Stress–strain index

Short description: Measure of bone strength

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]

## Contents

* 1 Definition
* 2 History and relation to moments of inertia
* 3 References

## Definition

It is calculated using the following formula:[2]

SSI=∑i=0nri2a(CDND)rmax

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,[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

1. ↑ "Bone geometry and density in the skeleton of pre-pubertal gymnasts and school children.". Bone 36 (6): 1012–8. 2005. doi:10.1016/j.bone.2005.03.001. PMID 15876561.
2. ↑ 2.0 2.1 "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. 2001. doi:10.1016/S0021-9290(00)00211-6. PMID 11266673.

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