The central element (1) is of a cylindrical shape and provided with a helical groove at its outer surface, creating a slight flexibility. A ring-shaped element (2) is attached to each of its edges. The elements (2) have toothed outer edges (3). Each tooth (3) has a plane crest. The rings (2) are provided with a multitude of diamond-shaped openings (4), adding to the flexibility of the device (10). All parts (1, 2) can be made of a rigid bio-compatible material.
