A constrictor for closing or constricting a passage through tissue of a hollow organ, includes: a ring having a thread-like body, which, viewed in the peripheral direction of the ring, extends in a wavy pattern； pins for fastening to tissue surrounding the passage to be closed or restricted； which pins are distributed over the periphery of the ring. Each pin has a fixed end rigidly attached to the ring, and a free pointed end. The constrictor is deformable from a first state into a second state, while a pretension is built up such that the constrictor, in the second state, is under a pretension acting in the direction of the first state. The pretension includes a torsional stress present in portions of the body at the fixed end of each pin, and inclined to want to pivot the respective pin back in the direction of the first state.
