Although multiplanar deformities can occur in the cervical spine, kyphosis is the most common may be attributable to one of several etiologies. These include degenerative spondylosis, iatrogenic (e.g., post-laminectomy), post-traumatic, infection, inflammatory arthritides (e.g., ankylosing spondylitis), neuromuscular diseases, and metabolic diseases [12345]. Cervical kyphosis is typically progressive, and leads to abnormal positioning of the skull center of mass anterior to the occiput-C1 joints. As the deformity progresses and becomes more rigid, patients can develop pain, myelopathy, radiculopathy, difficulty achieving horizontal gaze or upright posture, swallowing dysfunction causing aspiration, social impairment, and even chest pressure sores in severe cases [45]. Pain may be attributable to degeneration of facet joints and intervertebral discs, compression of neural structures, as well as to increased stress on the posterior the soft tissue constraints (i.e., ligaments/muscles) as they struggle to maintain normal alignment [45].

Although cervical deformity can occur in isolation, it is commonly accompanied by progressive myelopathy and/or radiculopathy [6]. As the cervical spine develops progressively more kyphosis, the spinal cord is stretched and flattened against the posterior aspect of the vertebral bodies, leading to microvascular disruption, demyelination, and neuronal loss [78]. Additionally, cadaveric studies have shown significant increases in intramedullary pressure in the cervical spinal cord as the degree of kyphosis passes a critical threshold [9]. Compounding the increase in direct anterior pressure, there is also an increase in longitudinal cord tension due to the tethering of the cervical cord by the dentate ligaments and the cervical nerve roots [610], which further increases intramedullary pressure leading to neuronal loss and demyelination [111213]. Deformity patients thus frequently experience symptoms typical of myelopathy, such as gait instability, neck pain, diminished manual dexterity, bladder and bowel dysfunction, paresthesias and extremity weakness [61415161718].

Studies of nonoperative management of cervical myelopathy reveal that myelopathy symptoms tend to progress without surgery [19]. While these studies were performed in patients suffering from myelopathy due to cervical spondylosis, not all of whom necessarily suffered from deformity, myelopathy secondary to cervical deformity likely has a similar natural history with a poor prognosis without operative intervention.

In addition to the deformity and neurologic symptoms, patients with cervical deformity also frequently suffer from neck pain. Progressive deformity leads to a shift of the weight-bearing axis of the head anterior to the cervical spine. This results in an increase in cantilever loads, necessitating increased cervical paraspinal muscular expenditure in an attempt to extend the neck to maintain horizontal gaze, which can cause muscular fatigue and resultant pain. Patients with cervical kyphosis frequently complain of neck and upper shoulder pain for these reasons. The semispinalis cervicis and capitis muscles, in particular, have been shown to be critically important in maintaining neutral head position [20]. In addition to neck pain, patients can also have lower back pain as they attempt to compensate for a rigid chin-on-chest deformity by bending their knees, increasing pelvic retroversion and increasing lumbar lordosis [21].

First, one must determine the primary location of the patient's deformity, which is not always trivial. Because of the close relationship of both global and regional spinal sagittal alignment to the alignment of the cervical spine, it is important to assess the alignment of the spinal column in its entirety with full-length standing 36" radiographs. For example, positive sagittal balance from a primary thoracolumbar deformity leads to hyperlordosis in the cervical spine [31] and conversely, thoracic hypokyphosis in patients with adolescent idiopathic scoliosis leads to a compensatory cervical kyphosis (Fig. 6) [41]. In a similar fashion, deformities at the craniocervical junction or in the upper cervical spine can cause secondary deformities in the subaxial spine and vice versa. Further, the presence of deformities elsewhere in the spine need to be accounted for when planning the deformity correction to maintain overall balance and function. For example, in a patient with subaxial cervical spine kyphosis, fusion of the occipitocervical junction alone in an "optimal" neutral position may leave them with difficulty maintaining horizontal gaze with a markedly positive chin brow vertical angle. Thus, it is important to evaluate the entire spinal axis as well as the lower extremities for all primary and compensatory deformities when planning a surgical correction. When possible, full length skull to foot radiographs should be utilized prior to planning corrective osteotomies for cervical deformity.

Many, if not most, patients presenting with cervical deformity will have had previous cervical procedures. The details of what operation was performed, the approach used, and the success of that operation with respect to decompression and fusion, and whether there were any perioperative or postoperative complications are critical when creating a preoperative plan for revision surgery. If patients have had a previous Smith-Robinson approach, an evaluation by an otolaryngologist is important to determine the innervation status of the recurrent laryngeal nerve (RLN). If a previous operation resulted in an injury to the RLN, then any future anterior approaches should be performed ipsilateral to the injured nerve to avoid the potential for bilateral RLN injuries resulting in devastating vocal cord dysfunction. However, if both RLNs are intact, and an anterior approach is indicated, we recommend an approach from the contralateral side to the previous surgery to avoid dissecting through scar; in essence, turning a revision procedure into a primary procedure to reduce the risk of operative complications.

Additionally, if a patient has had a previous operation, it is important to consider infection or pseudarthrosis, especially with new onset pain after a pain-free interval or in the setting of a newly worsening deformity. If infection is suspected clinically an erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) should be obtained with a further workup pursued if these markers are elevated. Thin-slice CT scans with coronal and sagittal reconstructions and/or flexion-extension radiographs can be helpful to evaluate for pseudarthrosis [42].

The rigidity of the curve is an important factor to assess when formulating a preoperative plan for correction of a cervical deformity. If the curve is flexible, then gaining the necessary amount of correction is trivial as the head can be positioned intraoperatively in the desired position and then fused using an anterior, posterior, or a combined approach depending on the amount of immediate stability needed. If the curve is rigid but not ankylosed, then anterior releases through discectomies followed by anterior +/– posterior fusion is often sufficient for achieving the desired result. If however the curve is rigid and ankylosed, then some type of osteotomy will be required to obtain the necessary curve correction. In the setting of a previous operation, a careful review of preoperative imaging studies will allow the surgeon to determine where the spine has been previously fused (anterior or posterior). In general, the spine will need to be approached from the side where a previous fusion has been performed (e.g., posterior approach with posterior-based osteotomy in the setting of a prior posterior fusion) to mobilize the spine followed by either same-sided fusion alone, or a combined approach.

The amount of angular and translational deformity in rigid deformities will also affect the surgical plan. Different types of osteotomies can achieve differing amounts of correction (see section on Osteotomy types). A thorough analysis of sites of previous fusion as well as the amount of correction needed will allow the surgeon to formulate an appropriate plan for location and type of osteotomy to be performed. It is important to remember that in patients with a large C2–C7 SVA, osteotomies performed lower in the cervical spine or even upper thoracic spine will allow for the largest amount of translational correction at the time of surgery (Fig. 7).

Lastly, the presence of myelopathy or radiculopathy in a patient with cervical deformity is important for at least two reasons. First, decompression of the neural elements becomes an important goal of the procedure which could have implications for the bony surfaces available for fusion after deformity correction, or may necessitate both an anterior and posterior fusion due to the iatrogenic instability caused by the decompression. Second, there may be a higher risk of neurological complications after deformity correction in patients who already have a "sick cord" [43].

Anterior-based osteotomies can range from partial uncovertebral joint resection as performed during an anterior cervical discectomy and fusion (ACDF), partial or complete corpectomy, or an osteotomy which exits through the uncovertebral joints out laterally into the transverse foramina. These are classified by Ames as grade 1, 3, or 4 osteotomies [22]. These osteotomies depend on motion through the facets to gain correction, and therefore cannot be performed in isolation in patients with fused posterior elements.

The technique for an anterior osteotomy (grade 4) has been described in detail recently by Kim et al. [44]. Briefly, the patient is positioned supine with support under the head in the case of a fixed, kyphotic deformity. Gardner-Wells tongs are placed with 2.2 kg of traction to stabilize the head. A Smith-Robinson approach is used to approach the convexity of the deformity if there is a concomitant scoliosis in addition to kyphosis. Blunt dissection lateral to the uncinates is performed with a Penfield 4 and/or 2 and is used to identify the fused disc space, which is located just medial to the cranial border of the costal process. Caspar pins are placed in a divergent fashion (perpendicular to the anterior plane of the deformed cervical spine) which can help produce lordosis after completion of the osteotomy. The osteotomy is initiated using a 3mm matchstick burr, with the carpentry being tailored to match the patient's unique deformity so that there is proper alignment when the osteotomy is closed down. The bony resection is performed all the way back to the posterior longitudinal ligament. When burring laterally it is imperative to protect the vertebral arteries using a Penfield 4 or 2, or a cottonoid patty. It is advisable to also perform a foraminotomy at the level of the osteotomy as well to prevent nerve root compression with closure of the osteotomy. With the osteotomy complete, the deformity reduction is performed by removing the supports under the head and pushing on the forehead through the sterile drapes. This can be done in conjunction with distraction along the Caspar pins or by using a laminar spreader in the osteotomy defect. The traction weight is increased to 9 to 11.3 kg to maintain the correction. A graft with large surface area is shaped to fit the defect to reduce the risk of subsidence, and an anterior cervical plate is placed with fixed angle screws. Adjunctive posterior fusion is performed as necessary for increased stability.

Kim et al. [44] demonstrated that anterior osteotomies provide on average 23° of angular correction and 1.3 cm of translational correction. If performed in combination with posteriorly-based Smith-Petersen osteotomies (SPO), they obtained an average angular correction of 33° and translational correction of 3.7 cm although most of these were performed only for additional stabilization.

Posterior-based osteotomies can range from partial facet joint resection, total facet resection (SPO or Ponte-type), or a closing-wedge pedicle subtraction osteotomy (PSO) involving both the posterior and anterior columns. These have been classified by Ames as grades 1, 2, 5 or 6 depending on the degree of instability produced [22].

The SPO or Ponte-type osteotomy is performed by completely resecting both the superior and inferior articulating facets at a given spinal segment. This may be performed in conjunction with resecting the ligamentum flavum, lamina, and spinous processes. These osteotomies depend on residual anterior column mobility to obtain the necessary correction. These osteotomies are typically performed at multiple levels to obtain increased correction (Fig. 8). Importantly, these osteotomies do not involve any resection of the pedicles or vertebral body. The foramen transversarium is left intact as the vertebral artery runs just anterior to the level of the resection anterior to the facets and exiting nerve roots.

An opening wedge osteotomy, or grade 5 resection, involves complete resection of the posterior elements, lamina, spinous process, and facets followed by osteoclasis with subsequent lengthening of the anterior column. This was originally described by Mason et al. [45] and modified by several people including Urist [46] and McMaster [47], and typically used in the treatment of patients with ankylosing spondylitis. Because it lengthens the anterior column, it is a relatively dangerous osteotomy with significant risks to the anterior soft tissue structures (trachea, esophagus) and is rarely performed.

Alternatively, a pedicle subtraction osteotomy (PSO, grade 6) is a spinal shortening closing wedge osteotomy which is inherently safer than the opening wedge osteotomy, in addition to being more stable upon osteotomy closure. Both of these resections are typically performed at the cervicothoracic junction at C7 for several reasons: (1) the safe location of the vertebral artery in front of the transverse processes of C7, (2) the size of the spinal canal at C7–T1, (3) the long lever arm provided by performing an osteotomy low in the cervical spine which allows for greater translational correction, (4) the mobility of the spinal cord and eighth cervical nerves in this region, and (5) the limited impairment in hand function if a C8 nerve root injury were to occur. Prior to performing a PSO at C7, however, it is critical to evaluate the preoperative imaging studies carefully to make sure the vertebral artery does not take an aberrant course through the C7 foramen transversarium, which occurs in 2%–5% of patients [48].

The technique for cervical PSO has been previously described in detail by Wollowick et al. [49]. Briefly, the patient is positioned prone on a Jackson table on bolsters positioned to accommodate the deformity, with the table in maximum reverse Trendelenburg. The head is suspended in Gardner-Wells tongs with 7 kg traction, with a second traction rope positioned to provide an extension moment after completion of the osteotomy to hold the reduction. Posterior cervical and upper thoracic exposure is performed per routine. If motion is preserved at the craniocervical junction the fusion will stop proximally at C2, but is taken to the occiput in a completely ankylosed spine for improved fixation. Distal fixation is to T3 or T4. Instrumentation is placed segmentally with lateral mass screws and pedicles screws where appropriate skipping C6 or T1. The osteotomy is begun with a complete laminectomy of C7 as well as a partial laminectomy of the inferior half of C6 and superior half of T1. The lateral masses of C7 are then removed using a rongeur and a high-speed burr. Great care must be taken to remove any overhanging bone on osteotomy closure from the facets of C6 and T1 to prevent C8 nerve root impingement. After identifying and protecting the thecal sac medially, the C7 nerve root superiorly, and the C8 nerve root inferiorly, a burr is used to decancellate the C7 pedicle and vertebral body. Pituitary rongeurs and reverse-angle curettes are used to remove the bony walls of the pedicle, along with further decancellation of the body. Finally, the dorsal cortex of the vertebral body is impacted into the previously created decancellation cavity to complete the osteotomy. The osteotomy is closed and the deformity corrected by fixing pre-bent rods to the thoracic pedicle screws, and then extending the neck by pulling up on the Gardner-Wells tongs and seating the rods into the cervical lateral mass and/or pedicle screws. The C7 and C8 nerve roots again must be meticulously checked to rule out any signs of impingement. The neuromonitoring should also be checked carefully to ensure no signs of spinal cord compromise. Once the deformity is corrected, adjunctive anterior plating with screws above and below the osteotomy can be performed for additional stability.

Kim et al. [50] showed that with isolated cervical PSOs, the mean angular correction achieved was 34.5° with a mean translational correction of 2.5 cm. Interestingly, they found that with combining an anterior osteotomy with posterior SPOs, they achieved a mean angular correction of 27.8° with a mean translational correction of 2.6 cm, which were both statistically equivalent to those achieved with an isolated PSO, and with significantly less blood loss. Because of this, if technically feasible, they recommend combining anterior osteotomies with SPOs rather than an isolated PSO.

For severe deformities, both anterior and posterior osteotomies can be combined to achieve significant curve correction. As stated above, it's been shown recently that combining an anterior osteotomy with posterior SPOs can provide similar correction to an isolated PSO [50]. In certain, rare, situations a severe deformity may require removal of a complete vertebral segment (grade 7), consisting of the entire vertebral body, the adjacent disks, the complete uncovertebral joint, lamina, and facets. This type of osteotomy, called a vertebral column resection (VCR), is rarely performed in the cervical spine except for oncologic resections due to the need to protect the vertebral arteries. If necessary, it is performed via a combined approach.