A switching node 3 for providing admission control in a network comprises a number of separate priority queues 6 for receiving packets having different priority levels. The tissue retractor is removably couplable to the bone screw. The guide cylinder is rotatable and provided with a diametrical face groove open from a side of the faceplate, and one or more working vanes mounted on the faceplate such that the faceplate is rotatable and movable in the grooves of the guide cylinder as the faceplate rotates.