Disassembly tool

0001The present invention relates generally to the field of orthopaedics, and more particularly, to a disassembly tool for disassembly of components of a prosthesis.

0002Patients 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.

0003Such 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.

0004Many designs and methods for manufacturing implantable articles, such as bone prostheses, are known. Such bone prostheses include components of artificial joints such as elbows, hips, knees and shoulders.

0005During 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's age, size and gender. For example, in the case of a femoral prosthesis, the patient'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's femoral canal.

0006Such 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 centre 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'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'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.

0007As 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'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's anatomy. Such a design allows the distal stem component to be selected and thereafter implanted in the patient's bone in a position that conforms to the patient's anatomy while also allowing for a limited degree of independent positioning of the proximal neck component relative to the patient's pelvis.

0008One 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.

0009Current 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.

0010Because the modular pieces need to be securely joined, it is sometimes difficult to separate them if the need arises. For example, in some cases, during the hip replacement surgery, the surgeon may determine (after assembling the modular pieces) that a different proximal body needs to be used. In such a case, the original proximal body needs to be removed. Alternatively, the need may arise to remove the proximal body if the version (or angle) needs to be corrected. In such a case, the surgeon will need to remove the body and adjust the angle of the proximal body relative to the distal stem.

0011A disassembly tool may also be needed if, after implantation, the implant needs to be removed. One method of removing the implant would be to attach the entire construct to a slap hammer and remove both pieces at once. However, if there has been substantial bone in-growth into the stem, this can be very difficult. Another method would be to disassemble the proximal body from the stem, and then use a trephine to cut out the distal stem. This option preserves more bone and is the most feasible option. Thus, a tool needs to be developed that can disassemble the proximal body from the distal stem while the stem is implanted and fixed in the femur.

0012<patcit id="pcit0001" dnum="EP1000595A"><text>EP-A-1000595</text></patcit> disclsoes an extractor for removing a bioceramic insert from the acetabular cup of a hip prosthesis. The insert has a through hole formed in it. The extractor includes a tubular shaft whose end has three slits formed in it to define three claws. The end of the shaft can be screwed into the threaded hole in the insert. A bush having a threaded bore and a flared outer surface can slide in tne end of the shaft. A stem with a threaded end can extend through the tubular shaft to engage the bore in the bush. Rotating the stem relative to the bore causes the bush to be retracted into the end of the shaft so that the slit end of the shaft expands to engage the insert. Continued rotation of the stem causes its end to act on the internals surface of the acetabular cup so that a separating force is applied to the insert relative to the cup.

0013The invention provides a disassembly tool for disassembly of a first component of a prosthesis and a second component of the prosthesis for use in joint arthroplasty, as defined in claim 1.

0014The invention also provides a method for disassembling a first component of a prosthesis and a second component of the prosthesis for use in joint arthroplasty, as defined in claim 10.

0015Optionally, the first member includes a radially expanding collet and exerting a radial force includes causing the radially expanding collet to expand into a recess in the first component.

0016Optionally, the first member includes an external collar coupled to the radially expanding collet, and causing the radially expanding collet includes moving the external collar along the longitudinal axis.

0017Optionally, the first member includes an internal sleeve coupled to the external collar and the radially expanding collet and moving the external collar includes moving the internal sleeve along the longitudinal axis.

0018Optionally, the drive mechanism includes a moveable handle coupled to the second member and a stationary handle coupled to the first member and activating the drive mechanism includes rotating the moveable handle while the stationary handle remains stationary.

0019Optionally, the step of rotating the moveable handle includes causing the second member to move distally in the longitudinal direction.

0020The invention also provides a kit for use in hip arthroplasty as defined in claim 5, which includes a first component of a prosthesis and a second component of a prosthesis. The second component is coupled to the first component. The kit further includes a disassembly tool for disassembling the first component from the second component of the prosthesis for use in joint arthroplasty.

0021Embodiments of the invention are described below by way of example with reference to the accompanying drawings, in which: <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">FIG. 1</figref> is a perspective view of a disassembly tool.</li><li><figref idref="f0002">FIG. 2</figref> is a perspective view of the disassembly tool of <figref idref="f0001">FIG. 1</figref> coupled with an implant assembly.</li><li><figref idref="f0003">FIG. 3</figref> is a see-through view of <figref idref="f0002">FIG. 2</figref>.</li><li><figref idref="f0004">FIG. 4</figref> is a cross-sectional view of a first member component of the disassembly tool.</li><li><figref idref="f0004">FIG. 5</figref> is a cross-sectional view of a cap component of the disassembly tool.</li><li><figref idref="f0005">FIG. 6</figref> is a perspective view of a second member component of the disassembly tool.</li><li><figref idref="f0006">FIG. 7</figref> is a perspective view of a kit which includes a disassembly tool.</li><li><figref idref="f0007">FIG. 8</figref> is a cross-sectional view of an expandable collet component of the disassembly tool.</li><li><figref idref="f0008">FIG. 9</figref> is a flow chart illustrating the method of using a disassembly tool.</li></ul>

0022Referring to the drawings, <figref idref="f0001">FIG. 1</figref> shows a disassembly tool 10 which includes a first member 12 and a second member 14. The second member 14 has a second member longitudinal axis 16. A drive mechanism 18 is coupled to the second member 14. As the drive mechanism 18 is activated, it causes the second member 14 to move relative to the first member 12 along the second member longitudinal axis 16. In the illustrated embodiment, the drive mechanism 18 is a handle that is rotated about the second member longitudinal axis 16. However, in other embodiments, the drive mechanism could be a longitudinal handle, a Hudson connection that connects to a power source, or other known drive mechanism that would cause the second member 14 to move relative to the first member 12 along the second member longitudinal axis 16. In the illustrated embodiment, the first member 12 is coupled to a cap 19 that will be described in more detail below. In some embodiments, there may not be a cap 19.

0023Multiple views of the disassembly tool 10 coupled to a modular implant 20 are shown in <figref idref="f0002">FIGS. 2</figref> and <figref idref="f0003">3</figref>. As shown, the modular implant 20 includes a first component 22 (or a proximal or neck component) and a second component 24 (or a distal or stem component). The first member 12 of the disassembly tool 10 includes a radial portion 25 at a distal portion 26. The radial portion 25 radially engages a recess 28 of the neck component 22. In the illustrated embodiment, the first member 12 includes a distal portion 26 that is a radially expanding collet 44 (<figref idref="f0004">FIG. 4</figref>). The radially expanding collet 44 engages the walls defining the recess 28.

0024As shown in <figref idref="f0003">FIG. 3</figref>, a distal portion 30 of the second member 14 engages a proximal portion 32 of the stem component 24. In this embodiment, the distal portion 30 of the second member 14 fits inside a bore of the proximal portion 32 of the stem component 24. In some embodiments, threaded bores and threaded ends may be used.

0025A close-up view of the first member 12 is shown in <figref idref="f0004">FIG. 4</figref>. The first member 12, according to this embodiment, includes a housing 36, a stationary handle 38 coupled to the housing 36, an expansion member 40, an internal sleeve 42 coupled to the expansion member or external collar 40, and an expandable collet 44. The stationary handle 38 may be one piece with the housing 36, or a portion of the housing may connect to the stationary handle 38 via any known connection methods. In one embodiment, a quick-connect connection 39 (<figref idref="f0003">FIG. 3</figref>) is used, but other embodiments may include threads, tapers, or other known connection methods. The quick-connect feature saves time, since it is an easy way to snap the stationary handle together, although other connections as described above may be used.

0026The housing 36 includes an internal bore 41 having female threads 43. The female threads 43 will engage corresponding male threads 45 (<figref idref="f0004">FIG. 5</figref>) of the cap 19. The male threads 45 of the cap 19 slide into the bore 41 and engage the female threads 43 of the bore 41. As shown in <figref idref="f0004">FIG. 4</figref>, the bore 41 extends all the way through the first member 12 and is designed to receive both the cap 19 and the second member 14 as shown in <figref idref="f0003">FIG. 3</figref>.

0027The external collar 40 is coupled to the internal sleeve 42, such that as the external collar 40 is moved along the second member longitudinal axis 16 relative to the housing 36, the internal sleeve 42 also moves along the second member longitudinal axis 16 relative to the housing 36. As the internal sleeve 42 moves distally along the second member longitudinal axis 16, a tapered distal portion 46 of the internal sleeve 42 engages a cylindrical inner wall 48 of the expandable collet 44. As the tapered distal portion 46 moves along the cylindrical wall 48, the increasing diameter of the tapered distal portion 46 causes the collet 44 to expand and grasp the inner wall of the proximal body 22 (as shown in <figref idref="f0004">FIG. 4</figref>). Similarly, as the external collar 40 is moved upwardly (or proximally) along the second member longitudinal axis 16, the internal sleeve 42 also moves upwards along the second member longitudinal 16, disengaging the cylindrical inner wall 48 of the expandable collet 44, allowing the expandable collet to return to its original size.

0028Although the above embodiment describes a collet 44 having a cylindrical inner wall 48 and the sleeve 42 having a taper, other embodiments may be used. For example, both the collet 44 and the distal portion 46 of the sleeve 42 may have a taper, so long as the tapers differ enough in diameters so that the insertion of the sleeve 42 causes the collet 44 to expand radially. Alternatively, the distal portion 46 of the sleeve 42 may not be tapered, it may have a cylindrical shape, so long as the diameter is greater than that of the inner wall of the collet 44.

0029Details of the second member 14 and drive mechanism 18 are shown in <figref idref="f0005">FIG. 6</figref>, in which it can be seen that the second member 14 includes, in this embodiment, a push rod 50 and a torque shaft 52. The drive mechanism 18 is coupled to the torque shaft 52. In this embodiment, the drive mechanism 18 includes a central bore 54a, and two outer bores 54b, 54c. A complementary portion 56 of the torque shaft 52 is configured to engage any of the three bores 54a, 54b, 54c, coupling the torque shaft 52 to the drive mechanism 18. In other embodiments, the bore and complementary portion may be switched. Alternatively, other known coupling mechanisms may also be used.

0030The torque shaft 52 includes male threads 57. The male threads 57 engage female threads 59 on the cap 19 (<figref idref="f0004">FIG. 5</figref>). The threads 57, 59 connect the torque shaft 52 to the cap 19. The torque shaft 52 is inserted into the central bore 54a of the drive mechanism 18. The drive mechanism 18 is then rotated, causing the torque shaft 52 to rotate. This in turn threads the male threads 57 of the torque shaft 52 into the female threads 59 of the cap 19.

0031A proximal portion 58 of the push rod 50 is coupled to the torque shaft 52. In the illustrated embodiment, the proximal portion 58 fits within a recess 60 of the torque shaft 52. The proximal portion 58 of the push rod 50 may have an interference fit within the recess 60 of the torque shaft 52. In other embodiments, the proximal portion 58 of the push rod 50 may be coupled to the recess 60 of the torque shaft 52 via a taper lock. Alternatively, the recess may be located on the push rod 50 and the torque shaft 52 may include a protrusion. In other embodiments, other connection mechanisms may be utilized. As shown in <figref idref="f0003">FIG. 3</figref>, the distal portion 30 of the second member (which coincides with the distal portion of the push rod 50) fits within the recess 28 of the stem 24.

0032As the drive mechanism 18 is rotated about the second member longitudinal axis 16, the female threads 59 of the cap 19 engages the threads 57 on the torque shaft 52 causing the torque shaft 52 to move downwardly (or distally) along the second member longitudinal axis 16. As the torque shaft 52 moves, the engagement between the torque shaft 52 and the push rod 50 also causes the push rod 50 to move distally (or downwardly) along the second member longitudinal axis 16.

0033In order to increase the moment when turning the drive mechanism 18, the user can insert the complementary portion 56 of the torque shaft 52 into one of the outer bores 54b, 54c. The user then can utilize the moment of the length of the drive mechanism 18, making the drive mechanism 18 easier to turn. In the illustrated embodiment, the complementary portion 56 is a hex-shaped protrusion and the bores are hex-shaped recesses, such that the two pieces engage one another when the drive mechanism 18 is turned. In other embodiments, different shaped bores and complementary portions may be used. In some embodiments, threaded bores and threaded complementary portions may be used. By utilizing the drive mechanism 18 to do the initial threading and to do the final threading by increasing the moment arm, OR time is reduced. The user only has to pop the drive mechanism 18 on and off. Multiple tools are not needed.

0034As shown in <figref idref="f0003">FIG. 3</figref>, the distal portion 30 of the push rod 50 will then abut the bottom of the recess 28 of the stem 24. As the push rod 50 is advanced distally, the stem 24 will also be moved distally. Because the collet 44 is engaging the proximal body 22 and holding the proximal body 22 in position, the engagement between the proximal body 22 and the stem 24 will be broken, allowing the proximal body 22 to be removed separately from the stem 24.

0035Generally, the disassembly tool 10 may be made from stainless steel. In some embodiments, the various parts of the disassembly tool may be made of different types of stainless steel. In one embodiment, the torque shaft 52 may be made of a hard stainless steel, such as custom 455 stainless steel. The cap 19 may be made of a nitrogen rich stainless steel that is very soft, such as Nitronic 60 stainless steel. The drive mechanism 18 may be made from a stiffer stainless steel. The collet 44 and the push rod 50 may be made from 465 stainless steel. By making the various parts of different materials having different strengths, the wear on the various threaded parts is reduced. Also, the tool 10 may be able to withstand more force without the parts galling or binding. In other embodiments, some of the pieces may be made of other biocompatible materials such as plastics or other metals. In some embodiments, the various parts may be all made from the same material.

0036<figref idref="f0006">FIG. 7</figref> shows a kit 70 which includes a drive mechanism 18, a plurality of torque shafts 52a, 52b, 52c, a push rod 50, and a first member 12. The plurality of torque shafts 52a, 52b, 52c are of differing lengths. The differing lengths allow the instrument to be used with proximal implants 22 that are of differing lengths. In other words, because the length of the proximal implants 22 varies, the distance between the top of the proximal implant 22 and the stem 24 varies. Although the present kit 70 shows a particular number of torque shafts 52a, 52b, 52c, it should be understood that any number of torque shafts 52a, 52b, 52c may be included in the kit 70. Having multiple torque shafts 52a, 52b, 52c is advantageous because it reduces the time in the operating theatre for the surgeon. The torque shafts 52a, 52b, 52c may be color coded to match a particular proximal body length. The surgeon can then quickly grab the correct torque shaft 52a, 52b, 52c and assemble the tool. If only a single torque shaft 52 were included, it would need to be adjustable to account for the various lengths of different proximal bodies.

0037As shown in <figref idref="f0007">FIG. 8</figref>, a cross-sectional view of the collet 44 according to one embodiment of the present invention is illustrated. As shown in <figref idref="f0007">FIG. 8</figref>, the collet 44 includes a plurality of ribs 72 that extend radially from the collet 44. In this embodiment, the ribs 72 reinforce the strength of the collet 44. In other embodiments, the collet 44 may not have the ribs 72.

0038<figref idref="f0008">FIG. 9</figref> shows steps in a method of using the disassembly tool 10. At step s100, the distal portion of the second member is inserted into the recess of the second component. The first member then abuts the proximal portion of the first component (step s102). A radial force is exerted on the first component at step s104. This may be achieved by activating the radially expanding collet as described above. At step s106, the drive mechanism is activated, causing the second member to exert a force in the longitudinal direction against the second component. Activating the drive member in some embodiments includes rotating a moveable handle about the longitudinal axis, causing the second member to move distally along the longitudinal axis, engaging a bottom wall of the recess in the second component.