Ankylosing spondylitis is characterised by inflammation of the spine and the entheses followed by bone formation. Excessive bone formation in ankylosing spondylitis leads to the formation of bone spurs, such as syndesmophytes and enthesiophytes, which contribute to ankylosis of joints and poor physical function. This process is based on increased differentiation of osteoblasts from their mesenchymal precursors, which allows to rapidly build up new bone. Prostaglandins, bone morphogenic proteins and Wnt proteins play an essential role in this process. By contrast, tumour necrosis factor (TNF) does not appear to be the direct trigger for osteophyte formation in ankylosing spondylitis. The article reviews the current knowledge regarding the mechanisms and clinical role of ankylosis and explains strategies on how to prevent it in patients with ankylosing spondylitis.
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