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SIJ - differentials
OCI / postpartum changes
Strain-related SIJ changes
Degenerative / DISH changes
Infectious sacroiliitis
Fractures
Inflammatory conditions
Metabolic disorders
Tumors
Spine - differentials
Degenerative changes
Infectious spondylitis
Hyperparathyroidism
Disorders of mineral metabolism can involve the SIJ region. As part of hyperparathyroidism, there can be subchondral bone resorption usually presenting as bilateral and symmetrical irregularities of the iliac joint margins with gross erosion-like lesions and pseudo-widening of the joint due to the bone resorption, changes mimicking structural lesions in axSpA sacroiliitis as shown on the radiograph beneath. By MRI there may in addition to the structural damage be subchondral BME which in combination with pseudo-widening of the joints can simulate axSpA changes (doi:10.3390/DIAGNOSTICS11112001). However, joint space narrowing or bone bridging/ankylosis and sacral-sided erosions are not features of changes caused by hyperparathyroidism, doi:10.1038/s41598-021-83989-1.
 
Frontal pelvic radiograph in a patient with low back and right-sided hip pain, and secondary hyperparathyroidism due to renal insufficiency. Both SIJs are irregular with subchondral iliac sclerosis. There is in addition a tumorous process corresponding to the right femur representing a biopsy verified Brown tumor related to hyperparathyroidism.
 
MRI of the SIJs, semi-coronal T1 and T1FS and semi-axial STIR image, revealed irregular iliac joint margins with subchondral sclerosis intermingled with small fat depositions (arrow). There are well-delineated sacral joint margins, but an area with BME in the right side of sacrum (open arrow).
 
Hypoparathyroidism
Hypoparathyroidism is characterized by reduced bone remodeling resulting in increased osseous mineralization (doi:10.1148/RG.2016160004) and occasionally also ligamentous ossification, which can simulate paravertebral ossification in DISH and axSpA, especially psoriatic SpA. The SIJ is generally spared, but ligamentous changes may occur simulating those observed in DISH.
 
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