Patent Publication Number: US-2022218361-A1

Title: Mastoid bone start drill bit

Description:
BACKGROUND 
     Field of the Invention 
     The present invention relates generally to implantation of prostheses, and more particularly, to the implantation of an implantable component of the prosthesis. 
     Related Art 
     For persons who cannot benefit from traditional acoustic hearing aids, there are other types of commercially available hearing prostheses such as, for example, bone conduction hearing prostheses (commonly referred to as “bone conduction devices”). Bone conduction devices mechanically transmit sound information to a recipient&#39;s cochlea by delivering vibrations to recipient&#39;s skull. This enables the hearing prosthesis to be effective regardless of whether there is disease or damage in the outer or middle ear. 
     Traditionally, bone conduction devices transfer vibrations from an external vibrator to the skull through a percutaneous bone conduction device that penetrates the skin and is physically attached to both the vibrator and the skull. Typically, the bone conduction implant is located behind the auricle facilitating the efficient transfer of sound via the skull to the cochlea. The bone conduction implant connecting the vibrator to the skull generally comprises two components: a bone attachment component such as a bone fixture that is attached directly to the skull, and a skin penetrating component attached to the bone attachment component, commonly referred to as an abutment. 
     SUMMARY 
     In one aspect of the present invention, there is a drill bit for drilling into bone, comprising first and second longitudinally extending substantially straight flute blades, and third and fourth longitudinally extending substantially straight flute blades, wherein the first and second flute blades extend further in the distal direction of the drill bit than the third and fourth longitudinally extending flute blades. 
     In another aspect of the present invention, there is a drill bit for drilling into bone, comprising at least a first, second and third longitudinally extending substantially straight flute blades, wherein the drill bit has an extrapolated outer profile established by rotation of the first, second and third flute blades 360 degrees about a longitudinal axis thereof, the extrapolated outer profile includes a first surface having tangents more perpendicular than parallel to the longitudinal axis, and the extrapolated outer profile includes a second surface having tangents more parallel than perpendicular to the longitudinal axis. 
     In another aspect of the present invention, there is a method of implanting a prosthesis in a skull of a recipient, comprising obtaining access to the skull through skin of a recipient, and boring a compound conical hole into the skull of the recipient with a drill bit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects and embodiments of the present invention are described herein with reference to the attached drawings in which: 
         FIG. 1  is a perspective view of a percutancous bone conduction device to which embodiments of the present invention may be relevant; 
         FIG. 2  is a perspective view of a transcutaneous bone conduction device to which embodiments of the present invention may be relevant; 
         FIG. 3A  is an isometric view of a drill bit according to an exemplary embodiment; 
         FIG. 3B  is a side view of the drill bit of  FIG. 3A ; 
         FIG. 4A  is a close-up view of a portion of  FIG. 3B ; 
         FIG. 4B  is a cross-sectional view through the portion depicted in  FIG. 4A ; 
         FIG. 4C  is an end view of the drill bit of FIB.  3 A; 
         FIG. 5  is a bottom view of the drill bit of  FIG. 3A ; 
         FIG. 6  is an exemplary flow chart representing an exemplary method; and 
         FIGS. 7 and 8  are exemplary holes drilled with the drill of  FIG. 3A . 
     
    
    
     DETAILED DESCRIPTION 
     Aspects of the present invention are generally directed to a guide drill bit for drilling into the skull mastoid bone, also referred to herein as a mastoid bone start drill bit. The drill bit has, a distal end that is curved (e.g., semi-circular) and a side extending proximally from the face that is straight (e.g., extends linearly outward). The drill bit may have three or more longitudinally extending substantially straight flute blades. Two of these flute blades may extend further in the distal direction of the drill bit than at least one other flute blade. Use of at least some of the exemplary drill bits detailed herein and/or variations thereof result in less heat generation (which may have a deleterious effect on the bone that adversely affects the bone&#39;s ability to osscointegrate with a component implanted in the drill hole. improved drill times (e.g., drill times to a given depth reduced by ½ or more), improved transport of bone shavings out of the hole, a reduction in so-called “drill wander” at the start of the drilling process, and improved hole roundness, as compared to current ball tip drills used to drill holes in the mastoid bone. 
     The mastoid bone start drill bit may be used during a surgical procedure for placement of a bone fixture in the skull for use with a bone conduction device. Specifically, the mastoid bone start drill bit may be used to drill a hole into the skull as described in greater detail below. A bone fixture may be placed, substantially permanently, into this hole, and thus attached to the skull. It is noted that some start drill bits detailed herein and/or variations thereof may be used at other locations on the skull beyond the mastoid bone to drill holes used for other types of prostheses other than bone conduction devices. Moreover, some start drill bits detailed herein and/or variations thereof may be used on other bones at other locations of the human body. 
       FIG. 1  is a perspective view of a percutaneous bone conduction device  100  with which embodiments of the present invention may be pertinent. As shown, the recipient has an outer ear  101 , a middle ear  102  and an inner ear  103 . Elements of outer ear  101 , middle ear  102  and inner ear  103  are described below, followed by a description of bone conduction device  100 . 
     In a fully functional human hearing anatomy, outer ear  101  comprises an auricle  105  and an ear canal  106 . A sound wave or acoustic pressure  107  is collected by auricle  105  and channeled into and through ear canal  106 . Disposed across the distal end of ear canal  106  is a tympanic membrane  104  which vibrates in response to acoustic wave  107 . This vibration is coupled to oval window or fenestra ovalis  210  through three bones of middle ear  102 , collectively referred to as the ossicles  111  and comprising the malleus  112 , the incus  113  and the stapes  114 . The ossicles  111  of middle ear  102  serve to filter and amplify acoustic wave  107 , causing oval window  210  to vibrate. Such vibration sets up waves of fluid motion within cochlea  139 . Such fluid motion, in turn, activates hair cells (not shown) that line the inside of cochlea  139 . Activation of the hair cells causes appropriate nerve impulses to be transferred through the spiral ganglion cells and auditory nerve  116  to the brain (not shown), where they are perceived as sound. 
       FIG. 1  also illustrates the positioning of bone conduction device  100  relative to outer ear  101 , middle ear  102  and inner ear  103  of a recipient of device  100 . As shown, bone conduction device  100  is positioned behind outer ear  101  of the recipient and comprises a sound input element  126  to receive sound signals. Sound input element may comprise, for example, a microphone, telecoil, etc. In an exemplary embodiment, sound input element  126  may be located, for example, on or in bone conduction device  100 , or on a cable extending from bone conduction device  100 . 
     In an exemplary embodiment, bone conduction device  100  comprises an operationally removable component  120  and a bone conduction implant  140 . The operationally removable component operationally removably attaches to the bone conduction implant. By operationally removably attaches, it is meant that it is removable in such a manner that the recipient can relatively easily attach and remove the operationally removable component  120  to the bone conduction implant  140  during normal use of the bone conduction device  100 . This as contrasted with how the bone conduction implant  140  is attached to the skull, as will be detailed below. The operationally removable component  120  includes a sound processor (not shown), a vibrating electromagnetic actuator (not shown) and/or various other operational components, such as sound input device  126 . More particularly, sound input device  126  (e.g., a microphone) converts received sound signals into electrical signals. These electrical signals are processed by the sound processor. The sound processor generates control signals which cause the actuator to vibrate. In other words, the actuator converts the electrical signals into mechanical motion to impart vibrations to the recipient&#39;s skull. 
     As illustrated, the operationally removable component  120  of the bone conduction device  100  further includes a coupling apparatus  130  configured to operationally removably attach the operationally removable component to a bone conduction implant (also referred to as an anchor system and/or a fixation system) which is implanted in the recipient. In the embodiment of  FIG. 1 , coupling apparatus  130  is coupled to the bone conduction implant  140  implanted in the recipient in a manner that is further detailed below with respect to exemplary embodiments of the bone conduction implant  140 . Briefly, an exemplary bone conduction implant  140  may include a percutaneous abutment  142  attached to a bone fixture  162  via a screw (not shown), the bone fixture  162  being fixed to the recipient&#39;s skull bone  136 . The abutment extends from the bone fixture  162  which is screwed into bone  136 , through muscle  134 , fat  128  and skin  132  so that coupling apparatus  130  may be attached thereto. Pertinently, the bone fixture  162  may be screwed into a hole prepared using a drill bit as detailed herein and/or variations thereof. Such a percutaneous abutment provides an attachment location for coupling apparatus  130  that facilitates efficient transmission of mechanical force. 
       FIG. 2  is a perspective view of a transcutaneous bone conduction device  200 , as differentiated from the percutaneous bone conduction device  100  of  FIG. 1 , with which embodiments of the present invention are also pertinent. Like reference numbers of  FIG. 2  correspond to those of  FIG. 1 . 
       FIG. 2  also illustrates the positioning of bone conduction device  200  relative to outer ear  101 , middle ear  102  and inner ear  103  of a recipient of bone conduction device  200 . As shown, bone conduction device  200  is positioned behind outer ear  101  of the recipient. Bone conduction device  200  comprises an external component  240  and implantable component  250 . The bone conduction device  200  includes a sound input element  126  to receive sound signals 
     Bone conduction device  200  comprises a sound processor (not shown), an actuator (also not shown) and/or various other operational components. In operation, sound input device  126  converts received sounds into electrical signals. These electrical signals are utilized by the sound processor to generate control signals that cause the actuator to vibrate. In other words, the actuator converts the electrical signals into mechanical vibrations for delivery to the recipient&#39;s skull. 
     In accordance with embodiments of the present invention, a fixation system  262  which includes a bone fixture may be used to secure implantable component  250  to skull  136 . As described below, fixation system  262  may be a bone fixture fixed to skull  136 , and also attached to implantable component  250 . Pertinently, the bone fixture of fixation system  262  may be screwed into a hole prepared using a drill bit as detailed herein and/or variations thereof. 
     In one arrangement of  FIG. 2 , bone conduction device  200  is a passive transcutaneous bone conduction device. That is, no active components, such as the actuator, are implanted beneath the recipient&#39;s skin  132 . In such an arrangement, the active actuator is located in external component  240 , and implantable component  250  includes a magnetic plate, as will be discussed in greater detail below. The magnetic plate of the implantable component  250  vibrates in response to vibration transmitted through the skin, mechanically and/or via a magnetic field, that are generated by an external magnetic plate. 
     In another arrangement of  FIG. 2 , bone conduction device  200  is an active transcutaneous bone conduction device where at least one active component, such as the actuator, is implanted beneath the recipient&#39;s skin  132  and is thus part of the implantable component  250 . As described below, in such an arrangement, external component  240  may comprise a sound processor and transmitter, while implantable component  250  may comprise a signal receiver and/or various other electronic circuits/devices. 
     As may be seen from  FIGS. 1 and 2 , in at least some embodiments, the percutaneous bone conduction device  100  and the transcutaneous bone conduction device  200  include a fixture ( 162  and  262 , respectively) that screws or is otherwise fitted into a hole that extends into the skull  136 . As noted, some embodiments of the present invention are directed to a drill bit that may be utilized to form the hole into which the fixtures extend, as will now be described. 
       FIG. 3A  depicts an exemplary mastoid bone start drill bit  300  (hereinafter, sometimes referred to as the “drill bit”). In an exemplary embodiment, the drill bit may be made from hardened stainless steel and/or made from other suitable materials, such as, for example, any material acceptable for surgical grade bone drills. The drill bit  300  includes a shank  302  to which a dill chuck of a hand-held and/or mounted drill motor grips the drill bit  300 , as will be described below. With reference to  FIG. 3B , which depicts a side view of drill bit  300 , shank  302  has a generally constant diameter D 1  of about 2 mm to about 2.5 mm, and a length L 1  of about 16 mm. It is noted at this time that the dimensions and geometries detailed herein are but exemplary, and other embodiments may have other dimensions and geometries. In this regard, Applicants provide details to further the art and note that alternate embodiments may have other dimensions and geometries beyond those detailed herein. However, its is further noted that in some embodiments, the figures shall be considered to be to scale unless otherwise noted, but again, other embodiments will have features that differ in scale. It is further noted that dimensions modified by the term “about” also include those exact dimensions. Embodiments include drill bits having dimensions of any value within ranges specified in 0.01 mm increments. 
     Drill bit  300  includes cylindrical portion  304  that serves as a stop which limits or otherwise prevents a user of the drill bit  300  from drilling too far into the skull, as will be detailed below. The cylindrical portion  304  may have a generally constant diameter D 2  of about 4 mm, and some other embodiments may be within a range of about 3.5 mm to about 4.5 mm. Cylindrical portion  304  may have a length L 2  of about 6 to about 7 mm. Extending from the cylindrical portion  304  is a bone removal portion  310  that extends below the surface of the skull  136  during the drilling operation. The collective components of bone removal portion  310  extend a length L 3  of about 5.1 mm and in some alternate embodiments may have a length anywhere within a range of about 4 mm to about 6 mm in 0.1 mm increments, from the cylindrical portion  304 . 
     Bone removal portion  310  includes a relief portion  312  and a cutting head  320 . The cutting head  320  extends a length L 4  of about 4 mm, and in some other embodiments may be within a range of about 3 mm to about 5 mm. Additional features of the relief portion  312  and cutting head  320  will now be described with respect to  FIGS. 4A, 4B and 4C , which depict more detailed views of the bone removal portion  310 , along with continued reference to  FIGS. 3A and 3B . 
     As may be seen in  FIG. 4A , cutting head  320  comprises a plurality of substantially straight flute blades  321 ,  322  and  323 .  FIG. 4B  depicts a cross-sectional view of the cutting head  320  taken at a location slightly distal from the proximal end of the cutting head  320 . As may be seen, the exemplary cutting head  320  includes four substantially straight flute blades, with the fourth flute blade  324  eclipsed in  FIG. 4A , which may be seen in  FIGS. 4B and 4C . The substantially straight flute blades extend along the longitudinal axis  301  of the drill bit  300 . It will be understood that the scope of “substantially straight” includes both straight flute blades as depicted in  FIGS. 3A-4B  and blades having a twist of about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 degrees per mm or less. As may be seen from  FIG. 4B , respective cutting edges of the flute blades are arranged about a longitudinal axis of the drill bit  301  at substantially equidistant locations. In an exemplary embodiment, this is the case with respect to a drill bit having two, three, four, five, six, seven, eight or more flute blades, each of which may or may not have a cutting edge, and thus the drill bit may have one, two, three, four, five, six, seven, eight or more cutting edges. Also as may be seen, the respective cutting edges of the flute blades  321  and  322  effectively opposite one another, just as is the case with the respective cutting edges of flute blades  323  and  324 . 
     With regard to the substantially straight flute blades, some embodiments of the drill bit include no spiral flute blades and/or no helix structure. 
     As may be see, each flute blade comprises four generally planar surfaces that extend a substantial length along the longitudinal axis of the cutting head  320 . Surfaces  330  and  332  meet to form cutting edge  334 . The angle between the two surfaces falls within a range of about 70 to about 80 degrees, at any value therebetween in about 0.1 degree increments.  FIG. 4C , which depicts a view of the drill bit  300  looking towards the distal tip, further depicts the angle A 1  between these two surfaces as measured from the tangent line formed by rotation of the drill bit, where angle A 1  falls within a range of about 10 to about 20 degrees, at any value therebetweeen in about 0.1 degree increments.  FIG. 4C  also includes arrow  390 , which depicts the rotation direction of the drill bit  300  during use. Surface  336  extends away from surface  330  at about a 30 to about a 60 degree angle to surface  338 , which extends away from surface  336  at about a 30 to about a 60 degree angle, again at any value therebetween within these ranges in about 0.1 degree increments. In view of the rotation direction of the drill bit  300 , surface  332  corresponds to a leading surface on the leading side of the flute blade  321 , and surfaces  330 ,  336  and  338  correspond to trailing surfaces on the trailing side of the flute blade  321 . 
     Cutting edge  334  is configured to cut human bone in general, and the mastoid bone in particular. 
     Referring to flute blade  321 , it may be seen that surface  330  extends towards the distal tip of the cutting head  320 . In this regard, the distal end of the drill bit  300  corresponds to a location where a tangent plane of the curve forming the cutting edge of the flute  321  (and  322 ) is normal to the longitudinal axis  301  of the drill bit  320 . The respective cutting edges  334  of flute blades  321  and  322  extend proximally from this distal tip. 
     At a distance L 5  from the distal tip, which may correspond to about 1 mm, and in other embodiments, may correspond to any value within a range of about 0.5 mm to about 1.5 mm in 0.01 mm increments, the surface  330  begins to curve towards the center of the drill bit  300 . In this regard, the location at the distance L 5  from the tip represents a transition point where the cutting edge  334  transitions from a substantially linear cutting edge to a curved cutting edge. 
     As will be understood, rotation of the drill bit  320 , in general, and the flute blades  321 ,  322 ,  323  and  324  in particular, 360 degrees about the longitudinal axis  301  results in an extrapolated outer profile of the drill bit  320 . In an exemplary embodiment, this extrapolated outer profile corresponds to a “negative” of the hole drilled by the drill bit  320 . That is, in a perfect use scenario, the extrapolated outer profile would perfectly conform to the inner profile of the resulting hole. The extrapolated outer profile includes a first surface having tangents more perpendicular than parallel to the longitudinal axis and contiguous thereto, a second surface having tangents more parallel than perpendicular to the longitudinal axis. These first and second surfaces respectively correspond to the portions of the drill head  320  distal and proximal of the distance L 5  from the distal tip (i.e., the transition point where cutting edge  334  transitions from a substantially linear edge). The first surface forms a rounded front end face of the extrapolated outer profile owing to the fact that the cutting edge  334  is curved distally of the location at L 5 . In an exemplary embodiment, the curvature of the cutting edge  334  distally of the location at L 5  is such that the first surface forms a portion of a sphere (e.g., a hemisphere or a truncated hemisphere). Because of the contiguous nature of the first and second surfaces, the surfaces respectively form a cone and a truncated cone, thus collectively forming a compound cone of increasing diameter along the longitudinal axis thereof. As will be understood, the second surface forms a conical side face that is substantially uniformly expand outward owing to the fact that cutting edge  334  extends linearly outward. However, in alternate embodiments, the cutting edge  334  extends substantially parallel to the longitudinal axis  301 , and thus the second surface may instead form a cylindrical side face. Note further that the location at L 5  may not be the only transition point on the cutting head  320 . In this regard, the cutting edge  334  may have multiple varying geometries. By way of example, the cutting edge  334  may extend linearly outward to a location about 2.5 mm from the distal tip, and then extend parallel to the longitudinal axis  301 . Thus, the extrapolated outer profile may include a third surface having tangents that are parallel to the longitudinal axis  301 . As may be seen, the first surface forms an end face surface of the drill bit and the second surface forms a side face surface of the drill bit. By end face, it is meant that the first surface predominantly faces the distal end of the drill bit, and by side face, it is meant that the second surface predominantly faces the side of the drill bit. 
     Referring back to  FIG. 3B , the diameter D 3  of the drill bit at the transition point has a value that falls within a range of about 2 mm to about 3 mm in increments of 0.01 mm (e.g., 2.3 mm). The diameter D 4  of the drill bit at the end of the cutting head  320  has a value that falls within a range of about 2.5 mm to about 3.5 mm in increments of 0.01 mm (e.g., 3.1 mm). 
     Surface  332  of flute blade  321  continues to extend to the surface tip, and thus cutting edge  334  curves towards the distal tip of the cutting head  320  as well. Conversely, surface  336  does not continue to the distal tip of the cutting head  320 . Instead, another surface  331  extends towards the center of the drill bit  300 . As may be seen, surface  331  links surface  338  to surface  330 . 
     The configuration of flute blade  322  is substantially identical to flute blade  321  except for position relative to the longitudinal axis  301  of the drill bit  300 . In this regard, as will be seen from  FIGS. 4B and 4C , the surfaces  332  of flute blade  321  and  322  are offset from one another relative to the longitudinal axis  301  (centerline) of the drill bit  300 . Because flute blade  322  has a substantially identical (including identical) configuration but oriented 180 degrees about the longitudinal axis  301  of the drill bit  300  and has the surfaces  332  offset as just detailed, a relief surface  333   a  is provided on the trailing side of flute blade  321 . The relief surface  333   a  extends to relief surface  333   b  located on the leading side of the flute blade  322 . 
     With regard to surfaces  332  being offset, as may be seen from  FIG. 4B , the cutting head  320  of the drill bit  300  has an outer cross-section in the form of an offset cruciform. As further may be seen, the flute blades extend outward away from the longitudinal axis  301  from a central section of the drill bit. This central section is bounded by planes lying on surfaces  338  of the flute blades, and has a square cross section having a width WI. The general shape of the offset cruciform is constant along most of the longitudinal axis of the cutting head  320 . By way of example,  FIG. 4B  may also correspond to a cross-sectional view of the cutting head  320  lying on a plane normal to the longitudinal axis taken at about location L 5 . Thus the outer cross-section of the cutting head  320  at L 5  and the outer cross-section at the location slightly distal from the proximal end of the cutting head  320  (i.e., the location corresponding to the plane which corresponds to that from which  FIG. 4B  is based) are scaled substantially identical profiles of one another and corresponding features of the outer cross-sections are rotationally aligned about the longitudinal axis with one another. This may be the case for any planes lying along the longitudinal axis between the location L 5  and the proximal end of the cutting head  320  and any distance range therebetween in 0.01 increments (e.g., 1 mm, 1.01 mm, 1.11 mm, etc.). As will be understood from the figures, cross-sectional views of the cutting head  320  lying on respective planes normal to the longitudinal axis  301  between about section L 5  and about the proximal end of the cutting head  320  will be are scaled substantially identical profiles of one another and corresponding features of the outer cross-sections are rotationally aligned about the longitudinal axis with one another. It is noted that the cross-sectional profiles at and proximate to the location L 5  will include a portion of relief  333   b . However, the depth of relief  333   b  into the leading surface of the flute blades is sufficiently shallow that the cross-sectional profiles may still be considered to be scaled substantially identical profiles to those without the portion of the relief  333   b  (such as that depicted in  FIG. 4B ). 
     In an exemplary embodiment, width WI varies linearly along the length of the drill bit (consistent with the cross-sections being scaled substantially identical profiles). For example the width may fall within a value in a range of about 0.5 to about 0.8 mm in 0.1 mm increments (e.g., 0.7 mm) at the location L 5 , and may fall within a value in a range of about 0.7 to 1.1 mm in 0.1 mm increments (e.g., 0.9 mm) at the location slightly distal from the proximal end of the cutting head  320  (i.e., the location corresponding to the plane which corresponds to that from which  FIG. 4B  is based). 
     Flute blades  323  and  324  share many of the same features as flute blades  321  and  322 . In the exemplary embodiment of these blades that will now be described, unless otherwise noted, the features of flute blades  321  and  322  just described are also applicable to flute blades  323  and  324 . 
     Flute blades  323  and  324  do not extend to the distal tip of the drill bit  300 . Instead, the cutting edges of those blades curve towards the center of the drill bit  300  and truncate at a location about 0.05 mm, about 0.1 mm, about 0.15 mm, about 0.2 mm, about 0.25 mm, about 0.3 mm, about 0.35 mm, about 0.4 mm, about 0.45 mm, about 0.5 mm, about 0.55 mm or about 0.6 mm, where “about” as just used corresponds to a range of about plus or minus 0.03 mm. However, in other embodiments, one or both of the flute blades  323  and  324  may extend to the distal tip. In the same vein, one or both of the flute blades  321  and  322  may not extend to the distal tip. In an exemplary embodiment, this reduces a requisite minimum force that a surgeon or the like need apply on the drill bit in the direction of the skull during the drilling operation. 
     Referring to  FIG. 5 , flute blades  323  and  324  also differ from flute blades  321  and  322  in that the most distal portions of the cutting edges of the of the flute blades do not converge into the flute blades  321  and  322  and do not converge into each other (as is the case with the flute blades  321  and  322 ). That is, the respective distal tips  572  and  574  of the third and fourth flute blades  323  and  324  are located away from a distal tip  570  of the drill bit  300  formed by the first and second flute blades  321  and  322 . In an exemplary embodiment, when viewed from a side of the drill bit  320  such that the sides of flute blades  323  and  324  are seen extending away from the center of the drill bit in an equidistant manner (i.e., flute blades  321  and  322  extend towards and away or away and towards, respectively, from the viewer), such as that depicted in  FIG. 5 , reliefs  540  and  541  in flute blades  323  and  324 , respectively, may be seen. Thus, flute blades  321 ,  322 ,  323  and  324  form a trident configuration, as may be seen in  FIG. 5 . In this regard, it is noted that the trident feature does not create an enclosed passageway within the drill bit. That is, in the embodiment of the FIGs., there is no enclosed passageway or the like within the drill bit, at least not one having an opening to an exterior of the drill bit (e.g., the drill bit may, in some embodiments, have a cavity, but this cavity is entirely sealed). In this regard, in an exemplary embodiment, an outer profile of a cross-section of the drill bit and/or the cutting head taken on any plane lying on and parallel to the longitudinal axis  301  of the drill bit entirely encompasses any cavity in the drill bit. 
     Also as may be seen in  FIG. 5 , a relief  533  is located in the leading side of flute blade  323 . Relief  533  substantially corresponds to reliefs  333   a  and  333   b . As with reliefs  333   a  and  333   b , relief  533  is optional in some embodiments. 
     Referring back to  FIG. 3B , as noted above, the drill bit  320  includes a relief portion  312 . Relief portion  312  connects the cutting head  320  to cylindrical portion  304 . In the exemplary embodiment depicted in the FIGs., relief portion  312  has a maximum diameter D 6  having a value falling within the range of about 2 mm to about 3 mm in about 0.1 mm increments (e.g., 2.5 mm). As may be seen, relief portion  312  is a compound structure having a surface  350  that curves away from surface  338  of the flute blades into the surface  352  that serves as the stop of cylindrical portion  304 . In this regard, surface  350  links the surface  338  to the surface  352 . While surface  350  is depicted as a curved surface having a substantially uniform radius of curvature, in other embodiments, surface  350  may have a compound curvature and/or may have straight sections. In an exemplary embodiment, the surface  350  extends outward on a plane that is normal to the longitudinal axis  301  of the drill bit. 
     It is noted that in some alternate embodiments, surface  350  is instead is located on cylindrical portion  304 . That is, surface  338  may extend all the way to cylindrical portion  304 , and surface  350  may be formed on cylindrical portion  304 , thus resulting in four gaps in the stop surface  352  and the sides of the cylindrical portion  304  proximate to the stop surface  352 . 
     As is noted above, the drill bit  300  is configured to drill into bone in general and into the mastoid bone in particular. In an exemplary embodiment, the flute blades are configured to remove bone shavings in general and mastoid bone shavings/chips in particular from a bore drilled by the drill bit via rotation of the drill bit, and, in at least some embodiments detailed herein and variations thereof, this is accomplished only by rotation of the drill bit (e.g., not by moving the drill bit in and out of the bore). More particularly, the leading side of a first flute blade and the trailing side of the flute blade facing the leading side of the first flute blade form a groove that results in a longitudinal passageway for the loose bone shavings/chips to travel through. In the embodiments depicted in the FIGs., there are four such passageways. 
     In an exemplary embodiment, the compound conical nature of the cutting head and/or the fact that the portion of the cutting head proximal from location L 5  expands outward results in an improved drilling speed (i.e., time to reach the desired depth), translating to shorter operations and/or reduced heat transfer into the skull as compared to, for example a ball drill. In some embodiments, this improved drilling speed is obtained as a result of improved bone shaving/chip transport due to the expansive nature of the drill bit. In this regard, in at least some embodiments of the drill bits detailed herein and variations thereof, the drill bit avoids significant packing of bone shavings/chips in the hull due to the improved shaving/chip transport features of the drill bit. 
     In an exemplary embodiment, all or part of (e.g., the portions proximate the cutting edges) of the cutting head  320  of the drill bit may be coated with a hard carbon coating film or other coating configured to provide a surface finish that provides a reduced/low friction surface, at least one that is relatively lower as compared to an uncoated drill bit. Such coating may correspond to a diamond like carbon (DLC), amorphous diamond or the like, such as, for example, that detailed in WO 0027301 or variations thereof. 
     In an exemplary embodiment, the geometries and dimensions of the drill bits detailed herein and variations thereof are configured so as to limit heat generation produced when drilling into bone such as the mastoid bone. In this regard, some embodiments of the drill bits detailed herein and variations thereof provide a mastoid bone guide drill bit having geometries and dimensions that limit friction between the drill bit and the mastoid bone during the drilling procedure, thus limiting heat generation. By way of example and not by way of limitation, some embodiments detailed herein and variations thereof are configured such that contact between the outer drill surface (e.g., the cutting edges) and the bone is relatively minimized. In some embodiments, this may be achieved by minimizing the cutting edges on the drill bit to three or four (e.g., utilizing three or four flute blades). 
     By way of example only and not by way of limitation, at least some drill bits detailed herein and variations thereof reduce the air temperature increase within the hole by about a 3 degree Celsius, or, in some embodiments, more than about a 3 degree Celsius, as compared to the increase resulting from a ball drill in the same bone. 
     Embodiments include a surgical procedure for placement of the bone fixture of the percutaneous and/or transcutaneous bone conduction devices.  FIG. 6  details a flow chart  600  representative of an exemplary method. Step  610  of the exemplary method includes preparation of the implant site behind the outer ear. Specifically, step  610  entails obtaining access to the mastoid bone by utilizing a scalpel to make an incision through the periosteum to reach the mastoid bone. In an alternate embodiment, another surgical instrument may be utilized. It is noted that at least some embodiments of the drill bits detailed herein and/or variations thereof permit this step to be performed in a relatively less invasive manner than that which would be the case with a ball drill. Step  620  entails utilizing a mastoid bone start drill bit according to any of those detailed herein and/or variations thereof to perform an initial drilling of a hole in the mastoid bone. Step  620  may be executed without boring a pilot hole or the like. That is, step  630  entails drilling into solid bone, meaning that this is the first drill of any meaningful depth into the mastoid bone. In an exemplary embodiment utilizing any of the drill bits detailed herein and/or variations thereof, the result of this step corresponds to that depicted in  FIG. 7 , where element  710  is the resulting hole in bone  136 . In an exemplary embodiment, hole  710  may have a different configuration than that depicted in  FIG. 7 , such as may be the case if the surgeon elects not to drill into the bone a distance that causes the stop surface  352  to contact the surface  720  of bone  136 /come close to the surface  720  of the bone. 
     Step  630  entails widening the drilled hole utilizing a drill countersink, and step  640  entails inserting a bone fixture or the like into the hole and thereby securing the bone fixture to the mastoid bone. 
     In an exemplary embodiment, there is a method of drilling a hole that results in a hole having the configuration depicted in  FIG. 7 , corresponding to drilling down into the bone until the stop surface  352  contacts or about contacts the surface  720  and/or a configuration indicative of drilling a hole with any of the drill bits detailed herein and/or variations thereof a depth having a value falling within a range of about 1 mm to about 6 mm in increments of about 0.1 mm. By way of example, utilizing the drill bit of  FIG. 3B  to drill a hole about 2.5 mm deep would result in a hole  810  having a compound conical surface corresponding to that depicted in  FIG. 8 . 
     In an exemplary embodiment, the flute blades (or other components of the drill bit) extending to the distal tip provide a relatively sharp tip as compared to, for example, a ball-tip geometry that may have about the same or the same extrapolated outer profile at the distal tip and/or the areas proximate thereto (e.g., the “end face” as detailed below). This improves the ease with which a surgeon may accurately position the drill bit and initiate drilling by, for example, reducing oscillating tendencies which may exist with the ball-tip geometry. These oscillating tendencies may result in a hole having an elliptical cross-section lying on a plane normal to the longitudinal axis of the hole, as opposed to a substantially circular cross-section on that plane. In an exemplary embodiment, there is a drill bit that provides for, over the course of 100 operations in any of Australia, the United States, Canada, Japan, the United Kingdom, France, Sweden, Germany, Italy or Austria, or the European Union, as those entities are defined as of the filing date of this application, by physicians licensed to perform surgery in those entities associated with hearing prosthesis implantation including drilling a hole into a skull, drilled holes into a skull of at least 4 mm in depth and between about 2 mm and about 5 mm in diameter having an inner diameter lying within two concentric circles spaced about 3 mm apart for at least about 75% of the depth of the hole about 80% of the time. By providing a hole having roundness achieved by the drill bits detailed herein and variations thereof, holes can be prepared that correspond to the perceived exactness of hole geometries required for self-tapping bone fixtures. 
     In yet a further exemplary embodiment, some of the drill bits detailed herein and variations thereof provide for, over the course of the aforementioned  100  operations with drilled holes drilled to a depth of 4 mm or more, at least about 80% of the time the depth of 4 mm being reached, in some embodiments, within one minute from commencement of drilling into the skull, in some embodiments, within 40 seconds from commencement, in some embodiments, within 30 seconds from commencement of drilling into the skull, and, in some embodiments, within 20 seconds from commencement of drilling into the skull. In some embodiments, these temporal performance features are also achieved while meeting the above-mentioned roundness performance features. 
     In some embodiments, there is a method which includes performing the aforementioned  100  operations and meeting the above-mentioned temporal and/or roundness performances at the listed percentages. In this regard, in an exemplary embodiment, there is a method of implanting a series of prostheses in respective skulls of respective recipients, comprising a) obtaining access to the skull through skin of the respective recipient, b) boring a hole into solid bone of the skull of the recipient with a drill bit to a depth having a value within a range of about 3 mm to about 5 mm implanting a bone fixture into the drilled hole (which may be executed after a widening action is executed. In this exemplary method, steps a, b an c are repeated at least 25 times, at lest 50 times, at least 75 times, at least 100 times, at least 150 times and/or at least 200 times on respective recipients. In alternate methods, step b is performed within about 1 minute, within about 45 seconds, within about 30 seconds and/or within about 20 seconds for all of the times specified. 
     An embodiment includes a method of drilling a hole into the mastoid bone for the bone fixture while avoiding over-heating of the surrounding (remaining) mastoid bone tissue. In an exemplary embodiment, this avoidance of the over-heating may provide utilitarian value in that it increases the chances of successfully osseointegrating the bone fixture to the mastoid bone, which reduces the chance that the bone fixture may later loose its utility. 
     Thus, in an exemplary embodiment, at least method step  630  includes cooling of the drill bit and/or the mastoid bone in order to prevent or at least reduce the heat induced into the mastoid bone during the drilling process. By way of example, cooling of the drill bit and/or bone through irrigation of a coolant fluid directed towards the distal tip of the drill bit may be executed during a portion of the drilling procedure (a single instance or a plurality of separate instanced) or during the entire drilling procedure, and may encompass temporal periods before and/or after the drilling procedure. In an exemplary embodiment, the coolant fluid is a saline liquid solution, although other coolant fluids may be utilized. 
     It is further noted that method step  630  may be practiced by moving the drill bit in and out (towards and away from the mastoid bone) slightly when enlarging the hole, thus permitting visual inspection and/or facilitating coolant flow to the distal tip of the drill bit. 
     Step  630  may entail drilling into the mastoid bone a depth of 4 mm from the surface (or, more appropriately, an extrapolated surface) of the mastoid bone, at least if mastoid bone thickness permits such. The embodiments of the drill bits detailed herein and variations thereof may be used, in some embodiments, to achieve this depth. This depth may be suitable for placement of a bone fixture configured to extend about 4 mm into the mastoid bone. If a more shallow hole is desired, a spacer (e.g., a plastic spacer) may be placed on the drill bit. This spacer may be configured to abut the surface  352 , and thus extend distally therefrom. This spacer may limit the drill depth to a different depth, such as, for example, to a depth within a range of 4 mm to 1 mm in about 0.1 mm increments, such depth being suitable for the desired bone fixture. Greater depths may also be drilled as well, using variations of the drill bits detailed herein. 
     The desired depth may be based on the thickness of the mastoid bone at the implant location. In this regard a depth may be selected such that the wall of the sigmoid sinus in the recipient is not penetrated. Having said that, the end face of the drill bits detailed herein and variations thereof may be configured such that even if the drill penetrates to the dura or sigmoid sinus, the dura and/or the signoid sinus is not harmed. In some embodiments, the dura or sigmoid sinus may be cut but still not be harmed. It is believed that at least some of the embodiments of the drill bits detailed herein are configured to meet these features, although these embodiments may not meet these features. 
     It is noted that in an alternate embodiment, instead of adding spacers (or removing spacers) from the drill bit, drill bits may be prefabricated to have different lengths from the distal tip to the stop surface  352 , and thus the surgeon may select from a plurality of drill bits to obtain the desired depth. By way of example, a drill bit set may be provided to the surgeon including a plurality of drill bits, where respective lengths L 5  of the drill bits may be any lengths from about 1 mm to about 8 mm in increments of 0.1 mm. By way of example, a set may include five drill bits having lengths L 5  of 1 mm, 2 mm, 3 mm, 4 mm and 5 mm, respectively. 
     In this regard, there is a method of preparing a hole as detailed herein which includes determining the desired depth and at least one of placing a spacer on the drill bit corresponding to the desired depth or selecting a drill bit having a length L 5  corresponding to the desired depth. 
     It is also noted that in some alternate embodiments, instead of the stops/spacers or in addition to the stops and spacers, the drill bit may be may be provided with depth markings on its outer surface to provide an indication to the surgeon of the depth of drilling. Such may be done through the use of, for example, laser markings and/or etched markings, etc.) 
     Some embodiments of the drill bits detailed herein and variations thereof have utility in that the drill bits reduce the minimum size space in the skin needed to reach the bone to drill therein (e.g., the incisions of method step  610  detailed above are smaller than that which may be needed for, for example, a ball drill). Thus, some embodiments involve a less-invasive procedure than previous drilling procedures. Indeed, in some embodiments, step  610  is achieved by opening a hole in the skin less than about 10 mm in diameter. 
     While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.