Psoriatic Arthritis

For years psoriatic arthritis was considered part of the spectrum of rheumatoid arthritis. The classification of psoriatic arthritis as a “rheumatoid variant” persists today. However, the radiographic manifestations, along with clinical and laboratory data, establish psoriatic arthritis as a separate and distinct articular disorder. Psoriatic arthritis occurs in 5 to 8 percent of patients with severe and longstanding psoriatic skin disease. However, the arthropathy may coincide with or antedate the appearance of skin disease. In these patients, the radiographic examination becomes the determinate diagnostic study. The distinguishing radiographic features are as follows:

• 1.

  Fusiform soft tissue swelling

• 2.

  Maintenance of normal mineralization

• 3.

  Dramatic joint space loss

• 4.

  Bone proliferation

• 5.

  “Pencil-in-cup” erosions

• 6.

  Bilateral asymmetrical distribution

• 7.

  Distribution primarily in hands, feet, sacroiliac (SI) joints, and spine, in decreasing order of frequency

Although psoriatic arthritis differs from rheumatoid arthritis radiographically in many ways, the most significant difference is the presence of bone proliferation.

The hands

The hands are most commonly involved in psoriatic arthritis. Although there may be nonspecific periarticular fusiform soft tissue swelling, there may be soft tissue edema beyond the joint, causing swelling of the entire digit in approximately 25 percent of patients. This dactylitis is described as sausage-like or resembling a cocktail hot dog ( Fig. 10-1 ) on clinical examination and is highly specific for a spondyloarthropathy. Juxta-articular osteoporosis may occur in the early phases of the disease; however, it is transient. Normal mineralization is usually maintained even in the presence of severe erosive disease. Erosions occur initially at the margins of the joint but with time progress to involve the central area ( Fig. 10-2 ). The erosion may become so extensive, destroying so much of the underlying bone, that the joint space may actually appear to be widened. The ends of the bones may become pointed, appearing as if destroyed by a pencil sharpener. The bone articulating with the pointed bone may become saucerized through erosion, producing the classic “pencil-in-cup” or “cup-and-saucer” appearance ( Fig. 10-3 ). Erosions may also be appreciated involving the distal tufts of the fingers producing a pattern of acro-osteolysis.

Bone proliferation is one of the most important features of psoriatic arthritis and is almost always present in some form. Bone proliferation takes place in four areas: adjacent to erosions, along shafts, across joints, and at tendinous and ligamentous insertions. The bone proliferation adjacent to erosive changes is observed as irregular excrescences with a spiculated, frayed, or fluffy appearance. With time these excrescences become well-defined bone ( Fig. 10-4 ). Bone proliferation may be observed along shafts as a periostitis ( Fig. 10-5 ). Initially it is exuberant and fluffy in appearance. Eventually it becomes solid new bone along the shaft of the phalanx, causing the widened appearance to the phalanx (see Fig. 10-1 ). Bone ankylosis across a joint is a common occurrence in distal interphalangeal (DIP) and proximal interphalangeal (PIP) joints (see Fig. 10-2 and 10-6 ). Bone proliferation occurring at tendinous and ligamentous insertions in the hand and wrist will be seen as a continuation of the periosteal response.

In the hand, psoriatic arthritis has three different patterns of distribution. The first pattern is primarily DIP and PIP involvement, with relative sparing of the metacarpophalangeal (MCP) and carpal joints ( Fig. 10-7 ). The second pattern is ray involvement, wherein one to three fingers will be involved in all joints while the other fingers are spared. The carpal bones may or may not be involved ( Fig. 10-8 ). The third pattern is similar to rheumatoid arthritis. In this distribution, other features will distinguish psoriatic arthritis from rheumatoid arthritis ( Fig. 10-9 ). There is usually DIP involvement or evidence of bone proliferation ( Fig. 10-10 ).

The feet

The radiographic changes described in the hand are also found in the feet. An entire digit may be swollen and resemble a sausage ( Fig. 10-11 ). Although there may be early juxta-articular osteoporosis, generally the mineralization is maintained. Severe erosive changes with pencil pointing are observed. Extensive destruction of the interphalangeal (IP) joint of the great toe is more common in psoriatic arthritis than in any other arthritis ( Fig. 10-12 ). Bone proliferation is identified as periostitis, new bone formation around erosions, and bone ankylosis of IP joints ( Fig. 10-13 ). Bone proliferation around the distal phalanx of a toe may produce an “ivory” phalanx ( Fig. 10-14 ).

As with the hand, three different patterns of distribution occur in the foot. Distal IP and PIP involvement is common (see Fig. 10-12 ); however, metatarsophalangeal (MTP) involvement is more common than MCP involvement ( Fig. 10-15 ). Again, one to three rays may be affected, with the other rays being spared ( Fig. 10-16 ).

Radiographic changes are frequently seen on the posterior and inferior aspects of the calcaneus. Erosion and bone proliferation occur at the site of the Achilles tendon attachment posteriorly and superiorly ( Fig. 10-17 ). Similar changes occur at the attachment of the plantar aponeurosis inferiorly, creating irregular and ill-defined spurs ( Fig. 10-18 ). The spurs tend to point upward toward the calcaneus rather than downward along the course of the plantar aponeurosis as a normal heel spur points. Occasionally the entire inferior aspect of the calcaneus may be involved.