Patent Publication Number: US-2016228130-A1

Title: Surgical Drill Bit

Description:
FIELD OF THE INVENTION 
     This invention relates to material drilling tools and more particularly to surgical drill bits. 
     BACKGROUND OF THE INVENTION 
     Drilling of bone is a very common operation in many fields of surgery such as orthopedics, plastic surgery, neurosurgery and the like. In the field of dentistry the fixing of implants requires drilling of bone. Generally the purpose of bone drilling is to create a bore or socket to accommodate a screw or other implant device. The bore or socket is formed using a drill bit secured in a drill mechanism which is usually powered by an electric motor. 
     The drilling of bone presents several problems for the surgeon. The chief problem being an increase in temperature caused by friction during the drilling process. Other risks include drill bit failure inside the bone that sometimes must be left in the bone because it cannot be extracted. Yet another problem is so called “bit walking” that is difficulty in holding the drill bit at the desired location, especially when drilling into bone at an acute angle. Bit walking presents a risk of severing soft tissue such as muscle, blood vessels and nerves. In addition it can also result in a bore larger than the diameter of the bit so that the implant is loosely fitting causing the fixture or implant to subsequently fail that requires the patient to undergo the expense and discomfort of another procedure. 
     Multiple studies have shown that the rise in temperature of bone during drilling can cause thermonecrosis of the bone resulting in the subsequent failure of the installed implant. The threshold temperature is 47° C. and temperatures above this value may cause osteonecrosis. The time at which bone is held at a temperature value above the threshold is another very important parameter that can affect the response of bone to the drilling operation. It widely accepted in the surgical field that the maximum holding time above 47° C. is 1 minute. Maintaining bone above these levels can result in osteonecrosis with irreversible bone damage and detrimental effects on the solidarity of the implant. 
     In the drilling process the surgical bit is subjected to torsional and bending loads that can produce significant shear and normal stresses. Surgical drill bits are available in a variety of diameters ranging from 0.76 mm to 17 mm but the vast majority falls in the range of 2 mm and 4 mm. The small diameter of most surgical drill bits coupled with the weakening of the cross section due to the flute shape and the stresses increase the chances of bit failure during the drilling operation. Many times due to the complexity of the operation and the limited time and space, the surgeon may decide not to remove the broken part of the bit but to leave it in the bone. 
     The round shape and the smooth surface of the bone, as well as the presence of liquids, can complicate the drilling start, especially if drilling at a small acute angle. In this case the bit may wander from its starting position which, as discussed above, may result in damaging soft tissue and producing a bore too large for the implant. 
     The design of the flute and drill point of the bit as well as the condition of the surface of the bit can enhance or reduce the negative factors discussed above. The increase in bone temperature is an effect of the heat generated during the drilling process that in turn is the result of the friction forces between the bit and the bone. 
     The present invention offers an improved surgical bone drill over conventional surgical bone drills. 
     OBJECTS OF THE INVENTION 
     It is an object of the present invention to provide a surgical drill bit, particularly for drilling bone, which develops less heat buildup during the drilling operation than conventional drill bits. 
     Another object of the invention is to provide a drill bit that is less subject to failure due to torsional and flexural stress developed during a drilling operation. 
     Still another object of the invention is to provide a surgical drill bit point that is less subject to wander during the start of a drilling operation. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a new and improved surgical drill bit designed minimize the deficiencies found with prior art surgical drill bits, such as the buildup of heat in the object being drilled. In addition the surgical drill bit of the present invention has improved resistance to torsional and flexural stress and thus is less likely to fail during a drilling procedure. The drill point of the surgical drill bit of the invention is designed to maintain the drill point at the desired position during startup. 
     The drill bit is a cylindrical body that comprises a drill section, a midsection and a drive transmission shank. The drive transmission shank defines the proximal end of the bit and is adapted for attachment to a driving hand piece of conventional design. The surface of the drive transmission shank is smooth and normally is not in contact with bone. 
     The drill section defines the distal end portion of the surgical bit and comprises one or more flutes which define radially, helically extending lands along the length of the drill section and a drill point. The distal end of the drill section forms the drill point of the surgical bit and comprises at least two opposed cutting edges that extend from the tip of the drill point to the leading edge of a land defined by the outer surface of a respective drill point flute. The angle between the opposed cutting edges may be between 90° and 180°. The diameter of the drill section at the extending end of the lands forms the outside diameter of the bit and defines the diameter of the bore formed by the bit. The width of the land of the drill section at the outer edge is held at a minimum for minimum bearing surface area to reduce contact between the surface and the wall of the bore being formed. A portion of the cutting edges adjacent the tip may be concave to form a centric starter tip to aid in maintaining the position of the drill bit upon startup. 
     The midsection is defined by one or more helical flutes that extend radially from a central web and extend radially, spirally to the drill section. The midsection is adapted to remove bone fragments during the drilling operation and is not necessarily involved in the actual drilling. The outside diameter of the midsection is less than the outside diameter of the drill section so that the edges of the flute lands are not in contact with the wall of the bore during the drilling operation. 
     In cross section each flute in both the drill section and the midsection defines a radially extending land and an enlarged central web of at least 40% of the outside diameter of the bit at the drill section. The land radially decreases in width to the outer surface of the land. 
     The condition of the surface of the drill bit is a factor in heat buildup. The surface of the bit may be polished, electro polished, fine ground or coated with a low friction coating. 
     These and other advantages of the present will be apparent from the following description of the invention and the attendant drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a prior art surgical drill; 
         FIG. 2  is a sectional view of the drill bit of  FIG. 1  taken along line  2 - 2 ; 
         FIG. 3  is a perspective view of a two fluted surgical drill bit in accordance with the invention; 
         FIG. 4  is a sectional view of the drill bit of  FIG. 3  taken along line  4 - 4 ; 
         FIG. 5  is a sectional view of the drill bit of  FIG. 3  taken along line  5 - 5 ; and 
         FIG. 6  is an enlarged perspective view of the drill bit of  FIG. 3  particularly illustrating a preferred drill point in accordance with the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
     As illustrated in  FIG. 1  and  FIG. 2 , a conventional prior art surgical drill bit shown generally as  10  consists of a drive section  12  for attachment to a power tool and a fluted section  14  comprising two opposed helical flutes comprising lands  16  extending radially to the axis of the drill bit. As shown most clearly in  FIG. 2  the leading edges  18  of the lands  16  are cutting edges for contacting and cutting away bone being drilled. The trailing portion  19  of each edge is sloped slightly inwardly to define body clearance between the wall of the bore and the surface  19  for the collection and removal of bone chips. At the distal end of the drill bit the opposed cutting edges  18  are inwardly sloped to form a drill point  20 . The outside diameter of the fluted section  14  is maintained at nominal value along the length of the fluted section and the edges  18  of the lands  16  are in contact with wall of the bore being drilled. The contact between the cutting edges  18  and bone being drilled as well as contact between the bone chips created by the drilling operation and the trailing portion  19  generates a substantial amount of heat in the bone that, as mentioned above, can rapidly reach the limiting temperature of about 47° C. If held at 47° C. for more than one minute substantial damage and even necrosis of the bone will result. 
     Accordingly, the surgeon must exercise extreme care when drilling bone to ensure that the temperature of the bone does not exceed about 47° C. for more than one minute. This may be accomplished by drilling for short periods of time with intervals between drilling periods to allow the bone to cool. Alternatively, a cooling fluid may be introduced to the drilling site to reduce the buildup of bone temperature. Some prior art drill bits are provided with a coolant channel (not shown) that opens to the drill  20  point and its use is commonly understood by those skilled in the art. 
     Surgical drill bits must also withstand substantial torsional and flexural stress applied during the drilling procedure. However surgical drill bits are normally small, on the order of 1 mm to 5 mm in cross section and due to the flutes the core of the drill body is even smaller in cross section. Thus breaking or bending of the drill bit may be a frequent occurrence. In some cases the broken portion of the drill bit cannot be recovered and must be left in the bone. As illustrated in  FIG. 2  the area between the lands  16  of the flutes, referred to as the web  34 , forms the core of the drill bit body. The web  34  substantially contributes to the torsional and flexural strength of the bit but conventionally it is substantially smaller in cross section than the extending land  16  thus limiting the stress and bending resistance of the drill bit  10 . 
     The present invention provides a more robust surgical drill bit that minimizes the buildup of heat in the bone and reduces the chance that necrosis of the bone will occur. In addition, surgical drill bits are usually of small diameter and the pressure that must be applied during drilling can result in bending or breaking the drill bit. This results in an interruption of the operation while the bit is replaced and in some cases the broken portion of the drill bit in the bone cannot be removed. The drill bit of the invention is more resistant to flexural and torsional stress and can more readily withstand stresses encountered during drilling and therefor is less likely to bend or break during a procedure. 
     Referring to  FIG. 3  and  FIG. 6  there is shown a surgical drill bit  30  in accordance with the invention. The drill bit comprises an elongated cylindrical body  31  comprising three sections; a drill section  32 , a midsection  34  and a drive transmission shank  36  adapted for attachment to a power tool of conventional design, not shown. 
     As illustrated, the drill section  32  includes an opposed pair of flutes  40  that helically extend along the length of the drill section and a drill point  38  at its distal end. Each flute  40  defines a land  42  that extend helically, outwardly from an enlarged core  41 . As most clearly shown in  FIG. 4  the extending outer surface  43  of the land  42  forms a cutting edge  44  and a trailing portion  45 . The trailing portion  45  is biased inwardly with respect to the cutting edge  44  to provide body clearance  46  between it and the bore wall. The cutting edge  44  of the land  42  defines the outer diameter of the drill bit  30  and the resulting diameter of the bore formed by the drill bit. 
     Referring to  FIG. 6  the distal end of the drill section  32  defines a drill point  38 . The drill point  38  comprises cutting edges  44  sloping at an angle from the distal tip of the drill section  32  to the edge  43  of a respective land  42 . The angle A 2  defined between the edges  44  forming the drill point  38  and the edge  43  of the land is a matter of choice and may range from about 90° to about 180°. The wider the point angle the shorter the cutting edge at the drill point  38  and therefore less contact with the bone and less heat generation. A point angle of between 100° and 170° is preferred for drilling bone. The drill point  38  may comprise any conventional drill point configuration and the prior art is replete with various configurations depending on the material being drilled. For the purposes of the invention a split point configuration is preferred as generating the least amount of heat. 
     A major source of heat buildup is due to friction between the outer surface  43  of the land and the material being drilled. Conventionally, with surgical drills, the outer surface of the lands bear against the inner face of the bore along the entire length of the bit producing heat due to friction. In addition, debris from the drilling procedure can accumulate in the body clearance between the trailing surface of the land and the wall of the bore creating more friction and heat buildup. Since the function of the drill section  32  and the drill point  38  is to provide the cutting action of the bit  30 , the axial length L 1  can be kept to a minimum, preferably between 1 to 2 times the outside diameter of the drill section. Direct contact between the outer surface  43  of the flutes  40  and the wall of the bore is kept to a minimum. 
     Although the drill section  32  is illustrated with a pair of flutes  40  it will be understood the drill section may comprise a single flute or more than 2 flutes for example, 3 or 4 flutes. 
     As shown in  FIG. 3  flutes  52  that extend along the length of the midsection  34  have a smaller outside diameter than the drill section  32 . As most clearly shown in  FIG. 5  flutes  52  comprise lands  54  that extend from the core  41  to a lesser extent than lands  42  of the drill section  32  so that the outer surfaces  58  define a larger body clearance  60  for removal of bone chips without unduly producing friction by bone chips becoming compacted between the surface  58  and the wall of the bore. Good results are achieved when the outside diameter of the midsection  34  is reduced about 5%-10% as compared to the outside diameter of the drill section  32 . The larger body clearance  60  does not interfere with the chip removing function of the midsection  34 . The flutes  52  may be integrally formed with the flutes  40  or may be separately formed. Additionally, the number of flutes  52  on the midsection  34  may be different than the number of the flutes  40  on the drill section  32 . For example it may be advantageous to provide the midsection  34  with four flutes  40  to aid in removal of bone cuttings while providing the drill section  32  with 2 or 3 flutes in order to minimize friction during the drilling procedure. 
     In accordance with the invention the drill bit  30  has improved flexural and torsional strength. Referring to  FIG. 4  and  FIG. 5 , in cross section the drill bit body  31  comprises a web  41  that is substantially enlarged while the width of the lands  42  and  54  at their outer surfaces,  43  and  58  respectively, is reduced. In accordance with the invention, the web  41  comprises between about 40% and 70% of the diameter of the drill bit body  31  and decreases gradually to form the lands  42  and  54  respectively. The land width at the outer surface  44  and  58  is reduced since land width is not essential to the strength of the drill bit  30 . In particular the reduced width of the lands  42  and  54  reduces the outer surface area of the lands  42  and  54 . In addition the reduced diameter of the mid-section  34  provides additional body clearance  60  for chip removal while permitting the reduced land width. In this manner there is less area in contact with the bore wall and bone chips resulting in a reduction of friction and resultant heat buildup. With conventional surgical drill bits, such as shown in  FIG. 2 , where the flutes are of equal diameter along the entire drill bit, the width of the lands must be wide enough to provide flexural strength to the drill bit. In addition the width of the lands  16  provide land edges to form a sufficient trailing surface area  19  for receiving and removing bone chips 
     The enlarged web  41  substantially increases torsional and flexural strength of the drill bit  30 . Calculations show that for equal cross section and equal material drill bit  30  with the increased web  50  has up to 20% lower normal stress (bending) and over 30% lower shear stress (torsion). 
     Centering and maintaining the position of the drill bit  30  on bone can be difficult especially with the wider preferred point angle. As illustrated in  FIG. 6  a concentric tip  45  is formed on the drill point  38 . The axial height H 2  of the concentric tip  45  is preferably about ⅓ of the axial height H 1  of the drill point  38 . The concentric tip  45  is conical in shape and defines an angle A 1  of between 50° and 80°. The concentric tip  45  serves to aid in maintaining the position of the drill bit during the start of the drilling procedure to reduce “bit walking” on the bone being drilled. 
     Several factors have to be considered when selecting the material for the surgical drill bit in view of the medical applications for which the drill bit will be used. Martensitic stainless steel ( 410  or  420 ) is preferred for the surgical drill bit. Martensitic stainless steel is preferred because it is resistant to corrosion, can be easily machine in annealed condition possesses a high ultimate tensile strength and very good edge-keeping ability in the hardened condition. 
     It is preferred that the entire drill bit  30  be formed from hardened martensitic steel to reduce manufacturing costs without sacrificing the effectiveness of the bit. However, the drill section  32  can be formed of a different material such as carbide. Likewise, the midsection  34  and drive transmission shank  36  may be formed of a different material such as a high strength plastic while the drill section is hardened martensitic steel. 
     Surface condition of the surgical drill bit is a consideration in reducing friction and heat buildup in bone. A smooth, low coefficient of friction, surface results in lower friction and, as consequence, a lower temperature increase. The best surface condition can be achieved by a polishing process but processing times and associated costs are unjustifiably high. Fine grinding of the external diameter and flute offers a better cost/benefit combination. Some products are electro polished but this process, while assuring a low surface roughness, rounds off the cutting edges thus reducing the cutting ability. Coating the drill bit with a material having a low coefficient of friction has been commonly used with conventional drill bits. Thus, it is preferred to coat the surgical drill bit of the invention with a suitable coating to reduce the coefficient of friction. 
     Some of the existing drill bits are offered with a titanium nitride (TiN) coating. TiN is an extremely hard ceramic material and is applied as a layer of less than 5 micrometers thickness. In a preferred embodiment of the invention a-C:H coating, which is a diamond-like carbon layer with very high density, is applied over at least the drill section  32 . Compared to TiN, a-CH is slightly harder (2500 vs. 2300 HV), is thinner (2-3 vs. 5 micrometers) and, most important, has a friction coefficient on steel of 0.1-0.2 vs. 0.4 for TiN or 0.5-0.8 of steel. The lower friction force between this coating and the material being drilled results in lower heat generation and, thus, lower increase in temperature of the material. 
     From the foregoing description and drawings it will be seen that the present invention provides an improved drill bit for drilling bone that results in lower temperature buildup in the bone for a given period of time during the drilling procedure. While the embodiments of the invention have been disclosed herein have been particularly shown and described with reference to the exemplary embodiments thereof, those of ordinary skill in the art will understand that various changes may be made in the form and details herein without departing from the spirit and scope of the disclosure and the appended claims. Those of ordinary skill in the art will recognize or be able to ascertain many equivalents to the exemplary embodiments described specifically herein by using no more than routine experimentation. Such equivalents are intended to be encompassed by the scope of the present disclosure and the appended claims.