Osteoarthritis and psoriatic arthritis do share some simple clinical characteristics. Both commonly affect the spine. More importantly, in the hand both osteoarthritis and psoriatic arthritis show a very strong tendency to afflict the distal interphalangeal joints.
“So when you look at it from a very simplistic, clinical perspective, there are strong similarities between psoriatic arthritis and osteoarthritis,” said Dr. Dennis McGonagle of the University of Leeds (England).
Dr. McGonagle, Dr. Ai Lyn Tan, and their colleagues have used high-resolution magnetic resonance imaging to look more closely at how psoriatic arthritis (PsA) and osteoarthritis (OA) affect the distal interphalangeal (DIP) joints (Arthritis Rheum. 2006;54:1328–33). This type of imaging involves conventional 1.5-T MRI scanners coupled with coils that are designed specially for hand imaging that give very high quality images.
Seeing patients with definite OA or PsA, “the most striking abnormalities that we saw were related to the ligaments and the adjacent attachments or entheses,” he said.
Ligament and tendon insertions were involved in patients with either disease; however, enhancement on scanning (evidence of inflammation) of these structures was greater in patients with PsA. In these patients, ligament origins/insertions and extensor tendon insertions appeared to be the epicenter of the inflammatory response with diffuse involvement of adjacent structures. While ligaments tended to be thickened and abnormal in patients with OA, even when the cartilage appeared normal, there was less postcontrast enhancement compared with patients in the PsA group.
The ligaments are sites of very high mechanical stress, particularly in small joints. Both OA and PsA “are localized to sites of very high mechanical stress and 'wear and tear.' A simplistic explanation is that this mechanical stress leads to joint degeneration in osteoarthritis but joint inflammation in psoriatic arthritis,” said Dr. McGonagle.
“Inflammation is well recognized in osteoarthritis but it's thought to be secondary to mechanical factors. Psoriatic arthritis is much more an inflammatory disease. What we speculate is that normal or low levels of microdamage and repair of joints—that damage is misinterpreted by the immune system as some sort of severe damage and then you get this autoimmune reaction.”
There is evidence that most normal people walk around all of the time with low-grade microdamage of the joints, based on studies of insertions and ligaments, carried out in conjunction with Dr. Mike Benjamin of Cardiff University, Wales.
However, in the psoriatic phenotype, the body may misinterpret that damage, resulting in an overexaggerated immune response. After age 50 years, the cumulative microdamage to the joints becomes more pronounced, resulting in the secondary and less severe inflammation that accompanies wear and tear. This would fit with known data that PsA is a disease of younger individuals, while OA is a disease of older individuals said Dr. McGonagle.
It's unclear why the immune system is inadvertently activated in patients with PsA or why cumulative microdamage results in OA in some older individuals but not all. However, the hypothesis that microdamage may trigger both PsA and OA provides fertile ground for research. It is especially relevant for an appreciation that inflammation in the joints may not be primarily the result of immune system malfunction but could be a result of some intrinsic problem with the joints' response to normal locomotion.
Joint space in the distal interphalangeal PsA joint (left) is well preserved versus the OA joint (right). Photos courtesy Dr. Dennis McGonagle/Dr. Ai Lyn Tan
