Table 2.

Congenital disorders and normal variants of the sacroiliac joints and spine that mimic axial spondyloarthritis.

Condition	Type	Characteristic features
Blood vessels	–	Location–ligamentous portion of the SIJ, adjacent, adjacent to anatomical variants, lower ilium (partial volume)
Normal marrow changes	–	Location–lower iliac bone Low SPARCC scores
Healthy individuals	–	Location–anterior upper sacrum, posterior lower ilium
Sports/exercise related	–	Topographic distribution overlaps with axSpA
Port-partum	–	Extent and distribution indistinguishable from axSpA Structural changes are rare
Schmorl nodes	–	Location–lower thoracic and upper lumbar vertebrae, along the nucleus pulposus axis
Block vertebra	Congenital	Location–cervical segments Other associated conditions
	Acquired	Other findings–post-surgical, degenerative disc disease, advanced axSpA
SIJ normal variants	Iliosacral complex	Location–Ilium opposite posterolateral sacrum, extra-articular, ligamentous portion Other–women
	Paraglenoid sulci	Location–inferior ilium Other–women
	Ossification centers sacral wings	Location–postero-superior border, cartilaginous portion Other–triangular shape
	Bipartite iliac bony plate	Location–postero-inferior segment Other–unilateral, women
	Accessory iliac joints	Location–between iliac and sacral surfaces at posterior joint
	Semicircular defect articular surface	Location–ligamentous portion, postero-superior, focal sacral depression Other–women, bilateral
	Isolated ankylosis	Location–mid-third of the SIJ
Transitional vertebrae/Bertolotti syndrome	–	Variable presentation (Castellvi classification) Types II and IV correlate with symptoms and disc herniation
Spina bifida occulta	–	Location−5th lumbar segment Other–correlation with spondylolysis
Intra-osseous pneumatocyst	–	Location–iliac bone adjacent to SIJ, lumbar or cervical spine
Tarlov cysts	–	Location–sacrum Other–bilateral, women, 40 years-old