"Hanging Drop Sign" in pelvis X-ray.

Background: Diagnosis of Seronegative Rheumatoid Arthritis (SNRA) is based on clinical, radiological, and MRI findings with set criteria. High specificity is provided by plain X-rays in the differential diagnosis of rheumatic disorders Blum A et al. in Journal de Radiologie 90(12):1789-17811, 2009, https://doi.org/10.1016/s0221-0363(09)73586-3. In our study of pelvis X-rays, we found that ischiopubic ramus enthesopathy was more common than sacroiliitis and we have coined that enthesopathy finding as "hanging drop sign" because of its characteristic appearance.

Methods: A total of 152 proven (92 Females, 60 Males) SNRA cases (based on ACR*/EULAR**criteria and ASAS***criteria) were studied for enthesopathy features in their plain X-ray of pelvis. In addition to sacroiliitis, calcification/ossification of attachment of pelvic muscle and ligaments was documented. Hanging drop enthesopathy was noted in ischiopubic ramus in most of the cases. A cadaveric study was done on 10 specimens just to understand the corresponding anatomical origin of this enthesopathy.

Results: Of the 152 pelvic radiographs studied, 110 revealed radiological evidence of SNRA. Among them, 64 patients had hanging drop sign (58.18%), 25 patients had sacroiliitis (22.72%), and 21 patients had other features like hip arthritis, calcifications around greater trochanter, iliac crest, and acetabular region. The cadaveric study revealed that the hanging drop sign corresponds to the adductor part of adductor magnus muscle and gracilis muscle origin in the ischiopubic ramus.

Conclusion: In pelvic radiology, hanging drop sign is a common radiological finding in patients with seronegative rheumatism. It is along the origin of adductor part of adductor magnus and gracilis. It is more common than sacroiliitis and this radiological feature can be a useful tool to diagnose SNRA.