ANKYLOSING SPONDYLITIS: WHAT REMAINS OF THE STANDARD RADIO- GRAPHY ANNO 2004?

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1 ANKYLOSING SPONDYLITIS: WHAT REMAINS OF THE STANDARD RADIO- GRAPHY ANNO 2004? V. Lambrecht, F.M. Vanhoenacker, P. Van Dyck, J. Gielen, P.M. Parizel 1 In this review, the radiographic features of ankylosing spondylitis of the axial skeleton will be discussed shortly. Three pathologic processes, including inflammation, bony repair and ossification occurring consecutively or simultaneously, will contribute to the radiographic picture of ankylosing spondylitis. Typical target sites at which these processes take place are the synovial joints, discovertebral joints and ligamentous attachments or entheses of the axial skeleton. Key-words: Spondylitis Spine, radiography. Ankylosing spondylitis (AS) is part of a spectrum of overlapping disorders characterized by arthropathy, enthesopathy, certain extraskeletal manifestations, familial occurrence, and the expression of HLA-B27. The cardinal feature of the disease, from both the clinical and radiographic point of view, is articular pathology. Characteristically, the axial skeleton is involved earlier and more severely than the peripheral skeleton. After a brief discussion of the epidemiology, the basic radio-pathological correlation and clinical signs of AS, we will describe the typical radiographic features of the disease as it manifests in the axial skeleton. Definition and epidemiology AS is a chronic, idiopathic, inflammatory arthritis with a predilection for the axial skeleton. The prevalence of disease has been estimated to be as high as 0.2 to 1.6% (1). The disease is most common in young men, with age of onset usually in the range of 20 to 40 years. Males are more commonly affected than women and the male/female ratio is estimated about 4 to 1 (2). Moreover, radiographic changes of AS are more pronounced in men and there has been concern that early AS is underreported in women, as the disease may be more subtle and difficult to diagnose in woman (2). Although the mode of inheritance remains unknown, there is clearly a genetic component to the disease, with a strong association between AS and the HLA-B27 allele. This histocompatibility antigen is expressed in up to 96% of patients with AS, although the exact role of HLA-B27 in the pathogenesis of AS remains unknown (1). Approximately 10% of HLA-B27 positive individuals will ultimately develop the disorder (3). Because the overall prevalence of HLA-B27 expression (9% of the general population) outnumbers largely the amount of affected patients, numerous exogenous factors, including infectious etiologies, have been incriminated as inciting or exacerbating factors. Clinical features Classically, AS presents as an insidious onset of low back pain persisting for more than 3 months and associated with morning stiffness. Pain or tenderness over the gluteal or sacroiliac regions can be prominent in the early phases of the disease. Other possible early clinical signs include chest pain and later mild to moderate reduction in chest expansion, owing to costovertebral and costosternal involvement. From: 1. Department of Radiology, UZ Antwerpen, Edegem, Belgium Address for correspondence: Dr F.M. Vanhoenacker, M.D., Dept. of Radiology, UZ Antwerpen, Wilrijkstraat, 10, B-2650 Edegem. Peripheral articular manifestations, most commonly synovitis of hips or shoulders, are eventually seen in as many as 50% of patients, but are mild and overshadowed by more prominent manifestations in the central skeleton (4). Painful tendinopathy occurs in about 10% of patients (3). The complete picture of AS leads to a painful stiffening of the entire spine, severe thoracic kyphosis and a rigid thorax. Associated extra-articular manifestations include acute anterior uveitis (25-30%), cardiovascular involvement, especially aortitis (10%), inflammatory bowel disease, amyloidosis and rarely pulmonary fibrosis (4). Basic radio-pathologic correlation Three pathologic processes, namely inflammation, bony repair and ossification are the basis of the radiopathological picture of AS and may occur either consecutively or simultaneously in the individual patient (Fig. 1 and table I). Different target sites, including the entheses and the synovial as well as the cartilaginous joints, will be involved (Table II). Each of these basic histopathological processes are reflected by non-specific radiographic changes, occuring at the different target sites (Table I). Generally, the inflammatory destruction presents on standard radiography as erosions, whereas subchondral sclerosis and ligamentous ossification are the radiological counterparts of reactive bone formation, leading to ankylosis due to osseous bridging of the articulations.

Basic histopathological process Inflammation Bony repair Progressive ossification Radiographic counterpart Erosions, joint widening/ dilatation, erosive fibroosteitis Sclerosis, productive

2 26 JBR BTR, 2005, 88 (1) Fig. 1. Basic histopathological processes occurring consecutively or simultaneously. Table 1. Basic histopathological features occurring in AS and their radiological counterparts. Basic histopathological process Inflammation Bony repair Progressive ossification Radiographic counterpart Erosions, joint widening/ dilatation, erosive fibroosteitis Sclerosis, productive fibroosteitis Ankylosis Fig. 2. Schematic drawing of inflammatory changes at the sacroiliac joint. Blurring of the articular contours and hazy structure of the subchondral bone at the right sacroiliac joint, resulting in pseudodilatation of the joint. On the left side typical erosions resembling the perforated edge of a postage stamp (1), a saw blade (2), and a rosary (3). Table 2. Target sites for AS involvement of the axial skeleton. Synovial joints Cartilaginous joints Entheses Sacroiliac joints Discovertebral joints Spine: posterior and interspinous ligamentous attachments, Sharpey s fibers anteriorly Facet joints Symphysis pubis Pelvis Costovertebral joints Manubriosternal joint Shoulder Atlanto-axial joint Shoulder, hip The different locations of the disease have quite characteristic radiographic features. The resulting radiographic picture will depend on the ongoing histopathological processes, either alone or in combination. Concomitant destruction, subchondral sclerosis, and osseous ankylosis may exist in any combination and lead to the typical variegated picture of the disease. In the next paragraph, the typical radiographic features of AS in each of the different target sites of the axial skeleton will be discussed in detail. Sacroiliac joints and pelvis Sacroiliitis is the hallmark of ankylosing spondylitis. Previous studies have shown that the earliest radiographic sign in 99% of cases of AS is sacroiliitis (2, 5, 6). Although sacroiliitis alone is a non-specific finding and is not sufficient for the diagnosis of AS, changes in the sacroiliac joints are considered ubiquitous among patients with AS (1, 2). Both the synovial and ligamentous (superior and posterior) portions are involved. Classically, the involvement is symmetric and bilateral (6), but - particularly very early in the course of the disease - unilateral disease may occur. The above mentioned basic radiopathological processes will contribute to the ultimate radiographic picture of sacroiliac joint involvement (Table I). Among the signs of sacroiliac inflammation (Fig. 2) are blurring of the subchondral cortex with illdefined articular contours and hazy structure of the subchondral cancellous trabeculae, resulting in pseudodilatation of the joint (5). This finding is followed by the development of small, succinct erosions, giving the joint margin the appearance of the perforated edge of a postage stamp or a saw blade. Small erosions lined up one behind another at corresponding sites of the ilium and the sacrum resemble a string of beads or rosary. The erosions usually develop earlier on the iliac than on the sacral side of the joint, possibly due to the fact that the cartilage covering the sacrum is approximately twice as thick as that covering the ilium (1 to 4 mm vs. 0.5 to 2 mm) (1). Reactive bone proliferation (Fig. 3) is seen radiographically as

PROCEEDINGS OF THE SRBR-KBVR OSTEOARTICULAR SECTION MARCH MEETING 27 Fig. 3. A. Schematic drawing of the variable appearance of reactive bone proliferation.

3 PROCEEDINGS OF THE SRBR-KBVR OSTEOARTICULAR SECTION MARCH MEETING 27 Fig. 3. A. Schematic drawing of the variable appearance of reactive bone proliferation. Different types of subchondral sclerosis on the ilial and sacral side of the right sacroiliac joint: bandlike (1), triangular (2) and spotty (3). At the left joint different types of ossification according to the location towards the linea terminalis. Above the linea terminalis a striated appearance (4) A is seen, resulting in a star sign (5) at the upper part of the joint. Hook-like bony projections (6) below the linea terminalis, at the inferior part of the joint. Bony bridges (7) across the joint, resulting in complete ankylosis of the joint, sometimes appearing as a phantom joint (8). B. Standard radiograph of the sacroiliac joints, demonstrating bilateral ankylosis. Note the phantom sign at the inferior part of the joint (white arrow). Ankylosis of the facet joints gives rise to the so called trolley-track sign (arrowheads). Ossification of the interspinous ligaments can be seen as the dagger sign (arrowhead). Bilateral star sign (black arrows) due to ossification of capsuloligamentous structures at the superior part of the sacroiliacal joint. B sclerosis of the adjacent cancellous bone with variable pattern (diffuse, band-shaped, spotty, triangular). Ossifications of the joint capsule and its reinforcing ligaments present themselves as bony hooks, below the linea terminalis and above this line as a striated structure, forming the so-called starsign at the superior border of the joint. In addition to the destruction of cartilage with narrowing of the joint space, slim bony bridges connecting the ilium with the sacrum enlarge and become fused with one another, resulting in complete bony ankylosis. Radiographically, parts of the articular contour that remain visible following the advent of ankylosis, are described as phantom joint. The simultaneous triad of sacroiliac destruction, sclerosis and discrete ankylosis reflects a mode of reaction that is characteristic of AS and is described on plain radiography as the variegated sacroiliac picture (Fig. 4) (5). Early sacroiliac changes can be occult on plain radiography with plain radiographic findings of sacroiliitis lagging behind the clinical findings by several years (2). Because an early diagnosis of AS allows earlier institution of appropriate treatment, other imaging techniques, namely, CT and MRI, have been proposed for the evaluation of the sacroiliac joints (1). Other changes in the pelvis may be associated with the changes in the sacroiliac joints. There may be ossification and/or erosion of the ligamentous attachments to the iliac crest and ischial tuberosities, resulting in the so-called whiskered appearance. The pubic symphysis may be involved in 23% of patients with AS (1), with small succinct erosions and adjacent sclerosis, followed by total ankylosis (Fig. 5). The spine Abnormalities of the spine can be seen in the discovertebral junctions, apophyseal joints, costovertebral joints, posterior ligamentous attachments, and atlantoaxial joints. Classically, changes initially are noted at the thoracolumbar and lumbosacral junctions. Spinal extension to the midlumbar, upper thoracic and cervical vertebrae occurs with disease chronicity, but may be arrested at any stage (4). During the course of AS, the spine may present destructive changes as well as features due to bone proliferation and ankylosis. There is a spectrum of inflammatory and destructive lesions of the spine in AS that predominantly involves the cartilaginous discovertebral junction. One of the earliest plain film findings of spinal involvement, typically seen at vertebrae T10 through L2, is the so-called Romanus lesion, also known as spondylitis anterior (Fig. 6B). Inflammation (active enthesitis and fibroosteitis) at the attachments of the outer fibers of the annulus fibrosus and longitudinal ligaments at the vertebral body, causes small erosions at the anterosuperior and anteroinferior vertebral body corners. Less frequently, this defect can be seen at the posterior vertebral body corners. As the erosions heal, reactive sclerosis produces a shiny corner configuration. Bony proliferation in the connective tissue between the anterior longitudinal ligament and the anterior surface of the vertebral body, fills the concavity of the anterior vertebral surface. In combination with osseous erosion of the vertebral body rim, this results in a flattened or even convex anterior surface, creating a squared or barrel-shaped contour (Fig. 6A). The squaring phenomenon is best identified on a lateral view of the lumbar spine because a straightened anterior contour also occurs as a normal variant in the cervical and thoracic spine (5).

28 JBR BTR, 2005, 88 (1) B A Further along the spectrum of destructive discovertebral lesions are the Andersson lesions. Two types have been described.

disc material through the vertebral endplate.

4 28 JBR BTR, 2005, 88 (1) B A Further along the spectrum of destructive discovertebral lesions are the Andersson lesions. Two types have been described. Andersson lesion, type A is an inflammatory reaction with focal destruction of the intervertebral disc and the adjacent vertebral endplates, characterized histologically by herniation or invasion of disc material through the vertebral endplate. Plain film radiographs characteristically show disc space narrowing (usually), a circumscribed defect in the neighboring vertebral bodies or irregularities along the endplates, and a wide area of reactive sclerosis in the surrounding cancellous bone (1). Differentiation from mechanically induced erosive osteochondrosis, which has predilection for L4-S1, is sometimes difficult (3). The inflammatory type develops primarily during the first 9 years of the disease (5) and may be asymptomatic or cause acute, focal back pain that, in contradiction to the typical pattern of AS pain, worsens with movement (1). The noninflammatory type B usually develops later in the evolution of the disease, after ankylosis has occurred. The ankylosed spine is vulnerable to fracture. The type B Andersson lesion is thought to be a pseudarthrosis following a discovertebral fracture of the demineralized and extensively ossified, stiff axial skeleton (3). It is usually located at the thoracolumbar or cervicothoracic junction, around an area of skipped ossification which has become the single point of motion in the entire spine and therefore undergoes degeneration with extensive, irregular destruction of the vertebral endplates extending into the subchondral bone of two neighboring vertebral bodies. Alternatively, it may develop around an area of true fracture through the ankylosed apophyseal joints. On plain film radiographs, there is destruction of the entire discovertebral junction with a normal or widened disc space Fig. 4. Variegated sacroiliac picture. A. Schematic drawing. B. AP radiograph of bilateral variegated sacroiliitis. Erosive and hazy subchondral cortex at the right side (little arrow), subchondral sclerosis (arrowhead) and beginning ankylosis with bony bridges (arrow) at the left side. Fig. 5. Standard AP radiograph of the pelvis. The symphysis pubis demonstrating erosions and adjacent sclerosis. Note also ankylosis of the lumbar spine and the sacroiliac joints. A combination of erosive changes, cartilage loss, subchondral sclerosis and marginal osteophytosis are seen at the right hip joint. Irregular delineated bony proliferation at both ischial tuberosities, known as whiskering (arrow). and reactive sclerosis in the adjacent vertebral bodies (1). The radiographic findings may mimic infectious spondylodiscitis or neuropathic spine (7). The frequency of Andersson lesions in AS patients is estimated between % (1, 7). The erosive vertebral abnormalities are associated with bone formation. Ossification first takes place in the outer portion of the annulus fibrosus, where Sharpey s fibers attach to the vertebral body. This

PROCEEDINGS OF THE SRBR-KBVR OSTEOARTICULAR SECTION MARCH MEETING 29 A

The evolution from a normal vertebral body to squared or barrel-shaped appearance is illustrated (see text for further comment). B.

Some are completely sclerotic (shiny corner) (arrow), others consist of a small erosion with surrounding sclerosis (small arrow).

Fig. 7. Late stage AS. A. AP-view of the lumbar spine in two different patients. Multiple syndesmophytes bridging adjacent vertebral bodies.

Lateral view of the lumbar spine in a third patient. Severe osteoporosis of the vertebral bodies with discal calcifications in a completely ankylosed bamboo spine.

5 PROCEEDINGS OF THE SRBR-KBVR OSTEOARTICULAR SECTION MARCH MEETING 29 A Fig. 6. A. Schematic drawing of the inflammatory process and reactive sclerosis at the anteroinferior and anterosuperior corner of the vertebral bodies. The evolution from a normal vertebral body to squared or barrel-shaped appearance is illustrated (see text for further comment). B. Lateral radiograph of the lumbar spine demonstrating several Romanus lesions at the anterior corners. Some are completely sclerotic (shiny corner) (arrow), others consist of a small erosion with surrounding sclerosis (small arrow). At the level L3 and L4 there is extension of the bony production along the anterior margin of the vertebra resulting in a barrel shaped-appearance (arrowhead). B A Fig. 7. Late stage AS. A. AP-view of the lumbar spine in two different patients. Multiple syndesmophytes bridging adjacent vertebral bodies. Note also ankylosis of the facet joints and ossification of the interspinous ligaments, resulting in a trolleytrack (combining a central dagger sign and a peripheral tram-like track). B. Lateral view of the lumbar spine in a third patient. Severe osteoporosis of the vertebral bodies with discal calcifications in a completely ankylosed bamboo spine. B ossification will extend from Sharpey s fibers along the deep layers of the longitudinal ligaments, forming initially thin, vertical outgrowths along the contour of the disc. These syndesmophytes ossify one vertebral body to the adjacent vertebral body in a succinct fashion (Fig. 7A). The disc spaces are generally preserved. With disease progression, the syndesmophytes thicken and involve the anterior longitudinal ligament and paravertebral soft tissues. The end result is the socalled bamboo spine (Fig. 7B) (3). Syndesmophytes must be differentiated from vertebral osteophytes which are (sub) marginal ossifications caused by disk degeneration and grow horizontally. Disuse osteoporosis of the spine may result from immobilization due to ankylosis. Decreased nutrition of the intervertebral disk may promote premature disk calcification in late stage disease. Furthermore, due to osteoporotic deformity of the vertebral body, the intervertebral disc appears biconvex, better known as discal ballooning (Fig. 7B). Other target locations in the spine are summarised in table II and will be briefly reviewed. The apophyseal joints may demonstrate erosions due to inflammation and reactive bone formation with subchondral sclerosis and ankylosis. Narrowing and osseous fusion of these joints can be apparent on standard radiography. Especially in the cervical spine, apophyseal joint ankylosis can be very striking (Fig. 8). On the other hand, the triad of erosion, sclerosis and ankylosis is more difficult to demonstrate on standard radiographs at the level of the costovertebral joints, because of superposition of adjacent structures. Like in rheumatoid arthritis, inflammatory changes of the synovial and adjacent ligamentous structures in the atlantoaxial articulations can lead to erosions of the dens (Fig. 8) and even, although much less frequently, to atlantoaxial subluxation. Enthesitis of the posterior ligamentous attachments of the spine leads to subligamentous erosion

6 30 JBR BTR, 2005, 88 (1) spine, with less frequent involvement of other joints, including the pubic symphysis, hips, shoulders and knees. The manifestations of AS in the axial skeleton are numerous, but the overall radiographic hallmark of the disease is concomitant destruction, subchondral sclerosis, and osseous ankylosis leading to the typical variegated picture of the disease. Whereas newer techniques, such as MRI, will certainly have a role in the early diagnosis of AS, standard radiography is still the mainstay in the overall evaluation of the different manifestations of AS in the axial skeleton. References Fig. 8. Lateral spot view of the upper cervical spine. Severe osteoporosis due to longstanding ankylosis with very striking ossifications of the facet joints. There are also erosive changes and concommitant sclerosis at the dens axis, resulting in an increased atlanto-dental distance. and ossification. On frontal radiographs ossification of the supraspinous and interspinous ligaments can be seen as a single central radiodense line, known as the dagger sign (Fig. 3B). Ossification of the apophyseal joint capsules forming two vertical radiodense lines lateral to this central line is apparent as the trolley- or tramtrack sign on frontal view (6). Other joints Outside of the axial skeleton and the pelvis, shoulders and knees are the most commonly involved joints, but virtually every joint can be affected. The calcaneus is the most common site of extra-axial enthesopathy. Radiographically, these enthesopathies appear as whisker- and blister-like fibro-osteoses, serrated contour irregularities and erosions. Further discussion of the peripheral manifestations of AS is however beyond the scope of this brief review. Conclusion The main sites of involvement in AS are the sacroiliac joints and 1. Vinson E.N., Major N.M.: MR Imaging of Ankylosing Spondylitis. Semin Musculoskelet Radiol., 2003, 7: El-Khoury G.Y., Kathol M.H., Brandser E.A.: Seronegative spondyloarthropathies. Radiol Clin North Am, 1996, 34: Haller J. Hofmann J.: Ankylosing spondylitis. In: Musculoskeletal Imaging. A concisce multimodality approach. Edited by Bohndorf K. Georg Thieme Verlag, Stuttgart, 2001, pp Resnick D.: Ankylosing Spondylitis. In: Diagnosis of Bone and Joint disorders, 4 edition. Edited by Resnick D., WB Saunders Company, Philadelphia, 2002, pp Dihlman W.: Joints and Vertebral Connections. Georg Thieme verlag, Stuttgart, Bennett D.L., Ohashi K., El- Khoury G.Y.: Spondyloarthropathies: Ankylosing Spondylitis and Psoriatic Arthritis, Radiol Clin North Am, 2004, 42: Brower A.C.: Ankylosing Spondylitis. In: Arthritis in black and white. Edited by Brower A., WB Saunders Company, Philadelphia, 1988, pp

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Ankylosing spondylitis: A Pictorial Review

Ankylosing spondylitis: A Pictorial Review Poster No.: P-0009 Congress: ESSR 2012 Type: Scientific Exhibit Authors: J. Acosta Batlle, B. Palomino Aguado, M. D. Lopez Parra, S. 1 2 3 2 4 1 2 Hernandez Muñiz,