The major objective of the proposed research programme is to investigate, using a multispecimen hip function simulator specifically designed for this project, the long term friction and wear behaviour of joints containing compliant surfaces produced by a variety of novel routes.
New theoretical analyses of the lubrication and contact mechanics for thin layer, heavily loaded point contacts were developed from which the acetabular cup was designed.
Finite element analysis on a three dimensional model of the hemi-pelvis showed that implanting a low modulus acetabular cup would reduce the stresses experienced by the subchondral bone.
A comprehensive database of biocompatible polymers was generated and several segmented polyurethanes chosen for further study. Hygrothermal ageing, irradiation and fatigue testing suggested that the Tecothane family of polyurethanes showed the best biocompatibility and biostability.
Porous materials improved the interfacial bonding between the compliant layer and the metallic substrate by 2-3 fold over dense materials. As a result of the materials selection exercise Ti-6A1-4V was identified as the most promising material for this application despite concerns over pore distributions and notch sensitivity.
A 5-station hip joint simulator was designed and constructed which simulated three degrees of freedom (vertical, flexion-extension and pelvic rotation). This machine was used to test the prototype cups for wear.
Prototype acetabular cups containing compliant layers were produced using both sintered powder metal substrates and polymeric substrates. Sintered substrate cups demonstrated friction levels of f < 0.01 better than friction levels currently available with metal-plastic systems of f = 0.05 although wear rates were still high, attributed to the condition of the femoral heads used to conduct the tests.
It has been demonstrated that the incorporation of impervious elastomeric layers onto the surface of the acetabular cups of model artificial hip joints generates fluid film lubrication conditions. Friction levels comparable to those of healthy human joints have been observed and the surfaces of the joint remain separated under a wide range of simulated human activities. Long term wear resistance should therefore be good, however, to date poor adhesion between (fully dense) stainless steel substrate and the elastomer has prevented exploitation of this system.
The tasks will include the fixation of biocompatible elastomers to porous metal substrates produced by powder metallurgy and the development of heterogeneous polymeric systems with compliant surfaces. Powders of approved biocompatible metals will be processed to give prototype porous components to enhance adhesion by interlocking of the elastomeric layers. Effective combinations will be used to produce prototype joints containing compliant surfaces on the acetabular cup and/or femoral head for long term friction and wear testing.
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BD7 1DP Bradford
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