15 Spondyloarthropathies

Spondyloarthropathies (SpA) are a family of chronic, inflammatory rheumatic diseases involving axial or peripheral arthritis. Progression of SpA is often associated with enthesitis, dactylitis, as well as extra-articular symptoms such as uveitis and skin rash. The hallmark of diagnosis is radiographic evidence of sacrolitis.¹ The family of SpA includes ankylosing spondylitis (AS), psoriatic arthritis, juvenile-onset spondyloarthritis, reactive arthritis, SpA associated with inflammatory bowel disease, and undifferentiated SpA. This chapter will focus specifically on axial SpA, or SpA of the spine. The most common presentation of axial SpA is AS.

15.2 Background

15.2.1 Ankylosing Spondylitis

Ankylosing spondylitis is a slowly progressive inflammatory disease that mainly affects the spinal column and sacroiliac joints. As inflammation of the vertebral column progresses, peripheral arthritis, enthesitis, and anterior uveitis also become common. Patients typically present with chronic inflammatory back pain at a mean age of onset of less than 30 years.² Men are affected twice as often as women and the prevalence rate of AS is between 0.1 and 1.4%.^2,3 Incidence is highest among Caucasian patients.⁴

There is a known genetic predisposition toward AS associated with the major histocompatibility complex (MHC) group of molecules—specifically the HLA-B27 allele. Over 90% of AS patients are HLA-B27 +, but only 5% of HLA-B27 + individuals go on to develop AS.^2,5 This discrepancy is not yet fully understood, but might be accounted for by the variety in HLA-B27 subtypes that exist. Certain subtypes such as the most common parent subtype, B*2705, are more closely linked to AS than others.^3,5 Another explanation for the discrepancy in AS susceptibility might be attributable to differences in expression levels. The antigen-presenting cells of patients with AS tend to more heavily express HLA-B27 than the cells of unaffected HLA-B27 + patients.⁵

The exact mechanism by which the HLA-B27 allele confers disease susceptibility is also not yet known. It may be that the allele may uniquely bind and present arthritogenic peptides leading to CD8 + T-cell–directed responses against self-antigens. Alternatively, it may be that HLA-B27 is more likely to fold aberrantly in a manner that has been shown to lead to increased, unnecessary binding and recognition by KIR3DL2 + NK and CD4 + T cells, triggering NK and CD4 + T-cell–mediated autoreactivity.⁵ On the other hand, aberrant protein folding, if it leads to misfolding and improper aggregation, may instead trigger nonspecific inflammatory pathways against protein aggregates.^3,5

Although the mechanism behind what triggers AS is not fully understood, what is known is that widespread inflammation is a hallmark of disease. In fact, in diagnosing AS, pain resulting from inflammatory processes allows one to differentiate between AS and mechanical chronic back pain.^1,6 Together, this extensive inflammation along with increased new bone formation leads to characteristic vertebral column remodeling. Upregulation of inflammatory processes promotes new ectopic bone formation at ligamentous insertion points throughout the axial skeleton. The enthesopathy causes ossification of the ligaments, intervertebral discs, endplates, and apophyseal structures while the ectopic bone formation results in the formation of syndesmophytes through the nucleus pulposus within each intervertebral disc.² In AS, syndesmophytes cause ankylosis across both zygapophysial joints and between vertebral bodies, permanently restricting the patient’s capacity for movement at the spine.⁶ As the inflammation progresses, the cortex and spongiosa of the vertebral bodies themselves are repeatedly destroyed and rebuilt, resulting in the formation of square vertebral bodies.^6,7 Over time, vertebral remodeling and ankylosis lead to spinal deformity in the form of the characteristic hyperkyphotic “bamboo spine.”²

Despite the upregulation of new bone formation, AS is also associated with early osteoporosis due to the uncoupling of bone resorption and formation processes.^2,7 Increased osteoclast activity and syndesmophyte formation are linked to lower bone mineral density, weakening the spine.⁶ Biomechanically, increased spinal fusion and fragility greatly increase susceptibility to vertebral column fractures. Spinal fusion prevents the spine from adequately dissipating the force of traumatic events and impairs mobility. This limited mobility when combined with the peripheral joint arthritis usually seen in AS causes gait unsteadiness which increases susceptibility to falls.² Thus, the rate of spinal cord injuries and fractures is greatly increased in patients with AS. While spinal deformity and trauma-related injuries are the most common conditions requiring surgical intervention in AS patients, spinal stenosis, cauda equina syndrome, and degenerative axial spine pain may also occur in a minority of patients.⁶

15.2.2 Patient Presentation

AS is most commonly diagnosed in the primary care setting. It manifests as sacroiliitis, spondylitis, and enthesitis that present as chronic and progressive inflammatory axial pain, usually during adolescence or early adulthood. Extra-axial manifestations include peripheral arthritis (25–50%), inflammatory bowel disease (26%), and psoriasis (10%).⁸ The mainstays of medical treatment of AS revolve around goals of symptom relief, preservation of functionality and quality of life, and delaying or avoiding disease progression.⁹ Physical therapy and nonsteroidal anti-inflammatory medications are the first and primary treatments for chronic AS. Other more aggressive disease-modifying antirheumatic medications such as sulfasalazine and methotrexate have limited roles in AS, as there are no empirical data that definitively support their use. Tumor necrosis factor and interleukin inhibitors have demonstrated efficacy in several randomized control trials, but carry a significant immunosuppressive risk profile.¹⁰ Thus, the chronic aspects of AS are more typically managed nonsurgically with medical treatments aimed at alleviating symptoms and slowing disease progression.

Presentation for neurosurgical care most commonly occurs in the acute setting following spinal trauma.¹¹ The most common fractures seen are of the midcervical spine or cervicothoracic junction.¹² These fractures are highly unstable and are often displaced due to large bone lever arms created in the spine due to autofusion.¹³ Unfortunately, severe neurological damage is common with these fractures, with up to 75% with neurological compromise in some series.^12,14,15 The risk of fracture progression, even with halothoracic plaster or jacket immobilization is significant given the large bone lever arms. Worsening fracture displacement can also lead to devastating neurological injury.¹⁶ Thus, surgical stabilization is complex and warranted in most AS fractures.

15.3 Surgical Considerations

Surgical management of AS generally carries an increased risk of morbidity given the many unique challenges to performing surgery on patients with AS. Imaging studies can be difficult to obtain and interpret. The osteopenia and kyphosis make it difficult to interpret X-ray evidence.¹¹ Computed tomography (CT) scans and magnetic resonance imaging (MRI) are generally the recommended modality to allow for precise visualization in patients with AS.¹¹ Additionally, lying in the supine position is often intolerable due to pain or deformity. In these cases, the patient should either be positioned in the right decubitus position or a pillow should be used to raise the pelvis relative to the head.¹¹

Intubation also becomes more challenging in patients with AS. Endotracheal intubation may be obstructed in patients where the presence of large anterior cervical osteophytes blocks the larynx or cervical flexion deformities restrict movement of the mouth or neck.¹¹ Imaging studies should be performed to detect the presence or absence of an obstruction prior to intubation. If endotracheal intubation fails, nasotracheal fiberoptic intubation can be used.^11,17,18 Head manipulation during intubation should be done with extreme caution. Given the brittle nature of the spine in AS, manipulation of the neck and thorax during intubation can cause iatrogenic cervicothoracic fractures.¹⁹ Proper positioning in general is complicated by the increased risk for iatrogenic injury.²⁰ AS patients are at risk for complete spinal cord injury and possible death in cases where the extent of cervical kyphosis and overall sagittal alignment is inappropriately assessed.¹¹ To reduce the risk of injury, halo placement and increased traction prior to surgery, as well as extensive thoracic bracing have been shown to improve patient stability in cervical and thoracic fractures, respectively. Adaptations to patient beds such as the use of circle electric beds allows increases to the range of movement possible.¹¹ Head stabilization through a Mayfield head frame reduces inappropriate head movements.¹¹ When used in conjunction with a reverse Trendelenburg position, it may decrease the risk of postoperative blindness by reducing intraocular and ocular venous pressure.¹¹ Finally, an important adjunct for positioning AS patients with potentially unstable spines is neuromonitoring. Confirmation of pre- and postpositioning somatosensory evoked potentials and motor evoked potentials can alert the surgeon to any worsened alignment causing neurological compression during positioning, especially when moving from supine to prone position.

Aside from the challenges posed by patient positioning, patients with AS are also at an increased risk for intraoperative blood loss and epidural hematomas.¹⁹ The spinal deformities can prevent achievement of a free-hanging abdomen.¹¹ The resulting increased pressure placed on the chest leads to increases in peak inspiratory pressure and ventilation issues.¹¹ To compensate for this, padding is sometimes used. This increased abdominal pressure can impact central venous pressure and cause distention of the epidural venous plexus.¹¹

Given the patient’s increased risk of both intraoperative blood loss and iatrogenic fractures, neurological monitoring is exceptionally important throughout surgery.^11,18,19 Corrective osteotomy procedures carry high risk of neurological deficits postsurgery, and local anesthesia is generally the ideal option to allow the patient to continually provide neurological feedback.^11,18 In some cases, however, such as surgeries with extensive soft-tissue exposure and prolonged periods of maintaining the prone position, awake surgeries are infeasible. In these cases, techniques like the wake-up test and continuous neurophysiological monitoring methods should be used in conjunction with general anesthesia.¹⁸ Given the variability in sensitivity of neurophysiological monitoring strategies, it is recommended 2that multiple methods be used together. These methods include spinal cord evoked potentials, somatosensory cortical evoked potentials, spinal somatosensory evoked potentials, and muscle motor evoked potentials.^11,18

15.4 Surgical Tools and Techniques

The surgical goals in treatment of AS patients with acute injury are for stabilization of fractures and correction of deformity. Standard surgical techniques aimed at improving outcomes in patients with AS include external fixation with halo immobilization and bracing, internal fixation with spinal instrumentation, and anterior release. In patients in whom only halo immobilization is utilized, the risk of delayed subluxation is greatly increased. Supplementation of halo immobilization with internal fixation significantly reduces this risk. Thus, the mainstay of AS treatment is spinal instrumentation involving lateral mass and pedicle screw and rod constructs for posterior cervical fixation, and long-segment thoracic fusion with pedicle screw and rod constructs for posterior thoracic fixation. This is supplemented with typical arthrodesis techniques using allograft or autograft and intervertebral spacing devices such as polyetheretherketone (PEEK) or titanium cages when appropriate. Optimal internal fixation in this way may obviate the need for postoperative halo immobilization entirely with cervical fractures. However, because of poor bone quality, it may be difficult to identify normal bony landmarks for screw entry points and trajectories, and thus the use of shorter-length screws and CT image guidance is recommended. Depending on the severity of fracture and deformity, different osteotomy techniques may need to be employed to achieve optimal alignment and fracture reduction. These include, but are not limited to Smith–Peterson osteotomy (SPO), pedicle subtraction osteotomy (PSO), or vertebrectomy. Other techniques to prevent acute subluxation include the use of sublaminar wires,¹⁸ or utilization of a provisional rod threaded through posterior cervical and thoracic screws to prevent acute translation during extension.¹⁸ These posterior fixation techniques may be combined with an anterior approach to help achieve the goals of deformity correction and improve fusion fixation with the addition of an anterior cervical discectomy and fusion (ACDF) implant. Aside from spinal instrumentation, several authors have recommend performing an initial anterior release when possible before posterior extension osteotomy and/or fusion.²¹ An initial anterior release allows the surgeon to determine the level of the anterior wedge osteotomy and permits controlled correction with neck extension, reducing the risk of fracture at a random, undesired level.^18,21 That being said, given the propensity of ankylosed bone toward fracture even without excess manual force, it is not clear that an initial anterior release would provide significant clinical advantage. Additionally, in situations where performing an initial anterior release would be technically challenging such as with severe cervical kyphosis, the costs of the procedure may outweigh the benefits.¹⁸

With the many unique challenges of operating on a patient with AS, certain surgical tools and techniques have been developed or implemented to improve surgical outcomes in this patient population. More recently, research has focused on the use of algorithms and computational methods to guide operations. A study conducted by Kanter et al studied the impact of a clinical problem-solving algorithm designed to assist with surgical management decisions around injury and deformity.²² The algorithm takes into account fracture site and pattern, degree of dislocation, facet pathology, cord compression, neurological deficits, deformity, and surgical urgency. Imaging with plain films, CT, and MRI are also conducted. With the use of the algorithm, 92% of patients achieved postoperative stability or improvement with a 38% complication rate.²² Along the same lines, Pigge et al studied the effectiveness of biomechanical and mathematical preoperative planning for patients with thoracolumbar kyphotic deformity.²³ The authors utilized the ASKyphoplan computational program to perform preoperative surgical planning. The planning procedure was aimed at predicting postoperative balance and view angle.²³ The computational planning improved understanding of the biomechanical and clinical effects of lumbar spine correction osteotomy and was associated with clinical improvements in balance and view angle in all patients.²³ However, the methodology was unable to precisely predict clinical results. Achieved correction angles were between −8 and 7 degrees of the planned correction angles.²³

While algorithms and modeling methods have shown to be useful for the preoperative stage of surgery, MIS techniques have been shown to improve outcomes in spinal surgery for fractures in patients with AS. Use of MIS is thought to reduce blood loss, physiological stress, and perioperative morbidity.¹⁹ MIS techniques, as described by Nayak et al, involve percutaneous posterior instrumentation three levels above and three levels below the fracture.¹⁹ Screw placement and positioning postsurgery can be guided by CT imaging. CT image guidance is recommended over intraoperative X-ray as the osteoporosis and joint fusion seen in AS complicate the interpretation of X-rays.^11,19 In patients with AS, CT image guidance can be used to bring greater accuracy to screw trajectories in order to counteract the impact of poor bone quality on the risk of screw loosening and hardware migration. To ensure a good screw–rod interface, it is recommended to pre-bend the rods to the exact contour of the screw extensions about the skin.¹⁹

As a standard approach to strategies for surgical stabilization of fractures and correction of significant deformity associated with AS, we maintain several specific surgical principles. First, given the highly unstable nature of AS fractures ( Fig. 15.1) and the difficulty in positioning AS patients, great care and consideration is taken in positioning these patients. Preservation of pre- and postpositioning neuromonitoring potentials is essential. Patient alignment must also be ideal prior to beginning the case as there is very little other laxity in the spine after surgical stabilization is completed and patient will be effectively fused in situ to that specific alignment. Next, given the fusion of the rest of the spine superior and inferior to the fracture or targeted area of correction in an AS patient, we have to consider the significant biomechanical forces to be exerted in a ”lever arm” fashion around this target level. Aggressive fusion of multiple levels (often at least three levels above and below) surrounding this target level will help redistribute the tension of this “lever arm.” Also, when possible, 360-degree fusion with a combined anterior–posterior approach can also strengthen the stabilization construct significantly against this ”lever-arm.” Finally, as described above, the surface bony anatomy is often severely distorted making discerning normal entry points and determining screw trajectories challenging, even for the most experienced spine surgeons ( Fig. 15.1). Utilization of intraoperative CT guidance for navigation of screw placement and for delineating osteotomy boundaries has been extremely useful. Confirmation of the construct intraoperatively can also be helpful for correct suboptimal screw placements or alignment prior to closure.

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