SEPARATE EFFECTS OF OSTEOPOROSIS AND DENSITY ON THE STRENGTH AND STIFFNESS OF HUMAN CANCELLOUS BONE

The question this paper attempts to answer is whether osteoporosis has any effect on the mechanical properties of cancellous bone, except that produced by osteoporotic bone being generally less dense. The general density of the heads of the tibia, which we took as a surrogate measure of osteoporosis, was measured on the tibias of nine women using dual energy X-ray absorptiometry, and cubes cut from these tibias were tested mechanically. Equations were derived relating Young's modulus of elasticity and strength to the densities of the cubes, to their 'fabric' (a measure of anisotropy) and to 'mineral volume density' (a measure of the general density of the bone from the head of each tibia). Density and fabric between them accounted for about 80% of the variance in strength and in stiffness of the cubes, and adding mineral volume density as an explanatory variable had no effect on the explanatory power of the equations. Fabric was completely uncorrelated with density or mineral volume density. We conclude that although, obviously, osteoporosis weakens bones by making them less dense, there is no evidence that osteoporosis weakens bones in any other way. If there is an effect, it must be subtle.