Trabecular bone was studied both microscopically using metallographic techniques and mechanically, under cyclic sinusoidal loading at low strain-rates and small deformations. Special attention was paid to the function of trabecular bone as a stress distributer and shock-absorber, and to the relationship between structure-property changes in trabecular bone and the etiology of osteoarthritis.
Under conditions of very small deformation and low strain rate, trabecular bone from the human knee behaves purely elastically in the low audio range, except at several sharp frequencies where it exhibits marked viscous behavior. These resonances are believed to be controlled by momentum-wave modes of calcium and phosphorous atoms in the lamellae of the trabeculae. Trabecular bone therefore can serve as a filter to reduce the amplitudes of certain frequencies present in force waves transmitted through the human skeleton. The elastic behavior is consistent with accepted rheological models for cortical bone, which is assumed to be the constituent material of trabecular bone.
The elastic modulus of trabecular bone is roughly 10psi. The modulus varies about this value depending on the structure of the bone in the following manner: If the structure is comprised of uniform sheets of bone, the module is inversely proportional to the contiguity of the open spaces between the sheets. If the structure contains patches of dense bone, the modulus is proportional to the volume fraction of bone. The uniform structure deforms elastically by a plate-bending mechanism; the nonuniform structure deforms in a plate compression mode.
A model for trabecular bane is developed to investigate the mechanical behavior of the region just beneath the cartilage in the human knee. The stresses in the structure are calculated for a physiological static load using the finite-element method. In certain areas of the structure, the ultimate tensile and compressive strengths of the constituent material are exceeded. The modulus of the structure varies locally by a factor of ten. An increase in thickness of a trabecula from 0.003 to 0.005 inch, as through normal remodelling, results in a fourfold increase in modulus,
Fracture is to be expected in trabecular bone. Buckling as well as Griffith-type brittle fracture are considered. Micro fracture has been observed in samples of trabecular bone from rabbits and humans. It is reasoned that fatigue fracture is the most important mode of fracture in trabecular bone, and that the fatigue life of certain areas in the model is less than 10- cycles.
A proposed etiology for osteoarthritis is developed from studies of structure, properties, and cartilage degeneration in a group of twenty patients. Early arthritic joints contain trabecular bone that is significantly stiffer than that in normal or advanced arthritic joints. This stiffening is due to remodelling of the sheets of bone to resist the plate-bending deformation resulting from slightly increased impulsive loading. The bone remodels to decrease the contiguity of the open spaces. Increased stiffness implied decreased deflections of the bone under stress, and reäuces its effectiveness as a shock-absorber. Under these conditions the cartilage loses mucopolysaccarhides in the weight bearing area, the first sign of the disease. The cartilage is severely damaged locally through increased punishment above dense parches of bone that form and serve as regions of increased local modulus as in the model. The bone subsequently undergoes fracture, necrosis, and resorption, resulting in lowered stiffness and progression of the disease.
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