The invention relates to the use of a spongy material for the production of an implant (10) designed to obstruct a cyst neck (3). As spongy material especially the polyvinyl alcohol Ivalon can be employed. The invention furthermore relates to a set of instruments for the introduction of such an implant (10) made of a spongy material into a cyst neck (3) with said set of instruments consisting of a puncture needle (5) for the insertion of a guide wire (6), a guide wire (6) the distal end of which can be introduced into the cyst neck (3), an implant needle (7) provided at its proximal end with a connection element, a feeding unit (8) that can be attached via the connection element to the implant needle (7), and the implant (10) proper which consists of a spongy material and can be introduced via said feeding unit (8) through the implant needle (7) into the cyst neck (3). The inventive set of instruments may in particular be used for the treatment of Baker's cysts.