Septic arthritis of the right sacroiliac joint and left L3-4 facet joint.
Slightly increasing in size 10 cm abscess anterior to the right sacroiliac joint.
Extension into the greater sciatic foramen with some adjacent inflammation of the sciatic nerve.
Further involvement anterior exit foramen of right S1-S3.
Small multi fragment sequestered segment of bone anterior to the right sacroiliac joint.
No involvement of the epidural space around the lumbar spine soft
Involvement of the adjacent left L3 and L4 nerve roots in the exit foramen.
Further subtle osteomyelitis L5 spinous process, left posterior superior iliac spine, left subchondral bone S2 sacral alar.
Any history of immunocompromise?
Atypical infection TB should be considered.
Three features of infective sacroiliitis are particularly helpful in differentiating infective from inflammatory sacroiliitis. First, bone marrow oedema in infective sacroiliitis tends to be more intense and there is more intra-articular fluid (Figure 17A). Second, inflammation in infective sacroiliitis spreads to involve the peri-articular soft tissues, particularly the iliacus and gluteal muscles (Figure 17B) (50). Third, peri-articular fluid collection or abscess is practically pathognomonic of an infective sacroiliitis
As in our case, together with clinical findings, unilateral involvement, bone marrow edema adjacent to the SIJ surfaces and edema in the neighborhood soft tissues help to diagnose and distinguish infectious from noninfectious sacroiliitis (i.e., some spondyloarthropathies).