Position- and Measurement-Dependent Differences in Cervical Lordosis: A Comparative Analysis of Standing Radiographs and Supine MRI

Cervical lordosis is a key determinant of sagittal spinal alignment and plays a critical role in maintaining horizontal gaze, biomechanical efficiency, and overall spinal balance. Accurate assessment of cervical alignment is essential for clinical decision-making, particularly in the evaluation of degenerative conditions, deformity, and surgical planning. However, despite its clinical importance, there remains substantial variability in both the measurement techniques and the reported normative values of cervical lordosis.

Among the available measurement methods, the Cobb technique and the posterior tangent method are the most commonly used approaches. While the Cobb method provides a global estimate of cervical curvature, the posterior tangent method allows for a more segmental analysis. Nevertheless, these techniques may yield significantly different results even within the same patient, contributing to inconsistency across studies. Furthermore, the absence of a universally accepted standard complicates comparisons between studies and limits the generalizability of findings.

One of the most important determinants of cervical lordosis is the T1 slope, defined as the angle between the superior endplate of the T1 vertebra and the horizontal plane. The T1 slope has been shown to function analogously to pelvic incidence in the lumbar spine, serving as a morphological parameter that dictates the required degree of cervical lordosis for maintaining sagittal balance. Several studies have demonstrated a strong linear relationship between T1 slope and cervical lordosis, suggesting that cervical curvature is largely T1 slope-dependent. This relationship has led to the development of predictive models for estimating ideal cervical lordosis based on T1 slope values.

Despite this established biomechanical relationship, another critical factor influencing cervical alignment is patient positioning during imaging. It is well recognized that standing radiographs and supine magnetic resonance imaging (MRI) produce systematically different measurements of cervical lordosis. Under weight-bearing conditions, the cervical spine adopts a more lordotic configuration to maintain head posture and balance, whereas in the supine position, the loss of axial loading leads to a relative reduction in lordosis. Previous studies have shown that MRI tends to underestimate cervical lordosis compared to standing radiographs, with reported differences ranging between 2° and 6° depending on the measurement technique and patient population.

However, most of the existing literature on imaging-related differences in cervical alignment has been conducted in populations with degenerative cervical pathology, such as cervical spondylotic myelopathy. These pathological conditions may introduce confounding factors, including stiffness, pain-related muscle guarding, and structural deformity, which can alter the natural biomechanical relationship between T1 slope and cervical lordosis. Consequently, the extent to which imaging modality alone influences cervical alignment in individuals without structural pathology remains incompletely understood.