Regulation of cartilage metabolism by dynamic tissue shear strain and the mechanical characterization of cartilage

Abstract

I investigated the physical regulation of cartilage metabolism induced by dy­namic tissue shear strain, and also the mechanical behavior under shear strain, espe­cially focused on shear modulus, under different shear strain, frequency, compressive offset, and physiochemical environment. For this purpose, a new instrument was developed to apply axial deformations as small as lµm and sinusoidal rotations as small as 0.5% up to 4% based on 1 mm thickness of tissue under feedback control. This apparatus is small enough (30 cm high x 25 cm x 20 cm) to be placed in a standard incubator for long-term tissue culture loading studies. Consistent with previous studies, articular cartilage showed a typical viscoelas­tic material behavior under shear strain, and the shear modulus increased when the frequency and compressive offset was increased, or the applied shear strain was de­creased. This shear softening effect was found to be related to the transient response of cartilage. The equilibrium stress was linear with shear strain. Under different ionic strengths, articular cartilage showed a decrease in the shear modulus up to 1.0 M NaCl bath concentration, but interestingly above this point the shear modulus began to increase while axial stiffness monotonically decreased. Biosynthetic response of chondrocytes under 0.1 Hz and 1 % sinusoidal shear strain, which was measured by the incorporation rate of 35 S-sulfate and 3 H-proline, was significantly increased compared to the incorporation level of statically com­pressed or unloaded free-swelling controls. To check the local stimulation by relative fluid flow which can be induced in the outer peripheral region, the incorporation rate of 2 mm center region and outer ring region was compared to those of static and free swelling controls. Unlike axial compression, where the incorporation rate in the outer ring region was greater than the 2 mm center region due to fluid flow and cell deformation, the effect of shear strain was uniformly distributed over the entire area, so the increased biosynthetic effect under shear strain is more related with direct me­chanical deformation of chondrocytes rather than fluid flow, changes in hydrostatic pressure, or electrical or chemical environment.