Evaluation and Treatment of Cervical Spine Deformity

Abstract

The cervical spine plays a pivotal role in activities of daily living by allowing a wide range of motion while supporting the cranium. Cervical spine deformity (CSD) can cause significant negative impact on the patient’s functional status and quality of life. Surgical correction of cervical spine deformity can be challenging due to the complexity of the regional neurovascular anatomy, which necessitates a comprehensive understanding of the cervical spine anatomy and biomechanics. Goals of cervical deformity surgery include decompression of neural elements, and restoration of cervical alignment, and achieving solid arthrodesis. Cervical spine deformity correction can involve various anterior and posterior surgical techniques. Careful surgical planning and meticulous surgical techniques are essential to minimize complications and optimize clinical outcomes in cervical deformity correction. In this chapter, we provide an in depth review of pre-operative evaluation, surgical planning, and treatment strategies for cervical spine deformity.

1. Introduction

The cervical spine is a highly mobile segment of the spinal column with the important role of supporting the weight of the head, maintain a horizontal gaze and allowing for a wide range of physical activities. Cervical deformity can cause significant disability and dysfunction, and has been correlated with poor health-related quality of life (HRQOL) [1, 2, 3, 4, 5, 6], as well as ranked in the bottom 25th percentile of the EuroQol-5 Dimension (EQ-5D) HRQOL metric, along with heart failure, stroke, renal failure, emphysema, blindness, breast and prostate cancer when compared to other serious chronic diseases [7].

With an aging population and growing focus on cervical spine disorders and their impact on patient outcomes, utilization of cervical spine decompression and fusion procedures in the United States has been steadily rising each year [8]. Over the past decade, substantial progress has been made in the diagnosis and treatment of cervical deformities in adults. Established parameters now define both normal and pathological cervical alignment, and classification systems for cervical deformities, along with a cervical osteotomy classification, has been introduced [9, 10, 11, 12, 13, 14, 15, 16, 17]. Despite an increase in the cervical deformity knowledge base, this work is ever expanding and continues to evolve. The consolidation of the most relevant and clinically applicable information for adult cervical deformity is needed to aid the practicing surgeon as well as those in training to better understand and treat these complex patients and ultimately improve the clinical outcome.

In this chapter, we provide a comprehensive review of cervical biomechanics, radiographic alignment parameters, deformity classification, clinical evaluation, and pre-surgical planning, surgical techniques, and complication avoidance.

2. Cervical spine biomechanics

The primary function of the cervical spine is to support the head’s position above the body and facilitate a level horizontal gaze. To accomplish this, the cervical spine has six distinct degrees of freedom in its movement. These encompass flexion/extension, axial rotation, and lateral bending, in addition to minor anterior/posterior translational movements along the Cartesian coordinates [18]. The head’s center of mass (COM) is situated slightly above and forward of the external auditory canal, approximately 1 cm above the occipital condyle [19]. Any deviations from the typical cervical alignment that lead to a displacement of the head’s center of mass result in an elevated cantilever load on the cervical spine. Consequently, this provokes an increased expenditure of energy by the paraspinal muscles [15]. Within the neutral zone of movement, the cervical spine can move with minimal energy demand from the paraspinal muscles. However, any motion beyond this neutral zone necessitates a greater force and energy input to overcome the elastic forces of the surrounding tissues. This region outside the neutral zone is aptly referred to as the elastic zone [18]. The physiologically accepted global ROM of the cervical spine typically encompasses approximately 90° of flexion, 70° of extension, 20° to 45° of lateral bending, and a maximum of 90° of rotation on each side [18, 20].

The atlanto-occipital and atlanto-axial joints unique joints of the cervical spine and because of their anatomy, they allow for significant specific motion at these segments. The atlanto-occipital joint is a synovial joint composed of the interface between the convex shaped occipital condyles and the concave shaped superior articular process of C1 [18]. This type of joint anatomy results in a large amount of flexion/extension, however, there is very little lateral bending movement or axial rotation [21]. In contrast, the atlanto-axial joint consists of four synovial joint interfaces. These include: the anterior arch of C1 and the odontoid process, the odontoid process and the transverse ligament, and the bilateral C1-2 articular surfaces. This type of joint results in a significant amount of axial rotation, but with more limited flexion/extension and lateral bending [18]. A study by Panjabi et al. [22] demonstrated that the flexion, extension, lateral bending, and axial rotation ROMS were the following: 3.5°, 21.0°, 5.5°, and 7.2°, respectively, at the atlanto-occipital joint, and 11.5°, 10.9°, 6.7°, and 38.9°, respectively, at the atlantoaxial joint. Therefore, the largest ROM between two cervical vertebral segments is the axial rotation at the atlantoaxial joint, with a neutral zone of 29.6° resulting in 75% of this motion.

The initial transfer of the axial load stemming from the weight of the head commences with its transmission from the occipital condyles to the lateral masses of C1. It subsequently progresses to the C1-2 facet joints, then to the lateral masses of C2, and ultimately disperses throughout the remainder of the spinal column by way of the intervertebral disc at C2-3 and the facet joints. This load is further distributed via the C2 articular pillars to engage the anterior column, the C2-3 disc, the posterior column, and the C2-3 facet [23]. The posterior columns of the cervical spine mostly handle the load distribution with 36% in the anterior column and 64% in the two posterior columns [23]. This is in contrast with the lumbar spine where the anterior loads 67–82% have been reported as higher than the posterior loads 18–33% [24, 25]. To counterbalance the thoracic spine’s inherent kyphotic curvature, the cervical spine typically has a lordotic shape in the neutral position [26]. The lower part of the cervical spine, characterized by its lordotic curvature, meets the inflexible kyphotic thoracic inlet at the cervicothoracic junction (CTJ). Deviations from the normal cervical lordosis, such as the emergence of cervical kyphosis, are linked to pain and diminished functionality [6, 26, 27, 28, 29]. In the presence of cervical kyphotic deformity, the center of mass (COM) of the head shifts forward, resulting in the formation of a lever arm, thereby generating a greater bending force [18]. The resulting increased bending force necessitates heightened contraction of the paraspinal muscles to uphold the head’s upright position and sustain a level horizontal gaze. As previously noted, this heightened muscle effort can lead to muscle fatigue and discomfort [18]. Furthermore, a cervical kyphotic deformity will shift the forward movement of the head’s axial load, which has the potential to expedite cervical disc degeneration. Additionally, a reduction in disc height due to degenerative changes can, in turn, exacerbate cervical kyphosis [18]. Moreover, pronounced cervical kyphosis can potentially cause elongation of the spinal cord, leading to heightened intramedullary pressure and compromised microcirculation. This, over time, can progress to spinal cord ischemia and subsequent myelopathy, as elaborated upon later. Nevertheless, it’s crucial to note that not all instances of cervical kyphosis are classified as deformities, as many of them are symptomatic. Estimates suggest that cervical kyphosis can be observed in 2–35% of asymptomatic individuals [30, 31, 32].

3. Cervical alignment radiogrpahic measurements

3.1 Cervical radiographic alignment parameters

Cervical lordosis (CL) is an important cervical alignment parameter. There are three methods to measure CL and they include Cobb angles, Jackson’s physiologic stress lines, and the Harrison posterior tangent method (Figure 1) [33].

Figure 1.

Shows the three different ways to measure the cervical lordosis (CL). A) the cobb method involves drawing two parallel lines paths: The first runs parallel to the base of the second cervical vertebra (C-2), and the second parallel to the lower edge of the seventh cervical vertebra (C-7). Next, lines perpendicular to the first two are drawn, and the point where these perpendicular lines intersect determines the cervical curvature’s angle. B) Jackson’s physiological stress line technique involves sketching a pair of lines parallel to the posterior surfaces of the C-7 and C-2 vertebrae. Subsequently, the angle formed by these two lines is precisely measured. C) Harrison’s posterior tangent approach: This method involves tracing lines parallel to the posterior surfaces of each cervical vertebra, from C-2 to C-7. The individual segmental angles are then summed up to calculate the total curvature angle of the neck.

The Cobb angle measurement technique entails drawing a line that is either parallel to the lower endplate of C2 or extending from the anterior tubercle of C1 to the posterior edge of the spinous process. Simultaneously, another line is drawn parallel to the lower endplate of C7. To determine the angle, perpendicular lines are then drawn from each of the aforementioned lines, and the cervical lordosis angle is measured as the angle formed where these perpendicular lines intersect [33]. The Jackson’s physiologic stress lines method involves drawing a parallel line perpendicular to the posterior surface of each of the C7 and C2 vertebral bodies and calculating the angle between them [34]. Lastly, the Harrison posterior tangent method requires drawing lines that are parallel to the posterior surfaces of all cervical vertebral bodies from C2 to C7. The individual segmental angles are then summed to derive an overall cervical lordosis angle [35]. In the sagittal plane, the translation of the cervical spine is primarily assessed using cSVA. cSVA can be regionally defined as the distance between a plumb line dropped from the centroid of C2 and the superior posterior aspect of the C7 vertebral body (refer to Figure 2).

Figure 2.

C2-7 SVA defined as the distance from the superior posterior edge of C7 vertebral body to the C2 plumb line.

An alternative approach to evaluating global sagittal alignment is the use of the gravity line, measured from the center of gravity (COG) of the head, alongside the C7 plumb line (COG SVA) [36, 37, 38, 39, 40, 41, 42] This method can also be applied regionally for cervical SVA by drawing a plumb line from the center of gravity of the head instead of C2 (COG-C7 SVA). On lateral radiographs, approximating the center of gravity of the head can be achieved by initiating the plumb line at the anterior part of the external auditory canal [19]. However, it’s worth noting that the C2 plumb line holds particular clinical significance, as it has a direct correlation with Health-Related Quality of Life (HRQOL), with larger C2 SVA values associated with poorer HRQOL [6].

For the measurement of horizontal gaze, the chin-brow-vertical-angle (CBVA) is employed. This measurement proves especially valuable in the management of severe, rigid cervical kyphotic deformities, as the loss of horizontal gaze significantly impacts daily activities and overall quality of life [43]. The CBVA is defined as the angle formed between a line drawn from the patient’s chin to brow and a vertical line (refer to Figure 3).

Figure 3.

The CBVA is defined as the angle formed between a line drawn from the patient’s chin to brow and a vertical line.

The angle is assessed using clinical photographs of the patient standing with their hips and knees extended, maintaining a neutral or fixed neck position [43]. Notably, deformity correction that takes into account the chin-brow-vertical-angle (CBVA) has demonstrated a significant connection with favorable postoperative results, including enhanced gaze, walking ability, and daily activities [43, 44, 45, 46, 47, 48]. A contemporary and highly significant parameter for evaluating cervical alignment is the T1 slope. This angle is defined as the measurement formed between a line parallel to the superior endplate of T1 and a horizontal reference line (refer to Figure 4).

Figure 4.

T1 slope is defined as the measurement formed between a line parallel to the superior endplate of T1 and a horizontal reference line.

Once the T1S angle is determined, it becomes possible to calculate the discrepancy between CL and T1S, denoted as CL-T1S. Schwab and colleagues have highlighted that an individual’s optimal lumbar lordosis (LL) should ideally fall within 10° of the pelvic incidence (PI), signified as LL = PI ± 10° [49, 50] A PI-LL mismatch exceeding 10° has been associated with notably inferior Health-Related Quality of Life (HRQOL), encompassing increased pain and disability, particularly among adults with thoracolumbar deformities [50, 51, 52]. In the cervical spine, a higher T1 slope necessitates a greater degree of cervical lordosis to achieve equilibrium in positioning the head over the thoracic inlet and trunk [15, 53]. This is akin to the concept of the mismatch between pelvic incidence and lumbar lordosis. In the cervical spine, this mismatch between T1 slope and cervical lordosis is termed TS-CL and has been proposed as a comparable parameter to the PI-LL mismatch [54, 55].

3.2 Normal cervical alignment

A broad spectrum of normal alignment has been documented, attributable to the cervical spine’s remarkable mobility, as outlined in Tables 1 and 2 [28, 29, 31, 56]. Among asymptomatic individuals, approximately 75–80% of cervical standing lordosis is concentrated within the C1-C2 segment [28, 57] while lower cervical levels exhibit comparatively less lordosis. This distribution mirrors the lumbar spine, where the preponderance of lumbar lordosis is concentrated at the caudal end, with L5-S1 featuring the most significant segmental lordotic angle [58]. The predominance of cervical lordosis in the C1-C2 region can be elucidated by the findings of Beier et al. [19] which indicate that the head’s center of gravity aligns closely with the centers of the C1 and C2 vertebral bodies. On average, the total cervical lordosis measures approximately −40 degrees, with the occiput-C1 segment typically displaying a kyphotic curve [28]. Merley 6 degrees (15%) of lordosis occurs at the lowest three cervical levels (C4-C7) [28]. In terms of total cervical lordosis, there is no discernible difference between asymptomatic men and women, and there exists a positive correlation between cervical lordosis and advancing age [28, 31]. The average distance for cSVA falls within the range of 15–17 ± 11.2 mm, while the characterization of normal CBVA remains undefined. However, postoperative values ranging from +10 to −10 degrees have been well tolerated by patients [43, 44, 45, 46, 47, 48].

Segmental cervical angles [28]		C2-C7 lordosis [31]
Level	Angle (deg)	Age group	Men (deg)	Women (deg)
C0-C1	2.1 ± 5.0	20–25	16 ± 16	15 ± 10
C1-C2	−32.2 ± 7.0	30–35	21 ± 14	16 ± 16
C2-C3	−1.9 ± 5.2	40–45	27 ± 14	23 ± 17
C3-C4	−1.5 ± 5.0	50–55	22 ± 15	25 ± 11
C4-C5	−0.6 ± 4.4	60–65	22 ± 13	25 ± 16
C5-C6	−1.1 ± 5.1
C6-C7	−4.5 ± 4.3
C2-C7	−9.6
total (C1-C7)	−41.8
cervical sagittal vertical axis [28]
odontoid marker at C7	15.6 ± 11.2 mm
odontoid marker at sacrum	13.2 ± 29.5 mm

Table 1.

Normal cervical spinal values in asymptomatic adults from the literature.

[Values presented as the means ± SD and the negative sign indicates lordosis in the segmental values].

	20–39 years		40–59 years		>60 years		P value
	Mean	SD	Mean	SD	Mean	SD
C2-C7 Cervical Lordosis (°)	+9.4	9	+6.6	9	+22.2*	9	<0.001
T4-T12 Thoracic Kyphosis (°)	−38.1	11	−36	9	−45	14	NS
L1-S1 Lumbar Lordosis (°)	+61.5	12	+60.3	7	+55.7	13	NS
Pelvic Tilt (°)	12.1	7	14.5	5	15.1	8	NS
Pelvic Incidence (°)	52.1	10	54.3	8	53.5	10	NS
Sacral Slope (°)	40	9	39.9	7	36.5	10	NS
SVA (mm)	−28.5	28	−18.2	39	22.4*	40	<0.001
T1-Slope (°)	−22	7	−21.1	8	−31.6*	9	0.001

Table 2.

Mean sagittal parameters among the volunteers stratified by age group, p-values refer to ANOVA comparison between groups.

NS = non significant. Adapted from Blondel et al. [56].

Cervical lordosis seems to be intricately linked to the anatomy of the cervicothoracic junction (CTJ), typically encompassing C7 and T1 vertebrae, the C1-C7 disc, and the surrounding ligaments. In the context of osteotomy planning, the CTJ definition can extend to include T2 and T3 [59]. This region also encompasses the thoracic inlet, a stable bony circle formed by the first ribs on both sides, the T1 vertebral body, and the upper part of the sternum. From a biomechanical perspective, the CTJ marks the transition from the highly mobile cervical spine to the relatively rigid thoracic spine. Additionally, it’s the point where cervical lordosis transforms into thoracic kyphosis. This curvature shift exerts significant stress on the CTJ, both in static and dynamic conditions [59, 60].

The sagittal alignment of the cranium and cervical spine is influenced by the shape and orientation of the thoracic inlet, crucial for maintaining a balanced, upright posture and horizontal gaze, akin to the relationship between pelvic incidence and lumbar lordosis (LL) [55]. Lee et al. [55] revealed significant associations between the thoracic inlet angle and cranial offset as well as craniocervical alignment. The neck typically tilts at around 45 degrees to minimize the energy expended by neck muscles. This implies that a smaller thoracic inlet angle results in a smaller T1 slope and reduced cervical lordosis angle to sustain the natural neck tilt, and vice versa. The T1 inclination is instrumental in determining the requisite subaxial lordosis to maintain the head’s center of gravity in equilibrium. This value varies depending on global spinal alignment, assessed through SVA, and inherent upper thoracic kyphosis. In cases of scoliosis, the T1 sagittal angle has been shown to directly correlate with SVA, measured from the C2 dens plumb line, offering a measure of overall sagittal alignment [61].

It’s important to recognize that the cervical, thoracic, lumbar, and pelvic spinal regions are interrelated, with several significant correlations identified [56]. Blondel et al. [56] explored various spinal parameters in asymptomatic volunteers and observed that pelvic incidence (PI) correlated with LL, LL correlated with thoracic kyphosis (TK), and TK correlated with cervical lordosis (CL). Consequently, an increase in PI corresponded to an increase in LL, which, in turn, correlated with an increase in TK, subsequently influencing CL. However, there was no observed correlation between PI and TK, which adds complexity to the correlation chain from the pelvis to the cervical spine. The prevailing view suggests that LL is proportionate to PI and TK, given that PI is a fixed parameter with limited flexibility. Individuals with a smaller PI or lesser TK exhibited reduced LL compared to those with small PI and more substantial TK. This underscores that TK does not result from LL; instead, LL is a consequence of TK and PI. As mentioned earlier, CL was linked to TK, indicating that as TK increases, so does CL. Nevertheless, this CL adjustment, while substantial, is often insufficient to maintain the head’s alignment over the pelvis, though it does suffice to ensure a horizontally oriented gaze. Moreover, CL was found to correlate with SVA, pelvic tilt (PT), and T1 slope. Subjects with a positive SVA displayed increased cervical lordosis, regardless of whether their SVA fell within the normal range of values. This adaptation in cervical lordosis to sagittal global alignment represents a compensatory mechanism aimed at preserving a horizontal gaze, similar to the roles of thoracic kyphosis and lumbar lordosis. When both LL and TK adjust in response to the patient’s PI, cervical lordosis aligns proportionately with the other spinal curves. However, when there’s an anterior malalignment of the spine due to a loss of lumbar lordosis and/or an increase in thoracic kyphosis, an augmented cervical lordosis serves as a compensatory mechanism.

4. Clinical evaluation of the cervical deformity patient

When evaluating a patient with cervical deformity in the preoperative setting, the primary goal is to determine the ideal amount and type of correction as well as how to achieve this desired correction. As of now, an established optimal postoperative cervical lordosis does not exist, however, a general guideline for correcting any cervical kyphosis is to achieve at least a neutral alignment [62]. In addition to the medical history and physical exam, a comprehensive assessment of the previously mentioned radiographic parameters should be conducted: specifically CL, cSVA, CBVA, and T1S-CL. Understanding these values for a given patient will aid in determining the surgical techniques required to achieve these specific goals. In cervical deformity surgery, there is no gold standard for “normal values” of these parameters, however, the current available evidence suggests that generally accepted targets include cSVA <40 mm, T1S-CL <15°, and CBVA between −10° and + 20° [14, 15, 43, 63, 64, 65].

4.1 Medical history and physical exam

The medical history and physical exam is vital to the proper assessment of the adult cervical deformity patient. This will lead to an appropriate evaluation of the preoperative risk as well as the correct deformity classification and subsequently the surgical management. The initial phase of this procedure commences with a comprehensive medical history review and physical examination. The medical history plays a pivotal role, particularly in cases involving high-risk and fragile surgical patients. Consequently, an extensive medical history becomes instrumental in tailoring the treatment approach and facilitating discussions about potential risks. Pertinent aspects include significant medical comorbidities, smoking history, and the utilization of nonsteroidal anti-inflammatory drugs (NSAIDs. Therefore, preoperative medications and social habits should be reviewed thoroughly. Patients suffering from severe cervical deformities that have significant neck pain are many times on chronic opioids. It is thus critical to make every effort to wean off of these medications prior to surgery to help improve the postoperative outcomes and to minimize the risk of narcotic overdose [65]. Furthermore, medications as well as dietary supplements that can result in excessive bleeding such as ginger, gingko, fish oils, aspirin, etc. should all be stopped. Smoking cessation and normalization of vitamin D levels must also be addressed. And finally, the patient’s general medical status and medical comorbidities must be carefully evaluated. Due to a significant number of because of complex comorbidities, some patients may not be medically fit for an extensive surgical correction and less invasive procedure or medical management may be needed instead.

The physical exam should start with a complete neurological exam and also include an assessment of the patient standing upright with hips and knees fully extended as well as in the sitting and supine positions [15, 16]. These additional physical exam positions can add a significant amount of insight into the patients’ type of cervical deformity. For the sitting position, the effect of any lumbar and/or pelvic/hip deformity on the cervical spine. When the patient is in the supine position, the examiner can assess the rigidity of the deformity under the direct effect of gravity. Depending on the flexibility/rigidity of the deformity, the patients head may or may not be able to lay flat on the exam table. Patients with a fixed primary cervical deformity exhibit enduring cervical flexion even when in a supine position, which is in contrast to individuals with thoracic, lumbar, or hip deformities, as they tend to correct their posture when seated or lying down. Additionally, it is essential to measure CBVA both before and after surgery using the chin-brow to vertical angle method, utilizing clinical photographs of the patient in a standing position with their hips and knees extended, and their neck maintained in a neutral or fixed position. The CBVA measurement assists in determining the extent of posterior osteotomy wedge removal based on the angle, contributing to surgical planning. Patients experiencing head ptosis or neck drop, a condition known as cervical camptocormia, often present with a flexible sagittal spinal deformity that corrects itself when they lie in a supine position. Cervical camptocormia can stem from various causes, including different myopathies, amyotrophic lateral sclerosis (ALS), parkinsonism disorders, and idiopathic factors [66]. Therefore, the initial evaluation of a patient with camptocormia (or any other flexible deformity) should encompass relevant electromyography and nerve conduction studies, coupled with a referral to Neurology to rule out a primary myopathy or ALS. Furthermore, it is advisable to refer these patients to physical therapy before considering surgical correction and fusion as potential treatment options.

4.2 Cervical deformity and myelopathy

If a cervical deformity patient shows signs of myelopathy on the physical exam, this could be from direct spinal cord compression as evidenced on the MRI or from the deformity itself [67]. However it is likely a result of a combination of both and it’s important to understand the relationship of cervical deformity and myelopathy. Progressive cervical kyphosis has been associated with myelopathy even without central stenosis [67]. The myelopathy is a consequence of cervical kyphosis, which leads to the sagging of the spinal cord against the vertebral bodies. This sagging results in anterior spinal cord damage and an escalation in the longitudinal tension on the cord due to its tethering by the dentate ligaments and cervical nerve roots [67, 68, 69]. As the cervical kyphosis continues to progress over time, the spinal cord undergoes compression, becoming flattened against the posterior aspect of the vertebral bodies. This results in the tethering of the cervical cord, leading to an increase in intramedullary pressure, subsequently causing neuronal loss and cord demyelination [70, 71, 72]. Furthermore, the deformity also has a flattening effect on the small blood vessels located on the spinal cord’s surface, resulting in reduced blood supply and ischemia [67, 70, 72, 73, 74]. Smith and colleagues [75] investigated myelopathy scores via the modified Japanese Orthopedic Association (mJOA) scale with cervical sagittal alignment and spinal cord volume in patients with myelopathy. They found a moderate negative correlation in patients with cervical kyphosis between cord volume and cross sectional area to mJOA scores (i.e., as the kyphosis is larger the cord volume gets smaller). A positive correlation (opposite) was found for patients with cervical lordosis (as the cervical lordosis increases so does the spinal cord volume) thus indicating a relationship of cord volume to myelopathy that differs on the basis of cervical spine sagittal alignment [75]. Therefore sagittal myelopathy in the cervical deformity patient may be largely form the cervical kyphosis and must be considered in the clinical evaluation and preoperative surgical planning.

4.3 Radiographic evaluation

After a comprehensive medical history and physical exam, every patient should have a 36-inch standing plain film x-ray (or EOS scan described below) and dynamic flexion/extension cervical plain x-ray films. The 36-inch standing film (or EOS) allows for cervical alignment measurements in the context of critical global spinal alignment parameters. The flexion/extension xrays aid in determining the relative rigidity of the cervical spine as well as any atlantoaxial instability. With regard to the standing 36 inch x-rays, the EOS system gives single planar standing x-rays from the skull to the feet and allows a 3-dimensional (3D) configuration of the entire spine [76]. The advantages of this system include: (1) the entire body can be imaged at once thus obviating the need for joining multiple separate x-rays, (2) the x-ray images can be re-formatted into 3D configuration, (3) the delivered radiation dose is 1/10th of the standard x-rays (4) all the points of alignment and related compensation throughout the spine, pelvis, and/or lower extremities are imaged simultaneously [77]. Furthermore, studies have shown that the EOS system is highly accurate, reproducible, and precise in the measurement of spinal curvature [78, 79, 80, 81]. The x-rays mentioned above proved the radiographic foundation for any cervical deformity patient, however, additional radiographic studies are generally performed which include computed tomography (CT) and magnetic resonance imaging (MRI). The CT scans are used to assess for osseous landmarks to plan for instrumentation insertion, evaluate bone quality, and any prior instrumentation which the MRI can aid in evaluating areas of stenosis resulting in neural compromise and spinal cord tethering. Sometimes a CT scan can help determining the extent of fusion and osteophyte changes at the disc and can be important in decision making to assess the need for osteotomies to correct the deformity.

4.4 Cervical deformity classification

Once the surgeon has a comprehensive assessment of the patient form a medical history, physical exam, and radiographic stand point, the patients’ deformity can then be classified to help guide surgical planning. A classification system has recently been proposed by Ames and colleagues [14] whereas until then, there was no comprehensive cervical spine deformity classification system. Not only does this system aid in surgeon planning, but it also provides a mechanism for clear communication among surgeons and places the cervical deformity in the context of global spinal alignment and clinically relevant radiographic parameters as well as myelopathy. The classification system involves a primary deformity descriptor that describes the location of the apex as five different modifiers (Table 3). The modifiers include radiographically measured parameters and a clinical score for the degree of myelopathy per the mJOA scale. All of the five modifiers used in this classification had been previously shown to have a clinical impact [14].

Deformity Descriptor

C: Primary sagittal deformity apex in the cervical spine

CT: Primary sagittal deformity apex at the cervico-thoracic junction

T: Primary sagittal deformity apex in the thoracic spine

S: Primary coronal deformity

CVJ: primary cranio-vertebral junction deformity

5 Modifiers

C2-C7 sagittal vertical axis

0: C2-C7 SVA <4 cm

1: C2-C7 SVA <4 cm to 8 cm

2: C2-C7 SVA >8 cm

Horizontal gaze

0: CBVA 1°-10°

1: CBVA −10° to 0°

2: CBVA <-10° to >25°

Cervical lordosis minus T1 slope

0: CL-T1 < 15°

1: CL-T1 15° to 20°

2: CL-T1 > 20°

Myelopathy

0: mJOA = 18 (None)

1: mJOA = 15–17 (Mild)

2: mJOA = 12–14 (Moderate)

3: mJOA = <12 (Severe)

SRS-Schwab Classification

T, L, D, or N: Curve type

0, +, or ++: PI-LL grades

0, +, or ++: Pelvic tilt grades

0, +, or ++: C7-S1 SVA grades

Table 3.

Cervical deformity classification system by Ames et al. [14].

CBVA = chin brow vertical angle, CL = cervical lordosis, mJOA = modified Japanese Orthopedic Association, T = thoracic, L = lordosis, D = double, N = none, LL = lumbar lordosis, PI = pelvic incidence. SRS = Scoliosis Research Society.

The classification system commences by using a deformity descriptor to establish a fundamental categorization of various deformity types and their respective locations. The initial three types pertain to primary sagittal deformities, distinguishing them based on the deformity apex’s position. Type C corresponds to cases where the curve apex is situated in the cervical spine, Type CT denotes instances with the apex at the cervico-thoracic junction, and Type T designates those with the apex located in the thoracic spine. Primary coronal deformities are defined as those exhibiting a C2-C7 coronal Cobb angle exceeding 15°, and they are denoted as Type S. As for the remaining deformity types occurring at the cranio-vertebral junction, they are categorized under Type CVJ.

Among the five modifiers present in the classification system, three pertain to sagittal alignment parameters. These include cSVA, horizontal gaze (assessed through CBVA), and the variance between cervical lordosis and the T1 slope (CL-T1S). Each of these parameters was specifically chosen due to its direct correlation with patients’ HRQOL14, and they are each divided into three distinct severity grades, as indicated in Table 3. The inclusion of cSVA is justified by the substantial influence of sagittal alignment on HRQOL in patients with thoracolumbar spinal deformities, supported by studies like those conducted by of Tang et al. [54]. and Smith et al. [75], which establish the connection between cervical sagittal alignment and HRQOL. CBVA is integrated into the system because horizontal gaze holds a fundamental significance in basic human functioning, with previous research underscoring the importance of accounting for horizontal gaze in spine deformity surgery [15, 43, 44, 45, 46, 47, 48, 82]. Furthermore, the connection between the T1 slope and cervical lordosis can be likened to the relationship between pelvic incidence (PI) and lumbar lordosis (LL) in the lower back. Just as a higher T1 slope necessitates a more substantial degree of cervical lordosis to align the head properly over the thoracic inlet and trunk, a greater PI demands a greater degree of lumbar lordosis to achieve balance in the lumbar spine [15, 53]. And moreover, since cervical kyphosis is the most common type of cervical deformity having a large clinical impact, a classification parameter reflective of cervical lordosis is included as a modifier for the classification.

The last two modifiers pertain to myelopathy and the overall spinal alignment, as defined by the SRS-Schwab thoracolumbar deformity classification. As previously mentioned, understanding the relationship between myelopathy and cervical alignment is pivotal when assessing patients with cervical deformities. The presence of myelopathy can significantly influence the surgical approach, as it necessitates addressing spinal cord compromise when it is present. This can be accomplished through direct decompression, deformity correction, or a combination of both interventions. Consequently, the classification system includes a measure of myelopathy, employing the mJOA score, which is a widely recognized and accepted quantitative assessment of spondylotic myelopathy severity. The scores on this scale range from 0 to 18, with lower scores indicating a more severe impact from myelopathy. As a result, three distinct groups of scores with increasing severity were integrated into the classification system (Table 3).

Lastly, the incorporation of global spinal alignment through the SRS-Schwab classification was deemed essential because spinal segments do not operate in isolation. Cervical deformities can influence thoracolumbar deformities, and conversely, thoracolumbar deformities may generate or contribute to cervical deformities [15]. This interdependence is underscored by Smith et al’s findings, which reveal that adults with positive sagittal spinopelvic malalignment adjust by increasing cervical lordosis to maintain a horizontal gaze. Moreover, surgical correction of thoracolumbar sagittal malalignment leads to the amelioration of abnormal cervical hyperlordosis through reciprocal adjustments [83]. Ha and colleagues subsequently corroborated this observation, pinpointing key radiographic parameters associated with these compensatory alterations [84]. Furthermore, there is a notable prevalence of concomitant cervical deformities among adults with thoracolumbar deformities [85]. As a result, it became evident that the assessment and classification of cervical deformities should not be conducted in isolation. Instead, it is imperative to consider the alignment of the thoracolumbar spine and pelvis. Consequently, the SRS-Schwab classification for adult thoracolumbar spinal deformities was integrated as a modifier within the cervical deformity classification (Table 1). The validity of the SRS-Schwab classification has been established, showcasing its correlation with HRQOL measures at the outset and its sensitivity to changes in disease state [49, 86, 87].

5. Surgical planning

Following a thorough examination that encompasses a detailed medical history, an extensive neurological evaluation, and a comprehensive physical assessment, as well as the necessary imaging and radiographic measurements, and the categorization of the deformity, the groundwork for preoperative surgical planning can commence if the patient is deemed suitable for surgery. The key considerations for surgical planning encompass the following factors:

1. The evaluation of neural compression and any accompanying neurological symptoms.

2. The determination of deformity flexibility or rigidity.

3. Assessment of anterior or posterior ankylosis.

4. Examination of the location of the deformity.

5. Review of any previous cervical spine surgeries.

6. Inspection for degenerative changes at other spinal levels, notably the proximal and distal vertebral levels and the cervicothoracic junction.

7. Evaluation of the patient’s overall medical condition and the presence of any relevant medical comorbidities, as previously mentioned [65].

Some surgeons have advocated cervical traction pre-operatively to facilitate deformity correction prior to surgery. In most cases, a period of 3 to 5 days under traction is typically adequate for achieving a reduction in the deformity [62]. In the event of a successful reduction of the cervical deformity, posterior only fixation and fusion techniques may be adequate to treat the deformity.

In general, anterior approach enables better and more harmonious correction of deformities through interbody release and instrumentation that leverages both biomechanical principles and posture adjustment to achieve the desired postoperative cervical alignment. Any ventral compression of the spinal cord should also be noted on the MRI. If anterior compression is present, then a ventral approach may be necessary in the overall strategy in order to achieve anterior spinal cord decompression [15, 65]. In the setting of infection or neoplasm, if the integrity of the anterior column is compromised, then the anterior approach is preferred to establish anterior column support [65].

The assessment of cervical spine flexibility and rigidity plays a pivotal role in determining the appropriate surgical course, as discussed previously. When clinical examination, dynamic x-rays, and CT scans confirm the cervical spine’s flexibility and absence of ankylosis, either an anterior or posterior correction approach can be considered.

For many flexible deformities, a posterior-only correction method might be applicable. However, in cases of severe kyphotic deformities, relying solely on a posterior approach might prove insufficient to rectify the deformity, necessitating an additional anterior approach. In situations where the deformity is identified as rigid, it is advisable to obtain a high-resolution CT scan of the cervical spine to assess the presence of anterior and posterior ankyloses. If the spine exhibits rigidity without ankylosed facets or has a history of prior instrumentation, employing an anterior-only approach may suffice, but if anterior fusion is more than three levels, posterior fixation should be considered to minimize risk of pseudarthrosis [65].

When the anterior column of the spine is rigid and features ankylosed facets, a combination of both anterior and posterior strategies may be employed to address the deformity. In such instances, either a 360- or 540-degree approach may be utilized to achieve circumferential release and deformity correction. The exact choice may depend on the specific deformity and surgeon’s training and preference.

In the senior author’s experience, anterior release and interbody fusion followed by posterior osteotomy and fixation is typically adequate to treat even circumferentially ankylosed cervical deformities. Some surgeons may prefer to initiate the correction process with a posterior release, followed by anterior release and fusion, and then followed by posterior fixation. This posterior-anterior-posterior, or 540-degree approach can be useful in revision cases where posterior instrumentation is present.

The precise location of the kyphotic deformity holds significant importance, influencing the choice of approach, type of osteotomy, and correction strategy for the specific cervical deformity under treatment. In general, when dealing with focal cervical sagittal plane deformities in the subaxial region (referred to as cervical kyphosis), a combination of anterior release and posterior fixation frequently yields the most effective results.

For patients presenting with cervical deformities arising from issues at the cervicothoracic junction (characterized by a high T1-slope, neutral to normal cervical lordosis, and a high low C2-slope), the appropriate strategy typically involves a three-column osteotomy in cervicothoracic junction with a posterior approach.

In situations where cervical deformity coexists with severe thoracic kyphosis, additional osteotomies in the thoracic spine may be necessary to achieve the desired correction in the cervical region. Notably, increased kyphosis in the upper thoracic region can negatively impact the T1S (T1 slope), and a comprehensive correction of T1S and cSVA (cranial sagittal vertical axis) may necessitate a significant surgical intervention extending into the mid-thoracic level [65].

Nonetheless, this approach remains a subject of controversy, and additional data is needed to substantiate the viability of such assertive surgical methods. However, as a general guideline, when patients necessitate deformity correction above the T2 level and concurrently present with lower thoracic deformities, it is advisable to address the lower deformity initially. In cases where the reverse sequence is pursued, rectifying the cervical deformity first to achieve a horizontal gaze might subsequently result in an overcorrected positioning of the head and neck when addressing the lower thoracic deformity [65].

Coronal cervical deformities can occur independently or in conjunction with sagittal deformities, often exhibiting a range of severity levels, which are collectively referred to as fixed multiplanar deformities. Patients presenting with such deformities may necessitate the implementation of three-column osteotomies to rectify the spinal alignment in both the coronal and sagittal planes, as well as to alleviate pressure on the spinal cord and nerves [15]. Furthermore, a multifaceted approach involving multiple stages might be imperative. This could entail the adoption of a 540-degree circumferential osteotomy or even a cervical vertebral column resection (VCR) [88, 89, 90].

Lastly, in the case of patients with a history of previous cervical spine fusion, it is of paramount importance, to ensure the surgery’s safety and smooth execution, to acquire the operative report. This report is instrumental in identifying the specific instrumentation employed during the prior procedure, as well as any intraoperative discoveries. Additionally, a comprehensive evaluation of the previous fusion mass should be conducted using CT scans to assess the presence of any pseudarthrosis. If an anterior approach was done previously, it may be preferable to enter the contralateral side for the planned upcoming surgery as prior scar tissues may make the tissue dissection much more difficult [65]. An otolaryngology consultation may be obtained preoperatively to confirm vocal cord motility bilaterally if planning on entering the contralateral side. Should injury to the recurrent or superior laryngeal nerve occur or if, for any reason, an evaluation by Otolaryngology is needed, it is imperative to err on the side of caution by opting for the same side to minimize the risk of potential bilateral damage to the laryngeal nerves [65]. Depending on ones’ comfort level and the institution, the Otolaryngology colleagues may assist in exposure to help minimize risk to the nerves and surrounding structures given the amount of scar tissue. In the event that a prior posterior approach was undertaken, it becomes imperative to examine the surgical site meticulously for the integrity of the surrounding soft tissues. If any observation reveals the separation or detachment of the posterior paraspinal muscles, or if a significant void within the wound or inadequate soft tissue coverage is identified, the risk of potential complications related to the wound increases. In such cases, it becomes necessary to mobilize and reattach any detached muscles from their lateral positions back to the midline. If the surgeon lacks experience in these specialized techniques, it is advisable to seek a consultation with a plastic surgeon who can provide expertise in managing the complexities of wound closure [65].

6. Surgical techniques

Cervical deformity surgery has multiple goals and includes the correction of deformity, re-establishment of horizontal gaze, stabilization of the cervical spine, and decompression of the neural elements. The choice of surgical technique for correction involves a careful assessment of the patient including a history and physical, neurologic examination, and evaluation of radiographic data. Careful surgical technique is critical to decreasing the risk of complications related to cervical spine deformity correction. Methods to correct cervical spine deformity include anterior-only, posterior-only, and a combination of anterior and posterior approaches. In this paper, surgical techniques including anterior and posterior techniques are discussed.

6.1 Anterior surgical techniques

6.1.1 Anterior cervical discectomy and fusion

Anterior cervical discectomy and fusion (ACDF) is perhaps one of the most familiar procedures to spine surgeons. The use of ACDF allows the sequential induction of lordosis via distraction over multiple segments. Furthermore, additional lordosis can be induced via sequential screw tightening, which pulls the spine towards a lordotic cervical plate [91]. A single level ACDF may increase the overall C2-C7 lordosis by 3.5 degrees after 1 year [92]. If the patient has had prior ventral cervical spine surgery, vocal cord function should be assessed by otolaryngologists prior to performing another anterior surgery to determine the approach to the cervical spine. If there is no history of prior injury, then the authors of this paper approach the cervical from the left side to reduce the risk of injury to the recurrent laryngeal nerve, which has a longer course on the left side. If the patient is shown to have prior vocal cord function impairment, then the approach should be performed from the ipsilateral side as prior surgery to avoid placing the contralateral vocal cord at risk.

The patient is placed under general endotracheal anesthesia and maintained in the supine position. Bumps may be placed under the shoulders to encourage extension of the head but care must be taken to ensure that the head is not floating. The traditional approach to the anterior cervical spine is well described via the Smith-Robinson approach [93]. The incision can be performed utilizing either a transverse or longitudinal incision. During this approach, the omohyoid muscle is identified and divided by monopolar cautery. If the superior and inferior thyroid arteries are encountered, they may be ligated and divided to maximize the surgical exposure. However, if the superior thyroid artery is encountered, then it should be separated from adjacent structures to prevent injury to the superior laryngeal nerve, a portion of which often accompanies the superior laryngeal artery. The trachea and esophagus are mobilized carefully, and the longus colli muscles are released with bipolar cautery. A table-mounted retractor system is utilized to provide retraction of one disc space at a time.

Once the disc space is identified using lateral fluoroscopy, the anterior osteophytes are first removed using combination of Leksell rongeur and high-speed burr. Subsequent to this, Caspar pins are placed in the vertebral bodies above and below the disc space along with a retractor to distract the disc space. The anterior longitudinal ligament and disc annulus are incised and the disc material is removed utilizing a combination of curettes, pituitary rongeurs, and high-speed burr. The posterior osteophytes should be removed utilizing a high-speed burr and the posterior portion of the uncinate processes. The PLL is identified by looking for fibers traversing in a rostral-caudal direction and removed using small curettes and Kerrison rongeurs.

Mechanical correction of kyphotic deformity can be achieved by utilization sequentially disc spacers in combination with intervertebral body spreaders. The graft with autograft/allograft is subsequently placed into the disc space with sequential lateral fluoroscopic guidance to confirm placement. This process is repeated for each disc level. If an anterior only procedure is performed, then the authors prefer utilizing anterior plates (Figure 5). If the surgery involves a posterior fixation in addition, then standalone cages may be used (Figure 6).

Figure 5.

Cervical deformity correction via anterior only approach with anterior plating.

Figure 6.

Cervical deformity correction via combined anterior-posterior approach with standalone cages in front with additional posterior instrumentation.

Additional deformity correction may be achieved by utilizing a long anterior cervical plate which is first fixed into place with screws at the apex of the deformity followed by sequential placement and tightening of screws further and further away from the apex to “pull” the spine towards the lordotic cervical plate [91].

6.1.2 Anterior cervical corpectomy and fusion

The approach for an anterior cervical corpectomy is the same as an ACDF. An anterior cervical discectomy is performed above and below the intended corpectomy level. A Leksell rongeur is utilized to remove the vertebral body up to the posterior portion of the vertebral body, which is removed with a high-speed burr. The remaining PLL is subsequently resected.

There are several choices for graft into the corpectomy site, which include iliac crest, fibula, or a cage. It is critical to ensure that the graft is not so deep as to impinge into the spinal canal. An anterior cervical plate, similar to that in an ACDF, should be placed to prevent cage/graft migration (Figure 7). It is important to note that anterior cervical corpectomy performed at more than 2 levels has a very high pseudarthrosis rate and therefore posterior fixation should be performed in these situations [94, 95]. The patient should be immobilized in a hard cervical collar for 4–6 weeks to encourage fusion.

Figure 7.

Cervical deformity correction via anterior corpectomy at C4 with revision posterior spinal instrumentation and fusion.

6.1.3 Anterior cervical osteotomy

The usage of anterior osteotomy in the cervical spine, performed through an anteriorly fused spine to the level of the transverse foramen bilaterally can be a powerful correction technique applicable throughout the cervical spine. Symmetric anterior osteotomies can be used to “chin-on-chest” deformities while asymmetric anterior osteotomies can assist with “ear-on-shoulder” deformities [96]. This technique has been described by Kim et al. [97]. The patient undergoes general endotracheal anesthesia in the supine position, with bumps placed under the shoulders to suspend the head which is subsequently supported by a foam donut placed on sheets. Traction utilizing Gardner Wells tongues of a chin strap may assist with intubation and allow easier initial approach to the cervical spine.

The choice of laterality for exposure should be determined as described above under the ACDF technique with utilization of preoperative vocal cord examination as necessary. If there is a “ear-on-chest” deformity of significant coronal deformity, it may be easier to approach the spine from the convex side. Dissection is carried out according to the Smith-Robinson approach. Once the disc level of intended anterior osteotomy is identified, Caspar pins may be placed, perpendicular to the anterior cervical spine to induce lordosis when distraction is performed using the Caspar pin retractor.

Using a high-speed burr, bony resection is started. If there is only kyphotic deformity, then the osteotomy should be performed in a symmetric fashion to avoid inducing coronal deformity. If there is coronal deformity, then an asymmetric resection of bone may facilitate correction of coronal deformity. This bony resection should be performed all the way back to the PLL. As the resection nears the lateral uncinates, the vertebral artery is at risk of injury, and thus a Penfield #2 dissector should be placed at the lateral border of the uncinates to protect the vertebral artery. Once the bone of the lateral uncinate is “egg-shelled” the remainder of the resection should be performed with a microcurette.

Once the osteotomy is performed, the correction of deformity occurs as the surgeon pushes gently on the forehead through the drape. At the same time, sheets may be removed under the head and distraction on the Caspar pins performed to induce lordosis. Other methods to induce lordosis include placement of a Cobb elevator into the osteotomy site with rotation or usage of vertebral body spreaders with sequentially larger disc spacers.

Deformity correction is complete if the occiput touches the operating room table, but if inadequate, the shoulders may be bumped to provide further lordosis and the cervical traction via Chin strap or Gardner Wells tongs may be increased to 25 lb. If there was complete deformity correction, then an anterior cervical plate with fixed angle screws are placed but if there is incomplete correction, then a trapezoidal graft in the anterior portion of the osteotomy site without cervical plate is performed so that further lordosis induction can be performed via a posterior operation (Figure 8). It is important to place a buttress plate or an interference screw in this situation however, to prevent the graft from being extruded anteriorly during the posterior portion. Posterior augmentation is recommended for anterior osteotomies, as the endplates are trabecular in nature and at high risk of subsidence. Patients are almost always placed in a hard cervical collar for immobilization for 4–6 weeks to encourage fusion.

Figure 8.

Cervical deformity correction of a rigid deformity using anterior osteotomies to release fused levels with posterior spinal release, instrumentation and fusion.

6.2 Posterior surgical techniques

6.2.1 Posterior instrumentation and fusion with or without decompression

Patients who have a deformity curve which is flexible without a significant source of ventral compression may benefit from a posterior-only instrumentation and fusion. This procedure is quite commonly performed among spine surgeons.

The patient undergoes general endotracheal anesthesia. Subsequently, pinning for fixation of the head occurs, either utilizing a Mayfield head holder and system or Gardner-Wells tongs with a bivector setup utilizing two separate ropes at different angles to stabilize the neck prior to instrumentation, and another rope to induce extension after osteotomy/correction with 15–20 lb. of weight on the rope. After pinning, the patient is flipped prone and the table is subsequently placed in a reverse Trendelenburg position.

A midline incision is performed with meticulous division into the midline raphe. Careful subperiosteal exposure of the laminae and lateral masses are performed to minimize blood loss. At C2 there are a variety of options for instrumentation, including the placement of pars screws, pedicle screws, or translaminar screws. From C3-C6 typically lateral mass screws are placed, and at C7 there is an option for either lateral mass of pedicle screw fixation, although typically if fixation is extended into the upper thoracic spine, C7 is skipped to allow space for screw fixation of T1. At T1 and T2, pedicle screw fixation is utilization.

Decompression via laminectomy can be performed either before or after placement of screws, although the placement of rods and deformity correction itself usually takes place after the laminectomies are performed. Arthrodesis is performed after copious irrigation of the surgical cavity via thorough decortication and subsequent bone graft placement. Vancomycin powder may be used to decrease the chance of infection. One or more deep subfascial drains are placed. Occasionally, a more superficial subdermal drain may be added if the wound is particularly deep with excess adipose tissue.

During closure, muscles are reapproximated and multi-layer closure is performed with interrupted sutures.

6.2.2 Smith-Peterson osteotomy

Smith-Peterson Osteotomies (SPOs) also known as “Ponte Osteotomy” when performed at multiple levels, are performed as part of a posterior instrumentation and fusion with or without decompression. The SPO is performed by completely resecting the superior articulating facet, inferior articulating facet at a joint, along with removing the ligamentum flavum, lamina, and spinous processes (Figure 9). It is important that there is residual anterior column mobility to obtain correction of kyphotic deformity during this osteotomy. After the SPO is performed, compression of the posterior elements is performed with rod and screw fixation to close the osteotomy. Risks of SPO include compression of nerve roots as the osteotomy is closed due to compression from pedicles or residual superior articulating facets.

Figure 9.

Cervical Smith-Petersen osteotomies for posterior release of a rigid deformity.

6.2.3 Pedicle subtraction osteotomy

When there is severe, fixed cervical kyphotic deformity the cervical pedicle subtraction osteotomy may be the preferred technique for deformity correction. The patient is induced with general endotracheal anesthesia and flipped into the prone position after fixation with Mayfield or Gardner Wells tongs as previously described. As described previously, the Gardner Wells tongs are used with two ropes, which allows extension for osteotomy closure after the pedicle subtraction osteotomy is performed. If a Mayfield clamp is utilized, then the clamp should be released and the head should be placed in a gently extended position to close the osteotomy.

Ideally, the PSO is performed at the cervicothoracic junction for several different reasons. At the cervicothoracic junction, the vertebral artery is further away, the spinal cord is more mobile due to the larger size of the canal, and a C8 nerve root injury is typically less debilitating for hand function. However, it is important to assess the vascular course of the vertebral artery as there can be aberrant courses of vertebral arteries which transverse the C7 transverse foramen.

Exposure and instrumentation of the cervical spine is placed as described above for posterior instrumentation and fusion. Subsequent to this, a C7 laminectomy is performed along with the caudal C6 lamina and the cranial T1 lamina. Utilizing high-speed burr and Leksell rongeur, the C7 lateral masses are removed along with the caudal aspect of C6 inferior facet and the cranial aspect of the T1 superior facet. The pedicles of T1 are fully exposed to ensure there is no superior articular process cranial to the T1 pedicle that remains as this can cause injury to the C8 nerve root during osteotomy closure. The C7 and C8 nerve roots should be visualized.

The thecal sac and C7/C8 nerve roots are protected with cottonoid patties and the cancellous bone of the C7 pedicle is drilled out with burr while leaving the cortical walls intact. The burr should be passed deeper into the body of C7 to remove the cancellous bone of the C7 body. The cortical rim of the C7 pedicles is subsequently removed with small pituitary rongeur and small reverse-angle curettes. The entire pedicle should be removed to avoid compression of nerve roots during osteotomy closure. Utilizing small round tamps and reverse angled curettes, a cavity is formed within the posterior superior portion of the vertebral body and the bone is subsequently pushed anterior into this cavity. Following this, the cortical posterior vertebral wall should be pushed into the cavity using Woodson and angled dural elevators.

Rods are then placed into the thoracic pedicle screws and the osteotomy closure performed by extension of the neck. If the Gardner-Wells tongs have been used, then this is performed by pulling on the tongs through the drape while the traction weight is switched by assistant to the extension rope. It is important to examine and C7 and C8 nerve roots during this closure to make sure there is no compression. If there is compression, additional resection of the inferior portion of C6 facet and superior aspect of T1 facet may be needed. The rod is than engaged into the cervical lateral mass and C2 pars/pedicle screws superior to the osteotomy and fixed into the screws with locking caps.

Upper thoracic PSO or VCRs can be a good alternative for C7 PSO for many patients as it poses less danger for hand function and can offer powerful deformity correction (Figure 10).

Figure 10.

“Chin-on-chest” deformity due to DJK corrected via posterior only approach by performing a T2 VCR.

Confirmation of correction is performed with lateral X-rays. Arthrodesis is performed with autograft and allograft as described above with careful decortication. Careful multilayer closure is performed along with placement of one or more subfascial drains. The patient should remain in a hard cervical collar for 4–6 weeks after surgery to encourage fusion.

7. Clinical outcome and complication avoidance and future directions

7.1 Clinical outcomes

Clinical outcomes after cervical deformity correction include the physical correction of the kyphosis and the functional outcome of the patient.

7.1.1 Radiographic outcome

In terms of kyphosis correction, the most used measure of horizontal gaze utilized within studies is the Chin-Brow Vertical (CBV) angle, which is determined from photographs. The CBV angle measure the angle between the vertical axis of a patient that is standing upright and a line from the chin to the brow. In severe chin-on-chest type of cervical deformity, this CBV angle may even approach 90 degrees. Multiple studies support a good outcome of kyphosis correction. Simmons and colleagues found that CBV angle improved from 56 degrees to 4 degrees in 114 patients treated with smaller osteotomy and from 49 degrees to 12 degrees in a smaller subset of 17 patients treated with a wider osteotomy, all from a posterior approach [98]. Likewise, Langeloo et al. reported in a group of 16 patients treated via posterior-only approach that CBV angle improved from 42 degrees to 5 degrees resulting in restoration of horizontal gaze in all patients [99]. Tokala et al. found similar results with the posterior approach also, specifically a C7 closing wedge osteotomy technique, with CBV angle improving from 41 degrees to 6 degrees in a group of 8 patients [100].

Another measurement of deformity correction commonly utilized in the literature is the Ishihara index, which is measured on a lateral X-ray. A vertical line is drawn between the posterior-inferior edges of C2 and C7. The perpendicular distances from the line to the posterior-inferior edges of C3, C4, C5, and C6 are summed up and divided by the length of A and multiplied by 100 to determine the Ishihara index. One study by Mummaneni et al. shows that the average Ishihara index improved from a pre-operative value of −17.7 (indicated kyphosis) to +11.4 (indicating lordosis) [101]. This study included 30 patients with cervical kyphotic deformity who underwent circumferential (anterior-posterior combined) spine surgery. Likewise, a review of the literature indicates that there is significant correction of kyphosis as measured by the Cobb angle. Mean correction of kyphosis ranged from 11 to 32 degrees for the anterior only approach, 23.3 degrees to 54 degrees for a posterior only approach, and 24 to 61.3 degrees in a combined anterior-posterior approach in a review of the literature by Etame et al. in 2010 [102]. Radiographically then, cervical deformity surgery appears to result in good outcomes in terms of kyphotic angle correction and CBV angle.

7.1.2 Functional outcomes

Prior studies on cervical deformity correction have measured different functional outcomes after deformity correction. Outcomes measured in prior studies include horizontal gaze, measures of myelopathy (Nurick grade, modified Japanese Orthopedic Association score), measures of spinal cord injury (Frankel grade), and neck pain.

Generally, cervical deformity surgery has excellent outcomes with respect to restoration of horizontal gaze with almost all studies reporting good restoration of horizontal gaze with correction [64, 98, 99, 103, 104].

8. Complications

Surgical correction of cervical spinal deformity is technically challenging as discussed in part 3 and is associated with a high complication rate [105, 106]. A study by Passias and colleagues evaluating a prospective cervical deformity database found a 39.9% intraoperative complication rate [105].The authors further categorized the complications into various types revealing that the rate of neurological complications stood at 13.9%. Meanwhile, the rates of dysphagia and respiratory issues were 9.8 and 8.2%, respectively. The infections accounted for 8.2%, and the dural tears were documented at 4.1% [105]. In a similar investigation by Smith et al. [106], the comprehensive complication rate in cervical deformity surgery was reported at 43.6%. Among the most frequently encountered complications were dysphagia (11.5%), deep wound infections (6.4%), new C5 palsy (6.4%), and respiratory failures (5.1%).

Smith and colleagues performed another investigation into focusing on all-cause mortality following surgical correction and demonstrated that at 1.2 yrs. postop the all-cause mortality was 9.2%. It goes without saying that the reported rates listed above are high and it is critical to discuss these potential risks and rates of undergoing a surgery for cervical deformity with the patient and to devise an appropriate surgical plan. The complication rate increases in patients undergoing a 3-comumn osteotomy for cervical deformity correction. Smith and colleagues again studied this population and discovered an overall complication rate of 56.5% with neurological deficits being the most common at 17.4% [107]. Among the various surgical procedures, the occurrence of C5 palsy was most frequent when utilizing SPOs, while C8 palsy predominated in cases involving 3-column osteotomies [107].

Postoperative distal junctional kyphosis (DJK) is another critical complication that must be addressed [108, 109]. DJK is typically characterized by a deviation from the expected radiographic alignment, occurring either at one or two levels below the lower-most instrumented vertebra (LIV). This condition has been linked to a range of factors, including adjacent level problems, fixation issues, spondylolisthesis, and various other causes [109]. Passias and colleagues have showed that DJK can occur at a rate of 23.1% 1 year following the cervical deformity correction [109]. The authors showed that DJK is likely to occur at 3 months after the surgery. Furthermore, the patients suffering from DJK were more likely to have severe cervical deformity at baseline [109].

Given the notable prevalence of DJK (Distal Junctional Kyphosis), proactive measures are frequently taken to mitigate its occurrence. These preventive strategies encompass the administration of bone-strengthening medications like teriparatide, the meticulous selection of the Lower Instrumented Vertebra (LIV) with respect to the curvature apex, and the implementation of ligament reinforcement [110]. Furthermore, the positioning of the LIV in relation to the curvature apex plays a pivotal role in DJK prevention. When certain criteria are met, such as a cSVA (cranial sagittal vertical axis) exceeding 4 cm, a T1 slope surpassing 30 degrees, or a global SVA exceeding 8 cm, as well as in cases where patients undergo a 3-column osteotomy, it is imperative for the LIV to be situated below the thoracic apex, typically at T9 or T10. In situations meeting these criteria, the risk of DJK escalates if the LIV is positioned near or above the thoracic apex [111]. Many different considerations must be taking into account for preoperative surgical planning and any possible attempt to reduce potential risks to the patient should be pursued given the high complication rate associated with these surgeries. With the recent expansion of large datasets and complex computation methods, future directions of risk stratification and reduction include predictive models and artificial intelligence that may augment surgical planning and aid in shared surgical decision making with the patients.

9. Conclusion

Cervical deformity correction can be challenging. Goals of cervical deformity surgery include correction of cervical spine deformity to maintain horizontal gaze, decompress neural elements, and restore alignment of the cervical spine. Cervical spine deformity correction can involve several anterior and posterior techniques such as ACDF, anterior osteotomy, posterior instrumentation and arthrodesis, and posterior osteotomies. Careful surgical planning and surgical technique are essential to minimize the risk of complication and maximize clinical outcomes in cervical deformity correction.