Use of C7 Slope as a Surrogate Marker for T1 Slope: A Radiographic Study in Patients with and without Cervical Deformity

T1 slope has emerged as an important radiographic parameter in the evaluation and surgical management of adult cervical spinal deformity. Given the high rates of nonvisualization of T1 slope on upright cervical radiographs, however, this study examined the evaluation of C7 slope as a potential surrogate marker.

This is a retrospective review in adult patients with and without cervical deformity to examine the correlation of C7 and T1 slopes on routine upright cervical radiographs. In secondary analysis, correlations of C7 and T1 slopes were made amongst various demographic variables, different surgical groups, and various measures of cervical alignment. Cervical deformity was defined as sagittal vertical axis >40 mm, coronal Cobb angle >10°, and sagittal Cobb >10° in kyphosis.

C7 slope was visualized in 93% of patients as opposed to T1 slope in 68% of patients, leading to a final study population of 129 patients. Mean values of C7 and T1 slopes were 26.5° and 28.1°, respectively. Significant correlation was found in patients with and without cervical deformity (r = 0.9, P < 0.01). This correlation remained significant amongst demographics, surgical groups, and measures of cervical alignment.

Results demonstrated that C7 and T1 were in direct correlation in a variety of different cohorts regardless of deformity status or prior fusion. This study indicates that C7 slope may be reliably used a surrogate marker especially when visualization of T1 slope is not possible.