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.

It is calculated using the following formula:

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

Where:

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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