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 th at produced by osteoporotic bone being generally less dense. The gener al density of the heads of the tibia, which we took as a surrogate mea sure of osteoporosis, was measured on the tibias of nine women using d ual energy X-ray absorptiometry, and cubes cut from these tibias were tested mechanically. Equations were derived relating Young's modulus o f elasticity and strength to the densities of the cubes, to their 'fab ric' (a measure of anisotropy) and to 'mineral volume density' (a meas ure of the general density of the bone from the head of each tibia). D ensity 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 po wer of the equations. Fabric was completely uncorrelated with density or mineral volume density. We conclude that although, obviously, osteo porosis weakens bones by making them less dense, there is no evidence that osteoporosis weakens bones in any other way. If there is an effec t, it must be subtle.