Overview
Ankylosing spondylitis (AS) is a chronic inflammatory disease of unknown etiology that affects approximately 197 people per 100,000 per year in the United States with a male-to-female predominance of approximately 3 : 1. An idiopathic seronegative spondyloarthropathy, AS is strongly associated with the class I antigen HLA-B27. The sacroiliac joints and the axial skeleton are most commonly affected by AS, but clinically significant peripheral joint involvement may also occur. The chronic inflammatory nature of the condition causes stiffness and loss of lordosis or increased kyphosis of the spine through destruction and autofusion of the vertebral motion segments. This can ultimately lead to a dramatic imbalance of sagittal alignment, causing severe deformities throughout the cervical, thoracic, and lumbosacral spine ( Fig. 68-1 ).
Osteotomy for Correction of Kyphotic Deformity of Ankylosing Spondylitis
Indications
Involvement of the cervical spine in patients with AS is often overlooked, because lumbar spine and appendicular sequelae may be more obvious. Cervical kyphosis may slowly progress in an insidious fashion, resulting in the development of a chin-on-chest deformity. When cervical kyphosis progresses to the point of causing difficulty with function and hygiene, a corrective cervical osteotomy is indicated. Functional difficulty may present as dysphagia or inability to lift the head to allow forward gaze. Inability to achieve forward gaze inhibits activities of daily living, such as forward ambulation and ascending and descending stairs. Also, difficulty swallowing solids may cause malnourishment. In addition to the cervical spine, the thoracic and lumbar spine may contribute to the kyphotic deformity.
Kyphosis of the ankylosed thoracic spine usually does not progress sufficiently to require surgical intervention. When it occurs in AS, increased thoracic kyphosis often goes unnoticed owing to the naturally occurring thoracic kyphosis. The combination of this natural kyphosis and the relatively small thoracic spinal canal places the thoracic spinal cord at higher risk of traction and ischemic injury with osteotomy.
In the lumbar spine, AS presents as low back pain for more than 3 months duration, stiffness, diminished range of motion, and hypolordosis. Osteotomy is the procedure of choice to address severe, fixed kyphotic deformities in the lumbar spine that result from AS. The midportion of the lumbar spine is the safest area for correction, below the termination of the spinal cord (i.e., the conus medullaris at the L1–L2 level). Correction of kyphotic deformity of the lumbar spine via osteotomy may be safer than in the cervical spine because of the potential to perform the correction below the level of the spinal cord. In cases of coexisting deformities of the cervical and lumbar spine, osteotomy of the lumbar spine is generally preferred for this reason, and also because it can simultaneously correct the forward gaze and lumbar kyphosis. Patients with preexisting cervical kyphosis who come to medical attention with acute cervical spine fractures that require surgical stabilization may occasionally be treated with osteotomy to simultaneously stabilize the acute fracture and correct the functionally limiting deformity.
Contraindications
Extraarticular manifestations of AS include aortic incompetence, cardiac conduction defects, fibrotic lung disease, and renal amyloidosis. These extraskeletal expressions of AS can present a significant risk for serious perioperative complications, including cardiopulmonary compromise and renal insufficiency. In some instances, the severity of these concomitant medical conditions may result in an unacceptably high level of morbidity for the patient to tolerate surgery, and careful preoperative evaluation is warranted.
Appendicular skeletal manifestations of AS, such as hip-flexion contractures and osteoarthritis, may exaggerate a kyphotic spinal deformity by tipping the patient forward, thereby worsening effective sagittal balance. Correction of hip pathology in AS is safe and efficacious and should be addressed before spinal management.
Posterior Cervical Osteotomy for Correction of Kyphotic Deformity of Ankylosing Spondylitis
Advantages
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High degree of correction
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Single-level surgery for correction
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Availability of segmental internal fixation for precise control and maintenance of osteotomy correction
Disadvantages
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Risk of neurologic injury (infrequent)
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Risk of vascular injury
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Technically demanding
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Inability to correct concomitant lumbar deformities
Preoperative Evaluation
Preoperative evaluation begins with a thorough history and physical evaluation. The medical history should focus on premorbid conditions that may compromise surgical treatment and recovery. In addition to evaluating the entire spinal column, the musculoskeletal examination should also evaluate the hip joints, which are often affected by osteoarthritis in AS. As a result, loss of the normal chin-brow vertical angle either occurs as a result of flexion deformity throughout the spine or through fixed hip-flexion contractures ( Fig. 68-2 ). Surgical correction should be planned for the area of the major deformity after conducting a thorough medical evaluation.
Measurement of the chin-brow and sagittal vertical angles is necessary for preoperative planning of the degree of correction needed from the osteotomy ( Fig. 68-3 ). In addition, the inion-wall angle can also be helpful in preoperative planning; it is measured by standing the patient against the wall and measuring the distance between the wall and the inion at the base of the skull. This provides another useful tool for monitoring the progression and subsequent correction of cervical kyphosis.
Once the chin-brow angle has been determined, the angle necessary to bring the face to within 10 degrees of horizontal gaze can be calculated. The goal is to provide a reasonably safe correction to improve the patient’s ability to perform the activities of daily living that rely on forward gaze, such as grooming, eating, reading, and ambulation. Correction beyond neutral (overcorrection) is not recommended, because a fixed gaze above the horizon creates significant difficulty for the patient during straightforward ambulation and stair descent.
To view the entire spine and assist with determining the level for corrective osteotomy, lateral radiographs of the entire cervical, thoracic, and lumbar spine on a 36-inch cassette are necessary. A sketch of the spinal osteotomy can be drawn on tracing paper, which can also be used to outline the surgical algorithm. Alternatively, the osteotomy and operative plan can be written on radiographs with a wax pen ( Fig. 68-4 ).
One day before surgery, the patient should be admitted to the hospital. This allows for the application of a halo cast with a contoured plaster jacket. After the halo cast is applied, the patient is placed in a seated position to evaluate for potential areas of undue pressure that will be placed on the patient intraoperatively. If necessary, areas of the cast that could contact pressure points are trimmed, and these are well padded.
The restrictive nature of a kyphotic posture causes many patients with AS to have a protuberant abdomen, which affects their breathing ( Fig. 68-5 ); therefore the abdominal portion of the contoured body cast must avoid compression that might inhibit respiratory function. The cast is also modified posteriorly to allow access to the cervical and upper thoracic spine. Preadmission also allows for the establishment of central venous access, which is necessary for intraoperative hemodynamic monitoring by the anesthesiologist. Completing these procedures before surgery maximizes patient safety and surgical efficiency when the patient arrives in the operating room.
Intraoperative Technique
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After awake fiberoptic intubation and induction of anesthesia, the patient should be placed in a beach chair position.
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It is imperative to have the appropriate neurophysiologic monitoring available throughout the entire procedure; if available, this should include transcranial-evoked motor potentials, somatosensory-evoked potentials, and C8 dermatomal-evoked potentials.
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If neurophysiologic monitoring is unavailable, the procedure should be performed under local anesthesia.
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The patient is positioned on the operating table in the seated position, and the halo cast is secured to the table. Cervical traction is then applied to the halo to stabilize the head and cervical spine ( Fig. 68-6 ). The posterior support bars of the halo may be removed if they hinder the approach to the cervical spine.
Figure 68-6
Patient with ankylosing spondylitis in upright position and cervical traction in preparation for posterior cervical osteotomy.
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Following the induction of general anesthesia, transesophageal echocardiographic monitoring should be established to assist with detection of a possible air embolus, which may occur secondary to the patient’s upright position. Air can be removed via the central venous pressure line if necessary.
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The posterior cervical spine is approached through a standard midline incision, and Gelpi retractors are used to provide exposure. Meticulous hemostasis is essential. Subperiosteal dissection should widely expose the laminae, facet joints, and transverse processes.
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The C7–T1 interval is identified with intraoperative radiographs. Performing the osteotomy at the cervicothoracic junction minimizes the risk of injury to the vertebral artery, which enters the transverse foramina at the C6 level in the majority of patients ( Fig. 68-7 ).
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