Malignant mixed tumor of the soft tissue occurring after total knee arthroplasty.

A 70-year-old woman developed a malignant mixed tumor of the soft tissue 2 years after total knee arthroplasty. A 5×3×3-cm elastic hard tumor at the lateral side of the surgical scar was resected. The tumor showed focal infiltration into surrounding adipose and fibrous tissues, focal necrosis, and vascular infiltration. It was diagnosed as malignant. Mixed tumor, or myoepithelioma, of the soft tissue is a relatively rare tumor that was recently recognized as a disease entity; the vast spectrum of myoepithelial cell differentiation and the resultant morphologic diversity might increase the difficulty of the histological diagnosis. Postoperatively, the patient did not receive adjuvant therapy and no recurrence of the tumor was observed for 6 years. Range of motion of her left knee is -5° extension and 90° flexion; however, her activities of daily living are restricted because of general fatigue, partly due to hepatoma and chemotherapy.Despite the increase of artificial implant use worldwide, reports of peri-implant tumor formation are rare. Although we do not know the exact mechanism of tumor genesis, we consider the fibroblast formation in the routine healing process to be a possible mechanism. Further investigation is necessary to identify coexisting factors that increase the risk of tumor formation after implantation.