Biomedical subjects

R Paul Crawford

Publications and source records attributed to R Paul Crawford.

4 recordsLinked to original sources

PubMed2005

Development of residual strains in human vertebral trabecular bone after prolonged static and cyclic loading at low load levels.

Development of irreversible residual strains in trabecular bone may be a mechanism by which age-related non-traumatic vertebral fractures occur. To investigate this concept, static and cyclic loading tests were conducted at low loading levels for cylindrical cores of cadaveric vertebral trabecular bone. Stresses were applied equivalent to elastic strains of either 750 or 1,500 microstrain. Creep strains were measured during the tests, which lasted for 125,000 seconds (about 35 h), and for an additional 125,000 seconds after complete unloading. Emphasis was placed on the residual strains that developed, defined as the strain remaining at the end of the unloading phase. The results indicated that appreciable residual strains did develop, and were similar for static and cyclic loading. Irrespective of the applied load levels and loading modes, the residual strains that remained after the unloading phase were similar in magnitude to the originally applied elastic strain. Extrapolation of the observed residual strains to full recovery indicated that the time that would be required for full recovery was over 20 times longer than the duration of the applied loads. These results indicate that human vertebral trabecular bone does not creep in a linear viscoelastic fashion at low stress levels, and that creep mechanisms dominate the residual strains regardless of the loading mode. Taken together, these findings support the concept that non-traumatic vertebral fractures may be related to long-term creep effects because the trabecular bone does not have sufficient time to recover mechanically from creep deformations accumulated by prolonged static or cyclic loading.

Aged↗

PubMed2004

Relationship between axial and bending behaviors of the human thoracolumbar vertebra.

STUDY DESIGN: The authors studied the mechanical behavior of vertebrae through the use of finite element analyses. OBJECTIVES: To determine the relation between axial and bending rigidity, and to determine the geometric and densitometric factors that affect this relation. SUMMARY OF BACKGROUND DATA: Metrics of vertebral body mechanical properties in bending have not been established despite evidence that anterior bending loads play a significant role in osteoporotic vertebral fracture. METHODS: Voxel-based finite element models were generated using quantitative computed tomography (QCT) scans of 18 human cadaveric vertebral bodies, and both axial and bending rigidities of the vertebra were computed. Both rigidity measures and their ratio were correlated with vertebral geometric and densitometric factors obtained from the QCT scans. RESULTS: Bending rigidity was moderately correlated with axial rigidity (r2 = 0.69) and strongly correlated with the product of axial rigidity and vertebral anteroposterior depth squared (r2 = 0.88). The ratio of bending to axial rigidity was independent of bone mineral density (P = 0.20) but was moderately correlated with the square of vertebral depth (r2 = 0.69). CONCLUSIONS: Vertebral anteroposterior depth plays an important role in bending rigidity. The scatter in the correlation between bending and axial rigidity suggests that some individuals can have vertebrae with a normal axial stiffness but an abnormally low bending stiffness. Because whole-bone stiffness is indicative of bone strength, these results support the concept that use of more than one metric of vertebral strength, for example, compression and bending strengths, may improve osteoporotic fracture risk prediction.

Adult↗

PubMed2003

Finite element models predict in vitro vertebral body compressive strength better than quantitative computed tomography.

The correlation between bone mineral density and vertebral strength is not based on mechanical principles and thus the method cannot reflect the effects of subtle geometric features and densitometric inhomogeneities that may substantially affect vertebral strength. Finite element models derived from quantitative computed tomography (QCT) scans overcome such limitations. The overall goal of this study was to establish that QCT-based "voxel" finite element models are better predictors of vertebral compressive strength than QCT measures of bone mineral density with or without measures of cross-sectional area. QCT scans were taken of 13 vertebral bodies excised from 13 cadavers (L1-L4; age: 37-87 years; M = 6, F = 7) and used to calculate bone mineral density (BMD(QCT)). The QCT voxel data were converted into linearly elastic finite element models of each vertebra, from which measures of vertebral stiffness and strength were computed. The vertebrae were biomechanically tested in compression to measure strength. Vertebral strength was positively correlated with the finite element measures of strength (r(2) = 0.86, P < 0.0001) and stiffness (r(2) = 0.82, P < 0.0001), the product of BMD(QCT) and vertebral minimum cross-sectional area (r(2) = 0.65, P = 0.0008), and BMD(QCT) alone (r(2) = 0.53, P = 0.005). These results demonstrate that highly automated "voxel" finite element models are superior to correlation-based QCT methods in predicting vertebral compressive strength and therefore offer great promise for improvement of clinical fracture risk assessment.

Adult↗

PubMed2003

Quantitative computed tomography-based finite element models of the human lumbar vertebral body: effect of element size on stiffness, damage, and fracture strength predictions.

This study investigated the numerical convergence characteristics of specimen-specific "voxel-based" finite element models of 14 excised human cadaveric lumbar vertebral bodies (age: 37-87; M = 6, F = 8) that were generated automatically from clinical-type CT scans. With eventual clinical applications in mind, the ability of the model stiffness to predict the experimentally measured compressive fracture strength of the vertebral bodies was also assessed. The stiffness of "low"-resolution models (3 x 3 x 3 mm element size) was on average only 4% greater (p = 0.03) than for "high"-resolution models (1 x 1 x 1.5 mm) despite interspecimen variations that varied over four-fold. Damage predictions using low- vs high-resolution models were significantly different (p = 0.01) at loads corresponding to an overall strain of 0.5%. Both the high (r2 = 0.94) and low (r2 = 0.92) resolution model stiffness values were highly correlated with the experimentally measured ultimate strength values. Because vertebral stiffness variations in the population are much greater than those that arise from differences in voxel size, these results indicate that imaging resolution is not critical in cross-sectional studies of this parameter. However, longitudinal studies that seek to track more subtle changes in stiffness over time should account for the small but highly significant effects of voxel size. These results also demonstrate that an automated voxel-based finite element modeling technique may provide an excellent noninvasive assessment of vertebral strength.

Adult↗