Testing the daily stress stimulus theory of bone adaptation with natural and experimentally controlled strain histories.

Theories of bone adaptation generally consider that a departure in some feature of the normal homeostatic mechanical stimulus governs mechanical adaptation. Specifically, the 'daily stress stimulus' theory commonly used in computational models of bone adaptation suggests that the mechanical stimulus arises from a synthesis of the peak magnitudes from each loading event during a day. In this study, the homeostatic daily strain history of the adult turkey ulna was established by categorizing and counting the natural wing activities of adult male turkeys over a full 24h period. Strain signals were recorded in vivo for each activity type at three mid-diaphysis sites using stacked rosette strain gages. Following surgical isolation and transverse metaphyseal pinning of the ulnae, additional strain signals were recorded during controlled axial and torsional loading regimens associated with documented maintenance, loss, or addition of bone mass. When the present data were incorporated into the daily stress stimulus formulation, the theory did not consistently discriminate maintenance versus formation regimens, i.e., some maintenance regimens were associated with a substantially higher daily stimulus than some regimens causing bone formation.

Nonlinear viscoelastic properties of articular cartilage in shear.

The strain dependence of the intrinsic viscoelastic properties of the cartilage matrix in shear was investigated. Stress relaxation experiments were performed on bovine articular cartilage at shear strains ranging from approximately 3% to 16%. The tissue was found to exhibit nonlinear strain-dependent viscoelastic behavior, with the nonlinearity occurring primarily in the short-time transient during stress relaxation. In addition, the equilibrium stress was found to fit a quadratic relation with strain. This relationship was noted to be nearly linear with strain from 3% to 16%. The instantaneous stress was seen to be highly nonlinear, and followed a cubic relationship with applied shear strain. Fung's quasilinear theory can be used to describe the stress relaxation response over the range of strains examined when a nonlinear regression is performed to determine an "average" normalized relaxation function. Alternately, strain dependence can be incorporated into the model to describe and predict more accurately the strain-dependent stress relaxation response.