Table 2.

Hehne und Zielke classification in ankylosing spondylitis [9].

Type	Definition	Syndesmophyte characteristics	Synonymes
I	Dorsal ossification	Simple ossification of the dorsal spine bodies, no syndesmophytes	Spondylathritis type
Iia	Incomplete anular ossification	Tender, the anulus fibrosus ventrally with or without lateral syndesmophytes, without overlapping of the vertebral bodies (incomplete)	Anulus-type
Iib	Complete anular ossification	As type IIa, but with syndesmophytes overlapping the vertebral disc (complete)	Anulus-type
IIIa	Partial ostotic ossification	Thick and wide syndesmophytes, often with cortical- and spongiosa structure, corresponding the bamboo-type, but not present in all segments (incomplete)	Ligament/sub-ligament type
IIIb	Total ostotic ossification	As type IIIa, but several thoracic and lumbar segments are affected	Bamboo-spine