Patent Abstract:
Instrumentation for osteofixation including a locking bone plate and a surgical drill guide. The plate has a plurality of fastener holes with inner walls of a preselected hole diameter. The drill guide has a guide member, for guiding a drill bit, and a hollow collet disposed substantially coaxially with the guide member. A radially expandable forward end of the collet comprises a radially expandable neck and an outwardly projecting rim disposed forward of the neck. This rim defines a contracted outer rim diameter that is smaller than the hole diameter in a contracted collet position, and an expanded outer rim diameter that is larger than the hole diameter in an expanded position. Thus, the rim is freely extractable through the plate hole in the contracted position, but is unreceivable through the plate hole in the expanded position. The collet neck is configured and dimensioned to press outwardly against an inner wall of the plate hole when the neck is expanded.

Full Description:
TECHNICAL FIELD 
     The present invention relates to a surgical drill guide and a surgical plate that are attachable to each other for retaining a precise alignment therebetween. More particularly, the invention relates to a bone plate with a fastener hole and surgical drill guide with an expandable collet having a rim that, when contracted, is smaller than the fastener hole. 
     BACKGROUND OF THE INVENTION 
     Surgical fixation plates are used in many procedures to mend, align, and alter compression of patients&#39; bones. These plates are primarily secured to the patient&#39;s bones by a plurality of fasteners such as screws. Proper orientation and alignment of fasteners and secure surgical fixation of the plates is crucial to avoiding future complications after implantation. This is especially the case for cervical spine locking-plates, such as sold by SYNTHES Spine. These plates are used for long term, intravertebral fixation, bone-fragment fixation, and anterior decompression in the cervical region of the spine. Locking plates enable secure monocortical implantation, meaning that their screws need only penetrate the anterior bone cortex. In conventional plates, screws must pass through both the anterior and posterior bone cortices to attain sufficient support. In passing through both cortices, conventional plates risk penetrating the spinal chord. 
     Surgeons implanting vertebral plates operate within a fine margin of error. Fairly little vertebral bone is available for setting fasteners. Each plate hole should coaxially align with its screw, i.e., each plate hole has an axis that must align with the screw axis. Otherwise, screws do not seat correctly with the plate. Thus, misalignments can potentially damage tissues, including the spinal cord, or lead to improperly secured plates. 
     Locking plates in particular demand precise fastener alignment. Cervical locking plates are generally about 2 mm thick. Some screw holes in these plates are inclined by 12° to the surface of the plate to permit optimal screw placement in the cervical region of the spine. 
     Anchor screws secure the locking plate to the vertebral body. Anchor screws have hollow, longitudinally slotted expansive heads that must fit snugly within a plate&#39;s screw hole. These screws are externally threaded to secure to the vertebral bone and the plate. These screws are also threaded internally from their head through a shallow portion of their shaft. Once a surgeon implants an anchor screw, he or she screws a small locking screw into the head of the anchor screw. This locking screw expands the head of the anchor screw so that the head presses outwardly against the locking plate&#39;s hole for a compression fit. This compression fit locks the screw in place and creates a solid coupling between the plate and the screw, preventing motion between them and preventing the screw from backing out from the plate, which may damage the esophagus. 
     This locking mechanism demands extremely precise screw alignment. If the holes drilled in the bone prior to anchor screw insertion are misaligned or off center, anchor screws and locking-plate holes will not seat correctly. Forcing a misaligned anchor screw into the plate hole can collapse the expansive head and prevent insertion of a locking screw. Thus, accurate drill guides for use in drilling the screw hole into the bone are critical to successful operations. 
     Known drill guides for locking plates, such as disclosed in a SYNTHES Spine catalog dated 1991, are generally a cylindrical tube shaped to receive and guide a drill bit. Most known guides also have a handle. A tip of the tube is shaped to slide into screw holes. A shoulder near the guide tip rests against a modest countersink in the screw hole to limit the guide&#39;s insertion into the hole. Constant axial pressure against the plate is required to maintain the guide in the hole, although it is sometimes beneficial to limit unnecessary pressure against the spine during drilling. Also, a clearance between the tip of the guide and the hole is provided to ease insertion into the hole. Due to this clearance, the diminutive thickness of the plate, and the small size of the countersink, an amount of angular play exists in this system. Other similar guides, though shown with femur fixation-plates, are disclosed in U.S. Pat. Nos. 2,494,229, and 5,417,367. 
     A more accurate drill guide is sold by SYNTHES Spine and shown in its catalog dated 1995, in which angular play is reduced and which does not require a constant force against the plate. This drill guide has an expanding collet formed with a plurality of fingers disposed coaxially about a drill guide sleeve. The sleeve is conical, and when it is slid forward, it spreads the collet fingers to lock them against the inside walls of a screw hole in a cervical spine locking plate. A scissoring handle linked to the collet and the sleeve controls the relative forward and backward motion therebetween. 
     At the forward tip of the drill guide, the collet has a neck, designed to press against the inside walls of the screw hole. Adjacent this neck is a radially extending rim, which, in a naturally assumed contracted position, has a diameter slightly larger than the screw hole, providing an interference fit. As a surgeon inserts the tip of the collet into the screw hole, the greater diameter of the rim provides a surgeon with a detectable snap and decreased resistance to insertion of the collet as the rim passes to the far side of the hole. To extract the collet from the screw hole, the surgeon must apply a slight force to pry the rim back through the smaller diameter walls of the hole, as these force the rim to contract to the smaller diameter. 
     A problem frequently arises when using this drill guide during surgery. Once the plate has been carefully positioned in the desired implantation position within the incision, when the surgeon attempts to remove the drill guide from the bone plate, the collet rim often catches on the plate. This catching prevents the drill from releasing the plate, and the surgeon often pulls the plate out of the incision along with the drill guide. As a result, any temporary fixation pins that were holding the plate to the bone could be stripped out of the vertebra, weakening the supporting bone structure, or in the best scenario, the plate would merely become misaligned with previously drilled holes. Even if the plate only becomes misaligned, however, careful realignment of the plate is required before the implantation procedure can continue. 
     Due to the precise nature of the relationship between the dimensions of screw hole and the rim and neck of the collet, the above problem cannot be avoided by simply using a particular drill guide in combination with any available plate that has larger screw holes. The drill guide and its corresponding locking plates are precisely size-matched and are sold in kits. A drill guide of this type cannot adequately lock and function as a guide with available plates with differently sized holes than those for which the guide was designed. Slightly large holes, for instance, permit excessive play between the plate and the guide, even when the guide is expanded. 
     Thus, a drill guide is needed that can disengageably lock to a surgical plate fastener hole, but without catching as the drill guide is extracted therefrom. 
     SUMMARY OF THE INVENTION 
     The invention is directed to instrumentation for fixing bones or bone fragments to each other. The instrumentation includes a bone plate for attaching to the bones, and a drill guide. The bone plate has at least one fastener hole through which fasteners, such as locking bone screws, fasten the plate to the bones. The hole has an inner wall with a predetermined hole diameter. 
     The drill guide has a guide member for guiding a drill bit. A hollow collet disposed coaxially with the guide member has a radially expandable forward end with a neck and outwardly projecting neck and an outwardly projecting rim forward of the neck. The neck is configured to press outwardly against an inner wall of the plate hole when collet is in the expanded position. The rim is freely extractable through the plate hole when the collet is in a contracted position. However, when the collet is in an expanded position, the rim does not fit through the plate hole. 
     To achieve this, the rim defines a contracted outer rim diameter smaller than the hole diameter when the rim is in a contracted position, rendering the rim freely extractable from the hole. When the rim is in an expanded position, it defines an expanded outer rim diameter larger than the hole diameter, rendering the rim impassable through the plate hole. The contracted rim diameter is preferably between 0.1 mm and 0.3 mm smaller than the hole diameter, or about 95% of the hole diameter. In the preferred embodiment, the rim protrudes radially from the neck by less than 0.1 mm. In one embodiment, the diameter of the rim is equal to that of the neck. 
     To further facilitate extraction of the rim from the hole, the rim has a rounded cross section in a plane extending through the axis of the neck and rim, preventing the rim from catching on the plate during its extraction therefrom. Also, a surface of the rim substantially adjacent the neck and configured at a first angle thereto of preferably less than about 55°, and more preferably of about 45°. 
     The guide member includes a guide sleeve movably axially and telescopically received within the collet. The sleeve defines a guide bore through which it axially receive and guide a drill bit. In a forward position within the collet, the sleeve biases the collet towards the expanded position. Preferably, the sleeve has a surface tapered inwardly at a second angle of between 3° and 5° to its axis to effect the expansion of the collet. More preferably this taper angle is about 4°. 
     As a result, the invention provides a surgical drill guide and a bone plate that are securable to one another, but which do not catch on each other upon drill guide extraction. The guide is unfetteredly and freely removable from the plate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a side view of a surgical drill guide according to the invention; 
         FIG. 2  is a cross-section, cutaway view of an expandable collet in a contracted position and a guide sleeve according to the invention; 
         FIG. 3  is an enlarged cross-section of the collet being inserted into a locking plate; 
         FIG. 4  is a further enlarged view of the front of the collet; 
         FIG. 5  is a cross-section of a drill guide assembly of the invention locked coaxially to a screw hole and aligned at an angle to the surface of a locking plate; 
         FIG. 5A  is an expanded cross-section of the forward portion of the drill guide assembly of  FIG. 3 ; 
         FIG. 6  is a cross-section of a drill guide assembly according to the invention locked coaxially to a screw hole extending perpendicularly to the surface of the locking plate; 
         FIG. 6A  is an expanded cross-section of the forward portion of the drill guide assembly of  FIG. 4 ; 
         FIG. 7  is a flow chart of the method of implanting a cervical spine locking plate; and 
         FIG. 8  is a flow chart of the method for using the drill guide assembly to drill an aligned hole. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  shows an embodiment of a surgical drill guide assembly  8  according to the invention, which is adapted for use with a cervical spine locking plate. At a forward end of the drill guide assembly is a collet  10 . Telescopically and slideably engaged within collet  10  is a guide sleeve  12 . Preferably, a tissue protector  14  extends rearwardly from the sleeve  12 . The collet  10 , sleeve  12 , and tissue protector  14  are adapted to axially receive a drill bit  16 , and the guide sleeve  12  is sized to retain the spinning bit  16  in a precise coaxial alignment. 
     The collet  10  is fixed to a remote rear handle-member  18 . The handle member  18  is pivotably attached to a scissor grip  20  by a handle pin  22 . Together, handle member  18  and scissor grip  20  form a drill guide assembly handle  23 , which allows a user to maneuver and use the drill guide assembly. The scissor grip  20  has an arm  24  that extends to the opposite side of the handle pin  22  from the grip  20  to pivotably attach to an actuation bar  26  at actuation pin  28 . An end of the bar  26  is pivotably attached with the sleeve  12  at sleeve pin  30 . 
     Thus, the entire drill guide assembly in this embodiment forms a four bar linkage. When a surgeon squeezes scissor grip  20  towards handle member  18 , the arm  24  forces the actuation bar  26  forward. This in turn forces the sleeve  12  to slide forward, deeper into collet  10 . Preferably, however, no part of the sleeve  12  can slide further forward than the front of the collet  10 . The scissor grip  20  has a forward wall  32  and a rear wall  34  to help the surgeon manually force the sleeve  12  forward or backward by closing or opening the guide sleeve assembly with only one hand. Preferably, leaf springs  36  are fastened to the handle member  18  and the scissor grip  20  to further assist rearward motion of the sleeve  12  by biasing the handle  23  towards an open position. 
     The collet  10  has a forward end  40  that is radially expandable. In this embodiment, the collet has a plurality of fingers  38  that can be spread apart to expand the forward end  40  of the collet  10 . 
     Referring to  FIG. 2 , the collet  10  coaxially receives the sleeve  12  about an axis  37 . Also, a guide bore  39  extends along axis  37  for guiding a drill bit coaxially therein. 
     The forward end  40  of collet  10  is preferably comprised of longitudinally extending fingers  38 . The fingers  38  are divided by slots  42  extending longitudinally between adjacent fingers  38 . These fingers  38  are resiliently biased inwardly and naturally assume an inward disposition when in a relaxed state and when the sleeve  12  is in the unlocked position, as shown in the figure. In the figure, a portion of the sleeve  12  has been cut away to better illustrate the slots  42 . 
     At a frontmost portion of the expandable forward end  40  of the collet  10 , the fingers  38  form a radially expandable circumferential neck  44 . At the back end of and adjacent to neck  44  is a shoulder  46 , and at the front end of and adjacent to neck  44  are protrusions that form a radially expandable rim  48 . These portions of the collet  10 , i.e., the neck  44 , the shoulder  46 , and the rim  44 , are preferably a single piece of material of unitary construction, in the interest of minimizing the size of the drill guide that must be inserted into an incision. 
     In the contracted, unlocked position shown in  FIG. 2 , the neck  44  and the rim  48  are sized to fit freely through screw holes in a locking plate.  FIG. 3  shows the collet  10  being inserted into a screw hole  64  in a locking plate  56 . In the drawing, the collet is in its natural, contracted position. The collet  10  is resiliently biased towards this position, in which the neck  44  has a contracted diameter d 1  and the rim has a contracted rim diameter d 2 . The screw hole  64  has an inner wall with a hole diameter d 3 . 
     The contracted rim diameter d 2  is smaller than the hole diameter d 3  to permit free and unfettered extraction of the rim  48  from the hole  64 . Preferably, the contracted rim diameter measures between 0.1 mm and 0.3 mm less than the hole diameter d 3 . More preferably, the rim diameter d 2  is 0.2 mm smaller than the hole. The contracted rim diameter d 2  is preferably between 4.2 mm and 4.4 mm in a drill guide that functions with a hole diameter d 3  of about 4.5 mm. Thus, the contracted rim diameter is approximately 95% the size of the hole diameter. Also, the contracted rim diameter d 2  is preferably about between 1 mm and 2 mm larger than the contracted neck diameter d 1 . Thus, the rim  48  protrudes from the neck  44  by a preferred 1 mm. Hence, the contracted neck diameter d 1  is preferably more than 95% as large as the contracted rim diameter d 2 . 
     These diameters permit a surgeon to extract, and most preferably also insert, the rim  48  of the collet  10  through a screw hole  64  without the rim  48  catching in the far side  57  of the plate  56  when the collet  10  is contracted. This arrangement virtually eliminates the possibility of collet  10  failing to disengage from a bone plate  56 , reducing the likelihood of unintentional extraction of temporary fixation pins or misalignment of a previously positioned plate  56 . 
     At the same time, having a rim  48 , provides the surgeon with a detectable feel for when the rim has completely passed the through the hole  64 . In alternative embodiments, the rim  48  may be eliminated completely, for instance by reducing the contracted rim diameter d 2  to an equal size as the contracted neck diameter d 1 . These embodiments, though, would lack the signal to the surgeon produced by full passage of the rim  48  through the hole  64 . 
     As shown in  FIG. 4 , to further foment free removal of the rim  48  from the hole  64 , the rim  48  is rounded in a cross-section taken parallel to axis  37 . The cross section preferably curves around a radius  49  of about 0.15 mm. Also, in this embodiment, a surface of the rim  48  disposed adjacent the neck  44  is configured at an angle  51  of less than 55° to the neck  44 , and most preferably at about 45° thereto. In some embodiments, this angled surface is preferably joined to the neck  44  via a narrow surface  47  of concave radius. 
     Referring again to  FIG. 3 , shoulder  46  has a diameter d 4  that is greater than the contracted rim diameter d 2 . Thus, the shoulder  46  has a diameter that is greater than the hole diameter d 3  such that the shoulder  46  cannot be inserted therethrough. Still further, in the preferred embodiment, the neck  44  is slightly longer than the thickness of the hole wall  65 , such that the neck can abut the wall of the locking plate hole and the rim  48  can abut the inside surface of a locking plate  56 . In this manner, the drill guide assembly can be secured to the locking plate  56 , restricting relative movement. 
     The inside of the expandable forward end  40  the collet  10  preferably has a variable inner diameter. Preferably, the fingers  38  have a step  50  or a taper, resulting in a smaller inner collet  10  diameter forward of the step  50 . 
     The guide sleeve  12  includes a forward portion  52  that cooperates with the fingers  38  to expand the fingers  38  when the guide sleeve  12  is moved into a locked position. Preferably, the guide sleeve  12  is tapered at taper angle  53  to the axis  37  to form a conical forward portion  52 . The conical section  52  of guide sleeve  12  pushes outwardly against the inner surface of the collet  10  as the guide sleeve  12  is moved forward to expand the forward end  40 . In this embodiment, the conical section mates with and pushes against the inner collet  10  surface forward of step  50  to push the fingers  38  radially outward. When the guide sleeve  12  is in the unlocked position as shown in  FIG. 2 , the conical section  52  allows the fingers  38  to return to a relaxed, contracted position. This allows the collet  10  to be inserted and retracted from the plate hole. The taper angle  53  is preferably between 3° and 5°, and more preferably about 4°. The inner surface of the collet  10  forward of the step  50  is also preferably tapered at an angle  55  to axis  37  that is substantially equal to taper angle  53 . This range of angles provides a desirable amount of movement of the sleeve  12  within the collet  10  to bias the collet  10  from a contracted position to an expanded position. 
     When the surgeon squeezes the handle  23 , the guide sleeve  12  is moved forward and the conical section  52  cooperatively forces the inner surface of the collet  10  beyond step  50  and fingers  38  radially outward. Thus, the forward motion of the guide sleeve  12  towards a forward position expands the forward end  40  of the collet  10  to an expanded position. In this manner, the neck  44  can be expanded to abut the inner wall of the plate screw hole and the rim  48  is expanded to abut the inner surface of the locking plate. In the expanded position, the expanded outer diameter d 5  of the rim  48  is greater than the plate hole diameter d 3  so that the guide cannot be retracted from the plate hole, as shown in FIG.  6 A. 
       FIGS. 5-6A  show the sleeve  12  in a locked, forward position, and the expandable end  40  in an expanded position and locked to different screw holes of the same predetermined diameter d 3 . Referring to  FIGS. 5 and 5A , screw hole  54  in locking plate  56  is disposed at an angle of about 12° to the locking plate&#39;s  56  outside surface  58 . The drill guide assembly is configured so that when the collet  10  is expanded, as shown, the neck  44  presses outwardly against interior wall  60  of screw hole  54 , positively gripping the wall  60 . The rim  48  preferably abuts the back surface of the plate  56  so that the neck positions the guide. The shoulder  46 , on the other hand, preferably does not abut the outside surface  58  of the plate  56 . A firm locking against the plate  56  results, and precise co-axial alignment through the center of screw hole  54  is achieved even though the surface area of wall  60  is small. In this embodiment, the axis of the drill guide is aligned with the axis of the plate screw hole  54 . Thus, the axis of the hole drilled into the bone will also be aligned with the axis of the plate screw hole  54 . In this manner, an anchoring screw inserted into the drilled hole will be centered and aligned with the plate screw hole  54 , i.e., they too will be substantially co-axially aligned. 
     The plate  56  and the guide may become slippery during use when blood and drilled tissue residue cover the instruments. In this situation, rim  48  aids in preventing the collet  10  from sliding backwards, out of the hole  54 . The rim  48  is adapted to rest against the far side of the plate  56 , near the perimeter of the hole  54 . Note that when the drill guide of this embodiment is locked to an angled hole  54 , as shown, only a segment of rim  48  may actually contact the back of the plate  56 . This small contact surface suffices to retain the collet  10  within the hole  54 . 
     Preferably, a gap  62  remains between the forwardly facing surface of shoulder  46  and the plate  56 . This is because, in the preferred embodiment, the shoulder  46  is not necessary for achieving a proper drill alignment or a secure locking. Consequentially, a surgeon need not press the drill guide against the locking plate  56  to keep the guide properly seated within the hole  54 . 
       FIGS. 6 and 6A  show the same embodiment of the invention locked to a screw hole  64  in a different part of locking plate  56 . Hole  64  is perpendicular to the locking plate&#39;s  56  surface  66 . In this application, most of the rim  48  is in contact with the back of plate  56 . Similarly to the applications shown in  FIGS. 5 and 5A , a gap  62  preferably remains between the forwardly facing surface of shoulder  46  and the plate  56 . 
     As seen in  FIGS. 5 and 6 , the internal diameter of the tissue protector  14  is preferably wider than that of the sleeve  12 , forming a step  68 . This step  68  may alternatively be formed in a different place along the length of the tissue protector  14  or the sleeve  12 . Step  68  is adapted to stop a surgical drill bit  16  that is inserted through the rearward end of the tissue protector from advancing beyond a predetermined depth. This stopping action occurs when the step  68  contacts a portion  70  of the drill  16  that is wider than the internal diameter of the sleeve  12  or the tissue protector  14  forward of the step  68 , as illustrated in FIG.  6 . 
     Referring again to  FIG. 1 , the drill bit  16  illustrated has a safety stop  72  with a wider diameter than the interior of the tissue protector  14 . The rear  72  of the tissue protector  14  also preferably prevents advancement of the drill bit  16  when the tissue-protector rear  74  contacts the bit&#39;s  16  safety stop  72 . By selecting a bit  16  with an appropriately located safety stop  72  or safety step  68 , the surgeon is assured that the bit  16  will penetrate the vertebral body no further than necessary for insertion of a screw. 
     The flow chart in  FIG. 7  provides the procedure for implanting a cervical spine locking plate. After making an incision, and measuring the cervical vertebra to be fixed with the plate, a surgeon places a cervical locking plate of a correct estimated length on the vertebral body. The surgeon then bends the plate to contour it to the correct lordotic curvature. Once the plate is properly positioned on the vertebra, it is secured with a temporary fixation pin, which is monitored under lateral imaging. The surgeon then locks the drill guide to the plate and drills into the bone. He or she then taps the hole, inserts an anchor screw, and inserts a locking screw to lock the anchor screw to the plate. The locking and drilling process is repeated for the remaining screws. The last hole is drilled through the plate hole in which the locking pin was located. Finally, the surgeon closes the wound. 
     The chart in  FIG. 8  shows the procedure for using the drill guide. A surgeon inserts the collet into the plate screw hole and squeezes the handle to slide the sleeve forward, expanding the collet with the conical portion of the sleeve and locking the drill guide to the plate. The surgeon then inserts the drill through the drill guide sleeve, drills the hole, and removes the drill. He or she opens the handle of the drill guide, sliding the sleeve backwards and releasing the collet from the hole, and then freely and unfetteredly removes the guide from the plate. 
     Before and during locking-plate implantation, the surgeon may insert the expandable end  40  of the collet  10  into a screw hole in a locking plate  56 . By squeezing the handle  23 , the surgeon may grasp and manipulate the plate  56  without an additional plate holder if he or she so desires. 
     Preferably, friction between the forwardly moved conical portion  52  and the inner surface of fingers  38  beyond step  50  retains the expandable end  40  of the collet  10  in an expanded, locked position. This provides a presently preferred travel of scissor grip  20  required to expand and contract the collet  10 . In this embodiment, the inward bias of fingers  38  is selected to produce the desired friction, while allowing operation of the handle  23  with only one hand. Alternative taper angles of conical portion  52  and inner finger  38  surfaces, and alternative finger  38  resiliencies may be chosen according to the purposes of other embodiments. 
     The tissue protector  14  is preferably sized so that once the plate  56  is properly positioned over the implantation site and the collet  10  is locked to the plate, the tissue protector  14  extends to the outside of the patient&#39;s body. As a result, a spinning bit  16  will not laterally reach or harm surrounding tissues that the surgeon does not intend to drill. 
     Also, the handle  23  is preferably located remotely from the drilling site. This frees space near the plate  56  and permits insertion of the drill guide into narrow incisions. 
     Various changes to the above description are possible without departing from the scope of the invention. For example, in embodiments for use with plates that have noncircular screw holes, the outer cross-section of collet  10  may match the shape of the holes. It is intended that the following claims cover all modifications and embodiments that fall within the true spirit and scope of the present invention.

Technology Classification (CPC): 8