The effect of insert conformity and material on total knee replacement wear

The mean average life is increasing; therefore, there is a need to increase the lifetime of the prostheses. To fulfil this requirement, new prosthetic designs and materials are being introduced. Two of the design parameters that may affect wear of total knee replacements, and hence the expected lifetime, are the insert conformity and material. Computational models have been used extensively for wear prediction and optimisation of artificial knee designs. The objective of the present study was to use a previously validated non-dimensional wear coefficient-based computational wear model to investigate the effect of insert conformity and material on the predicted wear in total knee replacements. Four different inserts (curved, lipped, partial flat and custom flat), with different conformity levels, were tested against the same femoral and under two different kinematic inputs (intermediate and high), with different levels of cross-shear. The insert bearing materials were either conventional or moderately cross-linked ultra-high molecular weight polyethylene (UHMWPE). Wear predictions were validated against the experimental data from Leeds knee simulation tests. The predicted wear rates for the curved insert (most conformed) were more than three times those for the flat insert (least conformed). In addition, the computationally predicted average volumetric wear rates for moderately cross-linked UHMWPE bearings were less than half of their corresponding conventional UHMWPE bearings. Moreover, the wear of the moderately cross-linked UHMWPE was shown to be less dependent on the degree of cross-shear, compared to conventional UHMWPE. These results along with supporting experimental studies provide insight into the design variables, which may reduce wear in knee replacements

Nanowear of polymers

The use of viscoelastic materials, such as polymers, constantly increases in the field of nanotechnology. These materials are softer than metallic and inorganic ones, and, because of that, they are easier to deform and wear off. The wear mechanisms occurring for viscoelastic materials are rather complex, and, generally, present more complications for a direct investigation with respect to metals or ceramics materials. With the advent of Scanning Probe Microscopy (SPM), well characterized forces can be applied to a surface with a nanometer-scale spatial resolution. In particular Atomic ForceMicroscopy (AFM), working at high contact forces, can significantly modify many surfaces. Polymers are soft enough to be modified by hard AFM tips, such as those of silicon, silicon nitride or diamond. For these reasons, the AFM is today the main tool employed to investigate wear occurrence on polymer surfaces. The wear of a polymer surface caused by an AFM tip in a regime of single asperity contact is an articulate process that depends on conditions such as, namely, the applied forces, the tip shape, size and the relative velocity. Since the influence of all these parameters is in close connection with the sample properties, one can expect a dependence of the wearing process on the mechanical properties of the sample surfaces. These properties can vary significantly from the bulk properties, if cross linking is made or, on contrary, residual solvents are present in the specimens. This chapter is divided in three sections following a general introduction. Specifically, the first section deals with wear induced by means of AFM tips to study the mechanical properties of films at the nanoscale; the second one regards the exploitation of wear for the creation of nanolithographic patterns; the last one is finally dedicated to an applicative field such as the characterization ofwear of polymers for biomedical applications at the meso- and nanoscales