Ankylosing spondylitis (AS) is a frequent chronic inflammatory rheumatic disease that affects the axial skeleton, starting in the sacroiliac joints and spreading to the spine in most patients. Non-steroidal anti-inflammatory drugs (NSAIDs) are the primary treatment for AS. Even if the use of anti-TNF agents has demonstrated good clinical efficacy in controlling inflammation, in contrast to other conditions such as rheumatoid arthritis and psoriatic arthritis, anti-TNF treatment has failed to demonstrate any benefit on the structural progression of AS, some data even suggesting that it may accelerate the formation of syndesmophytes that seems to be an independent process of TNF. Conversely, NSAIDs inhibit ossification phenomena independently of their anti-inflammatory properties, owing to a specific action on bone formation via prostaglandin inhibition.

Several features suggest that a continuous NSAID therapy is needed, in addition to anti-TNF treatment, to prevent syndesmophyte formation in AS patients.

Inclusion Criteria:

• Patients aged over 18 years old
• Patients who fulfil 1984 modified New York criteria for ankylosing spondylitis:
• SA patients with indications for anti-TNF treatment initiation according to the French society of Rheumatology guidelines.
• Being affiliated to a health insurance system
• Having signed an informed consent form (later than the day of inclusion and before any examination required by the research)

Exclusion Criteria:

• Patients who present another chronic systemic inflammatory disorder, different from ankylosing spondylitis.
• Patients who present contraindications to treatment with NSAIDs.
• Patient with daily corticosteroid treatment at a dosage ≥ 10 mg/day
• Pregnant or breastfeeding women
• Contra-indication to MRI (pacemakers, aneurysm clips, artificial heart valves, ear implants, metal fragments or foreign objects in the eyes, skin or body, claustrophobia) or refusing to undergo MRI
• Women that refuse to an effective contraception method for all the study duration