BIOMECHANICAL MODELING OF SCOLIOTIC SPINE INSTRUMENTATION USING FLEXIBLE MECHANISMS - FEASIBILITY STUDY

Surgical instrumentation of the scoliotic spine is a complex procedure with many parameters, such as the spinal segment to operate on, the n umber and position of the hooks and screws, etc. Biomechanical modelin g is a tool which can be used to determine the influence of these para meters. However, technical difficulties due to the large stiffness ran ge of involved components and the large deformations associated with s urgical maneuvers are encountered when using the finite elements metho d. Thus, the objective of this study is to adapt a modeling approach u sing analysis of flexible mechanisms and evaluate its feasibility. The model combines rigid bodies for the vertebrae and flexible elements r epresenting intervertebral structures. The mechanical properties were calculated from published data and the geometry was personalized with intraoperative measurements. Following the installation of the hooks a nd screws on the modeled spine, two steps were used to simulate the su rgical maneuvers: 1) translation and attachment of the hooks/screws on the first rod; 2) rod rotation. The feasibility of this modeling appr oach was evaluated by simulating the surgical maneuvers on 2 cases: 1) a physical model; 2) a clinical case. The agreement between intraoper ative measurements and simulation results (frontal curvatures are repr oduced with over 80% accuracy) shows the feasibility of the modeling a pproach. This approach also reduces computational convergence problems because of its limited sensitivity to stiffness differences between e lements, which demonstrates the advantage of flexible mechanism modeli ng relative to finite element modeling. Long term goals of subsequent refinements are the development of a tool for surgical correction pred ictions and for the design of more efficient instrumentation.