Stress–strain index

The stress–strain index (SSI), of a bone, is a surrogate measure of bone strength^[1] 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.^{[citation needed]}

It is calculated using the following formula:^[2]

$${\displaystyle {\text{SSI}}=\sum _{i=0}^{n}{{r_{i}^{2}a({\frac {CD}{ND}})} \over {r_{\text{max}}}}}$$

Where:

• r_max is the distance of voxel from centre
• CD is the apparent cortical (bone) density
• ND is the normal (cortical bone) density
• r_i is the pixel position from the centre
• a is the area of a pixel

It was developed by the manufacturer of a peripheral quantitative CT (pQCT) scanner,^[2] 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.