Active spinal changes may, like abnormalities of the sacroiliac joint, have two different appearances.
There can be mainly osseous changes localised adjacent to the vertebral plates or anterior in the vertebral body beneath the attachment of the longitudinal ligament.
Bone marrow oedema at the corner of vertebral bodies and signs of new bone formation around the discs can also occur and may be indistinguishable from changes occurring as part of reactive arthritis.