A medical instrument for cutting tissue has a tubular outer shaft, which comprises at least one window in the region of its distal end. The medical instrument further comprises a tubular inner shaft which is rotatable about a longitudinal axis and housed in the outer shaft and which has at its distal end a window arranged in the region of the at least one window of the outer shaft and provided with at least one cutting element. The medical instrument has a fixing element for unrotatably holding of the inner shaft relative to the outer shaft in a preset position. The fixing element is irreversibly detachable through rotational movement of the inner shaft.