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Theoretical Calculation of Lubrication Regimes in a Metal-on-Metal First Metatarsophalangeal Prosthesis Evaluated against an Ex Vivo Sample

Lookup NU author(s): Professor Tom Joyce

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Abstract

Replacement of the first metatarsophalangeal (MTP) joint is a relatively uncommon procedure. A cobalt chrome-on-cobalt chrome MTP prosthesis, which had a diamond like carbon (DLC) coating applied to its articulating faces, 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. Modeling the ball and socket implant as an equivalent ball-on-plane model and employing elastohydrodynamic theory allowed the minimum film thickness to be calculated and in turn the lambda value to indicate the lubrication regime. Boundary lubrication is indicated for lambda less than 1, mixed lubrication for lambda between I and 3, and fluid film lubrication or lambda greater than 3. The calculations were undertaken for a 10 to 800N range of loading values, and a 0 to 30mm/s range of entraining velocities, for both worn and unworn situations. Calculations showed that, for the range of loads and entraining velocities considered, the implant would almost always operate in the boundary lubrication regime, both when new and when worn. Therefore surface to surface contact would most frequently take place, with little separation due to lubrication between the articulating surfaces. 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.


Publication metadata

Author(s): Joyce TJ

Publication type: Conference Proceedings (inc. Abstract)

Publication status: Published

Conference Name: World Congress on Medical Physics and Biomedical Engineering 2006

Year of Conference: 2007

Pages: 3315-3318

Date deposited: 19/02/2010

ISSN: 9783540368410

Publisher: Springer

URL: http://dx.doi.org/10.1007/978-3-540-36841-0_837

DOI: 10.1007/978-3-540-36841-0_837

Notes: Conference held in 2006; proceedings published 2007.

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