The dependence of ultrasound velocity and attenuation on the material properties of cancellous bone.

There is an increasing interest in evaluating the role of ultrasound in the identification and management of osteoporosis. We may measure the velocity of ultrasound through bone and the frequency-dependent attenuation, generally referred to as broadband ultrasound attenuation (BUA). The dependence of these parameters upon osteoporotic changes in density and architecture(total loss or thinning of trabeculae width) is still not well defined. A physical model for cancellous bone was developed by introducing an array of cylindrical voids of defined diameter and configuration into polymethylmethacrylate (Perspex). Experimental studies on the cancellous bone model demonstrated that the relationship between BUA and porosity is approximately parabolic, with low BUA values obtained at both low (cortical bone) and high (bone marrow) porosities. This explains the discrepancies in the correlation between BUA and density for different bone structures reported in the literature. BUA was also found to be dependent on the number of pores and the pore distribution(structure). Velocity was found to be dependent on pore size only. BUA and velocity were also found to be temperature dependent. Permeability provides quantitative information related to structure, validated using the perspex model.In vitro studies were carried out on bovine and human cancellous bone (calcaneus and vertebrae). The relationship between Young's modulus, strength and density followed the power law predicted by theoretical models. Measurements on bovine and vertebrae samples were carried out in three orthogonal directions. Young's modulus, strength, BUA, velocity and permeability were shown to be direction dependent and hence dependent upon structure. The relationship between BUA and density followed the parabolic trend observed in the physical model, with the human samples on the rising phase and the bovine on the falling phase of the parabola. BUA in the calcaneus was found to follow a power law relationship with density (BUA = rho[1.99]). BUA was a goodpredictor of strength in both the bovine (R[2] = 74%) and calcaneus (R[2] = 75%) samples. Velocity was a good predictor of both Young's modulus and strength whenapplied to the bar wave equation (E = V[2]rho) with an R[2] of 94% and 88% respectively for the calcaneus and 91% and 92% respectively for the bovine samples. For thecalcaneus samples an R[2] of 83% and 80% for Young's modulus and strength were obtained when density in the bar wave equation was substituted by BUA. The cortical end plates have a significant offset effect on BUA in the calcaneus. Permeability was highly correlated to strength. BUA and velocity were shown to be good predictors of cancellous bone strength in vitro. Future work should concentrate upon the investigation of controlled structural models of cancellous bone and also on the extrapolation of this study to the in vivo prediction of bone strength.