Patient-specific rods for thoracic kyphosis correction in adolescent idiopathic scoliosis surgery: Preliminary results

Introduction: Restoring a degree of kyphosis consistent with good sagittal alignment of the spine is a key concern when performing surgery to correct adolescent idiopathic scoliosis (AIS). The objective of this study was to assess the preliminary outcomes of posterior fusion for AIS using patient-specific rods that were pre-contoured based on pelvic incidence. The primary evaluation criterion was thoracic kyphosis at last follow-up. Hypothesis: The use of pre-bent patient-specific rods has a favourable effect on thoracic kyphosis at last follow-up. Material and methods: A total of 37 patients with AIS, including 17 with hypokyphosis, managed with patient-specific rods were included in a prospective study. The rod contouring angles were based on predefined pelvic incidence criteria (25° to 40° for the rod on the convex side and the same value plus 10° for the rod on the concave side). Thoracic kyphosis was assessed before surgery and at last follow-up, after 12–36 months (mean, 19 months). Student's t test was applied to compare means. Multivariate linear regression analysis was performed. Results: At last follow-up, the mean increase in kyphosis was 14° and was comparable to the planned increase (mean difference = 0, p = 0.85). Factors associated with kyphosis at last follow-up were the concave rod contouring angle and the pre-operative kyphotic angle of the thoracic segment to be instrumented (p < 0.05). Mean differences between kyphosis of the instrumented thoracic segment at last follow-up and target kyphosis were −5° in the subgroup with hypokyphosis (< 20°) before surgery and +4° in the subgroup with normal kyphosis before surgery. Conclusion: With patient-specific rods, kyphosis at last follow-up was close to the target value. Predictors of kyphosis at last follow-up were the concave rod contouring angle and pre-operative kyphotic angle of the thoracic segment to be instrumented. Over-contouring of the concave rod seems necessary in patients with preoperative hypokyphosis but not in patients with normal kyphosis. Level of evidence: III, prospective non-comparative study.