Biomechanical analysis and simulation of scoliosis surgical correction.

In scoliosis, presurgical determination of each patient's segmental spinal stiffness will permit model simulation of the surgical correction. It also permits calculation of the optimal combination of corrective forces applied to the spine by the internal fixation system. This information can help to optimize the surgical procedure, especially if calculated forces and corrections can be monitored during surgery. To this end, a finite-element model of the spine has been developed that determines segmental stiffness from data of spinal geometries, derived from the radiographs of the patient, taken with incremental distraction applied to the head. On this model, custom-tailored combinations of corrective forces are applied incrementally until optimal correction is obtained, for safe levels of distraction and lateral forces. With variations in the computer software, any desired surgical instrumentation system can be simulated. Instrumented distractor and compression forceps have been fabricated to monitor surgical forces and displacements, and to permit eventual implementation of the model recommendations. Comparison of surgically acquired correction with that predicted by the model is then carried out. Data on complete analysis of two cases of idiopathic scoliosis are presented. They show significant variations in segmental and interpatient stiffness values, and how spinal stiffness governs the amount of correction gained by the nature and magnitude of corrective forces.