Kinematics of the MATCO hip simulator and issues related to wear testing of metal-metal implants.

Metal-metal hip implants have been used clinically in Europe to reduce the risk of wear particle induced osteolysis. Joint simulator devices could provide useful information for design improvement of the modern generation of metal-metal hip implants. Early wear results for metal-metal hip specimens were obtained using a MATCO hip simulator. A detailed kinematic analysis was developed for the MATCO simulator and applied to two of the wear experiments to predict the starting surface motion, contact zone and lubricant film thickness. It was shown that points on cup surfaces were not subjected to a reciprocating interaction with the head during wear at the beginning of testing but as wear proceeded, it was suggested that, in some cases, reciprocating interaction did occur on the cup surface. Comparison between simulator and in vivo kinematics suggested a more realistic representation for cup than for head wear. In the simulator, the Hertzian contact zone moved in a circular path over the cup surface and changed in size in correspondence with the applied load. Elastohydrodynamic lubrication was considered to be possible in the simulator, with estimated fluid film thickness as great as 0.1 micron. However, such thick films were not likely to have occurred at the start of the two wear tests which were examined in detail, although some mixed film lubrication might have accounted for the relatively low wear of one of the specimens. The inclusion of kinematic details, contact mechanics and elastohydrodynamic lubrication analysis in simulator testing protocols and in design of metal metal hip implants was recommended.