Fig. 9.

Facet involvement. a Axial CT section demonstrates incongruity at the facet joint (solid white arrow) due to an involved enlarged facet of one vertebra articulating with an uninvolved non-expanded adjacent vertebral facet. b Axial CT section in a different patient demonstrates advanced facet joint arthropathy (dashed white arrows) across two pagetic facets with complete loss of joint space and new bone formation contributing to spinal canal stenosis. This can progress to fusion across the facet joints