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A retrospective evaluation.To evaluate the change in lumbar lordosis in spinal deformity patients who underwent an instrumented posterior spinal fusion on the Orthopedic Systems Inc. (OSI) “Jackson” frame.Intraoperative prone positioning with hip extension may posturally increase lumbar lordosis during adult spinal deformity reconstructive surgery, as has been shown in adult lumbar degenerative surgery.Radiographs of 44 operative spinal deformity patients (43 females/1 male; mean age, 57.4 years) were analyzed. Diagnoses included idiopathic scoliosis (29), degenerative lumbar scoliosis (9), and other (6). Total lumbar lordosis (T12–S1), segmental disc angles, and C7 plumbline were measured on preoperative upright and supine, intraoperative prone, and postoperative upright lateral radiographs. All patients were positioned intraoperatively with hip extension on the OSI frame.Average preoperative upright and supine, intraoperative prone, and postoperative upright lumbar lordosis (T12–SAC) measurements were −38.1°, −46.0°, −46.2°, and −51.8°, respectively (P < 0.05 for preoperative upright to all other comparisons). Two groups were noted: those with increased lumbar lordosis (>5°) during intraoperative prone positioning (n = 25, increased lordosis group) as compared to the preoperative measurement versus those with minimal to no change in lordosis (≤5°) during intraoperative prone positioning (n = 19, unchanged lordosis group). The corresponding lumbar lordosis measurements for the increased lordosis group were −25.9°, −40.0°, −43.1°, and −48.9° (P < 0.05 for preoperative upright to all other comparisons). The corresponding lumbar lordosis measurements for the unchanged lordosis group were −54.2°, −53.8°, −50.3°, and −55.7° (no significant differences). Preoperative upright lumbar lordosis in the unchanged lordosis group was substantially higher than increased lumbar lordosis group (P < 0.05).Adult spinal deformity patients with preoperative hypolordosis who were positioned prone during reconstructive surgery had an enhanced lumbar lordosis via positioning alone compared with theirpreoperative upright radiographs. Conversely, those with substantial preoperative lordosis remained unchanged with intraoperative prone positioning. This knowledge will help in the surgical planning of adult spinal deformity reconstructive surgery to optimize sagittal alignment and balance.