Psoriatic arthritis is a systemic inflammatory disease that involves joints and extra-articular manifestations. Episodes of arthritis are often insidious, but can also be acute. Most cases of psoriatic arthritis present as polyarthritis, in which all peripheral joints of the psoriatic arthritis can be involved. Most patients with psoriatic arthritis have typical psoriatic scaly skin lesions, and nail lesions, including pits, longitudinal valleys and nail fragments, are the only psoriatic skin lesions that are clearly associated with the development of psoriatic arthritis. It is the only clinical manifestation of psoriatic skin clearly associated with the development of psoriatic arthritis. Nail damage is seen in 90% of patients with psoriatic arthritis and only in 41% of patients without arthritis.