The effect of contact area on wear in relation to fixed bearing and mobile bearing knee replacements

It is generally assumed that the wear rates in knee replacements are reduced as the contact area is increased. Hence, fixed bearing or mobile bearing designs with large contact areas throughout the full range of flexion wear less than partially conforming fixed-bearing designs. This hypothesis was investigated in an experimental model, where flat-ended ultra high molecular weight polyethylene pins of varying diameters were reciprocated and rotated on polished metal plates under a constant load with serum lubrication. The pin diameters ranged from 8-23 mm, giving nominal contact pressures from 23.9-2.8 MPa, covering the range associated with a wide spectrum of total knees including mobile-bearings. For pin diameters of 8-12 mm, the mean wear rates were in the range of 5.0-16.0 E-10 g/cycle. For diameters of 17 and 23 mm, the mean wear rates were approximately 1.0 E-10 g/cycle. The latter wear rates were significantly less than the former. Scanning electron microscopy indicated milder wear processes with the larger diameters, while the smaller diameters exhibited transverse ripples and cracks and detachment of thin layers from the surface. The percentages of granules (mostly submicron), fibrils and flakes, and the sizes of these particle types were similar among all pin diameters, except that, for the 23 mm pin diameter, the percentage of fibrils increased and of flakes decreased. This work supports the hypothesis that larger contact areas, up to the maximum area tested in our study, produce lower wear rates, and suggests that there is no disadvantage regarding particle type or size associated with the larger areas of contact.