Sagittal balance of the cervical spine

Physical exam

The primary goal during the preoperative evaluation of a patient with a cervical deformity is to determine the ideal amount of correction required and the best method to achieve the desired amount of correction. There is no predefined amount of cervical lordosis one should attempt to correct to given the wide range of normal values, however a general rule of correcting the cervical kyphosis to at least neutral has been accepted. ³² When seeing a patient with cervical deformity for the first time, the evaluation begins with the basics such as a complete medical history and physical exam. Many of these patients are frail and at high-risk for surgical complications. Therefore, a relevant medical history (e.g. major medical comorbidities, smoking, use of nonsteroidal anti-inflammatory drugs (NSAIDs, etc.) must be obtained and can aid in tailoring treatment and discussion of potential complication risks. The physical exam should include having the patient standing upright with hips and knees fully extended and in the sitting and supine positions as well as perform a detailed neurological exam. The sitting position removes the effect of lumbar and pelvic/hip deformity and the supine position can helpful to assess the rigidity of semi rigid curves under the direct effect of gravity. Chin-brow vertical angle should be measured before and after surgery (Figure 3) with the patient standing with the hips and knees extended and the neck in its neutral or fixed position. The extent of the posterior osteotomy wedge can be determined based on the CBVA. Despite lying flat, patients with fixed primary cervical deformity have persistent cervical flexion (Figure 4), whereas patients with thoracic, lumbar, or hip deformities correct in the sitting or supine positions.

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Figure 4.

Clinical photograph demonstrating the supine physical exam component demonstrating a patient with fixed cervical kyphosis. Reprinted with permission from Scheer JK, Ha Y, Sears WR, et al. Cervical deformity and treatment algorithms. In: Shen FH, Samartzis DS, and Fessler RG (eds) The cervical spine. Philadelphia, PA: Elsevier, 2014, pp. 146–154.

Imaging

After the physical exam, the initial radiographic evaluation should always include a 36 inch standing x-ray as well as dynamic flexion/extension cervical x-ray films (Figure 5). The 36 inch standing film allows for measurements of global spinal alignment, which is necessary when classifying the type of cervical deformity (discussed below) and identifying if the cervical deformity may be compensatory method for a thoracolumbar deformity. The dynamic films aid in determining the presence of any atlantoaxial instability as well as the relative flexibility of the cervical spine. With regard to standing x-rays, the EOS system is a new device that can provide single planar standing images of the patient from the skull to the feet as well as a three-dimensional (3D) configuration of the spine. ³³ The benefits to this new system include: 1) the entire body can be imaged at once therefore obviating the need for taking and joining multiple x-rays, 2) the x-ray images can be re-formatted into 3D configuration with semi-automatic contouring of individual vertebra, 3) the delivered radiation dose is much lower than standard x-rays, thus reducing the dose by a factor of 10, and 4) all the points of alignment and related compensation throughout the spine are concurrently imaged. ³⁴ Moreover, studies have shown that the EOS system is highly accurate, reproducible, and precise in the measurement of spinal curvature and that it even compares reliably to CT scanning. ^{35 –38} These systems are new and not every institution may have one, however they are recommended if one can be obtained. If not, the standard standing 3 foot x-rays mentioned above can work well. Rigid cervical deformities may need additional or more aggressive osteotomies to correct the deformity. Additional imaging studies are generally performed and include computed tomography (CT) and magnetic resonance imaging (MRI) to assess for osseous landmarks to plan for instrumentation insertion and spinal cord tethering or impingement, respectively. CT scans are very helpful in determining the extent of facet fusion and osteophytic bridging at the discs in order to evaluate the need for anteriorly and posteriorly osteotomies in fixed deformity.

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Figure 5.

(a) Preoperative lateral radiograph of a patient with fixed cervical kyphosis. Dynamic radiographs assessing the extent of rigidity by (b) flexion and (c) extension.

Relationship of cervical deformity and myelopathy

In the context of the physical exam, one must understand the relationship between cervical deformity and myelopathy. The surgical plan may change if the patient is found to have myelopathy. Cervical deformity, especially progressive cervical kyphosis, has been associated with myelopathy even without central stenosis. This phenomenon is a result of the kyphosis leading to spinal cord being draped against the vertebral bodies. The draping causes anterior spinal cord pathology from compression against the vertebral bodies. In addition, it causes an increase in the longitudinal cord tension due to the cord being tethered by the dentate ligaments and cervical nerve roots. ^39,40 (Figure 6) Tethering of the cervical cord can produce increased intramedullary pressure leading to neuronal loss and cord demyelination. ^{41 –43} Chronic compression of the spinal cord against the posterior aspects of the vertebral bodies results in flattening of the small feeder blood vessels on the surface of the cord become that leads to reduced blood supply and ischemia. This can lead to a large reduction in the number of vessels and network size, as well as produce an abnormal arrangement of the vessels. ⁴⁴ Animal models have shown that greater cord tension increases intramedullary cord pressure ^{41 –43,45} and leads to neuronal apoptosis. ⁴⁴ Shimizu et al. ⁴⁴ analyzed the severity of demyelination and neuronal loss in spinal cord histological sections following induction of cervical kyphosis small birds. The authors discovered a significant correlation between the degree of kyphosis and the amount of cord flattening. ⁴⁴ Angiography demonstrated that the vascular supply to the anterior portion of the cords was decreased. ⁴⁴ Additionally, atrophy of the anterior horn, neuronal loss, as well as demyelination of the anterior fasciculus was observed. The extent of demyelination progression was directly related to a greater kyphosis angle. ⁴⁴ The demyelination pattern began with the anterior fasciculus but then progressed to the lateral and posterior fasciculi. ⁴⁴

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Figure 6.

Sagittal cervical spine model showing that the spinal cord tension and length changes in response to sagittal alignment. The distance between marks (black dots) on the cord were measured and were 1.2 cm for kyphosis. Note a multilevel laminectomy has been performed to help visualize the concept. (a), 1.1 cm for the neutral position (b), and 1.0 cm for the cervical lordosis with C3–C5 laminectomy (c). Reprinted with permission from Scheer JK, Tang, JA, Smith JS, et al. Cervical spine alignment, sagittal deformity, and clinical implications: a review. J Neurosurg Spine 2013; 19(2): 141–159.

A study by Smith and collegues ⁴⁶ evaluated myelopathy scores [Japanese Orthopedic Association (mJOA)] with cervical sagittal alignment and spinal cord volume in patients with myelopathy. From 56 patients, the authors found a moderate negative correlation in patients with cervical kyphosis between cord volume and cross sectional area to mJOA scores. A positive correlation was found for patients with cervical lordosis thus indicating a relationship of cord volume to myelopathy that differs on the basis of sagittal alignment. ⁴⁶ Therefore, sagittal alignment can contribute substantially in the development of cervical myelopathy.

Cervical deformity classification

Not until recently did a cervical deformity classification system exist. Such as system can aid in the communication between physicians as well as assist in facilitating the appropriate clinical and radiographic evaluation of patients with cervical spine deformity. Therefore, Ames et al. ¹³ developed a cervical deformity classification system in order to provide a mechanism to assess cervical deformity patients within the context of global spinopelvic malalignment and clinically relevant parameters (Figure 7). The classification system consist of a primary deformity descriptor characterizing the curve apex location as well as five different modifiers. The modifiers include radiographically measured parameters and a clinical score for the degree of myelopathy. All of the five modifiers used in this classification had been previously shown to have a clinical impact. ¹³

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Figure 7.

Description of cervical deformity classification system, which includes a deformity descriptor and 5 modifiers. 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.

The classification system is comprised of a deformity descriptor, three sagittal alignment parameters, a myelopathy modifier, and lastly the SRS-Schwab thoracolumbar deformity classification. The deformity descriptor was selected to provide a basic categorizing of the various deformity types. The first three types are primary sagittal deformities that are differentiated by the location of the deformity apex. Type C is for the apex in the cervical spine, Type CT for apex at the cervicothoracic junction, and Type T for apex in thoracic spine. Primary coronal deformities are those with a C2–C7 coronal Cobb angle >15° and are designated as Type S. And lastly, type CVJ corresponds to primary cranio-vertebral junction deformities.

The three sagittal alignment parameters include cSVA, horizontal gaze (measured by CBVA), and the difference between the cervical lordosis and the T1 slope. The cSVA was selected as a modifier for the CSD classification given the significant impact of sagittal alignment on HRQOL among patients with thoracolumbar spinal deformities and the studies of Tang et al. ⁶ and Smith et al. ⁴⁶ demonstrating correlations between cervical sagittal alignment and multiple measures of HRQOL. Tang et al found that a C2–C7 SVA threshold of 4 cm was found to correlate with moderate disability based on the Neck Disability Index (NDI). ⁶ Therefore, there are three scores for the C2–C7 SVA modifier focused around the 4 cm threshold (Figure 7). Because horizontal gaze has a fundamental impact on basic human function and previous reports demonstrating the importance of accounting for horizontal gaze in spine deformity surgery, ^{14,24 –29,47} the CBVA was selected as a modifier for the classification system. Although thresholds of normal CBVA have not yet been rigorously defined, a CBVA of 10° is accepted as an optimal value. ¹⁴ Therefore three scores were selected for the horizontal gaze modifier (Figure 7). The relationship between T1 slope and cervical lordosis is similar to the relationship between PI and LL in that a greater T1 slope requires a greater magnitude of cervical lordosis to balance the head over the thoracic inlet and trunk. ^14,48 And given that cervical kyphosis is the most common type of cervical deformity with studies showing the clinical impact of this kyphosis, a parameter reflective of cervical lordosis was added as a modifier for the classification. The mismatch between T1 slope and cervical lordosis (TS-CL) was added with three scores (Figure 7).

As mentioned above, the clinical importance of cervical myelopathy is well known and its relationship to cervical alignment is critical to evaluating cervical deformity patients. Having myelopathy may affect the surgical plan in deformity patients as it should address spinal cord compromise when present, which may be through direct decompression, deformity correction, or both. Thus a measure of myelopathy was included in the CSD classification since myelopathy can be directly related to cervical deformity and since it can profoundly impact patient function. The mJOA score is a recognized and a widely accepted quantitative functional assessment of the severity of spondylotic myelopathy severity. ⁴⁹ Scores range from 0 to 18, with lower scores reflecting worse impact and three groups of scores were added to the classification (Figure 7).

And finally, the SRS-Schwab modifier was added because the spinal segments are all related and are not insolation of one another. Cervical deformities may contribute to thoracolumbar deformities and thoracolumbar deformities may produce or contribute to cervical deformities. ¹⁴ Smith et al demonstrated that adults with positive sagittal spinopelvic malalignment compensate with increased cervical lordosis in an effort to maintain horizontal gaze and that surgical correction of the thoracolumbar sagittal malalignment results in improvement of the abnormal cervical hyperlordosis through reciprocal changes. ⁵⁰ Ha and colleagues subsequently confirmed this observation and further identified key radiographic parameters associated with these compensatory changes. ⁵¹ The authors studied 49 patients that underwent surgical correction for thoracolumbar deformity and measured the cervical lordosis for a high and low preoperative C7 SVA groups. ⁵¹ In the lower C7 SVA group, cervical lordosis was significantly increased after thoracic/lumbar deformity correction and the high C7 SVA group showed decreased cervical lordosis postoperatively. ⁵¹ Furthermore, a high prevalence of concomitant cervical deformity has been reported among adults with thoracolumbar deformity. ⁵² Therefore, based on these and other studies that emphasize that assessment and classification of cervical deformity should not occur in isolation and that alignment of the thoracolumbar spine and pelvis should also be assessed. Therefore SRS-Schwab classification for adult thoracolumbar spinal deformity was added as a modifier for the cervical deformity classification. The SRS-Schwab classification includes five thoracolumbar coronal curve types and three sagittal modifiers (Figure 8). The SRS-Schwab classification has been validated, shown to correlate with HRQOL measures at baseline, and shown to be sensitive to changes in disease state. ^{53 –55}

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Figure 8.

SRS-Schwab classification system for adult spinal deformity. The classification consists of four coronal curve types and four sagittal modifiers. PI = pelvic incidence; LL = lumbar lordosis; SVA = sagittal vertical axis; PT = pelvic tilt; T = thoracic; TL = thoracolumbar.

Nonoperative management

Surgery should not be the first line of treatment and the patient may attempt a number of conservative options. Conservative treatment of cervical spinal deformity is mostly aimed at symptom reduction and the majority of cases that is addressing neck pain. Other options include chiropractic care and/or physical therapy, steroid injections, brace therapy, cervical traction, and nonsteroidal anti-inflammatory drugs (NSAIDs). Surgical treatment indications include patients with severe mechanical neck pain, any neurological deficit, and progressive deformity resulting in significant disability (i.e. loss of horizontal gaze and/or dysphagia).

Patients presenting with head ptosis or neck drop (cervical camptocormia) have a flexible sagittal deformity of the cervical spine which is corrected when the patient is laying supine. ⁵⁶ The various etiologies of camptocormia include various myopathies, parkinsonism disorders, amyotrophic lateral sclerosis (ALS), and idiopathic. ⁵⁶ Therefore, the initial work up of a patient with camptocormia (or other flexible deformity) should include appropriate electromyography, nerve conduction studies and referral to a Neurologist to rule out a primary myopathy or ALS. Moreover, they should be referred to physical therapy prior to considering treating with surgical correction and fusion.

Surgical management

When considering a surgical intervention, evaluation of the flexibility and rigidity of the cervical spine may determine the type of surgery needed. For a cervical spine that is flexible and is without ankyloses based on clinical exam, flexion/extension x-rays, and CT, an anterior or posterior alone correction strategy may be used. However, if the spine is rigid without ankylosed facets or have prior instrumentation, an anterior alone strategy may be sufficient. A combination of anterior and posterior approaches may be required to correct a deformity that is rigid in the anterior column and has ankylosed facets. The surgical approach, osteotomy type, and correction strategy also depends on the cervical deformity type being treated. In general, anterior release and posterior fixation is often most effective for subaxial cervical sagittal deformities (cervical kyphosis). For patients with cervical deformity as a result of CTJ pathology (high T1-slope, neutral to normal CL, and high low C2-slope), a three-column osteotomy and posterior approach would be the correct strategy to use.

Cervical traction with a halo may be used to initially attempt deformity correction prior to surgical intervention. Typically, 3–5 days of traction may be sufficient to reduce the deformity. ³² There is no current standard for the amount of weight to add, however, starting with 4 kg and adding 1 kg per day up to 9 kg has been described. ⁵⁷ If the deformity is not reduced after day 5, further traction is not likely to be of any benefit. In addition to the traction, once can add various muscle relaxing agents to aid in the deformity reduction. If successful reduction of the cervical deformity does occur, posterior fixation and fusion may be used to prevent the deformity from progressing.

When able to use the anterior alone approach, this allows for deformity correction via interbody release as well as instrumentation that maintains the correction. This method uses both biomechanics and posture to obtain the targeted postoperative cervical alignment. The patient is placed in the supine position with their head extended. In almost all cases, cervical traction is utilized with Gardner well tongs with 15 to 20 lbs. of weight (depending on patient size and deformity). After adequate exposure, anterior release may be performed by a combination of the osteotomy grades listed below. They generally include releasing the disc, osteophytes and uncovertebral joints. But may include corpectomies if indicated.

When the cervical deformity is rigid with ankylosed facets, a combined anterior-posterior strategy may be employed. ⁵⁸ With modern instrumentation and technology, a 360-degree approach is often sufficient to gain adequate correction. However a 540-degree approach can be utilized in uncommon circumstances. In this scenario, the side of deformity correction is first chosen for release and then the opposite side is released. Generally, the posterior approach is performed first with placement of screws and various osteotomies performed. The patient is then turned to the supine position for the anterior approach that includes disc release, anterior osteotomy and lordotic plating. The patient is then turned again to the prone position and instrumentation and posterior compression is applied. For significant fixed subaxial kyphosis it is recommended to first perform an anterior osteotomy (including release of the vertebral arteries from the foramen transversarium) then followed by posterior osteotomies with correction using head manipulation after the circumferential release. In patients requiring significant deformity correction, it is strongly recommended to use the halo ring instead of the Mayfield clamp. The halo has more fixation points and allows a stronger grip on the patient’s head during manual correction resulting in a lower chance of it slipping off the patient. Usually more lordosis is possible during an anterior release followed by posterior correction compared with the converse of posterior release followed by an anterior fixation. This is because it is typically easier to generate more lordosis from the posterior position. If the cervical deformity is very kyphotic and rigid with ankylosed facets, larger osteotomies or a combination of different osteotomies may be necessary to correct the deformity.

Cervical deformities in the coronal plane may be isolated or in combination with sagittal deformities with varying degrees of severity. Patients with fixed multiplanar deformities may require three-column osteotomies in order to allow for correction of the spine in both planes as well as to decompress the spinal cord and nerves. In addition, these patients may need multiple combined approached and/or stages. A 540 degree circumferential osteotomy or possibly a cervical vertebral column resection (VCR) may be used. ⁵⁹

With the evolution of cervical deformity types and the surgical treatment becoming more complex, a cervical osteotomy classification was developed by Ames et al. ¹⁶ (Table 1 and Figure 9). This system is focused on being anatomically based, graduated in degree of resection, and comprehensive. ¹⁶ It’s not designed to describe surgical indications for correction but more to aid in determining the optimal surgical approaches and osteotomies used. Moreover, it allows for a common language to more effectively and objectively communicate cervical osteotomy resections between surgeons and in the literature when describing techniques or research. In the classification system, there are seven anatomical grades of resection that increase in amount of tissue resection and degree of spine destabilization (Table 1 and Figure 9). Furthermore, since surgical correction of complex deformities may involve combinations of resection types, modifiers are included in the system to designate the surgical approach(es) that may be used in conjunction with the osteotomy grade. These include anterior [A], posterior [P], anterior-posterior [AP], posterior-anterior [PA], anterior-posterior-anterior [APA], and posterior-anterior-posterior [PAP] are listed in Table 1.

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Figure 9.

Illustrations depicting the cervical osteotomy classification system, which includes seven grades of resection, representing progressive degrees of potential destabilization. Reprinted with permission from Ames CP, Smith JS, Eastlack R, et al. Reliability assessment of a novel cervical spine deformity classification system. J Neurosurg Spine 2015; 23(6): 673-683

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Table 1.

Description of cervical osteotomy classification system developed by Ames et al. ¹⁶

Grade 1	Partial facet joint resection	Anterior cervical discectomy including partial uncovertebral joint resection, posterior facet capsule resection or partial facet resection	A, P, AP, PA, APA, PAP
Grade 2	Complete facet joint/Ponte osteotomy	Both superior and inferior facets at a given spinal segment are resected; other posterior elements of the vertebra including the lamina and the spinous processes may also be resected	P, AP, PA, APA, PAP
Grade 3	Partial or complete corpectomy	Partial or complete corpectomy, including discs above and below	A, AP, PA, APA, PAP
Grade 4	Complete uncovertebral joint resection to foramen transversarium	Anterior osteotomy through lateral body and uncovertebral joints and into foramen transversarium	A, PA, AP, APA, PAP
Grade 5	Opening wedge osteotomy	Complete posterior element resection with osteoclastic fracture and open wedge creation	P, PA, AP, APA, PAP
Grade 6	Closing wedge osteotomy	Complete posterior element resection and pedicle resection with closing wedge creation	P, PA, AP, APA, PAP
Grade 7	Complete vertebral column resection	Resection of one or more entire vertebral bodies and discs including complete uncovertebral joint and posterior lamina and facets	AP, PA, APA, PAP

* A = anterior, P = posterior, AP = anterior-posterior, PA = posterior-anterior, APA = anterior-posterior-anterior, PAP = posterior-anterior-posterior.