This 53-years old lady presented with left sided backache with tenderness over the SI joint.

Her radiograph showed a possible erosion. The MRI showed extensive marrow edema with large articular margin erosions (STIR and T1W axial). The CT scan done at the time of biopsy showed a sequestrum anteriorly with large erosions.

This is definite sacroilitis, but it is unilateral. All mono-articular arthropathies should be assumed to be infective unless proven otherwise.

The biopsy was performed using an 18G coaxial biopsy gun introduced into the joint and up to the area of the sequestrum. Sometimes a light tap with a mallet helps push the needle and cannula further along. This was done under intravenous sedation.

The diagnosis was tuberculosis, on histology and on culture.

Rule 1: Any mono-articular arthropathy should be assumed to be infective unless proven otherwise and unless it is definitely degenerative in nature.
Rule 2: All infective arthropathies should biopsied for etiology.