This is an observational study aiming to study the prevalence and risk factors for osteoporosis and vertebral fractures in patients with ankylosing spondylitis attending three Rheumatology clinics in Western Sweden.

Background Ankylosing spondylitis (AS) is a common inflammatory rheumatic disease with a prevalence of 0.5-1.0%. Men are more commonly affected by the disease as compared to women, ratio 2.6-4:1. Pain in the back is a frequent symptom of debut. This pain is often associated with sacroilitis. In later stages also the lumbar, thoracic and cervical spine are hit by the disease. Peripheral joints, eyes, heart, lungs and urinary tract may also be influenced.

Osteoporosis in AS In AS the risk of osteoporosis is increased. However, this field has not yet been significantly studied probably due to several reasons such as the predominance of men with the disease and men are more seldom investigated for osteoporosis compared to women. When AS progresses syndesmophytes of the spine are developed which makes it difficult to assess bone mineral density (BMD) correctly with the conventional method, dual energy x-ray absorptiometry (DXA). Fractures in the spine are easy to foreseen since the pain of the patient might be misjudged to be related to increased disease activity. Fractures are also overlooked in radiographs in AS. AS is associated with both increased bone formation and increased bone resorption. The bone remodeling process in the spine renders the spine less flexible and stiffer and as a consequence also a quite small trauma may result in a fracture. These fractures are often instable risking injuring the spinal cord and nerves.

Aims of the study

• To investigate the prevalence of osteoporosis and osteoporosis related fractures in patients with AS in Western Sweden.
• To identify risk factors for osteoporosis and osteoporosis related fractures in AS.
• To study how fractures in the spine may influence the pain in the back, the flexibility of the spine and the posture of the patients.
• To study to which extent the patients with AS are investigated and treated for osteoporosis.
• To investigate which method is most reliable for measuring BMD in AS.

Design and methods This is an observational study consisting of questionnaires, examination of the patients, blood tests, measures of BMD with different techniques, DXA, both frontal-dorsal and lateral projections, Quantitative Computed Tomography (QCT) and Xtreme CT. Patients with AS from three rheumatology clinics in western Sweden with definite AS will be invited to participate. It is estimated that about 250 patients will be included in the trial. All patients gave informed written consent according to the Declaration of Helsinki. The study has been approved by the Regional Ethics Committee.

Inclusion Criteria:

• Definitive Ankylosing spondylitis according to the New York criteria
• At least 18 years old

Exclusion Criteria:

• Pregnant
• Dementia
• Not able to understand, speak or read Swedish

Patients with definitive ankylosing spondylitis attending three rheumatology clinics in Western Sweden.