Ankylosing spondylitis is a chronic, progressive inflammatory disease that affects the medial bones, including the spine, sacroiliac joints, and hip joints, causing local pain and progressive joint stiffness, and eventually leading to bony ankylosis and deformity of the joints.
The pathology of AS itself makes AS patients prone to spinal fractures, and when fractures occur, they often lead to serious consequences.
Spinal fractures associated with AS have their own characteristics in terms of mechanism of injury p incidence p prevalence p location p imaging features diagnosis treatment and comorbidities, which are different from the general spinal fractures. Therefore, misdiagnosis p underdiagnosis or improper management are frequently reported.
I. Mechanisms of spinal fractures in AS
Normal intervertebral discs and ligaments are flexible, making it possible for the spine to move in all directions and to slow down shock forces. After ossification of the intervertebral discs and ligaments in AS patients, the elasticity and mobility of the discs and ligaments are significantly reduced, and the texture of these ossified ligaments is often brittle, which can easily cause rupture of the ossified ligaments when the spine is stretched backward with force. In addition, AS often causes osteoporosis of the vertebral body, which significantly weakens the ability of the vertebral body to resist compression and tension. These changes increase the risk of spinal fracture in AS patients, and minor trauma or chronic strain can cause transverse fractures of the vertebral body or rupture of the ossified discs and ligaments.
Because the anterior longitudinal ligament, p intervertebral disc, p posterior longitudinal ligament, p interspinous ligament, and joint capsule ligament of the spine can ossify, causing the ankylosed spine to move toward a long bone, a fracture in a patient with AS often involves the anterior, middle, and posterior columns of the spine at the same time, making the fracture very unstable. Also, because AS
is often accompanied by posterior derangement, which increases the leverage force of the spine, and the strong leverage force is concentrated on the fracture line, making the fracture easy to cause dislocation and pseudo-joint formation.
II. Characteristics of AS spinal fractures
The incidence of the disease has been reported differently, ranging from 1.5% to 23%. Although the overall incidence is not too high, the incidence of AS with spinal fracture is 3.5 times higher than the normal incidence of spinal fracture.
The incidence is 3.5 times higher than normal.
The disease is most likely to occur in the lower cervical spine and cervicothoracic junction, which may be related to the anatomical features of this area. The lower cervical p cervicothoracic junction and the thoracolumbar segment are the junction of the relatively fixed thoracic spine and the more mobile cervical p lumbar spine, where stresses are relatively concentrated.
Traumatic injuries resulting in AS spine fractures tend to be mild. The mechanism of injury is mostly hyperextension injury. They are mostly three-column fractures and are prone to subluxation.
AS spine fractures are prone to spinal cord (nerve) injury, especially cervical spine fractures with cervical spinal cord injury. This may be due to the fact that AS combined with spinal fractures mostly involves the anterior, middle, and posterior columns, so the fractures are often relatively unstable, and the strong leverage of the ossified spine, which resembles a long bone, makes the fracture prone to dislocation. In addition, patients with ankylosing spondylitis have osteoporotic vertebrae, and more cancellous bone bleeds after fracture, which can easily complicate epidural hematoma and may also contribute to their vulnerability to nerve injury after fracture. Compared to the cervical spine, the incidence of thoracolumbar spine fractures with nerve injury is less in AS.
The mortality rate of AS spinal fractures is high. The main causes of death are pulmonary failure and cerebrovascular accidents.
Most AS spinal fractures are transverse to the intervertebral space, which is the weakest point of the ankylosing spine.
Imaging of AS spine fractures
AS spine fractures can be imaged in three categories: fresh fractures and/or dislocations, also known as shear fractures; pseudarthrosis or stress fractures; and vertebral compression fractures.
1. Fresh fractures and their radiographic features
These fractures occur mostly in the cervical spine, often with a clear history of minor trauma, and the mechanism of injury is mostly hyperextension. After the injury, there are symptoms such as local pain and limitation of movement, and most of them are associated with nerve (spinal cord) injury.
Fresh fractures can be seen as fracture lines on radiographs, and the fracture usually involves all three columns. When fractures of the anterior and middle columns are transverse to the vertebral body, transverse or oblique fracture lines are seen on the vertebral body. When the fracture passes through the intervertebral space, the fracture line is often difficult to see within the intervertebral disc, but an ossified anterior longitudinal ligament rupture may be seen. Posterior column fractures often present as fractures of the vertebral plates in the same segment, fractures of the articular processes in the same or adjacent segments, and also as interrupted continuity of the interspinous ligaments with ossification and fusion. Due to altered biomechanical properties of the spine, fractures are often associated with displacement, including anterior separation of the intervertebral space and posterior angular displacement, and in severe cases, dislocation or subluxation, although the degree of displacement and dislocation is often mild.
2. Stress fractures (or pseudarthrosis) and their radiographic manifestations
This type of lesion often occurs in the thoracic and lumbar spine, especially in the thoracolumbar segment. There is often no clear history of trauma. It is usually found incidentally during the radiographs for low back pain or other diseases. Nerve injuries are rare and, if present, are often mild.
The characteristic change on radiographs of stress fractures is the presence of a destructive disc-body lesion at the disc in the fracture plane, which is characterized by extensive subchondral bone destruction on the endplates of the two adjacent vertebral bodies, with irregular margins and surrounding osteochondral sclerosis. The intervertebral disc space may be irregularly widened.
IV. Diagnosis of AS spine fractures
Although AS spine fracture is not a difficult case. However, because the trauma causing the fracture is often mild, or even without obvious trauma, it is easy to miss or misdiagnose the fracture, especially when the patient does not have symptoms of paraplegia after the injury. Once the diagnosis is missed or misdiagnosed, it can often lead to serious consequences. There have been reports of pseudarthrosis being misdiagnosed as vertebral tuberculosis and vertebral tumor, and there are also reports of lower cervical fractures being misdiagnosed as clavicle fractures.
The main reasons for delayed diagnosis are the following: ( 1 ) the violence causing the fracture is often small, mostly falls during walking, and some have no obvious history of trauma, which does not attract the attention of patients and physicians; ( 2 ) the long-term pain of ankylosing spondylitis increases the patient’s tolerance for pain, and the pain caused by the fracture is either masked by the pain of AS itself, or mistaken by patients and physicians as ankylosing spondylitis “( 3 ) These fractures tend to occur in the lower cervical spine and at the cervicothoracic junction, and the obstruction of the shoulder often makes it difficult to detect fractures here in time on plain radiographs; ( 4 ) Calcification of the ligaments and osteoporosis in the ankylosing spine itself sometimes make the fracture line difficult to identify. ( 5 ) Ischemic necrosis at the fracture end and traumatic osteosclerosis are easily misdiagnosed as chronic bacterial inflammation of the vertebral body; ( 6 ) Some physicians lack sufficient knowledge of the fracture.
Therefore, the possibility of an accompanying spinal fracture should be highly suspected in a patient with a history of AS who has newly developed or worsened neck pain p thoracic back pain or low back pain that is not significantly relieved by bed rest and a history of mild or severe (often milder) trauma. The possibility of a spinal fracture should also be thought of in the above cases without a clear history of trauma. Once a spinal fracture is clinically suspected, a full frontal p-lateral spine film should be routinely taken on the basis of careful clinical examination to clarify the presence and location of the fracture and to avoid missing less symptomatic or asymptomatic fractures. If there is a high clinical suspicion of fracture and plain X-rays fail to provide evidence of fracture, additional tomography or CT (including 3D reconstruction) should be performed. If the tomogram and CT remain normal and the fracture cannot be completely excluded clinically, a bone scan may be useful to clarify or exclude the diagnosis. MRI is also a valuable adjunct in patients with concomitant nerve injury, or in cases of pseudarthrosis, and in those with clinical suspicion of an epidural hematoma.
V. Treatment of spinal fractures in AS
Since most spinal fractures complicated by ankylosing spondylitis are unstable, once the diagnosis is established, close attention should be paid to the patient’s position and reliable fixation of the fracture site during transport and delivery, otherwise it may lead to serious consequences. Unlike general spinal fractures, the direction of preoperative traction for ankylosing spondylitis spinal fractures should be consistent with the direction of the original deformity, and the weight should not be excessive.
Regarding the treatment of spinal fractures associated with ankylosing spondylitis, the early literature mostly recommended conservative treatment. The main reason for this is the belief that ankylosing spondylitis is a systemic disease, often associated with damage to other organs, especially the respiratory organs, and that surgery is associated with many complications, high surgical risk, and high mortality. However, in recent years, as anesthesia techniques and surgical skills have improved, more and more scholars have advocated surgical treatment. These authors believe that surgical treatment can better stabilize the spine than conservative treatment and also more directly relieve the compression of nerves, which can more effectively avoid complications caused by long-term traction and external fixation. Therefore, for patients with ankylosing spondylitis combined with spinal fractures, surgical treatment is feasible as long as the patient is physically able to do so.
The main goal of surgical treatment for this disease is also decompression and fusion stabilization. The choice of decompression is not very different from that of a typical spinal fracture, depending on where the compression is coming from, but most scholars believe that simple laminectomy decompression is unacceptable for patients with ankylosing spondylitis spinal fractures.
Because of the poor stability of the three columns involved, ankylosing spondylitis spine fractures require more fusion and stability than the usual spine fractures. Most scholars advocate 360-degree fusion in cases of three-column fractures and pseudoarthrosis formation. Internal fixation is also necessary for patients with ankylosing spondylolisthesis spine fractures. In order to reduce the stress on the internal fixation and to avoid loosening of the fixation due to osteoporosis, it has been suggested that for posterior fixation, the fixation should not be limited to the fracture gap, but should extend 1-2 segments up and down.
Because of the relatively low stress on the cervical spine, simple anterior interbody fusion with plate fixation may also be able to stabilize the fracture locally and thus promote fracture healing.
Because ankylosing spondylitis is a systemic disease, the incidence of pulmonary and cardiovascular complications after fracture is higher than that of a typical spinal fracture, and the mortality rate is much higher than that of a typical spinal fracture. In addition, osteoporosis of the vertebral body causes more intraoperative bleeding and ossification of the ligaments makes the normal bone structure illegible, all of which make the procedure more risky. Therefore, care should be taken to avoid such complications.