Biomechanical simulations for planning of scoliosis surgery

This paper summarizes a series of investigations to determine whether biome chanical simulations can be used to plan scoliosis surgery bq predicting th e outcome as a function of choices available to the surgeon. In order to te st this idea, simulations have initially been performed with Drier knowledg e of the real outcome (from postoperative radiographs). In addition, intra- operative measurements of the spine shape and displacement of instrumentati on have been used to improve documentation of the influence of different st eps in the surgical procedure. Several difficulties have emerged which have given new understanding of both the mode of action of surgical procedures, as well as the biomechanical modeling, 1. important reduction of the Cobb angle occurs in the anesthetized patient compared to pre-operative (standin g) radiographs. 2. For an elastic analysis the stiffness of the spine and r ib cage should be known for each anatomical level of each patient. 3. Shape changes in the non-instrumented part of the spine are mainly due to the pa tient and their muscles, not the instrumentation directly, and are difficul t to analyze biomechanically. 4. The large difference in stiffness between the stiffest and the least stiff parts of the model, as well as the large d isplacements which occur in surgical procedures, create technical difficult ies in simulations using finite element modeling. 5. Segmental instrumentat ion offers surgeons many variables and multistep maneuvers to adapt to indi vidual patients' needs, but conversely create many unknown inputs for the b iomechanical analyses, and difficulties in validation of model predictions. It is concluded that in order to be a reliable tool to assist with pre-oper ative planning, deterministic modeling of scoliosis surgery will require mo re information to formulate the models, better specification of inputs, and improved;analysis tools.