PENTA-MERS (PENTA – Modular Extremity Reconstruction System) is a set of implants designed to reconstruct irreparable defects involving the hip, knee, shoulder and elbow joints caused by tumor resection, revision arthroplasty or trauma. Different parts of the implant system are made of A Ti6Al4V, CoCrMo. and ultra high molecular weight polyethylene (UHMW-PE). The design rationale and main advantages of the implant system can be described in relation to its anchorage, modularity and articulation characteristics. The name of the implant `PENTA` is derived from the pentagonal cross-section of its stem, which particularly favors cementless implantation. Modular parts are secured with 3 different features combined: a toothed-connection which allows rotational adjustment, a conical press-fit connection and an axial transfixation bolt. PENTA knee megaprosthesis has a rotational hinge mechanism, which connects the femoral and tibial components through a yoke assembly (tibial rotation piece). Hyperextension is curbed by a bumper insert on the yoke while the rotational parts are designed to provide a smooth stop at the endpoints of rotation.