PSORIATIC ARTHRITIS The hallmark of psoriatic arthritis is a combination of productive and aggressive erosive changes with severe joint space narrowing. Normal materialization is maintained.

1. Distribution:
There is asymmetric bilateral distribution, primarily involving hands. In addition, feet, SI joints, and spine are involved in decreasing order of frequency. There are three patterns of involvement of psoriatic arthritis in the feet. First pattern primarily involves DIP, PIP, and MTP. Second pattern is single ray involvement where all the joints of one digit are involved while other fingers are spared. The third pattern is similar to rheumatoid arthritis distribution. Fusiform soft tissue swelling involving a single ray can occur giving the appearance of 'sausage digit'.

2. Erosion pattern:
Erosions appear peripherally and progress to involve central area. Erosions are described as having fuzzy margins. 'Pencil-in-cup' deformity occurs when distal head of a bone becomes pointed appearing as if it had been sharpened and the adjacent articulating surface becomes 'saucerized' through erosions. Resorption of terminal tufts may also occur. In the feet, erosions with fuzzy margins and bony proliferation may be observed along the posterior and inferior aspect of the calcaneus at the attachment of Achilles tendon and plantar aponeurosis. Bone proliferation may occur adjacent to erosions, along shafts, across joints, and at tendinous or ligamentous insertion. Bone proliferation along the shaft is observed as periostitis, usually described as fluffy. Bone proliferation around distal phalanx of the great toe leads to 'ivory phalanx" appearance. Bone production adjacent to erosions leads to appearance of "mouse ears" .

3. Differential diagnosis:
The presence of bone proliferation and bony ankylosis, and the lack of osteoporosis are common finding of all seronegative arthritis and are useful in the differentiation of psoriatic arthritis from rheumatoid arthritis. The distinction among seronegative arthritis is based on the distribution, psoriatic arthritis involves hands and feet, Reiter's has a predilection for the lower extremity, while ankylosing spondylitis has a predilection for the axial skeleton with only rare involvement of the small joints of the appendicular skeleton.