Abstract
Introduction: Pre-operative 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. The aim of this study was to explore how the mechanical properties of spinal ligaments and intervertebral discs affect coronal curve flexibility in the fulcrum bending test. To this end a biomechanical analysis of a scoliotic thoracolumbar spine and ribcage was carried out using a three dimensional finite element model.
Methods: CT-derived spinal anatomy for a 14 year old female adolescent idiopathic scoliosis patient was used to develop the 3D finite element model. Physiological 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 six separate analyses, the disc collagen fibre 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 to T12.
Results: Reducing disc collagen fibre 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 fibre stiffness and ligament stiffness produced no clinically measurable increase in fulcrum flexibility (increase of 1.2%). By contrast, following removal of the discs, the simulated fulcrum flexibility increased by more than 80% compared to the initial case.
Discussion: Disc collagen fibre and ligament stiffness both have minimal influence on scoliotic curve flexibility. However, discectomy simulation shows that the intervertebral discs are of critical importance in determining spinal flexibility.
Correspondence should be addressed to Dr Owen Williamson, Editorial Secretary, Spine Society of Australia, 25 Erin Street, Richmond, Victoria 3121, Australia.
