An apparatus for the mobile fixation of bones in one plane, which has a first assembly plate that can be fixed on the bone by a suitable first fastener and which has a first connecting part and a second assembly plate which can be fixed on the bone by a suitable second fastener, and which has a second connecting part. Optionally, the apparatus has at least one further assembly plate, which can be fixed on the bone by a suitable fastener and it has a further connecting part which is mounted on a connecting element. The connecting element is mounted on the connecting parts so that the movement of the assembly plates is possible in relation to each other in substantially only one plane, and a change of spacing of the assembly plates relative to each other is possible by a predetermined length. All the components of the apparatus must consist of physiologically compatible materials (such as, for example, precious metals, ceramics, plastics, gold-vitallium alloys, titanium, vanadium, high quality steels and alloys thereof, etc.) to prevent a defensive reaction of the human or animal body. At the same time, the materials must be osseointegratable, to allow the bone to grow undisturbed.