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: 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.