- Onset in young adulthood
- Male:female 3:1
- HLA-B27, unclear etiology
- Shiney corners (sclerosis of endplates)
- Bridging syndesmophytes
- Flattening of the anterior vertebral body then later bamboo spine.
- First appears in the SI joints with bilateral symmetric sacroiliitis
- 10% with AS have IBD and vise versa