Characterization of growth plate cartilage compressive properties.

Abstract

The process of endochondral ossification involves incremental units of longitudinal bone growth added through the proliferation and hypertrophy of growth plate chondrocytes. The extracellular matrix synthesized by these chondrocytes must also withstand the applied physiological loads that occur within the growth plate. It was the purpose of this thesis to characterize the relationship between the compressive material properties of the growth plate and its matrix. Compressive stress relaxation tests were performed on eight week New Zealand white (NZW) rabbit proximal tibial growth plate specimens. Included were tests to determine the effects of strain rate and environmental testing conditions (freezing/thawing, inhibitor solution/Ringers). Significant differences were found in non-equilibrium properties based on environmental testing conditions. Equilibrium properties were unaffected. While the strain rate dependency was similar to articular cartilage, the average equilibrium modulus of 0.149 MPa was five times less than previously reported for articular cartilage. There were also qualitative differences between these tissues respective morphologies. To address the contribution of morphology to mechanical properties, growth plate specimens from the proximal tibia, distal radius, and distal ulna of one, three, and eight week old NZW rabbits were tested. In addition to stress relaxation tests, each specimen was subjected to morphological evaluation which included measurement of zone heights, zone matrix fractions, and an averaged matrix fraction. Significant variations between the different ages and epiphyses were found for both the mechanical and morphology properties. However, the average matrix fraction explained only 12-22% of the variance in the equilibrium modulus, suggesting further quantification of the morphology, including chondrocyte/matrix organization, was needed. In addition to morphology evaluation, each specimen was analyzed for water, proteoglycan, and collagen content. Correlations were found between these biochemical constituents and compressive properties. The best relationships found were between the proteoglycan content and the equilibrium moduli. Depending on the epiphysis, between 29-56% of the variance was explained. In conclusion, these experiments demonstrated variations in the compressive material properties, morphology, and biochemical composition in rabbit growth plate cartilage and serve as a basis for further research into the biomechanical nature of growth plate cartilage.