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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 strucural parameters determined by analysis of QCT/pQCT cross-sectional scans to the results of three point bending test.

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Definition

It is calculated using the following formula:^[2]

$SSI = \sum_{i=0}^n {{r_i^2 a ({{CD} \over {ND}})} \over {r_{max}}}$

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. PMID 15876561.
^ ^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. PMID 11266673.