An Unusual Case of a Mass-Like Hydroxyapatite Deposition Disease Causing Forearm Flexor Tendon Calcific Tendinitis

Hydroxyapatite deposition disease (HADD) is a relatively common disorder characterised by periarticular deposition of calcium hydroxyapatite, most frequently involving tendons of the glenohumeral joint. Although calcific deposits typically produce a self-limiting inflammatory response, mass-like presentations at atypical sites can mimic trauma, infection or soft tissue neoplasia, posing a significant diagnostic challenge. We report an unusual presentation of mass-like calcific tendinitis within the flexor carpi ulnaris (FCU) tendon of the distal forearm-an atypical location for HADD presenting as a painful soft tissue swelling. Multimodality imaging, including radiography, MRI and CT, was instrumental in characterising the deposit and excluding alternative diagnoses. The patient was treated with ultrasound-guided barbotage using a large bore needle system, with significant reduction in deposit volume and symptom relief. This case highlights the importance of considering HADD in the differential diagnosis of painful soft tissue masses at atypical sites, where multimodal imaging is essential for accurate characterisation and ultrasound-guided barbotage represents an effective minimally invasive treatment alternative to surgery.