The risk of osteoporosis is increased in AS. 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.