Abstract
Introduction
Osteolysis and aseptic loosening in total hip replacement (THR) is often associated with polyethylene (PE) wear. This caused interest in alternative bearing surfaces. Since the mid nineties, research focused on hard-hard bearings like metal-on-metal (MOM) or ceramic-on-ceramic (COC). However, concerns remain about biological reactions to metallic wear debris or failure of the ceramic components. A new approach to reduce wear with a minimized risk of failure may be the use of a metallic cup in combination with a ceramic head, the so called ceramic-on-metal bearing (COM). The aim of this study was to estimate the wear behaviour at an early stage of this COM bearing type in comparison to COC bearings using a hip simulator.
Material and Methods
Simulator studies were carried out on a single station hip simulator (MTS 858 Mini Bionix II, Eden Prairie, USA) in accordance to ISO 14242-1. Bovine serum was used as the test medium. Four COM and four COC bearings were used, both 36mm in diameter. The heads were made of a mixed-oxid ceramic (Biolox Delta(r)) paired with a high carbon wrought CoCrMo cup in the COM group whereas both components were made of Biolox Delta(r) in the COC group. Simulation was run to a total of 2.4×106 cycles. Wear measurements were performed in intervals of 0.2x106 cycles using a gravimetric method (Sartorius Genius ME235S, measuring solution: 15 μg, Sartorius, Göttingen, Germany).
Results
Wear of the COM and COC pairings is shown in Figure 1. During the first 200,000 cycles a mean wear rate of 0.16mm3/106 cycles was found followed by a decreased wear rate of 0.04mm3/106cycles for the COC bearings. The overall wear ranged from 0.08mm3 to 0.17mm3, with a mean of 0.12 mm3. There was found a high variability in the wear progression between the four COM implants (Figure 1). A mean wear rate of 0.13mm3/106 cycles was determined during the first 200,000 cycles followed by a decreased wear rate of 0.05mm3/106 cycles. The overall wear of the COM implants ranged from 0.02mm3 to 0.21mm3, with a mean of 0.13 mm3. All ceramic heads from the COM bearings showed metallic material transfer in form of stripes whereas no visible wear traces were found on the COC heads.
Discussion and Conclusion
The COM implants showed very low wear levels that were similar to the COC bearings and far below wear levels of conventional MOM bearings. However, there was a spreading up to the thirteen fold between lowest and highest wear volume of the COM at the end of the study. Concerning COM implants such high variability was also seen by other investigators. The simulator conditions are highly reproducible (as seen for the COC bearings). Considering the high variations in patients demand, the influence of patient related activity parameters should be further investigated in terms of wear. Moreover, this study was performed without implementing subluxation, impingement or malalignment which might also increase wear. These effects together with the influence of third body wear needs to be further considered.
