TNF and arthritic joint disease: Spondylarthritis (SpA) is a chronic form of arthritis that shares with RA both a similar burden of disease (i.e., peripheral arthritis, age of patients, impact on the quality of life, etc....), some similar cytokines pathways (i.e., the TNFa pathway) and similar therapeutical approaches (i.e., leflunomide, methotrexate and TNFa inhibitors).