The Central Hip Vertical Axis A Reference Axis for the Scoliosis Research Society Three-Dimensional Classification of Idiopathic Scoliosis

Study Design. Reliability comparison of 2 radiographic axis systems by inter- and intraobserver variability. Objective. To determine whether the central hip vertical axis (CHVA) provides a more reliable reference axis for the evaluation of scoliosis. Summary of Background Data. Current practices in the evaluation of the scoliotic spine use the central sacral vertical line (CSVL), a true vertical drawn upward from the middle of S1, to assess the spinal deformity. However, the CVSL is defined only in the coronal radiographic view and has no corresponding definition in the sagittal view. Therefore, it represents a 2-dimensional positioning of the scoliotic segments relative to the pelvis. In view of this limitation, the Scoliosis Research Society 3-dimensional (3D) scoliosis committee proposed the CHVA, a true vertical bisecting the line segment joining the centers of the 2 femoral heads, as a reference line for the 3D evaluation of the spinal deformity. Unlike the CSVL, the CHVA can be identified in both radiographic views (coronal and sagittal) and has been shown to represent the physiologic center of balance of the spino-pelvic unit. Methods. A vertical axis was established on preoperative radiographs of 68 Lenke 1 main thoracic curves twice by 5 members of the Scoliosis Research Society 3D scoliosis committee assisted by dedicated software. The user digitized separately on the postero-anterior radiographs, the lateral borders of the S1 facets (for the CSVL), and 3 points on the 2 femoral heads (for the CHVA). The software then drew lines representing both axes. Then the observers determined the lumbar modifier (A, B, and C) using both axes. Results. There was no intra-and interobserver difference in the position of the CHVA (P > 0.1; SD: 0.4 mm), whereas intraobserver differences were found for the CSVL (P < 0.00007; SD: 0.9 mm). The CHVA was more reproducible and showed better intra- and interobserver agreement (kappa: 0.86/0.75; both excellent reliability), when compared with the CSVL (kappa 0.77/0.61; excellent and good reliability, respectively) for the identification of the lumbar modifier. The CSVL was on average 3.2 mm to the left, when compared with the CHVA generating a shift (A -> B -> C) in the assignment of the lumbar modifier. Conclusion. The CHVA is more reproducible and showed better intra- and interobserver agreement, when compared with the CSVL for the identification of the lumbar modifier. The CHVA can be easily computed in 3D and represents the physiologic center of balance of the spino-pelvic unit because it takes into account femoral head support. We recommend keeping the CSVL for 2-dimensional measurement to adapt the measures relative to the CSVL to the proposed CHVA axis and adopting CHVA as the reference axis for 3D evaluation of idiopathic scoliosis.