Introduction: Restoration of the sagittal alignment in scoliosis correction surgery is generally achieved by rod pre-contouring intra-operatively. However clinical studies suggested that it could be affected by the inherent properties of the spine rather than the instrumentation or surgical maneuver.
Objectives: To investigate the natural coupling behavior between the frontal and sagittal deformities, and to examine how the post-operative thoracic sagittal realignment relates to this natural coupling behavior.
Methods: Ninety eight idiopathic scoliosis patients with thoracic curves treated with one-stage posterior spinal fusion using segmental spinal instrumentation were investigated. Pre- and postoperative frontal and sagittal alignments were measured by fulcrum bending and standing anteroposterior and lateral radiographs.
Results: When thoracic frontal deformity was corrected under fulcrum bending, coupled changes in the thoracic kyphosis demonstrated three different patterns: thoracic kyphosis increased in 25 patients, decreased in 45 patients and remained unchanged in 28 patients. After surgery, the direction of correction of thoracic kyphosis significantly correlated with the coupling patterns demonstrated on fulcrum bending radiographs (r=0.579, p<0.001). However, the actual postoperative thoracic kyphosis angle cannot be predicted by the pre-operative lateral fulcrum bending radiograph.
Conclusions & significance: Changes in thoracic kyphosis on fulcrum bending due to natural coupling of deformities are directed towards “self-normalization”. There is no difference in the sagittal plane deformity correction with the use of hook-rod system or pedicle screw-rod constructs. The findings also suggest that it is not how big or strong the implants are, but rather the natural curve behavior will at least partially determine the final sagittal outcome.