Introduction: Cervical kyphosis is often considered a marker of cervical deformity, but this may not be valid since recent studies are suggesting that: 1) Cervical curvature (CC) is affected by thoracic and global alignment; 2) There is a large variability in normative CC ranging from lordotic to kyphotic alignment in the setting of asymptomatic subject. This study investigates the effect of thoracic and global alignment on CC in maintenance of horizontal gaze. The investigators hypothesized that cervical kyphosis may be a physiologic alignment necessary for the maintenance of horizontal gaze depending on underlying thoracolumbar (TL) alignment.
Methods: This is a retrospective review for patients who underwent full‐body imaging between 2012 and 2014. For formula development, full body x‐rays of 744 patients without presenting cervical complaints or existing fusions higher than T3 were studied. Only patients who maintained their horizontal gaze (CBVA ‐5° and 17° or McGregor's slope between ‐6° and 14°) were included. Patients were stratified based on thoracic kyphosis (TK) into (> 50, 40‐50, 30‐40 and < 30). Patients were sub‐stratified by SRS‐Schwab sagittal vertical axis (SVA) modifier into (posterior alignment SVA < 0, aligned 0‐50 and malaligned > 50mm). C2‐C7 cervical curvature was assessed among SVA grade in every TK group. Stepwise linear regression analysis was applied. A simplified formula was validated on random selection of 1905 patient visits from same database.
Results: In each thoracic kyphosis group (n = 265, 172, 163, 144), cervical curvature was significantly more lordotic by increased Schwab SVA grade (Figure 1). In SVA < 0, CC was neutral for TK 40‐50°, and kyphotic for TK < 40°. All patients with SVA < 50 mm, and TK < 30° were kyphotic. Regression analysis revealed lumbar lordosis LL minus TK (LL‐TK) as an independent predictor (r = 0.653, r2 = 0.426) with formula: CC = 10 ‐ (LL‐TK)/2. Validation of the formula revealed error of 1.2° between predicted CC and real CC (r = 617, r2 = 381).
Conclusions: Kyphotic cervical alignment is necessary in the maintenance of horizontal gaze in some well aligned and some sagittal backward patients depending on thoracic curvature. Questioning the ability of kyphotic cervical alignment to maintain the gaze for patients with thoracolumbar malalignment (SVA > 50 mm). CC can be predicted from underlying TK and lumbar lordosis, which can be clinically relevant in cervical deformity correction with respect to patient specific thoracolumbar alignment.