Imaging Findings.

Plain film radiography, predominantly digitized images today, is an inexpensive tool and may be readily used by practitioners on a frequent basis in clinical practice to support clinical and laboratory findings consistent with a diagnosis of RA and aid in eliminating possible differential diagnoses. Although the radiographic changes admittedly fall behind the disease process, plain film does have the advantage of being readily reproducible and permitting easy serial comparison to follow the progression of RA and the effectiveness of its treatment.601,625 Ultrasonography and MRI are superior in detecting synovitis, and many of the initial subtle joint changes are visible much earlier with MRI⁵⁰⁰; however, studies have not yet shown MRI and/or sonography to sufficiently distinguish between early rheumatoid and nonrheumatoid arthritis.⁶⁸ In addition, MRI is not yet used routinely because of its high cost, and depending upon the area to be examined, the small size of the joints involved and the inability to image several joints simultaneously (i.e., hand) may limit widespread use.^142,188,538 At this time, the major role for MRI in patients with RA is in the preoperative assessment for surgical intervention where patients are experiencing neurologic deficits and/or severe pain.^103,341 Consequently, this section primarily focuses on plain film findings.

RA is classically symmetric and may involve any synovial joint. The general radiographic findings reflect the underlying pathologic change of chronic synovial joint inflammation with associated hyperemia, edema, and pannus formation. The first feature, which is usually the sole finding for the first few months, is fusiform periarticular soft-tissue swelling arising from capsular distension caused by excessive fluid accumulation seen clinically. Normal fat planes may be displaced or obliterated as a consequence of joint effusion, edema, and/or tenosynovitis. Increased blood flow to the synovium leads to a second early radiographic finding of juxtaarticular osteoporosis, which later becomes more generalized because of patient inactivity.^324,405

Joint spaces eventually narrow uniformly as the cartilage is destroyed by the enzymatic nature of pannus. Osseous erosions usually become apparent within the first 2 years of the disease,87,237,537 first occurring at the unprotected bone margins or “bare areas” (Fig. 9-2) in which the pannus has direct osseous contact, and later involving the subchondral bone. Radiographic erosions appear to occur most quickly in the first year of the disease.²⁵² The appearance of an alternating pattern of erosions and normal cortical bone has been termed a “dot dash” appearance. Multiple, nonmarginated subchondral cysts or geodes typically develop and may communicate with the synovium.^426,686

Later stages of the disease give rise to joint deformities resulting from tendon and ligament laxity, ruptures, and contractures. During periods of prolonged remission, radiographic signs of secondary osteoarthritis (OA) may develop, fibrous ankylosis may occur, and bony ankylosis may develop on rare occasions.

Hands and Feet.

The earliest clinical and radiographic changes are characteristically found in the hands and feet.⁶⁸⁶ Although traditional evaluation methods of RA focus on radiographs of the hands, the feet appear to show osseous erosions first and to a greater extent.^{87,301,310,537,685} The initial involvement is of the head of the fifth metatarsal, with erosion and narrowing of the fifth metatarsophalangeal (MTP) joint.

The classic joint distribution of RA in the hands is bilateral and symmetric involvement of the metacarpophalangeal (MCP), proximal interphalangeal (PIP), and thumb interphalangeal (IP) articulations. Some or all of the general radiographic features may be seen. Initially, the joint spaces may appear widened secondary to effusion, however as the cartilage is destroyed, joint space narrowing occurs. The earliest osseous erosions in the hands occur at the MCP joints of the second and third digits of the dominant hand,³⁸³ and at the radial aspect of the metacarpal heads (Figs. 9-3 and 9-4). The Norgaard and Brewerton views may aid in visualization of these early joint erosions, although it is difficult to duplicate the exact positioning on serial radiography.⁶⁸⁶ In the Norgaard view, the hands are 45 degrees supine, with the fingers straight,⁴⁸⁷ and in the Brewerton view (ball catcher’s view), the MCP joints are flexed at 65 degrees.⁸² Characteristic joint deformities include the swan neck deformity, boutonnière deformity, ulnar deviation of the fingers (doigts en coup de vent), and the hitchhiker’s or Z-shaped deformity of thumb (Figs. 9-5 to 9-7). The swan neck deformity results from hyperextension of the PIP and hyperflexion of the distal interphalangeal (DIP) joints, whereas the boutonnière deformity represents the opposite configuration of hyperflexion of the PIP joint and hyperextension of the DIP joint. The hitchhiker’s thumb is secondary to MCP flexion and IP extension (Fig. 9-5). Ulnar deviation at the MCP joints is called ulnar drift; when combined with radial deviation in the radiocarpal articulations it results in a “zigzag” deformity (Fig. 9-5). Loosening or disruption of the distal attachment of the extensor tendon to the terminal phalanx leads to a “mallet” finger.

Changes in the feet tend to parallel those of the hands, with the MTP articulations commonly affected first and other deformities, including lateral or fibular deviation at the MTP joints of the first through fourth digits, flexion of the DIP joints (hammer or claw toes), and extension of the MTP joints eventually developing (Figs. 9-8 to 9-11). Pathologic changes in the deep transverse tendons may lead to spreading of the metatarsals or forefoot. Pathologic changes in supporting ligaments also may lead to hallux valgus. Retrocalcaneal bursitis, plantar fasciitis, Achilles tendonitis, or Achilles tendon rupture may occur in the heel.

Wrist.

One of the earliest findings of RA may involve tendonitis of the wrist, as suggested in a study by McQueen et al,⁵⁰² which found evidence on MRI that 81% of patients with early RA demonstrated tendinopathy at baseline. Pancompartmental involvement is typical in the wrist, with the earliest erosions usually involving the radial and ulnar styloid processes; distal radioulnar and radiocarpal joints; and waist of the scaphoid, triquetrum, and pisiform (Figs. 9-12 to 9-15).⁴²⁵ Ligamentous instability results in patterns that parallel posttraumatic lesions, such as scapholunate dissociation, distal radioulnar dissociation, and dorsiflexion or volar flexion instability (Fig. 9-15). Eventual carpal ankylosis is common.

Elbows.

Joint effusion is recognized by a positive fat pad sign. Osseous erosions may be noted at each of the articulating surfaces, although the main destruction usually affects the humeral–ulnar joint. Antecubital cysts may occur in which the synovium herniates through the weakened anterior capsule of the joint.

Shoulders.

The glenohumeral and acromioclavicular (AC) joints may be affected. Resorption of the distal clavicle, in addition to erosions at the coracoclavicular ligament insertion on the undersurface of the clavicle, is not uncommon. Erosions may be observed on the medial aspect of the humeral neck as it abuts the glenoid process from elevation of the humerus, which is caused by a rotator cuff tear secondary to a chronically inflamed supraspinatus tendon. The acromiohumeral distance narrows with progressive elevation of the humeral head, and sclerosis and cyst formation are noted on the adjacent portions of the humeral head and acromion process.

Hips.

Abnormalities of the hips generally are bilateral and symmetric. Axial migration of the femoral head (concentric joint space narrowing) and bilateral acetabular protrusion are common findings that accompany the other general features of erosions, subchondral cysts, and osteopenia associated with RA (Fig. 9-16).¹⁵⁵ There is an absence of sclerosis and osteophyte formation unless secondary OA has developed.

Knees.

Tricompartmental involvement that includes the general features of joint space narrowing and osseous erosions is typical (Figs. 9-17 to 9-19). A genu valgus deformity is more likely to develop than a varus joint deformity (Fig. 9-20). Soft-tissue swelling may be present in the form of suprapatellar effusion or a large popliteal (or Baker) cyst. Subchondral sclerosis and osteophytes may be noted with the development of secondary OA.

Cervical Spine.

RA involvement is rare in other regions of the axial skeleton but affects the cervical spine in more than half the patients within the first 10 years of disease onset,557,686 with a preference for the apophyseal and atlantoaxial joints. The more chronic the disease, the greater is the likelihood of cervical involvement.⁵⁵⁴ Cervical spine progression appears to parallel clinically and radiologically with peripheral joint involvement. The greater the structural or erosive damage in the peripheral joints, the greater the tendency for severe structural damage in the cervical spine. Radiographs of the cervical spine are a prudent consideration in all patients with RA regardless of symptoms, because many patients are asymptomatic despite cervical involvement.¹³⁵ These should include lateral views with the patient in flexion and extension to assess level of involvement and evidence of instability.

Atlantoaxial subluxation or instability is the most common radiographic abnormality encountered in the cervical spine, with the prevalence varying from 19% to 80% depending on the patient selection and radiographic examination (Fig. 9-21).³³⁴ Movement may occur in several directions (listed in order of decreasing frequency): anterior (most frequent, 9.5% to 36%), lateral, vertical, or posterior.^56,115 Movement is usually the result of odontoid erosions or transverse ligament laxity, although the alar and apical ligaments also may be involved.

The degree of anterior subluxation shown on conventional radiography correlates poorly with neurologic signs and the presence of cord compression, which is in part a result of the unknown thickness of the synovial pannus not visible on plain films.^56,566 The standard method for determining anterior atlantoaxial subluxation has been an anterior atlantodental interval* (ADI) of greater than 3 mm. An ADI of 3 to 6 mm is indicative of early instability with damage of the transverse ligament. An ADI greater than 6 mm indicates the addition of alar ligament damage. Surgical stabilization is likely necessary with an ADI greater than 9 mm.

It has been shown, however, that a posterior atlantodental interval* (PADI) of 14 mm or less may be a more reliable predictor than utilizing the ADI for neural compression, and necessitates an MRI for evaluation of true cord space (Fig. 9-22).⁵⁶ It has also been shown that neurologic recovery from surgery was likely if the surgery was performed with a PADI of greater than 14 mm, but unlikely if the PADI was less than 10 mm.

Vertical atlantoaxial subluxation, also known as cranial settling, atlantoaxial impaction (AAI), and pseudobasilar invagination, is superior migration of the odontoid, and occurs with erosion of the occiput-C1 and C1-2 articulations. It results in approximation of the dens and brainstem, which can lead to direct compression or cause neurologic damage by excessive kyphosis.97,566 The McGregor line may be used to assess superior migration, in which vertical subluxation is designated when the odontoid tip lies more than 4.5 mm above the line; however, this is not always accurate as there may be odontoid erosion. The Ranawat index may also be used to evaluate vertical subluxation on the lateral cervical radiograph, in which a measurement is made from the center of the C2 pedicles to a line connecting the midpoint of the anterior and posterior arches of C1. Normal values are greater than 15 mm for men and greater than 13 mm for women. The Sakaguchi-Kauppi method may be used, which was also developed for screening purposes and evaluating the position of C1 in relation to C2. This method divides AAI into four grades based on the relationship of the cranial tip of the C2 pedicle shadows, as visualized on the lateral cervical radiograph, to the line drawn between the caudal parts of the anterior and posterior arches of atlas.³³⁵

Subaxial subluxations may occur in 10% to 20% of patients and often are at multiple levels. C3-4 and C4-5 are the most common areas of involvement, producing a “stepladder” or “doorstep” deformity and associated kyphosis on the lateral radiograph.⁵⁰⁴ Although these are less common than upper cervical spine subluxations, they are potentially more important neurologically because of the smaller spinal canal dimensions.^56,115 A canal measurement may be taken from the lateral radiograph in a similar way that the PADI is taken. The normal sagittal diameter from C3 to C7 is 14 to 23 mm. If the subaxial canal diameter measures less than 14 mm, MRI is advised.⁵⁶ Another method of measuring subaxial subluxations is the cervical height index which evaluates the impact of the combination of changes that may occur throughout the cervical spine (reduced disc height, bony collapse, and subluxations). On the lateral projection, the distance from the center of the sclerotic ring formed from the C2 pedicle to the midpoint of the inferior C7 endplate is measured. This is divided by the measurement taken from the center of the C2 pedicle shadow to the tip of the C2 spinous process. A cervical height index less than 2 mm is specific and sensitive for predicting neurologic compromise.

Disc height narrowing, mild subchondral sclerosis, and erosions of the vertebral endplates, facet joints, and spinous processes are other less-severe signs of rheumatoid involvement that occur in the cervical spine, and much less commonly the thoracic and lumbar spine. These signs often are subtle and difficult to diagnose on plain film radiographs. Patients treated with corticosteroids are susceptible to avascular necrosis which may affect segments of the thoracic and lumbar spine. Vertebral body collapse, fragmentation, and radiolucent fracture lines may be visible.

Lungs.

Lung involvement is common, although not always clinically significant. Pleural involvement (e.g., pleurisy, pleural effusion) is the most common lung manifestation and usually is asymptomatic.¹⁴ Other pulmonary manifestations include pulmonary fibrosis or honeycombing, constrictive bronchiolitis, bronchiectasis, pulmonary nodules, and subpleural micronodules (Fig. 9-23). High-resolution computed tomography (HRCT) is useful for this avenue of investigation. These findings have been observed on CT in patients with symptoms, and those asymptomatic patients with no abnormalities noted on chest plain film.⁵⁷⁰ Caplan syndrome describes inflammation and scarring of pulmonary tissue in people with RA who have exposure to coal dust.