Ankylosing spondylitis (AS) is one of the sero-negative spondylo-arthropathies.1 This group of arthritides is characterised by specific skeletal imaging findings and, biochemically, by the absence of rheumatoid factor or nodules, and the presence of the HLA-B27 gene.
The lesions are preceded by a Romanus lesion, and are symmetrical and marginal. Calcification of the interspinous ligaments can also be seen (open arrowhead).
About 15% of affected patients show evidence of syndesmophytes. Ossification of the supraspinous and interspinous ligaments produces a characteristic single or double radio-dense sign on the AP view of the spine (Fig. The trolley track sign derives from the 2 lateral lines of ossification representing the apophyseal joint capsules, and the dagger sign from the central line of ossification visible on the AP view. Ankylosing spondylitis: Patterns of radiographic involvement – a re-examination of accepted principles in a cohort of 769 patients.
The second category typically occur near the cervico-thoracic or thoraco-lumbar junctions.
Delay in treatment can cause pseudoarthrosis, which can be seen on conventional X-rays as subchondral sclerosis and vertebral endplate erosions.