Image Interpretation I: Arthritic Disorders

Arthritis is the most common thing you will see on a musculoskeletal film, and the interpretation section of Part III expects you to name it from its pattern rather than from a single sign. Each arthritis leaves a signature made of distribution, symmetry, bone density, joint space, erosions and new bone, and reading those features in order separates degenerative from inflammatory and crystal disease. The stakes are practical: the same film that shows a harmless osteophyte may show an unstable atlantoaxial joint in rheumatoid arthritis or a fused, fracture prone spine in ankylosing spondylitis, and those findings change whether and how you treat.

Section 1Degenerative joint and disc disease

The radiographic language of osteoarthritis

Osteoarthritis (degenerative joint disease) is a failure of articular cartilage followed by a reparative response of the underlying bone. Every radiographic sign follows from that sequence. Cartilage loss is greatest where load is greatest, so the joint space narrows nonuniformly (one compartment more than another). The bone beneath the thinned cartilage takes more stress and becomes dense: subchondral sclerosis, or eburnation when bone rubs on bone. Synovial fluid forced through cracks in the subchondral plate produces subchondral cysts (geodes) with sclerotic margins. At the joint margins, where cartilage meets periosteum, the bone grows outward as osteophytes. Bone density stays normal, erosions are absent, and soft tissue swelling is slight. These features (nonuniform narrowing, sclerosis, osteophytes, cysts, normal density) are the "productive" signature that separates degenerative disease from the "destructive" erosive signature of inflammatory arthritis.

Osteoarthritis by site

The Kellgren and Lawrence scale grades OA on radiographs: grade 0 none; grade 1 doubtful narrowing and possible osteophyte; grade 2 definite osteophytes and possible narrowing; grade 3 moderate multiple osteophytes, definite narrowing, some sclerosis and possible bone end deformity; grade 4 large osteophytes, marked narrowing, severe sclerosis and definite deformity. Radiographic severity and pain correlate only loosely, so the film describes structure, not symptoms.

Degenerative disease of the spine

The spine has two kinds of joints: the intervertebral disc (an amphiarthrosis, a symphysis) and the paired zygapophyseal (facet) joints, which are synovial. The cervical spine adds the uncovertebral joints (joints of Luschka) from C3 to C7. Each degenerates in its own way, and the vocabulary matters because the board uses the terms precisely.

• Intervertebral osteochondrosis is degeneration of the nucleus pulposus and endplates: disc space narrowing, the vacuum phenomenon (a dark linear nitrogen collection in a fissured disc, which essentially excludes infection), endplate sclerosis and Schmorl nodes. On MRI it shows as a dark (desiccated) disc and Modic endplate changes (type 1 edema, type 2 fatty, type 3 sclerotic).
• Spondylosis deformans is degeneration of the outer annulus fibers (Sharpey fibers): the annulus bulges, pulls on its attachment, and osteophytes form at the anterior and lateral vertebral body margins. They begin a few millimeters from the endplate, project horizontally, then turn vertically toward the disc (claw osteophytes), and may bridge. Disc height may be relatively preserved early.
• Traction spurs are small horizontal osteophytes arising 2 to 3 mm from the endplate margin; they are traditionally taught as a sign of segmental instability.
• Facet arthrosis shows joint space narrowing, sclerosis, hypertrophic articular processes and periarticular osteophytes, best seen on oblique lumbar views and on the lateral view as overlapping density. Hypertrophied facets and a buckled ligamentum flavum narrow the lateral recess and canal (degenerative stenosis).
• Degenerative spondylolisthesis is forward slippage with an intact pars, almost always at L4 on L5, usually Meyerding grade 1, and most often in women over 50 with sagittally oriented facets. Because the whole vertebra slips, the canal narrows (unlike isthmic spondylolisthesis, which leaves the posterior arch behind and widens the canal).
• Uncovertebral arthrosis produces sharpening and hypertrophy of the uncinate processes on the AP lower cervical view and posterolateral osteophytes that narrow the intervertebral foramina, best seen on oblique cervical views. Combined with facet arthrosis it is the usual cause of cervical foraminal stenosis and radiculopathy at C5 to C6 and C6 to C7.
• Baastrup disease (kissing spinous processes) shows contact, sclerosis and flattening of adjacent lumbar spinous processes, often with interspinous bursitis.

Diffuse idiopathic skeletal hyperostosis (DISH)

DISH (Forestier disease) is a noninflammatory enthesopathy in which ligaments and tendon insertions ossify. The Resnick criteria are: (1) flowing ossification along the anterolateral aspect of at least four contiguous vertebral bodies; (2) relative preservation of disc height without extensive degenerative disc disease; and (3) absence of facet joint ankylosis and of sacroiliac erosion, sclerosis or intra-articular fusion. It is most common in the thoracic spine, on the right side (the descending aorta on the left seems to inhibit it), in men over 50, and is associated with obesity, type 2 diabetes and hypertension. A thin lucent line often separates the ossified ligament from the anterior vertebral body, and the ossification bulges at the disc levels like candle wax. Extraspinal "whiskering" enthesophytes appear at the iliac crests, ischial tuberosities, trochanters, olecranon, patella and calcaneus.

DISH matters clinically for four reasons: dysphagia from bulky anterior cervical ossification; a strong association with ossification of the posterior longitudinal ligament (OPLL), which can cause myelopathy; stiffness that turns minor trauma into unstable fractures through the ossified segment (the long lever arm behaves like a fused spine); and postoperative heterotopic ossification after hip arthroplasty.

Describing degenerative findings

A good degenerative report names the joint or level, the specific findings, their severity (mild, moderate, severe), alignment and its effect on canal or foramina. For example: "Moderate disc space narrowing at L5 to S1 with vacuum phenomenon, endplate sclerosis and anterior osteophytes (intervertebral osteochondrosis and spondylosis). Bilateral L4 to L5 facet arthrosis with grade 1 degenerative anterolisthesis of L4." Avoid alarming labels such as "severe degeneration" without clinical context, and remember that degenerative findings are common in asymptomatic adults.

Section 2Rheumatoid arthritis and juvenile idiopathic arthritis

The rheumatoid signature

Rheumatoid arthritis (RA) is a synovial inflammation. Hypertrophied synovium (pannus) releases enzymes and cytokines that dissolve cartilage evenly and attack bone where synovium touches it directly without a cartilage cover: the bare areas at the joint margins. Increased blood flow and disuse thin the bone around the joint. The radiographic signature follows:

• Distribution: bilateral and symmetric, polyarticular, favoring the MCP joints (especially the second and third), PIP joints, the wrist (with early erosion of the ulnar styloid) and the MTP joints of the feet (the fifth MTP is often the first to erode). The DIP joints are characteristically spared.
• Soft tissue: fusiform, symmetric periarticular swelling, the earliest finding.
• Bone density: periarticular (juxta-articular) osteopenia early, becoming generalized later and worsened by corticosteroids.
• Joint space: uniform, symmetric narrowing because the whole cartilage surface is attacked.
• Erosions: marginal erosions at the bare areas, seen first as loss of the thin white cortical line (dot dash appearance), then as defined notches. No sclerotic margins and no new bone formation.
• Alignment (late): ulnar deviation and volar subluxation at the MCP joints, swan neck and boutonniere deformities, carpal collapse and, sometimes, carpal ankylosis.

The cervical spine in rheumatoid arthritis

The cervical spine is involved in a large proportion of patients with long standing RA, and it is the finding with the most direct effect on chiropractic care. The synovial joints around the dens, and the transverse ligament that holds the dens against the anterior arch of the atlas, are destroyed by pannus.

• Anterior atlantoaxial subluxation is the most common pattern. The atlantodental interval (ADI), measured on the lateral view from the posterior surface of the anterior arch of C1 to the anterior surface of the dens, should be no more than 3 mm in adults (5 mm in children). Instability may be hidden in neutral and appear only in flexion, so flexion and extension laterals are obtained. An ADI over about 9 to 10 mm, or a posterior atlantodental interval (space available for the cord) of 14 mm or less, indicates high risk of cord compression and surgical consultation.
• Vertical subluxation (cranial settling, basilar invagination) occurs when the lateral masses of C1 and C2 collapse and the dens rises toward the foramen magnum. It is assessed with lines such as McGregor's line (hard palate to the lowest point of the occiput; the dens tip should not project more than about 4.5 mm above it) and Chamberlain's line (hard palate to opisthion).
• Dens erosion may leave a thin, pointed or completely resorbed odontoid.
• Subaxial subluxation at several levels gives a stepladder appearance, with disc narrowing and endplate erosion but little osteophyte formation.

Laboratory correlation and report recommendations

Imaging findings suggest RA; the diagnosis rests on the clinical picture with rheumatoid factor (sensitive but not specific), anti-cyclic citrullinated peptide antibodies (anti-CCP, highly specific, about 95%), and ESR and CRP. Radiographs are often normal in the first months, when ultrasound or MRI show synovitis and bone marrow edema earlier. A typical recommendation reads: "Findings consistent with an inflammatory arthropathy in a rheumatoid distribution. Recommend serologic correlation (RF, anti-CCP, ESR, CRP) and rheumatology referral. Flexion and extension lateral cervical views are recommended before cervical manipulation to assess atlantoaxial stability."

Juvenile idiopathic arthritis

Juvenile idiopathic arthritis (JIA) is arthritis lasting at least six weeks with onset before age 16. Subtypes include oligoarticular (the most common, four or fewer joints, often the knee, with ANA positivity and a high risk of silent uveitis requiring ophthalmic screening), polyarticular (RF negative or RF positive, the latter resembling adult RA), systemic (Still disease, with quotidian fever, rash and organomegaly), enthesitis related arthritis (HLA-B27, the juvenile spondyloarthritis) and psoriatic. Most children are RF negative.

Because the skeleton is still growing, the hallmarks of JIA are growth disturbances produced by chronic hyperemia:

• Epiphyseal overgrowth (ballooned epiphyses) and accelerated maturation, often with limb length discrepancy (a longer limb early from hyperemia; shortening later from premature physeal closure).
• At the knee, a widened intercondylar notch and squared inferior patella; the same findings occur in hemophilic arthropathy, the main differential.
• Periostitis of the phalanges and metacarpals, and brachydactyly from early physeal fusion.
• Joint space loss and erosions appear late because the cartilage is thick in children; ankylosis is more common than in adult RA, especially in the carpus and tarsus.
• In the cervical spine, ankylosis of the apophyseal joints, classically at C2 to C3, with hypoplastic vertebral bodies and discs at the fused levels (bodies small in both height and AP diameter), plus atlantoaxial subluxation. Mandibular underdevelopment (micrognathia) may accompany temporomandibular involvement.

Section 3Seronegative spondyloarthropathies

What the group shares

The seronegative spondyloarthropathies (spondyloarthritides) are ankylosing spondylitis, psoriatic arthritis, reactive arthritis and enteropathic arthritis (with Crohn disease or ulcerative colitis). They are "seronegative" because rheumatoid factor is absent, and they share a strong association with HLA-B27, inflammatory back pain, sacroiliitis, peripheral arthritis that favors the lower limb, dactylitis, anterior uveitis and, above all, enthesitis: inflammation where ligaments, tendons and joint capsules attach to bone. Enthesitis explains why these diseases make bone as well as destroy it. Where RA erodes and stops, the spondyloarthropathies erode and then heal with new bone: fluffy periostitis, enthesophytes ("whiskering"), syndesmophytes and eventually ankylosis. Bone density is preserved early (osteoporosis appears late in ankylosing spondylitis from immobility).

Sacroiliitis: views and grading

The sacroiliac joint is the first and most characteristic target. Its lower two thirds are synovial and its upper third is ligamentous. Inflammation begins on the iliac side, where the cartilage is thinner. The sequence on radiographs is: blurring and loss of the white subchondral line; small erosions that make the joint look paradoxically wider (pseudowidening) with a "postage stamp" serrated margin; reactive sclerosis, more on the iliac side; then bridging and complete ankylosis, after which the sclerosis fades and the joint becomes a faint ghost line. The joint is best seen on an AP view angled cephalad (about 15 to 30 degrees, the Ferguson view) or with oblique views; MRI (STIR sequences showing bone marrow edema) detects sacroiliitis years before radiographs and defines nonradiographic axial spondyloarthritis. CT shows erosions well but with substantial radiation.

Under the modified New York criteria, radiographic sacroiliitis counts toward ankylosing spondylitis when it is bilateral grade 2 or higher, or unilateral grade 3 or 4. The distribution of sacroiliitis is itself a diagnostic clue.

Ankylosing spondylitis

Ankylosing spondylitis (AS) begins in late adolescence or early adulthood, is more common in men, and over 90% of white patients with AS carry HLA-B27. The radiographic progression is orderly: bilateral symmetric sacroiliitis first, then ascending spinal disease starting at the thoracolumbar and lumbosacral junctions.

• Romanus lesions: erosions at the anterior corners of the vertebral bodies that heal with sclerosis, the shiny corner sign.
• Squaring of the vertebral bodies on the lateral view, as corner erosions and periosteal bone fill in the normal anterior concavity.
• Marginal syndesmophytes: thin, vertical ossification of the outer annulus arising at the body corners, bilateral and symmetric, eventually bridging every level to form the bamboo spine.
• Ossification of the facet capsules and of the interspinous and supraspinous ligaments: three vertical lines on the AP view (trolley track sign), or a single central line (dagger sign).
• Andersson lesion: a destructive spondylodiscitis of the disc and endplates, often from a stress fracture through an otherwise fused segment; it mimics infection.
• Hip disease is common, bilateral and symmetric, with concentric (axial) narrowing and a collar (ring) osteophyte at the head neck junction. Enthesitis produces whiskering at the iliac crests, ischial tuberosities and trochanters, and erosion with spurring at the calcaneus.
• Late osteoporosis of the fused spine sets up the most dangerous complication: the transverse (carrot stick or chalk stick) fracture, usually cervical or cervicothoracic, after minor trauma, unstable because it passes through all three columns, often missed on radiographs, and frequently associated with epidural hematoma and cord injury.

Psoriatic, reactive and enteropathic arthritis

Psoriatic arthritis keeps normal bone density, which helps separate it from RA in the hand; its erosions start at the margins and extend centrally until the proximal bone tapers to a pencil and the distal base cups around it. Severe disease produces arthritis mutilans with telescoping digits. Reactive arthritis looks like psoriatic arthritis moved to the foot. In enteropathic arthritis, peripheral joint flares track bowel activity, but the spinal and sacroiliac disease progresses independently of it, even after colectomy.

Section 4Crystal and metabolic arthropathies

Gout

Gout is inflammation caused by monosodium urate crystals, which form when urate exceeds its solubility (about 6.8 mg/dL) in tissue. It affects mostly men over 40 and postmenopausal women, and is promoted by renal underexcretion, diuretics, alcohol (especially beer), purine rich meat and seafood, fructose, obesity and high cell turnover (myeloproliferative disease, chemotherapy). The first attack is usually monoarticular at the first MTP joint (podagra), followed by the midfoot, ankle, knee, olecranon bursa and hand. Serum urate may be normal during an acute attack; the diagnosis is made by aspiration showing needle shaped, negatively birefringent crystals.

Radiographs are normal or show only soft tissue swelling for the first several years. Chronic tophaceous gout then produces a distinctive picture:

• Asymmetric, polyarticular distribution with a lower limb predominance; the first MTP is the classic site.
• Tophi: lumpy, eccentric soft tissue masses, slightly denser than muscle, occasionally calcified.
• Punched out erosions that are juxta-articular or away from the joint, with sclerotic margins and an overhanging edge (the Martel sign), where the cortex is lifted around an adjacent tophus.
• Joint space preserved until late, and bone density normal, both unlike RA.
• Olecranon and prepatellar bursal swelling; dual energy CT can map urate deposits directly.

Calcium pyrophosphate deposition (CPPD)

CPPD disease is caused by rhomboid, weakly positively birefringent calcium pyrophosphate dihydrate crystals. It is mainly a disease of older adults, and when it appears before about 55 it should prompt a search for hyperparathyroidism, hemochromatosis, hypomagnesemia or hypophosphatasia. It presents as asymptomatic chondrocalcinosis, as acute attacks (pseudogout, most often of the knee or wrist), or as a chronic degenerative arthropathy.

• Chondrocalcinosis: calcification of fibrocartilage (knee menisci, the triangular fibrocartilage of the wrist, the pubic symphysis, the acetabular and glenoid labra) and of hyaline cartilage, where it appears as a thin line parallel to the subchondral bone.
• Pyrophosphate arthropathy: looks like OA (narrowing, sclerosis, osteophytes) but in unusual places: the radiocarpal joint, the second and third MCP joints (with hook like osteophytes), isolated patellofemoral disease, the elbow and the glenohumeral joint. Large subchondral cysts and rapid fragmentation are characteristic.
• SLAC wrist (scapholunate advanced collapse): scapholunate dissociation with the capitate migrating proximally between them, a classic CPPD pattern.
• Crowned dens: calcification around the dens on CT, causing acute neck pain and stiffness with fever that can mimic meningitis or giant cell arteritis.

Hydroxyapatite deposition disease (HADD)

Calcium hydroxyapatite deposits in tendons and bursae rather than cartilage. The radiographic sign is a homogeneous, cloudlike, amorphous periarticular calcification without trabeculae. The supraspinatus tendon just above the greater tuberosity is the most common site (calcific tendinitis); others include the gluteal insertions at the greater trochanter and the flexor carpi ulnaris near the pisiform. The acute phase is very painful, and the calcification may become fluffy and then resorb. Longus colli calcific tendinitis deserves special mention: amorphous calcification anterior to C1 and C2 with prevertebral soft tissue swelling produces acute neck pain, stiffness and painful swallowing, and must be distinguished from a retropharyngeal abscess. Milwaukee shoulder is a destructive hydroxyapatite arthropathy of older women with a large effusion and rotator cuff loss.

Hemochromatosis

Iron overload (hereditary HFE mutation, more often expressed in men, or secondary to transfusions) produces an arthropathy that resembles CPPD. Hallmarks: narrowing, subchondral cysts and hook like (drooping) osteophytes on the radial side of the second and third metacarpal heads, with chondrocalcinosis and generalized osteopenia. Unlike CPPD it may also involve the fourth and fifth MCP joints and progresses more uniformly. Recommend iron studies (transferrin saturation over about 45%, ferritin) and physician referral, because early phlebotomy prevents cirrhosis, diabetes and cardiomyopathy even though the arthropathy responds poorly.

Neuropathic (Charcot) joint

Loss of protective pain and position sense allows repeated unrecognized trauma that destroys a joint. The classic radiographic features are the six Ds: Density increased (subchondral sclerosis), Destruction of articular surfaces, Debris (bone fragments), Disorganization, Dislocation and Distension (large effusion). The hypertrophic form shows sclerosis and debris; the atrophic form shows resorption that ends the bone as if surgically amputated, typical of the shoulder in syringomyelia. The site points to the cause.

In a diabetic foot, the main differential is osteomyelitis; infection favors involvement beside an ulcer and marrow signal away from the joint on MRI, while neuropathic change centers on the joints of the midfoot.

Section 5Approach to the arthritic film

A systematic search: ABCDS

Arthritis is diagnosed from a pattern, so read every arthritic film in the same order and write down each feature before naming a disease. The widely taught ABCDS scheme works for any joint.

Two summary contrasts organize most of the answers. Productive versus destructive: OA, DISH and the spondyloarthropathies add bone, RA removes it. Symmetric and proximal versus asymmetric and distal: RA is the former; OA (distal rows), psoriatic and reactive arthritis and gout tend toward the latter. Monoarticular disease is its own category: in a single inflamed joint, think first of infection, then crystals and trauma, and remember that septic arthritis destroys cartilage and subchondral bone within days, so a rapidly narrowing single joint with indistinct cortex is an emergency.

Writing the report

The imaging report has three parts. Findings describe what is seen, in order, without diagnosis (alignment, density, joint spaces, erosions, new bone, soft tissues, incidental findings). The impression gives the most likely diagnosis and a short differential, in order of likelihood, with any finding that affects safety stated plainly. Recommendations say what should happen next: laboratory tests, further views or advanced imaging, referral, and precautions for care. For arthritic films the recommendation almost always includes laboratory correlation (RF, anti-CCP, ESR, CRP, HLA-B27, uric acid, iron studies, as the pattern suggests), and for the spine it states whether stability views are needed and whether manipulation of a region is contraindicated or should be modified.

Worked interpretation cases

Words worth knowing