Background: Several variables can have effect on sagittal balance. The changes that occur between standing and sitting have been inadequately studied, especially in the, pediatric population.
Methods: Preoperative sagittal radiographs were obtained in both standing and sitting positions for 26 patients with idiopathic scoliosis before spinal fusion and instrumentation. Standard measurements of thoracic kyphosis, lumbar lordosis, sacral slope (SS), pelvic incidence, pelvic tilt, and lumbar intervertebral angles were, recorded. Differences were compared between positions using 2-sided paired t tests.
Results: When moving from standing to a seated position, the spine loses 5-degree thoracic kyphosis (P=0.007), 29-degree lumbar lordosis (P<0.0001), and the sacrum rotates 20 degrees (P<0.0001) to a more vertical position. The greatest change in sitting sagittal balance occurs due to increased pelvic tilt with decreased SS. The next greatest change is increased forward flexion of the lowest 2 lumbar vertebrae, 6.5 degrees between L4-L5 (P<0.0001) and 5.9 degrees between L5-S1 (P<0.0001). Flexion occurs throughout the lumbar spine but its magnitude decreases in the more proximal lumbar segments, 1.6 degrees between L1-L2 (P=0.028). The sagittal vertical axis also moves more anterior by 44 mm (P<0.0001).
Conclusions: Sitting significantly straightens the spine with decreases of thoracic kyphosis, lumbar lordosis, and SS. The majority of the changes occur in the lumbar spine and pelvis. As humans spend much of their time sitting, this difference should be considered when spinal instrumentation is performed. These findings may be important to those who only sit, especially when instrumentation is extended to the pelvis.
Level of Evidence: Level II—retrospective prognostic study.