Stress–strain index

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]

## Definition

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It is calculated using the following formula:[2]

SSI = ∑ i = 0 n r i 2 a ( C D N D ) r max {\displaystyle {\text{SSI}}=\sum _{i=0}^{n}{{r_{i}^{2}a({\frac {CD}{ND}})} \over {r_{\text{max}}}}}

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

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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

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1. ^ 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.
2. ^ a b 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.