Psoriatic arthropathy tends to occur in patients who already have skin manifestations of psoriasis, especially nail involvement (90%). This is a seronegative spondyloarthropathy hence patients are frequently HLA-B27 positive and usually Rheumatoid factor negative. Unlike most other arthropathies there is no sex predilection. Hand involvement is more common than feet involvement (as opposed to Reiter’s another seronegative arthritis). The hand pattern varies from a symmetric polyarthropathy, mimicking rheumatoid, to an asymmetric oligoarthropathy. Involvement of the distal interphalangeal joints typically exceeds that of the proximal interphalangeal joints or metacarpophalangeal joints. The erosive arthritis involves both the articular and juxta-articular joint margin. Resorption of the terminal phalanges is common and in combination with joint erosions this can become pronounced resulting in the “pencil-in-cup” appearance. In turn malalignment of the joint with angulation and even telescoping of phalanges often referred to as “arthritis mutilans”. Ankylosis can occur. Soft tissue swelling of an entire digit, a “sausage digit”, is also recognised.
Despite the severity of disease in contradistinction to rheumatoid there is usually no osteoporosis present. Blurring of the phalanges may be seen due to periosteal reactions or whiskering at sites of tendinous insertion due to enthesopathy. These effects can be pronounced and result in “cloaking” of the phalanx, or an ivory phalanx, more typically in the foot than the hand.
Apart from rheumatoid arthritis which is differentiated by distribution, and overall osteoporosis with periarticular accentuation one can also consider erosive osteoarthritis as a differential diagnosis. In erosive osteoarthritis “gull wing” central erosions are present that are differentiated from the peripheral bare erosision in psoriatic arthropathy sometimes referred to as “mouse ears”.