Rheumatoid arthritis (RA) is the prototype of polyarticular inflammatory disease, affecting ~1% of the world population. Other forms of arthritis specifically in children affect a single or very few joints. Pigmented villonodular synovitis (PVNS) is a tumour that occurs inside the synovial membrane, with a high tendency of recurrence despite surgery. The use of biological drugs has been a major advance for the treatment of RA. However, ~30% of RA patients do not respond to these drugs, which are expensive and can cause severe side-effects1, 2. Intra-articular treatment with radio-isotopes for instance, has been effective in RA and PVNS but has major restrictions related to the use of radio-active material. There is therefore a necessity for improvement or alternatives in the local treatment of arthritis.