Intraosseous pseudotumour of tibial plateau following migration of viscosupplementation material: a rarely documented complication.

Intraosseous pseudotumour following intra-articular injection of hyaluronic acid polymers (viscosupplementation) is very rare and has only once previously been documented in the tibial plateau. Herein reported is a further example in a 63-year-old female who underwent total knee replacement 11 months following a diagnosis of breast carcinoma for which low-level suspicion for intraarticular metastasis persisted following extensive investigation. On imaging and gross examination, there was moderate osteoarthritis. Symptoms warranted total knee arthroplasty which was performed with subchondral bone sampling. Histologically, there was mild degenerative articular change and within the subchondral marrow space, there was deposition of uniform, Alcian blue positive mucoid material, surrounding a small degenerative cyst. No associated epithelial cells were demonstrated. A subsequent history of viscosupplementation administration was obtained by pathology, and the material was determined to represent intraosseous displacement of hyaluronic acid polymer. The breast carcinoma was also discovered to represent in situ disease. This case is a cautionary tale of the importance of careful history taking and a multidisciplinary approach to patient management.