stress–strain index stress–strain index 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. definition it is calculated using the following formula: s s i = ∑ i = 0 n r i 2 a ( c d n d ) r m a x ^^a( \over )} \over where: 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.