Methods and modeling for the reduced platen compression of cancellous bone in the rodent proximal tibia

Abstract

This study focused on the reduced platen compression (RPC) test of cancellous bone in the rodent proximal tibia. The objective was to improve methods for this mechanical test, specifically in the areas of specimen location, specimen preparation, and platen sizing. Analytical studies were utilized to assess the effect of platen size and shape on the resultant values obtained for elastic modulus of cancellous bone induced by compression. RPC specimens are made from transverse slices of the proximal tibial metaphysis. The specimen location was determined using radiographic methods in which the distance from the proximal end of the bone to the distal extent of the growth plate was measured. Using this distance, 2-mm thick specimens were cut from the metaphysis and then platen sized for testing. Three platen sizing methods, which were based on various imaging methods, were studied. The cancellous bone was then tested under compression to failure. Three-dimensional finite element models of the specimens under the loading scenario were created with the ANSYS package. The models incorporated specimen geometry in the cross-sectional plane and through the thickness of the sample. The models were used to conduct parametric studies of the effects of platen size and shape on the percent error in elastic modulus as determined by the reduced platen compression test. Based on this study, several improvements to the reduced platen compression test were recommended. An improved method for specimen location was developed, however, it requires a corrective distance to account for the tissue not present in the radiograph analysis. The specimen preparation included minor changes like gripping fixtures, to ensure precise cuts. In platen sizing, it was found that the best-fit endocortical circle method is best suited for the proximal tibia compared with previous methods. The finite element models of the RPC specimen provided support for the use of an anatomically shaped platen in experiments and a foundation for further study. Recommendations for future study include using advanced technology like rapid prototyping, and custom shaped platens to enhance RPC testing.