Nanotribology of CoCr–UHMWPE TJR prosthesis using atomic force microscopy

Abstract

Ultra high molecular weight polyethylene (UHMWPE) submicron wear debris particles in total joint replacement (TJR) prosthesis have been observed clinically to cause osteolysis. In this study, the formation of UHMWPE submicron wear debris was investigated by modeling a nanoscale asperity on the surface of a CoCr femoral component of a total knee replacement (TKR) prosthesis using a Si 3N 4 AFM commercial tip. Contact mode atomic force microscope (AFM) was used as a tribosystem to represent CoCr–UHMWPE asperity contact in TKR prostheses in order to evaluate nanoscale coefficient of friction ‘ μ’. Nanoindentation was used to understand the mechanical response of the TJR prosthesis components. Moreover, the threshold for UHMWPE plastic deformation and subsequently wear as a function of frictional and normal load at a nanoscale between CoCr alloy and UHMWPE asperity-to-asperity contact was investigated. Results from energy dispersive X-ray (EDX) analysis and back scattering energy (BSE) technique illustrated the presence of silicon carbide (SiC) particles in the polished surface of a femoral component of total knee replacement (TKR) prosthesis. Sliding of the model asperity on direct compression moulded UHMWPE for a total normal load range of 1–11 nN caused contact pressures ranging from 14 to 80 MPa which resulted in the dynamic ‘ μ’ at the interface of the nanotribocontact equal to 0.22±0.01. Transients in frictional response of the UHMWPE as a function of normal load range (17–340 nN) during sliding against the modeled CoCr asperity were observed. The first transient in frictional force was observed at a normal load of 197±9 nN, corresponding to a contact pressure of 462±9 MPa. The second transient in the frictional force was observed at a normal load of 288±13 nN corresponding to a contact pressure of 518±13 MPa. Following the second transition in frictional response for the normal load range selected, plowing of the UHMWPE surface by the hard asperity was observed. This indicated that the deep scratches noticed in retrieved TJR prosthesis could be due to the abrasion from protruding hard carbide asperities that exist within the cast CoCr alloy of TJR prosthesis. From this study it can be stated that the plastic deformation of UHMWPE under multiaxial loading conditions (normal and shear) during sliding can be used to explain the initiation of failure of the UHMWPE insert in TJR prosthesis eventually leading to submicron wear debris generation.