A fusion device for fusing a joint of a human or animal patient, wherein the joint is a synovial joint comprising two articular surfaces and a gap therebetween, the fusion device comprising: at least one anchorage portion and at least one stabilization portion, wherein the anchorage portion(s) and the stabilization portion(s) constitute separate parts wherein the fusion device has an overall depth parallel to an implantation direction, the overall depth extending from a proximal face to a distal end of the fusion device, an overall width and a thickness profile (T1 and T2) perpendicular to the implantation direction, wherein the at least one anchorage portion comprises a thermoplastic liquefiable material being arranged on a surface of the anchorage portion or inside a perforated sheath constituting a part of the anchorage portion, wherein the anchorage portions are configured to be positioned and anchored between the articular surfaces in the joint first, and the stabilization portion is capable of being mounted on the proximal ends of the anchorage portions or wherein the stabilization portion is configured to be positioned between the articular surfaces first, and the anchorage portions are capable of being pushed through or past the stabilization portion and anchored in the bone tissue beside and/or beyond the stabilization portion, wherein the fusion device is configured for being implanted comprising the step of transmitting energy to the liquefiable material for a time sufficient to liquefy at least part of the liquefiable material and to make the liquefied material to penetrate into bone tissue of both articular surfaces in the region of said gap to form a positive fit connection with the bone tissue on re-solidification, and wherein the stabilization portion comprises a plurality of through bores for receiving the anchorage portions, the bores extending from a proximal face to a distal face of the stabilization portion and having a diameter that is smaller than a thickness of the stabilization portion at the proximal face and larger than a thickness of the stabilization portion at the distal face or wherein the stabilization portion comprises a plurality of grooves for receiving the anchorage portions, and two of the anchorage portions are connected with a bridge element to form a twin anchorage portion. at least one anchorage portion and at least one stabilization portion, wherein the anchorage portion(s) and the stabilization portion(s) constitute separate parts wherein the fusion device has an overall depth parallel to an implantation direction, the overall depth extending from a proximal face to a distal end of the fusion device, an overall width and a thickness profile (T1 and T2) perpendicular to the implantation direction, wherein the at least one anchorage portion comprises a thermoplastic liquefiable material being arranged on a surface of the anchorage portion or inside a perforated sheath constituting a part of the anchorage portion, wherein the anchorage portions are configured to be positioned and anchored between the articular surfaces in the joint first, and the stabilization portion is capable of being mounted on the proximal ends of the anchorage portions or wherein the stabilization portion is configured to be positioned between the articular surfaces first, and the anchorage portions are capable of being pushed through or past the stabilization portion and anchored in the bone tissue beside and/or beyond the stabilization portion, wherein the fusion device is configured for being implanted comprising the step of transmitting energy to the liquefiable material for a time sufficient to liquefy at least part of the liquefiable material and to make the liquefied material to penetrate into bone tissue of both articular surfaces in the region of said gap to form a positive fit connection with the bone tissue on re-solidification, and wherein the stabilization portion comprises a plurality of through bores for receiving the anchorage portions, the bores extending from a proximal face to a distal face of the stabilization portion and having a diameter that is smaller than a thickness of the stabilization portion at the proximal face and larger than a thickness of the stabilization portion at the distal face or wherein the stabilization portion comprises a plurality of grooves for receiving the anchorage portions, and two of the anchorage portions are connected with a bridge element to form a twin anchorage portion.