Biomechanical and Biotribological Correlation of Induced Wear on Bovine Femoral Condyles

Characterizing the biomechanical and biotribological properties for articular surfaces in healthy, damaged, and repaired states will both elucidate the understanding of mechanical degradation and lubricating phenomena and enhance the development of functional tissue engineered cartilage and surgical repair techniques. In recent work, a new methodology involving concomitant linear translational and oscillating rotational motion was developed to determine the frictional and wear characteristics of articular cartilage. The impetus of this work was to further characterize the biomechanical characteristics from stress relaxation and dynamic cyclical indentation testing of normal and damaged articular cartilage and to correlate the biotribological characteristic findings with the biomechanical data. Quasilinear viscoelastic (QLV) theory was used to curve fit the stress-relaxation data, while the dynamic data were used both to determine the dynamic properties through fast Fourier transform analysis and to validate the dynamic behavior based on the properties obtained from the QLV theory. Comparisons of the curve-fit parameters showed a significant decrease in pre- versus postwear elastic response, $A$$(p<0.04)$⁠, and viscous response, $c$$(p<0.01)$⁠. In addition, the short term relaxation time, $τ1$$(p<0.0062)$⁠, showed a significant decrease between surfaces with and without a defect. The magnitude of the complex modulus from dynamic tests revealed a decrease due to wear, $lGlpostwear∕lGlprewear<1$$(p<0.05)$⁠. The loss factor, $tanδ$⁠, was generally greater while $lGl$ was less for those specimens experiencing rotation. A linear regression analysis was performed to correlate $μstatic$ and $μinitial$ with the curve-fit QLV parameters, $A$⁠, $B$⁠, $c$⁠, $τ1$⁠, and $τ2$⁠. Increasing coefficients of friction correlated with decreases in the elastic response, $A$⁠, viscous response, $c$⁠, and the short term relaxation time constant, $τ1$⁠, while $B$ became increasingly nonlinear and $τ2$ became shorter postwear. Qualitatively, scanning electron microscopy photographs revealed the mechanical degradation of the tissue surface due to wear. Surfaces with a defect had an increased amount of wear debris, which ultimately contributed to third body wear. Surfaces without a defect had preferentially aligned abrasions, while those surfaces not within the wear path showed no signs of wear. The efficacy of various repair techniques and innovative repair tissue models in comparison to normal and worn articular surface tissue can be determined through experimental designs involving both biomechanical and biotribological parameter characterizations. The development of this comprehensive testing scenario involving both biotribological and biomechanical characteristics is essential to the continued development of potential articular repair tissue.