How low can you go? Implant density in posterior spinal fusion converted from growing constructs for early onset scoliosis

Study design Retrospective, multicenter comparative. Objectives Our purpose was to compare early onset scoliosis (EOS) patients treated with ultra-low, low, and high implant density constructs when undergoing conversion to definitive fusion. Summary of background data Larson et al. demonstrated that implant density (ID) at fusion does not correlate with outcomes in the treatment of adolescent idiopathic scoliosis, but did not address growth-friendly graduates. Methods EOS patients treated with growth-friendly constructs converted to fusion between 2000 and 2017 were reviewed from a multicenter database. ID was defined as number of pedicle screws, hooks, and sublaminar/bands per level fused. Patients were divided into ultra-low ID (< 1.3), low (>= 1.3 and < 1.6), and high ID (>= 1.6). Exclusion criteria: < 2 years follow-up from fusion or inadequate radiographs. Results A total of 152 patients met inclusion criteria with 39 (26%) patients in the high ID group, 33 (22%) patients in the low ID group, and 80 (52%) in the ultra-low ID group. Groups were similar in operative time (p = 0.61), pre-fusion major curve (p = 0.71), mean number of levels fused (p = 0.58), clinical follow-up (p = 0.30), and radiographic follow-up (p = 0.90). Patients in the low ID group (11.6 +/- 1.5 years) were slightly younger at the time of definitive fusion than patients in the ultra-low ID group (12.9 +/- 2.2 years) and high ID group (12.5 +/- 1.7 years) (p = 0.009). There was significantly more blood loss in the high ID group than the other two groups (high ID: 946.8 +/- 606.0 mL vs. low ID: 733.9 +/- 434.5 mL and ultra-low ID: 617.4 +/- 517.2 mL; p = 0.01), but there was no significant difference with regard to percent of total blood volume lost (high ID: 59.3 +/- 48.7% vs. low ID: 54.5 +/- 37.5% vs. ultra-low ID: 51.7 +/- 54.9%; p = 0.78). There was a difference in initial improvement in major curve between the groups (high ID: 21.6 degrees vs. low ID: 18.0 degrees vs. ultra-low ID: 12.6 degrees; p = 0.01). However, during post-fusion follow-up, correction decreased 7.1 degrees in the high ID group, 2.6 in the low ID group, and 2.8 in the ultra-low ID group (p = 0.19). At final follow-up, major curve correction from pre-fusion was similar between groups (high ID: 14.5 degrees vs. low ID: 15.5 degrees vs. ultra-low ID: 9.7 degrees, p = 0.14). At final follow-up, there was no difference in T1-T12 length gain (p = 0.85), T1-S1 length gain (p = 0.68), coronal balance (p = 0.56), or sagittal balance (p = 0.71). The revision rate was significantly higher in the ultra-low ID group (13.8%; 11/80) versus the high ID group (2/39; 5.1%) and low ID group (0/33; 0%) (p = 0.04). Conclusions Although an ID < 1.3 in growth-friendly graduates produces similar outcomes with regard to curve correction and spinal length gain as low and high ID, this study suggests that an ID < 1.3 is associated with an increased revision rate.