A cutting instrument comprising: an outer tube comprising a distal end and a proximal end and a longitudinal axis extending therebetween; an inner tube rotatably disposed within said outer tube, said inner tube comprising a distal end and a proximal end and a longitudinal axis extending therebetween; said inner tube comprising an opening having a leading edge disposed proximal to said distal end of said inner tube, and a trailing edge disposed proximal to said leading edge; and said outer tube comprising a single window formed therein, said window of said outer tube having a leading edge disposed proximal to said distal end of said outer tube, and a trailing edge disposed proximal to said leading edge; at least one of said leading edge of said opening and said trailing edge of said opening comprising a canted edge, said canted edge being canted relative to said longitudinal axis of said inner tube; at least one of said leading edge of said window and said trailing edge of said window comprising a canted edge, said canted edge being canted relative to said longitudinal axis of said outer tube; and said window of said outer tube being adapted to be completely blocked by said inner tube during a phase of the rotation of said inner tube within said outer tube, characterised in that said leading edge of said opening, said trailing edge of said opening, said leading edge of said window and said trailing edge of said window all comprise a helix, wherein at least one of said leading edge of said opening and said trailing edge of said opening comprises a different pitch than at least one of said leading edge of said window and said trailing edge of said window. an outer tube comprising a distal end and a proximal end and a longitudinal axis extending therebetween; an inner tube rotatably disposed within said outer tube, said inner tube comprising a distal end and a proximal end and a longitudinal axis extending therebetween; said inner tube comprising an opening having a leading edge disposed proximal to said distal end of said inner tube, and a trailing edge disposed proximal to said leading edge; and said outer tube comprising a single window formed therein, said window of said outer tube having a leading edge disposed proximal to said distal end of said outer tube, and a trailing edge disposed proximal to said leading edge; at least one of said leading edge of said opening and said trailing edge of said opening comprising a canted edge, said canted edge being canted relative to said longitudinal axis of said inner tube; at least one of said leading edge of said window and said trailing edge of said window comprising a canted edge, said canted edge being canted relative to said longitudinal axis of said outer tube; and said window of said outer tube being adapted to be completely blocked by said inner tube during a phase of the rotation of said inner tube within said outer tube, characterised in that said leading edge of said opening, said trailing edge of said opening, said leading edge of said window and said trailing edge of said window all comprise a helix, wherein at least one of said leading edge of said opening and said trailing edge of said opening comprises a different pitch than at least one of said leading edge of said window and said trailing edge of said window.