Introduction: Intraarticular injection of metallic mercury (Hg) such as those found in thermometers is very rare. According to available data in literature, an arthroscopic presentation and treatment of knee injury that had intraarticular elementary mercury seems not to have been published yet.
Material and methods: We report a first case of arthroscopically treated chronic right knee synovitis after accidental intraarticular mercury infusion by a broken thermometer in an 11-year-old boy.
Result: Mercury levels in blood and urine were slightly increased, but no systemic mercury intoxication developed. Inflamed synovia as well as synovia with incorporated metal particles was excised. A histological analysis of macroscopically inflamed synovia revealed an unspecific inflammation reaction. The symptoms gradually subsided and the knee regained full function. However, all the metal particles were not successfully removed from the injured knee; therefore a careful follow-up of the patient was provided.
Conclusion: It seems that arthroscopic excision and lavage could be suitable treatments for knee injury, infused with elementary mercury, and toxic knee synovitis.