Abstract
Background
Finite element (FE) models have become a standard pre-clinical tool to study biomechanics of spine and are used to simulate and evaluate different strategies in scoliosis treatment: examine their efficacy as well as the effect of different implant design parameters. The goal of this study is to investigate, in a system of rods and laminar wires, the effect of the number of wires and their pre-stress on whole spine stiffness.
Methods
A generic FE model was developed to represent a full human spine, including vertebrae, intervertebral discs, ligaments, facet and costovertebral joints, and ribcage. Intervertebral discs were modeled with 3D rebar elements with linear elastic material properties. Vertebrae, ribs, sternum, facet joints, cartilage and endplates were modeled with brick elements, and costal muscles with shell elements with linear elastic properties. Furthermore, ligaments were modeled with truss elements with nonlinear hypo-elastic properties. The spine model was instrumented from T7 to T12 with rods and wires modeled as titanium. Nonlinear contact properties were defined for rib neck-vertebra, transverse processes-rib and facet joint sets. The FE model was loaded in flexion and the whole spine instantaneous stiffness was calculated for different wire pre-stressing levels (0.1 to 2 MPa). Similar analyses were performed with changed numbers of wires and whole spine stiffness was calculated.
Results
The results show that with increasing the pre-stress level the whole spine instantaneous stiffness increases by up to 6%. Reducing the number of wires decreases the whole spine stiffness almost linearly by 5%. These changes also alter center of rotation of the spine. The results suggest that pre-stressing and number of wires have an effect on whole spine stiffness.
Conclusions
In summary, the develop FE model can be used to simulate different treatment strategies and to improve implant designs used in surgical treatment of scoliosis.
Level of evidence
FEA study
