The Effect of Soft Tissue Properties on Spinal Flexibility in Scoliosis

In Brief Study Design. Biomechanical analysis of the scoliotic thoracolumbar spine and ribcage using a three-dimensional finite element model. Objective. To explore how the mechanical properties of spinal ligaments and intervertebral discs affect coronal curve flexibility in the fulcrum bending test. Summary of Background Data. Preoperative coronal curve flexibility assessment is of key importance in the surgical planning process for scoliosis correction. The fulcrum bending radiograph is one flexibility assessment technique which has been shown to be highly predictive of potential curve correction using posterior surgery; however, little is known about the extent to which soft tissue structures govern spinal flexibility. Methods. CT-derived spinal anatomy for a 14-year-old female adolescent idiopathic scoliosis patient was used to develop the three-dimensional finite element model. Physiologic loading conditions representing the gravitational body weight forces acting on the spine when the patient lies on their side over the fulcrum bolster were simulated. Initial mechanical properties for the spinal soft tissues were derived from existing literature. In 6 separate analyses, the disc collagen fiber and ligament stiffness values were reduced by 10%, 25%, and 40% respectively, and the effects of reduced tissue stiffness on fulcrum flexibility were assessed by comparison with the initial model. Finally, the effect of discectomy on fulcrum flexibility was simulated for thoracic levels T5–T12. Results. Reducing disc collagen fiber stiffness resulted in a greater change in segmental rotations in the fulcrum bending test than reducing ligament stiffness. However, reductions of up to 40% in disc collagen fiber stiffness and ligament stiffness produced no clinically measurable increase in fulcrum flexibility. By contrast, after removal of the discs, the simulated fulcrum flexibility increased by more than 80% compared with the initial case. Conclusion. Homogeneous reduction in either the disc collagen fiber or ligament stiffness had minimal influence on scoliotic curve reducibility. However, discectomy simulation shows that the intervertebral discs are of critical importance in determining spinal flexibility. Assessment of preoperative flexibility is important in surgical planning for scoliosis correction; however, the extent to which spinal soft tissues govern flexibility is unknown. A biomechanical model of a scoliotic spine during fulcrum bending demonstrated that the intervertebral discs are of critical importance in determining spinal flexibility.