Cervical Alignment Variations in Different Postures and Predictors of Normal Cervical Kyphosis

A New Understanding

Hwee Weng Dennis Hey, MBBS (Sing), MRCS (Ire), MMED (Orth), MCI (Sing), FRCSEd (Orth), FAMS (Orth); Eugene Tze-Chun Lau, MB BChir (Cantab); Gordon Chengyuan Wong, MBBS (Sing); Kimberly-Anne Tan, MBBS (Aus), BSc (Med) Hons; Gabriel Ka-Po Liu, MBBCh (Ire), MSc (Ire), FRCS (Ire), FRCSEd (Orth); Hee-Kit Wong, MBBS (Sing), MMED (Surg), FRCS (Glas), MCh (Orth) Liv, FAMS (Orth)

Disclosures

Spine. 2017;42(21):1614-1621. 

In This Article

Abstract and Introduction

Abstract

Study Design. Comparative study of prospectively collected radiographic data.

Objective. To predict physiological alignment of the cervical spine and study its morphology in different postures.

Summary of Background Data. There is increasing evidence that normal cervical spinal alignment may vary from lordosis to neutral to kyphosis, or form S-shaped or reverse S-shaped curves.

Methods. Standing, erect sitting, and natural sitting whole-spine radiographs were obtained from 26 consecutive patients without cervical spine pathology. Sagittal vertical axis (SVA), global cervical lordosis, lower cervical alignment C4-T1, C0-C2 angle, T1 slope, C0-C7 SVA and C2-7SVA, SVA, thoracic kyphosis, thoracolumbar junctional angle, lumbar lordosis, sacral slope, pelvic tilt, and pelvic incidence were measured. Statistical analysis was performed to elucidate differences in cervical alignment for all postures. Predictive values of T1 slope and SVA for cervical kyphosis were evaluated.

Results. Most patients (73.0%) do not have lordotic cervical alignment (C2-C7) upon standing (mean −0.6, standard deviation 11.1°). Lordosis increases significantly when transitioning from standing to erect sitting, as well as from erect to natural sitting (mean −17.2, standard deviation 12.1°). Transition from standing to natural sitting also produces concomitant increases in SVA (-8.8–65.2 mm) and T1-slope (17.4°–30.2°). T1 slope and SVA measured during standing significantly predicts angular cervical spine alignment in the same position. SVA < 10 mm significantly predicts C4-C7 kyphosis (P < 0.001), and to a lesser extent, C2-C7 kyphosis (P = 0.02). T1 slope <20° is both predictive of C2-C7 and C4-7 kyphosis (P = 0.001 and P = 0.023, respectively). For global cervical Cobb angle, T1 slope seems to be a more significant predictor of kyphosis than SVA (odds ratio 17.33, P = 0.001 vs odds ratio 11.67, P = 0.02, respectively).

Conclusion. The cervical spine has variable normal morphology. Key determinants of its alignment include SVA and T1 slope. Lordotic correction of the cervical spine is not always physiological and thus correction targets should be individualized.

Level of Evidence: 3

Introduction

Normal cervical spinal alignment is not necessarily lordotic,[1–3] contrary to established belief.[4–6] It varies from lordosis to neutral to kyphosis, and may take on complex forms such as S-shaped or inverted S-shaped curves.[1] Although there exist a number of studies demonstrating predictors for cervical alignment in a pathological spine,[7–11] the factors accounting for its highly varied behavior in a normal spine remain poorly understood. Apart from our previous study,[3] there has yet to be literature on predictors of normal cervical spine alignment. From this aforementioned study, more radiographic parameters were found to be predictive of cervical alignment during the more commonly assumed erect sitting posture[12] than during standing.[3]

Unlike lumbar alignment, which is generally lordotic due to the caudal sacral slope (SS),[13] cervical alignment may be more variable as it not only depends on the caudal T1 slope,[14,15] but also may be influenced by sagittal plane translations such as the sagittal vertical axis (SVA). Following on from the previous study, which investigates only the erect sitting posture,[3] this study evaluates the changes in cervical spine alignment in healthy normal individuals when they transition between standing, erect sitting, and natural sitting, to identify predictors of cervical alignment in these common physiological postures. The secondary objective of the present study is to test the hypothesis that T1 slope and SVA are predictors of cervical kyphosis in the normal spine.

Knowing the physiological alignment of the cervical spine and how it changes across commonly assumed postures such as erect and natural sitting is crucial to our understanding of the cervical spine as a dynamic structure. Moreover, as lordotic correction may not always advantageous, the ability to predict cervical alignment and the awareness of factors determining physiological kyphosis will guide deformity correction strategies.

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