Human vertebral cancellous bone surface distribution.

The three-dimensional distribution of bone surface and the bone volume fraction (BV/TV) of 110 human vertebral cancellous bone specimens from seven individuals were measured using a three-dimensional radiographic method (microcomputed tomography). The ratios of the three principal projections of bone surface per total volume were found to be relatively constant for specimens examined in this study. The constancy of the projected surface ratios means that the fraction of the total bone surface oriented in any direction does not change markedly with BV/TV. Bone volume fraction was a good predictor of bone surface per total volume (BS/TV) for a one-parameter nonlinear model (r2 = 0.92). The results of this pilot study suggest that the changes in surface distribution which occur during age-related bone loss are largely predetermined rather than adaptive. The results are also consistent with the idea that cancellous bone tends to maintain a constant ratio of trabecular number for the principal directions. If these inferences from the data are correct, the morphogenetic processes which create the initial adult trabecular pattern become of primary interest. A model was developed which explained the strong relationship between BS/TV and BV/TV. The model was used to demonstrate the importance of morphogenetic processes.

The effect of defect size on the stress concentration and fracture characteristics for a tubular torsional model with a transverse hole.

The maximum stress location and crack resistance of a tubular torsional model with varying transverse circular defects were determined by the use of experimental and global-local finite element modeling techniques. The experimental results showed that the reduction in torsional strength was inversely proportional to defect size. In addition, the maximum stress location around the defect was closely related to the normalized defect diameter. By measuring the shifted angle associated with each defect ratio, a linear relationship, delta theta = -6.28 + 0.55*(d/D), was determined. Finite element results indicated that the stress concentration factor, Kg, for a single-cortex defect is similar to that of a double-cortex defect of identical dimension. Application of the strain energy density (SED) theory proposed by Sih and Oliveira Faria (Fracture Mechanics Methodology, Martinus Nijhoff, The Hague, 1984), indicated that the fracture toughness, KIC, for large defects was greater than that for small defects. This implies that tubular structures with large defects have a greater resistance to crack initiation and growth.