Fusion of the fourth digit of both hands in a patient with psoriatic arthritis

Bony fusion across a joint or ankylosis, often noted in the spine of patients with axial spondyloarthritis (SpA) and with axial involvement in psoriatic arthritis (PsA), can also take place in the peripheral joints of PsA patients. Note the fusion across the fourth proximal interphalangeal (PIP) joints in this patient with PsA. These findings can also be observed in the interphalangeal joints of the toes. Interestingly, bony ankylosis is not a feature of rheumatoid arthritis, where excess pathologic resorption in the form of erosions is not accompanied by new bone formation.