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.

Three types of psoriatic arthritis are 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 suffer from one or more of these forms simultaneously.

The causes of this disease are still poorly defined. However, it does have 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 suffering from osteoarthritis, movement is restricted to some extent. This leads to a reduction in physical capabilities and 25% of patients are unable to perform their main 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 sporting or professional reasons when the joints are excessively used over many years. Osteoarthritis is also linked to ageing. Its prevalence increases with age and mainly affects people aged 65 and over. Repeated traumas such as professional gestures 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, causing 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 sensitises nociceptive terminals.

• The treatments: Anti-TNF agents used in the case of psoriatic arthritis have proven to be less effective in treating and alleviating 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 soothe 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).