Tribological Assessment of Hydrogels for Replacing Damaged Articular Cartilage

In joint disorders, lesions may be limited to the joint surface. In such cases, replacing only the affected surface to preserve healthy tissue and cancellous bone is preferable to total joint arthroplasty. By employing a cartilage replacement material in focal defect repair or hemiarthroplasty applications, joint stability may be preserved while patient pain and joint dysfunction may be reduced. Hydrogels have been studied to replace damaged articular cartilage tissue. The motivation is that hydrogels may maintain natural joint lubrication due to their biphasic nature and their structure can be modified to mimic mechanical properties of articular cartilage. In order to assess the tribological properties of such a biphasic material, its lubrication mechanisms, the damage it causes on the opposing articular cartilage, and its wear properties under clinically relevant conditions were evaluated in the current dissertation. A biphasic model with linear-elastic solid matrix sufficiently predicted the material behavior of the family of tested hydrogels. Also, Stribeck analysis suggested that hydrogel-on-ceramic articulation was lubricated by a fluid film. Together, these findings suggested that, similar to articular cartilage, interstitial fluid pressurization was crucial to the viscoelasticity and lubrication properties of this biphasic material. Results indicated that biphasic materials with smaller aggregate moduli (spearman's rho=0.5; p<0.001) and larger permeability values (rho=-0.3; p<0.001) than those of the tested hydrogels in this study would produce lower coefficients of friction. Furthermore, collagen maturity and proteoglycan content as obtained by Fourier transform infrared spectroscopy were shown to decrease at the onset of in vitro cartilage wear before surface damage occurred. Cartilage pins that articulated against cartilage and hydrogel yielded higher collagen maturity than cartilage on CoCr articulation in a physiologic pin on disc (POD) wear tester. However, only cartilage-on-cartilage articulation yielded higher proteoglycan content than cartilage-on-CoCr articulation. It was postulated that the cartilage articulations against cartilage, hydrogel and CoCr in the current research represented three distinct stages of in vitro wear of articular cartilage. Finally, submerged weights were found to be more suitable than wet weights in quantifying wear of hydrogels in spite of unwanted effects of swelling. Based on submerged weights, the wear rate of hydrogel articulations was -1.4 +/- 8.3 mm3 / MC, which was not statistically different than undetectable wear. The combination of coefficient of friction measurements, white light interferometry, and environmental scanning electron microscopy supported that wear generated was undetectable up to 5 million cycles of physiologic POD testing.