edge 90 and trailing edge 95 have a configuration which is a segment of an ellipse it should be appreciated that leading edge 90 of inner tube 25 extends at an angle to trailing edge 60 of outer tube 20 and or trailing edge 95 of inner tube 25 extends at an angle to leading edge 55 of outer tube 20 so as to create a shearing action between such surfaces i e edges when inner tube 25 is rotated relative to outer tube 20 as will hereinafter be discussed inner tube 25 is rotatably disposed within outer tube 20 such that rotation of inner tube 25 within outer tube 20 causes opening 85 of inner tube 25 to repetitively sweep across window 50 of outer tube 20 whereby to shear off tissue extending into window 50 of outer tube 20 more particularly in accordance with the present invention as inner tube 25 is rotated within outer tube 20 leading edge 90 of inner tube 25 is swept across trailing edge 60 of outer tube 20 or depending on the direction of rotation of inner tube 25 relative to outer tube 20 trailing edge 95 of inner tube 25 is swept across leading edge 55 of outer.