Unicompartmental knee arthroplasty (UKA) with extrusion of cement into the posterior compartment of the knee is uncommon. Various problems after a UKA procedure, such as aseptic loosening, polyethylene wear and progressive arthritis have been reported. This study will report on a patient with extrusion of cement fragments into the posteromedial compartment of the knee after a UKA procedure. This complication was treated successfully with the direct posterior-posterior triangulation arthroscopic visualization method. In cementing the prosthesis, it is of paramount importance to take caution to completely remove extruded cement remnants in order to prevent this complication during UKA.