within the housing 140 and inhibits undesirable deviation of the cutter into the blood vessel wall in fig1 use of an atherectomy catheter in removal of stenotic material sm along the inside radius of curvature of a blood vessel bv is illustrated when the housing window 142 is on the inside radius of curvature the work element 138 tends to cut into the inside wall of the housing 140 rather than cutting along the optimal path for removal of stenotic material sm the shaft 170 again defines the proper travel path for the work element 138 so that it avoids cutting into the housing 140 and is properly disposed to remove the stenotic material sm as illustrated referring now to fig1 and 14 an embodiment of the catheter with a guiding structure 100 is illustrated the embodiment includes housing 140 formed with a window 142 a work element 138 a cable 134 a ribbon 180 and a slider 152 the housing includes a keyway 185 the work element 138 includes a slider 152 having an outer surface 163 with an extension 187 the extension 187 defines guide hole 195 which receives the ribbon 180 the slider 152 holds the work element 138 in a desired position with respect to the housing 140.