A medical instrument serves for creating an access for a minimally invasive intervention and has a body having a longitudinal axis. The body is assembled of at least two parts, each of said parts having edges extending along a direction of said axis. Each of said parts having edges extending along a direction of said axis. Each part has a distal section merging into a proximal section standing outwardly from said longitudinal axis. In a first position, the distal end section of the parts are joined along said edges to form a distal body with the proximal section standing away. In a second position, the proximal sections of said parts are joined along said edges to form a hollow proximal body with said distal end section spreaded outwardly. The edges are provided with interacting features which are designed to block a lateral sliding of said joined parts transversely to said edges.