One last idea.
The spine as a whole could be viewed as giant enthesis organ so that in conventional AS, as inflammation slowly crept upwards from the initiating point at the SI joints, attempting to fuse each vertebrae in upwards direction, the longitudinal ligament would be progressively inflamed, altered and shortened such that over time the thoracic spine would be pulled forward into familiar hyper-hyper kyphosis!

Dx Oct 2006 B27+ undifferentiated spondlyarthropathy (uSpA) with mild sebhorrhoeic dermatitis and mild Inflammatory Bowel Disease (IBD) controlled by NSD since 2007.