INDENTATION ASSESSMENT OF BIPHASIC MECHANICAL PROPERTY DEFICITS IN SIZE-DEPENDENT OSTEOCHONDRAL DEFECT REPAIR

The apparent biphasic material properties of 10-month osteochondral de fect repair tissue were determined for a series of full thickness defe cts of 1, 3, or 5 mm diameter, created in weight-bearing regions of 48 canine femoral condyles. Load cell recordings from indentation tests were compared with resultant contact forces computed using a correspon ding linear biphasic finite element model. The spread of cartilage eng agement by a spherical ended indentor was modeled by successively impo sing an impenetrability kinematic boundary condition at cartilage surf ace nodes for which incipient indentor surface penetration was detecte d. For each indentation test, a]east-squares-error curve fitting proce dure was used to identify a set of biphasic coefficients (aggregate mo dulus, permeability, Poisson ratio) that closely modeled experimental behavior. In the near neighborhood of best-fit, the finite element sol utions were found to be much more sensitive to aggregate modulus pertu rbations than to permeability permutations, suggesting that perceived permeability increase may be of lesser value as a discriminant of repa ir tissue inadequacy. Compared to surrounding cartilage, the repair ti ssue for all defect sizes had statistically significant decreases in a ggregate modulus and in Poisson ratio (much more so for 3 and 5 mm def ects than for 1 mm defects). The two larger defect diameters had signi ficant increases in permeability, whereas the 1 mm defects did not. Wh ile the material property deficits were consistent, substantial and co mparable to those in other recent animal models of osteochondral defec t repair, the size-dependence per se the observed constitutive differe nces was modest.