The application of optics polishing to free form knee implants

Free form surfaces have for some time played a part in the latest developments in the exploitation of new products in optics, from micro lenses up to ELT telescopes. The demands of the required tolerance on these "new" optics have led to significant improvements in manufacturing and metrology, as the capability to produce form in the tens of nanometre range coupled with nanometre surface roughness have become the critical enabler the their use. Free form surfaces are not limited to optics, and other product developments such as bio-implant surfaces in the prosthetics industry are utilising free form surfaces to make significant strides implant life. The prosthetic industry, have for some time, known from research into hip replacements, that significant reductions in the production of biologically damaging wear debris could be achieved by implementing hard on hard bearing couples rather than the traditional metal on polymer bearing couples. In these "new" knee joints the bearing couples comprise free form alumina on alumina or CoCr alloy on CoCr alloy surfaces. Unfortunately the application of these new materials for knee implants has been largely halted by the inability of manufacturers to polish their knee surfaces to the more demanding tolerances placed on free form hard on hard bearings of both form and roughness. These tolerances require micron level form deviation combined with nanometre surface finish in order to maintain full fluid film lubrication during in vivo functioning of the joint. This paper discusses the development of the capability to machine biomaterials for use as hard on hard knee joints. The surfaces are machined utilising a 7 axis CNC optics polishing machine, the Zeeko IRP200. The machine uses a novel precision technique to correctively polish components [1].