Arthritis occurs in about 5% of psoriatics. Several patterns are recognized. Distal arthritis involves the terminal interphalangeal joints of the toes and fingers, especially those with marked nail changes (Fig. 5.14). Other patterns include involvement of a single large joint; one which mimics rheumatoid arthritis and may become mutilating (Fig. 5.15); and one where the brunt is borne by the sacro-iliac joints and spine. Tests for rheumatoid factor are negative and nodules are absent. In patients with spondylitis and sacroiliitis there is a strong correlation with the presence of HLA-B27.