An improved metric for quantifying the stiffnesses of intact human vertebrae

Accurately quantifying the compressive stiffnesses of whole human vertebrae is important in the development of new treatment regimes for fractures due to osteoporosis or metastatic involvement. Two methods are commonly used to quantify compressive stiffnesses of whole vertebrae: first, the maximum slope of the force-deformation curve over a 0.2 per cent strain window; second, the slope of the best-fit line to the load-deflection curve over a specified loading range. Because the whole bone load-displacement response is non-linear, these two measurement systems yield different stiffness values for the same set of experimental data. Thus, the goal of this study was to develop and validate a standard method for deriving the whole bone stiffnesses of human vertebrae. Data from uniaxial compression tests on isolated human thoracic vertebrae (N=30 from 24 donors; T7-T10; age, 84 +/- 10, seven male, and 17 female) were analysed using the two aforementioned stiffness measurement techniques. A sensitivity analysis was also conducted whereby stiffness values were calculated for strain windows ranging from 0.05 per cent to 10 per cent. The results showed that the whole vertebra stiffness was sensitive to the calculation method. Using strain window approaches, the calculated stiffness was erratic at small strain ranges (less than 0.75 per cent), but it began to stabilize at 1 per cent strain. Comparing the historical measurement techniques versus the new standard, it was found that the 1 per cent and 0.2 per cent strain window techniques were well correlated (R2 = 0.91; p < 0.01); however, compared with the 1 per cent strain window method, the 0.2 per cent technique consistently overestimated stiffness and had five times the sensitivity to small changes in strain window magnitude. In conclusion, it is recommended that the 1 per cent strain window technique is adopted as a new standard for measuring the whole bone compressive stiffnesses of human vertebrae based on this method's superior level of accuracy and repeatability when compared with current techniques. The adoption of such a standard in the biomechanics field is important because it allows for inter-study comparisons of new orthopaedic treatments, such as vertebroplasty products.