Abstract:
A bone drill extending along an axis has (1) a stabilizing point with first and second cutting edges which extend to a tip, conical wall sections and flutes on opposite sides of each of the first and second cutting edges and (2) first and second lands extending radially outwardly from the conical wall sections, each land having a radially outwardly extending cutting edge.

Description:
RELATED APPLICATION 
     The present application claims priority to U.S. Provisional Patent Application Ser. No. 60/186,492 filed Mar. 2, 2000. 
    
    
     BACKGROUND OF THE PRESENT INVENTION 
     The present invention relates to a drill specifically designed for drilling cavities or apertures in bones during the performance of surgical procedures. 
     In performing surgical procedures involving the repair of bones, the setting of broken bones or the replacement of joints, it is frequently necessary to drill cavities or apertures suitable for receiving bone screws. For example, in the case of fractures, the screws may be utilized for securing bone plates for fixation of the fractured sections. Screws may also be used in a variety of joint replacements and compression hip plates among other applications. 
     Prior art drills for use in bone surgery are difficult to precisely position during the drilling process. The prior art drills are derivative of metal cutting drills and have a tendency, at the beginning of the drilling operation, to move slightly from the desired axis of the aperture being drilled. This difficulty is particularly pronounced when the surgeon is attempting to start the drilling process on a portion of the bone which has a non-flat surface. Prior art bone drills have points which are not self-retaining on the desired axis. This is particularly true for drills for forming apertures having a diameter greater than about ⅛41 . 
     Additionally, the prior art bone drills have tips which do not immediately start the cutting operation when the tip contacts the bone surface. In order to overcome the problem with respect to accurate positioning of the prior art drills, the surgeons were forced to either (1) using a bushing to guide the drill, (2) use a relatively smaller drill as a starter and use the desired diameter drill in a subsequent operation or (3) attempt to start the drilling at a flat part of the bone, at an oblique angle to the desired axis of the cavity or aperture, and then straighten the drill to the desired axis. None of these solutions is particularly satisfactory and frequently result in the cavity or aperture being drilled having an irregular, non-cylindrical surface. This can lead to premature loosening of the screws. The prior art drills, when used with drill guides, result in excessive wear between the drill and drill guide causing wear debris to be generated which can lead to bone necrosis or infection which could lead to loosening of the screws. The other prior art procedures using prior art bone drills can also result in excessive bone debris being generated which can rapidly fill the flutes leading to generation of excessive heat and more bone necrosis. 
     Under the present invention, there is provided a drill which can be used in a one-step drilling operation with precise positioning and immediate retention along the desired axis upon contact of the tip with the bone. The drill of the present invention has a stabilizing point having tapered cutting edges with adjacent flutes. The tapered cutting edges extend to the tip and begin the cutting action immediately upon contact of the stabilizing point with the bone. Spaced axially from the tip of the stabilizing point are spaced apart cutting edges lying on a plane perpendicular to the axis of the drill which are arcuately displaced from one another approximately 180°. These spaced apart cutting edges follow a straight line path and, after the start, cooperate with the stabilizing point (a) to prevent the drill from “walking” or moving off the desired axis, (b) to assure that the cavity being cut is round or cylindrical even if the aperture is being cut at an oblique angle on a circular surface and (c) to prevent packing of bone chips by causing the chips to be readily expelled from the site being drilled. The drill of the present invention permits the drilling of an aperture or cavity in the bone in a one-step operation even though the origin of the drilling is a non-flat bone surface. It has a fluted design which permits bone chips to be augured away from the bone interface with the result that less heat is transferred to the bone interface than with prior art drills thus lessening bone necrosis. 
     The present invention is also directed to a method for forming an aperture or cavity in a bone. 
    
    
     IN THE DRAWINGS 
     FIG. 1 is an elevational view showing the drill of the present invention. 
     FIG. 2 is an end view of the drill. 
     FIG. 3 is a sectional view taken through line  3 — 3  of FIG.  1 . 
     FIG. 4 is an enlarged perspective view of the end portion showing the stabilizing point and cutting edge immediately adjacent thereto. 
     FIG. 5 is a view similar to FIG. 4 but with the drill rotated approximately 90°. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings, there is shown a drill generally designated by the numeral  10  extending along an axis A from a first free end  12  of a shank  13  intended to be engaged to a power source such as a drill to a second end  14  defining the tip of a stabilizing point  20 . Spaced axially from the second end  14  defining the tip are two spaced apart cleared lands  22  and  23  disposed substantially perpendicular to the axis A. The leading portion of each cleared land  22 ,  23 , as the drill rotates in a counterclockwise direction as viewed in FIG. 2 (as shown by the arrow), defines a cutting edge  24 ,  25 . The cutting edges  24 ,  25  are disposed on a plane perpendicular to the axis A and at an angle of approximately 180° relative to one another. The drill  10  is preferably a right hand helix, right hand cutting tool. 
     The stabilizing point  20  is provided a pair of angled cutting edges  31 ,  32 , each of which has a flute forwardly thereof, namely, a flute  33  for cutting edge  31  and a flute  34  for cutting edge  32 . The angled cutting edges  31 ,  32  meet at the second end  14  in a very sharp point. The angled cutting edges  31 ,  32  and their respective flutes  33 ,  34  ensure that the stabilizing point  20  will immediately begin cutting bone as soon as the second end  14  contacts the bone. The sharpness of the tip defined at the second end  14  coupled with the feature of the angled cutting edges  31 ,  32  and their respective flutes  33 ,  34  ensures that the drill will form the aperture or cavity at the precise location of initial contact of the bone by the tip defined by the second end  14  irrespective of whether that portion of the bone being contacted has a flat or non-flat surface. The maintenance of the drill at precisely the location initially contacted by the second end  14  permits the surgeon to form the desired aperture or cavity in the bone while maintaining the drill  10  precisely along the axis at which the drill was positioned at the time of initial contact between the second end  14  and the bone. 
     As may be seen in FIGS. 2 and 4, cutting edges  24  and  31  lie on substantially the same plane, which plane is parallel to the axis A. Similarly, the cutting edges  25  and  32  lie on substantially the same plane parallel to axis A. As can be seen in FIG. 2, plane of cutting edges  24  and  31  is slightly offset from the plane of cutting edges  25 ,  32 ; however, it is within the contemplation of this invention that all four of the cutting edges  24 ,  25 ,  31  and  32  could lie on the same plane. 
     The stabilizing point  20 , in the area between the flutes  33  and  34 , defines a pair of spaced apart wall surfaces  36 ,  38 , each of which forms a section of a cone. As viewed in profile, the wall surfaces  36 ,  38  define an included angle of 60°±30°; however, best results are obtained when the included angle is 60°±10°. As will be appreciated, the greater the angle, the less stabilization which is provided upon contact of the bone by the tip of the second end  14 ; however, such greater angle will provide greater strength to the stabilizing point. Conversely, the smaller the angle, the greater will be the stabilization but with less strength. 
     Extending from each cutting edge  24 ,  25  is a helical flute  40 ,  41 . Each turn of each helical flute  40 ,  41  is separated by a helical land or helical thread  42 ,  43 , hereinafter referred to as “helical thread”. As may be seen in cross-section in FIG. 3, each helical thread  42 ,  43  has a first cylindrical wall section  44  or  45  which is parallel to the axis A and a curved wall surface portion  46 ,  47 . Each curved wall surface portion  46 ,  47 , in section as viewed in FIG. 3, extends between end points,  50 ,  51  for curved surface portion  46  and end points  52 ,  53  for curved surface portion  47 . A line extending between end points  50 ,  51  is disposed at an angle of approximately 30°±10° relative to the axis A. Similarly, a line extending between end points  52 ,  53  is disposed at an angle of approximately 30°±10° relative to the axis A. 
     The presence of the stabilizing point  20  and its angled cutting edges  31 ,  32  which commence cutting immediately upon penetration of the bone by the tip at the second end  14  coupled with the cutting edges  24 ,  25  being disposed on a plane perpendicular to the axis A along with the design of the helical flutes  40 ,  41  and helical threads  42 ,  43  with their cylindrical wall surface portions  44 ,  45  and curved wall surface portions  46 ,  47  cooperate to prevent the drill from walking thereby maintaining the drill  10  precisely on the desired axis A. This assures that the cavity being cut has a circular cross-sectional configuration even if it is being cut on an irregular surface or being cut at an oblique angle of the circular or rounded surface, as on a patella. 
     Additionally, the orientation of the flutes  33 ,  34  of the stabilizing point  20  causes the chips generated by its angled cutting edges  31 ,  32  to flow in one direction, for example, at an angle of approximately 30° to the axis A. In contrast, the direction of flow of the bone chips caused by the cutting edges  24 ,  25  is substantially parallel to the axis A as a result of directing the chips into the helical flutes  40 ,  41 . The action of causing the chips to flow in two directions ensures that the chips are expelled from the drill  10  thereby preventing packing of bone chips which frequently occurs with prior art drills. This further prevents the build-up of undesirable heat which causes bone necrosis with subsequent loosening of screws. A larger surface area in the flutes than is available with prior art bone drills permits the bone chips exit faster than is possible with prior art bone drills. The larger surface area of the flutes also is a factor in minimizing build-up of heat. 
     The helix angle of the helical flutes  40 , 41  is preferably on the order of 60°; however, anything greater than 45° relative to the axis A is satisfactory. 
     The drill of the present invention can manufactured of any durable metal which is biocompatible. 
     In using the drill of the present invention, the surgeon may simply cut the soft tissue overlying the bone and proceed directly to drilling the bone along the desired axis without the necessity of (1) first drilling a small pilot hole or (2) starting the drilling at an angle oblique to the desired axis of the aperture. The bone, having no pilot hole or initial drilling at an oblique angle, may be considered as having a “virgin surface” in the area of drilling, in other words, a surface as appears immediately upon cutting the overlying soft tissue. It is not necessary to use a bushing to guide the drill; however, the surgeon may use one if he so desires. 
     Many modifications will be readily apparent to those skilled in the art.