The delicate balance among the curves of the spinal column has allowed man to accomplish a unique feat among living organisms: upright bipedal ambulation. When examining human embryology, it is clear that the predominant curve of the nascent spinal column is one of kyphosis from the thoracic and sacral segments. The lordotic compensatory curves of the cervical and lumbar spines develop secondarily and allow for balancing of the occiput over the pelvis, a necessity for balanced upright gait. Any factors that alter the alignment of these curves, or their relationship to one another, has clinical consequences in regard to balance, ambulation, pain, and load sharing. The normal lordotic curvature of the cervical spine has been measured in various studies, but no general consensus exists on a range of “normal values.” To measure the kyphosis in the cervical spine, a tangent is drawn to the posterior cortex of the most cephalad and caudad vertebral bodies of the curve, typically the C2 and C7 vertebral bodies. The angle formed by the intersection of these two tangents, theta, is the degree of cervical kyphosis ( Fig. 56-1 ).
Gore and colleagues evaluated the values of cervical kyphosis from C2 to C7 in osteoarthritis patients and found 16 to 22 degrees of lordosis in men and 15 to 25 degrees of lordosis in women. Ganju and colleagues define normal values of cervical lordosis as being from 10 to 20 degrees. Irrespective of the true value, when evaluating the cervical segment, the global overall alignment of the patient’s spinal column must be taken into account. A plumb line drawn from the C2 body and extended caudally should pass through the S1 body and rest just anterior to the S2 vertebral body ( Fig. 56-2 ).
Biomechanical evaluation of the spinal column underscores the importance of both alignment and secondary stabilization structures. The vertebral bodies themselves primarily resist compressive forces and bear 36% of the axial load. The posterior bony elements, facet joints, and soft tissues resist predominantly tensile loads and bear approximately 64% of the axial load. Loss of integrity of either one of these stabilizing structures results in altered load-bearing mechanics and eventual loss of normal sagittal alignment. When devoid of all osteoligamentous attachments, a cadaveric spinal column fails at much lower loads than those observed in vivo. During flexion, extension, and other activities of daily living, the forces in the cervical spine can approach 1200 N. Cadaveric studies have demonstrated failure of the cervical column with vertical loads of as little as 10 N when all osteoligamentous attachments are removed. To withstand these physiologic loads, the cervical spine must have anatomic alignment and intact musculature. This allows appropriate load bearing and distribution to occur. In the lordotic cervical spine, the weight-bearing axis of the head falls behind the vertebral bodies, thus decreasing energy expenditure by the paraspinal muscles.
In the kyphotic cervical spine, the weight-bearing axis is translated anteriorly, and the force acting upon the spinal segments induces a bending movement. The increased forces acting upon the anterior column results in compression of the disks and places more stress upon the posterior tension band. Distribution of forces in this manner will result in continued worsening of kyphosis ( Fig. 56-3 ).
Because normal cervical lordosis depends on a delicate balance of forces and alignment between the anterior and posterior columns of the spine, disruption of either of these two regions can result in kyphotic alignment. The anterior column may lose its integrity as a result of trauma, tumor, spondylosis, spondyloarthropathies, or metabolic abnormalities. Patients with advanced spondylosis of the cervical spine will have collapse of their disk spaces and will gradually develop kyphosis, whereas changes in patients with a traumatic injury occur quite suddenly.
Postlaminectomy kyphosis is the most commonly encountered form of cervical kyphosis, and it results from complete disruption of the posterior tension band ( Fig. 56-4 ). The incidence of postlaminectomy kyphosis has been reported from 6% to 30%. Kaptain and colleagues performed a retrospective review to examine patients who underwent cervical laminectomy for myelopathy and found an overall incidence of 21% for postlaminectomy kyphosis within a 4-year follow-up period. Patients with preoperative loss of lordosis were twice as likely to develop postoperative kyphosis.
Patients with cervical kyphosis typically present with complaints of shoulder and neck pain. This is due to the cervical and back musculature attempting to maintain sagittal alignment. In severe cases, patients may have difficulty with forward gaze and even swallowing. These patients will attempt to compensate through their lumbar spine, and lumbar hyperlordosis with concomitant lower back pain is often noted. The presence of cervical radiculopathy and myelopathy is variable, depending on the amount of kyphosis and spondylosis present. Kyphosis leads to draping of the ventral aspect of the spinal cord over the posterior aspect of the vertebral bodies. Studies show that kyphosis in the cervical spine leads to compression of microvascular feeding vessels to the spinal cord, potentially leading to the development of myelopathy.
A complete neurovascular examination should be completed with emphasis on long-tract signs such as Hoffman or Babinski signs or clonus. A detailed and complete radiographic workup is essential to determine key facts about the deformity, such as 1) the origin of the deformity, 2) the severity of the curve, and 3) the flexibility of the deformity and whether it corrects passively. All of this information is essential for surgical planning, and the answers to these questions help determine whether the surgical approach should be anterior, posterior, or a combined intervention.
Initial films should include anteroposterior (AP), lateral, and flexion-extension views of the cervical spine. These dynamic radiographs are of utmost importance, as they allow determination of whether the deformity is fixed or mobile. The patient’s global sagittal alignment should be evaluated with a full-length standing lateral radiograph. In patients with severe kyphosis, we position them supine on an examination table to assess whether the deformity will correct passively and to what degree. In addition, a thin-slice CT scan of the cervical spine should be obtained to evaluate the facet joints posteriorly. If the facet joints are ankylosed, this will likely necessitate some form of posterior intervention. An MRI scan of the cervical spine should be included to assess for any pressure on the spinal cord and neural elements. The presence of large anterior osteophytes will typically require anterior intervention.
No firm surgical indications exist for patients with cervical kyphosis. Because not all patients with this condition require surgical intervention, the clinician must adeptly extract and synthesize key information from the history, physical exam, and imaging studies. These key pieces of information allow tailoring of an individualized treatment plan for each patient. In our experience, the presence of myelopathy, radiculopathy refractory to conservative treatment, or progressive deformity with worsening clinical symptoms are all indications for surgical intervention. Patients with severe deformity that interferes with their activities of daily living should also be closely evaluated for surgical intervention.
Surgical Approach: Anterior, Posterior, or Combined?
Once the decision has been made for surgical intervention, the decision for an anterior, posterior, or combined approach must be addressed. Much of this hinges on the key details about the deformity.
What is the origin of the deformity? Anterior column integrity can be compromised by tumor, trauma, or multilevel spondylosis. A deficient anterior column will require an anterior approach. Patients with postlaminectomy kyphosis have a deficient posterior tension band.
Are neural elements compressed ventrally? Ventral compression on the spinal cord and neural elements will likely need to be addressed anteriorly.
Is the deformity passively correctable? This key piece of information must be gleaned not only from the dynamic radiographs but also by having the patient lie supine. A kyphotic cervical spine that corrects passively can be addressed entirely from a posterior approach.
Are the facet joints ankylosed posteriorly? A rigid deformity with ankylosed posterior joints will likely require a combined AP approach.
The answers to the aforementioned questions permit a stepwise approach to surgical decision making. A rigid deformity with no ankylosis of the posterior facets can be addressed anteriorly. If the deformity is rigid, and the facets are ankylosed posteriorly, a combined AP approach will be necessary. Lastly, in the absence of ventral compression, a passively correctable deformity can be addressed posteriorly alone. The flowchart shown in Figure 56-5 demonstrates this in algorithmic format.
The utility of the anterior approach to the cervical spine has expanded since its introduction in the 1950s by Smith and Robinson. Advances in spinal fixation have allowed surgeons to treat more complex spinal pathology through more limited approaches. The primary indication for ventral-only cervical kyphosis deformity correction is a fixed deformity without ankylosis of the posterior facet joints.
The anterior approach offers several advantages to the surgeon: direct access to the ventral spinal cord is available to decompress the spinal column, and ankylosed segments can be directly released ventrally, assuming the posterior facet is mobile. The anterior approach offers the ability to correct the deformity with reconstruction of the load-bearing anterior column. The anterior approach is associated with less morbidity and mortality than the combined anterior and posterior approach. In cases of postlaminectomy kyphosis, the anterior approach avoids the surgical risk of exposure and instrumentation with an exposed spinal cord. The major disadvantages of this approach are less kyphotic deformity correction than combined approaches, loss of correction, graft-related complications, and approach-related complications. In general, the approach is implemented when the posterior facets are not fused.
Anterior Surgical Technique
The patient is positioned supine on a radiolucent table that provides radiographic evaluation of all planned surgical segments. The patient is intubated with the assistance of fiberoptic guidance to reduce the amount of cervical extension prior to decompression. Our current practice is to perform the procedure with neuromonitoring. A roll is placed under the patient’s shoulders, and a donut roll is placed under the head with several towels. This allows for varying amounts of extension to be attained after decompression is performed.
The cervical spine is approached by standard methods, utilizing either an oblique incision along the anterior sternocleidomastoid or a transverse incision. The decompression is performed with diskectomy, corpectomy, or a combination based on preoperative imaging. It is paramount to adequately release the uncovertebral joints and to ensure mobility of the posterior facets prior to attempting correction of the deformity. If present, intermediate vertebral bodies should be left during the decompression to provide intervening points of fixation during reconstruction. As described by Steinmetz and colleagues, an intermediate vertebral body is a vertebral body that has cerebrospinal fluid present posterior to the vertebral body on T2-weighted MRI sagittal imaging. This implies a lack of compression at this level, and the vertebral body may be left for additional fixation.
After decompression and anterior release is complete, attention can be turned to kyphotic deformity correction. The previously placed towels or donuts are removed to increase cervical lordosis. Distraction posts are then placed in a convergent manner, such that as distraction is performed, the cervical spine is corrected into a lordotic position ( Fig. 56-6 ).
The posterior longitudinal ligament (PLL) may be left intact as long as no disk fragments are evident that could cause spinal cord or nerve compression. The PLL can be used to aid in restoration of lordosis as the anterior column lengthens in relation to the posterior column. Bone graft should be placed after contouring to maintain lordosis. In diskectomy, the graft should be trapezoidal in shape with the larger segment in the anterior cervical spine. Anterior cervical plating is generally from the most cephalad vertebral body to the most caudal vertebral body involved in the kyphotic deformity. The cervical plate is then secured first at the most cephalad and caudal vertebral body. If present, the intermediate vertebral bodies are then drawn to the implant, recreating cervical lordosis. Steinmetz and colleagues advocated the use of dynamic cervical plating in older populations. The advantage of dynamic plating is that it controls sagittal plane deformity while allowing for minimal subsidence in the axial plane. This decreases the stress on the construct, thereby decreasing graft- and plate-related complications.
The wound is thoroughly irrigated and is closed over a small drain brought out through the incision. The patient is placed in a cervical orthosis prior to emergence from anesthesia. The patient may remain intubated overnight, depending on the length of the surgery and the number of levels involved. The cervical orthosis is typically kept for 6 to 8 weeks, depending on radiographic evidence of consolidation of the fusion mass. Cervical radiographs including AP, lateral, and flexion-extension views are obtained at 3 months to evaluate for pathologic motion.
The current literature on anterior-alone treatment for cervical kyphotic deformity correction is composed entirely of retrospective reviews with a relatively small number of patients. Furthermore, available instrumentation has evolved over the last several decades, which has led to significantly different treatment modalities among the studies for the anterior-alone procedures.
Zdeblick and colleagues published their results of 14 patients treated with fibular or iliac graft with a follow-up of 27.9 months. The reconstruction utilized placement of a graft into a 5 mm deep hole centered in the cephalad and caudal vertebral body. Postoperatively, cervical orthosis or a halo-vest was used for stabilization. The average preoperative kyphosis was 45 degrees, which was corrected postoperatively to 13 degrees, with final follow-up revealing 17 degrees of kyphosis (loss of 4 degrees). The average Nurick grade improved from 3.6 to 1.3 at follow-up, and 9 of 14 patients had complete neural function recovery. All patients demonstrated fusion on lateral radiographs with flexion and extension views.
Herman and Sonntag reported on their results treating postlaminectomy kyphosis in 20 patients treated with anterior decompression, bone graft, and anterior cervical plate. The mean preoperative kyphosis was 38 degrees, which was corrected postoperatively to 16 degrees of kyphosis. Complete resolution of preoperative symptoms occurred in 10%, and improvement in pain and neurologic function occurred in 55%. Pain improved in 30% of the patients without change in neurologic function. One patient (5%) developed late progressive neurologic symptoms.
Steinmetz and colleagues treated cervical kyphosis with anterior decompression, reconstruction, and dynamic cervical plating in all but two young patients. The average preoperative kyphotic angle was 13 degrees, and the authors were able to obtain a mean of 20 degrees of correction; postoperative measurement averaged 6 degrees of lordosis. Over the follow-up period, an average of only 2.2 degrees of lordosis was lost. Clinical improvement was found in all patients, three of whom had complete resolution of their symptoms. All 10 patients went on to achieve osseous union.
Ferch and colleagues presented their results in 26 of 28 patients treated for cervical kyphosis with a minimum of 18 months follow-up (2 patients died before 18-month follow-up). Cervical kyphosis was addressed with anterior decompression, reconstruction with strut grafts, and static anterior cervical plating. The preoperative local kyphosis was 12 degrees, and postoperative local lordosis of 2 degrees was obtained. Improvement in myelopathy scores, assessed by the modified Japanese Orthopaedic Association Myelopathy Scale, occurred in 41% of patients. No improvement occurred in 56% of patients, and one patient (4%) experienced deterioration. The authors noted that the patient who suffered deterioration was the only patient in whom more that 20 degrees of local correction was attempted. The overall pain scores were not different preoperatively and postoperatively.
Park and colleagues were able to correct preoperative kyphosis by an average of 20.9 degrees to 14.0 degrees of lordosis, with final follow-up showing 9.6 degrees of lordosis in patients with postlaminectomy cervical kyphosis. The treatment consisted of anterior decompression, anterior reconstruction, and placement of a static anterior cervical plate. The mean angle of correction was 30.5 degrees. Improvement of neck disability index, visual analog scale, and Nurick grades was 27.09 to 10.48, 6.22 to 2.3, and 2.52 to 1.04, respectively.
Zdeblick and colleagues reported that 3 of 14 patients had graft-related complications with dislodgement. The graft dislocations required revision surgery in two of the patients (2/14, or 14.3%). The mortality rate was 14.3%, but the two deaths were not related to the surgical procedure itself. Riew and colleagues reported specifically on the short-term complications of anterior cervical corpectomy in postlaminectomy patients. In this series of 18 patients, a graft-related complication rate of 50% occurred when counting for extrusion, collapse, pseudarthrosis, or progressive kyphosis. The purpose of this paper was to highlight that postoperative immobilization in a halo-vest did not prevent graft-related complications. It is noteworthy that none of the 18 patients were treated with anterior cervical plating techniques. Herman and Sonntag reported vocal cord paresis (15%), pneumonia (10%), deep vein thrombosis (5%), reintubation (5%), graft site wound dehiscence revision (5%), and screw pullout (5%) treated successfully with revision screw and halo vest orthosis for 2 months. Steinmetz and colleagues reported one transient complication of feeling a “lump in the throat” that completely resolved by 6 months. The two long-term complications were of hoarseness in the patient’s voice. No graft-related complications were reported in this series. Ferch and colleagues reported two deaths (7%), two patients with prolonged dysphagia (7%), one infection (4%), one hardware loosening (4%), and one persistent neural compression (4%). Park and colleagues had a complication rate of 30.4% in 26% of patients treated. Graft-related complications comprised 14.3% of the complications that included implant displacement, graft dislodgement, and pseuodarthrosis. Other complications included swallowing difficulty, wound infection, dural tear, and pneumonia. Overall, the implementation of anterior cervical plating has helped to decrease graft-related complications; however, this has created the possibility of hardware-related loosening as demonstrated in several series.
The author’s current practice is to perform a stand-alone anterior procedure for fixed cervical kyphosis without evidence of posterior facet ankylosis. Careful preoperative evaluation is performed with cervical AP, lateral, flexion-extension, CT, and MR imaging. Attention is focused on levels that need decompression and intermediate vertebral bodies available for supplemental fixation. Reconstruction of the vertebral column is performed with the use of iliac crest or fibular strut graft (greater than two-level corpectomy) with implementation of a dynamic cervical plate for fixation. Cervical orthosis is worn for 6 to 8 weeks postoperatively.
Combined Anterior-Posterior Approach
A combined AP surgical approach is primarily indicated in patients with fixed cervical kyphosis and ankylosed facet joints. This disease pattern typically prevents adequate decompression and deformity correction through a single approach. Additional consideration for a combined AP approach is given in patients whose deformity involves the cervicothoracic junction, given the physiologic stresses that occur across this transitional area of the spine. To minimize risk of pseudarthrosis and loss of correction, other authors prefer the combined approach when there is a multilevel kyphotic deformity. The combined approach has the potential to decompress the neural elements both anteriorly and posteriorly, to control correction with focused osteotomies as necessary, and to provide rigid internal fixation in a 360 degree fashion.
When considering a combined approach, it is imperative that the goals of surgery and the predominant pathologic features are clearly defined in each individual case. This will allow for optimal sequencing of the overall surgical procedure. Many techniques have been described for the combined approaches, depending upon the specific clinical scenario and surgeon preference. Location of both spinal cord compression and ankylosis, along with extent of kyphosis, need be considered in sequence planning. Regardless of technique, the overall goals are decompression of the neural elements and restoration of sagittal balance. This is essentially achieved through the lengthening of the anterior column and shortening of the posterior column.
Anterior-Posterior Surgical Technique
The surgical sequence is typically anterior–posterior or posterior–anterior–posterior, although posterior–anterior and anterior–posterior–anterior have all been described. Most surgeons prefer that final correction be carried out posteriorly, because this ensures adequate decompression of both the spinal cord and nerve roots at the time of final alignment and fixation. There is a theoretic risk of anterior graft loosening when finishing with the posterior approach, but this has not been observed clinically.
Intraoperative neurologic monitoring is performed through the use of somatosensory evoked potentials (SSEPs) and transcranial motor-evoked potentials (TcMEPs). Awake fiberoptic intubation is preferred, although some authors advocate the use of preoperative and intraoperative traction, because there may be correction of the deformity, even if it is not complete. The head and neck must be supported appropriately during the anterior approach, because the deformity typically prevents standard supine positioning. The arms are secured at the patient’s sides with gentle traction to facilitate adequate fluoroscopic visualization.
Depending on the number and location of the levels to be addressed anteriorly, a transverse or longitudinal incision may be used. Decompression is performed using diskectomies, corpectomies, or a combination thereof. If the cervical spine is significantly ankylosed, osteotomies must be carried far enough laterally through the uncinate processes to ensure complete anterior release. In cases where the osteotomy must be performed at a focal area of kyphosis, skeletonization of the vertebral artery can be considered to limit the risk of kinking during subsequent correction of the kyphosis.
Instrumented fusion is performed with the use of grafts or cages and a plate. Fusion is augmented with the use of local or remote autograft and/or allograft. Use of bone morphogenetic protein is controversial; because it carries the potential for catastrophic airway compromise, it is not recommended in the cervical spine and is not approved for such applications. A soft drain is left in place, and the wound is closed in the standard layered fashion.
If the anterior surgical correction afforded by multilevel diskectomies or corpectomies provides a significant correction, it may obviate the need for additional osteotomies. In that case, the posterior approach may be performed simply to reinforce the posterior tension band and back up the anterior instrumented fusion with a posterior instrumented fusion. However, more severely rigid kyphotic deformities may require posterior osteotomies. If posterior osteotomies are necessary, they can be performed either at the levels of maximal kyphotic deformity or at the cervicothoracic junction. These posterior osteotomies have the added risk of kinking of the vertebral artery during correction in addition to the risk of neurologic injury. Simmons described the ideal location for osteotomy at the C7 level, where the vertebral artery remains anterior to the transverse processes; the canal is large relative to the cord, limiting potential for injury, and neurologic injury at this level can still allow for meaningful upper extremity function.
To perform the posterior approach, the patient is repositioned prone with the head held in position using the Mayfield tongs. Again, the arms are positioned at the patient’s sides with gentle traction to facilitate adequate imaging. A standard midline approach to the cervical spine is performed with subperiosteal dissection to expose the lateral edge of the lateral masses and facet joints. The posterior construct and fusion needs to extend far enough cephalad and caudad to maintain the correction and limit the risk of failure or junctional kyphosis, typically at least one to two levels above or below the osteotomies or deformity. Typical posterior osteotomy options include limited facet osteotomies, Smith-Peterson osteotomies, pedicle subtraction osteotomies, or posterior extension osteotomy as originally described by Urist ( Fig. 56-7 ).