There are 4 sites in the posterior spine to assess for changes of AS.

1. Facet Joints.
    
2. Superspinous Ligament.
    
3. Costovertebral joints.
    
4. Costo transverse joints.

Changes can be at 1 or more of these sites.

• Spinal changes start anteriorly and progress posteriorly.

• All the examples are from chronic AS where bony fusion has occurred.

• But these are also the areas to look at on MRI for the earlier changes of enthesitis and inflammation.