Abstract:
An instrument for use in drilling a hole into a bone is provided. The instrument monitors the depth of the hole and prevents the drill from drilling too deep. Additionally, the instrument ensures that the hole reaches the desired depth. The instrument includes a sleeve, including a depth gauge for setting the depth of the hole to be drilled, a slide, including a releasable locking mechanism and being axially disposed within the sleeve and a drill including a shaft defining a shoulder. The desired depth of the hole is set on the depth gauge and the drill is axially inserted into slide. The instrument contacts the bone via teeth on the sleeve and the drill advances into the bone until the shoulder contacts the slide. Additionally, the instrument may be used to monitor the depth of tapping of the hole.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention generally relates to surgical devices and more specifically relates to drill guides for use in drilling bones.  
           [0002]    The spinal column is a highly complex system of bones and connective tissues that provides support for the body and protects the delicate spinal cord and nerves. The spinal column includes a series of vertebral bodies stacked one atop the other, each vertebral body including an inner or central portion of relatively weak cancellous bone and an outer portion of relatively strong cortical bone. Situated between each vertebral body is an intervertebral disc that cushions and dampens compressive forces exerted upon the spinal column. A vertebral canal containing the spinal cord and nerves is located behind the vertebral bodies.  
           [0003]    There are many types of spinal column disorders, including scoliosis (abnormal lateral curvature of the spine), kyphosis (abnormal forward curvature of the spine, usually in the thoracic spine), excess lordosis (abnormal backward curvature of the spine, usually in the lumbar spine), spondylolisthesis (forward displacement of one vertebra over another, usually in a lumbar or cervical spine) and other disorders caused by abnormalities, disease or trauma, such as ruptured or slipped discs, degenerative disc disease, fractured vertebra, and the like. Patients that suffer from such conditions usually experience extreme and debilitating pain, as well as diminished nerve function.  
           [0004]    Surgical techniques commonly referred to as spinal fixation use surgical implants and/or mechanical immobilization to fuse two or more vertebral bodies of the spinal column. Spinal fixation may also be used to alter the alignment of adjacent vertebral bodies relative to one another so as to change the overall alignment of the spinal column. Such techniques have been used effectively to treat the above-described conditions and, in many cases, to relieve pain.  
           [0005]    One spinal fixation technique involves immobilizing the spine using orthopedic stabilizing rods, commonly referred to as spine rods, which are positioned generally parallel to the spine. This may be accomplished by exposing the spine posteriorly and fastening bone screws to the pedicles of vertebral bodies. The pedicle screws are generally placed two per vertebra and serve as anchor points for the spine rods.  
           [0006]    In order to fasten pedicle bone screws, a hole must be drilled and, preferably tapped into the pedicles. It is important in orthopedic applications, such as inserting bone screws into the pedicles of vertebral bodies, that the depth of the hole being drilled is monitored. For example, if a hole is drilled or tapped into a bone too deeply, major damage may be caused to arteries or tissue surrounding the bone. The depth of a hole for receiving a pedicle screw is in the range of about 6 mm to about 54 mm and pedicle screws are readily available in these sizes. Preferably, the depth of the hole is about 10-14 mm to receive pedicle screws of the same length. Moreover, it is equally as important to monitor the depth of tapping of the hole, since the flutes on the tap end can cause severe damage to arteries or other tissue surrounding the bone. Typically the depth of tapping a hole ranges from about 6 mm to about 24 mm. The preferred tapping depth depends on the depth of the hole and the length of the bone screw to be used.  
           [0007]    Attempts have been made in the prior art to provide a guide for monitoring the depth of holes drilled into pedicles and other bones. Typically, a device usually referred to as a drill guide is utilized in connection with an orthopedic drill to gauge the depth of the hole. Such devices usually include a threaded collet and threaded slide, thus requiring the screwing on and off of the slide. The slide is screwed to a point on the collet that corresponds to the desired depth of the hole. A drill is then used in conjunction with the guide, so that the slide prevents the drill from drilling a hole past the desired depth.  
           [0008]    For example, Glossop, U.S. Pat. No. 6,203,543 B1 discloses a device for securing objects to bones using bone screws. Glossop recites drilling a hole prior to insertion of the bone screw. The device taught in Glossop is used to insert the bone screw into the hole. Glossop recites using a threaded nut on a sleeve. The screw is inserted axially in the sleeve. The threaded nut is set to prevent rotation of the screw into the bone beyond the desired depth.  
           [0009]    The prior art systems such as the Glossop device, utilizing threaded members, are difficult to operate, since two hands are required to thread the slide to the proper depth. Since the drill guides are used in conjunction with drills, only one hand may be available to operate the guide. Additionally, a typical threaded slide does not lock into position, and therefore the depth may change. A simple drill guide is therefore needed which locks in at the desired depth. Moreover, the drill guide should be operable with only one hand.  
           [0010]    Gisin et al., U.S. Pat. No. 5,507,801 discloses the visual monitoring of the hole depth in a compression drill guide for drilling screw holes in bone fragments. The guide includes an outer cylinder, an inner cylinder with a greater length than the inner cylinder, and a spring mechanism for advancing the inner cylinder. The inner cylinder is provided with optical markings so that a surgeon can visually control the depth of the drilling. Thus, when the surgeon observes that the inner cylinder has reached the appropriate marking, the surgeon ceases drilling. Visual monitoring of the drilling of a hole in a bone is likely to result in errors in the hole being drilled too deeply or not deeply enough.  
           [0011]    Kuslich et al., U.S. Pat. No. 5,899,908 discloses a drill tube guide for use in spinal applications. A drill tube sleeve is utilized so that a bore is reamed until a shoulder of the reamer abuts the sleeve. The kit taught in Kuslich, however, includes multiple drill tubes, drill tube sleeves and reamers in various lengths to provide an accurate depth of the bore, to correspond to the size of the implant. Thus, the kit requires multiple drill guides to monitor different depths of the hole, providing a complex system.  
           [0012]    Therefore, a guide is needed for monitoring the depth of a hole to be drilled in a bone that is simple to use and provides accurate results, so as to not cause damage to arteries, nerves and tissue surrounding the bone.  
           [0013]    Additionally, a drill guide is needed that may be operated using only one hand, providing a surgeon with a free hand for operating the drill.  
           [0014]    There is also a need for a drill guide that is adaptable to monitor the depth of a hole to various values.  
           [0015]    In addition, there is a need for a guide that may be utilized to monitor the depth of tapping as well as drilling.  
           [0016]    There is also a need for a drill guide that provides safety stops and locks to prevent the drill from drilling too deeply.  
         SUMMARY OF THE INVENTION  
         [0017]    A drill guide is provided that overcomes the disadvantages of the prior art.  
           [0018]    In accordance with the present invention, a drill guide or instrument for use in drilling a bone is provided. The instrument includes a sleeve with a depth gauge for setting the depth of a hole to be drilled into the bone. The sleeve defines an end for contacting said bone. The instrument further includes a slide having a releasable locking mechanism cooperable with the depth gauge. The slide is axially disposed within the sleeve. Additionally, the instrument includes a drill having a shaft defining a shoulder. The drill is axially received within the slide and advances to drill a hole in the bone until the shoulder abuts the slide.  
           [0019]    Preferably, the depth gauge comprises a channel formed in the sleeve. The channel includes a plurality of pairs of notches. In addition, the locking mechanism is a spring arm formed in the slide. The arm includes an edge extending radially from its free end. The arm is adaptable to lock into a pair of notches, thereby locking the slide in position. The arm may be deflected toward the center of the slide to unlock the edge from the notches.  
           [0020]    Preferably, the arm includes a tab extending radially from the arm and being aligned within the channel of the sleeve. Thus, a force may be applied to the tab to deflect the arm. Additionally, each of the pairs of notches may be labeled to identify the depth that the pair corresponds to  
           [0021]    In a preferred embodiment, the sleeve includes teeth extending axially therefrom for contacting the bone. The teeth contact the bone and ensure that the instrument is properly position. Moreover, the teeth prevent the instrument and the drill from slipping out of position.  
           [0022]    According to another aspect of the invention, the drill defines a first end, a middle portion having a diameter smaller than the inner diameter of the slide and a second end. The shoulder is formed between the first end and the middle portion. Moreover, the middle portion fits axially within the slide.  
           [0023]    In a preferred embodiment, the sleeve is comprised of stainless steel or titanium and the slide is comprised of stainless steel or titanium. Additionally, the slide may be formed via plastic injection molding. The depth gauge is capable of controlling the depth of the hole. In one embodiment of the invention used for the cervical spine, the depth gauge may be set to a depth of about 6 mm to about 54 mm, preferably between about 10 mm and 14 mm. Of course, the present invention may be utilized for other areas and therefore, the depth gauge may be set to various appropriate ranges.  
           [0024]    According to another aspect of the invention, the instrument further includes a tap including a shaft defining a first shoulder. The tap may be axially inserted into the slide and advances to tap the hole in the bone until the first shoulder of the tap contacts or abuts the slide. In one embodiment of the invention used for the cervical spine, the depth gauge can control the depth of tapping in the range of about 6 mm to about 24 mm. Of course, the present invention contemplates applications other than the cervical spine and therefore the depth gauge for tapping may be set to other appropriate ranges according to the application for which it is used.  
           [0025]    Additionally, the tap further defines a first end, a first middle portion having a diameter smaller than the diameter of the first end, a second middle portion having a diameter smaller than the first middle portion. The first shoulder of the tap is formed between said first middle portion and said second middle portion and a second shoulder is formed between the first end and the first middle portion. The first end has a diameter larger than the inner diameter of the sleeve such that the second shoulder contacts or abuts the sleeve when the hole is tapped to a depth of about 24 mm.  
           [0026]    According to yet another aspect of the invention, an instrument for use drilling a hole in a bone is provided comprising a sleeve, a slide, and a drill. The sleeve defines a channel including a plurality of pairs of notches. Each of the pairs of notches corresponds to a depth of the hole to be drilled into the bone. The sleeve further includes teeth extending axially therefrom for contacting the bone and preventing the instrument and drill from slipping from its position on the bone. The slide defines a spring arm having an edge extending radially from its free end. The edge, thus, locks into each of the pairs of notches in order to lock the slide in position. The arm may be deflected toward the center of the slide to unlock the edge from the notches and allow the slide to axially slide within the sleeve. The drill includes a shaft defining a shoulder. The shaft is axially inserted into the slide and advances to drill the hole in the bone until the shoulder contacts said slide. Since the depth of the hole has been set by lining the edge of the arm up with the pair of notches that corresponds to the desired depth, the hole will reach the desired depth when the shoulder abuts the slide.  
           [0027]    Preferably the arm further includes a tab extending radially from the arm. The tab is aligned within the channel of the sleeve. Applying force to the tab deflects the arm, thereby unlocking the edge and allowing for axial movement of the slide. In a preferred embodiment, each of the pairs of notches is labeled with the corresponding depth.  
           [0028]    According to another aspect of the invention, the drill defines a first end, a middle portion having a diameter smaller than the inner diameter of the slide and a second end, wherein the shoulder is formed between the first end and the middle portion.  
           [0029]    In a preferred embodiment, the sleeve is comprised of stainless steel or titanium and the slide is comprised of stainless steel or titanium. Alternatively, the slide may be formed by plastic injection molding.  
           [0030]    Preferably, when used for the cervical spine, the depth the hole may be set from about 6 mm to about 54 mm.  
           [0031]    In accordance with another aspect of the invention, the instrument further comprises a tap including a shaft defining a first shoulder. The tap is axially inserted into the slide and advances to tap the hole previously formed in the bone until the first shoulder of the tap contacts or abuts the slide. Preferably, when used for the cervical spine, the depth gauge is adaptable to control the depth of tapping in the range of about 6 mm to about 24 mm. According to a preferred embodiment, the tap defines a first end, a first middle portion having a diameter smaller than the diameter of the first end, and a second middle portion having a diameter smaller than the first middle portion. The first shoulder is formed between the first middle portion and the second middle portion and a second shoulder is formed between the first end and the first middle portion. Additionally, the first end has a diameter larger than the inner diameter of the sleeve so that the second shoulder contacts or abuts the sleeve when the hole is tapped to a depth of about 24 mm.  
           [0032]    According to another aspect of the invention, a method of drilling a hole in a bone using an instrument including a sleeve defining a channel including a plurality of pairs of notches and including teeth extending axially therefrom for contacting the bone, a slide defining a spring arm having an edge extending radially therefrom, the slide being axially disposed within the sleeve, and a drill including a shaft defining a shoulder is provided. The method includes creating a dimple on the surface of the bone at the location of the hole, setting the desired depth of the hole on the instrument, pressing the teeth to the bone to surround the dimple, inserting the shaft into the slide, drilling a hole in the bone with the shaft, and abutting the shoulder of the shaft on the slide.  
           [0033]    Preferably the depth of the holt is set by applying a pressure to the arm to deflect the arm toward the center of the slide, axially moving the slide to line the edge up with the pair of notches that corresponds to the desired depth of the hole, and removing the pressure from the arm thereby locking the edge into the notches. The desired depth of the hole may be set to a depth of about 6 mm to about 54 mm when the present invention is used on the cervical spine.  
           [0034]    In addition, another aspect of the invention contemplates tapping the hole, so that the hole can receive a threaded implant such as a bone screw. Thus, the method further includes removing the drill from the bone, removing the instrument from the bone, providing a tap defining a first shoulder and a second shoulder, setting the depth of the desired tapping on the instrument, pressing the teeth to the bone surrounding the hole, inserting the tap into the slide, tapping the hole with the tap, and abutting the second shoulder of the tap with the slide,  
           [0035]    Preferably, the depth is set for tapping by applying a pressure to the arm to deflect the arm toward the center of the slide, axially moving the slide to line the edge up with the pair of notches that corresponds to the desired depth of the hole, and removing the pressure from the arm thereby locking the edge into the notches. The depth of the tapping may be set to a depth of about 6 mm to about 24 mm when the present invention is used for the cervical spine.  
           [0036]    The maximum depth of the tapping is about 24 mm when applied to the cervical spine. Of course, the maximum depth of tapping may be increased or decreased depending on the application of the tap. When the depth is set on the instrument to 24 mm, the drill advances until the first shoulder of the tap contacts or abuts the sleeve. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0037]    [0037]FIG. 1 is a perspective view of a drill guide having a drill shaft inserted in accordance with the present invention.  
         [0038]    [0038]FIG. 2 is a perspective view of a sleeve in accordance with the present invention.  
         [0039]    [0039]FIG. 3 is a perspective view of a slide in accordance with the present invention.  
         [0040]    [0040]FIG. 4 is a sectional perspective view of the slide and sleeve in accordance with the present invention.  
         [0041]    [0041]FIG. 5 is a sectional view of the slide and sleeve having a drill shaft inserted in accordance with the present invention.  
         [0042]    [0042]FIG. 6 is a side view of a drill shaft in accordance with the present invention.  
         [0043]    [0043]FIG. 7 is a side view of a tap in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0044]    Referring to FIGS.  1 - 3 , a drill guide  10  for use with an orthopedic drill shaft  12  or tap is shown. The guide  10  generally comprises a sleeve  14 , a slide  16  and a handle  18 .  
         [0045]    The sleeve  14  is generally a hollow cylinder defining a proximal end  20  and a distal end  22 . Serrations or teeth  24  extend axially from the distal end  22 . The sleeve  14  further defines a channel  26  extending from the proximal end  20  and terminating at a point prior to the edge of the distal end  22 .  
         [0046]    The channel includes receiving members along its opposite edges. As illustrated herein, the receiving members comprise pairs of notches  28  disposed along the opposite edges of the channel. Of course, the present invention contemplates the receiving members being various shapes including a plurality of holes, indents, notches or other structures in the sleeve to engage the tab and prevent movement of the slide.  
         [0047]    Each pair of notches  28  corresponds to a desired depth of the hole. Preferably, each pair of notches is labeled with its corresponding depth. The interior diameter D 1  of the sleeve remains constant, as shown in FIGS. 4 and 5. The outside diameter Do of the sleeve is stepped, having the greatest diameter D o1  at the proximal end  20 , decreasing at the middle portion including the notches to D o2 , and decreasing yet again at the portion of the distal end from which the teeth extend to D o3 . Preferably, the sleeve is comprised of stainless steel or titanium.  
         [0048]    The slide  16  is also generally cylindrical and has an inner diameter d i  and an outer diameter d o . Outer diameter d o  is slightly smaller than the inner diameter D i  of the sleeve  14 . Thus, the slide is axially disposed with the sleeve  14  as shown in FIGS. 1, 4 and  5 . The slide  16  includes a first end  30 , a middle portion  31  and a second end  32 . The slide  16  defines a spring arm  34  formed via slots  36  extending from about the middle portion  31  towards the second end  32 . Thus, the arm has a fixed end  35  in the middle portion  31  and a free end  37  at the second end  32 . Since the arm  34  acts like a spring, it may be deflected toward a center  39  of the slide. An edge  38  extends radially from the arm  34  at its free end  37 . The spring arm  34  further includes a tab  40  positioned between the fixed end  35  and the free end  37 . The tab  40  includes a stem  42  and a pressing member  44  defining ridges  46 . Preferably, the slide is comprised of stainless steel or titanium. Alternatively, the slide may be formed of injection molded plastic.  
         [0049]    The slide  16  is positioned axially within the sleeve  16  with its second end  32  pointing towards the distal end  22  of the sleeve  14 . The channel  26  of the sleeve  14  receives the stem  42  of the tab  40  so that the edge  38  of the slide  16  cooperates with the pairs of notches  28  of the sleeve  14  and the channel. Thus, the edge  38  may be positioned into any one of the pairs of notches  28 . Since the arm  34  is a spring, the edge  38  locks into the selected pair of notches. Thus, when the edge  38  is locked into a pair of notches  28 , the slide  16  is stabilized in position and cannot be axially displaced within the sleeve. Applying a slight force to the tab  40  against the spring arm  34  causes the arm to deflect inwardly toward center  39 , thereby unlocking the edge  38  from the pair of notches. The slide  16  may then be axially moved to a different position. Thus, the desired depth of the hole may be set by deflecting the arm, axially displacing the slide to line the edge up with the proper pair of notches and releasing the arm from deflection to allow the edge to lock into the pair of notches corresponding to the desired depth.  
         [0050]    The drill shaft  12  for use with the drill guide  10  according to the present invention is illustrated in FIG. 6. The drill shaft defines a first end  52 , a middle portion  54  and a second end or tip  56  and is stepped in diameter. Thus, the diameter of the drill shaft  50  at its first end  52  is X 1 , decreasing to X 2  at its middle portion  54  and terminating at X 3  at tip  56 . The change in diameter between the first end  52  and middle portion  54  forms shoulder  58 . The diameter X 2  of middle portion  54  is slightly smaller than the inner diameter d i  of the slide  16 . Thus, the drill shaft  12  may be axially inserted into the slide  16 , which is positioned within the sleeve  14  as shown in FIG. 5. Since the middle portion  54  of the drill shaft  12  fits within the slide  16 , the middle portion  54  prevents the arm  34  of the slide from deflecting inwardly toward the center  39  and thus, unlocking. Thus, once within the sleeve  16 , the middle portion  54  assists in locking the edge  38  of the arm  34  into position within the pair of opposite notches. The diameter X 2  of the middle portion also has enough clearance to allow for rotation and movement of the drill within the slide. The shoulder  58  of the drill shaft abuts against the first end  30  of the slide  16  as drilling occurs to stop further advancement of the drill. Therefore, the shape of the drill shaft acts as a constraint to gauge the depth of the hole to be drilled. In one embodiment, the drill guide  10  is capable of controlling the drilling of a hole to a depth in the range of about 6 mm to about 54 mm. Such a range of depth is appropriate and preferable for the cervical spine.  
         [0051]    Referring now to FIG. 7, a tap  60  may also be used with the guide  10  to tap a hole that has been drilled in the bone. Thus, the guide  10  also regulates the depth at which the tapping is complete. The tap  60  includes a first end  62 , a first middle portion  64 , a second middle portion  66 , and a second end  68 . The tap  60  is also stepped in diameter. The first end  62  has the largest diameter of X 4 . The first middle portion has a diameter X 1  which is substantially equal to the diameter of the first end  52  of the drill shaft  12 , but less than the diameter X 4  of the first end  62 . A first shoulder  70  is formed at the junction of the first end  62  and the first middle portion  64 . The second middle portion  66  has a diameter X 2 , which is substantially equal to the diameter of the middle portion  54  of the drill shaft  12 , and smaller than the diameter X 1  of the first middle portion  64 . A second shoulder  72  is formed at the junction of the first middle portion  64  and the second middle portion  66 . Finally, the second end has a diameter X 3  which is substantially equal to the diameter X 3  of the second end  56  of the drill shaft  12 , and smaller than the diameter X 2  of the second middle portion  66 .  
         [0052]    Since the diameter X 2  of the second middle portion  66  of the tap  60  is slightly smaller than the inner diameter d i  of the slide  16 , the tap  60  may be inserted into the slide  16  at the second middle portion  66 . The diameter X 1  of the first middle portion is larger than the inner diameter d i  of the slide  16 ; therefore, the second middle portion should abuts the slide to prevent the tap from advancing. It is the first shoulder  70 , however that acts as the constraint. Specifically, the diameter X 4  of the first end  62  is larger than the outside diameter D o1  of the sleeve. Thus, the tap is constrained from tapping more deeply when the first shoulder  70  abuts the sleeve  14 .  
         [0053]    Preferably, when used for the cervical spine, the tap  60  is capable of tapping to a depth in the range of about 6 mm to about 24 mm. Thus, the first shoulder  70  is positioned such that it abuts the sleeve  14  when the tap reaches a depth of 24 mm. This is an added safety component, since tapping the bone too deeply could cause severe damage.  
         [0054]    The handle  18  is attached to the sleeve  14  at its proximal end  20 . The handle extends at an angle such that the guide  10  may be comfortably handled by a surgeon. The handle  18  includes a grip  48 .  
         [0055]    In operation, a surgeon performing an orthopedic procedure requiring the drilling and/or tapping of a hole in a bone to a specific depth first would prepare the surface of the bone, which is already exposed. The surgeon, preferably, marks the location of the hole using, for example, an awl to create a dimple in the bone.  
         [0056]    Next, the surgeon holds the guide  10  via the grip  48  in one hand. The guide is then set to control the desired depth of the hole to be drilled into the bone. The depth of the hole may be set, for example, within the range of about 6 mm to 54 mm, which is an appropriate range for the cervical spine. Preferably, each pair of opposite notches is marked with the depth of the hole to which the pair corresponds. In order to set the depth, the slide must be moved. Thus, the tab  40  is pressed to deflect the arm  34  inwardly, thereby unlocking the edge  38  from the notches  28 . Having unlocked the slide  16 , it may easily be moved axially within the sleeve by the surgeon using the thumb of the hand gripping the guide  10 . The ridges  46  on the pressing member  44  prevent the thumb from slipping off the tab and facilitate movement of the slide  16 . The edge  38  must be lined up with the pair of notches that corresponds to the desired depth of the hole. The tab may then be released to lock the slide into position.  
         [0057]    Once the desired depth of the hole has been set on the guide  10 , the teeth  24  are pressed onto the bone so that the dimple is centered within the teeth  24 . The teeth  24  ensure that the drill guide does not slip from its placement on the bone. Using the free hand, the surgeon then inserts the drill shaft  12  through the sleeve and slide until the tip  56  contacts the surface of the bone at the dimple. The surgeon then manually turns the drill using his or her free hand, which advances the drill shaft  12  axially within the slide  16  to drill the hole in the bone. As the hole is drilled, the shaft advances until the shoulder  58  contacts or abuts the first end  30  of the slide  16 . Since the diameter X 1  of the first end  52  of the drill shaft  12  is larger than the inside diameter d i  of the slide  16 , the drill cannot advance further. Thus, the hole has reached the desired depth, and the surgeon can stop drilling and remove the drill shaft from the drill guide  10 .  
         [0058]    Of course, the present invention also contemplates inserting the drill shaft into the sleeve prior to the guide  10  contacting the bone.  
         [0059]    If desired, the hole then may be tapped using tap  60 . Similar to using the drill, the depth of the desired tapping must first be set on the guide  10  by the surgeon. The depth of the tapping may be set within a range of about 6 mm to about 24 mm. Thus, at 24 mm, the first shoulder  70  will abut the sleeve  14 . The surgeon sets the depth of the tapping on the guide by pressing the tab  40  to deflect the spring arm  34  inwardly. The slide may then be moved axially within the sleeve to line the edge  38  up with the notches  28  that correspond to the desired depth of the tapping. Preferably, the depth of the tapping may be set to within a range of about 6 mm to about 24 mm. Once the edge is lined up with the pair of notches that correspond to the desired depth of tapping, the tab may be released so that the springing motion of the arm allows the tab to lock into position. The slide  16  is then locked into position.  
         [0060]    Next, the surgeon inserts the tap  60  into the slide until the second end  68  of the tap is approximately in line with the teeth  24  of the guide. The guide can then be positioned onto the bone using the teeth. The teeth are placed onto the bone, surrounding the hole to be tapped. The tap is then operated so that it advances axially within the guide to tap the hole. Thus, the tap advances until the second shoulder  72  abuts the slide  16 . In the alternative, if the depth of desired tapping is 24 mm, then the tap advances until the first shoulder  70  abuts the proximal end  20  of the sleeve  14 . This is an added safety feature to ensure that the tapping does not occur past this point.  
         [0061]    Of course, the drill guide of the present invention may be used for drilling holes for all types of bone screws and not just pedicle screws. For example, the drill guide may be used for trauma applications and other orthopedic applications.  
         [0062]    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.