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A retrospective study.To analyze the incidence, characteristics, risk factors, and Scoliosis Research Society Instrument-24 (SRS-24) outcome scores of pseudarthrosis in adult idiopathic scoliosis primary fusions.The healing of spinal fusion is complex and difficult to study in a clinical setting. There are no detailed reports on pseudarthrosis in primary fusion for adult idiopathic scoliosis since the introduction of “modern” segmental fixation techniques.A retrospective chart and radiographic review of 96 patients (average age 42.2 years; range 18.2–62.9 years) with adult idiopathic scoliosis undergoing first time (primary) spinal instrumentation and fusion with a minimum 2-year follow-up (average 5.9 years; range 2–16.8 years) treated at a single institution between 1985 and 2001 were analyzed.Sixteen patients had pseudarthroses (17%). Fifty-nine percent of the pseudarthroses occurred between T9 and L1, and 81% presented with multiple levels involved (2–6 levels). The site of crosslinks or dominoes correlated with pseudarthrosis site in 69%. Pseudarthroses were detected radiologically at 32.4 months (range 12–67 months) postoperatively. Patient age at surgery more than 55 years significantly correlated with pseudarthrosis (P = 0.007). The number of fused levels more than 12 vertebrae is also significantly correlated with pseudarthrosis (P = 0.03). Smoking history and comorbidity did not increase the pseudarthrosis rate (P = 0.71 and 0.19, respectively). A larger preoperative Cobb angle (≥70°) and a greater thoracic kyphosis (T5–T12 >40°) did not correlate with a higher pseudarthrosis rate (P = 0.76 and 0.73, respectively). Thoracolumbar kyphosis (T10–L2 ≥20°) correlated with a significantly higher pseudarthrosis rate (P < 0.0001). Preoperative global sagittal and coronal imbalance did not increase the pseudarthrosis rate (P = 0.45 and 0.62, respectively). Patients with pseudarthrosis had lower SRS-24 scores than those without (P = 0.01).The incidence of pseudarthrosis following adult idiopathic scoliosis primary fusion was 17%. The pseudarthrosis was most likely to occur at the thoracolumbar junction. Older patients (>55 years), longer fusion (>12 vertebrae), and those with thoracolumbar kyphosis (≥20°) demonstrated increased risk for pseudarthrosis. Patients’ outcomes as measured by the SRS-24 were “negatively” affected by the pseudarthrosis.