A device for fastening an orthesis or prosthesis, having a length-adjustable substantially rod-shaped bar assembly, which is pivotably connectable by two ends on exposed first ends of two implants fastened by a second end at a distance from each other in or onto a bone. With this bar assembly, a three-dimensional angle between the longitudinal axis of the bar assembly and the longitudinal axis of at least one implant can be set, so that, irrespective of the distance between and the relative position of the implants, each time, a fitting, preferably stress-free connection between these implants can be effected.