Hidden flexion injury of the cervical spine in ankylosing spondylitis

A 36.year-old man, who had been diagnosed as suffering from ankylosing spondylitis for the past ten years, was referred to our department for the evaluation and management of increasing flexion deformity of the cervical spine, 4 months after injuring his neck. He had fallen backwards and his occiput had hit the ground. Soon after the injury, he developed pain in the nape of the neck radiating to his right arm and paraesthesia in the C-6 dermatome. He attended another hospital where AP and lateral radiographs of the cervical spine were reported as normal apart from showing evidence of ankylosing spondylitis (Figure I). Over the subsequent 4 months the patient observed increasing pain and flexion deformity of his neck, and also revealed that supporting the chin with his hand partly relieved the pain and helped to correct his deformity. His neurological symptoms did not deteriorate. Repeated radiographs of the cervical spine showed forward subluxation of the C-5 vertebral body over C-6 and widening of the interspinous space in an ankylosed cervical spine (Figure 2). Review of the initial radiographs showed signs of the hidden flexion injury’. A magnetic resonance imaging (MRI) scan was performed. Interestingly the subluxation had reduced itself during the MRI examination. No extradural haematoma at the site of subluxation was identified (Figure 3). At operation, through an anterior approach, a ruptured anterior longitudinal ligament was found at the level of C-5/6. The disc structure was normal (the nucleus pulposus is not affected by ankylosing spondylitis)8. The disc was excised and a tricortical iliac bone graft was punched into the space. The C-5/6 vertebral bodies were fixed with a titanium locking screw plate (TLSP ~ Synthes)“. Paraesthesiae improved and radiographs taken 3 months post-operatively showed fusion of C-5/6 (Figure4).