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ANKYLOSING SPONDYLITIS
EDWARD D. SIMMONS, MD, EDWARD H. SIMMONS, MD

Ankylosing spondylitis affects about 0.2-0.3% of the U.S. population at any given time. A seronegative inflammatory arthritis of the spine that is more common In males, ankylosing spondylitis often presents with early pain in the sacroiliac joints and lower back and must be carefully distinguished from other causes of lower back pain. The major clinical problems related to ankylosing spondylitis occur with respect to ossification and potential deformity of the spine and hip joints. Thorough investigations include plain radiographs of the sacroiliac joints, which often will show early erosive changes, bone scan studies of the sacroiliac joints, and serologic studies such as HLAB-27.

FIGURE1: A and B. A, lateral view of a 57-year-old woman referred for correction of her "spinal deformity." A anterior view showing her knees held rigidly together with protective pad on the right knee.

FLEXION DEFORMITIES
With ossification and stiffening of the spine in ankylosing spondylitis, the spine may fuse in a flexed posture. A predominant area of concern is in the cervical spine, and sudden pain and forward flexion of the cervical spine most often indicates a fracture. Fractures are often difficult to detect on plain radiographs and must be ruled out with proper imaging studies. Early treatment of the fracture with realignment of the spine to its prefracture position with controlled halo traction and immobilization in the halo vest is the appropriate management.

FIGURE1: C and D. C, anteroposterior radiograph showing ankylosis of sacroiliac joints and spine. D, anteroposterior radiograph showing complete ankylosis of both hip joints.

If the spine does fuse in a flexed position, surgical correction can be undertaken if the deformity presents a significant functional handicap to the patient. The patient must understand the risks and rehabilitative measures required.

Examining the entire spine and hip joints to appreciate where the predominant flexion deformity exists is extremely important. The loss of the normal "chin-brow to vertical angle" can occur due to flexion deformity in any area of the spine or through hip flexion contractures that are fixed. Surgical correction should be planned for the area of the major deformity (Fig. 1).

Accurate measurement of the trunk deformity is carried out by measuring the chin-brow to vertical angle, which is a measurement of an angle formed by a line from the brow to chin through a vertical line and right angle to the floor (Fig. 2).

FIGURE 1E. Postoperative lateral standing view following bilateral total replacement arthroplasties, showing correction of the patient's main clinical deformity.

KYPHOTIC DEFORMITY OF THE LUMBAR SPINE
Correction of kyphotic deformity of the lumbar spine in ankylosing spondylitis was first recorded by Smith-Petersen et al. in 1945.¹ Additional authors reported on this; however, initial procedures done with the patient under general anesthesia were fraught with difficulties and high morbidity and mortality. A major complication of correction of flexion deformity of the lumbar spine postoperatively is abdominal aeus and gastric dilatation secondary to mesenteric artery obstruction as it impinges over the third part of the duodenum. It is imperative to avoid this complication by leaving a nasogastric tube in position postoperatively until free bowel sounds and intestinal motility are established. Surgery can be performed under general anesthesia using fiberoptic intubation and spinal cord monitoring. An endotracheal tube is inserted with the patient awake. The patient helps position himself on the operating table and signals with an "okay sign," at which point general anesthesia is commenced. We have used a tower table for these procedures, which allows for extension of the hips for closure of the osteotomy site.

FIGURE 2 The chin-brow to vertical angle as used to measure the degree of flexion deformity of the spine in ankylosing spondylitis. A, for thoracolumbar deformity. B, for cervical deformity. C, for postoperative assessment. The chin-brow to vertical angle is the angle from the brow to the chin to the vertical line with the hips and knees extended and the neck in its fixed or neutral position.

Various fixation techniques have been used, including Luque rectangle with Drummond wires, Cotrel-Dubousset instrumentation, and Texas Scottish Rite Hospital (TSRH) instrumentation-with hooks and screws or with screws only.

Bone Removal and Decompression
The fused spinous processes are resected and cut into small strips to be used for grafting. Bone is removed with bone cutters, chisels, rongeurs, and a power burr, allowing the surgeon to slowly cut throught the fused elements toward the spinal canal, which is opened. The dura is carefully stripped from the bone with a seeker and protected with cottonoid patties, which are applied with care. In many long-standing cases the dura is quite atrophic, similar to that of longstanding spinal stenosis. In rare instances it may adhere to the laminae, making its separation difficult. The amount of bone indicated by the wedge measured on the radiograph is removed from the tips of the spinous processes and laminae, decompressing the cauda equina completely in the area of the spinal canal. The cauda equina must be decompressed well laterally out to the level of the pedicles. Bone is removed laterally through the fused area of the posterior joints. The slot on each side is gradually widened depending on the amount of correction desired. The angle that has been transposed to the lateral film of the lumbar spine will indicate the amount of bone that must be resected at the level of the spinous processes, the laminae, the fused posterior joints, and, finally, the space that must be allowed at the level of the pedicles. The pedicles must be undercut, removing the inferior edge of the superior pedicle and the superior edge of the inferior pedicle to allow adequate room for the nerve roots following extension correction. Symmetrical amounts of bone must be removed from either side to allow symmetrical closure of the defect. The ossified ligamentum flavurn is invariably harder and more dense than the vertebral elements. The laminae must be undercut above and below to avoid impingement on the dura following extension.

FIGURE 3A. Preoperative standing lateral view of the patient with knees extended, indicating the extent of flexion deformity of lumbar spine and the angle of correction that will be required to straighten the deformity.

When the procedure is carried out under local anesthesia, the patient is advised following decompression that his or her spine is ready to be straightened. The anesthetist ensures that the patient is well oxygenated, and a short-acting barbiturate is given to allow osteoclasis to be performed. When the patient is no longer able to maintain conversation, the anesthetist indicates that the short-acting barbiturate has had effect, and the osteoclasis is performed. The hips are extended with the surgeon pushing forward in the midlumbar area. There is frequently an audible sound and a physical feeling of the osteoclasis occurring as the lateral masses come together. The patient is turned prone on previously prepared plaster molds. The osteotomy site is grafted by inserting autogenous bone that was removed during the resection; additional crushed cancellous allograft bone can be used if necessary.

Postoperatively the patient is nursed on a roto-rest bed. The patient must be maintained in a well-molded posterior plaster shell because the spine is rigid and uneven. Lying on a flat mattress could result in displacement at the osteotomy site. In addition, the posterior shell is necessary to support the patient adequately when the trap-door in the mattress is removed for use of a bedpan. Nasogastric suction is maintained until intestinal motility and bowel gas passage have occurred (Fig. 3).

KYPHOTIC DEFORMITY OF THE THORACIC AND CERVICAL SPINE
Kyphotic deformity of the thoracic spine in ankylosing spondylitis does not usually reach proportions that require surgical correction. Patients with ankylosing spondylitis breathe with their diaphragms, and violation of the diaphragm by exposure through thoracoabdominal incisions should be avoided at all times. A thoracotorny can be undertaken for anterior diskectomy and releases, followed by posterior osteotomies at multiple levels along with compression instrumentation for correction. The patient must be carefully assessed to ensure that the thoracic kyphosis is the primary cause of the flexion deformity.

FIGURE 3B and C B, lateral preoperative radiograph showing the angle of correction and amount of bone to be resected. Removal of 2 cm of bone is required on each side at the level of the fused posterior joints, 2.5 cm at the level of the laminae, and 5 cm at the level of the tips of the fused spinous processes. C, postoperative lateral radiograph showing angle of correction obtained following closure of resected defect posteriorly with anterior osteoclasis through the L3-L4 disk space.

FIGURE 3D. Postoperative standing lateral view showing correction achieved following removal of calculated wedge of bone, indicated by preoperative defortnity.

If there is an area of incomplete anterior ossification such as a destructive spondylodiskitis, a period of careful halo-dependent traction can be carried out, followed by further correction with multiple posterior osteotorn~ies and stabilization with Harrington compression instrumentation and fusion.

The correction of primary thoracic kyphosis is the most hazardous of all procedures due to the fact that the spinal canal is smallest in this area and the blood supply is more tenuous. Staged procedures are usually performed with an anterior release and multiple diskectornies; performed initially, with an interval of halo-dependent traction, followed by multiple posterior resection osteotomies with compression instrumentation and fusion (Fig. 4).

With ankylosing spondylitis of the cervical spine, one must determine the history of the development of the kyphotic deformity. If the deformity presented following a painful event, even if the trauma was relatively trivial, a fracture must be considered as a cause because fracture is a common and often unrecognized cause of the development of a cervical flexion deformity in ankylosing spondylitis. If it is unrecognized, the potential for severe neurologic injury is high. The site of the injury is usually in the lower cervical spine or at the base of the cervical spine at the cervicothoracic junction. The fracture is usually a transversely oriented sheer fracture, and any displacement can result in severe neurologic injury. The patient may put his head in his hands to ease the distress and pain.

FIGURE 4 A, B,andC  A, lateral view of a 34-year-old woman with increasing thoracic kyphotic: deformity associated with ankylosing spondylitis. Her height had diminished 5 inches over the preceding 5 years. B, anterior view showing restricted field of vision and rib impingement in pelvis. Pulmonary function was 54% of predicted normal. C, lateral bending view showing localized thoracic kyphosis.


It is often quite difficult to recognize the fracture with plain radiographs. Tomographic studies are a great help in this regard. When a fracture is recognized, a halo should be applied and traction supplied to restore the alignment of the head and the neck to its prefracture position. If the head was in a previously flexed position, pulling it with traction into a neutral alignment may cause severe neurologic injury. When the appropriate alignment has been obtained, the patient should be immobilized in a halo vest or a well-molded halo cast for 4 months. A high union rate is associated with this protocol. If pseudarthrosis develops, posterior fusion or anterior cervical fusion can be undertaken.

TECHNIQUE OF CERVICAL OSTEOTOMY

FIGURE 4F. Postoperative lateral 3-foot radiograph of spine showing correction of major deformity and restoration of normal spinal alignment.

For patients with fixed flexion deformity of the cervical spine that requires corrective surgery, a cervical osteotomy can be performed. The operation is carried out under local anesthesia and in the sitting position. This allows the best form of active spinal cord monitoring and immediate assessment of the patient's vital functions and neurologic status. The level for the osteotomy, C7-TI, is below the entry point of the vertebral arteries, which enter through the foramen transversarium at C6. Also any injury to the C8 nerve root would cause less disability than injury to other nerve roots.

Preoperative planning is important. The chin-brow to vertical angle is calculated, and, based on this, the size of the wedge to be resected posteriorly is determined. A dental type of chair is used to position the patient; the halo is placed and a 9-pound weight of traction applied in direct line with the neck to stabilize the head throughout the procedure. The patient is allowed to listen to the radio or other music throughout the procedure and usually converses with the anesthetist. Fentanyl and Versed are used by the anesthetist to supplement the local anesthesia. Pulse oximetry, C02 analyzer, and systemic blood gases are used by the anesthetist to monitor the patient, and a Doppler apparatus is fixed to the patient's chest to detect any possible air embolisms.

FIGURE 4G and H.  G, postoperative lateral view showing restoration of normal alignment. H, postoperative anterior view showing elevation of the ribs from pelvis and restoration of normal field of vision.

Exposure is carried out using local infiltration with 1% lidocaine with epinephrine in a concentration of 1:200,000. The C7 spinous process is usually identifiable, as is C6, which is the last bifid process. Radiographic confirmation can be used if necessary. The entire posterior arch of C7, the inferior half of C6, and the superior half of TI posterior arches are resected. The spinal canal is opened, and the dura and spinal cord are carefully protected with patties. The eighth cervical nerve root canal is identified, and bone is removed overlying the eighth nerve root, carrying the decompression laterally and undercutting the C7 facet joints and pedicles adequately to allow ample room for the eighth cervical root when the osteotomy site is closed. The amount of bone to be resected is carefully assessed preoperatively and intruoperatively to avoid any compression of the nerve roots on closure of the osteotomy. The laminae of C6 and TI must be undercut to avoid any kinking of the spinal cord on closure of the osteotomy site (Fig. 5).

FIGURE 5. Diagrammatic outline of the posterior view of the area of resection. The lines of resection of the lateral fused joints are beveled slightly away from each other, extending posteriorly so that the two surfaces will be parallel and in apposition following correction. The pedicles must be undercut to avoid impingement on the C8 nerve roots. The midline resection is beveled on its deep surface above and below to avoid impingement against the dura following extension correction.

Following the decompression, in preparation for the osteoclasis and closure of the osteotomy site, the patient is given a dose of short-acting barbiturate, usually Brevital sodium or sodium pentathol. The patient's mental status is closely monitored by the anesthetist and, at the point that the patient becomes unable to converse, the osteoclasis is performed by extending the neck carefully, grasping the halo through the drapes, and tilting the neck backward. An audible snap and palpable sense of osteoclasis is felt as the neck is extended. The lateral masses and osteotomy site close and oppose together. The patient is given supplemental oxygen while waking, and confirmation of the neurologic status is undertaken, observing motion of upper and lower extremities and mental status. While the surgeon holds the head firmly in the corrected position, the assistant stabilizes the halo to a preoperatively made cast, locking the correction in place. Portions of bone that had been removed during the course of the posterior compression are now fragmented and used as posterolateral bone grafts on each side.

Closure is carried out with deep sutures and suction drainage. Ideally the sutures are placed before the osteotomy site is closed and are tied following the osteotomy to ease insertion of the deep sutures. The skin is closed with interrupted nylon sutures. Final connections are made and tightening of the halo to the halo plaster cast carried out. The patient stands and walks toward the Circoelectric bed, which is in the vertical position and then tilted to the horizontal position. The patient is taken to the intensive care unit (Fig. 6).

The Circoelectric bed facilitates postoperative mobilization, allowing the patient to change from a horizontal to vertical position, making standing and walking easier. When sufficiently mobile to get in and out of a regular bed, the patient is transferred to a regular bed with or without a trapeze attachment. The patient is immobilized in the halo cast for 4 months. When the cast and halo are removed, careful clinical and radiographic assessment of the osteotomy and fusion site are obtained. Lateral tomography centered at C7-Tl may be necessary to evaluate the radiographic union. Further bracing with a sternal occipital mandibular immobilization (SOMI) brace can be carried out for an additional 2 months.

FIGURE 6 A, B, and C   A, posterior view of a man with rigid flexion deformity of the cervical spine of such magnitude that the head is not visible from behind. B, lateral view showing his chin rigidly fixed against his chest, with marked restriction of the field of vision and interference with his ability to open his mouth. C, anterior view showing complete restriction of his field of vision.


FIGURE 6D and E.   D lateral view of cervical spine showing complete ossification of posterior joints and previous subluxation of C6-C7. E, postoperative lateral view of the cervical spine showing extension osteotomy correction.

FIGURE 6 F and G.   F, postoperative anterior view showing return of normal field of vision. G, postoperative lateral view showing establishment of normal chin-chest interval and alignment of head to the vertical.

REFERENCE
1. Smith-Petersen MN, Larson CB, Aufranc OE: Osteotomy of the spine for correction of flexion deformity in rheumatoid arthritis. J Bone Joint Surg 27:1, 1945.


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