf serum was used as the lubricant, diluted to give a protein content of 17g/l. Friction tests were carried out on the unworn joints and worn joints after 7.5 million cycles using lubricants containing protein (bovine serum based carboxymethyl cellulose (CMC) fluids) and without protein (water based CMC fluids). Temperature measured near every hip joint over a continuous wear testing period of 0.5 million cycles was recorded using PICO TC-08 data logger. One K-type thermocouple was placed carefully and consistently in each wear station and two were used to record the ambient room temperature. After stopping the wear test, the data logger continued recording the temperature for a further ten hours to indicate the cooling period. Additionally surface analyses were undertaken before and after wear testing using a non-contacting profilometer and atomic force (AFM) microscope.
Results and Discussion
Throughout the wear testing, different degrees of fluid absorption were observed for the load control and soak control MOTIS® cups. After normalising the wear data in the post-processing analysis, the corresponding volumetric wear rates, averaged among five worn cups, were 0.551±0.115 mm3/106 cycles taking account of the load control and 0.493±0.107 mm3/106 cycles taking account of the soak control respectively. In contrast there was no difference in the fluid uptake for the ceramic heads between the load control and soak control. Normalised by the mass changes of the load control, the worn heads produced a volumetric wear rate of 0.243±0.031mm3/106 cycles. Interestingly, varying inclination angles had no effects on the wear of ceramic heads and statistically little effects for the MOTIS® cups. The mean lubricant temperature during wear testing varied from 40°C to 45°C with a mean of 43°C. However, friction testing on worn joints produced a mean friction factor of 0.089 which is relatively low for CFR-PEEK bearings. Further investigation is ongoing. Atomic Force Microscopy showed some partial grain pull-out on the ceramic heads.
Compared with the literature, addressing the hard-on-soft hip bearings, the hemispherical MOTIS® cups assessed in the present study have produced the lowest wear [1-4]. Statistically, high inclination angles appeared to have little effect on the wear performance of the hemispherical MOTIS-PEEK-on-ceramic joints.
