The structure model index (SMI) is a method intended for determining the plate- or rod-like geometry of trabecular structures. It uses the change in surface area (BS, from Isosurface) as volume increases infinitesimally to calculate SMI = 0 for plates, 3 for rods and 4 for solid spheres. Unfortunately SMI is negative in the case of concave surfaces, which are common in trabecular bone, so SMI is of no use to quantify real bone structures. Structure Model Index is included in BoneJ because the bone research community frequently uses SMI. Instead of SMI, please consider using ellipsoid factor. Two methods are implemented; one uses surface mesh dilation and is preferred, whereas the other uses voxel dilation (Dilate 3D) and is only implemented because that is how the SkyScan™ technical manual describes the algorithm in CTAn (it doesn't work very convincingly in our hands).
Hildebrand T, Rüegsegger P (1997) Quantification of bone microarchitecture with the structure model index. Comput. Methods Biomech. Biomed. Engin. 1: 15-23. doi:10.1080/01495739708936692.
Structural parameters measured by the SkyScan™ CT-Analyser software. SkyScan AB. http://www.skyscan.be/next/CTAn03.pdf
Salmon PL, Ohlsson C, Shefelbine SJ, Doube M (2015) Structure model index does not measure rods and plates in trabecular bone. Frontiers in Endocrinology 6:162. doi:10.3389/fendo.2015.00162
