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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
Malignant and benign tumors

The pelvic bones are common sites for metastasis and myeloma, but the changes will rarely be located directly adjacent to the SIJ and simulate sacroiliitis. Primary mesenchymal tumors such as osteochondromas, giant cell tumors and sarcomas will usually imply an expansive osseous lesion and/or a soft tissue extension, but the presence of osteoid osteoma may give rise to primarily osseous edema and simulate sacroiliitis if the tumor nidus is not identified. Clinically the symptoms can also simulate those of sacroiliitis with pain especially during rest at night. Supplementary CT will often be necessary for this differential diagnosis as shown beneath.

 
Osteoid osteoma

MRI in a young patient with low back pain especially in the night. Semi-coronal STIR and T1-weighred image with supplementary CT reconstructed in the semi-coronal plane corresponding to the MRI plane and in the axial plane. There is relative diffuse BME in the sacrum, especially on the right side where a delineated round osseous lesion is seen located to the posterior part of the sacrum (arrows). The supplementary CT clearly delineate the osteoid osteoma (pile).

 
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