A perforated sheath is anchored in a tissue opening with the aid of a tool, wherein the anchorage is achieved with the aid of mechanical vibration and a material which is liquefiable by the vibration. The tool includes a vibrating element and a counter element. Distal portions of both elements are introduced into the sheath to be in contact with each other at an interface. The vibrating element is connected to a vibration source and the vibrating element and the counter element are held against each other for effecting liquefaction of the liquefiable material at the interface. Under the effect of the force applied to the vibrating and counter element for holding them against each other, the liquefied material flows from the interface through the sheath perforation and penetrates the tissue.