Odontoid pannus formation in a patient with ankylosing spondylitis causing atlanto-axial instability.

Ankylosing spondylitis is one of the commonest inflammatory diseases of the axial skeleton and can be complicated by atlanto-axial instability. This serious and likely underestimated complication can be easily overlooked. However, there are clear features which can help alert suspicion to initiate the appropriate investigations with imaging that is very effective at diagnosing and assessing this complication. The authors report an unusual case where odontoid pannus formation, akin to that seen in rheumatoid arthritis, was the underlying cause.