Septic sacroiliitis can be difficult to differentiate from SpA sacroiliitis by radiography, but MRI demonstrates oedema and contrast enhancement in the soft tissue around the joint, a feature not seen in SpA.

Insufficiency fracture often involves the sacrum and sometimes also the adjacent iliac bone. It can be differentiated from sacroiliitis by MRI or CT.

MRI showing pronounced oedema in the sacrum at STIR with dark intraosseous streaks on T1-weighted image compatible with insufficiency fractures. Multislice CT (images on the right) confirmed the sacral fracture lines.

Degenerative and stress induced changes can be difficult to differentiate from sacroiliitis if there is concomitant bone oedema due to strain.

Ligamentous calcification seen in DISH may simulate inactive enthesopathy as part of SpA. In the illustration occuring proximally at the sacroiliac joint.

Spinal changes as part of SpA have to be differentiated from the following disorders:

• Other inflammatory disorders, e.g. rheumatoid arthritis
• Septic spondylitis
• Degenerative changes
• Metabolic disorders
• Tumour or tumour-like conditions

MRI is especially valuable for differentiating septic spondylitis form SpA changes.

Radiography showing erosion of vertebral plates without subchondral sclerosis.

MRI demonstrating enhancing tissue within the vertebral disc in addition to enhancement of the surrounding bones compared to the pre-contrast images to the left. This is not a feature occurring in SpA.

Disc degeneration may be accompanied by subchondral osseous inflammation and in later stages also fatty marrow degeneration. The changes often involve the entire subchondral area, but may predominate at vertebral corners (arrow) and can then be difficult to differentiate from SpA changes.