Introduction Both congenital and acquired aetiologies for Os Odontoideum have been proposed leading to confusion in a medico-legal setting. No large series with CT and MRI features has been reported.

Methods Clinical, radiological, CT and MRI data from 25 cases of cranio-cervical anomalies (including 18 with Os Odontoideum) was collected prospectively. The hypothesis that Os Odontoideum is associated with signs of dysplasia such as arch hypertrophy (as quantified by the arch-peg ratio) and a ‘jigsaw’ like atlanto-dens joint configuration was tested.

Results Only one of 18 cases with Os Odontoideum gave a history of remote trauma. None of the cases with other post traumatic conditions showed arch hypertrophy or dysplasia of the joint. Os Odontoideum was associated with two distinctive features on mid-sagittal CT reconstruction: arch hypertrophy and the presence of a ‘jigsaw sign’. A quantitative definition of arch hypertrophy (an arch/ peg ratio greater than 0.5) provided a sensitive and specific test for Os Odontoideum. This was further enhanced in combination with the jigsaw sign to give a test accuracy of 97%. Motor changes with MRI signs of cord damage were observed in one of 12 cases associated with trauma.

Conclusions The lack of a history of remote trauma and the dyplastic atlanto-dens joint seen on CT are more suggestive of a congenital than a post-traumatic aetiology for Os Odontoideum. Patients with Os odontoideum are able to tolerate moderate to severe levels of injury without sustaining significant acute cord damage.

Degenerative spondylolisthesis is four times more common in women than in men. Although this gender difference has long been recognised there has been no explanation for it. We have examined the radiographs and CT scans of 118 patients over the age of 55 years and of a control group under the age of 46 years.

Our findings confirmed the presence of more sagittally-orientated facet joints in patients with degenerative spondylolisthesis but did not show that the gender difference can be explained by the morphology of the facet joint.

Furthermore, we conclude that the increased angle of the facet joint is the result of arthritic remodelling and not the primary cause of degenerative spondylolisthesis. It is more likely to be due to loss of soft-tissue resilience with subsequent failure of the facet joints which are acting as the last restraints to subluxation.