For removal of obstructive material from a body lumen or cavity an interventional catheter assembly comprises an operating head operably connected to a rotatable, translatable drive shaft which is driven by a drive motor and is received through an elongated catheter (32). The operating head comprises a plurality of differential blades (93) and at least one of the blades has a stepped blade cutter configuration having a leading edge cutting surface (99) adjacent or in proximity to a shoulder region (97) which is adjacent to a leading surface (91) of the blade (93). The shoulder region (97) is positioned between the leading edge cutting surface (99) and the leading surface (91), and the shoulder region has a width that is greater than a height of the leading edge cutting surface. Risk of damage to tissue being acted upon is thereby reduced.
