µCT-Based, Anatomical Finite Element Model of a Unilateral Fixator for Bone Reconstruction in Rabbit Tibia

Karunratanakul, Kavin Kerckhofs, Greet [UCL] Lammens, Johan Vanlauwe, Johan Schrooten, Jan Van Oosterwyck, Hans

Introduction When reconstructing a large segmental bone defect by means of a porous scaffold, a fixator is used to stabilize the reconstruction. The overall stiffness of the bone-fixator system is determined not only by fixator design but also by the way the fixator is fixed to the bone. This stiffness is an important factor as it will influence the biomechanical environment to which tissue engineering scaffolds are exposed. A validated finite element model can help to quantify this environment. The goal of this study was to develop and validate a finite element model of a unilateral fixator developed for a large segmental defect in the rabbit tibia [Bakker, 2008] Materials and Methods A custom-made unilateral fixator with two proximal and distal screws was fixed to the tibia of skeletally mature female New Zealand White rabbits (n=9) and a segmental defect was created. The axial stiffness of the bone-fixator system was measured. In addition, cortical bone specimens were prepared from the mid-shaft of tibiae (n=6). Screws were inserted at mid-height of each specimen and the bending stiffness of the screw-bone fixation was measured. 3D FE models, consisting of linear tetrahedral elements, were created. First, µCT-based case specific FE models of screw-bone specimens were created. Appropriate screw-bone interface parameters (amount of screw-bone contact, peri-screw bone modulus) were obtained by comparison to the experimental bending stiffness of the screw-bone fixation. Then an FE model of the tibia-fixator system was created, applying the interface parameters derived from the case specific FE models. The same boundary conditions as in the experiments were applied. Results A good correspondence was obtained between the measured and predicted stiffness of the screw-bone fixation when screw-bone contact area was set to 10% and the peri-screw bone modulus equalled 5 GPa. For the tibia-fixator system, a very good correspondence was found between measured and predicted stiffness (difference of 7.85%), when applying the same interface parameters. Discussion and conclusion The results demonstrate that the interface parameters between bone and fixator screws play an important role for the prediction of the stiffness of the bone-fixator system. These findings must be taken into account when using FE models of bone-fixator systems for virtual mechanical testing of tissue engineering scaffolds. References Bakker et al, Tissue Eng Part C Methods, 14:251-60, 2008