13.1 Epidemiology

Cervical spondylosis is an extremely common condition of the aging cervical spine. ¹ However, the prevalence of cervical deformity in the general population is less well known. A recent study demonstrates that 53% of patients previously diagnosed with degenerative thoracolumbar deformity have cervical deformity. ² But observation would suggest that clinically symptomatic patients are less common. Unfortunately, the prevalence of patients with cervical deformity who are clinically symptomatic is not known.

Cervical deformity contributing to spinal imbalance presents a unique diagnostic and therapeutic challenge in the context of the aging spine. Whether it is due to degenerative disease; inflammatory conditions; or postsurgical, sagittal cervical kyphotic deformity, it can induce pain and/or neurologic deficits, thereby limiting functional status and health-related quality of life (HRQOL). This chapter considers the etiological and biomechanical factors that contribute to cervical spinal imbalance. Additionally, the evaluation and treatment of these patients with emphasis on surgical planning is discussed.

13.2 Biomechanical Considerations

The cervical spine is responsible for maintaining the weight of the head, performing multiplanar movement through its flexibility, housing the spinal cord as well as its exiting nerve roots, and allowing for passage of the vertebral arteries into the skull base. Physiologic cervical lordosis is counteracted by the kyphotic thoracic spine; the horizontal slope of T1 corresponds to the subaxial lordosis necessary to maintain the head’s center of gravity in balance. ^{3 , 4} The head serves as an axial load, producing a bending moment through the cervical spine. The bony anatomy, musculature and ligamentous structures provide support of the head; without these structures, studies have demonstrated that the cervical spine would fail at one-fifth of the weight of the human head. Deviation from normal alignments, as is seen in patients with cervical deformity, causes increased loading forces on the cervical spine, leading to a cycle of progressive deformity and placement of large amounts of stress on paraspinal musculature. ^{5 , 6}

Cervical facet joints and their capsule contribute significantly to the overall cervical spine stability, as demonstrated in multiple cadaveric studies. They are oriented at about 45°in the coronal plane and about 80 °in the sagittal plane, which allows for a large range of motion in flexion and extension, but simultaneously limits translation and lateral bending. ⁷ Studies have demonstrated that surgical resection of 50% or more of the facet joint leads to instability in the sagittal plane. ⁸

Significant increases in flexion-extension motion following multilevel cervical laminectomy have been demonstrated where there is removal of posterior bony and ligamentous elements. The same study showed 80% reduction in motion after the decompressed cervical spine underwent facet wiring for stabilization, further illustrating the extent of stability offered by the facet joints. ⁹

Ligamentous structures also provide stability and resist forces, which help maintain the posture and function of the cervical spine. The anterior longitudinal ligament (ALL) and posterior longitudinal ligament (PLL) are fibrous bands extending from C2 down to the sacrum, anteriorly and posteriorly to the vertebral bodies, respectively. Because, unlike the ALL, the PLL narrows over vertebral bodies and widens at the intervertebral disc spaces, the ALL offers twice the strength. ¹⁰

The ligamentum flavum has some amount of strain at baseline so that it does not buckle or generate force on the cord during neck extension. The weight of the head creates a compressive force on the anterior aspects of the cervical spine and a tensile force on the posterior aspects. The anterior elements of the spine, including the vertebral body and the disc, transmit about 35% of the total load, while the posterior elements, including the pedicles, facet joints, lamina, spinous process, and posterior ligamentous structures, transmit approximately 65% of the load. By comparison, in the lumbar spine, the anterior elements transmit about 80% of the load. ¹¹ This can be attributed to the differing functions of the cervical and lumbar spines; the cervical spine needs to bear a large amount of force posteriorly in order to provide mobility and functional range of motion. As the posterior elements of the cervical spine become compromised, the anterior elements are forced to take on increasing amounts of load, causing stress on paraspinal musculature, leading to a cycle of progressive deformity through a positive feedback loop. ^{11 , 12 , 13 , 14}

13.2.1 Etiology and Pathomechanics

Cervical spine deformity is most often due to iatrogenic causes. ^{14 , 15} Sagittal plane kyphosis primarily occurs after a decompressive laminectomy without fusion. When the posterior arch is removed as part of a decompressive laminectomy, including disruption of the interspinous ligaments, laminae, and facets, the end result is a loss of stability. The anterior elements are required to bear more of the load, and cervical paraspinal muscles also undergo an increase in their load-bearing role, often leading to constant contraction and contributing to symptoms of neck pain, stiffness, and fatigue. This redistribution of biomechanical forces occurs in order to maintain the upright position of the head and overall spinal balance. ¹⁶

Kyphotic deformity can lead to myelopathy due to increased tension on the spinal cord in the cranial-caudal axis. The cord becomes tethered due to stress on the cervical nerve roots and the dentate ligaments. Eventually, the cord becomes compressed and there is a rise in intramedullary pressure, leading to neuronal damage and restriction of blood supply. ¹⁷

Postsurgical cervical kyphosis is not exclusive to posterior approach operations. Iatrogenic kyphosis can also be a result of anterior decompression, typically due to a failure to restore proper cervical lordosis, or due to pseudarthrosis, which causes increased load-bearing forces on the anterior elements. ^{18 , 19 , 20}

Cervical deformity can also be caused by degenerative disease. As the nucleus pulpous of the intervertebral discs becomes desiccated and loses its elasticity, the force distribution on the discs is altered. The annulus fibrosis becomes more load bearing, overall disc height is reduced, and encroachment into the epidural canal may occur, providing a mechanism for myelopathy and/or radiculopathy. ^{10 , 21} The attachment of the posterior longitudinal ligament to the vertebral bodies is compromised as the PLL also bulges into the posterior canal. These changes lead to progressive kyphotic deformity, laxity of the PLL, and overall cervical spinal imbalance.

Systemic inflammatory diseases, such as rheumatoid arthritis (RA) or ankylosing spondylitis (AS) can cause specific patterns of cervical spinal deformity. The autoimmune destruction of connective tissue leads to ligamentous compromise and bony erosion. The occiput–C1 and C1–2 joints are synovial and more affected in RA due to destruction and inflammation of the synovial cells. This can lead to atlantoaxial subluxation, multilevel subaxial instability, or migration of the odontoid process. ²² With AS, chronic inflammation leads to the formation of syndesmophytes, bridging osteophytes, ossification of joints, and eventually, bony fusion of the entire spinal column. Most typically, AS is associated with low back pain, loss of lumbar lordosis, and involvement of the sacroiliac joint. Additionally, disease progression in patients with AS can impact the global spinal alignments and the cervical sagittal balance, leading to loss of cervical lordosis, which causes chin-on-chest deformity and loss of horizontal gaze. ^{22 , 23}

Less common etiologies of cervical deformity include trauma, neoplasm, and infection. Cervical spinal deformity exists in the coronal plane as well, typically due to bony abnormalities, and is associated with several congenital disorders, such as congenital scoliosis and neuromuscular diseases seen in the pediatric population.

Clinical Assessment

The history and physical exam are critical in guiding the clinical decision-making process. The goals of clinical assessment are to determine the etiology of the patient’s symptoms, determine optimal treatment strategies and whether surgical management is indicated, and establish the location and degree of correction necessary. Given the complexity of surgical treatment of cervical deformity, special consideration should be given to medical comorbidities. Patients with histories that include tobacco usage, diabetes, or chronic nonsteroidal anti-inflammatory drug (NSAID) use may need lifestyle modifications and/or medical optimization prior to consideration for surgical treatment. Additionally, special consideration should be given to the bone health of each patient, and appropriate treatment for osteoporosis or osteopenia should ensue.

Patients may present with a variety of complaints, such as, neck pain, myelopathy, radiculopathy, dysphagia, inability to maintain horizontal gaze, and, in severe cases, respiratory compromise. Often, patients will complain of axial neck pain. Obtaining detailed history about pain is critical in making accurate clinical diagnoses. It is important to determine whether the pain is truly mechanical, that is, repeatedly and reliably occurs with movement but is not present at rest. Pain can also be present due to compensation by paraspinal musculature in the interscapular, thoracic, and lumbar regions. Range of motion, both active and passive, must be assessed to establish whether the patient has a fixed or flexible deformity. Often, what is believed to be cervical deformity may actually be a compensatory mechanism due to the global spine misalignment from the thoracolumbar spine or hip pathology. It is critical to perform a thorough clinical assessment to ensure that patients are not recommended unnecessary surgery. Patients with a flexible deformity that exists when upright but corrects when lying supine need to be assessed for various nonsurgical neuromuscular conditions, such as amyotrophic lateral sclerosis (ALS), Parkinsonian disorders, or myopathies. Such evaluation should include electromyography (EMG) or nerve conduction studies (NCS) and referrals to neurology and physical therapy prior to the consideration of any surgical intervention. ²⁴ Evaluation should include a full neurological exam to assess for the presence of myelopathy, radicular symptoms, or neurologic deficits, which may impact the goals of surgery. The apex of kyphotic deformity is at the highest risk for cord compression due to maximum tension on the cord. ¹⁷

Radiographic Assessment

Radiographic assessment of the deformity is important in determining a treatment plan. Upright and dynamic (flexion-extension) radiographs, in addition to 36-inch standing scoliosis films (with visualization of the skull base and femoral heads), help assess whether deformity is fixed or flexible, and enables a look at local and global spinal alignment. When obtaining the standing scoliosis films, patients should stand with knees fully extended to obtain accurate representation of sagittal plane deformity. At our institution, patients are instructed to place hands in the contralateral supraclavicular fossa to ensure no postural supportive devices are used.

Several radiographic metrics have been established as objective measures of spinal balance. Assessment of these metrics is essential to surgical planning because they have been demonstrated to be predictive of patient HRQOL and complications. The Cobb angle can be measured from C1 to C2 (Fig. 13‑1) or C2 to C7 (Fig. 13‑2) Preoperative measurements can be used as comparison to intraoperative imaging to establish whether adequate deformity correction has been obtained. Global sagittal alignment is determined by the distance from the posterosuperior corner of S1 to the plumb line extending down from the center of C7, or the distance from the center of the femoral head to the plumb line extending down from the center of C2. This value should be less than 5 cm. Numerous studies have demonstrated correlation between HRQOL metrics and the degree of global sagittal alignment with the C2 or C7 plumb line. ^{4 , 25 , 26 , 27 , 28} Cervical sagittal vertical axis (SVA) looks at local alignment with a plumb line drawn from the center of the C2 vertebral body and calculating the distance from this line to the posterior superior corner of the C7 vertebral body from standing radiographs (Fig. 13‑3). This measure has also shown to predict postoperative outcomes in patients undergoing multilevel cervical fusion surgery. ²⁸

The T1 slope, defined as the angle between the superior endplate of T1 and a horizontal line, can provide insight into the degree of cervical lordosis necessary to preserve sagittal balance (Fig. 13‑4). The chin brow vertical angle (CBVA) is a measure of cervical deformity that is useful in instances where patients have loss of horizontal gaze. The CBVA is the angle between the vertical and a line drawn between the glabella and anterior-most point of the chin. Improved outcomes (gait, activities of daily living) have been demonstrated when surgical intervention is focused on improving the CBVA and restoring horizontal gaze. ²⁹

Computed tomography (CT) is used to assess bony quality, osteophyte bridging, auto fusion or ankylosis of facet joints, which can determine the need for posterior osteotomies, landmarks, and sizing of instrumentation. Many patients who present with kyphotic deformity have histories of cervical spine surgery, and in such cases thin-slice CT can help assess prior fusion sites. Magnetic resonance imaging (MRI) is valuable in evaluating compression of neural structures such as cord injury/swelling, soft tissue compression, or presence of syrinx. Patients who cannot receive MRI may require CT myelogram. Depending on the degree of deformity and levels involved, computed tomography angiography (CTA) or magnetic resonance angiography (MRA) may be useful in assessing the course of vertebral arteries and other anomalous vascular structures for surgical planning.

13.3 Treatment Options

Generally, conservative treatments should be attempted before considering surgical intervention. These involve symptomatic treatments to target pain and include NSAIDs, muscle relaxants, steroid injections, bracing, and/or physical therapy. Cervical traction can be used to correct deformity either on its own, or as a preoperative intervention. If the deformity does not reduce after five days, it is unlikely to provide further reduction. Muscle relaxants and sedatives should be used in conjunction with traction. ³⁰

Surgery is usually considered when patients have neurologic compromise, instability, and/or deformity resulting in severe mechanical neck pain, and physical deformity leading to difficulty with ambulation, loss of horizontal gaze, dysphagia, or respiratory dysfunction, adversely impacting HRQOL.

Evaluation of the flexibility of the deformity with flexion-extension radiographs is important preoperatively. An anterior or posterior correction alone may suffice for a flexible deformity. However, if the deformity is rigid or involves ankylosis of the facet joints, a combined surgical approach may be necessary. The etiology of deformity is important to consider. For example, a bony tumor compromising the anterior column will require an anterior approach, while a flexible postlaminectomy kyphotic deformity may be best served with a posterior approach. The extent and quality of the kyphosis also impacts operative planning; focal kyphosis is more amenable to an anterior-alone approach, while longer-segment kyphosis may need posterior intervention.

The anterior-only approach can be used to correct deformity and place instrumentation to fixate the spine in the corrected position. It is most often utilized for rigid cervical kyphotic deformities when there is no ankylosis of the facet joints. The surgeon is able to directly visualize and decompress the anterior aspect of the spinal cord in addition to any anterior column reconstruction that may be necessary due to bony compromise.

Patients are kept supine in slight extension, and decompression is performed, including osteophytectomy, discectomy, and/or corpectomy. It is critical that the uncovertebral joints are released to obtain mobility. Deformity can segmentally be corrected by placing distraction pins in a convergent fashion to generate lordosis upon distraction of the pins. The patient may require gradual repositioning at this time. Lordotic cages can be used to promote fusion and provide additional deformity correction. Further lordosis can be obtained by contouring plates and using three-point bending to bring the middle-segment vertebral body up to the plate. Although an anterior approach is associated with decreased morbidity and mortality compared to combined approaches, the main drawback to this strategy is limited deformity correction.

The posterior-only approach is useful in cases of flexible cervical deformity where sagittal plane imbalance is due to insufficient posterior ligamentous structures or ossified posterior longitudinal ligament (OPLL); a posterior cervical fusion may suffice for correction. If adequate cervical alignment can be achieved with extension radiographs, there is consideration for a posterior approach using three-point skull fixation and extension positioning. Most typically, posterior alone approaches are used as a prophylactic measure after multilevel decompression to prevent the development of sagittal plane deformity as a result of ligamentous interruption, as seen in postlaminectomy kyphosis.

A combined posterior-anterior-posterior approach may be necessary in cases of rigid or incompletely reducible deformity with ankylosed facet joints. ³¹ Such an approach is frequently necessary in patients with AS. Generally speaking, a posterior-based facet release and osteotomy is completed first. Following this, anterior release, discectomy, and graft placement is undertaken. Finally, posterior instrumentation and deformity correction completes the surgery. This may be done in a staged fashion, with in-line cervical traction employed during each portion of the planned surgery, or, less typically, in one setting. In the setting of AS, posterior instrumentation is critical to prevent further deformity due to the large moment arms surrounding the fusion site.

In instances where osteotomy is necessary, if the degree of correction needed is less than 30°, posterior column osteotomies (facet resections) can be performed. There is a small increased risk of pseudarthrosis stemming from the need for multiple osteotomies to correct larger deformity. A flexible anterior column is needed for full closure of the osteotomy, necessitating either movement through the intervertebral disc, or, in cases with significant spondylosis, a combined anterior-posterior approach. The planned resections are completed through the facets with a high-speed burr down to the exiting nerve roots. Care must be taken to resect laterally through the lateral edge of the lateral masses, or remaining bone will pervent closure of the ostetomies. After all planned resections are completed, desired cervical alignment is achieved through extension positioning using three-point head fixation. Lateral radiographs are taken to ensure alignment goals have been met prior to placement of hardware. Alternatively, for larger deformity correction, a pedicle subtraction osteotomy (PSO) can be used. PSOs allow for up to 30°of correction without the need for an anterior-based procedure. PSOs allow for three-column correction and typically are completed in the lower cervical or upper thoracic segments, thereby correcting the overall position of the cervical spine. The downsides to performing a PSO are that it is technically demanding and has increased morbidity and perioperative complications. Surgeries involving osteotomies often have greater than average blood loss, so careful assessment of the patient’s medical comorbidities is needed.

Multilevel fusion typically should not terminate at the cervicothoracic junction (CTJ). CTJ bears the high mechanical stresses between the mobile cervical spine and the relatively fixed thoracic spine. Moreover, it is a transitional zone between the cervical lordosis and thoracic kyphosis, further accentuating the high biomechanical forces. ³² As these special characteristics cause a predisposition for instrumentation failure and increased risk of revision surgery, fusion should be extended to at least the T1 level. ³³ In the cervical and thoracic spines respectively, lateral mass screws and pedicle screws are usually used. The use of pedicle screws may be considered in the cervical spine, despite the increase in difficulty and risk of damaging neurovascular structures, due to the potential benefits from increasing the load sharing across the disc space as well.

Multimodal intraoperative neuromonitoring is often used in deformity surgery. Both motor evoked potentials (MEPs, recording intermittently from corticospinal tracts) and somatosensory evoked potentials (SSEPs, recording continuously from dorsal columns) are used. Changes in the MEPs intraoperatively may guide deformity correction, such as timely reversal of reduction maneuvers. However, neuromonitoring changes should be interpreted in context and with a degree of suspicion. Studies have not reliably shown any changes to safety or outcomes with the use of intraoperative neuromonitoring. ^{23 , 34}

Additionally, the use of SSEPs alone should be avoided, as these readings can be misleading and insufficient. Studies have demonstrated that even in instances of normal recordings, patients can have significant deficits postoperatively. ^{35 , 36} The gold standard for assessing changes to neuromonitoring is the wake-up test; however, due to the complexity of these surgeries, oftenperformed with the patient in prone position, utilizing the wake-up test requires both time and an experienced anesthesiologist and may not always be an option intraoperatively.

Deformity surgery should always involve fluoroscopy, which allows for intraoperative adjustment of correction. Anesthetic considerations are also important during cervical deformity surgery. An experienced anesthesia team with expertise in spinal anesthesia is preferable. Depending upon the severity of myelopathy or the stability of the cervical spine, awake fiber-optic intubation may be required. Intraoperatively, the maintenance of mean arterial pressure to minimize any poor perfusion to the spinal cord is required. A preoperative huddle or meeting should be considered as a method whereby the details of the surgery and requirements of each member of the operating room team are checked and verified.