Understanding sagittal compensation in adult spinal deformity patients: relationship between pelvic tilt and lower-extremity position

Abstract

OBJECTIVE In adult spinal deformity (ASD), quantifying preoperative lower-extremity (LE) compensation is important in formulating an operative plan to achieve optimal global sagittal alignment. Whole-body radiographs are not always available. This study evaluated the possibility of estimating LE compensation without whole-body radiographs. METHODS In total, 200 consecutive ASD patients with full-body radiographic assessment were categorized into the following three groups according to their cranio-hip balance (distance from the cranial sagittal vertical axis to the hip axis [CrSVA-H]): group 1, anterior-shift (A-shift) group (CrSVA-H > 40 mm); group 2, balanced group, −40 mm < CrSVA-H < 40 mm; and group 3, posterior-shift (P-shift) group, CrSVA-H < −40 mm. After analyzing the correlation between CrSVA-H, pelvic tilt (PT), and LE parameters, the cutoff PT and PT/pelvic incidence (PI) values that correlated with the presence of LE compensation were determined. Previously published data from asymptomatic volunteers were used as a baseline threshold (sacrofemoral angle [SFA] > 217.0° and knee flexion angle [KA] > 11.0°). RESULTS Among the hip, knee, and ankle, only KA showed a significant increase in the A-shift group compared to the other two groups (p < 0.01). With a wide threshold (SFA > 208.0° and KA > 5.0°), 84.9% of the A-shift group showed LE compensation (hip or knee or both), which was a significantly greater percentage than those in the balanced and P-shift groups (48.4% and 51.9%, p < 0.01). With a narrow threshold (SFA > 217.0° and KA > 11.0°), 62.2% of the A-shift group showed any LE compensation, which was also a higher percentage than the other two groups. The CrSVA-H was moderately correlated with KA (r = 0.502), but had no correlation with PT, SFA, and ankle dorsiflexion angle (AA). PT showed a moderate/strong correlation with SFA, KA, and AA (r = 0.846, 0.624, and 0.622, respectively). With receiver operating characteristic curves, the authors determined that a 23.0° PT with PT/PI > 0.46 predicts the presence of any type of LE compensation with use of the wide threshold. CONCLUSIONS ASD patients with increased CrSVA-H, which represents cranio-hip anterior imbalance, demonstrated a higher prevalence of LE compensation, especially knee flexion, compared to those with neutral and posterior shift of CrSVA. PT represents the extent of LE compensation in patients with spinal sagittal malalignment. Using the cutoff value of PT determined in this study, surgeons can preoperatively estimate the extent of LE compensation without obtaining a full-body radiograph.