A COMPARISON OF EX VIVO AND THEORETICAL FIRST METATARSO-PHALANGEAL LUBRICATION REGIMES

Abstract

Introduction First metatarsophalangeal (MTP) arthroplasty is a relatively uncommon procedure compared with hip and knee joint replacement. A range of different designs of first MTP prostheses have been proposed including metal hemi-arthroplasties, single-piece double-stem silicone designs, and multi-component designs. Of the latter group, a cobalt chrome-on-cobalt chrome prosthesis, which had a diamond like carbon (DLC) coating applied to its articulating faces and hydroxyapatite-coated stems, was implanted. However, due to poor clinical results the cohort of implants were removed and one was obtained for ex vivo analysis. In addition, calculation of predicted lubrication regimes applicable to this implant design was undertaken.

Materials and Methods The ex vivo MTP implant was examined using standard microscopy as well as by using an environmental scanning electron microscope and a non-contacting profilometer. The latter device also allowed values of surface roughness to be determined while the radii of the articulating faces were measured using a co-ordinate measuring machine. Modelling the ball and socket implant as an equivalent ball-on-plane model and employing elastohydrodynamic theory [1] allowed the minimum film thickness to be calculated and in turn the lambda value to indicate the lubrication regime [2]. These calculations were undertaken for a 0 to 800N range of loading values, and a 0 to 50mm/s range of entraining velocities. The viscosity of the synovial fluid lubricant was taken to be 0.01Pa s, while for the cobalt chrome a Young’s modulus of 210GPa and a Poisson’s ratio of 0.3 were assumed.

Results and Discussion The implant was measured to have a nominal radius of 10mm and a radial clearance of 0.1mm. Calculations showed that, for the range of entraining velocities and loads considered, the implant would almost always operate in the boundary lubrication regime. Therefore surface to surface contact would most frequently take place, with little if any separation between the articulating surfaces. This result is in contrast to resurfacing designs of hip prosthesis which can operate in the fluid film lubrication mode [3]. This outcome is due to their larger radii, greater entraining velocity and reduced surface roughness values compared with the MTP implant considered here. It is felt that these design differences, inherent in different joints around the body, should be appreciated by those concerned with such implants. The presence of scratches on the articulating faces of the ex vivo sample further implied boundary lubrication. The DLC coating had been removed from the entire face of the phalangeal component and from most of the face of the metatarsal component. From the latter it appeared as if the coating had been scratched and then flaked away parallel to the scratches. In turn this suggested a corrosion based failure of the interface between the DLC coating and the cobalt chrome subsurface, a result noted recently elsewhere [4].