TECHNICAL FIELD OF THE INVENTION 
     The present invention relates generally to the field of orthopaedics, and more particularly, to an implant for use in arthroplasty. 
     BACKGROUND OF THE INVENTION 
     Patients who suffer from the pain and immobility caused by osteoarthritis and rheumatoid arthritis have an option of joint replacement surgery. Joint replacement surgery is quite common and enables many individuals to function properly when it would not be otherwise possible to do so. Artificial joints are usually comprised of metal, ceramic and/or plastic components that are fixed to existing bone. 
     Such joint replacement surgery is otherwise known as joint arthroplasty. Joint arthroplasty is a well-known surgical procedure by which a diseased and/or damaged joint is replaced with a prosthetic joint. In a typical total joint arthroplasty, the ends or distal portions of the bones adjacent to the joint are resected or a portion of the distal part of the bone is removed and the artificial joint is secured thereto. 
     There are known to exist many designs and methods for manufacturing implantable articles, such as bone prostheses. Such bone prostheses include components of artificial joints such as elbows, hips, knees and shoulders. 
     During performance of a joint replacement procedure, it is generally necessary 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. Such variations may be due to, for example, the patient&#39;s age, size and gender. 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 femoral canal. 
     Such a need for prostheses of varying shapes and sizes thus creates a number of problems in regard to the use of a one-piece prosthesis. For example, a hospital or surgery center must maintain a relatively large inventory of prostheses in order to have the requisite mix of prostheses needed for certain situations, such as trauma situations and revision surgery. Moreover, since the bow of the stem must conform to the bow of the intramedullary canal of the patient&#39;s femur rotational positioning of the upper portion of the prosthesis is limited thereby rendering precise location of the upper portion and hence the head of the prosthesis very difficult. In addition, since corresponding bones of the left and right side of a patient&#39;s anatomy (e.g. left and right femur) may bow in opposite directions, it is necessary to provide (left) and (right) variations of the prosthesis in order to provide anteversion of the bone stem, thereby further increasing the inventory of prostheses which must be maintained. 
     As a result of these and other drawbacks, a number of modular prostheses have been designed. As its name implies, a modular prosthesis is constructed in modular form so that the individual elements or figures of the prosthesis can be selected to fit the needs of a given patient&#39;s anatomy. For example, modular prostheses have been designed which include a proximal neck component which can be assembled to any one of numerous distal stem components in order to create an assembly which fits the needs of a given patient&#39;s anatomy. Such a design allows the distal stem component to be selected and thereafter implanted in the patient&#39;s bone in a position which conforms to the patient&#39;s anatomy while also allowing for a limited degree of independent positioning of the proximal neck component relative to the patient&#39;s pelvis. 
     One issue that arises as a result of the use of a modular prosthesis is the locking of the components relative to one another. In particular, firm reproducible locking of the proximal neck component to the distal stem component is critical to prevent separation of the two components subsequent to implantation thereof into the patient. The need for the firm locking is particularly necessary if the design does not provide for positive locking with weight bearing. As such, a number of locking mechanisms have heretofore been designed to lock the components of a modular prosthesis to one another. For example, a number of modular prostheses have heretofore been designed to include a distal stem component, which has an upwardly extending post, which is received into a bore defined distal neck component. A relatively long fastener such as a screw or bolt is utilized to secure the post with the bore. Other methods of securing modular components include the impacting of one component onto the other. This method has highly variable results. 
     Current designs of modular stems include designs in which the modular connection utilizes a tapered fit between the two components. For example, the proximal body may include an internal taper, which mates with an external taper on the distal stem. Such a taper connection may be used in conjunction with additional securing means, for example, a threaded connection or may be used alone. It is important that the tapered connection be secure. For example, the proper amount of force must be applied to the tapered connection to properly secure the tapered connection so that the connection can withstand the forces associated with the operation of the stem. 
     In certain instances, it may be desired to disassociate the two components. For example, after the surgery, if there are problems with the implant, the surgeon may need to do a revision that would require removing the original proximal body from the stem. Other times, the surgeon may discover after assembling the proximal body and the stem that a different sized stem or body would be more appropriate. In such instances, the surgeon would have to disassemble the stem from the body. In these cases, removing the proximal body from the stem can be very difficult. Because the tapered connection is so secure, it requires a great amount of force to disassociate the components. In some designs, the surgeon may have to apply 2000 pounds of axial force in order to separate the components. 
     There are currently some disengagement tools in use for disassociating the proximal body from the distal stem. However, these devices have proven to be unsuccessful. Some of the current devices may even break the components. 
     Therefore, there is a need for a tool that allows a surgeon to easily and safely disengage the taper lock between the proximal body and the distal stem. 
     SUMMARY OF THE INVENTION 
     According to one embodiment of the present invention, a disassembly tool for disassembly of a first component of a prosthesis from a second component of the prosthesis for use in joint arthroplasty is provided. The tool includes a body having an internal bore. At least a portion of the internal bore is threaded for engagement with a thread on the first component of the prosthesis. The tool also includes an expandable ring surrounding a portion of the body and adapted to engage the first component, such that when a radial force is applied to the body, the ring expands, applying an axial force against the first component. 
     According to another embodiment of the present invention, a method of disassembling a first and second component of a prosthesis used in joint arthroplasty is provided. The method includes providing a first component having a male taper and a second component having an internal bore with a female taper. The first component and second component are engaged via the male and female tapers. A disassembly tool is inserted into the inner bore of the second component. The disassembly tool has a body and an expandable portion. The body is coupled to the first component. Through a frictional force the expandable portion and an inner diameter of the second component become engaged. The threaded internal bore of the disassembly tool is partially dethreaded from the first component. The expandable portion and the inner diameter of the second component expand, disengaging the first component from the second component. 
     According to yet another embodiment of the present invention, a kit is provided. The kit includes a first component having a male taper and a second component having an internal bore with a female taper. The first component and second component are engageable via the male and female tapers. The kit also includes a disassembly tool having a body and an expandable portion separable from the body, such that the expandable portion has an initial outer diameter equal to the inner diameter of the internal bore of the second component. 
     Other technical advantages of the present invention will be readily apparent to one skilled in the art from the following figures, descriptions and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in connection with the accompanying drawings, in which: 
         FIG. 1  is a perspective view of a disengagement tool according to one embodiment of the present invention; 
         FIG. 2  is a top view of the disengagement tool of  FIG. 1 ; 
         FIG. 3  is a plan view of a two-pieced modular hip stem that may be disassembled with the disassembly tool of  FIG. 1 ; 
         FIG. 4  is a plan view of the modular hip stem of  FIG. 3 ; 
         FIG. 5  is an expanded view of the modular hip system of  FIG. 3 ; 
         FIG. 6  is a cross-sectional view of a disengagement tool according to one embodiment of the present invention; and 
         FIG. 7  is a cross-sectional view of a disengagement tool inserted into an implant according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention and the advantages thereof are best understood by referring to the following descriptions and drawings, wherein like numerals are used for like and corresponding parts of the drawings. 
     Referring now to  FIG. 1 , a disassembly tool  10  according to one embodiment of the present invention is illustrated. The disassembly tool  10  includes a body  11  having a top portion  12 , a tapered portion  14 , and a bottom portion  15 . The tool  10  also includes a ring portion  16  ( FIG. 3 ). As illustrated in the top view of  FIG. 2 , the top portion  12  is hex-shaped. The hex-shape of the top portion  12  allows the top portion to be grasped by common tools. 
     Turning back to  FIGS. 1 and 3 , as shown, the ring  16  is generally cylindrical and includes a plurality of perforations  18 . The perforations  18  allow the ring  16  to break into pieces when a force is applied against the ring  16 . The ability to break into pieces allows the ring  16  to be inserted into a space that has a greater diameter than the outer diameter of the ring  16 . However, if a force is applied that causes the ring  16  to break into pieces, the effective outer diameter of the ring  16  enlarges. As illustrated in  FIG. 3 , the ring  16  includes an inner bore  19 . The bore  19  is tapered in the embodiment illustrated in  FIG. 1 , but may be non-tapered in other embodiments. 
     The body  11  also includes an internal bore  20 . The internal bore  20  in the embodiment shown in  FIG. 1  extends through part of the bottom portion  15 . In other embodiments, the bore  20  may extend through the entire body  11 . In the embodiment illustrated in  FIG. 1 , the internal bore  20  includes a threaded portion  22  that extends along the length of the bore  20 . In some embodiments, the threaded portion includes clockwise threads—meaning that the threads are designed to engage other threads when rotated in a clockwise fashion and to disengage other threads when rotated in a counter-clockwise fashion. 
     Turning now to  FIG. 4 , a prosthesis  30  is shown in greater detail. The prosthesis  30  as shown in  FIG. 4  includes a proximal body  32  and a distal stem  34 , which have an interference connection that is, for example, an interference connection of a cylindrical bore to a cylindrical stem, as well as, a splined non-uniform cross-section stem to a splined or non-uniform cross-section opening. It should further be appreciated that the proximal body and distal stem of the prosthesis  30  for use with the assembly tool of the present invention may include a taper connection  36  in which the distal stem  34  has an internal taper  38  and the proximal body  32  has an external taper  40 . The taper connection  36  consists of an external taper  40  formed on the distal stem  34  that engages with internal taper  38  formed on the proximal body  32 . 
     The prosthesis  30  as shown may include external threads  42  formed on the distal stem  34 . The proximal body  32  may include a neck  44  to which a head  46  may matingly be fitted. As an additional precaution in assuring that the proximal body  32  remains secured to the distal stem  34 , the prosthesis  30  may further include a nut  48  which threadably engages the external threads  42  of the distal stem  34 . 
     Referring now to  FIG. 5 , the prosthesis  30  is shown with the proximal body  32  disassembled from the distal stem  34 . The external taper  40  of the distal stem  34  is defined by an included angle β 1 . In order that the proximal body  32  fits securely to the distal stem  34 , the proximal body  32  includes the internal taper  38  defined by included angle β 2 . The angles β 1  and β 2  may be generally the same. Alternatively the taper angle may be divergent. The angles β 1  and β 2  should be chosen, such that the fit of the proximal body  32  to the distal stem  34  is secure. 
     As discussed previously in the background section, in some instances, the internal and external tapers  38 ,  40  lock the proximal body  32  to the distal stem  34 . This can be problematic should the surgeon need to disengage the proximal body  32  from the distal stem  34 . 
     Turning now to  FIG. 6 , the tool  10  is shown inserted into the proximal body  32  and distal stem  34 . As shown, the threads  22  of the tool  10  engage the threads  42  of the distal stem  34 . The ring  16  is shown in place between the proximal body  32  and the body  11  of the tool  10 . The ring  16  may include serrated edges  50  to allow the ring  16  to grasp the inner diameter of the proximal body  32 . 
     As shown in  FIG. 7 , when the user turns the body  11  counterclockwise, the threads  22  disengage from the threads  42  of the distal stem and the body  11  moves upward relative to the proximal body  32  and distal stem  34 . Because of the serrated edges  50  of the ring  16  grasp the inner diameter of the proximal body  32 , the ring  16  does not advance with the body  11 . Instead, as the body  11  exerts a radial force against the ring  16 , the perforations  18  ( FIG. 3 ) break, creating three ring pieces that push against the inner diameter of the proximal body  32 . As the body  11  turns, the radial force applied to the body translates into an axial force against the inner diameter of the proximal body  32 , thereby breaking the taper lock, and allowing the proximal body  32  to be removed from the stem  34 . 
     Therefore, breaking the lock does not require pulling the taper off-center or using a great amount of force. Merely by utilizing a common wrench, the taper lock can be broken. 
     In other embodiments, the ring  16  may be broken apart by the insertion of a sleeve (not shown). The sleeve would have a diameter greater than the ring  16  and would force the ring  16  to break into pieces. In yet other embodiments, the ring  16  could include a groove with an elastomer band. As the body  11  is pulled away from the proximal body  32 , the elastomer band would keep the ring  16  in place. The user could then use a hollow tube or an impact to disengage the ring  16  from the body  11 . Alternatively, some sort of adhesive could be used to keep the ring  16  in place during insertion into the proximal body  32 . 
     In the illustrated embodiment, the bore  19  is tapered at an angle that matches the tapered portion  14  of the body  11 . However, in other embodiments, the bore  19  may not be tapered or may be tapered at a different angle. 
     In the illustrated embodiment the outside of the ring  16  is angled the same as the inside of the proximal body  32 . However, in other embodiments, the outside of the ring  16  may have an angle that is different than that of the proximal body  32 . 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made therein without departing from the spirit and scope of the present invention as defined by the appended claims.
A disassembly tool for disassembly of a first component of a prosthesis from a second component of the prosthesis for use in joint arthroplasty. The tool includes a body having an internal bore. At least a portion of the internal bore is threaded for engagement with a thread on the first component of the prosthesis. The tool further includes an expandable ring surrounding a portion of the body and adapted to engage the first component, such that when a radial force is applied to the body, the ring expands, applying an axial force against the first component.