Stress-Strain-Index

Stress - Strain index ( SSI) ( to German: deformation stress index), a bone is a surrogate for its strength and flexural strength. The SSI is primarily used to compare the analysis of cross-sectional scans QCT / pQCT systems and the results of three- point bending tests ( strength).

Definition

The SSI is calculated as follows:

Where:

• Rmax distance of the voxel to the center
• CD apparent (average ) bone density of the voxel
• ND is the typical density of bone as a material ( cortical bone )
• Ri pixels from the center
• A surface area of the pixel

Application

SSI has been developed in order to obtain a better measure of the strength of a bone, as it enables the calculation of the pure geometrical moment of inertia. Therefore, it allows easy comparison of in vivo measurement results to results of typical material tests such as: the 3-point bending test.