Background: Foreign bodies inside the knee joint are not uncommon. However, the literature has no reports of a foreign body inside the knee joint, with no history of trauma.
Case Report: Glass foreign bodies were found embedded inside the knee joint during arthroscopic washout for a middle aged male patient with knee osteoarthritis. The patient had no history of trauma and no scars or sign of entry of foreign bodies. It was found that these foreign bodies originated from glass vials broken while withdrawing medication for intra-articular injection of this knee in the past.
Conclusions: To avoid similar incidents, the authors recommend using filter needles to withdraw medications from glass vials.

Keywords: glass foreign body, intra-articular injections, Osteoarthritis, Arthroscopy, Knee