Optimization of a stiffness-graded Fracture Fixation plate

Abstract

It has been observed that fractured bones which are stabilized with a titanium fracture fixation plate, once healed, often refracture at an edge of the plate. This is believed to be caused by stress concentrations in the bone that take place at the location of the edges of the plate. Various implant parameters are known to affect these concentrations of stress, but we further hypothesized that the material of the plate has the most significant influence in reducing these concentrations, here referred to as ‘peak stresses’. Moreover, it was reasoned, based on relevant literature, that minimizing the peak stresses was not the only criteria that should be considered for the design of an optimal implant, the effect of the Interfragmentary strain on the healing outcome in the early stages after implantation is also crucial for the success of the implant. Otherwise the bone will never heal in the first place.

Thus, by assuming that different regions of the plate have a different influence in the peak stresses, it was suggested that a stiffness graded plate could minimize the peak stresses while still allowing for an acceptable interfragmentary strain at the early stages of healing, through an optimization. In order to prove or disprove all the hypotheses mentioned above, a Finite Element model of a fracture fixation construct was developed. For the selection of many of the modelling assumptions, a literature study was carried out. For the selection of the contact properties to be used, a graphical comparison study was done. For choosing an appropriate mesh, a mesh study was implemented.

Using this Finite Element model, it was possible firstly to show that while the material properties of the plate do in fact appear to be very influential in reducing the peak stresses, the thickness seems to be even more influential. For the latter a parametric study was carried out using the Taguchi method. Secondly, it was shown that different regions of the fracture fixation plate do have a different influence in the peak stresses of the bone. The outer most sections of the plate seem to always be the most influential. Lastly, optimisations were carried out in three different ways and although they all reduced the peak stresses, the method thought to be the simplest, yielded the most useful results. This consisted of dividing the plate only into three sections and assigning one material to the outer sections and another to the inner section.