A mechano-regulation model to optimize design of minimally invasive percutaneous fixation devices for treatment of fractured vertebrae

Abstract

Minimally invasive percutaneous fixation techniques play a role of crucial relevance in the clinical practice. In spite of their consolidated use, little is reported in the literature to provide a mechanobiological explanation on how design of fixation devices can affect the healing process within fractured vertebrae. The aim of the study is to develop a multi-scale mechano-regulation model capable of predicting how the patterns of tissue differentiation within a vertebral fracture change in the presence or in the absence of fixation devices and how the dimensions of the device, and the materials it is made from, can affect the outcome of the healing process. To this purpose, a multi-scale mechano-regulation model is developed that combines a macro-scale model representing the spinal segment L3-L4-L5 including the fractured body of the L4 vertebra, and a micro-scale model of a fractured portion of cancellous bone. The macro-scale model includes also a minimally invasive percutaneous fixation device. The above mentioned model allows us to investigate how spatial and temporal patterns of tissue differentiation in the fracture gap change for different dimensions of the fixation device components and for different materials (Ti-6A1-4V alloy and Co-Cr alloy). Furthermore, the model provides information on the stress state in the fixation device and hence allows the risk of failure of the device itself to be estimated. The mechanical properties of the forming tissue change as the healing process progresses. In order to validate the mechano-regulation model, displacement fields will be measured with moiré and holography and compared with numerical computations. The model predicts that fixation devices significantly shorten healing times. Increasing values of the rod diameter D and decreasing values of its radius of curvature R lead to shorter durations of the healing period. Manufacturing the rods in Cobalt-Chrome alloy is predicted to reduce slightly the healing period by providing greater mechanical stability within the fracture callus. © The Society for Experimental Mechanics, Inc. 2014.