The differences between psoriatic arthritis and osteoarthritis.

The psoriatic arthritis is a chronic inflammatory rheumatism (CIR) that is part of the spondyloarthritis group. It should not be confused with osteoarthritis, which is a joint disease. It manifests as a result of the immune system's reaction against ligaments, tendons, and joints. It is estimated that 30% of patients suffering from this disease also have another form of psoriasis.

There are three types of psoriatic arthritis distinguished:

• The axial form: affects the spinal column, the joints of the thorax, as well as those that connect the pelvis and the lumbar vertebrae;

• The peripheral joint form : affects the knees, hips, shoulders, fingers, or toes. This is the most common form;

• The form that affects the heels and elbows.

It is possible that some patients affected by psoriatic arthritis may have one or more of these forms at the same time.

The causes of this disease are still poorly defined. However, it has a hereditary nature. In 40% of cases, one of the patient's family members suffers from arthritis or psoriasis.

Osteoarthritis is a joint disease that results in the destruction of cartilage. The most frequently affected joints are the knee, hip, and those of the spine. However, it is possible that other joints, such as the shoulder, ankle, and wrist, may also be affected. The main morphological characteristic of osteoarthritis is a degenerative degradation of the cartilage that develops slowly with the onset of episodic synovitis.

Furthermore, changes occur in the bones, synovial fluid, and muscles. In 80% of patients with osteoarthritis, movement is limited to some extent. This leads to a decrease in physical abilities and 25% of patients are unable to perform their primary daily life activities.

Due to similarities in the distribution of joints involved in psoriatic arthritis and osteoarthritis, it can be challenging to distinguish between them. Both osteoarthritis and psoriatic arthritis can affect similar joint regions such as the DIP (distal interphalangeal) and PIP (proximal interphalangeal) joints in the hands, cervical and lumbar regions, the spine, and the large joints of the lower limbs. Although osteoarthritis and psoriatic arthritis share some similarities, they differ in other aspects.

• The causes: Both psoriatic arthritis and osteoarthritis can be linked to genetic factors. However, osteoarthritis is also caused by lifestyle-related factors, particularly for athletic or occupational reasons when the joints are excessively used over many years. Osteoarthritis is also related to aging. Its prevalence increases with age and primarily affects individuals over the age of 65. Repeated traumas such as occupational movements or accidents like fractures can accelerate this disease.

• Molecular effects: In the case of psoriatic arthritis, the IL-23/IL-17 and TNF pathways are involved in its onset. Interleukin 23 activates Th17 lymphocytes and the secretion of interleukin 17. These lymphocytes return to the dermis and are the origin of a complex inflammatory response.

  In the case of osteoarthritis, it is primarily the interleukin IL-1 that is not expressed in normal cartilage but is expressed in arthritic cartilage. It induces the degradation of matrix components by increasing
  the expression and activity of metalloproteinases (MMP) and stimulates the production of free radicals, involved in the apoptosis of chondrocytes, the resident cells of cartilage. It can trigger the production of pro-inflammatory prostaglandins, leading to inflammation. NGF also plays a crucial role in the chronic pain of osteoarthritis, and this is reflected in the consistently elevated levels of NGF in osteoarthritis compared to patients with psoriatic arthritis. NGF (nerve growth factor) binds to cellular receptors and sensitizes nociceptive terminals.

• The treatments: Anti-TNF agents used in the case of psoriatic arthritis have proven to be less effective in treating and relieving the pains of osteoarthritis. New studies need to be conducted to understand the reasons for this failure. Treatments for osteoarthritis are not yet well defined. A 2019 study showed that the inhibition of interleukin IL-1 did not constitute an effective treatment. The reasons for this ineffectiveness are still not well understood today. However, it is possible to alleviate osteoarthritic pain with analgesic medications or the intake of non-steroidal anti-inflammatory drugs.

• CHEVALIER X. & al. Traitements ciblés anti-cytokines dans l’arthrose. Académie Nationale de Médecine (2006).

• CHANDRAN V. & al. Serum-based soluble markers differentiate psoriatic arthritis from osteoarthritis. Annals of the Rheumatic Diseases (2019).

• KLOPPENBURG M. & al. Osteoarthritis year in review 2019: Epidemiology and therapy. Osteoarthritis and Cartilage (2019).