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 . <EOS>