It is well known that the mechanical properties of human bones change with aging, accompanied with the geometrical changes. While structure model index (SMI) is often employed to quantitatively evaluate the bone shapes, it is impossible that changes in shapes of bodies with concave and convex faces are accurately distinguished. In this study, SMI was extended to solve the problem, and this extended version of SMI (ESMI) was adapted to vertebral body models, where the models were built based on young and old aged vertebrae. Uniaxial compressive deformations of the vertebral body models were also numerically simulated by finite element method, and the relationship between the shapes and stress concentration factors was organized by ESMI. The results showed that ESMI distinguish shape changes of vertebral bodies more accurately than SMI, and the ease of stress concentration depending on the diameter of narrow part in vertebral body was successfully evaluated by ESMI.