Biomechanical principles of ventral and dorsal instrumentation correction in scoliosis

Abstract

Scoliosis is a complex deformity of the spine with an abnormal shape in all different planes. Purpose of the study is to describe different biomechanical aspects of the operative treatment on the basis of own experimental results and of a review of the literature. The corrective loads, the resistance of the curvature, principles of different implants, the stability of the instrumented spine and problems of implant fixation are discussed. Corrective loads are divided in forces of distraction, compression, translation and rotation. Distraction is more beneficial for severe curvatures (> 50 degrees), while transverse load is ideal for milder deformities (< 50 degrees). Compression is only efficient when applied segmental and anterior. Resistance increases with increasing of the curvature due to the deformation of vertebral bodies. Release is possible anteriorly by open technique or endoscopically and posteriorly by concave thoracoplasty. Principles of different implants are shown: Harrington- (distraction), Luque- (translation), VDS- (compression, rotation), CD- (combined loading--posterior) and Kaneda-System (combined loading--anterior). It is possible to perform a primary stable fusion of the spine anterior and posterior. The stability provided by three anterior devices (VDS, TSRH, CDH) has been studied in an in-vitro model. TSRH was found to be significantly (p < 0.05) higher in stiffness in flexion/extension and torsion than VDS. In bending there was no statistical difference. In all tests CDH was significantly more stable than VDS. In flexion/extension it proved higher in stiffness than TSRH, no statistical difference was observed in bending and rotation. Bone density plays a crucial role in implantfixation, this is more important anterior than posterior. Pedicular screws are superior in their fixational strength than lamina hooks and screws in the vertebral body.