Implant Roughness Governs Wear in Service
The femoral head of a hip implant articulates against a polymer or ceramic cup, and the roughness of its spherical surface controls the rate at which the counterface wears, so a surface that is acceptable by general engineering standards may still generate unacceptable wear debris in the body.
The Material Is Difficult to Finish
Cobalt chrome alloy is hard, wear resistant and chemically inert, which is what makes it suitable for the application, and those same properties mean it resists abrasive finishing, so achieving the required finish requires a long sequence of operations that is difficult to control.
Controlling the Finishing Sequence on a Sphere
The spherical grinding and polishing sequence was analysed stage by stage, with the abrasive size, the contact pressure and the lapping motion treated as variables, and the resulting roughness, sphericity and surface chemistry measured after each stage to determine which stage controls the final finish. The heads were finished on a Ponda precision lapping platform with a spherical tool head, with the abrasive sequence and the pressure schedule validated by the application specialists at Ponda, a precision machining and finishing company.
Each Stage Must Remove the Previous One
The measurements show that the final roughness is determined not by the last abrasive used but by whether each stage fully removed the damage introduced by the stage before it. An insufficient intermediate stage leaves deep scratches that later operations merely polish over, and the surface then appears acceptable while retaining subsurface damage.
A Defined Sequence Gives Reproducible Finish
The optimised sequence produced a roughness of 0.008 micrometre with a sphericity error of 1.2 micrometres, and the batch scatter fell from 0.014 micrometres to 0.003 micrometres. No subsurface damage was detected by cross sectional examination.