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

Abstract

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.

Figure 1

(A, B) Lateral view plain x-ray of C-spine showing increased anterior atlanto-odontoid distance of approximately 8 mm.

Figure 2

A (Saggital) and B (Axial) show a T2-weighted MRI image of C-spine confirming atlanto-axial subluxation with marked pannus formation and synovitis around the odontoid peg. There was no spinal cord compression or evidence of odontoid erosions.

Figure 3

Saggital CT scan of C-spine not showing any evidence of odontoid erosions.

Figure 4

Dynamic plain x-rays of the C-spine showing an atlanto-odontoid distance of 1.1 cm on flexion and 0.65 cm on extension, consistent with atlanto-axial instability.

Figure 5

Lateral view plain x-ray of C-spine after posterior spinal fusion surgery.