Cervical Lordosis: Imaging Position and Measurement

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.

Furthermore, although previous studies have demonstrated both (1) a strong association between T1 slope and cervical lordosis and (2) systematic differences between imaging modalities, these two aspects have rarely been evaluated together within a single analytical framework. In particular, there is a lack of studies that quantify the independent contribution of imaging modality after adjusting for T1 slope using multivariable regression analysis. This gap limits the ability to interpret MRI-based measurements in clinical practice and raises questions regarding the comparability of measurements obtained using different imaging techniques.

The present study aims to address this gap by evaluating cervical alignment in a cohort of individuals without structural cervical pathology, thereby minimizing confounding factors related to degenerative or deformity-related changes. Cervical lordosis is assessed using Cobb-based measurements (C1-C7 and C2-C7), which are widely accepted as clinically relevant parameters. The primary objective is to compare cervical lordosis values obtained from standing lateral radiographs and supine MRI. The secondary objective is to determine the independent effects of T1 slope and imaging modality on cervical lordosis using multivariable linear regression analysis.

In addition to group comparisons, a multivariable linear regression model will be constructed to evaluate the independent contributions of T1 slope and imaging modality to cervical lordosis measurements. Cervical lordosis (C2-C7 and C1-C7 Cobb angles) will be used as dependent variables, while T1 slope (continuous variable) and imaging modality (binary variable: supine MRI vs standing radiograph) will be included as independent predictors.

This modeling approach allows for quantification of the extent to which imaging modality influences cervical lordosis independently of underlying biomechanical alignment. Furthermore, the regression model provides a predictive framework for estimating cervical lordosis based on T1 slope and imaging conditions, addressing the lack of standardized conversion between supine MRI and standing radiographic measurements reported in the literature.

It is hypothesized that:

1. Cervical lordosis measurements obtained from supine MRI are significantly lower than those obtained from standing radiographs, and
2. Imaging modality has an independent effect on cervical lordosis measurements even after adjusting for T1 slope.

By integrating biomechanical parameters with imaging modality effects, this study aims to provide a more comprehensive understanding of cervical sagittal alignment and to contribute to the development of clinically applicable models for interpreting MRI-based measurements. Ultimately, these findings may help improve the accuracy of preoperative planning and the interpretation of cervical spine imaging in both research and clinical settings.

This study has been designed, conducted, and will be reported in accordance with the STROBE (Strengthening the Reporting of Observational Studies in Epidemiology) statement. As the study utilizes retrospectively collected routine clinical imaging data, additional adherence to the RECORD (REporting of studies Conducted using Observational Routinely-collected health Data) guidelines is ensured.

Sample size considerations were informed by previously reported differences in cervical sagittal alignment across imaging modalities and positions. Prior studies have demonstrated that differences in cervical lordosis between radiographic and MRI-based measurements may range from small to large, depending on imaging conditions and patient positioning. Considering this variability, a conservative moderate-to-large standardized effect size (Cohen's d = 0.75) was assumed for sample size estimation. Based on this assumption, a total sample size of 58 participants (29 per group) was considered sufficient to detect clinically meaningful differences between groups at a significance level of 0.05 with a statistical power of approximately 80%, while acknowledging that smaller effects may remain underpowered.

Descriptive statistics will be reported as mean ± standard deviation for continuous variables and count (percentage) for categorical variables. Normality will be assessed using the Shapiro-Wilk test. Between-group comparisons will be performed using independent-samples t test or Mann-Whitney U test, as appropriate. Sex distribution will be compared using the chi-square test or Fisher exact test, where appropriate. The primary analytical framework will include multivariable linear regression models with Cobb C2-C7 and Cobb C1-C7 angles as dependent variables and T1 slope and imaging modality as independent variables. Statistical significance will be defined as p < 0.05. All statistical analyses will be conducted using jamovi version 2.6.44 and RStudio version 2025.09.2.