Patent Publication Number: US-2010130983-A1

Title: Drill Guide for Angled Trajectories

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to repairing bone fractures, and more particularly, to a drill guide for angled trajectories. 
     BACKGROUND 
     When repairing a broken, fractured, or shattered bone, a physician may often be faced with the task of affixing a fixation plate to the bone in order to align the bone, and possibly, to hold bone fragments together. In order to affix the fixation plate to the bone, the surgeon may screw a bone screw into a predrilled pilot hole in the bone through one of a plurality of screw holes in the fixation plate. Since numerous screw holes may be spread out across the entirety of the fixation plate, the surgeon may affix virtually any portion of the fixation plate to the bone by inserting a suitable number of bone screws through the fixation plate and into the bone. 
     To prevent the bone screws from backing out of the fixation plate once inserted, the inner surface of each screw hole may include a set of locking threads configured to interfere with a corresponding set of locking threads on the head of each bone screw. Consequently, when a bone screw is screwed into one of the threaded screw holes in the fixation plate, the locking threads in the screw hole and/or the locking threads on the head of the bone screw may deform to lock the bone screw into the fixation plate. 
     In certain cases, proper placement and positioning of the fixation plate may call for inserting a bone screw into the fixation plate at an angle other than parallel to the central axis of the screw hole. For example, if the underlying bone beneath a particular screw hole is weak, for example, due to its proximity to a fracture line, the surgeon may wish to angle the bone screw away from the fracture line so as to anchor the bone screw into a more solid bony mass. In another scenario, the surgeon may wish to avoid a nerve underlying the screw hole. 
     SUMMARY 
     In particular embodiments, the present disclosure provides for a system and method for establishing a trajectory for a pilot hole. The system may include a plate having a screw hole. The screw hole may be defined by a rim and an inner surface surrounding the screw hole. The system may further include a drill guide comprising a body having a guide well formed therein. The guide well may be defined by a wide opening in the body disposed opposite a narrow opening in the body and an interior surface that tapers from the wide opening to the narrow opening. The narrow opening may be surrounded by a tip of the body that is configured to fit into the screw hole. 
     A method for establishing a trajectory for a pilot hole may include placing a plate onto a bone, the plate including a screw hole defined by a rim and an inner surface surrounding the screw hole. The method may further include fitting a drill guide into the screw hole, the drill guide comprising a body having a guide well formed therein. The guide well may be defined by a wide opening in the body disposed opposite a narrow opening in the body and an interior surface that tapers from the wide opening to the narrow opening. The narrow opening may be surrounded by a tip of the body that is configured to fit into the screw hole. 
     In particular embodiments, the method may further include inserting a drill bit into the guide well and drilling a pilot hole into the bone while keeping the drill bit within the confines of the interior surface. 
     Technical advantages of particular embodiments of the present disclosure may include providing a system and method for enabling a surgeon to effectively judge the maximum angle of insertion for which the locking effect between a locking bone screw and a locking fixation plate will be maintained. This technical advantage may be realized through the use of a drill guide placed on the fixation plate that physically defines the maximum angle of insertion, giving the surgeon a defined range of acceptable angles within which to drill a pilot hole. The drill guide may further include a tip that fits securely into the screw hole of the fixation plate to align the drill guide with the screw hole and to protect the threading inside the screw hole from the drill bit during creation of a pilot hole, yet another technical advantage. 
     Other technical advantages of the present disclosure will be readily apparent to one skilled in the art from the following figures, descriptions, and claims. Moreover, while specific advantages have been enumerated above, various embodiments may include all, some, or none of the enumerated advantages. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present disclosure and its advantages, reference is now made to the following descriptions, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates a system for establishing a trajectory for a pilot hole according to an example embodiment of the present disclosure; 
         FIG. 2  illustrates an isometric view of an example embodiment of a fixation plate according to an example embodiment of the present disclosure; 
         FIG. 3  illustrates an isometric view of an example embodiment of a drill guide according to an example embodiment of the present disclosure; and 
         FIG. 4  illustrates an example cross section view of a portion of the drill guide of  FIG. 3  exposing a more detailed view of the interior surface of the drill guide. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     Various fixation plates may include specially designed screw holes that may lockably engage a bone screw inserted (e.g., “screwed in”) along a trajectory other than parallel to (e.g., coaxial with) the central axis of such screw holes. In one example embodiment, the bone screw may include threading on the underside of the head that interferes with threading inside the screw hole to lock the bone screw into the fixation plate once the bone screw is screwed into the screw hole. 
     To facilitate insertion of the bone screw into the screw hole along an angled trajectory, a pilot hole may be drilled into the bone to establish the trajectory for the bone screw. After the pilot hole has been created, the tip of the bone screw may be inserted into the pilot hole through the screw hole in the fixation plate and rotated until the threaded portion on the underside of the head comes to bear on the threaded portion of the screw hole. At this point, further rotation of the bone screw may cause the threaded portion of the head to interfere with the threading inside the screw hole and lock the bone screw into the plate. 
     Depending upon design, the above-described locking effect between the bone screw and the fixation plate may only be effective up to a maximum angle of insertion of the bone screw relative to the central axis of the screw hole. If the trajectory of the bone screw exceeds the maximum angle of insertion, the locking effect may become unreliable or may fail, increasing the propensity of the bone screw to back out of the screw hole. In other words, depending upon design, the screw hole in the fixation plate may only be able to accommodate a certain angular range of screw engagement before the locking mechanism is weakened or rendered inoperable. 
     In the operating room, a surgeon may not be able to effectively judge the maximum angle of insertion by visual inspection and may need a tool to help him drill a pilot hole within the specified limits of the plate design (e.g., within the maximum angle of insertion). Consequently, there is a need for a system and method to reliably establish the trajectory for the pilot hole within the maximum angle of insertion and also a need to protect the threading inside the threaded screw hole from being deformed by the drill bit during creation of the pilot hole. 
       FIG. 1  illustrates a system  100  for establishing a trajectory for a pilot hole  102  according to an example embodiment of the present disclosure. System  100  generally includes a drill guide  104  for guiding a drill bit  106  and a fixation plate  108  for affixing portions of a bone  110  together and for holding drill guide  104  steady relative to bone  110 . 
     In the pictured embodiment, system  100  is being used relative to a single fractured bone  110 ; however, particular embodiments of system  100  may be applied equally as well to virtually any bone or group of bones in the body. System  100  may also be used to create a pilot hole  102  in a synthetic element such as a surgical implant. 
     To establish the trajectory for pilot hole  102  using system  100 , a surgeon may place fixation plate  108  onto bone  110 , after which the surgeon may fit the tip  112  (see  FIG. 3 ) of drill guide  104  into any one of a plurality of screw holes  114  disposed throughout fixation plate  108 . In particular embodiments, the surgeon may temporarily or permanently secure fixation plate  108  on bone  110  using pins, bone screws  116 , or other suitable means while creating pilot hole  102 . In any case, once the tip  112  of drill guide  104  has been inserted into a screw hole  114 , the surgeon may create pilot hole  102  by inserting drill bit  106  into a guide well  118  in drill guide  104  and drilling into bone  110  within the confines of guide well  118 . 
     If the surgeon is using a bone screw  116  having a locking feature, drill guide  104  may be used to ensure that the angle of insertion  120  for bone screw  116  (e.g. the trajectory of pilot hole  102  relative to the central axis  122  of screw hole  114 ) is less than or equal to the maximum angle of insertion  124  for which the desired locking effect between bone screw  116  and fixation plate  108  will be preserved. For example, drill guide  104  may designed such that, once the tip  112  of drill guide  104  has been positioned in a screw hole  114 , the aperture angle  126  (see  FIG. 4 ) of guide well  118  is coextensive with the twice the maximum angle of insertion  124 . One of ordinary skill in the art will appreciate that aperture angle  126  is coextensive with twice maximum angle of insertion  124  when aperture angle  126  is twice as large as maximum angle of insertion  124  since maximum angle of insertion  124  is measured with respect to central axis  122  while aperture angle  126  is measured with respect to opposing sides of guide well  118 . Consequently, by drilling within the confines of drill guide  104 , the surgeon may be assured that the angle of insertion  120  established for bone screw  116  is less than or equal to the maximum angle of insertion  124 . Once pilot hole  102  has been created using drill guide  104 , fixation plate  108  may be affixed to bone  110  by screwing bone screw  116  into pilot hole  102  through screw hole  114 . 
     Depending upon design, drill guide  104 , bone screw  116 , and fixation plate  108  may be formed from any one or more materials suitable for forming medical devices and implants, such as materials that have high strength-to-weight ratios and that are inert to human body fluids. In certain embodiments, drill guide  104 , fixation plate  108 , or bone screw  116  may be formed from one or more titanium alloys, which provide several benefits. For example, titanium alloys are relatively lightweight, provide adequate strength for withstanding high forces, are inert to human body fluids, and are visible in radiographs. In particular embodiment, bone screw  116  may be formed from the titanium based alloy Ti6Al4V ELI (per ASTM F136), which provides a desirable combination of benefits, such as those discussed above while fixation plate  108  may be formed from grade 2 or grade 3 titanium (per ASTM F67). In certain other embodiments, bone screw  116  or fixation plate  108  may be formed from one or more resorbable polymers, such as polylactides, polyglycolide, glycolide/lactide copolymers or other copolymers for example, or one or more implantable plastics, such as polyethylene or acetal copolymers for example. 
     One of ordinary skill in the art will appreciate that the above-described embodiments of system  100  were presented for the sake of explanatory simplicity and will further appreciate that the present disclosure contemplates using any suitable combination and number locking screws  116  and fixation plates  108  to repair bone  110 . 
       FIG. 2  illustrates an isometric view of an example embodiment of fixation plate  108 . For reference purposes, fixation plate  108  (as well as other components of system  100 ) may be referred to as having a bottom side intended to be placed closest to bone  110  (e.g., to be placed upon bone  110 ) and a top side intended to be place furthest from bone  110 . Though particular features of fixation plate  108  may be explained using such intended placement as a point of reference, this method of explanation is not meant to limit the scope of the present disclosure to any particular configuration of fixation plate  108  or to any particular placement or orientation of fixation plate  108  relative to bone  110  or any other components of system  100 . 
     Fixation plate  108  may typically be any fixture including one or more screw holes  114  for receiving a bone screw  116 . In the pictured embodiment, fixation plate  108  generally includes a plurality of screw holes  114  connected to each other in a web-like distribution by a plurality of ribs  128 . Each screw hole  114  may include a rim  130  (e.g., a flat surface surrounding screw hole  114 ). Though in the pictured embodiment, ribs  128  are thinned down relative to each rim  130 , particular embodiments of plate  108  may be designed such that the entirety of plate  108  is uniform in thickness. In any case, when drill guide  104  is fitted into a screw hole  114 , rim  130  may rest flush against the shoulder  132  of drill guide  104 , providing drill guide  104  with a steady foundation on fixation plate  108 . 
     To aid a surgeon in positioning fixation plate  108  relative to bone  110 , one or more ribs  128  may comprise a positioning hole  134 . As an example and not by way of limitation, a surgeon may insert a K-wire into bone  110  after which the surgeon may position fixation plate  108  on bone  110  by inserting the K-wire through positioning hole  134  and sliding fixation plate  108  down onto bone  110 . Additionally, the surgeon may rotate fixation plate  108  about the K-wire using positioning hole  134  to achieve a desired orientation of fixation plate  108  relative to bone  110 . Once fixation plate  108  has been properly positioned, the surgeon may use drill bit  106  in conjunction with drill guide  104  to create a pilot hole  102  through screw hole  114 . The surgeon may then secure fixation plate  108  to bone  110  by screwing bone screw  116  into screw holes  114  along the trajectory established by pilot hole  102 . 
     Each screw hole  114  may be any an opening in fixation plate  108  configured to accept a bone screw  116 . In particular embodiments, the inner surface of each screw hole  114  may be threaded to lockably engage bone screw  116  such that once bone screw  116  has been screwed into screw hole  114 , bone screw  116  is prevented from rotating within screw hole  114 . As mentioned above, to accomplish this locking feature, the underside of the head of bone screw  116  may include a locking thread configured to interfere with the threading inside screw hole  114 . Thus, when bone screw  116  is screwed into screw hole  114 , the locking thread on the head of bone screw  116  may deform against the threading inside screw hole  114  to lock bone screw  116  into fixation plate  108 . 
     In particular embodiments, screw holes  114  may be designed to enable bone screw  116  to be screwed in along a trajectory other than parallel to central axis  122  while still maintaining the ability to lockably engage bone screw  116 . One example system for achieving an angular locking interface between a bone screw and a fixation plate is described in U.S. Provisional Application 61/106,511, entitled “ANGULATED LOCKING PLATE/SCREW INTERFACE,” filed Oct. 17, 2008. 
     In particular embodiments, design constraints or other considerations may limit the range of insertion angles for which the locking effect between bone screw  116  and fixation plate  108  remains viable. For example, a manufacturer may design fixation plate  108  such that the locking interface between bone screw  116  and fixation plate  108  remains viable for insertion angles up to ten degrees from parallel the central axis  122  of screw hole  114 . Beyond this ten degree radius around central axis  122 , the locking effect brought about by the threadable interface between bone screw  116  and plate  108  may be unreliable; for example, the amount of contact between the threading on bone screw  116  and the threading inside screw hole  114  may be insufficient to overcome the mechanical forces that may cause bone screw  116  to back out of screw hole  114 . Consequently, the maximum angle of insertion  124  in this example situation is ten degrees. One of ordinary skill in the art will appreciate that the maximum angle of insertion  124  may be defined by the manufacturer according to any suitable criteria. For example, maximum angle of insertion  124  may be defined as the angle beyond which a certain percentage of locking failures occur at a give stress level. In the operating room, drill guide  104  may enable a surgeon to effectively judge the maximum angle of insertion  124  when drilling a pilot hole  102  for bone screw  116  by providing a physical barrier that physically defines maximum angle of insertion  124  for the surgeon. Consequently, by drilling within the confines of drill guide  104 , the surgeon may be assured that the trajectory of pilot hole  102  will be less than or equal to the maximum angle of insertion  124 . 
     One of ordinary skill in the art will appreciate that the above-described embodiments of fixation plate  108  were presented for the sake of explanatory simplicity and will further appreciate that the present disclosure contemplates any suitable configuration for fixation plate  108 . 
       FIG. 3  illustrates an isometric view of an example embodiment of drill guide  104 . Depending upon design, drill guide  104  may generally include a funnel-shaped body  136  rigidly coupled to a handle  138 . Body  136  may have a guide well  118  formed therein that is defined by a wide opening  138  at the top of body  136 , a narrow opening  140  at the bottom of body  136 , and an interior surface  142  (see  FIG. 4 ) that uniformly tapers (e.g., tapers in diameter) from wide opening  138  to narrow opening  140 . In particular embodiments, wide opening  138  and narrow opening  140  may be concentric circles disposed opposite one another on body  136 . Body  136  may further include a tip  112  that surrounds narrow opening  140  and a shoulder  132  disposed above tip  112  that includes a flat underside configured to rest flush against rim  130  when tip  112  is inserted into a screw hole  114 . 
     In particular embodiments, drill guide  104  may be fitted into fixation plate  108  by sliding tip  112  into screw hole  114  until shoulder  132  rests flush against the top surface of fixation plate  108  (e.g., the top surface of rim  130 ). To keep tip  112  from wobbling around in screw hole  114 , the outer surface of tip  112  may be generally cylindrical in shape and sized to fit securely (e.g., snugly) into screw hole  116 . In particular embodiments, the outer surface of tip  112  may be threaded to enable a surgeon to screw tip  112  into screw hole  116 . Tip  112  may be configured to align the central axis  144  of guide well  118  (see  FIG. 4 ) with the central axis  122  of screw hole  114 . In particular embodiments, a length  112 L of tip  112  may be approximately equal to the depth  114 D of screw hole  114  to protect the threading inside screw hole  114  from being deformed by drill bit  106  during the creation of pilot hole  102 . 
     Shoulder  132  may be any fixture or combination of fixtures on the outer surface of body  136  capable of providing a level footing for body  136  relative to fixation plate  108 . As an example and not by way of limitation, shoulder  132  may comprise a contiguous smooth flat surface surrounding tip  112 . When tip  112  is inserted into screw hole  114 , shoulder  132  may abut rim  130  and act as a stop that limits the penetration depth of tip  112  into screw hole  114 , and as leveling mechanism that levels body  136  relative to fixation plate  108 . As an example and not by way of limitation, the flat underside of shoulder  132  may be disposed perpendicular to the central axis  144  of guide well  118 , thereby ensuring the central axis  144  of guide well  118  is parallel to (e.g., coaxial with) the central axis  122  of screw hole  114  when tip  112  is inserted into screw hole  114 . Consequently, when drill guide  104  is fitted into a screw hole  114 , the outer surface of body  136  may be used to align and couple body  136  with fixation plate  108  while interior surface  142  may be used to limit the insertion angle of drill bit  106  into bone  110 . 
       FIG. 4  illustrates an example cross section view of a portion of drill guide  104  cut through the central axis  144  of guide well  118  to expose a more detailed view of the interior surface  142  surrounding guide well  118 . In particular embodiments, interior surface  142  may be defined, in part, by a maximum diameter  146  (e.g., the diameter of wide opening  138 ), a minimum diameter  148  (e.g., the diameter of narrow opening  140 ), and an aperture angle  126 . Although aperture angle  126  may be chosen according to any criteria, in particular embodiments, guide well  118  may be designed such that aperture angle  126  coincides with twice the maximum angle of insertion  124  for which the locking effect between bone screw  116  and fixation plate  108  is preserved. Thus, if aperture angle  126  coincides with twice the maximum angle of insertion  124 , drill bit  106  may be used to create pilot hole  102  along virtually any trajectory within the confines of interior surface  142  while still preserving the desired locking effect between bone screw  116  and fixation plate  108 . As an example and not by way of limitation, if the maximum angle of insertion  124  is approximately  10  degrees measured from the central axis  122  of screw hole  112 , aperture angle  126  may be approximately  20  degrees. Thus, once drill guide  104  is fitted into screw hole  114 , drill bit  106  cannot be inserted through screw hole  114  along any trajectory greater than  10  degrees from central axis  122  without wedging between the portion of interior surface  142  surrounding wide opening  138  and the portion of interior surface  142  surrounding narrow opening  140 . 
     In particular embodiments, to prevent drill bit  106  from grinding against the bottom edge  113  of tip  112  during the creation of pilot hole  102 , the portion of interior surface  142  disposed inside tip  112  may have a slight outward taper. In other words, interior surface  142  may come to its narrowest point somewhere in the inside tip  112  (e.g., at the uppermost portion of tip  112 ) and then grow slightly wider before reaching bottom edge  113 . This outward taper may provide clearance for drill bit  106  to be inserted through narrow opening  140  without grinding against bottom edge  113 . In this case, narrow opening  140  may refer to the narrowest point of interior surface  142 . 
     One of ordinary skill in the art will appreciate that the above described embodiments of interior surface  142  were presented for the sake of explanatory clarification and will further appreciate that the present disclosure contemplates interior surface  142  having any suitable size or shape. For example, although interior surface  142  was described and illustrated as having a generally circular base yielding a generally right angle conical shape, interior surface  142  could just as easily have a generally square base yielding a generally pyramidal shape or any other geometrically suitable configuration operable to limit the insertion angle of drill bit  106  into bone  110 . 
     Although the present disclosure has been described in several embodiments, a myriad of changes, substitutions, and modifications may be suggested to one skilled in the art, and it is intended that the present disclosure encompass such changes, substitutions, and modifications as fall within the scope of the present appended claims.