Coupling between sagittal and frontal plane deformity correction in idiopathic thoracic scoliosis and its relationship with postoperative sagittal alignment.

STUDY
DESIGN: Prospective clinical-radiographic study.
OBJECTIVES: To investigate the natural coupling behavior between frontal deformity correction and the simultaneous changes in thoracic kyphosis, and to examine how the postoperative thoracic sagittal realignment relates to this natural coupling behavior. SUMMARY OF BACKGROUND DATA: Restoration of the sagittal alignment is one of the fundamental goals in scoliosis correction surgery. It is generally achieved by rod precontouring intraoperatively. However, clinical studies suggested that postoperative sagittal realignment seems to be more affected by the inherent properties of the spine rather than the instrumentation or the surgical maneuver.
METHODS: Ninety-eight idiopathic scoliosis patients with thoracic curves treated with one-stage posterior spinal fusion, using corrective segmental spinal instrumentation (hook-rod or pedicle screw-rod constructs) were investigated. Pre- and postoperative frontal and sagittal alignments were measured by standing anteroposterior and lateral radiographs. Preoperative frontal plane flexibility was assessed by the fulcrum bending radiograph in the standard manner, an additional radiograph was taken in the lateral plane, to assess how this frontal correction force affects sagittal plane alignment (lateral fulcrum bending radiograph).
RESULTS: When thoracic frontal deformity was corrected under fulcrum bending, coupled changes in the thoracic kyphosis demonstrated 3 different patterns: thoracic kyphosis increased in 25 patients with a mean kyphosis of 9 degrees to 19 degrees, decreased in 45 with a mean of 34 degrees to 21 degrees and remained unchanged (within 3 degrees ) in 28 with a mean of 19 degrees to 18 degrees. 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 preoperative lateral fulcrum bending radiograph. There was no statistically significant difference (P = 0.263) between using pedicle screws and hooks in achieving the additional correction beyond what was demonstrated on the lateral fulcrum bending radiographs.
CONCLUSION: 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. This can be used as a guideline for exact preoperative rod contouring to reduce the stress on the bone-implant interface and the rate of postoperative failures. 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.