Imaging methods in rheumatology: imaging in psoriasis arthritis (PsA)

Conventional radiography is still the standard method of imaging in PsA since it displays many joints at the same time, thereby allowing different types of joint involvement to be recognized. Moreover, thanks to the high resolution of radiography, bony changes in a single joint are depicted in a brilliant way. Several features of psoriatic arthritis allow the distinction from rheumatoid arthritis, including the frequent involvement of the distal interphalangeal joints, asymmetry of joint involvement, axial involvement of finger joints, oligoarticular involvement, however, symmetric polyarthritis is also possible. At the level of the single joint, there are signs of severe destructive changes potentially leading to mutilation and at the same time signs of periostal bone proliferation and ankylosis may be present. Bony proliferation and/or osteolysis are not restricted to the joint region but can affect also the total phalanx with bone apposition or concentric osteolysis which may lead to a complete disappearance of phalanxes. For purposes of quantification of radiographic changes scoring methods are used that were originally developed for rheumatoid arthritis. So far, there is only one validated scoring method that was specifically designed for PsA and that takes into account both features of PsA, damage as well as proliferation of bone. In contrast to conventional radiography, MRI and sonography are able to visualize inflammatory processes within the soft tissue (joint capsules, tendon sheaths, tendon insertions, etc.), allowing an estimation of disease activity. Scintigraphy is nonspecific and can only be used to detect clinically silent inflammatory spots. The relatively frequent spinal (axial) involvement is similar to that seen in ankylosing spondylitis. However, unilateral sacroiliitis, asymmetry of syndesmophytes and development of parsyndesmophytes may distinguish PsA from ankylosing spondylitis. While conventional radiography demonstrates the bony consequences of inflammation in the spine, MRI also shows the active inflammatory changes in sacroiliacal joints and vertebrae.