Figure 2  .

MRI and SJ biopsy specimen of a 16 year old HLA B27+ uSpA patient with unilateral IBP located in the left buttock. (A) T1 weighted gradient echo sequence five minutes after application of gadolinium-DTPA showing some chronic changes and definite acute changes mainly in the left SJ (arrow) and in the periarticular left bone marrow (asterisk) (grade Ix right and Ib left). (B) The biopsy specimen shows a cellular infiltrate containing activated fibroblasts and CD45+ lymphocytes (white arrows) that seem to invade a degenerate cartilagineous area (black arrows). In the middle areas of calcification and bone formation (open black arrows), (immunohistology/haematoxylin and eosin staining, original magnification × 400).