Original Article
Copyright ©2013 Baishideng Publishing Group Co., Limited. All rights reserved.
World J Orthop. Oct 18, 2013; 4(4): 267-278
Published online Oct 18, 2013. doi: 10.5312/wjo.v4.i4.267
Experimental and finite element analysis of tibial stress fractures using a rabbit model
Melanie Franklyn, Bruce Field
Melanie Franklyn, Bruce Field, Department of Mechanical Engineering, University of Melbourne, Parkville, VIC 3010, Australia
Author contributions: Franklyn M and Field BW designed the research, analysed the results and wrote the paper; Franklyn M conducted the experiments and developed the FE model; Field BW provided expert advice on the experiments and finite element model.
Correspondence to: Dr. Melanie Franklyn, Land Division, Defence Science and Technology Organisation (DSTO), 506 Lorimer St, Fishermans Bend Vic 3207, Australia. melanief@unimelb.edu.au
Telephone: +61-3-96267171 Fax: +61-3-96267830
Received: May 10, 2013
Revised: August 21, 2013
Accepted: September 18, 2013
Published online: October 18, 2013

AIM: To determine if rabbit models can be used to quantify the mechanical behaviour involved in tibial stress fracture (TSF) development.

METHODS: Fresh rabbit tibiae were loaded under compression using a specifically-designed test apparatus. Weights were incrementally added up to a load of 30 kg and the mechanical behaviour of the tibia was analysed using tests for buckling, bone strain and hysteresis. Structural mechanics equations were subsequently employed to verify that the results were within the range of values predicted by theory. A finite element (FE) model was developed using cross-sectional computer tomography (CT) images scanned from one of the rabbit bones, and a static load of 6 kg (1.5 times the rabbit's body weight) was applied to represent running. The model was validated using the experimental strain gauge data, then geometric and elemental convergence tests were performed in order to find the minimum number of cross-sectional scans and elements respectively required for convergence. The analysis was then performed using both the model and the experimental results to investigate the mechanical behaviour of the rabbit tibia under compressive load and to examine crack initiation.

RESULTS: The experimental tests showed that under a compressive load of up to 12 kg, the rabbit tibia demonstrates linear behaviour with little hysteresis. Up to 30 kg, the bone does not fail by elastic buckling; however, there are low levels of tensile stress which predominately occur at and adjacent to the anterior border of the tibial midshaft: this suggests that fatigue failure occurs in these regions, since bone under cyclic loading initially fails in tension. The FE model predictions were consistent with both mechanics theory and the strain gauge results. The model was highly sensitive to small changes in the position of the applied load due to the high slenderness ratio of the rabbit’s tibia. The modelling technique used in the current study could have applications in the development of human FE models of bone, where, unlike rabbit tibia, the model would be relatively insensitive to very small changes in load position. However, the rabbit model itself is less beneficial as a tool to understand the mechanical behaviour of TSFs in humans due to the small size of the rabbit bone and the limitations of human-scale CT scanning equipment.

CONCLUSION: The current modelling technique could be used to develop human FE models. However, the rabbit model itself has significant limitations in understanding human TSF mechanics.

Keywords: Rabbit, Stress fracture, Tibia, Finite element analysis, Finite element model, Mechanics

Core tip: In the current study, experimental and finite element (FE) analysis demonstrated that under compression, the rabbit tibia exhibits linear behaviour. The stresses in the rabbit tibia are sensitive to small changes in load position due to its high slenderness ratio. Low tensile stresses occur at the anterior border of the midshaft, suggesting that this region fails in fatigue, as bone under cyclic loading initially fails in tension. The current modelling technique could be used to develop human FE models.