Necessary cervical kyphosis correction angle (NeckCA) for ideal alignment in cervical spinal deformity.

Abstract

The authors propose a formula to calculate the angle required for cervical kyphosis correction and validate its utility in achieving and maintaining optimal cervical alignment. The authors introduce a novel radiographic parameter, the necessary cervical kyphosis correction angle (NeckCA), defined as NeckCA = C2 slope (C2S) + center of gravity - T1 tilt (COG-T1 tilt) - 15. Using this formula, the authors retrospectively reviewed 29 cervical spinal deformity (CSD) correction surgical procedures performed at a single center from 2012 to 2024. Patients were categorized into two groups based on their radiological outcome: favorable (F group) and unfavorable (U group). The following criteria defined the U group: 1) T1 slope (T1S) - cervical lordosis (CL) > 25°; 2) C2-7 sagittal vertical axis (SVA) > 70 mm; or 3) segmental angle change > 10°. The authors compared clinical and radiological parameters between the groups and evaluated the discriminatory capacity of NeckCA using receiver operating characteristic (ROC) curve analysis. Among the 29 patients (19 degenerative, 6 oncological, and 4 infectious etiologies), 17 were classified into the F group and 12 into the U group. Preoperative radiological parameters such as CL, C2S, and segmental angle did not differ significantly between groups, except for C2-7 SVA, which was much greater in the U group. ROC curve analysis showed that NeckCA served as a critical predictor of radiological outcomes, with an area under the curve of 0.806 (p = 0.006). In CSD correction, increasing CL alone to meet a T1S - CL < 15° often results in compensatory increases in T1S, leading to undercorrection. To counteract this, preoperative planning should include an additional corrective angle represented by the COG-T1. The authors' analysis of 29 cases confirms the use of NeckCA as a practical and predictive parameter for achieving optimal cervical alignment.