Stress–strain index

The stress–strain index, of a bone, is a surrogate measure of bone strength determined from a cross-sectional scan by QCT or pQCT. 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.

Definition

It is calculated using the following formula:
Where:r_max is the distance of voxel from centre

CD is the apparent cortical (bone) density
ND is the normal densityr_i is the pixel position from the centrea is the area of a pixel

History and relation to moments of inertia

It was developed by the manufacturer of a peripheral quantitative CT 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.