Medicine:Stress–strain index
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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]
Definition
It is calculated using the following formula:[2]
[math]\displaystyle{ \text{SSI} = \sum_{i=0}^n {{r_i^2 a (\frac{CD}{ND})} \over {r_\text{max}}} }[/math]
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
- ↑ "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.
- ↑ 2.0 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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