A cutting instrument comprising: an outer tube; an inner tube rotatably disposed within the outer tube; the inner tube comprising an opening having a leading edge and a trailing edge; and the outer tube comprising a window having a leading edge and a trailing edge; at least one of the leading edge of the opening, the trailing edge of the opening, the leading edge of the window and the trailing edge of the window comprising a canted edge; and the window of the outer tube being adapted to be completely blocked by the inner tube during a phase of the rotation of the inner tube within the outer tube.