Stress–strain index

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The stressstrain 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.


It is calculated using the following formula:[2]


  • 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[edit]

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.


  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. PMID 15876561. doi:10.1016/j.bone.2005.03.001. 
  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. PMID 11266673. doi:10.1016/S0021-9290(00)00211-6.