The stress–strain index (SSI), of a bone, is a surrogate measure of bone strength determined from a cross-sectional scan by QCT or pQCT (radiological scan). The stress–strain index is used to compare the structural parameters determined by analysis of QCT/pQCT cross-sectional scans to the results of three point bending test.
It is calculated using the following formula:
- rmax is the distance of voxel from centre
- CD is the apparent cortical (bone) density
- ND is the normal (cortical bone) density
- ri is the pixel position from the centre
- a is the area of a pixel
History and relation to moments of inertia
It was developed by the manufacturer of a peripheral quantitative CT (pQCT) scanner, and is considered to be by some an improvement over the information provided by calculating the area moments of inertia and polar moments of inertia.
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