Contact Mechanics of Soft Layer Artificial Hip Joints: Part 2: Application to Joint Design

Contact Mechanics of Soft Layer Artificial Hip Joints: Part 1: General Solutions

Unlike natural synovial joints, which are lubricated with a full fluid film lubrication mechanism, conventional artificial hip joints are lubricated with a mixed lubrication mechanism. Recently, however, a new generation of artificial hip joints employing compliant layers to mimic the compliance of articular cartilage in natural synovial joints have been developed to provide fluid film lubrication in these joints. While satisfactory lubrication can be achieved by employing soft layers, compliant thin layers are susceptible to the debonding between the soft layer and its stiffer substrate and long-term mechanical fatigue failure. Stress analyses for different designs of such joints are therefore important. In the present paper, the circular contact between a rigid sphere and an elastomeric layer bonded on to a rigid substrate has been analysed with a novel semi-analytical approach. The detailed contact parameters (the contact radius, the maximum surface deformation, the contact pressure and the contact stress inside the layer) have been examined for a wide range of aspect ratios (0 ≤ a/ht ≤ 100).

Normalization of load and clearance effects in ball-in-socket-like replacements

A normalizing loading parameter useful in summarising the mechanical response of plane pin-in-plate-like contacts is extended to axisymmetric ball-in-socket-like contacts. An example addressing a compliant layered artificial hip joint is presented, and the usefulness of the normalizing loading parameter is evidenced.

Compliant layer acetabular cups: friction testing of a range of materials and designs for a new generation of prosthesis that mimics the natural joint

Total joint replacements (TJRs) have a limited lifetime, but the introduction of components that exhibit good lubricating properties with low friction and low wear could extend the life of TJRs. A novel acetabular cup design using polyurethane (PU) as a compliant layer (to mimic the natural joint) has been developed. This study describes a series of friction tests that have been used to select the most appropriate material, optimize the design parameters, and fine-tune the manufacturing processes of these joints. To determine accurately the mode of lubrication under which these joints operate, a synthetic lubricant was used in all these tests. Friction tests were carried out to assess the lubrication of four PU bearing materials. Corethane 80A was the preferred material and was subjected to subsequent testing. Friction tests conducted on acetabular cups, manufactured using Corethane 80A articulating against standard, commercially available femoral heads, demonstrated friction factors approaching those for full-fluid-film lubrication with only approximately 1 per cent asperity contact. As the joint produces these low friction factors within less than half a walking cycle after prolonged periods of loading, start-up friction was not considered to be a critical factor. Cups performed well across the full range of femoral head sizes, but a number of samples manufactured with reduced radial clearances performed with higher than expected friction. This was caused by the femoral head being gripped around the equator by the low clearance cup. To avoid this, the cup design was modified by increasing the flare at the rim. In addition to this the radial clearance was increased. As the material is incompressible, a radial clearance of 0.08 mm was too small for a cup diameter of 32 mm. A clearance of between 0.10 and 0.25 mm produced a performance approaching full-fluid-film lubrication. This series of tests acted as a step towards the optimization of the design of these joints, which has now led to an in vivo ovine model.

Design aspects of compliant, soft layer bearings for an experimental hip prosthesis

Currently, an artificial hip joint can be expected to last, on average, in excess of 15 years with failure due, in the majority of cases, to late aseptic loosening of the acetabular component. A realistic alternative to the problem of wear in conventional joints is the introduction of bearing surfaces that exhibit low wear and operate in the full fluid-film lubrication regime. Contact analyses and friction tests were performed on compliant layer joints (metal-on-polyurethane) and the design of a prototype ovine arthroplasty model was investigated. When optimized, these components have been shown to achieve full fluid-film lubrication.

Analytical modelling of the elastomeric layer in soft layer hip replacements

A mechanical analysis is carried out for a flat deformable layer, firmly anchored to a rigid substrate, and frictionlessly compressed by a rigid spherical indenter, with reference to the design of hip replacements whose cup is covered by an elastomeric layer. An available perturbation solution of differential character, developed for a layer subject to plane strain conditions, indented by a cylinder approximated as parabolic in shape and valid for high contact widths, is here extended to the axisymmetric problem of a layer compressed by a paraboloidal indenter. In addition to the paraboloidal idealization, an accurate mathematical description of the indenter spherical profile is incorporated into the modelling of this contact problem. Perturbed solutions up to the second order are developed for both compressible and incompressible layers. Selected numerical examples addressing actual joint geometries are included to assess the validity of the proposed analytical modelling, to explore the accuracy in describing the spherical profile and to compare the contact pressure profiles activated by paraboloidal and spherical indenters.

Soft Tissue Biomechanics

This overview paper summarizes recent work on the biomechanics of soft tissues relevant to biomedical problems, prosthesis design and interventions, with reports on the papers presented at the Nineteenth Annual Day Conference, held in Leeds on 7 January 1994, by the Bioengineering Group for the Study of Human Joints in association with the Biological Engineering Society.