Abstract:
A tool centers an elongated drill bit within an elongated canal surrounded by bone tissue. A sleeve having an expandable diameter is internally threaded. The sleeve threadedly engages an externally-threaded cylindrical guide. A key includes a handle at one end of an elongated rod. The rod includes transversely-protruding buttons for engaging the guide so that rotation of the handle produces rotation of the guide to cause radial expansion of the sleeve. This results in symmetrical contacting of the interior surfaces of the bone tissue, thereby creating a vertical, centered channel with respect to the bone, that extends through the tool for guiding the drill bit.

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention relates to tools for facilitating orthopedic procedures. More particularly, this invention pertains to a tool for reliably centering a drill bit within the femoral canal for facilitating removal of the plug of cementous material that remains after removal of a worn hip prosthesis. 
     2. Description of the Prior Art 
     While primary total hip arthroplasty (THA) generally produces a successful and long-lasting result, annually about one percent of existing hip replacements require revision.  FIG. 1  is a side elevation view in partial cross-section of the region of a hip replacement showing a prosthesis  10  fixed within a human femur  12 . The prosthesis may be fabricated of, for example, stainless steel and includes a metal ball or prosthetic head  14  that is, in turn, coupled to a prosthetic hip socket (not shown). The lower portion of the prosthesis  10  extends to and terminates in a elongated stem  16 . The stem  16  extends into the femoral canal. 
     The stem  16  is anchored to the surrounding bone of the femoral canal by means of cement  18 . The hardened cement  18  (a mixture of a monomer and a polymer) surrounds the stem  16  and extends beneath, forming a solid region known as a cement “plug”. A stop or restrictor  22  of plastic may have been inserted within the femoral canal during the procedure to increase backpressure (and thereby improving the quality of the cement-to-bone tissue bond) on the cement during hardening. 
     A gradual breakdown or loosening of the bond between the cement that holds the prosthesis within the patient&#39;s femur and the surrounding bone generally occurs over time, limiting the useful life of the existing procedure to about 10 to 20 years. Such loosening of the bond leads to the patient&#39;s experiencing pain. When such pain becomes significant, revision is required. This involves removal of the existing prosthesis and attachment of a new prosthesis in its place. The required surgical procedure involves removal of existing cement and the optional restrictor, in addition to the existing prosthesis, prior to insertion of a new device. The subsequent hip replacement often employs a non-cement fixation technique due to the thinning and reduced adhesion of existing bone tissue. The latter prosthesis may, for example, be formed with beaded or mesh-like surfaces that encourage interdigitation of bone tissue thereon. An alternative technique is to inject pressurized cement into a femoral canal that has been cleared of all debris. Generally, revision hip replacement can achieve a long lasting result and provide substantial pain relief. 
     The revision surgery is begun by removal of all foreign objects from the femoral canal. The existing prosthesis is readily removed, leaving the mantle of hardened cement and, possibly, the plastic restrictor in the canal. Removal of the structures associated with the existing prosthesis becomes more complicated and risky as one progresses further and further from the top or head of the bone into the femoral canal. While the uppermost cement is not as difficult to remove, visibility and accessibility limitations crop up as the surgeon proceeds further. Maintenance of adequate lighting is difficult and further obscuration is caused by the increased presence of blood once the upper regions of the cement have been removed (by, for example, power burrs, hand-held chisels, ultrasound, etc.), leaving the cement plug (and, perhaps, the plastic restrictor). 
     Removal of the remaining plug (and optional restrictor) is commonly accomplished by a number of tools and techniques including elongated drill bits, burrs or hand-held chisels to break through, fragment or create a hole in the plug. An elongated tool having a hooked end can then be inserted through the obstruction and an upwardly-directed force applied to lift and remove it. 
     The process of making a hole through the cement plug with an elongated drill bit is complicated by an environment that is obscure, small, remote and adjacent bone tissue whose integrity must be protected. Visibility and alignment difficulties in the plug region create a very dangerous environment in which to operate a drill bit, burr or chisel. Misalignment may result in the perforation of bone tissue. X-ray or like equipment is often employed to assure proper centering of potentially-harmful tools within the femoral canal. Another method of obtaining visibility is the excising of a “window” in the bone. This complicates the surgery, increasing its duration, the loss of blood and the risk of subsequent bone fracture. 
     U.S. Pat. No. 5,649,930 of Kertzner et al. covering “Orthopedic Centering Tool” teaches a tool for guiding a surgical drill bit through a cement plug within the femoral canal. Such tool provides a means for centering a drill bit that is not dependent upon direct observation of the drill bit within the interior of the femoral canal during surgery. Rather, it enables the surgeon to be assured that the bit is properly directed through an alignment procedure that occurs before the drill bit enters the canal. 
     The patented tool includes an adjustable frame comprising a pair of right angle sections mounted in a mirror image relationship. The sections are adjustably clamped to one another and, in turn, secure a vertical sleeve for guiding a surgical drill bit and a horizontal sleeve for accommodating an anchor pin. Various clamps permit a surgeon to adjust the tool so that the drill bit is guided to the approximate center of the femoral canal immediately below the plug while the pin anchors the frame to the bone. 
     While providing a number of advantages over such procedures as x-rays and the cutting of a window into the bone, the above-described tool is somewhat awkward to adjust during an operation. Such adjustment is undertaken within a difficult environment that includes blood, cement and bone fragments. Time limitations also complicate use of the tool. They may be due, for example, to the patient&#39;s tolerance for anesthesia. These and related factors complicate the surgeon&#39;s ability to perform the multiple adjustments required to assure that the drill bit has been positioned properly with respect to the bone. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the preceding and other shortcomings of the invention by providing a tool for centering a drill bit within an elongated canal at the interior of bone tissue. Such tool includes an exteriorly-threaded guide and a hollow, interiorly-threaded sleeve. 
     The sleeve is threadedly engaged to the guide, its diameter being responsive to rotation of the guide. A key is provided. Such key comprises an elongated rod having a handle fixed to an end. The elongated rod has a plurality of transversely-protruding tabs for engaging the guide so that rotation of the key produces rotation of the guide. In this way, the diameter of the sleeve is controlled by rotation of the handle. 
     The preceding and other features of the invention will become further apparent from the detailed description that follows. Such description is accompanied by a set of drawing figures. Numerals of the drawings, corresponding to those of the written description, point to the features of the invention with like numerals referring to like features throughout both the written description and the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partially sectioned side elevation view for illustrating the portion of a total hip arthroplasty that is addressed by the tool of the present invention; 
         FIGS. 2A and 2B  are exploded and fragmentary assembled perspective views of the tool and the guide and sleeve of the invention respectively; 
         FIG. 3  is a bottom plan view of the collar for a guide of a tool in accordance with the invention; 
         FIGS. 4A and 4B  are a series of views for illustrating the expansion of the sleeve in response to rotation of the key of the invention; 
         FIGS. 5A through 5D  are a series of partial section side elevation views for illustrating the operation of the tool of the invention; and 
         FIG. 6  is a perspective view of a sleeve for a tool in accordance with an alternative embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIGS. 2A and 2B  are exploded and fragmentary assembled perspective views of the tool of the invention. Viewed in combination, the tool generally comprises an exteriorly-threaded guide  24 , an interiorly-threaded sleeve  26 , and a key  28  that consists of an elongated rod  30  and a handle  32 . A pair of tabs  34  extend transversely from the surface of the elongated rod  30 . The tabs  34  may comprise, for example, a single pin whose length exceeds the thickness or diameter of the rod  30 . 
     The guide  24  comprises an annular collar  36  and an exteriorly-threaded body  38 . A pair of screws  40  secures the collar  36  to the body  38  by engaging aligned pairs of internally-threaded apertures  42 ,  44  respectively. 
     Once secured to one another, the circular void center  46  of the collar  36  is mated with a central aperture  48  of the body  38 . Such aperture  48  is aligned with the longitudinal axis of rotation  50  of the body  38 . The center  46  of the collar  36  and the central aperture  48  of the body  38 , in combination, provide a channel for receiving the key  28 . Entry into such channel is provided by opposed indentations  52  in an upper portion  54  of the annular collar  36 . Such indentations  52  are directed radially from the void center  46  of the collar  36  to permit passage of the tabs  34  that protrude outwardly from the elongated shaft  30  of the key  28 . 
     The interiorly-threaded sleeve  26  comprises an expandable configuration of three sleeve segments  56 . When mated, the sleeve segments  56  form a generally-cylindrical shape. Such shape may be radially expanded, by controllable expansion of the separation between individual sleeve segments  56 . This occurs in response to rotation of the shaft  30  of the key  28  when inserted into the channel formed by the void center  46  of the collar  36  and the central aperture  48  of the body  38 . The resultant expansion of the sleeve is described below. 
     The bottom edge  57  of each sleeve segment  56  (and of the sleeve  26 ) is serrated. As will be shown below, such serration of the edge  57  enables the sleeve to engage the cement and plug during operation. This permits the sleeve  26  to remain in place as the guide  24  is advanced downwardly with respect to the sleeve  26  by rotation of the key  28 . 
     As can be seen, the interiorly-threaded surface  58  of a representative sleeve segment  56  is tapered to create, in combination, a larger diameter at the top than at the bottom of the expandable sleeve  26 . As such, the internally threaded surface  58  of the sleeve  26  matches the exteriorly threaded surface  60  of the tapered guide body  38 . This enables the threads at the exterior surface of the guide body  38  to mate with those at the interior of the expandable sleeve  26  through rotation of the guide body  38  (in response to rotation of the shaft  30  of the key  28 ). 
     An elastic band  62  surrounds and exerts an inwardly-directed retention force upon the segments  56  of the expandable sleeve  26 . The band  62  is preferably of rubber or like elastic material. It may comprise, for example, commercially-available bicycle tubing manufactured by Trek Bicycle Corporation of Waterloo, Wis. and marketed under the trademark “BONTRAGER XXX LITE”. 
     The elastic band  62  is secured to the outer surfaces to the segments by means of an adhesive  63 . An appropriate commercially-available adhesive for securing the elastic band  62  to the segments  56  of the expandable sleeve  26  is the gel containing cyanoacrylate manufactured by Henkel Loctite Corporation of Rocky Hill, Conn. and marketed under the trademark “QUICKTITE SUPER GLUE”. 
     As mentioned above, rotation of the guide body  38 , which is fixed to the collar  36 , occurs in response to rotation of the key  28 .  FIG. 3  is a bottom plan view of the collar  36  with the elongated shaft  30  inserted. As can be seen, the bottom of the collar  36 , which is fixed to the body  38  of the guide  24  by means of the screws  40 , includes an annular channel  64  that is defined and bounded by a peripheral ridge  66 . The annular channel  64  is arranged to receive the tabs  34  of the elongated shaft  30  of the key  28  after insertion through the recesses  52  in the upper portion of the collar  36 . 
     Rotation of the guide  24  in response to rotation of the key  28  occurs by the coordinated rotational forces F exerted by the tabs  34  upon contact with the screws  40  that extend through the annular channel  64 . 
       FIGS. 4A and 4B  are a pair of views for illustrating the expansion of the sleeve  26  in response to rotation of the key  28 . Such rotation is occasioned by twisting of the handle  32 .  FIG. 4B  illustrates the tool after a quarter-turn of the handle  32  of the key  28 . As can be seen, upon rotation of the key  28 , the exteriorly-threaded body  38  of the guide, whose threaded exterior surface  60  is mated with the interiorly-threaded expandable sleeve  26 , travels downwardly with respect to the sleeve  26 . Due to the reciprocal tapered shapes of the outer surface of the body  38  and the interior surfaces of the sleeve segments  56 , the intrusion of the body  38  into the interior of the sleeve  26  causes separation, by an amount d, to occur between longitudinal edges  68 ,  68  of adjacent sleeve segments  56 . Such separation causes an expansion of the circumference of the expandable sleeve  26  by a total amount of approximately 3 d. 
       FIGS. 5A through 5D  are a series of partial section side elevation views for illustrating the operation of the tool of the invention. The tool is prepared for centering an elongated drill bit within a femoral canal  70  after removal of a prosthesis. This is begun by inserting the key  28  into the collar  36  that is fixed atop the tapered body  38  of the guide  24 . As illustrated in  FIG. 5A , the assembled device, with the tapered body  38  seated at the top of the expandable sleeve  26 , is ready to be inserted into the femoral canal  70 . It is gently lowered into the canal  70  until downward movement is terminated by contact with the plug  20 . 
     The sleeve members  56 , held together at this time by the elastic member  64 , are in substantial contact with each other along mating longitudinal edges  68 . As a result, the expandable sleeve  26  is of relatively minimal diameter, providing clearance with the inner wall  72  of the bone in the region of the cement plug  20 . 
       FIG. 5B  illustrates the configuration of the tool  10  upon rotation of the handle  32  of the key  28 . The serrated bottom edge  57  of the sleeve  26  create&#39;s a turning resistance force, occasioned by the application of downwardly-acting pressure upon the handle  32  and the weight of the tool, between the sleeve  26  and the cement plug  20 . Such force prevents the rotation of the sleeve  26  with the guide  24  that would otherwise occur when the key  28  is rotated. 
     By allowing rotation of the guide  24  to occur relative to the sleeve  26 , expansion of the diameter of the diameter and circumference of the sleeve  26  will occur within the canal  70  in accordance with the process described in conjunction with  FIGS. 4A and 4B  above. The surgeon continues to twist the handle  32 , gradually expanding the diameter of the sleeve  26  until resistance to further twisting is felt. This indicates that the periphery of the sleeve  26  has expanded to contact the inner wall  72  of the bone. 
     Once sufficient contact with the inner wall  72  of the bone is sensed, twisting of the handle  32  is terminated. As illustrated in  FIG. 5C , the key is then separated from the guide  24  and removed from the femoral canal. This is accomplished by reversing rotation of the key  28  while exerting a light upward force upon the handle  32 . This is done until the surgeon senses that the tabs  34  of the shaft  30 , rotated within the annular channel  64  are now aligned with the indentations  52  in the collar  36 . When this happens, the key is lifted from the other parts of the tool assembly as illustrated in  FIG. 5C . 
     Removal of the key  28  clears a centered, vertical channel through the guide  24  and the sleeve  26  comprising the circular void center  46  of the collar  36  and the central aperture  48  of the body  38 . Such channel permits the insertion of an elongated drill bit  76  into the combination of guide  24  and sleeve  26  as shown in  FIG. 5D . The surgeon may now confidently drill through the center of the cement plug  20  (and the optional restrictor  22 ) without concern about contacting the inner wall  72 . 
     Once a hole has been drilled through the plug  20  (and restrictor  22 ), the elongated drill bit  76  is removed and the key  28  re-inserted into the collar  36  so that the shaft  30 , and attached tabs  34 , are rotated within the annular cavity, as illustrated in  FIG. 3 , to cause reversal of the downward travel of the guide  24  with respect to the sleeve  26 . A reduction in the diameter of the expandable sleeve  26  occurs. Once the diameter of the sleeve  26  has been sufficiently reduced from prior contact with the inner wall  72  of the bone, upward pressure on the handle  32  will remove the tool  10  from the canal  70 . This permits insertion of an elongated hook through the hole formed by the elongated drill bit  76  followed by upward pressure on the hook to lift the cement plug  20  and the optional restrictor  22  from the canal  70 . 
       FIG. 6  is a perspective view of a sleeve for a tool in accordance with an alternative embodiment of the invention. Unlike the expandable sleeve  26  comprising three sleeve segments  56  as described above, the expandable sleeve  78  consists of a single piece that has been machined to comprise adjoining longitudinal segments  80  adjoined by alternating top and bottom edge segments  82  and  84  respectively. As in the case of the segmented expandable sleeve  26 , the sleeve  78  has an interiorly threaded surface  86  that is tapered for compatibility with the exteriorly-body  38  of the guide  24 . 
     While the sleeve  78  differs from that of the prior figures due, in part, to its integral nature, the presence of a threaded and tapered interior surface  86  coupled with the resilient character of the metal or like material for forming the sleeve  78 , assures its operation substantially in accordance with that of the expandable sleeve  26  discussed above. 
     While the invention has been described with reference to its presently preferred embodiment, it is not limited thereto. Rather, this invention is limited only insofar as it is defined by the following set of patent claims and includes within its scope all equivalents thereof.