Abstract:
A drill guide for drilling bone comprising a housing having a bore running through the housing is disclosed. In one embodiment, the drill guide may comprise a locking member coupled to the housing that is adapted to engage with a locking portion of a drill bit to lock the drill bit axially within the housing. The locking member may also be adapted to disengage with the drill bit to allow the drill bit to move within the bore of the housing and be inserted into bone. A movable guide may also be provided with the drill guide, such movable guide including a bore for receiving a portion of the drill bit and allowing the same to move axially within the housing. Methods of using the aforementioned drill guide are also disclosed.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates, in general, to a drill guide, and specifically to a drill guide having depth gauge features for controlling or gauging the depth to which a drill is inserted into bone. 
     Various bone drilling apparatus used, for example, to prepare a pilot hole(s) in bone for receiving a screw(s) or other fixation mechanism(s) are known. Such drilling apparatus, in some instances, may include a guide for receiving a drill bit and guiding the same into the bone to be treated. Further, the particular drill guide may also operate to limit the insertion depth of the drill bit. Examples of such drill guides are shown in U.S. Pat. Nos. 2,294,303, 6,699,253, and 7,163,542. These devices operate to, predominantly, prepare and/or initialize a hole in bone so that a screw or other fixation device may be easily inserted into the bone and through the hole. 
     Depth gauges for use in determining the depth to which a particular drill bit is inserted into bone are also known. An example of such a depth gauge is shown in U.S. Pat. No. 5,180,388, which discloses a depth gauge including a handle having a bore and adjacent sides with a scale for determining the insertion depth of a wire-type drill. In the &#39;388 patent, bone pins may be inserted into corresponding pre-drilled holes formed via the wire-type drill. Other depth gauge-type devices are known in the art, which provide stop-mechanisms for controlling the maximum insertion depth of a drill bit into bone. 
     In some instances, however, it may be difficult to utilize the aforementioned drill guides or depth gauges with another device, such as a bone plate being secured to the surgical treatment site. In other words, the particular constructs noted above may not be amenable to engaging with an aperture in a bone plate or other surgical device, which is being secured to bone. Other deficiencies of such devices may include the general mode of operation thereof (e.g., such devices may be cumbersome to use, due to the multitude of parts comprising the device, etc.) 
     Accordingly, there exists a need for an improved drill guide and/or depth gauge, which overcomes the deficiencies of the prior art. 
     BRIEF SUMMARY OF THE INVENTION 
     A first embodiment of the present invention includes a drill guide for cooperating with and aligning a drill bit for drilling bone. In some cases, the drill guide may comprise a housing having a proximal section and a distal section, the housing defining a bore extending through the housing from the proximal section towards the distal section. A locking member may also be coupled with the housing, the locking member being adapted to engage and disengage with a locking portion of a drill bit, such that, when the locking member is engaged with the locking portion of the drill bit, the drill bit is locked within the bore in the housing along an axis of the bore extending between the proximal and distal sections, and, when the locking member is disengaged from the locking portion of the drill bit, the drill bit is movable within the bore along the axis. 
     Other aspects of the first embodiment may include a locking member that is fixedly connected to the proximal section of the housing. The locking member may also include internal threads adapted to engage with corresponding threads on the locking portion of the drill bit. The bore of the housing may also be dimensioned such that the locking portion of the drill bit is movable within the bore along the axis and distal to the locking member. 
     A second embodiment of the invention includes a drill guide for cooperating with and aligning a drill bit for drilling bone. In some cases, the drill guide may comprise a housing having a proximal section and a distal section, the housing defining a bore extending through the housing from the proximal section towards the distal section. A locking member may also be coupled with the housing, the locking member being adapted to engage with a locking portion of a drill bit to lock the drill bit within the bore of the housing along an axis of the bore extending between the proximal and distal sections. The drill guide may also include a movable guide disposed within the bore of the housing, the movable guide having a flange for engaging with a portion of a wall defining the bore of the housing, wherein the movable guide includes a bore extending therethrough and a surface adapted to engage with a surface on the drill bit, such that the movable guide is movable along the axis within the bore in the housing while the drill bit is moved along the axis. 
     The locking member, in some aspects of the second embodiment, may be adapted to disengage with the locking portion of the drill bit, such that the locking portion is movable within the bore in the housing to a position past the proximal section and the locking member and adjacent the distal section. 
     In a third embodiment of the present invention, a method of drilling bone is provided, such method comprising the steps of: (1) providing a drill guide having a housing with a proximal section and a distal section, the housing defining a bore extending through the housing from the proximal section towards the distal section; (2) inserting a distal portion of a drill bit within the bore in the housing; (3) engaging a locking section of the drill bit with a locking member coupled with the housing, such that the drill bit is locked within the bore in the housing along an axis of the bore extending between the proximal and distal sections; (4) disengaging the locking section of the drill bit from the locking member; and (5) moving the locking section of the drill bit within the bore in the housing and distally along the axis. While these steps are numbered above, no particular order is contemplated. 
     The steps of engaging and disengaging, in certain aspects of the third embodiment, may also include rotating the drill bit about the axis such that threads on the locking section of the drill bit engage or disengage, respectively, with internal threads on the locking member. Other aspects of the third embodiment may include the steps of: (1) engaging a surface on the drill bit with a surface on a movable guide disposed within the bore in the housing; and (2) moving the movable guide along the axis within the bore in the housing and towards the distal section by contacting the surface on the drill bit with the surface on the guide and simultaneously moving the drill bit distally. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the subject matter of the present invention and of the various advantages thereof can be realized by reference to the following detailed description in which reference is made to the accompanying drawings in which: 
         FIG. 1  is an exploded view of a drill guide and drill bit according to one embodiment of the present invention. 
         FIGS. 2A and 2B  are cross-sectional views of the drill guide of  FIG. 1 , shown in assembled and exploded form, with the drill bit shown alongside the drill guide in  FIG. 2B . 
         FIG. 3  is a perspective view of the drill guide of  FIG. 1  with the drill bit inserted therein and in a locked position. 
         FIG. 4  is a cross section of the drill guide of  FIG. 1  with the drill bit inserted therein and in an unlocked position. 
     
    
    
     DETAILED DESCRIPTION 
     In describing particular embodiments of the present invention, specific terminology will be used for the sake of clarity. However, the invention is not intended to be limited to any specific terms used herein, and it is to be understood that each specific term includes all technical equivalents, which operate in a similar manner to accomplish a similar purpose. 
       FIG. 1  depicts: (1) a drill guide  10  according to the present invention in which the guide  10  includes a guide tube  20  having depth gauge features or markings  40  thereon; and (2) a drill bit  50  insertable into and through the guide tube  20  for drilling bone. The drill guide  10  may function as a combination guide and depth gauge for simultaneously drilling a pilot hole(s) into bone and indicating the depth at which such hole(s) is drilled. In some instances, the drill guide  10  may be configured to interact with an aperture in a bone plate or other implantable device  100  (hereinafter, “surgical device”) to drill a corresponding pilot hole(s) for such aperture(s). A screw or other fixation mechanism may thereafter be inserted through the relevant aperture and into the pilot hole to secure surgical device  100  to the bone. 
     Referring to  FIGS. 1 and 2A-2B , guide tube  20  of drill guide  10  may include an elongate cylindrical body  21  forming a main portion of the tube  20  and having a bore  22  running therethrough along an axis. The bore  22  may extend from a proximal end  23  to a distal end  24  of the tube  20 ; and, likewise, a slot  31  may be formed through the tube  20  and may extend, generally, between proximal  23  and distal  24  ends. The slot  31  may be used to view the drill bit  50  as it travels within bore  22 . In one embodiment, particular indicia or a scale  40  may be positioned on body  21  adjacent slot  31  for determining the exact depth to which the drill bit  50  is inserted into bone. 
     Projecting from the main portion of guide tube  20  (i.e., cylindrical body  21 ) adjacent distal end  24  may be an extension  25  of the tube  20  formed integral therewith or as a separate component part. Extension  25  may, in one embodiment, be generally cylindrical in shape and may have a chamfered section  33  on an external surface thereof for engaging with an aperture formed in a surgical device  100  as shown, for example, in  FIG. 3 . Extension  25  also, like guide tube  20 , has a bore  26  running therethrough. Thus, the combination of bore  22  through tube  20  and bore  26  through extension  25  allows the drill bit  50  to be inserted completely through drill guide  10 . In a particular embodiment, bore  22  of tube  20  may form a first relatively wider section  27 , and bore  26  of extension  25  may define a second relatively narrower section  28  (e.g., in terms of diameter) defining a stop surface  29  therebetween. In some embodiments, second section  28  defined by bore  26  may closely approximate the size and shape of a portion of drill bit  50  so as to serve as a guide and stabilize the bit  50  when inserted into bone. Stated differently, as second section  28  defined by bore  26  may closely approximate the size and shape of a portion of drill bit  50 , such section  28  may act as a longitudinal guide stabilizing drill bit  50  as it is inserted into bone. Stop surface  29  between first  27  and second  28  sections may also serve to limit the insertion depth of the drill bit  50 , as will be described in more detail below. 
     Housed within bore  22  of tube  20  may also be a spring  60 , as shown in detail in  FIGS. 2A-2B . Spring  60  may be a conventional helical spring, although other spring types are contemplated; and such spring  60  may extend, in its expanded state, along the entirety of bore  22  of tube  20 . A distal end  62  of spring  60  may also abut stop surface  29  so as to allow spring  60  to be compressed within bore  22 , when desired. 
     A movable guide  70  may also form part of drill guide  10 , such guide  70  being shown in detail in both  FIGS. 1 and 2A-2B . Guide  70  may generally be configured to be situated within bore  22  of tube  20 , such that a radial flange  71  extending from the guide  70  may ride along an interior surface of bore  22  ( FIGS. 2A-2B ). Guide  70  has a bore  72  extending therethrough, such bore  72  having a first tapered section  73  and a second generally straight section  74 . Second straight section  74  of bore  72  may be configured to receive and interact with a portion of drill bit  50 , while first tapered section  73  may be configured to interact with a separate section of drill bit  50 , described in more detail below. Radially extending flange  71  of guide  70 , and specifically a bottom surface  75  thereof, may also be configured to interact with a proximal end  61  of spring  60  to compress spring  60  in bore  22  of tube  20 , as shown best in  FIGS. 2A-B  and  FIG. 4 . A protrusion  77 , shown in  FIGS. 1 and 3 , may also extend from a surface of the guide  70  for interacting with the slot  31  or another channel in tube  20 , and thereby maintaining and/or aligning movable guide  70  appropriately as it travels along bore  22  of tube  20  (e.g., during drilling). 
     Referring again to  FIG. 2A-2B , a top surface  79  of radially extending flange  71  may interact with a cap  80  inserted into the proximal end  23  of guide tube  20 . In one embodiment, cap  80  may include external threading (not shown), which interacts with internal threading (not shown) formed on an interior surface of tube  20  adjacent proximal end  23 . Thus, cap  80  may be screwed into engagement with tube  20 . Once cap  80  is secured within tube  20  as such, top surface  79  of flange  71  may interact with a distal end  81  or other surface of cap  80  thereby preventing movable guide  70  from being pushed out of bore  22  in tube  20  via spring  60 . In other words, movable guide  70  may be compressed between spring  60  and distal end  81  of cap  80 , thereby retaining guide  70  in tube  20 . Accordingly, via the interaction between cap  80  and movable guide  70 , spring  60  may remain compressed (to some minor degree) at all times. 
     In one embodiment, cap  80  may include a bore  82  therethrough with multiple sections, one such section being a distal section  83  having a diameter that is sized to receive a proximal section  76  of movable guide  70 . Proximal section  76  of movable guide  70  may, therefore, be disposed within distal section  83  of bore  82  in cap  80 , and flange  71  of guide  70  may interact with distal end  81  of cap  80 . A second intermediate section  84  of bore  82  may be situated above distal section  83 , and may neck-down creating a reduced diameter section. Such section  84  of bore  82  may optionally include threading to interact with corresponding threading formed on a section of drill bit  50 . Depending on the dimensions of moveable guide  70 , intermediate section  84  of bore  82 , via the difference in diameter between distal section  83  and intermediate section  84 , may define a stop surface that contacts a proximal end  78  of movable guide  70  to assist in retaining the same within tube  20 . A final proximal section  85  of bore  82  through cap  80  may be sized similar to distal section  83 , thus defining a step between intermediate  84  and proximal  85  sections of bore  82 . One or more apertures  86  may also be disposed through cap  80  nearing proximal section  85  of bore  82  to allow a surgeon, nurse, or other skilled practitioner (hereinafter, “the user”) to view a portion of the drill bit  50  as it is inserted through drill guide  10 . A flange  87  may also be arranged on cap  80  for abutting against a portion of tube  20  to thereby limit the insertion of cap  80  within bore  22  of tube  20 . 
     Referring again to  FIG. 1 , there is shown a drill bit  50  having a cutting section  51 , a tool engaging section  52  (e.g., for connecting to a power tool-type device), and a shaft  53  running therebetween. Shaft  53  may have a series of markers  54  thereon for determining a depth to which the drill bit  50  is inserted into bone (i.e., by aligning with a feature of the drill guide  10 ), and/or for determining whether the drill bit  50  is in a locked or unlocked orientation, as described more fully below. Drill bit  50  may also include a tapered section  55  and a flange  56  adjacent the tapered section  55 . In one embodiment, flange  56  may include external threading thereon that may interact with the internal threading on intermediate section  84  of bore  82  through cap  80 . Drill bit  50  may, in one embodiment, be generally cylindrical in shape, and may be sized for insertion through cap  80  and guide tube  20  (including extension  25 ) of drill guide  10 . 
     To assemble drill guide  10 , as shown in detail in  FIG. 1 , a user may first insert movable guide  70 , and specifically second section  74  thereof, into the proximal portion  61  of spring  60 , such that flange  71  of guide  70  engages a portion of spring  60 . Then, the movable guide  70  and spring  60  construct may be inserted within bore  22  of guide tube  20  so that a distal end  62  of spring  60  may abut stop surface  29 . With these components assembled in the manner described, a user may then insert distal end  81  of cap  80  into proximal end  23  of tube  20  and screw cap  80  into tube until such a point as: (1) flange  87  of cap  80  abuts proximal end  23  of tube  20 ; (2) intermediate section  84  of bore  82  abuts proximal end  78  of movable guide  70 ; and/or (3) distal end  81  of cap  80  abuts flange  71  of guide  70 . In this configuration, movable guide  70  may move longitudinally along and within bore  22  of guide tube  20  to interact with drill bit  50  during drilling of bone. To be exact, since flange  71  of movable guide  70  contacts a portion of spring  60 , and a distal end  62  of spring  60  abuts stop surface  29 , such spring  60  may be compressed as guide  70  is moved axially within bore  22  of guide tube  20 . In other embodiments, drill guide  10  may be manufactured such that cap  80  is integral with tube  20 , creating a single monolithic structure in which moveable guide  70  and spring  60  are disposed. 
     With the components of drill guide  10  assembled, drill bit  50  may be inserted within and through drill guide  10  (e.g., for drilling) as follows. Cutting section  51  of drill bit  50  may first be inserted through bore  82  in cap  80 , bore  72  in movable guide  70 , and partway through bore  22  in guide tube  20 . During this insertion, drill bit  50  may be advanced until such a point as flange  56  of bit  50  engages or abuts intermediate section  84  of bore  82  in cap  80 . Since flange  56  may include external threading thereon, which may interact with the internal threading on intermediate section  84  of bore  82 , flange  56 , and thus drill bit  50 , may initially be precluded from traveling through intermediate section  84  of bore  82  of cap  80  (e.g., since the threading on intermediate section  84  may prevent movement of drill bit  50  distally without rotation thereof). Subsequently, a user may elect to rotate drill bit  50  about its longitudinal axis to screw flange  56  into engagement with intermediate section  84  of bore  82 . In this condition, i.e., during engagement of flange  56  with intermediate section  84 , drill bit  50  may be locked in position in an axial direction within bore  22 , such that drill bit cannot travel along the axis of bore  22  without rotating drill bit  50  about the axis. Such a locked configuration may be displayed to a user through the alignment of an indicator  54  on drill bit  50  with one or more apertures  86  in cap  80 , as shown in detail in  FIG. 3 . In particular, a specific reference marker  54  on drill bit  50  may be viewable through the one or more apertures  86  in cap  80  only when the drill bit  50  is situated in the locked condition. A user may therefore be informed as to when the drill bit  50  is disposed in the locked configuration and when it is not. 
     With flange  56  screwed into engagement with intermediate section  84  of bore  82  of cap  80 , drill bit  50  may act as a manipulation tool for manipulating drill guide  10  (e.g., since drill bit  50  is locked to drill guide  10 ). As such, it is possible for a user to manipulate drill guide  10  via drill bit  50  and engage chamfered section  33  of extension  25  with an aperture in surgical device  100  (shown in  FIG. 3 ) without fear of disconnection between drill bit  50  and drill guide  10 . That is, drill bit  50  may first be connected with a drill, and drill guide  10  may thereafter be engaged with drill bit  50 , such that the two are essentially locked together. This would allow a user to perform the step of engaging chamfered section  33  of extension  25  with an aperture in surgical device  100  and then to drill directly afterward without having to align the drill bit  50  into the engaged drill guide  10 . 
     Once chamfered section  33  of guide tube  20  is engaged to an appropriate aperture in surgical device  100 , a user may then continue to rotate drill bit  50  about its longitudinal axis to cause the threading on flange  56  to disengage with the corresponding threading on intermediate section  84  of bore  82  through cap  80 . In this condition, as shown in detail in  FIG. 4 , flange  56  of drill bit  50  may be engaged with proximal end  78  of movable guide  70 , and/or tapered section  55  of drill bit  50  may be engaged with tapered section  73  of bore  72  through guide  70 . Accordingly, movement of drill bit  50  in an axial direction may cause corresponding movement of movable guide  70  within bore  22  of guide tube  20 . Additionally, as flange  71  of movable guide  70  may be engaged with spring  60 , there may be some resistance to such movement, thereby providing a user with a tactile sensation that the drill bit  50  is smoothly gliding through drill guide  10 . Such resistance may also keep movable guide  70  disposed against tapered section  55  and flange  56  of drill bit  50 . 
     During movement of drill bit  50  through guide tube  20  via movable guide  70 , as discussed above, a user may also view the depth gauge features or markings  40  on tube  20  in conjunction with a particular reference marker  54  on drill bit  50 , which indicates the depth at which bit  50  is inserted into bone. Stated differently, as drill bit  50  is moved longitudinally or axially through guide tube  20 , a user may view a particular reference marker  54  on such bit  50  through slot  31  in tube  20 , and the position of such marker  54  in relation to the depth gauge markings  40  on tube  20  may indicate the depth at which drill bit  50  is inserted into bone. The same function may be provided by viewing protrusion  77  on movable guide  70  through slot  31 . For instance, a user may view the aforementioned reference marker  54  (or protrusion  77 ) traveling from a marking  40  on guide tube  20  reading zero (0) millimeters to a marking  40  reading twenty (20) millimeters and thereby know that the drill bit  50  has been inserted twenty (20) millimeters into bone. During such axial movement of drill bit  50 , movable guide  70  may also be stabilized within bore  22  of guide tube  20  via the interaction between protrusion  77  on guide  70  and slot  31  formed through tube  20  (e.g., such guide  70  may be restricted from unwanted rotation within tube  20  as drill bit  50  is rotated for drilling). Thus, in some instances, protrusion  77  may serve a dual function. 
     Referring still to  FIG. 4 , upon moving drill bit  50  (and thus movable guide  70 ) a sufficient distance in the distal direction, protrusion  77  on movable guide  70  may engage with a portion of slot  31  in guide tube  20 , thereby preventing further movement of drill bit  50  distally. Thus, such portion of slot  31  may act as a depth-limiting feature of drill guide  10  for limiting the maximum depth to which the drill bit  50  may be inserted into bone; although, as noted above, it is also contemplated that a user may simply stop drilling when the desired depth for drilling has been reached. 
     It is notable that, in the unlocked condition described above, the reference marker  54  on drill bit  50  indicating that such bit  50  is in a locked condition will no longer be viewable through apertures  86  in cap  80 . Thus, a user is informed that drill bit  50  is in the unlocked state and is freely movable within bore  22  in guide tube  20  once flange  56  on drill bit  50  is disengaged from intermediate section  84  of bore  82  through cap  80 . 
     With the initial pilot hole drilled according to the above-described method, a user may then disengage chamfered section  33  of extension  25  from the aperture in surgical device  100  and continue to drill another pilot hole(s) utilizing a different aperture(s) in surgical device  100 . This process may be repeated, as necessary, until all pilot holes are drilled. Subsequently, the user may insert a fixation mechanism (not shown), such as, for example, a bone screw or fixation pin through the aperture in surgical device  100  and into the corresponding pilot hole. Surgical device  100  may then be secured to bone via the fixation mechanism(s) inserted into the pilot hole(s). 
     In one embodiment, to remove drill guide  10  from engagement with surgical device  100 , a user may simply retract drill bit  50  proximally until such a point as flange  56  abuts intermediate section  84  of bore  82  through cap  80 . The user may then rotate drill bit  50  so that the threading on flange  56  reengages with the threading on intermediate section  84  of bore  82 . This places drill bit  50  back into its locked state, which may be conveyed to a user via a reference marker  54  on drill bit  50 , which is viewable through aperture(s)  86  in cap ( FIG. 3 ). With drill bit  50  in its locked state, a user may then simply remove the drill bit  50 /drill guide  10  construct (e.g., as one piece) from contact with surgical device  100 . Thereafter, the user may elect to continue to rotate the drill bit  50 , such that flange  56  disengages with intermediate section  84  of bore  82  and drill bit  50  is removed or separated from drill guide  10 . The drilling process, at this stage, may be complete, and a user may elect to utilize the drill guide  10  and drill bit  50  in another surgical procedure, if desired. 
     In the devices depicted in the figures, particular structures are shown that are adapted to provide an improved drilling apparatus and/or allow for an improved method of drilling bone. The invention also contemplates the use of any alternative structures for such purposes, including structures having different lengths, shapes, and/or configurations. For example, although drill guide  10  has been described as engaging an aperture in a surgical device  100 , it is equally contemplated that a portion of drill guide  10  (e.g., chamfered section  33 ) may be configured to interact directly with bone. In this embodiment, chamfered section  33  may include teeth or other such serrations so that, once engaged with bone, drill guide  10  may be stabilized with respect thereto. In this condition, drilling would take place as described above, except that drill guide  10  would directly contact bone instead of contacting an intermediate surgical device  100  (e.g., a bone plate aperture, etc.) It is also contemplated that the aforementioned teeth or serrations may be included on chamfered section  33 , and such section  33  may simultaneously engage an intermediate surgical device  100  and bone during drilling. 
     As another example, while drill bit  50  may be locked axially with respect to drill guide  10  through interaction of threading on the drill bit  50  with threading on a portion of guide  10 , other locking mechanisms are contemplated. For instance, a portion of drill bit  50  may be designed to be press-fit into a portion of cap  80  (or another portion of guide  10 ), such that drill bit  50  is lockable in an axial direction within bore  22 . In other words, slight differences in dimensions between one section of drill bit  50  and a corresponding section of cap  80  (or another section of guide  10 ) may cause drill bit  50  to become fixed within cap  80  (or the other section of guide  10 ) when such sections interact. Subsequently, upon application of sufficient force, drill bit  50  may be driven through the “press-fit” region, through the remainder of drill guide  10 , and into bone. Thus, with this mechanism, drill bit  50  may be provisionally locked within bore  22  in an axial direction. 
     In another example of this type of “press-fit” configuration, an outwardly extending flange on drill bit  50  may interact with two inwardly extending flanges on cap  80  (or guide  10 ). The outwardly extending flange may be dimensioned to create a slight dimensional interference with each of the inwardly extending flanges. Between the inwardly extending flanges may be an annular recess. The outwardly extending flange of drill bit  50  may pressed to pass the initial inwardly extending flange with a force that overcomes the dimensional interference and allows the outwardly extending flange to be temporarily retained within the annular recess, thereby locking the drill bit  50  to the drill guide  10  in a manner similar to that described above. 
     A protrusion and detent mechanism may also be used in place of the aforementioned locking structures. For example, a protrusion on drill bit  50  may selectively engage with a corresponding detent in cap  80  (or another portion of guide  10 ) so as to provisionally lock drill bit  50  axially within bore  22  of tube  20 . After such engagement, a user may then cause the protrusion on drill bit  50  to disengage with the detent in cap  80  (or other portion of guide  10 ) to allow drill bit  50  to move within bore  22  in tube  20  and into bone. Likewise, it is also contemplated that cap  80  (or the other portion of guide  10 ) may contain the protrusion and drill  50  the detent. It is therefore apparent that other locking mechanisms beyond that disclosed in the main embodiment may be used. 
     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims. 
     It will also be appreciated that the various dependent claims and the features set forth therein can be combined in different ways than presented in the initial claims. It will also be appreciated that the features described in connection with individual embodiments may be shared with others of the described embodiments.