Key Points

X-ray is relatively inexpensive, easily available, and reliable.

X-ray provides information on bone damage and, indirectly through joint space narrowing, on cartilage damage, whereas x-ray is neither sensitive nor specific for soft tissue change.

X-ray findings are important parts of the classification criteria for many rheumatic diseases, including RA, AS, SpA, PsA, and OA.

X-ray often should be the first imaging investigation in arthritis because the significance of positive and negative findings is well known.

X-ray can be used to follow structural damage progression in inflammatory and degenerative joint diseases but is less sensitive to change than MRI.

The main disadvantage of x-ray is its low sensitivity, particularly for soft tissue changes.

The first roentgen ray image was an “x-ray” of the hand. Since then, imaging of musculoskeletal structures has always been an important role of conventional radiography (x-rays).¹ The simple radiograph is relatively cheap, is available worldwide, and produces images that are almost identical regardless of technical parameters or whether the image is analogue or digital. The reliability of the image means that, despite the limitations of radiography, advances in clinical practice can rely on the good old x-ray, with the knowledge that it is largely independent of technologic advances. For many years to come, radiographs will appear much the same as they do now.

Technical Aspects

The conventional radiograph is a two-dimensional summation image that is dependent on variable absorption of x-rays by different tissues for its inherent contrast. It has a very high spatial resolution that is rarely surpassed by other modalities, but radiography offers high contrast only between a limited number of structures, namely, bone (calcium), soft tissue, and air. Fat is visible as a separate density, but the distinction between soft tissue and fat is often subtle, and radiography cannot distinguish between the other soft tissues because cartilage, muscle, tendon, ligament, synovium, and fluid all appear at the same density. These characteristics give the radiograph its inherent advantages and disadvantages.

In its favor, the x-ray shows skeletal structures very well, and because it is a summation image, it allows excellent overall assessment of skeletal trauma and alignment. The limited number of images produced facilitates rapid review, and the high bone–soft tissue contrast often produces radiographic manifestations of disease in specific patterns that make the test particularly useful in daily clinical practice. The biggest disadvantage of radiography is its inherent lack of soft tissue contrast, which makes it insensitive for the detection of soft tissue abnormalities. The first radiographic sign of inflammatory arthritis may be permanent structural damage (bony erosion); in degenerating joints, cartilage damage is not usually visualized until sufficient overall cartilage loss allows approximation of the bone ends, resulting in “joint space narrowing.”

Although x-rays use ionizing radiation, they should be regarded as relatively safe, especially in older patients. An exception may be spine and sacroiliac joint radiography, for which doses are higher to penetrate the trunk; in a younger population, MRI offers a safer and more informative alternative. In most examinations, two or more projections are required to adequately visualize the joint in question, and radiographic quality is improved by strict adherence to standard imaging protocols.

Rheumatoid Arthritis

Rheumatoid arthritis (RA) is the archetypal inflammatory joint disease that primarily targets synovium and peripheral joints. It is a systemic inflammatory disorder in which the typical clinical manifestations are usually symmetric, and the radiographic signs usually follow this pattern. As in all of radiology, the observed distribution of disease is often characteristic of the underlying cause; in RA, the metacarpophalangeal (MCP) and proximal interphalangeal (PIP) joints of the hands, the wrists, and the metatarsophalangeal (MTP) joints of the feet are most often involved.^1,2

Juxta-articular osteoporosis is a characteristic feature of RA best seen in early disease in small peripheral joints. Later, generalized osteoporosis is usually present and is exacerbated by disuse. Erosion of bone (Figure 58-1) is a characteristic feature of RA and usually appears first at the margins of the joint, where the proliferating synovium lies directly on the surface of the bone between the edge of the cartilage-covered articular surface and the capsular attachment—the “bare areas.” These marginal erosions may be subtle and first appear as disruption of the thin white cortical line, especially at the radial aspect of the metacarpal heads, where they may be seen best on dedicated radiographic projections such as the ball-catcher’s view of the hands. These erosive changes are a good indication of the aggressiveness of the arthritis. In large joints, synovial proliferation may be very severe before bony erosion is detectable on x-ray. This is especially notable in the knee, where pain and swelling due to arthritis and bursitis occur long before erosion. In addition to bare-area erosions, two other types of erosions have been described in RA. Compressive erosion refers to remodeling of inflamed and osteoporotic bone with gradual invagination of one bone into another, typified by protrusio acetabuli of the hip (Figure 58-1E). Surface erosion of bone may also be seen, usually resulting from inflammation of an adjacent tendon sheath. A typical location for this is at the outer margin of the ulnar styloid process secondary to extensor carpi ulnaris tenosynovitis.

Figure 58-1 Rheumatoid arthritis (x-ray). A, Multiple typical bone erosions are seen (e.g., in triquetrum, pisiform, scaphoid, radius, and ulnar styloid process). Diffuse cartilage loss also is evident in the radiocarpal compartment. B, Severe ulnar deviation is present at the metacarpophalangeal joints with extensive erosions. Severe erosion and bony ankylosis are seen in the wrist. C, Posterior oblique view of a shoulder shows severe glenohumeral joint space narrowing with marginal erosion and cystic change of the humeral head adjacent to the greater tuberosity (curved arrow). Elevation of the humeral head with respect to the glenoid indicates chronic rotator cuff tear. Tapering of the distal end of the clavicle and widening of the acromioclavicular joint (straight arrow) are also evident. D, Rheumatoid nodules appearing as lobulated subcutaneous soft tissue swellings (arrows) at the extensor surface of the elbow and forearm. E, Bilateral protrusio acetabuli. The medial acetabular margins protrude into the pelvis. Severe accompanying cartilage loss has occurred. F, Ankle with diffuse loss of cartilage space with erosions of the fibula (arrow and arrowhead). The hindfoot is in valgus alignment.

Joint space involvement is characteristic of RA. In many cases, inflammatory processes result in progressive destruction of articular cartilage, which in turn causes the radiographic finding of concentric joint space narrowing. This is usually diffuse in RA because all cartilage is involved at the same time; this feature may allow differentiation from the focal or asymmetric type of joint space loss that occurs with degenerative disorders. Occasionally cartilage destruction precedes synovial erosion of bone. Continued cartilage damage may result in partial or complete fibrous ankylosis, but progression to bony ankylosis is uncommon, although it may occur in end-stage disease in the wrist or midfoot. Subchondral radiolucent areas are common in RA and are often referred to as cysts, geodes, or pseudocysts. They may develop as a result of intraosseous extension of pannus, injury to the bone of any kind, or true intraosseous rheumatoid nodules. Mechanical factors accentuate their development, and very large cystic lesions may be seen in the elbow, femoral neck, or knee, occasionally precipitating pathologic fracture.

Symmetric soft tissue swelling around the small joints of the hands or feet is often the first clinical and radiographic sign of RA. Soft tissue swelling is caused by joint effusion, synovial proliferation, and periarticular inflammation, but the radiographic findings are nonspecific, and x-rays are much less sensitive for such changes than MRI and US. Eccentric soft tissue swelling may be due to adjacent bursitis, tenosynovitis, or rheumatoid nodules. Involvement of tendons and tendon sheaths is common; however, soft tissue changes usually are poorly visualized and are of lesser diagnostic importance.

Joint malalignment and deformity (see Figure 58-1) are very common in RA and occur as the result of laxity and disruption of capsule, ligaments, and tendons. These deformities are most characteristic in the hand, wrist, foot, and neck. Note that these malalignments may be transient and can reduce during positioning for radiographs. Late disease is associated with severe deformity, as is seen in arthritis mutilans.

Spine

Cervical spine involvement is common in RA and merits specific attention. It occurs mainly after several years of disease in rheumatoid factor–positive patients with severe peripheral RA and may cause severe pain, instability, and, ultimately, spinal cord compression.³ Prevalences of radiologic cervical involvement up to 70% have been reported but are much lower in recent studies in accordance with the fact that intensive disease-modifying antirheumatic drug (DMARD) therapy has been shown to reduce cervical radiologic progression.⁴

The upper cervical region is most affected, although abnormalities throughout the cervical spine are frequent (Figure 58-2). Upper cervical changes include odontoid process (dens) erosion and atlantoaxial subluxations (anterior, vertical [known as cranial settling], lateral, and posterior); subaxial manifestations include abnormalities of apophyseal and discovertebral joints, subluxation, and dislocation.

Figure 58-2 Cervical spine in rheumatoid arthritis (RA) (x-ray). A and B, Anterior atlantoaxial subluxation in RA. A, Lateral x-ray in flexion shows severe anterior atlantoaxial subluxation with a wide anterior atlantodental interval (asterisks) and a decreased posterior atlantodental interval (arrow). B, Almost complete reduction of subluxation is noted on the lateral view in extension. There also is subaxial subluxation at the level of C4-C5 (arrowhead) with erosive changes in various facet joints. O, odontoid. C, T2-weighted sagittal magnetic resonance (MR) image in RA shows low signal periodontoid pannus (P). The odontoid process appears irregular secondary to erosions (arrow). The atlantodental distance shows mild widening (solid line). There also is vertical subluxation without signs of cord compression. The anterior subarachnoid space is compromised by disk protrusions at multiple levels. Small erosions (arrowheads) are seen at the vertebral end plates at the C6-C7 level.

Adequate radiographic evaluation of the cervical spine requires lateral views in flexion and extension and should be performed in all RA patients with neck pain. Flexion-extension views are especially important for demonstration of the degree of atlantoaxial instability.

Atlantoaxial subluxation is caused by laxity or rupture of the transverse ligament. The characteristic finding is anterior subluxation (i.e., abnormal separation between the anterior arch of the atlas and the odontoid process of the axis). Reports of the exact lower limit of abnormal distance vary, but a distance greater than 2.5 mm is considered abnormal, as is distance that changes significantly between flexion and extension.¹ Cranial settling is diagnosed by radiographic measurement of the relationship between the tip of the odontoid process and landmarks at the base of the skull.¹ It may lead to protrusion of the dens through the foramen magnum and compression of neurologic structures. Significant atlantoaxial subluxations may require surgery.

Subaxial involvement is frequent and manifests as varying degrees of bone erosion, joint space narrowing, and subluxation of facet joints, as well as disk space narrowing and spinous process erosion. Subaxial subluxations may also lead to neurologic deficits.¹

MRI is useful for providing supplementary information on cervical spine involvement (Figure 58-2C), including spinal cord compression (see MRI section).

Ankylosing Spondylitis

Ankylosing spondylitis (AS) is the archetypal inflammatory joint disease that primarily targets fibrocartilage and the axial skeleton. Cartilaginous joints and sites of enthesis (tendon and ligament insertion) are involved early, and the characteristic involvement of the axial skeletal includes a predilection for the sacroiliac joints and all articulations and entheses of the spine.

Sacroiliac Joints

Erosion and ankylosis of the sacroiliac (SI) joints are the hallmarks of spondyloarthritis.^1,23 Sacroiliitis is usually the first manifestation and is characteristically bilateral and symmetric in AS.^24,25 Early radiographic findings predominate on the iliac side of the cartilage compartment, with erosion of subchondral bone causing loss of definition of the articular surfaces, usually accompanied by variable degrees of adjacent osteoporosis and surrounding reactive sclerosis. Bone erosion may result in the radiographic observation of focal joint space widening (Figure 58-3A), and as the disease progresses, the definition of the joint is completely lost with radiographic superimposition of erosion, sclerosis, and new bone formation, which fills in the erosions and the original cartilaginous “joint space.” The joint may disappear completely in late disease with ankylosis and remodeling of the bone (Figure 58-3B). The ligamentous compartment of the SI joint is frequently affected by bony erosion and entheseal proliferation, although these may be difficult to see radiographically.^24,25

Figure 58-3 Ankylosing spondylitis (x-ray). A, Sclerosis (arrows) is seen along the iliac sides of the sacroiliac joints, along with loss of portions of the iliac subchondral bone indicating erosion. B, Bilateral hip joint space narrowing is present. A ring of osteophytes is noted at the synovial insertion (arrows) on each femoral head. The sacroiliac joints are fused (ankylosis). C, Syndesmophytes (black and white arrows), some of which extend from the edge of one vertebral body to the next (bridging syndesmophytes, ankylosis; black arrows), are seen in this lateral view of the spine.

Spine

Traditional descriptions of the initial sites of spinal involvement are enormously influenced by the inability of radiography to adequately visualize many parts of the spine. Radiographic reports indicate that the lumbosacral and thoracolumbar junctions are first affected, whereas the MRI literature clearly indicates a predilection for early involvement of the midthoracic spine—an area that is extremely difficult to evaluate radiographically. The cervical spine is rarely affected first, but this can occur occasionally in women, and spinal disease rarely occurs in the absence of significant SI joint involvement.

Early radiographic manifestations of AS in the spine are most often due to enthesitis at the edges of the discovertebral joints.²⁴ Focal sclerosis (the “shiny corner”) and erosion (the “Romanus lesion”) develop at the attachment of the annulus fibrosus to the anterior corner of the vertebral end plate and are characteristic features of early AS (Figure 58-4A).²⁶ The anterior borders of the vertebrae may appear straight or “squared” owing to periosteal proliferation of new bone filling in the normal concavity or erosion at the anterosuperior and anteroinferior vertebral margins. This observation is much easier to make in the lumbar spine, where normal vertebrae are always concave anteriorly in comparison with the thoracic and cervical spine, where the normal contour is much more variable and may be square or occasionally convex.

Figure 58-4 Ankylosing spondylitis (x-ray and magnetic resonance imaging [MRI]). A, Lateral x-ray of the spine shows that marked erosion at the vertebral margins has produced straight or slightly convex anterior vertebral surfaces (“squaring”). New bone formation has resulted in “shiny corners” (arrows) and contributes to vertebral body squaring. The facet joints are fused. B, Sagittal fast spin echo T2-weighted magnetic resonance (MR) image in another patient shows bone marrow edema at the anterosuperior corners of the vertebral bodies (arrows), corresponding to early osteitis. C and D, Fracture in ankylosing spondylitis. C, X-ray shows disruption of the previously fused C6-C7 facet joints (arrow) and slight anterior subluxation. D, Sagittal T2-weighted MR image confirms a high signal fracture line (arrow) through the superior aspect of C7.

The hallmark of spinal disease in AS is the development of characteristic bony spurs known as syndesmophytes (see Figure 58-3C). These start as thin, vertically oriented projections of bone that develop as the result of ossification within the outer fibers of the annulus fibrosus of the intervertebral disk. Syndesmophytes are radiographically visible on the anterior and lateral aspects of the spine, starting from the corner of the vertebra. Progressive growth will bridge the intervertebral disk causing ankylosis, and extensive syndesmophyte formation produces a smooth, undulating spinal contour—the “bamboo spine.” The syndesmophytes that characterize AS must be differentiated from other spinal and paraspinal bone formations. Degenerative bony spurring in spondylosis deformans arises several millimeters from the discovertebral junction, is typically triangular in shape, and has a horizontally oriented segment of variable length at the point of origin. In diffuse idiopathic skeletal hyperostosis (DISH), bone formation in the anterior longitudinal ligament results in a flowing pattern of ossification that is usually thick, and the sacroiliac joints are not involved.

Erosion of the vertebral end plate is common in later stages of ankylosing spondylitis and may be focal or diffuse. It is also seen when pseudoarthrosis develops following a fracture in a previously ankylosed spine. Changes in the apophyseal joints are common and start with ill-defined erosion and sclerosis, but they may be difficult to see. Capsular ossification or intra-articular bony ankylosis frequently occurs in late disease (see Figure 58-4). The ankylosed spine is highly susceptible to fracture (Figure 58-4C and D), which should always be suspected in cases of unexplained pain exacerbation. Enthesitis at the interspinous and supraspinous ligamentous attachments is very common with bone formation, causing whiskering and interspinous ankylosis.

Psoriatic Arthritis

The presentation of psoriatic arthritis (PsA) is quite variable but is often distinctive. Peripheral joint involvement is common, and up to half of patients with PsA have evidence of joint damage on radiographs within 2 years of presentation.³¹ The hand and the wrist are most often involved and are affected in up to three-quarters of patients with PsA, but the pattern of joint involvement varies from patient to patient and over time in the individual patient. Radiographic appearances are usually asymmetric and may have a ray distribution, with affected joints involving a single digit, including the distal interphalangeal (DIP) joints (Figure 58-5), which are seldom involved in RA. Clinically, this manifests as dactylitis and is a common early presentation of PsA. Osteoporosis is frequently absent, and a distinguishing feature of PsA is the propensity for bone proliferation. This may be seen in the form of periostitis of the shafts of the phalanges, which is sometimes the first radiographic manifestation, or irregular bony spurring at joints or entheses. Fluffy new bone formation adjacent to marginal joint erosions may result in a particularly characteristic “whiskering” appearance. Severe marginal erosion of the heads of metacarpals or phalanges may produce the appearance of a whittled pencil; if combined with deep central erosion of phalangeal bases, this may be referred to as the “pencil-in-cup” appearance. Ankylosis of joints occurs frequently in advanced disease. Large joints are less often affected, and the findings are usually similar to those of RA. Spinal involvement is frequent in PsA. It may be seen in early disease, and sacroiliitis may be demonstrable in up to 75% of patients.³² Changes may be extensive and are more likely to be asymmetric than in AS.

Figure 58-5 Psoriatic arthritis (x-ray). A, Distal interphalangeal joint with classic radiographic findings, including soft tissue swelling, bone erosions with accompanying bone proliferation, and lack of osteoporosis. B, Arthritis mutilans resulting from psoriatic arthritis with destructive changes and joint deformity of the hand and pancompartmental ankylosis of the wrist. C, Psoriatic spondylitis. Thick asymmetric paravertebral ossifications (arrows), which are characteristic of psoriatic spondylitis and reactive arthritis.

(From Resnick D, Niwayama G: Diagnosis of bone and joint disorders, Philadelphia, 1988, WB Saunders.)

Gout

A variety of microcrystals can deposit in and around the joints, inducing an inflammatory response. In this chapter, the characteristics of gout (i.e., arthritis related to monosodium urate deposition) and of calcium pyrophosphate dihydrate (CPPD) crystal deposition disease will be presented. The reader is referred to chapters on individual diseases for descriptions of the remaining conditions, such as calcium hydroxyapatite crystal deposition disease, hemochromatosis, ochronosis, and Wilson’s disease.

Radiography is not helpful in the diagnosis of acute gouty arthritis because findings are limited to the soft tissues and are nonspecific. Chronic tophaceous gout is an asymmetric arthritis that frequently affects the feet, hands, wrists, elbows, and knees. The most common site of involvement is the first metatarsophalangeal (MTP) joint (Figure 58-6). Radiologic changes in long-standing gout are related to tophi and their effects on soft tissue and bone.³⁵

Figure 58-6 Tophaceous gout (x-ray, computed tomography [CT], and magnetic resonance imaging [MRI]). A, X-ray of forefoot in tophaceous gout. Extensive bone destruction is seen at the first metatarsophalangeal (MTP) joint with overhanging edges (arrowhead) and soft tissue swelling. Smaller erosions are present involving the first tarsometatarsal and second MTP joints (arrows). B, CT scan shows hyperdense tophus around the extensor digitorum muscle (arrow), with smaller deposits around the other extensor tendons. Axial precontrast (C) and coronal short tau inversion recovery (D) MR images show a large tophus (arrow) medial to the first metatarsal bone. Ca, calcaneus; Na, navicular.

(C and D Courtesy Professor Fiona McQueen, Auckland, New Zealand.)

Tophi appear as eccentric, nodular soft tissue masses around the joints with an amorphous increased density or patchy calcification.³⁵ Intraosseous tophi cause well-marginated subchondral cystic lucencies or intraosseous calcifications. Erosion of adjacent bone is common at the periosteal or endosteal surface. Erosions may be intra-articular, para-articular, or remote from the joint and typically are well defined with smooth sclerotic borders. An overhanging margin of bone at the edge of the erosion is a characteristic feature of gout. The joint space and bone density are usually preserved until late in the disease. Bursal inflammation commonly produces soft tissue swelling around the olecranon and in a prepatellar location.

Calcium Pyrophosphate Dihydrate Crystal Deposition Disease

The term CPPD crystal deposition disease refers to a variety of clinical situations, including asymptomatic crystal deposition, pseudogout, pyrophosphate arthropathy, and other, less frequent presentations, such as pseudo-RA. Chondrocalcinosis is a general term that refers to calcification of hyaline or fibrocartilage regardless of its origin (Figure 58-7).

Figure 58-7 Calcium pyrophosphate dihydrate crystal deposition disease (x-ray and magnetic resonance imaging [MRI]). X-ray of the knee (A) shows linear calcification within the lateral meniscus, whereas sagittal proton density MRI (B) of the same patient shows increased signal within the lateral meniscus, corresponding to chondrocalcinosis. C, X-ray of forefoot shows cartilaginous and capsular calcifications (arrows) within the second through fourth metatarsophalangeal joints. A nondisplaced fracture of the fourth metatarsal neck is noted.

An acute attack of pseudogout is often a monoarthropathy, and typical radiographic findings of soft tissue swelling and joint effusion are nonspecific, mimicking gout and infection. Often chondrocalcinosis is not radiographically visible. X-ray changes in more chronic pyrophosphate arthropathy are very similar to those in OA, with joint space narrowing, sclerosis, and subchondral cysts, with or without intra-articular or periarticular calcinosis.³⁹

The patellofemoral, radiocarpal, and second and third MCP joints are typical sites of involvement in pyrophosphate arthropathy. Distribution is usually bilateral and may be symmetric. In addition to the usual target sites, arthritic changes may be observed in non–weight-bearing joints and sacroiliac joints.⁴⁰

Pyrophosphate arthropathy may be associated with extensive and rapid subchondral bone collapse and fragmentation with intra-articular loose bodies, resembling neuropathic osteoarthropathy (pseudo-Charcot). Tumorous CPPD crystal deposits that resemble gouty tophi are observed occasionally, most frequently in digits, and are referred to as tophaceous pseudogout.⁴¹

Septic Arthritis

Septic arthritis usually is monoarticular and typically is associated with rapid onset of symptoms, systemic illness, obvious local clinical signs, and laboratory evidence of acute inflammation. The earliest clinical sign is symmetric soft tissue swelling around a joint due to soft tissue edema, hypertrophied synovium, and joint effusion. However, this may be difficult to discern on x-ray, and periarticular osteopenia may be the first radiographic feature. Initial widening of the joint space may be seen, as in septic arthritis of the pediatric hip. Marginal erosions develop quickly and have a very similar appearance to other forms of erosive inflammatory arthritis. As the infection progresses, hyaline cartilage and the subchondral cortex are rapidly destroyed, with progressive narrowing of the joint space (Figure 58-8). In late stages, ankylosis of the joint may be seen. Adjacent osteomyelitis occurs with increasing frequency as the infection becomes chronic. With granulomatous infection, the characteristic triad (Phemister’s triad) of radiographic findings consists of marked osteoporosis, marginal erosions, and absent or mild joint space narrowing. Periostitis and bone production are less common.

Figure 58-8 Septic arthritis (x-ray and magnetic resonance imaging [MRI]). A, X-ray in anteroposterior view of sacroiliac joints shows destruction of the right sacroiliac joint as part of septic arthritis (arrows). B, In another patient, short tau inversion recovery (STIR) coronal MRI through sacroiliac joints shows erosive changes in the right sacroiliac joint with extensive bone marrow edema. C and D, Tuberculous spondylitis. C, X-ray of the lumbar spine in lateral view shows disk space narrowing at the L3-L4 level with destructive changes involving the superior end plate of the L4 vertebral body (arrow). D, Coronal STIR MRI of the lumbar spine of the same patient confirms focal destruction of the L4 vertebral body (arrow). Enlargement of both psoas muscles (P) with increased signal is noted, owing to paraspinal extension of the infection.

Osteoarthritis

Osteoarthritis (OA) primarily targets hyaline cartilage, and its distribution is profoundly influenced by mechanical factors. OA is typically asymmetric in distribution both within and between joints. The most characteristic sites of osteoarthritis include hips, knees (Figure 58-9), proximal and distal interphalangeal joints of the hand, first carpometacarpal and trapezioscaphoid joints of the wrist (Figure 58-10), and first metatarsophalangeal joints.¹

Figure 58-9 Osteoarthritis of the knee (x-rays). Non–weight-bearing x-rays of the knee frequently underestimate the severity of cartilage loss. A 59-year-old man with pain and suspected osteoarthritis had supine anteroposterior (AP) (A) and erect weight-bearing AP (B) x-rays of the knee. The supine view demonstrates mild osteophytosis and possible subtle joint space narrowing in the medial compartment, whereas on the weight-bearing view, severe medial joint space narrowing is present, indicating severe cartilage damage (arrow). Usually the weight-bearing semi-flexed view is most sensitive to joint space narrowing (cartilage loss), but this is unpredictable. In a 53-year-old man, the erect AP view (C) shows more severe joint space narrowing (arrow) than is seen on the weight-bearing semi-flexed view (D).

Figure 58-10 Osteoarthritis of the hand (x-ray and magnetic resonance imaging [MRI]). A, X-ray shows asymmetric cartilage space narrowing at the proximal and distal interphalangeal joints, the first carpometacarpal joints, and the scaphoid-trapezium-trapezoid articulations. Subluxation at the first carpometacarpal joint and secondary hyperextension at the metacarpophalangeal joints are characteristic deformities of osteoarthritis. B, “Gull wing” deformities in erosive osteoarthritis: Cartilage loss and bone remodeling at the third and fourth proximal interphalangeal joints produce a gull wing appearance. X-ray (C) and coronal MRI (D) of distal interphalangeal joint show osteophyte and central erosions/collapse of the bone plate, representing severe osteoarthritis.

(C and D Courtesy Dr. Ida Haugen, Oslo, Norway.)

Joint space narrowing due to thinning of hyaline cartilage is a key feature of OA. Unlike the inflammatory arthropathies, resultant narrowing of the radiographic joint space is asymmetric, with cartilage thinning more pronounced in areas subject to greater mechanical stress, such as one compartment of the knee. Accurate assessment of cartilage loss is hugely influenced by radiographic projection, particularly in the lower limbs. Sensitive detection of cartilage thinning requires apposition of the most affected surfaces, which requires the joint to be optimally positioned with sufficient mechanical stress to bring the surfaces into contact. In the knee, for example, angular deformity tends to reduce on supine radiography, and severe cartilage loss may not be detected at all (see Figure 58-9). Bilateral weight-bearing projections are more consistent for the detection and measurement of cartilage loss, and the erect extended anteroposterior (AP) view has been established as the preferred projection for diagnostic utility. However, no view is completely reliable until global loss of cartilage occurs in late disease. Weight bearing in flexion is the most sensitive projection for cartilage thinning in the knee and is most reliable for detection of change, although the ideal degree of flexion for a specific individual varies according to the distribution of cartilage loss (see Figure 58-9).^43,44

Osteophytes, the most characteristic abnormality of OA, are more specific than joint space narrowing and often are the first radiographic sign of the disease. They start as osseous metaplasia of the hyaline cartilage best seen at the articular margins but also occur centrally on the articular surface. They can become large and often dominate the radiographic appearance. Thus, osteophytes are the key criterion in defining the presence of OA.⁴⁵

Bone density is usually normal or sclerotic. Bone erosion does not occur, except at the articular surface in late disease and occasionally earlier in the interphalangeal (IP) joints in inflammatory OA. The radiographic appearance of periarticular soft tissues is relatively normal in OA. Synovitis and effusion do occur but only rarely dominate the radiographic presentation.

Some deformity is common but tends to occur in particular joints such as the thumb and the knee. The deformity is due primarily to asymmetric loss of cartilage and bone, resulting in malalignment or subluxation.

Key Points

CT is a tomographic radiographic imaging technique that visualizes calcified tissue with high resolution. CT is a standard reference for detecting bone destruction.

In rheumatology, CT is useful mainly for detection of bone abnormalities in the axial skeleton (e.g., erosion, sclerosis, new bone formation, fracture).

CT is more sensitive for these purposes than x-ray and on most occasions MRI.

The role of CT of the axial skeleton is limited by a relatively large amount of ionizing radiation.

In practice, CT is rarely used unless x-ray is unclear and MRI is unavailable.

The main disadvantages of CT are the low sensitivity for soft tissue changes and the high exposure to ionizing radiation.

Computed tomography (CT) has been available for 40 years, and each decade has brought continued development in computing and engineering, resulting in remarkable changes in the technology. Although still limited in terms of soft tissue contrast, CT offers fast and reliable acquisition, high resolution, and multiplanar capabilities, which have enhanced its use in recent years. The modern CT scanner is a remarkable tool that might have an increasingly important role in arthritis imaging in the future.

Technical Aspects

CT image acquisition is no longer restricted to the axial plane of imaging, and its multiplanar capability is so versatile that many CT scans of the body are now interpreted primarily from coronal or sagittal images in the same way as MRI. Data acquisition is so fast that patient motion is rarely a problem because CT scans are now routinely acquired in a few seconds. In fact, CT is often faster than taking multiple radiographs. Complex requirements for patient positioning in musculoskeletal CT are now largely irrelevant in that almost all studies can be reconstructed to eliminate positioning problems. Patient tolerance is excellent and, unlike with MRI, no absolute contraindications to CT are known. Spatial resolution is high, usually higher than that of MRI, and contrast resolution between soft tissue and bone is unsurpassed by any other modality. However, despite all the advantages just listed, applications of CT in arthritis imaging will remain somewhat limited. CT still has two fundamental flaws that prevent its universal application. First, CT is constrained in the same way as radiography by its limited soft tissue contrast capability. Second, ionizing radiation is used (with increasing dose proportional to the size of the body part and the requirements for spatial detail).

Although this is not a problem with the more distal extremities because the exposure doses are smaller and the tissue more radioresistant, it remains an issue for spine, hip, and shoulder CT. Consequently, in most routine clinical situations, radiography provides sufficient information of a similar nature as CT for clinical decision making, and x-ray is cheaper than CT and is readily available; ultrasonography is a better and cheaper way to visualize and quantify superficial soft tissue pathology, and MRI offers superior soft tissue contrast and bone marrow imaging.

Rheumatoid Arthritis

The exquisite inherent contrast between bone and soft tissue that is afforded by CT makes it a gold standard reference for the detection of bone erosion; as such, CT is ideally suited to the investigation of erosion in inflammatory arthritides. Modern CT with isotropic voxel acquisition and three-dimensional visualization allows accurate detection and quantification of bone erosion with good intraobserver agreement, whereas radiography is limited by its two-dimensional projection and superimposition of structures. However, CT is very limited in its ability to visualize soft tissue changes, and even with contrast enhancement and complex subtraction techniques, CT remains inferior to MRI and ultrasound for assessment of synovial changes such as thickening and hyperemia. Furthermore, detection of erosion on CT and MRI shows very good agreement generally, although CT is slightly more sensitive.^48,49

Ankylosing Spondylitis/Axial Spondyloarthritis

CT allows visualization of the same pathologic processes as conventional radiography—erosion, osteoporosis/sclerosis, and new bone formation/ankylosis—with the added benefit of multiplanar imaging free from superimposition of overlying structures (Figure 58-11).

Figure 58-11 Bone erosion in sacroiliac joints in spondyloarthritis (computed tomography [CT]). Coronal (A) and axial (B) CT images of the sacroiliac joints in a patient with spondyloarthritis. Bone erosions (white arrows) and mild sclerosis and new bone formation (black arrows) are seen.

In AS, the pathologic processes start in the bone marrow and at sites of enthesis. However, CT has poor ability to detect soft tissue change and is usually normal until structural damage is present. CT can detect osteoporosis or osteosclerosis quite well, but these changes are very nonspecific. The primary value of CT in AS involves its ability to detect and clearly define erosion of bone at any joint or enthesis. New bone formation is also well visualized in the form of syndesmophytes, ligamentous ossification, and periarticular and intra-articular ankylosis, but use of CT in this regard is limited. CT can show these findings equally well in the axial and peripheral skeleton but is used primarily in areas where radiographic visualization or interpretation is problematic.

Psoriatic Arthritis

CT is rarely used for the investigation of PsA, and very few data have been published on this topic. In the peripheral skeleton, it would be expected that the application of CT for PsA would be similar to that for RA, and in the axial skeleton, use of CT would be similar to that for AS. In both cases, use of CT is limited by an inability to directly visualize inflammation in bone or soft tissue, and although it likely has superior ability versus radiography in detecting bone erosion or proliferation, in most circumstances the extra radiation dose is not warranted.

Gout

CT allows clear visualization of bone erosion, and recent developments in technology offer exciting prospects for crystal imaging.

Acute and chronic tophaceous gout is associated with deposition of urate crystals in soft tissues and the development of tophi. Chronic tophi are often partially calcified, and all CT scanners are sensitive for the deposition of calcium in soft tissues (see Figure 58-6). The condition frequently involves intraosseous deposition of crystals, which is much better visualized on CT than on radiography.⁵¹ CT shows quite well soft tissue swelling due to tophi, and because anatomic bone detail is very good on CT, erosions are clearly delineated. Dual-energy CT is a new technologic development whereby images can be reconstructed based on a two-material decomposition algorithm that can separate calcium from monosodium urate (Figure 58-12).⁵²

Figure 58-12 Gout (x-ray and dual-energy computed tomography [CT] scan). X-ray (A) and dual-energy CT scans (B through D) of left hand and wrist of a 39-year-old male patient with recent-onset bilateral pain in hands and wrists. X-ray (A) and laboratory investigations were all normal. Dual-energy CT was performed and confirmed the presence of sodium urate crystals in the left wrist and one finger. Left wrist aspirate was attempted but was unsuccessful. The patient responded well to treatment for gout. Imaging findings: After processing of CT data with an algorithm specific for detection of urate crystals (Siemens, Erlangen), the urate crystals are displayed green (arrows) on three-dimensional anterior (B), two-dimensional transverse (C), and two-dimensional sagittal (D) reconstructions.

Calcium Pyrophosphate Dihydrate Crystal Deposition Disease

CT is not used in the management of uncomplicated peripheral CPPD, although calcifications can be visualized. CPPD can involve the cervical spine and may cause neck pain. In such patients, CT may visualize CPPD deposition at the craniovertebral junction, often in the transverse ligament of the atlas (crowned dens syndrome).^55,56 Calcific deposits may also be found in other periodontoid structures and in the ligamenta flava in some patients, as may osseous abnormalities of the odontoid process such as subchondral cysts or erosions. CT may also be used for verifying the presence of calcifications at other rare locations (e.g., the temporomandibular joints).⁵⁷

Septic Arthritis

CT is rarely used for the investigation of septic arthritis. Acute septic arthritis of large joints is a surgical emergency, and although x-ray, fluoroscopy, and ultrasound may be useful for diagnosis or for image-guided aspiration, no use for CT in the peripheral skeleton is known. In chronic septic arthritis and infectious diskitis, CT may be used to investigate complications, particularly osteomyelitis. However MRI or a variety of scintigraphic studies are generally preferred.⁴² In complex cases where MRI scanning is problematic, contrast-enhanced CT can be very useful in detecting abscess formation in deep soft tissues that are not easily assessed by ultrasound. CT guidance may be preferred for spine biopsy as well.

Osteoarthritis

Even though CT can detect bone changes such as osteophytes more accurately than x-ray,⁵⁸ overall it offers little additional diagnostic utility in peripheral joints. It generally has poor soft tissue contrast capability, hyaline cartilage cannot be directly visualized, and spatial resolution of bone detail is inferior to that of radiography. It can be used in specific problem-solving situations where anatomy is complex and/or poorly visualized on x-ray, such as in the lumbar spine facet joints, but this is not usually the case in the peripheral joints. CT is used occasionally to assess extent of disease, as in the orthopedic assessment of large joints when detailed evaluation of bone morphology may be required preoperatively. CT scanning with intra-articular contrast material (CT arthrogram) may be the single most accurate modality for detecting cartilage thinning, fissuring, flaps, or volume⁵⁹ (Figure 58-13), as well as loose intra-articular bodies. This requires both a minimally invasive procedure and radiation exposure and is not widely used. CT arthrography may be used as a problem-solving tool in patients who are not able to undergo MRI.

Figure 58-13 X-ray, computed tomography (CT), and CT arthrogram of the knee of in a 46-year-old male patient with unexplained pain and locking. Initial x-rays (A and D) appear near normal with preserved joint space, a tiny osteophyte arising from the medial tibial spine, and no other definite abnormality. CT scan with sagittal (B) and coronal (E) reconstructions confirms the presence of osteophytosis (short arrow) and a small subchondral cyst (long arrow) but show no definite chondral changes. CT arthrogram with sagittal reconstruction (C) demonstrates several focal hyaline cartilage thinning (arrows) and a meniscal tear. On the coronal reconstruction (F), the tibial subchondral cyst is associated with complete loss of overlying hyaline cartilage.

Key Points

MRI allows visualization and assessment of peripheral inflammatory and destructive joint and soft tissue involvement in degenerative and inflammatory rheumatic diseases.

MRI is by far the best available method for detecting and monitoring inflammation in the spine and sacroiliac joints in AS and other spondyloarthritides.

MRI allows monitoring of inflammatory soft tissue changes (e.g., synovitis, tenosynovitis, enthesitis) during treatment of patients with peripheral inflammatory joint disease.

Through detection of early inflammatory changes, MRI can contribute to an earlier diagnosis of RA and SpA.

MRI bone edema is uniquely visualized by MRI; this provides prognostic information in RA, SpA, OA, undifferentiated inflammatory arthritis, and possibly other rheumatic arthritides.

The main disadvantages of MRI are its cost and lack of availability.

MRI provides multiplanar tomographic imaging with unprecedented soft tissue contrast, without the use of ionizing radiation, and it allows assessment of all structures involved in musculoskeletal disease. MRI is more sensitive than clinical examination and x-ray for detection of inflammation and damage in inflammatory and degenerative rheumatologic disorders. Disadvantages of MRI include higher costs and lower availability than radiography, longer examination times, and restriction to a limited anatomic area per session. It should be remembered, however, that costs of MRI represent only a small fraction of the cost of biologic treatment or of the indirect costs of sick leave and early retirement.

After the following section on key technical aspects, characteristics of MRI with respect to diagnosis, monitoring, and prognostication and its clinical utility in RA, SpA, PsA, gout, and OA will be described.

Technical Aspects

MRI is very safe. It involves no ionizing radiation and no increased risk of malignancy, and adverse effects of contrast agents are very rare. However, use of contrast agent should be avoided in patients with severely impaired renal function because of the risk of nephrogenic systemic fibrosis.

T1-weighted (T1w) imaging sequences are favored by relatively short imaging times, good anatomic detail, and the ability to visualize tissues with high perfusion and permeability, including the inflamed synovium, after intravenous contrast (paramagnetic gadolinium compound [Gd]) injection. Fat and Gd-enhanced tissues have high signal intensity on T1w images (Figure 58-14). T2-weighted (T2w) images depict both fat and fluid/edematous tissues with high signal intensity. They are used together with T1w images in degenerative spine disease, while in inflammatory conditions they are particularly useful when fat saturation (FS) techniques (Figure 58-15), in which the signal from fat is suppressed, are applied. This enhances detection of edematous tissue/fluid located in areas with fatty tissue (e.g., bone marrow edema). FS requires a homogeneous field and high magnetic field strength, which are not available on low-field (<1.0 T) MRI units. The only fat-suppressed sequence possible on low-field MRI, the short tau inversion recovery (STIR) technique, which is based on relaxation time differences, can provide information on bone marrow edema,⁶⁰ but with less detail.

Figure 58-14 Rheumatoid arthritis (magnetic resonance imaging [MRI]). Axial T1-weighted precontrast (A) and postcontrast (B) MRI of the second (left) through fifth (right) fingers at the level of the metacarpophalangeal joints in a patient with rheumatoid arthritis (RA). Synovitis (black arrows in B), joint effusion (white arrow in B), and severe bone erosion (arrow in A) are seen. Coronal T1-weighted precontrast MRI of the wrist (C) in another RA patient shows no bone erosions. Similar images 1 year later (D) show severe erosive progression (bone erosions marked by arrows).

Figure 58-15 Rheumatoid arthritis (RA) in clinical remission (magnetic resonance imaging [MRI]) and undifferentiated arthritis (MRI). A and B, RA patient in clinical remission. Despite clinical remission, coronal short tau inversion recovery (STIR) (A) and postcontrast T1-weighted fat-suppressed (B) MRI show marked bone marrow edema (osteitis, arrows in A) and synovitis (arrows in B). C through E, Patient with undifferentiated arthritis. Axial T1-weighted precontrast (C) and postcontrast (D) T1-weighted MRI views show synovitis in second and third metacarpophalangeal joints (arrows), whereas the STIR sequence (E) shows bone edema (arrow). X-ray was normal. One year later the patient had developed RA, according to the American College of Rheumatology 1987 criteria.

(Courtesy Dr. Anne Duer-Jensen, Glostrup, Denmark.)

Whereas MRI examination of axial joints requires whole body MRI units, MRI of the peripheral joints can be performed with whole body MRI units or with dedicated extremity MRI units (E-MRI). E-MRI increases the potential for widespread rheumatologic use through markedly lower costs, more comfortable patient positioning, and elimination of claustrophobia.

Some E-MRI units may provide information on synovitis and bone destruction that is not markedly inferior to what is obtained by standard sequences on high-field units,^60,61 but different machines may perform very differently, and for some units, smaller field of view, longer imaging times, and lack of certain imaging techniques (particularly FS at low field) should be considered.⁶²

Most MRI studies of the sacroiliac joint have used only one imaging plane—semi-coronal (i.e., parallel with the axis of the sacral bone). A supplementary T1w FS sequence may improve the evaluation of erosions,⁶³ and sequences designed for cartilage evaluation (e.g., three-dimensional [3D] gradient echo sequences) may be added.⁶⁴ For maximal sensitivity to changes in the ligamentous portion of the sacroiliac joints, imaging in the semi-axial plane is required.⁶³ This may be recommended when MRI is used for diagnostic purposes, although it is probably not essential when used as an outcome measure in trials. On some indications (e.g., suspected disk herniation), MRI should include axial images; however, MRI of the spine in SpA generally involves only sagittal images, but these should extend sufficiently laterally to include the frequently involved facet and costovertebral and costotransversal joints.⁶⁵ Bone marrow abnormalities in both sacroiliac joints and spine are detected almost equally well with STIR and contrast-enhanced T1w FS sequences in patients with SpA, so contrast injection generally is not needed.^66,67 In contrast, injection of contrast increases sensitivity for synovitis in peripheral joints.⁶⁸ For evaluation of structural (sometimes referred to as chronic) changes, such as bone erosion, new bone formation, and fat infiltration, T1-weighted images are mandatory. A supplementary T1w FS sequence may improve the evaluation of erosions.⁶³ In peripheral and axial joints, specific sequences designed for optimal cartilage evaluation (e.g., 3D gradient echo sequences) can be applied.⁶⁴

Rheumatoid Arthritis

Most MRI studies in RA have investigated knee, wrist, and finger joints. Although the knee joint is an excellent model joint for methodological studies, the clinical value of MRI is mainly dependent on its power to evaluate wrists, hands, and feet, which is also the primary focus of this section. Reports on other peripheral joints are few and are not essentially different. Definitions of key pathologies in RA and PsA are provided in Table 58-1.

Table 58-1 Magnetic Resonance Imaging Definitions of Inflammatory and Structural Lesions in Rheumatoid Arthritis, Psoriatic Arthritis, and Axial Spondyloarthritis

¹ OMERACT MRI in Inflammatory Arthritis Task Force recommendations for MRI definitions of important pathologies in rheumatoid arthritis and peripheral psoriatic arthritis.^86,121

² Canada-Denmark MRI Working Group and MORPHO Group recommendations for MRI definitions of important pathologies in spine and sacroiliac joints in axial spondyloarthritis.^10,104,105

From OMERACT MRI in Inflammatory Arthritis Task Force,^86,121 Canada-Denmark MRI Working Group, and MORPHO Group.^104,105,107

MRI allows assessment of all structures involved in RA (i.e., synovial membrane, intra-articular and extra-articular fluid collections, cartilage, bone, ligaments, tendons, and tendon sheaths) (see Figure 58-14). MRI and histopathologic and miniarthroscopic signs of synovial inflammation are closely correlated.^69,70 MRI bone marrow edema (see Figure 58-14) represents inflammatory infiltrates in the bone marrow (i.e., osteitis), as has been demonstrated by comparison with histologic samples obtained at surgery in patients with RA.^71,72 Whereas erosion reflects bone damage that has already occurred, bone marrow edema appears to represent the link between joint inflammation and bone destruction. A high level of agreement for detection of bone erosion in RA wrists and MCP joints (concordance at 77% to 90% of sites) between MRI and CT, the gold standard reference for detection of bony destruction, documents that MRI erosions signify true bone damage.^48,73

In the cervical spine, the primary imaging modality is CR, but MRI can provide detailed information on bone and soft tissue abnormalities, which can serve as a valuable supplement to findings revealed by radiographic evaluation^74–80 (see Figure 58-2C). MRI is able to directly visualize pannus tissue (e.g., around the odontoid process). Cord compression on MRI seems a better predictor of deterioration than initial clinical and plain radiographic features, supporting MRI as the imaging method of choice for evaluation of spinal cord involvement in RA.⁸¹

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