Abstract:
A tool for separating components of a modular prosthesis includes an upper and lower body, each defining opposing bills at one end that are sized to fit in juxtaposed relation within an initial gap between the components. The upper body is provided with a handle so that the tool may be manually held by the surgeon with the opposing bills in position in the prosthesis. The tool includes a jack assembly disposed between the upper and lower bodies of the tool that is configured to be driven into an extended position, gradually separating the upper and lower bodies, and consequently exerting a separation force on the prosthesis components through the bills of the tool. The jack assembly is driven by a threaded actuator rod that bears against one element of the jack assembly as the actuator rod is rotated within a threaded bore in one of the bodies of the tool.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates generally to modular orthopaedic prostheses, and more particularly to a tool for separating components of a modular prosthesis in situ. 
     BACKGROUND OF THE INVENTION 
     During the lifetime of a patient, it may be necessary to perform a joint replacement procedure to address damage to the joint due to, for example, disease or trauma. The joint replacement procedure may involve the use of a prosthesis which is implanted into at least one of the patient&#39;s bones. In the case of a hip replacement procedure, a femoral prosthesis is implanted into the patient&#39;s thigh bone or femur. The femoral prosthesis had been typically constructed as a one-piece structure having an upper portion that includes a spherically-shaped head which bears against the patient&#39;s pelvis or acetabulum, along with an elongated intramedullary stem which is utilized to secure the femoral component to the femur. In order to secure this prosthesis to the patient&#39;s femur, the medullary canal of the patient&#39;s femur is first surgically prepared (e.g. reamed and/or broached) such that the intramedullary stem of the femoral prosthesis may be subsequently implanted therein. The femoral prosthesis may be press fit into the medullary canal or, in the alternative, bone cement may be utilized to secure the femoral prosthesis within the medullary canal. 
     During performance of a joint replacement procedure, it is generally desirable to provide the surgeon with a certain degree of flexibility in the selection of a prosthesis. In particular, the anatomy of the bone into which the prosthesis is to be implanted may vary somewhat from patient to patient. For example, in the case of a femoral prosthesis, the patient&#39;s femur may be relatively long or relatively short thereby requiring use of a femoral prosthesis which includes a stem that is relatively long or short, respectively. Moreover, in certain cases, such as when use of a relatively long stem length is required, the stem must also be bowed in order to conform to the anatomy of the patient&#39;s femur. The trochanteric region of the femur may require a specially sized proximal portion of the prosthesis or a particular angle of the proximal portion to mate with the articulations of the hip joint. 
     Such a need for prostheses of varying shapes and sizes created a number of problems in regard to use of a one-piece prosthesis. As a result of these problems, the modular prosthesis was developed. As the name implies, a modular prosthesis is constructed in modular form so that the individual elements or features of the prosthesis can be selected to fit the needs of a given patient&#39;s anatomy. Examples of certain modular prostheses are found in U.S. Pat. No. 6,706,072 (the &#39;072 Patent), issued to DePuy Orthopaedics, Inc., including the two prosthesis illustrated in  FIGS. 1-2 . In general, the modular femoral prosthesis, such as the prosthesis  10  of  FIG. 1 , includes a proximal neck component  12  that includes a body  24  that terminates in a trunnion  26  onto which a variety of head bearing components (not shown) that mate with the patient&#39;s natural acetabulum or to a prosthetic socket. 
     The neck component  12  fits within a sleeve  14  that is configured to fit within a surgically prepared end of the femur. In a joint replacement procedure, the head of the natural femur is removed to form a flat surface across the trochanters of the femur. The medullary canal of the femur is reamed and/or broached and a cavity is formed in the proximal prepared end of the bone to receive the sleeve  14 . The sleeve component  14  may be provided in carrying angles and lengths to fit the needs of the patient&#39;s joint anatomy. A distal stem component  18  is provided that is implanted within the prepared medullary canal. As indicated above, the stem component  18  may be provided in various sizes and curvatures to accommodate the anatomy of the particular femur. 
     All of the components of the modular prosthesis are configured to be rigidly engaged when the prosthesis  10  is implanted. Thus, the prosthesis  10  described in the &#39;072 Patent includes a tapered shank  13  projecting from the body  24  of the proximal neck component  12 . The shank  13  extends into a tapered bore  30  of the sleeve  14  in a press-fit engagement. The two tapered elements (shank  13  and bore  30 ) may define a Morse taper for rigid fixation between the sleeve and neck component. The shank  13  also defines a tapered bore  22  for a press-fit engagement with a tapered post  19  of the distal stem component  18 . The end of the post  19  may be provided with threads  28  to be threaded into mating threads  29  in the shank  13  of the proximal neck component  12 . 
     A modified prosthesis  10 ′ is shown in  FIG. 2 . This modified prosthesis includes a sleeve  14  that is substantially the same as the sleeve shown in  FIG. 1 . The proximal neck component  12 ′ is similar to the neck component  12  in that includes a tapered shank  13 ′ for press-fit engagement within the mating bore of the sleeve  14 . However, the bore  22 ′ of the neck component  12 ′ extends entirely through the component. The distal stem component  18 ′ of  FIG. 2  is similar to the stem component  18  of  FIG. 1 , except that the tapered post  19 ′ is longer and terminates in a threaded end  28 ′ that is configured to be engaged by a threaded nut or cap  29 ′. 
     With either prosthesis  10  or  10 ′, the head of the femur is prepared as described above and the sleeve  14  pressed into the prepared proximal end. Bone cement may be used to help fix the sleeve  14  within the bone. The distal stem component  18 / 18 ′ is introduced through the sleeve bore  30  into the prepared medullary canal of the femur. The proximal neck component  12 / 12 ′ is then positioned with the sleeve bore  30  and advanced into the bore so that the tapered bore  22 / 22 ′ fits over the tapered post  19 / 19 ′ of the distal stem component. The threaded end  28 / 28 ′ is threaded into the appropriate mating component for the two prostheses  10 / 10 ′. One concern that has arisen with modular prostheses is the locking of the components relative to one another. The fixation of the distal stem component  18 / 18 ′ to the proximal neck component  12 / 12 ′ and of the neck component to the sleeve  14  in the prostheses  10 / 12 ′ of  FIGS. 1-2  address this concern. This fixation among the components allows the components to adequately absorb the axial functional loads exerted on the prosthesis without appreciable degradation or even breach of the mechanical integrity prosthesis. Ideally, the functional loads on these prostheses  10 / 10 ′ increase or improve the self-locking attributes of the devices. 
     In some joint replacements a revision of the prosthesis may be necessary. For instance, in some cases, a distal stem component is implanted that has a version that is determined to be less than optimal for the anatomy of the patient&#39;s femur. In other cases, the proximal neck component or distal stem requires replacement due to wear or damage, or because of changes in the patient&#39;s joint anatomy that can occur over time. At any rate, the otherwise beneficial self-locking characteristics of the modular prostheses  10 / 10 ′ become a disadvantage in a revision procedure where it is necessary to separate the neck and stem components from the stable well-fixed sleeve. 
     The current method of removing the neck and stem components from the sleeve is to use a bone chisel to wedge between the proximal surface  16  of the sleeve  14  and the opposing surface  25  of the neck component  12 / 12 ′ ( FIG. 1 ). A bone chisel is initially pushed into the space between the two surfaces  16 ,  25  to incrementally disassociate the components from the sleeve. When the gap is large enough, a second bone chisel is introduced into the space, and so on with multiple chisels until the gap is large enough to accept an impaction tool. The distal surface  25  of the neck component  12 / 12 ′ is impacted until it is dislodged from the sleeve. 
     This technique suffers from a few problems. Foremost is the inherent difficulties associated with using a bone chisel to wedge fixed components apart. The chisel may slip out of place and damage surrounding tissue or cut through the surgeon&#39;s gloves. In addition, the use of an impaction tool to separate the components from the sleeve tends to produce an axial load on the sleeve that can tend to disassociate the sleeve from the proximal end of the femur. 
     Consequently, there is a need for a tool that can be used to remove the neck and stem components of a modular prosthesis from the sleeve, while keeping the sleeve intact within the prepared bone. 
     SUMMARY OF THE INVENTION 
     In order to address this need, a tool is provided that permits separation of fixed components of a modular prosthesis without the need for use of a bone chisel and an impaction tool. The tool of one embodiment of the invention allows for the application of uniform pressure against both the sleeve and the component to be removed to gradually increase the gap between the modular components. The tool may be used in virtually any orientation without requiring undue manipulation of the patient&#39;s joint. 
     In one feature of the invention, the separation tool includes an upper and lower body, each defining opposing bills at one end that are sized to fit in juxtaposed relation within an initial gap between components of a modular prosthesis. In the case of a modular hip prosthesis, the juxtaposed bills fit in the gap between the sleeve and the proximal neck component. The upper body is provided with a handle extending outwardly from the body so that the tool may be manually held by the surgeon with the opposing bills in position in the prosthesis. 
     In a further feature of the invention, a jack assembly is disposed between the upper and lower bodies of the tool. Preferably the jack assembly is configured to be contained within cavities formed in the upper and lower bodies when the jack is in its retracted or un-extended orientation. The jack assembly is configured to be driven into an extended position, gradually separating the upper and lower bodies, and consequently exerting a separation force on the prosthesis components through the bills of the tool. In one preferred aspect, the jack assembly is driven by a threaded actuator rod that bears against one element of the jack assembly as the actuator rod is rotated within a threaded bore in one of the bodies of the tool. 
     In the preferred embodiment, the jack assembly is a scissors jack with crossed arms pivotably mounted at one end to a corresponding body and pivotably connected to each other at a mid-length position on each arm. The opposite ends of the arms are movably disposed within an opposite body (i.e., one arm is pivotably mounted to the upper body and slidably disposed in the lower body). The opposite end of at least one of the arms includes a pin that travels within a slot formed in one body to restrain the end of the arm to linear motion. Preferably, the non-pivotably mounted ends of both arms are similarly restrained to linear movement. The moving end of one arm may include a bushing to bear against the end of the threaded rod. Thus, in the operation of the tool, the threaded rod is threaded into the tool body to push against the bushing of the end of one arm. As that arm is pushed toward the opposite end of the body, the linkage formed by the pivot mounts and pivot connection between the arms causes the opposite arm to travel in the same manner. 
     Thus, the translating ends of the arms of the scissors jack move simultaneously toward the pivoting ends of the arms, thereby shortening the axial extent of the scissors jack while increasing the transverse or vertical height of the jack. It is this increase in vertical height that forces the upper and lower bodies, and ultimately the upper and lower bills, apart. Preferably, the proximal end of the threaded actuator rod includes a fitting for engagement with a driving tool. The driving tool may be manually operated, such as a crank, socket wrench or T-handle, but may instead be electrically or pneumatically powered. 
     It is one object of the invention to provide a tool that is operable to separate components of a modular prosthesis in situ. A further object of the invention is achieved by features that allow this separation to occur without compromising the integrity of components that are intended to remain within the bone or joint. 
     One benefit of the separation tool of the present invention is that it provides for controlled application of force between the components being separated. Another benefit is that the tool itself has a minimal profile so that it is not cumbersome to use and so that it does not interfere with the components of the prosthesis or the surrounding tissue. Other benefits and objects of the invention will become apparent upon consideration of the following written description taken together with the accompanying figures. 
    
    
     
       DESCRIPTION OF THE FIGURES. 
         FIG. 1  is a side partial cut-away view of a modular hip prosthesis that may be disassembled by the separation tool of the present invention. 
         FIG. 2  is an exploded perspective view of an alternative modular hip prosthesis that may be disassembled by the separation tool of the present invention. 
         FIG. 3  is a top elevational view of a separation tool according to one embodiment of the present invention. 
         FIG. 4  is a side elevational view of the separation tool shown in  FIG. 3  with the tool depicted in its expanded configuration. 
         FIG. 5  is a cross-sectional view of the tool shown in  FIG. 3 , taken along line A-A as viewed in the direction of the arrows. 
         FIG. 6  is a cross-sectional view of the tool shown in  FIG. 3 , taken along line B-B as viewed in the direction of the arrows. 
         FIG. 7  is a cross-sectional view of the tool shown in  FIG. 3 , taken along line C-C as viewed in the direction of the arrows. 
         FIG. 8  is an enlarged partial cross-sectional view of the separation tool depicted in  FIG. 3  being used to separate components of the modular hip prosthesis illustrated in  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the invention is thereby intended. It is further understood that the present invention includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the invention as would normally occur to one skilled in the art to which this invention pertains. 
     In accordance with one embodiment of the invention, a separation tool  50  is provided that includes a pair of separator bodies  56 ,  58  connected by and separable by a jack assembly  52 , as shown in  FIGS. 3-6 . The jack assembly  52  is operable to move the upper body  56  and the lower body  58  between an expended position depicted in  FIG. 4  and a contracted, or insertion, position shown in  FIG. 8 . The upper body  56  is provided with a handle  54  extending generally perpendicularly outward from the body. The handle is sized to be manually grasped by the surgeon to facilitate insertion of the tool into the joint space and to stabilize the tool while it is being used to separate the implant components. In a specific embodiment, the handle  54  includes a knurled outer surface, and has a length of about 90 mm and a diameter of about 14 mm. 
     One end of each separator body  56 ,  58  includes a corresponding bill  57 ,  59  extending along the longitudinal axis of the body. The bills  57 ,  59  are sized so that when the bills are juxtaposed, as depicted in  FIG. 8 , they can fit in the gap G between the proximal surface  16  of the sleeve  14  and the opposite surface  25  of the neck component  12 . The length of the bills  57 ,  59  from the end of the tool bodies is also sufficient to access the gap G in the joint prosthesis while the separator bodies are not impeded by the prosthesis or surrounding joint tissue. In a specific embodiment, the bills  57 ,  59  have a length from the end of each body  56 ,  58  of about 25 mm. In this specific embodiment, the thickness of each bill is about 2.54 mm so that their combined juxtaposed thickness is about 5.08 mm, which is sufficiently thin to fit within the gap G of most implanted prostheses. The separator bodies  56 ,  58  are further sized so that the handle  54  is accessible to be grasped outside the surgical site. In this illustrated embodiment, the bodies have a length of about 162 mm and may be provided with an angled surface leading to each bill to increase clearance around the prosthesis. 
     The operating mechanism of the separator tool  50  is a jack assembly  52  that operates to separate the bodies  56 ,  58 . In the preferred embodiment, the jack assembly includes a pair of arms  60  that are arranged in a crossed configuration, as shown in  FIGS. 4 and 6 . One end of each arm  60  is pivotably connected to a corresponding separator body at an end pivot mount.  62 . The two arms are connected to each other at a center pivot  64  situated at mid-length of each arm. In the illustrated embodiment, the center pivot  64  is at the center of each arm  60  for the optimum stroke and mechanical advantage. While one end of each arm is restrained to pivot relative to the corresponding separator body, the opposite end is constrained to translate within the opposite separator body. Thus, the arm pivotably connected to the upper body  56  is connected to the lower body  58  by way of a pin  66  extending through an elongated slot  68  formed in that body, as shown in  FIG. 6 . The pin and slot constrain end of the arm  60  to translate within the lower separator body  58  in the direction T. 
     Similarly, the top separator body  56  defines a slot  69  that receives a pin  70   a  connected to the translating end of the arm that is pivotably connected to the lower body. The pin  70   a  preferably includes a bushing  70  that provides a bearing surface for contacting the actuator rod  75 . In the preferred embodiment, the ends of the crossed arms  60  are contained within cavities  71 ,  72  defined in the two bodies  56 ,  58 , respectively. The actuator rod  75  is also disposed in the cavity  71 . The cavities allow the two bodies to contact flush as depicted in  FIG. 8 . Alternatively, the bills  57 ,  59  can be configured to achieve flush juxtaposition regardless of whether or not the upper and lower bodies  56 ,  58  are flush in their insertion configuration. 
     The actuator rod  75  includes a threaded stem  77  terminating in an end  78  that bears against the bushing  70 . The threaded stem  77  is threaded into and out of a threaded bore  79  defined in the proximal end of the upper separator body  56 . As the actuator rod  75  is threaded into the upper body, the end  78  of the stem  77  pushes on the bushing  70  causing the end of the arm  60  to translate along the slot  69 . This movement results in a rotation P of the arm about the pivot mount  62  in the lower body  58 , which in turn causes rotation of the other arm about the pivot mount in the upper body. The mechanism thus operates like a known scissors jack so that forced translation of one end of the linkage formed by the crossed arms cause the ends of the arms to approach each other. In so doing, the jack assembly increases the separation between the upper and lower bodies  56 ,  58 , and ultimately the separation between the bills  57 ,  59 . As the bills separate they contact and push against the surfaces  16  and  25  to drive the proximal neck component  12  away from the sleeve  14 . 
     The length of the arms  60  are established to separate the prosthesis components far enough to be easily dislodged manually or by an impact tool if necessary. In a specific embodiment for separation of a neck component from a sleeve of a hip prosthesis, the arms have a length of about 115 mm between the pivot mount  62  and the corresponding pin  66 ,  70   a.  If the arms are pivoted to a substantially overlapping orientation, the stability of the tool is compromised. Thus, the amount of pivoting of the arms  60  is preferably limited, or more appropriately the amount of translation of the non-pivoting end is constrained by the length of the corresponding slot  68 ,  69 . In the illustrated embodiment, the slots terminate about halfway into each body so that the overall travel of the arms in the slots is about 75% of the arm length. This results in a maximum expanded separation of about 75% of the arm length, or about 88 mm. The slots  68 ,  69  are further sized so that the arms  60  are fully contained within the cavities  71 ,  72 , which corresponds to a slot length of about 78 mm. 
     It can be appreciated that the jack assembly  52  and the actuator rod  75  provide means for a controlled and smooth separation of the components of the prosthesis  10  or  10 ′. The thread pitch between the threaded stem  77  and the threaded bore  79  can be sized to achieve a determinate amount of translation or separation of the bodies  56 ,  58  with each degree of rotation of the actuator rod  75 . In other words, a finer thread pitch will result in small changes in separation as the rod is rotated, while a larger pitch will produce greater separation movement with rod rotation. 
     The actuator rod  75  includes a stop  81  that is disposed outside the upper body  56  and serves to limit the depth of insertion of the threaded stem  77  into the upper body. The actuator rod further includes a fitting  85  that is configured for engagement by a driving tool that is operable to rotate the rod in the direction R. The fitting may be configured as a Hudson connection to mate with a T-handle that allows manual rotation of the rod. The fitting may also be configured to engage a crank arm, socket wrench or even a powered surgical driver. In the preferred embodiment, the mechanical advantage provided by the jack assembly  52  allows the use of a manually rotated T-handle to separate the press-fit components of a typical prosthesis. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the invention are desired to be protected. 
     For instance, the illustrated embodiment of the separation tool is shown and described for use in separating components of a modular femoral prosthesis for the hip joint. The tool  50  may be used to separate modular components of other orthopaedic implants. The bills  57 ,  59  may be modified to engage appropriate surfaces of the components to be separated. In the illustrated embodiment, the bills  57 ,  59  may be modified to include a cut-out at the end of each bill that is sized to fit around a part of the shank  13  of the proximal neck component  12 . 
     The tool is particular suited for modular implants where the tool can be introduced within a gap between the components and where it is desirable to maintain the position and stability of one of the components. Alternatively, the bills  57 ,  59  of the tool may be disposed within a gap between a prosthesis component and a prepared surface of a bone. 
     While the tool  50  is adapted for use in separating modular prosthesis components in situ, the tool may also be used outside the patient. In particular, the tool may be used to separate components that have been pre-assembled prior to implantation. 
     In the illustrated embodiment, the actuator rod  75  includes a threaded stem  77  that is rotated within a threaded bore  79  of the top separator body  56  to extend the rod into or retract the rod from within the cavity  71 , and more particularly to push on or withdraw from the bushing  70  on the crossed arm  60 . The threaded configuration optimizes the mechanical advantage to permit manual operation of the jack assembly. However, other actuator rod configurations are contemplated that produces linear movement of the rod end  78  to bear against the crossed arms to operate the jack assembly  52 . For instance, the actuator rod may incorporate a rack and pinion drive arrangement, or a ratchet and trigger assembly. As a further alternative, the actuator rod may incorporate a pneumatic cylinder in which the piston extends to bear against the bushing.