Title: Friction and wear measurements of bovine articular cartilage against non-native materials
Description: The three studies reported in this thesis investigate the friction and wear properties of articular cartilage. The aim of the first study presented was to examine the functional properties and biocompatibility of glutaraldehyde-fixed bovine articular cartilage for potential use as a resurfacing material in joint arthroplasty. Treated cartilage disks were fixed over a range of glutaraldehyde concentrations and incubated, along with an untreated control group, at 37 °C for up to 28 days. The equilibrium compressive modulus increased nearly twofold in the treated samples when compared to day 0 control, and maintained that property value from day 1 to day 28; the minimum friction coefficient did not change significantly with fixation and incubation time, whereas the time constant for the frictional response decreased twofold at most. Live explants co-cultured with fixed explants showed no qualitative difference in cell viability over 28 days of incubation. Cartilage-on-cartilage frictional measurements were performed under the configuration of migrating contact for a subset of treated explants over a period of 28 days exhibited either no significant difference or slightly lower friction coefficient values than the untreated control group.

These results suggest that a properly titrated glutaraldehyde treatment can reproduce the desired functional properties of native articular cartilage and maintain these properties for at least 28 days in-vitro. The aim of the second study was to determine whether the latest-generation particle analyzers are capable of detecting cartilage wear debris generated during in vitro loading experiments that last 24 h or less, by producing measurable content significantly above background noise levels otherwise undetectable through standard biochemical assays. Immature bovine cartilage disks were tested against glass using reciprocal sliding under unconfined compression creep for 24 h. Control groups were used to assess various sources of contamination. Results demonstrated that cartilage samples subjected to frictional loading produced particulate volume significantly higher than background noise and contamination levels at all tested time points. The particle counter used was able to detect very small levels of wear), whereas no significant differences were observed in biochemical assays for collagen or glycosaminoglycans among any of the groups or time points.

The aim of the final study was to measure the wear response of immature bovine articular cartilage tested against glass or alloys used in hemiarthroplasties. Two cobalt chromium alloys and a stainless steel alloy were selected for these investigations. The surface roughness of one of the cobalt chromium alloys was also varied within the range considered acceptable by regulatory agencies. Cartilage disks were tested in a configuration that promoted loss of interstitial fluid pressurization to replicate conditions believed to occur in hemiarthroplasties. Results showed that considerably more damage occurred in cartilage samples tested against smooth stainless steel and rough cobalt chromium alloys compared to smooth glass, and smooth cobalt chromium alloys. The two materials producing the greatest damage also exhibited higher equilibrium friction coefficients. Cartilage damage occurred primarily in the form of delamination at the interface between the superficial tangential zone and the transitional middle zone, with much less evidence of abrasive wear at the articular surface.

These results suggest that cartilage damage from frictional loading occurs as a result of subsurface fatigue failure leading to the delamination. Surface chemistry and surface roughness of implant materials can have a significant influence on tissue damage, even when using materials and roughness values that satisfy regulatory requirements.