Heterogeneous Tissue Modulus Improved Prediction of Mechanical Behavior in Osteoporotic Vertebral Cancellous Bone

The structural integrity of cancellous bone, which is essential to its skeletal load-bearing capacity, is governed chiefly by apparent density, trabecular architecture, and tissue material properties. Metabolic bone disorders such as osteoporosis can affect each of these factors, resulting in compromised load-bearing function and fracture. While the impact of apparent density and architecture on bone structural behavior is well-documented, much less is known about the influence of tissue material properties, particularly in osteoporotic bone. In this work, we isolated the influence of tissue modulus on normal and osteoporotic cancellous bone structural integrity, indicated by the apparent elastic modulus under uniaxial compression and patterns of internal tissue strain. Finite element (FE) models derived from 3D micro-computed tomography images were compared to physical testing data of the same samples. Three sets of FE models with increasing material detail were studied: 1) universal tissue elastic modulus (20 GPa), 2) specimen-specific average tissue modulus, and 3) heterogeneous tissue modulus. Applying a universal modulus resulted in overestimation of osteoporotic bone apparent modulus; applying specimen-specific material properties, either as a single average tissue modulus or heterogeneous distribution of tissue moduli, prevented significant apparent modulus overestimation. The greatest improvement in apparent modulus prediction resulted from incorporating a specimen-specific average tissue modulus, though using a specimen-specific heterogeneous tissue modulus provided the most reliable prediction of apparent modulus overall. In addition, median element strain in heterogeneous models also trended lower than in homogeneous models. This finding suggests that heterogeneous material properties may play a role in protective strain-concentrating mechanisms observed in cancellous bone. We conclude that future work exploring trabecular bone mechanics through finite element analysis should incorporate specimen-specific average tissue modulus at a minimum, but heterogeneous tissue modulus is recommended to maximize the functional similarity of bone in silico with bone in vivo.