16.1 Introduction

There has been new and engaging research in the understanding of how sagittal and coronal spinal imbalance affects patient function and satisfaction. The most robust data concentrates on spinal pelvic parameters in the thoracolumbar spine. More recently, there has been an accumulation of data demonstrating the importance of balance in the cervical spine (i.e., cervical sagittal alignment impacts quality of life). Each spinal region is unique because of their anatomical features and physiologic functions. Unlike the more rigid thoracic and lumbar spine, the cervical spine is very complex; it allows the widest range of motion while needing to support the head in a neutral position to allow horizontal gaze. Because of this complex nature, the cervical spine is inherently more susceptible to degeneration, injury, and dysfunction. ¹ Cervical kyphosis, a sagittal deformity, is the most common deformity in this region and is secondary to a multitude of etiologies: inflammatory spondyloarthropathies (ankylosing spondylitis and diffuse idiopathic skeletal hyperostosis), myopathies, degenerative disease, traumatic injuries, pathologic fractures, and iatrogenic causes (postlaminectomy kyphosis). ^{1 ,​ 2 ,​ 3 ,​ 4 ,​ 5}

16.2 Relationship between Sagittal Deformity and Cervical Myelopathy

Traditionally, cervical myelopathy is thought to be secondary to cervical stenosis and spondylosis. However, there is evidence suggesting that abnormal cervical sagittal alignment accounts for many cases of myelopathy. Abnormal cervical sagittal alignment, with the majority of cases being loss of lordosis or kyphosis, can lead to spinal cord injury and clinical myelopathy. ⁶ In cadaver models and animal studies, increased sagittal alignment (kyphosis) results in spinal cord tension and flattening, which leads to increased intramedullary pressure within the spinal cord. ^{7 ,​ 8 ,​ 9 ,​ 10} The end result of high intramedullary pressure is ischemia, demyelination, neuronal apoptosis, and injury. ¹⁰ In the setting of cervical kyphosis, the spinal cord can be further injured by being forced over the posterior aspect of the vertebral body causing direct mechanical pressure onto the spinal cord. This impingement causes direct injury to the underlying spinal tracts, most notably the corticospinal tracts that are located in the anterior portion of the spinal cord. This mechanical pressure can further result in greater tension within the spinal cord and contribute to even higher intramedullary pressures. ^{2 ,​ 11} This direct mechanical pressure onto the spinal cord has been observed in patients with cervical sagittal malalignment during flexion–extension studies on electrophysiology and magnetic resonance imaging (MRI). ^{12 ,​ 13 ,​ 14}

Cervical kyphosis is often a progressive process and can become quite severe if left untreated. The structural malalignment itself can result in significant patient dysfunction. These dysfunctions include debilitating neck pain, difficulties with eating (infrequently requiring a feeding tube in severe cases), inability in maintaining horizontal gaze (look straight), imbalanced gait, and restrictions of activities of daily living. ¹⁵ With regard to specific locations of pain other than the neck, patients that maintain a forward head posture can have increased incidence of interscapular head pains.

16.3 Cervical Spinal Curvature and Alignment Parameters

16.3.1 Cervical Lordosis

There are three general methods to measuring cervical lordosis: Cobb angle, the Harrison posterior tangent method, and Jackson physiological stress lines. ^{1 ,​ 16} Cobb angle is the clinical mainstay given its easy-to-use format and good intra- and interrater reliability. ^{17 ,​ 18} To measure cervical sagittal Cobb angle, either C1 to C7 or C2 to C7 can be used. The angle subtended between perpendicular lines drawn from the caudal line (C1 or C2) and rostral line (C7) is the cervical sagittal Cobb angle. The caudal line at C1 is drawn extending from the anterior tubercle of C1 to the posterior margin of the spinous process. If C2 is used, a line parallel to the inferior endplate of C2 vertebral body is drawn. ^{1 ,​ 16} The rostral line is drawn parallel to the superior endplate of C7. The method of measuring the C2 to C7 sagittal Cobb angle is shown in Fig. 16‑1. The Harrison posterior tangent method involves drawing parallel lines to the posterior surfaces of all cervical vertebral bodies from C2 to C7 and then summing up the segmental angles for an overall cervical curvature angle. ^{1 ,​ 16} The cervical lordosis measured by the Jackson physiological stress lines is performed by drawing parallel lines to the posterior surface of C2 and C7; the angle between those lines is the cervical lordosis measurement.

16.3.2 Cervical Sagittal Alignment

Like global sagittal alignment, regional cervical sagittal alignment can also be quantified as the cervical sagittal vertical axis (cSVA). ¹ cSVA is a regional measurement of sagittal alignment. This is the distance between a plumb line dropped from the centroid of C2 (or odontoid) and the posterior superior aspect of the C7 vertebral body (Fig. 16‑2). Another method of assessing regional cervical sagittal alignment is to use the center of gravity (COG) of the head (COG-cSVA) rather than the C2 centroid. To measure COG-cSVA, a plumb line is drawn from the anterior portion of the external auditory canal and the distance from the posterior–superior aspect of C7 is measured. ¹ The most commonly used method is the cSVA because past studies have shown that this parameter is directly correlated with health-related quality of life (HRQoL) outcomes (i.e., greater cSVA is associated with worse outcomes especially when greater than 40 mm). ¹⁹

16.3.3 Chin-Brow Vertical Angle

The chin-brow vertical angle (CBVA) is a measurement of horizontal gaze. This is an important parameter as horizontal gaze is essential for many of the activities of everyday life and ambulation. ^{20 ,​ 21 ,​ 22} The CBVA is measured by drawing a line from the patient’s brow to chin and a vertical line. The angle subtended between those two lines is the CBVA (Fig. 16‑3). ¹

16.3.4 Neck Tilt, Thoracic Inlet Angle, and T1 Slope

More investigational and novel methods of quantifying sagittal alignment in the cervical spine include neck tilt (NT), thoracic inlet angle (TIA), and T1 slope. ¹ NT is defined as an angle between two lines both originating from the upper end of the sternum with one being a vertical line and the other connecting to the center of the T1 endplate. The TIA is defined as the angle between a line originating from the center of the T1 endplate that is perpendicular to the T1 endplate and a line from the center of the T1 endplate to the upper end of the sternum. T1 slope is the angle measured between a horizontal plane and a line parallel to the T1 endplate. T1 slope is significant because it is a surrogate for the amount of subaxial lordosis required to maintain the COG of the head in a balanced position. Hence, this parameter may guide and predict physiological alignment following deformity correction. ²³ Examples of these three measurements can be seen in Fig. 16‑4.

16.3.5 C2 Pelvic Tilt

A novel measurement that has been shown to be valuable in preoperative evaluation of cervical deformity is C2 pelvic tilt (C2PT). C2PT considers the relationship of T1 slope and cervical lordosis with the extent of pelvic retroversion. C2PT is the sum of C2 tilt and pelvic tilt. C2 tilt is defined as an angle subtended by a line drawn parallel on the posterior aspect of the C2 vertebral body and a vertical line (Fig. 16‑5). ²⁴ Pelvic tilt is the angle formed by the intersection of a line drawn from the middle of the S1 endplate and a vertical line from the femoral head if aligned or bicoxofemoral axis (midpoint between the femoral heads) (Fig. 16‑5). The C2PT angle can be measured by extending the line parallel to the posterior edge of the C2 vertebral body until it intersects the line that is extended from the femoral head to the midpoint of the superior endplate of S1 (Fig. 16‑5).

16.3.6 Craniocervical Angle

Craniocervical angle (CCA) is a measurement that considers cervical compensation and sagittal inclination. CCA is the angle subtended by a line from the hard palate to the opisthion (McGregor’s line) and a line from the hard palate to the center of C7 vertebral body (Fig. 16‑6). ²⁴ As cervical deformity worsens, the CCA value gets smaller, and a CCA of less than 45° may be an indicator of the presence of a large cervical deformity.

16.4 Normative Cervical Curve and Alignment

Cervical sagittal curve is closely related to cervical sagittal alignment. The difference between the two is that cervical sagittal curve is a regional measurement while cervical sagittal alignment considers global spinal balance (thoracic, lumbar, and sacral spine). Normal or mean cervical lordosis can be quite variable depending on age and gender. According to past studies of asymptomatic patients, normative regional cervical lordosis from C1 to C7 in adults was 41.8°. ^{1 ,​ 25} Most of the cervical lordosis occurs at the C1-C2 junction (mean of 32.2°), while other individual cervical levels only contribute approximately 1° to 2° of additional lordosis. ^{1 ,​ 25 ,​ 26} It is also important to keep in mind that a mild physiologic cervical kyphosis is present in up to 34% of asymptomatic individuals, especially in younger individuals. ^{27 ,​ 28} Cervical sagittal alignment is dependent on the global spinal curvature and the anatomy of the cervicothoracic junction, because the body tries to maintain a neutral upright posture and horizontal gaze. In asymptomatic adults, mean cSVA has been reported to be 15.6 mm.