The instrument has a shank (20) encompassing an electrode surface (53) and a longitudinal slit (24). A blade (21) is made of an electrically conductive material and movably arranged relative to an elongate shaft. The blade is extended or retractable into the longitudinal slit. A drive element e.g. metal strip, extends from a housing through the elongate shaft to the blade. The drive element includes an isolator (40), which connects the blade and the drive element with each other. The elongate shaft extends away from the housing.
