Impact of metal density on deformity correction in posterior fusions for adolescent idiopathic scoliosis: A retrospective cohort study

INTRODUCTION: Optimal implant density for posterior spinal fusion in adolescent idiopathic scoliosis (AIS) remains controversial. We aimed to examine radiographic outcomes of AIS cases treated with limited density pedicle screw constructs. METHODS: This is a retrospective analysis of 96 patients (89 females and 7 males with mean age of 13.8 ± 4.4 years) with AIS who underwent posterior spinal instrumentation at Ghurki Trust Teaching Hospital between 2014 and 2016. Construct characteristics and radiographic measurements were compared preoperatively and at 2 year follow-up using paired t-test. Pearson's correlation coefficient between curve characteristics and metal density was calculated. RESULTS: Preoperative coronal Cobb angle was 68.5 ± 6.9°. Flexibility of the curve was 47.5 ± 10.3% based on push-prone films. The mean number of vertebrae in the fusion was 10.7 ± 1.6. The implant density was 62%. The mean postoperative Cobb angle was 18.6 ± 4.2°, giving a mean correction of 72.5 ± 6.8%. Metal density was not correlated with preoperative coronal or sagittal radiographic variables; MT Cobb angle (r = 0.02, p = 0.847), MT curve flexibility (r = 0.129, p = 0.210), preoperative thoracic kyphosis (r = -0.119, p = 0.247) or lumbosacral lordosis (r = −0.048, p = 0.645). There was a significant correlation between the flexibility of the curve as assessed by push-prone radiographs with the percentage correction achieved (r = 0.368, p < 0.0001) as well as absolute correction in degrees (r = 0.643, p < 0.0001). No significant correlations were present between metal density and MT curve coronal correction rate/percentage (r = 0.086, p = 0.407) or postoperative Cobb angle (r = 0.098, p = 0.344). CONCLUSION: Metal density does not influence the coronal and sagittal correction of AIS. Neither larger nor stiffer curves necessitate high metal density. LEVEL OF EVIDENCE: IV.