COMPRESSIVE STRESS-RELAXATION BEHAVIOR OF BOVINE GROWTH-PLATE MAY BE DESCRIBED BY THE NONLINEAR BIPHASIC THEORY

The compressive behavior of the bovine distal femoral growth plate was studied in vitro. Strain-rate controlled, compression stress relaxati on experiments were performed on cylindrical bone-growth plate-bone sp ecimens from the interior and periphery of the growth plate. The quest ions addressed in this study were (a) Can the nonlinear biphasic theor y one with strain-dependent permeability, be used to represent the com pressive stress-relaxation behavior of bovine growth plate? (b) How do different assumptions concerning the permeabilities of the chondro-os seous interfaces influence the inferred material properties of the gro wth plate? and (c) Are there any differences in these properties betwe en the periphery and the interior of the growth plate? Intrinsic bipha sic material properties-aggregate modulus (H-A), Poisson's ratio (v), and nonlinear strain-dependent permeability coefficients (k(o) and M)- were calculated from the compression stress-relaxation data with use o f a finite element model and a least squares curve-fitting procedure. To verify this constitutive model for the growth plate, an independent set of finite element analyses was performed with use of the determin ed intrinsic biphasic properties, and comparisons were made between th ese finite element predictions and two additional sets of experimental data subsequently obtained for the same specimens with use of two slo wer rates of compression. Excellent agreement was achieved between the se finite element predictions and the latter two sets of data. The agg regate modulus was found to be insensitive to the permeability of the chondro-osseous interface. The permeability coefficients were very sen sitive to, and the Poisson's ratio was only slightly sensitive to the interface permeability condition. Therefore, the periphery of the grow th plate is more compliant and permeable than the interior.