Patent ID: 7090678

Claim:
A positioning instrument for inserting an extension shell into a human or animal bone, in particular a pelvic bone, said instrument including a fixing device for attaching the extension shell to the positioning instrument, a displacement device including a disc element at least axially relative to the extension shell, said disc element during an axial displacement contacting beveled surfaces formed interiorly along a perimeter of the extension shell so as to tighten the extension shell and a radial extent of the disc element as viewed in a direction of insertion of the extension shell into said bone, which radial extent is within confines of radially outermost projecting elements of the tightened extension shell, and wherein said disc element includes rotatably mounted rollers which are rollable on said beveled surfaces formed in the extension shell to effectuate a tightening of said extension shell, the displacement device including a screwthreaded wheel for indirectly acting on the disc element, and wherein said screwthreaded wheel includes a lifting translation device for acting on the disc element, wherein said lifting translation device increases an exerting force in a direction of displacement of said disc element, said screwthreaded wheel being connected to a screwthreaded element which is formed on a base of the positioning instrument, said screwthreaded wheel comprising a screwthreaded sleeve having an external screwthread formed on at least axial sections thereof; the screwthreaded sleeve of the screwthreaded wheel having an external screwthread engage in an axially arranged transmission case, whereby upon rotation of the screwthreaded wheel, a lifting displacement of the screwthreaded sleeve operated through the external screwthread is greater than a lifting displacement of the transmission case in an opposite direction responsive to rotation of said external screwthread.