Experimental and finite element analysis of tibial stress fractures using a rabbit model

摘要

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 un der 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 anterio border of the tibial midshaft: this suggests that fatigue failure occurs in these regions, since bone under cycli loading initially fails in tension. The FE model predic tions were consistent with both mechanics theory and the strain gauge results. The model was highly sensi tive 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 curren study could have applications in the development o human FE models of bone, where, unlike rabbit tibia the model would be relatively insensitive to very sma changes in load position. However, the rabbit mode itself is less beneficial as a tool to understand the me chanical behaviour of TSFs in humans due to the sma 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 under standing human TSF mechanics.