Fractures in Ankylosing Conditions

Zachary A. Child and Richard J. Bransford

Introduction

The seronegative spondyloarthropathies, such as ankylosing spondylitis (AS) and diffuse idiopathic skeletal hyperostosis (DISH), present unique challenges in identifying and managing spine trauma. These challenges are unique with respect to this patient population, which is invariably elderly and subject to increased incidence of medical comorbidities and metabolic and physiological derangements. The structural complexity of the ankylosed spine and biomechanical demands of treatment constructs are major distinguishing characteristics of this cohort, and they pose additional challenges. This chapter seeks to clarify these points to aid spine surgeons in treating these unfortunate patients.

Background

Perhaps owing to a selection/detection bias, there appears to be an increasing incidence of patients with ankylosing conditions.1–6 Historically, there has been an emphasis placed on the distinction between the two dominant conditions—AS and DISH. Many trainees have been tested on the somewhat different radiographic appearances of the two to emphasize the physiological inflammatory basis of AS. More recent studies, however, have illuminated more similarities than differences. Similar fracture behavior and treatment pitfalls have allowed these groups to be combined with respect to trauma. Reactive arthritis, psoriatic arthritis, and colic arthritis (inflammatory bowel disease) can be included in this management chapter to the extent that they present with diffuse ankylosis of the spine.

Ankylosing spondylitis is perhaps the most notable of the seronegative spondyloarthropathies. Its origin is unknown, but it is estimated to have an overall incidence in the population of 0.1 to 1.4%.⁴ Diagnosis considers such factors as limited lumbar motion, decreased chest wall expansion, and persistent low back pain, with radiographic evidence of sacroiliitis and neo-ossification at sites of joint inflammation.⁷ The classic diffuse ossification, the so-called bamboo spine, is a relatively late finding. The mean patient age for diagnosing ankylosing spondylitis is 59.1 years.⁴ A recent study from the Danish Health Registries identified all subjects with a fivefold higher risk of clinical spine fracture and a 35% increased risk of nonvertebral fracture, most often within the first 2.5 years disease.⁴ There is a strong genetic component of the disease with a heritable risk of 9% among siblings and heritability of > 95% in children of afflicted adults. The presence of the human leukocyte antigen (HLA) B27 is classic and widespread in those affected, yet the disease is present in only 2% of HLA-B27–positive individuals.⁶ Twin studies also indicate that there are likely environmental components that have not yet been elucidated.

Diffuse idiopathic skeletal hyperostosis does not share as many of the systemic complications of AS and is likely part of a spectrum of the natural degeneration of the functional spinal unit. It has a linear relationship with age, with the mean age at diagnosis being 68 years. Similar to those for AS, the DISH diagnostic criteria 7 include flowing calcification and ossification along the anterolateral vertebral bodies of at least four contiguous levels, relative preservation of disk height but absence of ossification at the apophyseal joints, bilateral sacroiliac (SI) joints, and lack of joint erosions. Despite its association with obesity and type 2 diabetes mellitus, DISH is thought to be noninflammatory.^6,7

Ankylosing spondylitis has an earlier onset due to the inflammatory nature of the disease and has an earlier mean age at diagnosis.⁴ DISH progresses with age, but has a variable, individual expression. However, both AS and DISH steadily worsen with age, as do the medical comorbidities. The incidence, severity of neurologic injury, and complications associated with treatment all increase with age, making this an especially challenging entity to the treating surgeon. As the ossification of the functional spinal unit progresses, the spine naturally becomes increasingly rigid. This creates an increasingly stiff lever arm, with added stresses placed on the junctional levels. The mechanism of injury across almost all studies in the literature is low-energy trauma such as a ground-level fall, with the majority of patients sustaining extension injuries. This almost certainly reflects the cohort where the majority of injuries occurs, namely the medically morbid elderly. With a similar rationale, the mortality and surgical/postoperative medical complications also increase with age in both conditions.^2,4,9

Identification of injury in these disease entities has presented numerous diagnostic challenges. The first and perhaps most common challenge is identifying the underlying condition. Distinguishing end-plate and osteophyte fractures from subtle three-column injuries is a common difficulty. These minimally displaced injuries may pass unnoticed without an appropriate index of suspicion in the susceptible patient population. Missed injury rates as high as 21% have been reported, along with a high likelihood of neurologic deficit (86%).²

There are unique fracture patterns and complications, notably epidural hematoma specific in this cohort (Figs. 12.1 and 12.2).

Fig. 12.1 Low cervical fracture (arrows) in ankylosing spondylitis (AS) patient. Note the nondisplaced fracture pattern and seemingly benign injury.

Fig. 12.2 Magnetic resonance imaging (MRI) of the patient shown in Fig. 12.1.

The incidence of epidural hematoma is reported to be 5 to 10% in the literature.^4,10–12 Also very common is the presence of a neurologic injury, usually at a cervical or thoracic junctional level (Table 12.1), with obvious implications for the severity of neurologic injury. The American Spinal Injury Association (ASIA) neurologic status on presentation is evenly spread, but studies have shown that the severity of neurologic injury is correlated with rates of overall mortality.^2,4,5 Furthermore, a delay in diagnosis is associated with increased neurologic deficit. In clinical series, there was a reported 81% like lihood of neurologic worsening with a missed diagnosis.² The presence of additional injuries or noncontiguous fractures must be sought, as they are common (10%). Thus, the biggest challenge in the treatment of ankylosing conditions is in prompt identification and management.

Tools, Technique, and Special Considerations

There are many unique challenges in caring for the ankylosed spine patient. Some of these challenges relate to the patient’s preinjury and preoperative health. With a high rate of instability and neurologic injury, many patients present with the added urgency of spinal cord injury. Imaging, perioperative management, operative positioning, surgical approach and technique, and postoperative complications will all be discussed.

Identification

The first step prior to management is identification. As discussed, this can be a unique challenge in the ankylosed spine. Identification of ankylosed segments is in itself a very important step, as it raises the clinical suspicion of fracture and should encourage the clinician to apply greater scrutiny to the imaging obtained. In one study the average time to diagnosis was 2 days (± 2.7; range, 0–12). There is a high missed injury rate (20%) and high rates of neurologic injury, thus raising the stakes of diagnosis.² Close scrutiny of the cervicothoracic and thoracolumbar junctional levels is important (Table 12.1).

Additionally, noncontiguous injuries are common at rates estimated of about 10%.^2,4 Owing to the difficulties of identification on plain radiographs alone, routine use of magnetic resonance imaging (MRI) and mandatory use of computed tomography (CT) for the entire neuraxis are recommended. Seemingly benign fractures can mislead the clinician to dismiss potentially unstable three-column injuries. The presence of end-plate fractures or subtle disk space widening should guide the clinician in obtaining advanced studies in the ankylosed patient. Clinicians not familiar with this cohort or not maintaining an appropriate index of suspicion cannot be relied on to screen these patients. Additionally, the mechanism of injury alone is not a sufficient criterion for obtaining a CT, as low-energy injuries are the most common mode of injury across multiple series. Hyperextension or extension/distraction are the most common fracture patterns seen. A sequela of an old healed fracture or a preexisting kyphosis is common, especially in the AS cohort.