# 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.

## Definition

It is calculated using the following formula:[2]

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

## History and relation to moments of inertia

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.

## References

- Ward K, Roberts S, Adams J, Mughal M (2005). "Bone geometry and density in the skeleton of pre-pubertal gymnasts and school children".
*Bone*.**36**(6): 1012–8. doi:10.1016/j.bone.2005.03.001. PMID 15876561. - Hasegawa Y, Schneider P, Reiners C (2001). "Age, sex, and grip strength determine architectural bone parameters assessed by peripheral quantitative computed tomography (pQCT) at the human radius".
*J Biomech*.**34**(4): 497–503. doi:10.1016/S0021-9290(00)00211-6. PMID 11266673.

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