Analyse de la rupture des tiges chirurgicales de croissance en configuration simple

Details

Supervisors

Pardoen, Thomas

Faculty

Ecole polytechnique de Louvain

Degree label

Master [120] : ingénieur civil en chimie et science des matériaux

Abstract

enScoliosis is an abnormal deformation of the vertebral column in the frontal plane. This deformation can appear at any time during life but it is more problematic when it occurs during childhood. Depending on the amplitude of the deformation, different treatments are applied. In the most severe cases, surgery is required. During the surgery, one or two growth rods are placed along the previously straighten column to act as a support to correct the scoliosis. Since children keep growing and the column tries to return to its initial (deformed) position, the high stresses inside the rods can lead to failure. The aim of this master thesis is to find some progression axes in order to increase the rods lifetime. The H3S2 setup (3 hooks 2 screws), which uses one rod, has been analysed. Pure titanium, titanium alloy TiAl6V4 and cobalt-chromium alloy, that are typically used in surgery, were selected. The chemical and mechanical properties of the intact rods were initially characterised. The loss of properties and the internal stresses presents in the rods due to the bending of the rods during surgery were quantified by means of parametric tests. It appears that the forming process of the rods probably consisted of extrusion. To recover an isotropic microstructure and introduce a compressive stress state at the rod surface, some of them undergone an annealing process followed by a shot blasting process. The compressive stress state enhances the fatigue properties of the rods. Moreover, an increase of the hardness close to the surface highlighted the usage of shot blasting. Then, the Bauschinger ratios allowed to determine the type of hardening for each material. The cobalt chromium alloy showed the most kinematic hardening (BEF=0.216) while the titanium alloy and pure titanium rods showed a rather mixed hardening (BEF= 0.585 and 0.576 respectively). In addition, the bending simulations highlighted a tension area present in the concave part of the rod after elastic recovery and vice versa. In order to follow the lordosis of the column, the rod must be bent, inducing a traction area in the concave side of the rod. This traction area appears to be at the origin of the weakening and failure of the rod. Furthermore, it has been noticed that the internal stresses induced by the bending process increase drastically with the angle. Finally, the 3 points fatigue tests showed that the straight rods present a resistance to fatigue well above the bended rods which showed a failure after approximately 200 000 cycles. Considering that, according to the surgeons, the rods usually break after 14 months, this represent one solicitation every 3 minutes. Therefore, the fatigue process could be due to the movements of the child during the day (stand up, stoop, get dressed, play, …). Additionally, the rods which were successively bended and straightened up showed even worst properties. Surgeons should therefore avoid to bend the rods if deemed unnecessary and avoid to straighten them if a mistake during bending should occur. At this stage of the research, more results are required to select the material with best properties for this application