Ankylosing spondylitis is a chronic inflammatory disease affecting the medial skeleton, causing pain and progressive stiffness, and is associated with the histocompatibility antigen HLA-B27, which has been shown to be detected in 88% to 96% of patients with ankylosing spondylitis. It often includes the hip and shoulder joints. Rarely, peripheral joints are affected, and occasionally patients develop eye and heart damage. The movable and peripheral joints of the spine show synovitis similar to rheumatoid arthritis, with the formation of vascular opacities causing cartilage destruction and bone erosion. These changes finally trigger the body’s repair response process, leading to fibrous or bony ankylosis of the joints. Yang Cao, Department of Orthopedics, Wuhan Union Hospital
The onset of ankylosing spondylitis is between 15 and 20 years of age. It often begins as lower back pain, which can involve the buttocks or the back of the thighs, and is episodic, often accompanied by stiffness. Symptoms progress up the spine to the thoracic and shoulder joints. Most symptoms have resolved by the time the disease process appears to limit spinal motion, and inflammation has ceased by the age of 40 to 50. Patients with advanced ankylosing spondylitis have increased thoracic lordosis and decreased normal lumbar lordosis when upright. As the disease progresses, the patient loses the normal curve in the sagittal position and the spine becomes retroconvex.
X-ray changes are diagnostic for typical ankylosing spondylitis and occur first in the sacroiliac joint and are divided into three phases: (1) reduction of subchondral and periarticular bone; (2) loss of joint cavity due to erosion as inflammation progresses; and (3) sclerosis of subchondral bone and formation of new bone bridges until the joint cavity disappears, indicating a healing phase of inflammation. Early spine X-rays show a square anterior angle of the thoracolumbar vertebral body. Vertebral osteoporosis is often accompanied by a reduction in the concavity of the vertebral endplates. Paravertebral ossification manifests as ligamentous osteophyte formation. This ossification forms at the annulus fibrosus, with bone bridges forming between the affected vertebral bodies. In the early stages, the ligamentous osteoid is slender and well demarcated, and in the late stages, bamboo-like changes in the spine are formed. Posterior structures of the spine including the intervertebral joint capsule, interspinous ligament, supraspinous ligament and ligamentum flavum are also invaded, and late changes in these structures result in a tram-track-like appearance of the spine on X-ray. Peripheral joints show bone loss as the inflammatory process progresses, joint erosion and ossification, and later, healing of the joint inflammation common at the onset. Among the peripheral joints the hip is more susceptible than other joints to inflammatory damage, severe erosive changes and ossification, resulting in complete ankylosis of the hip joint.
(i) Indications for surgery
Most patients with ankylosing spondylitis do not require surgical treatment, and nonsteroidal drugs are the mainstay of treatment. Ankylosing spondylitis with fixed deformity of the spine in flexion may be considered for surgery if pain and neurological deficits are present. Pain is a common complaint and can be caused by compensatory curvature of the spine, especially if the patient has some cervical mobility with excessive anterior cervical convexity, while neurologic deficits are rarely present. The kyphosis limits the patient’s ability to perform some activities of daily living, and the main complaint of most patients is that they cannot look straight ahead. Severe kyphosis may result in abdominal complications due to compression of abdominal organs. At the same time, severe kyphosis may limit diaphragmatic movement and restrict pulmonary function due to ankylosis of the pectoral joints. The indication for surgery for ankylosing spondylitis is therefore related to the extent of the deformity and the degree of functional limitation.
There are no absolute contraindications to surgical orthopaedic surgery for fixed flexion deformities. Older age and poor general health are relative contraindications, and the patient’s body must be able to tolerate the blow of major spinal surgery as well as postoperative braking and functional rehabilitation and reconstruction. Patients with significant abdominal scarring and atherosclerosis of the great vessels cannot undergo extension osteotomy of the lumbar spine.
(B) Preoperative preparation
The patient’s general condition and special conditions related to ankylosing spondylitis should be evaluated preoperatively. Preoperative pulmonary function tests and arterial blood gas analysis should be performed because of restricted thoracic expansion and diaphragmatic restriction, impaired respiratory function, anterior cervical convexity with chin close to the chest wall, and temporomandibular joint ankylosis that restricts upper airway patency. Cardiac functional defects associated with ankylosing spondylitis include aortic closure insufficiency, conduction defects, and atherosclerosis should also be evaluated preoperatively. Because of renal amyloidosis, a special evaluation of renal function should be performed. Ankylosing spondylitis with colitis, a deformity that causes abdominal pressure to suppress appetite, should be examined preoperatively to determine the patient’s nutritional status, and any nutritional disorders should be addressed and corrected. Patients with ankylosing spondylitis have generalized osteoporosis, and preoperative bone densitometry can provide a reference for the quality and feasibility of bone fixation. Cervical and full spine stability should be considered before general anesthesia tracheal intubation and positioning of the patient. Preoperative standing anteroposterior and lateral full spine radiographs should be taken to evaluate the overall deformity and spinal balance, and CT is useful to visualize the cervicothoracic junction. dot radiographs can confirm the level of incomplete or “soft ankylosis” and help determine the optimal position for osteotomy. Imaging of the spinal canal can reveal arachnoid cysts associated with cauda equina syndrome, and MRI is most helpful in this case.
Kyphosis is often associated with a fixed flexion deformity of the hip joint. Hip ankylosis should be addressed before spinal orthopedics is performed. Sometimes the deformity is corrected by arthroplasty and the hip joint will be mobile enough to compensate for the residual deformity of the trunk. The primary location of the deformity should be determined preoperatively, and osteotomy in this plane will provide the greatest degree of correction with the least risk.
Thoracolumbar kyphosis deformities are divided into two main categories. The first category has significant thoracic kyphosis but normal anterior cervical and lumbar convexity. For this group of patients, multiple osteotomies are required to correct the major thoracic deformities. In the second category, the entire thoracolumbar spine is deformed and the anterior lumbar convexity disappears. In these patients, the spinal deformity can be corrected by extensional osteotomy of the lumbar spine. One of the major problems with kyphosis is that the curvature of the trunk causes the patient’s vision to change from horizontal to downward sloping. The kyphosis posture is either the result of lumbar, thoracic, or cervical kyphosis, hip flexion contracture (which is functional kyphosis), or a combination of these factors. Because the proximal spine acts as a long lever, the closer the deformity is to the distal end, the more severe the sagittal imbalance produced by each degree of kyphosis. The imbalance is most severe due to flexion contracture of the hip joint. In contrast, the downward slope of vision does not depend on the site of kyphosis, so corrective surgery to restore pronation should be performed caudally whenever possible. The lumbar spine is usually the ideal site for corrective surgery to restore pronation. As far as the spine itself is concerned, even if the patient has excessive kyphosis of the thoracic spine or kyphosis of the cervical spine, it is still preferable to perform a lumbar anterior convexity correction. Anterior lumbar correction is generally sufficient to correct the deformity, and only rarely is a cervicothoracic osteotomy required. Thoracic kyphosis correction is not worthwhile because of the stiffness of the thorax, which does not allow for significant improvement, and the risk of nerve damage is too high.
Because of the lack of compensatory cervical motion, overcorrection should be avoided regardless of the level of osteotomy. When the entire deformity is so severe that more than one osteotomy is required, cervical osteotomy should be completed first because correction of flexion deformity at the cervicothoracic junction is much more limited than correction at the low level. After the cervical level correction is completed, the final fine rotational adjustment of the force line is then performed in the lumbar osteotomy. The main goal of the osteotomy is to restore the longitudinal physiological sagittal axis of the spine.
(iii) Lumbar osteotomy
Smith-Peterson et al. first reported the use of posterior osteotomy in 1945 to correct fixed flexion deformity of the spine by osteotomy in a V-shape at one or more levels, followed by forceful hyperextension to bring the osteotomy surfaces together. The osteotomy site was chosen at the level of least ossification anterior to the vertebral body on the x-ray. Since then, La Chappelle has reported a two-stage approach to correcting the deformity. In the first stage, a posterior laminectomy was performed under local anesthesia, followed by an anterior wedge osteotomy and fusion two weeks later. Zeilke recommended multisegmental posterior osteotomy and transforaminal fixation, which he considered more conducive to restoring a smooth sagittal physiological curve.
A longitudinal midline incision is made in the lumbar region to expose the lumbar spine. The paravertebral muscle is stripped subperiosteal to the tip of the transverse process. Usually a single horizontal osteotomy is performed between the lumbar 3 and 4 vertebral plates. If the lumbar 2-3 plane is radiographically confirmed to have soft ankylosis, osteotomy in this plane is also possible. The top of the osteotomy angle is positioned posterior to the lumbar 3-4 disc. The osteotomy surface generally requires the amputation of 2.5 to 3 cm of posterior spinal structures and the amputation of the vertebral and posterior spinal structures within the intersection of the two side lines forming the osteotomy angle. As with high tibial osteotomies, the extent of the osteotomy and the angle to be corrected are determined first. A single horizontal osteotomy usually yields a degree of correction of approximately 60o.
During the osteotomy, care is taken to separate the dura from under the vertebral plate. The dura is usually adherent to the ligamentum flavum at the posterior arch attachment and can be easily torn. Usually the tear is not a laceration, but a defect in the dural sac. If direct sutures are not possible, fascia, muscle or fat may be used as a graft or filler. When the osteotomy surface is closed, the osteotomy surface of the vertebral plate must be occluded first to avoid damage to the cauda equina when closing. Likewise the arch should be partially or completely removed to avoid nerve root impingement. When the wedge osteotomy is closed, the apex of the wedge is located in the middle of the anterior longitudinal ligament. The posterior wedge closure shortens the posterior spinal column without elongating the anterior column.
Before the osteotomy is completed, the pedicles above and below the plane of osteotomy should be prepared with screws for internal fixation. Hook fixation may also be used, provided that there are transverse pedicle and lamina claw hooks proximally and distally. Harrington compression devices can also be used, but without fixation claw hooks and lateral attachments; Luque devices have no compression and are more dangerous to thread under the laminae due to more epidural adhesions. In the lumbar spine it is fixed with a pedicle nail, whereas in the thoracic spine it can be fixed with various hooks. Hook fixation should be considered if there is osteoporosis, enlargement of the medullary canal at the pedicle, and cortical atrophy. Sometimes the instrumentation can be extended to the upper thoracic spine in order to prevent progressive worsening of the kyphosis above the internal fixation instrumentation. Finally, autologous iliac bone is taken and implanted at the decorticated lumbar spine for fusion, and if autologous bone is insufficient, allogeneic cancellous bone graft can be used as a supplement to autologous bone graft.
After surgery, the spine is fixed with a thoracolumbosacral orthopedic brace and worn for 6 to 7 months until the spine is fused solidly.
(iv) Thoracic spine osteotomy
Primary thoracic kyphosis can be divided into two categories: one in which the kyphosis still has some tenderness, possibly due to anterior ossification or soft tonicity. The other category includes patients with a primary stiff thoracic kyphosis deformity. Thoracic segment correction deformities should not be routine because of the rigid thoracic structure, limited correction, and the greater risk of spinal cord injury in the plane of osteotomy. The primary indication for thoracic osteotomy is a patient with a progressive increase in stiff thoracic kyphosis with normal anterior lumbar convexity.
In patients with relatively soft primary thoracic kyphosis, it can be corrected by cephalic ring traction. When traction is achieved to a satisfactory spinal force line, posterior fixation and fusion with compression instrumentation is performed. Multi-segmental osteotomies to shorten the posterior spine and increase the degree of correction are feasible if necessary. Due to the large extent of the primary deformity, soft spine, and anterior column defects, posterior surgery alone is not sufficient, and anterior embedded support implants should be performed to strengthen the anterior column.
For primary thoracic stiff kyphosis, head ring traction is almost useless. The first step in the treatment plan for these patients is a multilevel anterior osteotomy of the extensively kyphotic spine, with discectomy and intervertebral implant fusion. Due to extensive posterior ankylosis, anterior instrumented internal fixation is not useful for orthopedic purposes. The aim of the second-stage procedure is a multilevel posterior osteotomy with fixation of the autogenous bone graft. CD instrumentation can orthose and maintain the shape. Because of bone fragility due to ankylosing spondylitis combined with osteoporosis, excessive orthosis will cause fractures at the interface between the built-in and the bone and result in surgical failure.
One level of correction is not advocated for thoracic kyphosis. The thoracic spinal canal is relatively small and the spinal cord is poorly tolerated. Because the transverse cribriform joints are straight and the movement between the osteotomy planes is limited, single-level orthosis is not easily successful, while multi-level osteotomy, with the accumulation of various planes, can result in satisfactory orthosis.