To investigate the process of wear in the total knee prosthesis consisting of alumina ceramics (Al-ceramics) and ultra-high molecular weight polyethylene (UHMWP) in vivo, we observed the fine structures of the articular surface of the prostheses used for four patients with osteosarcoma of the distal femur. We also examined the wear debris in the surrounding soft tissues. The prostheses were extracted at autopsy between 13 and 48 months after surgery. An increase of pores, indicating where Al-ceramics grains fell off, and many streaks on the UHMWP running parallel to the gliding direction of the joint were observed on the articular surface. The streaks had similar widths to the Al-ceramics grains. Al-ceramics debris was detected in the soft tissues around the joint mixed with UHMWP debris. We concluded that the wear of UHMWP in Al-ceramics prosthesis was promoted by interposition of Al-ceramics debris.