Abstract:
A flexible drill bit comprising:
       a proximal shaft portion for connecting to a source of turning;   a distal cutting tip portion for boring into a material; and   an intermediate shaft portion extending between the proximal shaft portion and the distal cutting tip portion, the intermediate shaft portion being characterized by (i) sufficient longitudinal flexibility so as to permit the flexible drill bit to be passed along a curve, and (ii) sufficient torsional strength to permit the flexible drill bit to bore into the material.

Description:
REFERENCE TO PENDING PRIOR PATENT APPLICATION 
       [0001]    This patent application claims benefit of pending prior U.S. Provisional Patent Application Ser. No. 61/583,265, filed Jan. 5, 2012 by J. Brook Burley et al. for FLEXIBLE DRILL BIT (Attorney&#39;s Docket No. FIAN-73 PROV), which patent application is hereby incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to surgical methods and apparatus in general, and more particularly to methods and apparatus for drilling a hole in bone. 
       BACKGROUND OF THE INVENTION 
     The General Trend Toward Treating Joint Pathologies Using Minimally-Invasive, and Earlier, Interventions 
       [0003]    The current trend in orthopedic surgery is to treat joint pathologies using minimally-invasive techniques. Such minimally-invasive, “keyhole” surgeries generally offer numerous advantages over traditional, “open” surgeries, including reduced trauma to tissue, less pain for the patient, faster recuperation times, etc. 
         [0004]    By way of example but not limitation, it is common to re-attach ligaments in the shoulder joint using minimally-invasive, “keyhole” techniques which do not require laying open the capsule of the shoulder joint. By way of further example but not limitation, it is also common to repair torn meniscal cartilage in the knee joint, and/or to replace ruptured ACL ligaments in the knee joint, using minimally-invasive, keyhole techniques. 
         [0005]    While such minimally-invasive approaches can require additional training on the part of the surgeon, such procedures generally offer substantial advantages for the patient and have now become standard procedures for treating many shoulder joint and knee joint pathologies. 
         [0006]    In addition to the foregoing, in view of the inherent advantages and widespread availability of minimally-invasive approaches for treating pathologies of the shoulder joint and the knee joint, the current trend is to provide such treatment much earlier in the lifecycle of the pathology, so as to address patient pain and so as to reduce the likelihood of exacerbating the pathology itself. This is in marked contrast to traditional surgical practices, which generally dictated postponing surgical procedures for as long as possible so as to spare the patient from the substantial trauma generally associated with invasive surgery. 
       Treatment for Pathologies of the Hip Joint 
       [0007]    Unfortunately, minimally-invasive treatments for pathologies of the hip joint have lagged far behind minimally-invasive treatments for pathologies of the shoulder joint and the knee joint. This is generally due to (i) the complex geometry of the hip joint itself, and (ii) the nature and location of the pathologies which are typically encountered in the hip joint. 
         [0008]    More particularly, the hip joint is generally considered to be a “tight” joint, in the sense that there is relatively little room to maneuver within the confines of the joint itself. This is in marked contrast to the shoulder joint and the knee joint, which are generally considered to be relatively “spacious” joints (at least when compared to the hip joint). As a result, it is generally relatively difficult for surgeons to perform minimally-invasive procedures on the hip joint. 
         [0009]    Furthermore, the pathways and approaches for entering the interior of the hip joint (i.e., the natural pathways which exist between adjacent bones and/or delicate neurovascular structures) are generally much more limited for the hip joint than for the shoulder joint or the knee joint. This limited access further complicates a surgeon&#39;s ability to effectively perform minimally-invasive procedures on the hip joint. 
         [0010]    In addition to the foregoing, the nature and location of the pathologies of the hip joint also complicate a surgeon&#39;s ability to perform minimally-invasive procedures on the hip joint. By way of example but not limitation, consider a typical labrum tear or detachment in the hip joint. In this situation, instruments must generally be introduced into the joint space at an angle of approach which is offset from the angle at which the instrument addresses the joint anatomy. This makes drilling into bone, for example, a significantly more complicated procedure than in a case where the angle of approach is effectively aligned with the angle at which the instrument addresses the joint anatomy, such as is frequently the case in the shoulder joint. Furthermore, since the working space within the hip joint is typically extremely limited, it is even more difficult to properly adjust the alignment of surgical instruments (e.g., a drill) where the angle of approach is not aligned with the optimal angle for the instrument to address the joint anatomy. 
         [0011]    As a result of the foregoing, minimally-invasive hip joint procedures are still relatively difficult to perform and hence less common in practice. Consequently, patients are typically forced to manage and endure their hip pain for as long as possible, until a resurfacing procedure or a partial or total hip replacement procedure can no longer be avoided. These resurfacing or replacement procedures are generally then performed as a highly-invasive, open procedure, replete with all of the disadvantages associated with highly-invasive, open procedures. 
         [0012]    As a result, there is, in general, a pressing need for improved methods and apparatus for treating pathologies of the hip joint. 
         [0013]    More particularly, there is a pressing need for improved methods and apparatus for introducing instruments into the joint space where the instruments will address the joint anatomy at an angle which is offset from the angle of approach. By way of example but not limitation, in some cases it may be desirable to drill into bone at an angle which is offset from the angle at which the drill is inserted into the joint space, in order to create a hole in the bone at an optimum location, e.g., at an optimum location to receive a suture anchor for use in effecting a labral repair. 
       SUMMARY OF THE PRESENT INVENTION 
       [0014]    These and other objects of the present invention are addressed by the provision and use of a new flexible drill bit, which may be used for drilling a hole in bone (or another material) where the flexible drill bit will enter the bone at an angle which is offset from the angle of approach. 
         [0015]    The flexible drill bit is particularly advantageous in situations where it is desirable to pass the drill bit into a joint in a curved configuration, such as where the drill bit is to be inserted into the joint through a curved guide or cannula. 
         [0016]    In accordance with the present invention, the flexible drill bit is constructed so that it is flexible enough to bend into a curved state, yet strong enough to transmit the torsional forces required for drilling into bone (or another material). 
         [0017]    In one preferred form of the present invention, there is provided a flexible drill bit comprising:
       a proximal shaft portion for connecting to a source of turning;   a distal cutting tip portion for boring into a material; and   an intermediate shaft portion extending between the proximal shaft portion and the distal cutting tip portion, the intermediate shaft portion being characterized by (i) sufficient longitudinal flexibility so as to permit the flexible drill bit to be passed along a curve, and (ii) sufficient torsional strength to permit the flexible drill bit to bore into the material.       
 
         [0021]    In another preferred form of the present invention, there is provided a method for forming a hole in a material, the method comprising:
       providing a flexible drill bit comprising:
           a proximal shaft portion for connecting to a source of turning;   a distal cutting tip portion for boring into a material; and   an intermediate shaft portion extending between the proximal shaft portion and the distal cutting tip portion, the intermediate shaft portion being characterized by (i) sufficient longitudinal flexibility so as to permit the flexible drill bit to be passed along a curve, and (ii) sufficient torsional strength to permit the flexible drill bit to bore into the material;   
           advancing the flexible drill bit to the material along a first angle of approach;   contacting the material at a second angle of approach; and   turning the flexible drill bit so as to form a hole in the material.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    These and other objects and features of the present invention will be more fully disclosed or rendered obvious by the following detailed description of the preferred embodiments of the invention, which is to be considered together with the accompanying drawings wherein like numbers refer to like parts and further wherein: 
           [0030]      FIG. 1  is a schematic view showing a flexible drill bit formed in accordance with the present invention; 
           [0031]      FIGS. 2-5  are schematic views showing the flexible drill bit of  FIG. 1  being used in conjunction with a curved drill guide to form a hole in bone; 
           [0032]      FIGS. 6 and 7  are schematic views showing another flexible drill bit formed in accordance with the present invention and being used in conjunction with a curved drill guide to form a hole in bone; 
           [0033]      FIG. 8  is a schematic view showing still another flexible drill bit formed in accordance with the present invention; 
           [0034]      FIG. 9  is a schematic view showing the flexible drill bit of  FIG. 8  being used in conjunction with a curved drill guide to form a hole in bone; 
           [0035]      FIG. 10  is a schematic view showing the flexible drill bit of  FIG. 1  with a helical coil disposed over a portion of the flexible drill bit; 
           [0036]      FIG. 11  is a schematic view showing the flexible drill bit of  FIG. 1  with another form of helical coil disposed over a portion of the flexible drill bit; 
           [0037]      FIG. 12  is a schematic view showing the flexible drill bit of  FIG. 1  with an over-molded sheath disposed over a portion of the flexible drill bit; 
           [0038]      FIG. 13  is a schematic view showing the flexible drill bit of  FIG. 1  with a metal braid or mesh disposed over a portion of the flexible drill bit; 
           [0039]      FIG. 14  is a schematic view showing another form of flexible drill bit formed in accordance with the present invention; 
           [0040]      FIG. 15  is a schematic view showing still another form of flexible drill bit formed in accordance with the present invention; 
           [0041]      FIGS. 16-19  are schematic cross-sectional views taken along lines A-A, B-B, C-C and D-D, respectively, of  FIG. 15  in one form of the invention; 
           [0042]      FIGS. 20-23  are schematic cross-sectional views taken along lines A-A, B-B, C-C and D-D, respectively, of  FIG. 15  in another form of the invention; 
           [0043]      FIG. 24  is a schematic view showing another form of flexible drill bit formed in accordance with the present invention; 
           [0044]      FIG. 25  is a schematic cross-sectional view taken along line A-A of  FIG. 24 ; 
           [0045]      FIG. 26  is a schematic view showing still another form of flexible drill bit formed in accordance with the present invention; 
           [0046]      FIG. 27  is an enlarged schematic view showing selected portions of the flexible drill bit of  FIG. 26 ; and 
           [0047]      FIGS. 28-32  are schematic views showing various forms of cutting tips which may be used with the flexible drill bit of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Flexible Drill Bit having a “Unibody” Construction 
       [0048]    Looking first at  FIG. 1 , there is a shown a flexible drill bit  5  formed in accordance with the present invention. Flexible drill bit  5  comprises three sections, i.e., a full diameter shaft portion  10 , a reduced diameter shaft portion  15 , and a fluted cutting tip portion  20 . Full diameter shaft portion  10 , reduced diameter shaft portion  15 , and fluted cutting tip portion  20  are all formed integral with one another so as to create a flexible drill bit having a “unibody” construction. If desired, a transition area  25  may be formed between full diameter shaft portion  10  and reduced diameter shaft portion  15 , and/or a transition area  30  may be formed between reduced diameter shaft portion  15  and fluted cutting tip portion  20 . 
         [0049]    The “unibody” construction eliminates the need for a mechanical joint connecting the cutting tip of the flexible drill bit (e.g., fluted cutting tip portion  20 ) to the flexible portion of the flexible drill bit (e.g., reduced diameter shaft portion  15 ), thereby eliminating a possible point of failure. Such a failure of a mechanical joint can be particularly problematic if the mechanical joint were to fail below the surface of the bone (i.e., subchondral); in this scenario, it would be unlikely that the portion of the drill bit left in the bone could be recovered. Thus, the possible failure of such a mechanical joint creates a serious clinical concern. In addition, the “unibody” construction eliminates the need for a mechanical joint connecting the flexible portion of the flexible drill bit (e.g., reduced diameter shaft portion  15 ) to the full diameter shaft portion (e.g., full diameter shaft portion  10 ) of the flexible drill bit, thus eliminating another possible point of failure. 
         [0050]    The flexible drill bit may comprise a material such as Nitinol, stainless steel, titanium, or other appropriate material, but is preferably Nitinol. 
         [0051]    The reduced diameter shaft portion  15  of flexible drill bit  5  provides flexibility in that portion of the drill bit while still providing the torsional strength needed to drill into bone. The diameter of the reduced diameter shaft portion  15  is preferably approximately 20-40% smaller than the diameter of the full diameter shaft portion  10 , and more preferably approximately 25% smaller than the diameter of the full diameter shaft portion  10 . 
         [0052]    The transition area  30  located between fluted cutting tip portion  20  and the reduced diameter shaft portion  15 , and/or the transition area  25  located between the reduced diameter shaft portion  15  and the full diameter shaft portion  10 , are preferably formed so as to distribute stress, whereby to minimize the possibility of mechanical failure at the transition areas. 
         [0053]    Full diameter shaft portion  10  provides a region, preferably at its proximal end, in which flexible drill bit  5  can be attached to a drill. 
         [0054]    Fluted cutting tip portion  20  is preferably sufficiently rigid to form a straight hole in the target bone. To that end, the length of fluted cutting tip portion  20  must be short enough so that the fluted cutting tip portion  20  may pass through the curve of a curved drill guide or curved cannula. In one preferred embodiment, fluted cutting tip portion  20  has a length which is approximately 6 times greater than its diameter. 
         [0055]      FIGS. 2-5  show flexible drill bit  5  being used in conjunction with a curved drill guide  35  to form a hole in a bone  40 . More particularly, as seen in the figures, the distal tip  45  of curved drill guide  35  is placed against the outer surface  48  of bone  40 , and then flexible drill bit  5  is passed through the lumen  50  of curved drill guide  35  and directed into bone  40  so as to make the hole in the bone at the desired location and with the desired angle. 
         [0056]    Note in  FIG. 5  how the curvature of curved drill guide  35  can combine with the differences in the diameters of the reduced diameter shaft portion  15  and lumen  50  so as to result in a non-perpendicular entry of flexible drill bit  5  into the bone, even where distal tip  45  of curved drill guide  35  is disposed substantially perpendicular to outer surface  48  of the bone. In other words, the curvature of curved drill guide  35  can combine with the differences in the diameters of reduced diameter shaft portion  15  and lumen  50  so that fluted cutting tip portion  20  is not perfectly coaxial with lumen  50  as fluted cutting tip portion  20  emerges from the distal end of curved drill guide  35 . It will be apparent to one skilled in the art that, depending on the bone surface contour and/or the angle of approach of curved drill guide  35 , the curved drill guide  35  may not always be disposed perpendicular to outer surface  48  of the bone. In this scenario, it is typically still desirable to have the fluted cutting tip portion  20  centered and aligned with the end of the curved drill guide  35 . 
         [0057]      FIGS. 6 and 7  show another form of the invention where the diameter of reduced diameter shaft portion  15  is sized so as to be closer to the diameter of fluted cutting tip portion  20  and so as to be somewhat closer to the diameter of lumen  50  of curved drill guide  35 . In this form of the invention, flexible drill bit  5  will tend to enter the bone closer to perpendicular. In other words, in this form of the invention, fluted cutting tip portion  20  will tend to remain more coaxial with lumen  50  as fluted cutting tip portion  20  emerges from the distal end of curved drill guide  35 . 
         [0058]    In one preferred form of the invention, full diameter shaft portion  10  has a length of approximately 12 inches and a diameter of approximately 0.063 inch; reduced diameter shaft portion  15  has a length of approximately 1.5 inches and a diameter of approximately 0.047 inch; fluted cutting tip portion  20  has a length of approximately 0.325 inch and a diameter of approximately 0.055 inch; and curved drill guide  35  has a radius of curvature of approximately 1.25 inches, a curve of approximately 25 degrees, and a lumen diameter of approximately 0.071 inch. In this preferred form of the invention, flexible drill bit  5  is capable of transmitting at least approximately 2 in-lbs (inch-pounds) of torque without failure, and more preferably approximately 3 in-lbs (inch-pounds) of torque without failure. 
         [0059]    In another form of the invention, and looking now at  FIGS. 8 and 9 , one or more enlargements  55  may be formed on the reduced diameter shaft portion  15  of flexible drill bit  5 . Enlargements  55  serve to keep flexible drill bit  5  centered in lumen  50  of curved drill guide  35  even where reduced diameter shaft portion  15  has a diameter which is significantly less than the diameter of lumen  50  of curved drill guide  35 . In this form of the invention, enlargements  55  will also keep flexible drill bit  5  closer to perpendicular as it enters bone  40 . In other words, in this form of the invention, fluted cutting tip portion  20  will tend to remain more coaxial with lumen  50  as fluted cutting tip portion  20  emerges from the distal end of curved drill guide  35 . 
         [0060]    In another embodiment, and looking now at  FIG. 10 , a helical coil  60  may be positioned over reduced diameter shaft portion  15  of flexible drill bit  5  so as to supplement the torque needed to drill into bone while still providing the flexibility needed to maneuver around a curve in a curved drill guide (e.g., curved drill guide  35 ) or curved cannula. Helical coil  60  also helps to keep flexible drill bit  5  centered in a curved drill guide (e.g., curved drill guide  35 ) and reduce the “mismatch” angle between flexible drill bit  5  and end of curved drill guide  35 . 
         [0061]    More particularly, helical coil  60  provides additional torsional strength and increased diameter to the reduced diameter shaft portion  15  of flexible drill bit  5  without significantly reducing the flexibility of the drill bit. The increased diameter of reduced diameter shaft portion  15  of flexible drill bit  5  (due to the presence of helical coil  60 ) creates a close fit within the drill guide or cannula, thereby ensuring that the drill bit remains coaxial with the curved drill guide or curved cannula as the flexible drill bit emerges from the distal end of the curved drill guide or curved cannula and engages the bone (or other material) which is being drilled. 
         [0062]    Helical coil  60  may form a close fit around reduced diameter shaft portion  15  and be sized so that it rests between transition area  25  and transition area  30 . Helical coil  60  may be resilient and may be stretched slightly (in its diameter) from its unbiased condition so as to allow the helical coil to be positioned onto reduced diameter shaft portion  15 ; in other words, in a free condition, the helical coil  60  has an inner diameter which is smaller than the outer diameter of the reduced diameter shaft portion  15 . Helical coil  60  may simply sit on reduced diameter shaft portion  15 , or it may be secured to reduced diameter shaft portion  15  (e.g., at one end of helical coil  60 , at both ends of helical coil  60 , and/or intermediate helical coil  60 , etc.). In one preferred embodiment, helical coil  60  is secured at both its ends to reduced diameter shaft portion  15  and forms a close fit with reduced diameter shaft portion  15  or is stretched slightly diametrically from its unbiased condition onto reduced diameter shaft portion  15 . Helical coil  60  may be secured to reduced diameter shaft portion  15  by soldering, adhesive, welding, mechanical interlock, or other appropriate attachment means. Helical coil  60  is preferably formed and positioned so that when the flexible drill bit is used to drill into bone, the helical coil will tighten onto reduced diameter shaft portion  15  during drilling. For example, if a flexible drill bit  5  rotates in a clockwise direction (when viewed from proximal to distal), the helical coil should have a counter-clockwise winding direction (again, when viewed from proximal to distal). This arrangement provides a preferred transfer of torque between reduced diameter shaft portion  15  and helical coil  60 . 
         [0063]    Helical coil  60  may comprise a material such as stainless steel, Nitinol or other suitable material. Helical coil  60  may comprise a wire of round or rectangular cross-section. Although  FIG. 10  depicts a closely wound helical coil (i.e., with substantially no space between the coils), an alternative embodiment comprises spacing between the coils. 
         [0064]      FIG. 11  shows a construction similar to that of  FIG. 10 , except that helical coil  60  comprises a multi-strand coil (i.e., multiple strands are coiled together). In this embodiment, adjacent multiple strands follow the same coil pitch. However, even with coils touching each other, the pitch can be greater than a single strand arrangement (e.g., as shown in  FIG. 10 ). This construction (i.e., larger pitch with coils touching) can be beneficial to reduce “play” in the coil; that is, as the flexible drill bit  5  starts drilling into bone, the helical coil  60  will more quickly respond in carrying a portion of the torque. 
         [0065]    In another embodiment, and looking now at  FIG. 12 , an over-molded sheath  65  may be positioned over reduced diameter shaft portion  15  of flexible drill bit  5 . Over-molded sheath  65  provides reduced friction (e.g., with curved drill guide  35  and/or bone  40 ) and increased diameter to reduced diameter shaft portion  15  of flexible drill bit  5 , while still enabling bending of the reduced diameter shaft portion  15  of flexible drill bit  5 . Over-molded sheath  65  may comprise a polymer such as Nylon or polytetrafluoroethylene (PTFE). Over-molded sheath  65  may be over-molded onto reduced diameter shaft portion  15  by injection molding or by diameter reduction (e.g., by shrinking or melting over-molded sheath  65  onto reduced diameter shaft portion  15 ). 
         [0066]    In another embodiment, and looking now at  FIG. 13 , a braid or mesh  70  (preferably but not necessarily formed out of metal) may be positioned over reduced diameter shaft portion  15  of flexible drill bit  5 . Metal braid or mesh  70  provides torsional strength and increased diameter to reduced diameter shaft portion  15  of flexible drill bit  5 , while still enabling bending/flexing of reduced diameter shaft portion  15  of flexible drill bit  5 . Metal braid or mesh  70  may comprise a material such as stainless steel or Nitinol. It may comprise wire having a rectangular cross-section. Metal braid or mesh  70  may be attached to reduced diameter shaft portion  15  of flexible drill bit  5  by attaching one or both of its ends to the reduced diameter shaft portion, or by attaching an intermediate portion of metal braid or mesh  70  to reduced diameter shaft portion  15 , or both (e.g., by welding, adhesive, etc.). Alternatively, or additionally, a polymer (e.g., Pebax) may be heated and melted into the metal braid or mesh  70  so as to create a solid structure atop reduced diameter shaft portion  15 . This polymer can provide a lower friction surface than the metal braid or mesh  70  alone, and can provide some torque transmission as well. 
         [0067]    Looking next at  FIG. 14 , there is shown a flexible drill bit  75  which is similar to the flexible drill bit  5  shown in  FIG. 1 , however, instead of providing a reduced diameter shaft portion (e.g., the aforementioned reduced diameter shaft portion  15 ) between the full diameter shaft portion (e.g., the aforementioned full diameter shaft portion  10 ) and the fluted cutting tip portion (e.g., the aforementioned fluted cutting tip portion  20 ) in order to create the desired flexibility in the drill bit, the full diameter shaft portion extends all the way to the fluted cutting tip portion and portions of material are removed from the full diameter shaft portion so as to create the desired flexibility in the drill bit while providing greater torque carrying strength as compared to simply a reduced diameter shaft portion  15 . 
         [0068]    More particularly, in this embodiment, and looking now at  FIG. 14 , flexible drill bit  75  comprises a full diameter shaft portion  10  and a fluted cutting tip portion  20 , with full diameter shaft portion  10  and fluted cutting tip portion  20  being formed integral with one another (i.e., a “unibody” design). In order to render the distal end  78  of full diameter shaft portion  10  flexible, material is removed from the full diameter shaft portion so as to create a flexible portion along the full diameter shaft portion of the drill bit. The material is removed in a pattern which enhances shaft flexibility but minimizes the reduction of torque transmission. In one preferred form of the invention, the material is removed in a spiral pattern as shown at  80  in  FIG. 14  and may be accomplished by laser cutting, electrical discharge machining (i.e., EDM), machining, grinding or other means. For a clockwise rotating flexible drill bit  5 , spiral cuts  80  are preferably formed in a clockwise pattern (when viewed from proximal to distal direction), but may also be formed in a counter-clockwise pattern. 
         [0069]    Material may also be removed from full diameter shaft portion  10  in other patterns so as to create a flexible, yet high torque transmitting, portion along the shaft of the drill bit. By way of example but not limitation, and looking now at  FIG. 15 , a series of transverse slots  85  (instead of the spiral cuts  80  shown in  FIG. 14 ) may be cut into the shaft, with the slots preferably following a spiral or other geometric pattern. Transverse slots  85  may be formed with various configurations.  FIGS. 16-19  show one way of configuring transverse slots  85 .  FIGS. 20-23  show another way of configuring transverse slots  85 . Still other ways of configuring transverse slots  85  will be apparent to those skilled in the art in view of the present disclosure. 
         [0070]    In this embodiment if the invention, flexible drill bit  75  may comprise a material such as stainless steel or Nitinol. 
       Flexible Drill Bit having a Multi-Body Construction 
       [0071]    In another embodiment of the present invention, portions of the flexible drill bit (e.g., the cutting tip) may comprise separate components which are connected to the remaining portions of the flexible drill bit (e.g., the solid shaft) in order to provide a flexible drill bit having a multi-body construction. 
         [0072]    More particularly, and looking now at  FIGS. 24 and 25 , there is a shown a flexible drill bit  90  comprising two components (i.e., full diameter shaft portion  10  and fluted cutting tip portion  20 ) which are connected together so as to form a flexible drill bit having three sections, i.e., a distal cutting tip, a proximal shaft and an intermediate flexible region. In this embodiment of the invention, fluted cutting tip portion  20  comprises an elongated solid shaft  95  which is received within a lumen  100  formed in full diameter shaft portion  10  and then secured therein (e.g., by welding, adhesive bond, swaging, etc. or a combination thereof or other means well known in the art). Full diameter shaft portion  10  is preferably secured to fluted cutting tip portion  20  at the distal end of full diameter shaft portion  10 , e.g., at  102 . Flexible drill bit  90  may comprise additional points of securement between full diameter shaft portion  10  and fluted cutting tip portion  20  (e.g., proximal of the intermediate flexible region, such as at  103 ). The drill bit is rendered flexible by removing material from full diameter shaft portion  10 , e.g., such as by forming spiral cuts  80  in full diameter shaft portion  10 . Although spiral cuts  80  are shown in  FIGS. 24 and 25  as being formed in a clockwise pattern (when viewed from proximal to distal direction), they preferably would be formed in a counter-clockwise pattern when used with a clockwise-rotating drill (when viewed from proximal to distal) so that the spiral cuts would tend to tighten down on the elongated solid shaft  95  during drilling. Alternatively, and looking now at  FIGS. 26 and 27 , the material may be removed as an interrupted spiral cut  105  so as to provide the desired flexibility to the drill bit. In one preferred form of this embodiment, the cuts are interrupted segment lengths of less an 120 degrees around the perimeter, have a opening—or width—which is less than the pitch distance (i.e., longitudinal distance between adjacent cuts), and have a gap between laser cuts which is approximately equal to the pitch distance. In one preferred form of this embodiment, the cuts have a slight angle relative to perpendicular to the longitudinal axis of the flexible drill bit  90 . 
         [0073]    Depending on the location(s) of securement between full diameter shaft portion  10  and fluted cutting tip portion  20  (e.g., at securement point  102 , securement point  103 , etc.), the torque may be transmitted through the full diameter shaft portion  10  (distal securement only), through solid shaft  95  of fluted cutting tip portion  20  (proximal securement only) or shared between the two (both the proximal and distal securements). 
       Cutting Tip Constructions 
       [0074]    Looking now at  FIGS. 28-32 , there are shown various shapes and designs of cutting tips which may be used in accordance with the present invention, e.g., a fluted cutting tip ( FIG. 28 ), a fluted cutting tip with a centering feature  110  similar to a center drill bit ( FIG. 29 ), a diamond shape ( FIGS. 30 and 31 ) or a forged or flattened tip ( FIG. 32 ). In the cutting tip embodiment of a fluted cutting tip (e.g.,  FIG. 28 ), the inclusive angle at the tip may be approximately 30-120 degrees, is more preferably approximately 60-90 degrees, and is most preferably approximately 70 degrees. 
       Helical Structures 
       [0075]    In the foregoing disclosure, various constructions are provided in which the flexible drill bit comprises a helical structure. By way of example but not limitation, a helical coil  60  is mounted over reduced diameter shaft portion  15  ( FIGS. 10 and 11 ), a helical groove is formed in full diameter shaft portion  10  (FIGS.  14  and  24 - 27 ), etc. These constructions are provided in order to maximize the flexibility of the drill bit while minimizing reduction of torque transmission through the drill bit. In this respect it will be appreciated that the configuration of the helical structure (i.e., the direction of the spiral) is preferably related to the direction of the applied torque, in order to maintain maximum torque transmission through the drill bit. However, the relationship of these may vary depending on the specific construction of the drill bit. 
         [0076]    In the embodiment of a helical coil mounted over a reduced diameter shaft portion ( FIGS. 10 and 11 ), where the torque is intended to be applied in a clockwise direction (when viewed from the proximal end of the drill bit), it is preferred that the helix rotate counter-clockwise as it advances down the drill bit, and where the torque is intended to be applied in a counter-clockwise direction (when viewed from the proximal end of the drill bit), it is preferred that the helix rotate clockwise as it advances down the drill bit. Such an inverse relationship between the direction of the applied torque and the direction of the spiral will ensure that any deformation of the helical coil from the applied torque will cause the helical coil to tighten, whereby to preserve torque transmission through the helical coil. 
         [0077]    In the embodiment of a helical groove formed in a full diameter shaft portion (FIGS.  14  and  24 - 27 ), where the torque is intended to be applied in a clockwise direction (when viewed from the proximal end of the drill bit), it is preferred that the helix rotate counter-clockwise as it advances down the drill bit, and where the torque is intended to be applied in a counter-clockwise direction (when viewed from the proximal end of the drill bit), it is preferred that the helix rotate clockwise as it advances down the drill bit. The appropriate relationship between the direction of the applied torque and the direction of the spiral will maximize torque transmission while maintaining drill bit flexibility. 
       General Construction 
       [0078]    The flexible drill bit may comprise Nitinol or stainless steel or any other material which is flexible enough to bend into a curved state, and strong enough to transmit the torsional forces required for drilling into bone. 
         [0079]    The entire shaft or portions of the shaft can be coated to reduce friction (e.g., with curved drill guide  35  and/or bone  40 ). 
       Modifications 
       [0080]    While the present invention has been described in terms of certain exemplary preferred embodiments, it will be readily understood and appreciated by those skilled in the art that it is not so limited, and that many additions, deletions and modifications may be made to the preferred embodiments discussed herein without departing from the scope of the invention.