Posterior spinal fusion in scoliosis: computer-assisted tomography and biomechanics of the fusion mass.

Thirty patients who had posterior vertebral fusion for scoliosis, performed without metallic implants, were examined using computer-assisted tomography to study the configuration of the evolving fusion mass. It was observed that the deposition of bone followed lines of force, according to Wolff's Law. The greatest amounts of bone at the apex of the curve on the concave side indicate that flexion-compression forces are greater at this point. Further, it was observed that usually the fusion mass, seen in section, has a hollow "box-section" shape, which means that a resorption of unstressed central bone occurs. The hypothesis that the torsional forces are the predominant forces acting on the scoliotic spine can explain the characteristic "box-section" of the CT scans obtained.