How the spine differs in standing and in sitting-important considerations for correction of spinal deformity.

Background Context: The current prevailing school of thought in spinal deformity surgery is to restore sagittal balance with reference to the alignment of the spine when the patient is standing. This strategy, however, likely accounts for increased rates of proximal junctional failure.

Purpose: The purpose of this study was to investigate the differences between the spine in standing and sitting positions as these may elucidate reasons for deformity correction failure.

Study Design/setting: A prospective, comparative study of 58 healthy patients presenting to a tertiary hospital over a 6-month period was carried out.

Patient Sample: All patients presenting with a less than 3-month history of first episode lower back pain were included. Patients who had radicular symptoms, red flag symptoms, previous spine surgery, or visible spinal deformity during forward bending test were excluded. Pregnant patients were also excluded.

Outcome Measures: Radiographic measurements including sagittal vertical axis (SVA), lumbar lordosis (LL), thoracolumbar angle (TL), thoracic kyphosis (TK), cervical lordosis (CL), pelvic incidence (PI), and pelvic tilt (PT) were collected. The sagittal apex and end vertebrae of all radiographs were also recorded.

Methods: Basic demographic data (age, gender, and ethnicity) was recorded. Lateral standing and sitting radiographs were obtained using EOS technology. Statistical analysis was performed to compare standing and sitting parameters using chi-square tests for categorical variables and paired t tests for continuous variables.

Results: Taking the standing position as the reference point, forward displacement of the SVA occurred during sitting by a mean of 6.39±3.87 cm (p<.001). This was accompanied by a reduction of LL and TK by a mean of 24.63±12.70° (p<.001) and 8.56±7.21°(p<.001), respectively. The TL became more lordotic by a mean of 3.25±7.30° (p<.001). The CL only reached borderline significance (p=.047) for increased lordosis by a mean of 3.45±12.92°. The PT also increased by 50% (p<.001). Despite relatively constant end vertebrae, the apex vertebra moved inferiorly for the thoracic curve (p<.006) and superiorly for the lumbar curve (p<.001) by approximately one vertebral level each.

Conclusions: Sagittal spinal alignment changes significantly between standing and sitting positions. Understanding these differences is crucial to avoid overcorrection of LL, which may occur if deformity correction is based solely on the spine's standing sagittal profile.