On the right at C4/5 is a well circumscribed heterogeneously calcified/ossified mass which measures 20 x 17 x 18 mm which appears to arise from and is continuous with the articular pillar, with involvement of the C4/5 facet joint which appears fused. Slight sclerosis is demonstrated along the right side at the C4 and C5 vertebral bodies.
The superior aspect of the mass remodels the pedicle of C4, and anteriorly results in severe stenosis of the neural exit foramen. Surrounding this mass is stranding and displacement of adjacent fat, and low density changes in the adjacent muscles consistent with the florid oedema seen on MRI. There is no convincing extension into the epidural space, and the cervical cord appears unremarkable and the canal capacious.
The remainder of the cervical spine is unremarkable.
Conclusion:
Calcified/ossified mass arising from the right C5 articular pillar, in the setting of previous trauma and probably a previous superior articular facet undisplaced fracture likely represents post-traumatic myositis ossificans. An underlying tumour is thought much less likely, but cannot categorically excluded. If clinical concern exists then a CT guided bone biopsy can be safely performed.