Assessing Alignment Using GAP Score and Complications for Pedicle Subtraction Osteotomy Revision Surgeries for Sagittal Deformity in Previously Fused Spines Using a Satellite Rod Technique

OBJECTIVE: Pedicle subtraction osteotomy (PSO) is a surgical technique to restore lumbar lordosis in patients with rigid sagittal deformity. Herein, we report on outcomes of a surgical technique utilizing a 4-rod PSO with satellite rods. METHODS: A retrospective study was performed for a cohort of patients who underwent 4-rod PSO revision surgery for sagittal deformity. Procedures were performed by one surgeon at 2 different academic centers between 2004-2018. Alignment outcomes and complications specifically for revision surgeries in previously fused spines were assessed. RESULTS: 40 patients underwent PSO with the satellite rod technique (n = 29 at L3, 72.5%; n = 7 at L4, 17.5%). Mean PSO angle was 28.7 +/- 7.6 degrees. Two patients (5%) had rod fracture necessitating revision surgery at 32 and 34 months. Three patients (7.5%) developed proximal junctional kyphosis (PJK), but none required revision. No patients developed pseudoarthrosis. Mean preoperative sagittal vertical axis (SVA) was 13.5 +/- 7.3 cm, which decreased to 4.8 +/- 7.3 cm (P = 0.002) postoperatively. Mean preoperative pelvic incidence-lumbar lordosis (PILL) mismatch improved from 35.9 +/- 16.6 degrees preoperatively to 11.8 +/- 14.6 degrees postoperatively (P < 0.001). Mean preoperative pelvic tilt (PT) improved from 34.7 +/- 9.8 degrees preoperatively to 30.1 +/- 9.0 degrees (P = 0.026) postoperatively. Mean global alignment and proportion (GAP) score improved from 10 +/- 2 preoperatively to 8 +/- 2 postoperatively (P < 0.001), which was still considered severely disproportioned. CONCLUSIONS: Rod failure rate after PSO with the 4-rod satellite configuration was low. SVA, PI-LL mismatch, and PT significantly improved for these previously-fused patients. Mean GAP score also improved, but was still considered severely disproportioned, likely due to the inability of PSO to correct low LL or PT in previously-fused spines.