Abstract
New generation alumina-on-alumina (A-A) prostheses have been introduced to try and overcome the problem of osteolysis often attributed to polyethylene wear particles liberated within conventional metal-on-ultra high molecular weight polyethylene (UHMWPE) joints. This study uses a hip simulator to compare the volumetric wear rates of five different radial clearances of A-A joints. Atomic force microscopy (AFM) provided topographic characterisation of the prosthesis surfaces throughout the wear test.
Materials and methods: The wear test was performed on the Durham hip joint wear simulator. The 28 mm diameter alumina ceramic couples investigated were manufactured by Morgan Matroc Ltd., in accordance with ISO 6474. Four radial clearances (33, 40, 48 and 74 μm) of A-A joints were tested to one million cycles with 25% new-born calf serum as the lubricant. Contact mode AFM (TopoMetrix Explorer SPM) was used to produce a topographical map of the poles of the four alumina heads every 0.1 million cycles. Every 0.2 million cycles the wear was assessed gravimetrically using a Mettler AE 200 balance (accurate to 0.1 mg).
Results and discussion: There was no measurable wear of either the heads or cups during this one million cycle wear period. Throughout the wear test the alumina equiaxed grain structure became apparent on the AFM images, the mean alumina grain size was 2 μm. The grain surfaces were below the mean femoral head surface height. Such topography was not observed on an as-received head. Some granular pull-out also took place. As the wear test proceeded, the average area surface roughness increased from 2.33 nm to 4.42 nm for the heads but stayed relatively constant for the cups (from 2.75 nm to 2.97 nm).
Conclusions: The very low wear produced by these A-A hip joints is very difficult to measure gravimetrically as it is close to the limits of resolution of the weighing equipment. The surface topography analysis, however, shows changes to the ceramic surfaces during the wear test and gives an indication of the wear processes and lubrication regimes acting within such joints. The authors wish to thank EPSRC for funding this research and Morgan Matroc Ltd. for supplying the joints.
The abstracts were prepared by Nico Verdonschot. Correspondence should be addressed to him at Orthopaedic Research Laboratory, University Medical Centre, PO Box 9101, 6500 HB Nijmegen, The Netherlands.
