Tribological Studies of Modem orthopaedic biomaterial Combinations

Abstract

Debris created during the wear of prosthetic joints is known to have an effect on the success of the implant. As such the factors affecting wear and lubrication need to be understood in order to reduce wear as much as possible in vivo. In this thesis a number of orthopaedic biomaterials were tested in vitro, and various factors affecting their tribology were investigated. Load was found to affect wear for stainless steel on UHMWPE contacts, but not the wear factor which has a load term incorporated into it. Increasing nominal contact stress, however, was found to cause a decrease in wear factor particularly at low values of contact stress. The wear factor was less affected at higher values of contact stress. Clearance and roughness were both seen to affect the lubrication mode under which ceramic-on-ceramic joints operated, as noted by extensive friction testing. Lower clearances and lower roughnesses respectively promoted fluid film lubrication. The wear remained very low in both cases and was undetectable gravimetrically. However, changes in the surface morphology throughout testing supported the fact that some wear did occur, but that this was so small as to be beyond the detection limits of the experiment. This is encouraging for the longevity of these joints in vivo. The "running-in" phenomenon seen in metal-on-metal combinations was investigated using large diameter metal-on-metal resurfacing prostheses. During the course of wear testing the initially higher wear rate dropped to a much lower steady state wear. Friction testing indicated that the joints were operating closer to fluid film lubrication and the topography of the joint surfaces became more negatively skewed as the test progressed.