Distal Adding-on Phenomenon in Scoliosis Secondary to Chiari Malformation Type I: Incidence and Risk Factors.

Study Design: A retrospective study.

Objective: The aim of this study was to investigate the incidence and risk factors for distal adding-on (AO) phenomenon after posterior spinal fusion (PSF) in scoliosis secondary to Chiari malformation type I (CMS) patients with right major thoracic curve.

Summary Of Background Data: Distal AO phenomenon is a common complication observed in adolescent idiopathic scoliosis (AIS), which is significantly associated with unsatisfactory outcomes. However, few studies specifically focused on the incidence and risk factors of distal AO in CMS.

Methods: Seventy-eight CMS adolescents were included with a minimum of 2-year follow-up after PSF. patients with distal AO at the final follow-up were divided into AO group and those without were No-AO group. The coronal and sagittal parameters were evaluated preoperatively, immediately after surgery, and at the last follow-up. Clinical outcome was analyzed using the Chicago Chiari Outcome Scale (CCOS).

Results: Distal AO was observed in 18 of 78 patients (23.1%). Compared with No-AO group, patients with distal AO had significantly lower Risser sign (P = 0.001), more flexibility of lumbar curve (P = 0.021), higher incidence of lowest instrumented vertebra-last substantially touched vertebra (LIV-LSTV) ≤0 (P = 0.001), smaller postoperative Cobb angle of lumbar curve (P < 0.001), and greater correction rate of lumbar curve (P = 0.001). Logistic regression analysis revealed that low Risser sign (Grade 1-2, odds ratio [OR] = 5.7, P = 0.029) and LIV-LSTV ≤0 (OR = 6.4, P = 0.019) were independent risk factors for distal AO. There was no significant difference of CCOS scores between two groups at the final follow-up (P > 0.05 for all).

Conclusion: The incidence of distal AO was 23.1% in CMS patients after PSF. Patients with low Risser sign and LIV-LSTV ≤0 were high risk factors for distal AO.Level of Evidence: 3.