Bone/Suture Response to Mechanical Loading Using a Combined FEA/Histology Technique

Finite element analysis (FEA) is a computer based analysis method used to predict the stress and strain characteristics of an object under applied mechanical loading. Objective: We present a novel use of the FEA technique to map the load environment of the rat cranial bone/suture interface in histological sections from mechanically loaded sutures in tissue culture. Methods: In this study we used 1 week old 4cm x 8cm rat calvarial explants bisected by the sagittal suture. A 50g spring was activated across each suture 5min on and 5min off 4 times/day for 5 days. Tissues were decalcified and sections of the calvaria were processed for immunohistochemistry using an antibody to MMP-9. Stained sections were imported into the WinTopo program (SoftSoft.net, Biggleswade, Bedfordshire, UK), which transforms and saves the images in the “dxf” file format. In this format, FEA can be performed on the images using the J.L Analyzer 9.0 FEA program (AutoFEA Engineering Software Technology Inc., Norwalk, California, USA). For our studies we created 2D models containing between 2000 and 4000 nodes incorporating published values for the modulus of elasticity and Poisson's ratio of composite bone and suture with an applied load of 50g. The resulting stress maps created were combined with the immunohistochemistry by image overlay to characterize the load environment, tissue characteristics, and protein expression in each tissue. Results: Sites of high compressive load correlate with sites of osteoclastic bone resorption, and sites of high shear stress correlate with the expression of MMP-9. Conclusion: This novel technique demonstrates a significant correlation between the loading environment at the bone/suture interface and the localization of changes in cellular, protein and gene expression.