Patent Publication Number: US-11020233-B2

Title: Tapered fixation device for a knee replacement

Description:
This application is the U.S. national phase of International Application No. PCT/US2018/042248 filed Jul. 16, 2018 which designated the U.S. and claims benefit to U.S. Provisional Application 62/533,251 filed Jul. 17, 2017, the entire contents of each of which are hereby incorporated by reference. 
    
    
     FIELD OF THE TECHNOLOGY 
     The subject matter disclosed herein relates to a prosthetic knee replacement and more particularly to a tapered fixation device for securing the prosthetic knee replacement to a patient&#39;s bones. 
     BACKGROUND 
     Many knee replacement surgeries are performed annually. Typical knee replacements include a femoral component that is fixed to the patient&#39;s femur. Knee replacements also include a tibial component that is fixed to the patient&#39;s tibia. A bearing insert is sandwiched between the femoral component and the tibial component and acts a replacement for the patient&#39;s cartilage. Most knee replacements are glued to the patient&#39;s bones with bone cement. Some knee replacements are simply press fit in place without cement. Either way, most knee replacements last 10-15 years, while some fail earlier for various reasons. 
     When a knee replacement fails, it must be fully revised. Over the life of a knee replacement, the patient&#39;s bone tissue can deteriorate resulting in bone loss. As such, when knee replacements are revised, they typically need additional mechanical fixation to compensate for the bone loss. This additional mechanical fixation takes the form of a stem that extends from the tibial component (or the femoral component) into the patient&#39;s bone canal. 
     Conventional stems are cylindrical. However, bone canals are rarely cylindrical. In addition, the portions of the bone canals that engage the cylindrical stems are typically tapered. Such a configuration allows for voids between the walls of the bone canal and the cylindrical stem, which allow the cylindrical stem to move or shift within the bone canal. Such movement or shifting can impede the cylindrical stem&#39;s ability to provide sufficient mechanical fixation, which in turn, can compromise the life of the revision knee replacement and can even cause pain to the patient. 
     Several solutions have been presented to compensate for the suboptimal fit of the cylindrical stem within the patient&#39;s bone canal. One solution uses bone cement to fill the voids between the cylindrical stem and the wall of the bone canal. However, all knee replacements (including revision knee replacement) have a limited life expectancy. In the event of knee replacement failure, the bone cement makes it more difficult to remove the previous knee replacement and contributes to even more bone loss with subsequent decreased structural support from the bone. 
     Another solution uses a hollow metallic support cone. The revision stem extends through the metallic support cone but is not affixed to the metallic support cone. Also, the metallic support cone is not fixed to the tibial component. This configuration allows for movement of the cylindrical stem relative to the metallic support cone, which in turn allows for movement of the revision stem relative to the bone canal. Cement is often used to fill this void. In addition, due to the unique nature of revision knee surgery, this system requires a large inventory. For example, upwards of 50 stems and 10 cones are required to ensure that the correct components are matched to the patient&#39;s specific geometry and fixation needs. 
     Another solution extends the length of the cylindrical stem so that it projects further into the patient&#39;s bone canal. Although this configuration may provide more support for the knee replacement, the extra length projecting deeper into the patient&#39;s bone canal can cause pain. 
     Yet another solution uses a stepped metallic sleeve that is fixed to the cylindrical stem. This configuration is not able to “wedge” into the patient&#39;s bone canal due to its “stepped” structure, thereby limiting its ability to support the prosthesis. In addition, the metallic sleeve is not versatile and is compatible with only one knee replacement system. Furthermore, due to the unique nature of revision knee surgery, this system requires a large inventory. For example, upwards of 50 stems and 10 sleeves are required to ensure that the correct components are matched to the patient&#39;s specific geometry and fixation needs. 
     BRIEF SUMMARY 
     Aspects of the tapered stem and the associated knee replacement described herein provide solutions to one or more problems or disadvantages associated with the prior art. 
     In one aspect of the technology, a fixation device for a knee replacement may be provided. The knee replacement may have a tibial component and a femoral component. The fixation device may have a stem configured to be fixedly attached to one of the tibial component and the femoral component. The stem may have a continuously and/or variably tapered outer surface and may have a distal end that is distal to said one of the tibial component and the femoral component. The stem may be configured so that the continuously tapered outer surface engages a patient&#39;s bone when the stem is inserted into a bone canal within the patient&#39;s bone. In addition, the continuously tapered outer surface of the stem at the distal end may be configured to mechanically fix the knee replacement to the patient&#39;s bone by being wedged within the bone canal. 
     In another aspect of the technology, a fixation device for a knee replacement may be provided. The knee replacement may have a tibial component configured to be secured to a tibia of a patient and a femoral component configured to be secured to a femur of a patient. The fixation device may include a stem configured to be wedged into one of the patient&#39;s bone canals. The stem may have an outer surface and at least a portion of the outer surface may be continuously tapered. In addition, the outer surface of the stem may be shaped to substantially match a shape of the patient&#39;s bone canal so that a furthest extension of the stem away from the knee replacement wedges in the patient&#39;s bone canal when the stem is inserted into the patient&#39;s bone canal. 
     In yet another aspect of the technology, a fixation device may be provided for a knee replacement. The knee replacement may have a tibial component and a femoral component. The fixation device may include a tapered stem configured to be attached to one of the tibial component and the femoral component. The fixation device may also include at least one projection on an outer surface of the tapered stem. The at least one projection may be configured to resist a rotational movement of the tapered stem when the tapered stem is inserted into a patient&#39;s bone canal. The fixation device may further include a fixation bolt configured to secure the tapered stem to the tibial component or the femoral component. The tapered stem may have a continuously tapered outer surface configured to be wedged within the patient&#39;s bone canal. 
     The fixation device with the tapered stem may provide the additional stability and fixation needed by patients with compromised bone stock. The “wedging” effect of the tapered design may increase bone loading, which in turn, may provide superior mechanical fixation without the need for cement in the bone canal. In addition, the use of projections (that may be in the form of splines or ribs) on the tapered stem may enhance rotational stability. It is contemplated that in addition to (or alternative to) the projections, the surface of the tapered stem may be roughened, polished or coated to add texture to the surface of the tapered stem to further enhance rotational stability. The coating may include (but may not be limited to) hydroxyapatite or similar elements. For the coating, any biocompatible surface finish may be applied. Providing a textured surface on the tapered stem in combination with the lack of cement may prevent bony ingrowth (or ongrowth) and make it easier to remove the knee replacement while also minimizing bone loss. 
     The additional stability and fixation may increase the longevity of the knee implant. Also, the design of the fixation device with the tapered stem is can be rigidly affixed to “off-the-shelf” knee implants. Accordingly, the configuration of the tapered stem may simplify installation of the knee replacement and reduce surgery times. It should be understood that the fixation device with the tapered stem may be used in most revision knee replacement surgeries or difficult primary surgeries. 
     In addition, the tapered design may reduce the amount of inventory needed to ensure that the correct components are matched to the patient&#39;s specific geometry and fixation needs. For example, the inventory may be reduced to no more than ten stems of different dimensions. This may save manufacturing and carrying costs. 
     In yet another aspect of the technology, a fixation device may be provided for a knee replacement. The fixation device may include a stem configured to be fixedly attached to one of a tibial component of the knee replacement and a femoral component of the knee replacement. The stem may have a continuously tapered outer surface and has a distal end that is distal to said one of the tibial component and the femoral component. A tapered projection may be positioned on the continuously tapered outer surface and may be tapered in the same direction as the continuously tapered outer surface. The stem and the tapered projection may be configured so that the continuously tapered outer surface and the tapered projection engage a patient&#39;s bone when the stem is inserted into a bone canal within the patient&#39;s bone. In addition, the continuously tapered outer surface of the stem at the distal end of the stem and the tapered projection may be configured to mechanically fix the knee replacement to the patient&#39;s bone by being wedged within the bone canal. 
     The patient&#39;s bone canal may become enlarged due to degradation of the bone or other issues. The tapered projection on the outer surface of the stem may provide additional structure that may be useful to fill the enlarged bone canal. 
     In yet another aspect of the technology, a knee replacement device may include a tibial component configured to be secured to the patient&#39;s tibia, a femoral component configured to be secured to the patient&#39;s femur, and a bearing insert sandwiched between the tibial component and the femoral component. A fixation device may be formed unitarily with one of the tibial component or the femoral component. 
     The fixation device may include a stem configured to be wedged into the patient&#39;s bone canal. The stem may include an outer surface and at least a portion of the outer surface may be continuously tapered. The outer surface of stem may be shaped to substantially match a shape of the bone canal so that a furthest extension of the stem away from the bearing insert wedges in the bone canal when the stem is inserted into the bone canal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded view of a knee replacement. 
         FIG. 2  is a perspective view of the knee replacement of  FIG. 1  without the tibial component but with a tapered stem attached. 
         FIG. 3  is an exploded view of the femoral component with the tapered stem. 
         FIG. 4  is a perspective view of the knee replacement of  FIG. 1  without the femoral component but with a tapered stem attached. 
         FIG. 5A  is a perspective view of an exemplary stem for use in the knee replacement of  FIG. 1 . 
         FIG. 5B  is a top view of the stem of  FIG. 5A . 
         FIG. 5C  is a sectional view of the stem of  FIG. 5A . 
         FIG. 5D  is a perspective view of another exemplary stem. 
         FIGS. 6A-6C  are perspective views of exemplary revision stems for use in the revision knee replacement of  FIG. 1 . 
         FIG. 7  is a side view of another exemplary stem. 
         FIG. 8  is another exemplary knee replacement without the femoral component. 
         FIG. 9  is a sectional view of another exemplary stem. 
         FIG. 10A  is a top view of another exemplary stem. 
         FIG. 10B  is a top view of another exemplary stem. 
         FIG. 10C  is a perspective view of another exemplary stem. 
         FIG. 10D  is a sectional view of another exemplary stem. 
         FIG. 11  is an exploded view of another exemplary knee replacement without the femoral component. 
         FIG. 12  is a side view of the knee replacement of  FIG. 11 . 
         FIG. 13  is an exploded view of another exemplary knee replacement without the tibial component. 
         FIG. 14A  is a perspective view of an exemplary stem with a tapered projection. 
         FIG. 14B  is a top view of the stem of  FIG. 14A . 
         FIG. 14C  is a side view of an exemplary stem with a tapered projection. 
         FIGS. 15A-18B  illustrate exemplary non-modular knee replacement devices. 
     
    
    
     DETAILED DESCRIPTION 
     It should be understood that the various numbers in the figures represent like components through the several views. 
       FIG. 1  illustrates a knee replacement  10 . The knee replacement  10  may be used as a primary knee replacement that forms a prosthetic replacement for a patient&#39;s knee. The knee replacement  10  may alternatively be used as a revision knee replacement for replacing a failed primary knee replacement or a failed revision knee replacement or difficult primary replacement. The knee replacement  10  may include a femoral component  12 , a tibial component  14 , a bearing insert  16 , one or two stems  18  and corresponding fixation bolts  20 . 
     The femoral component  12  may form an upper portion of the knee replacement  10  and may be secured to a lower end of the patient&#39;s femur (not shown). In addition, the femoral component may be made of biocompatible material such as, for example, biocompatible metal. In addition, an upper portion of the femoral component  12  may include a stem engagement portion  22  that may engage a surface of the stem  18  and receive the bolt  20 . It is contemplated that the stem engagement portion  22  and/or other parts of the femoral component  12  may interact with the stem  18  to resist rotational movement of the stem  18  relative to the femoral component  12 . It is further contemplated that the stem engagement portion  22  may be in the form of a cylinder or other hollow shape or tapered or solid shape. The cylinder may be received inside the stem  18 , and the bolt  20  may be received inside the hollow portion of the stem engagement portion  22 . It is contemplated that the hollow portion of the stem engagement portion  22  may be threaded to match a threading on the fixation bolt  20 . The hollow portion of the stem engagement portion  22  may include other features instead of threading that may mechanically fix the fixation bolt  20  and the stem  18  to the femoral component  12 . The hollow portion of the stem engagement portion  22  and the fixation bolt  20  may work together to fixedly secure the stem  18  to the top side of the femoral component  12  and resist any movement of the stem  18  relative to the femoral component  12 . The head of the fixation bolt  20  may vary in shape, size, length, surface finish and material. In addition, the head of the fixation bolt  20  may include a tuning fork/clothes peg end or other modified end to decrease terminal stiffness. 
     The tibial component  14  may form part of a lower portion of the knee replacement  10  and may be secured to an upper end of the patient&#39;s tibia (not shown). The tibial component  14  may also be made of biocompatible material such as, for example, biocompatible metal. In addition, a lower portion of the tibial component  14  may include a stem connection portion  23 . The stem  18  may connect to the tibial component  14  at the stem connection portion  23 . 
     The stem connection portion  23  may be the portion of the tibial component  14  that connects to the stem  18 . The stem connection portion  23  may include an engagement portion  24  and a bore  26 . The stem engagement portion  24  may engage a surface of the stem  18 . It is contemplated that, the stem engagement portion  24  may interact with the stem  18  to resist rotational movement of the stem  18  relative to the tibial component  14 . At least part of the engagement portion  24  may be rib-shaped (or any other shape) and may provide structural support to the rest of the tibial component  14 . 
     The bore  26  may receive the fixation bolt  20  to secure the stem  18  to the tibial component. It is contemplated that the bore  26  may be threaded to match a threading on the fixation bolt  20 . The bore  26  may include other features instead of threading that may mechanically fix the fixation bolt  20  and the stem  18  to the tibial component  14 . The bore  26  and the fixation bolt  20  may work together to fixedly secure the stem  18  to the underside of the tibial component  14  and resist any movement of the stem  18  relative to the tibial component  14 . 
     The bearing insert  16  may be positioned between the femoral component  12  and the tibial component  14  and may replace the patient&#39;s cartilage. In addition, the bearing insert  16  may be made of a biocompatible plastic. Preferably, the biocompatible plastic may be a low friction material so that the femoral component  12  may easily slide along the surface of the bearing insert  16 . 
     The stem  18  may extend from the tibial component  14  and may further secure the knee replacement  10  to the patient&#39;s tibia by way of a bone canal inside the patient&#39;s tibia (not shown).  FIGS. 1-4  illustrate one stem  18  extending from the tibial component  14  and another stem extending from the femoral component  12 . It is contemplated that the knee replacement  10  may include one or two stems  18 . In those configurations, the stem  18  may extend from the femoral component  12  and/or the tibial component  14 . 
     The stem  18  may include a central bore  30 , an internal receiving portion  32 , an outer surface  34  and at least one projection  36 . In addition, the stem  18  may be made of any biocompatible material. For example, the stem  18  may be made of biocompatible metal. It is contemplated that different components of the stem  18  may be made of different biocompatible materials. It is contemplated that the central bore  30  may also be offset as illustrated in  FIGS. 10A-10D  (offset bore  330 ). 
     The central bore  30  may be sized to receive the fixation bolt  20 . In addition, the central bore  30  may have a cylindrical or tapered shape and may be threaded to match a threading on the fixation bolt  20 . Alternatively, the walls of the central bore  30  may be smooth so that the fixation bolt  20  may simply slide through the central bore  30  rather than be screwed into the central bore  30 . It is further contemplated that the central bore  30  may include other features instead of threading that may facilitate mechanically fix the fixation bolt  20  and the stem  18  to the tibial component  14 . 
     The internal receiving portion  32  may be a recess at one end of the stem  18  that receives the engagement portion  24 . The internal receiving portion  32  may include a seat  38  and gaps  40 . 
     The central bore  30  may terminate at the seat  38 . In addition, when assembled to the tibial component  14  (or the femoral component  12 ), the stem engagement portion  24  may abut against the seat  38  so that the bore  26  and the central bore  30  together form a continuous cavity that receives the fixation bolt  20 . 
     The gaps  40  may be positioned to receive the stem engagement portion  24 . Although illustrated as relatively wide, the gaps  40  may be narrowed to substantially match the thickness (i.e., within manufacturing tolerances) of the engagement portion  24  so that the gaps  40  cooperate with the engagement portion  24  to resist rotational movement of the stem  18  relative to the tibial component  14 . 
     The outer surface  34  may be tapered from the internal receiving portion  32  to the opposite end of the stem  18 . As can be seen in  FIGS. 1-6C , the tapered shape of the outer surface  34  may be continuous. This may ensure that a tapered surface engages the patient&#39;s bone canal. Ensuring that a tapered surface engages the patient&#39;s bone canal allows the outer surface  34  to be wedged in the tapered portion of the patient&#39;s bone canal. The wedging of the outer surface  34  may be the primary way the stem  18  fixes the knee replacement  10  to the patient&#39;s bone canal. 
     It should be understood that the wedging of the outer surface  34  against the patient&#39;s bone canal may be facilitated by selecting a shape and/or material that provides some amount of elasticity so that the stem  18  may be “squeezed” or may be compressed when pushed into engagement with the walls of the patient&#39;s bone canal. The elasticity of the stem  18  may be closer to that of bone than conventional methods of fixation, thereby reducing implant related bone pain while reducing risks of stress shielding. It should be further understood that tapering the outer surface  34  to enable the outer surface  34  to be wedged against the walls of the patient&#39;s bone canal may substantially increase the force retaining the knee replacement  10  in place. In particular, the tapered outer surface  34  may substantially match the taper within the bone canal, which may substantially increase the surface area of the engagement zone between the stem  18  and the walls of the bone canal. For example, the engagement zone may be equivalent to the entire length of the outer surface  34  or a substantially large portion of the outer surface  34 . The ultimate result of the tapered configuration may be a more spread out and increased force holding the knee replacement  10  in place due to a much greater area of interaction between the walls of the bone canal and the outer surface  34  of the stem  18 . In addition, the stability of the knee replacement  10  may be increased by ensuring that at least the most distal end of the outer surface  34  (i.e., the end that is furthest from the tibial component  14  or the femoral component  12 ) is wedged against the walls of the bone canal. The stem  18  may also include radial steps and/or macrotexture that may convert axial forces to compression forces, thereby decreasing stress and increasing surface area. 
     It is contemplated that the outer surface  34  may be roughened, polished or coated to add texture to the surface  34  to provide some rotational support. For example, the surface  34  have a grit blasted finish and/or a coating that includes (but is not limited to) hydroxyapatite or similar elements or any other biocompatible material. The grit blasted finish and/or coating may prevent bony ingrowth (or ongrowth), which may facilitate easy removal of the stem  18 . It is further contemplated that instead of a grit blasted finish or in addition to the grit blasted finish, other textured features may be added to the outer surface  34  to increase the surface roughness of the outer surface  34 . 
     In addition, a cross-sectional shape of the outer surface  34  (as viewed along the axis of the stem  18 ) may be symmetrical (as illustrated in the drawings) or asymmetrical. The asymmetrical shape/configurations may be used when the patient&#39;s anatomy requires a relatively shifted or offset position of the baseplate  14  relative to the bony canal to provide an optimal fit of a baseplate of the tibial component  14  to the end of the bone. The cross-sectional shape of the outer surface  34  may be circular, trapezoidal or any other shape that may substantially match or compliment the cross-sectional shape of the patient&#39;s bony canal. It is contemplated that the cross-sectional shape of the outer surface  34  may vary along the axis of the stem  18 . For example, the cross-sectional shape of the portion of the outer surface  34  at the internal receiving portion  32  may be trapezoidal, while the cross-sectional shape of the portion of the outer surface  34  that is at the opposite end may be circular. The transition between cross-sectional shapes may be gradual or may be sudden. 
     The outer surface  34  illustrated in  FIGS. 1-5C and 6A-6C  are conical in shape. Thus, those figures illustrate an outer surface  34  in which the cross-sectional shape is circular throughout the stem  18 .  FIG. 5D  illustrates a trapezoidal cross-sectional shape. It should be understood that the cross-sectional shape of the outer surface  34  is not limited to the shapes illustrated in the figures. 
     The projections  36  may extend axially along the outer surface  34 . In addition, the projections  36  may be positioned to resist rotational movement of the stem  18  relative to the patient&#39;s bone canal. It is contemplated that the projections  36  may extend the entire length of the outer surface  34  or may extend only a fraction of the length of the outer surface  34 . Although each projection  36  is illustrated as being continuous, the projections  36  may be discontinuous (i.e., there may be multiple, axially aligned, distinct projections  36 ). It should be understood that the stem  18  may include as few as one projection  36 . Additionally, the projections  36  may be arranged symmetrically (equally spaced) or asymmetrically (not equally spaced) on the outer surface  34 . Although the projections  36  are shown as being axially aligned, they may have a different orientation. For example, the projections  36  may be angled relative to the central longitudinal axis of the stem  18  or may be fully radially aligned projections. 
     The projections  36  may be in the form of splines or ribs. The cross-sectional shapes of the projections  36  may be trapezoidal, semicircular, triangular, square or any other shape. 
     The fixation bolt  20  may mechanically secure the stem  18  to the tibial component  14  (or the femoral component  12 ). The fixation bolt  20  may be longer than the length of the stem  18  or may be shorter than the stem  18 . In addition, the fixation bolt  20  may be configured so that when fully received by the bore  26  and the central bore  30 , a portion of the fixation bolt  20  remains exposed to the patient&#39;s bone canal. In this configuration, the head of the fixation bolt  20  may be wider than the diameter of the central bore  30  so that the fixation bolt  20  may hold the stem  18  against the tibial component  14  (or the femoral component  12 ) by a compressive force. 
     Alternatively, the fixation bolt  20  may be configured so that when fully received by the bore  26  and the central bore  30 , a head of the fixation bolt  20  is inside the stem  18 . In this configuration, the stem  18  may include a second internal receiving portion (not shown) with a second seat (not shown) that abuts the head of the fixation bolt  20  when the fixation bolt  20  is fully received by the bore  26  and the central bore  30 . This may allow the fixation bolt  20  to hold the stem  18  against the tibial component  14  (or the femoral component  12 ) by a compressive force. 
     The dimensions of the stem  18  may be selected to substantially match or compliment the dimensions of the patient&#39;s bone canal. For example, a width of the stem  18  may be in a range between approximately 2 mm to approximately 80 mm. The length of the stem  18  may be within a range of approximately 10 mm to 300 mm. For example, the stem  18  may be 70 mm.  FIGS. 6A-6C  illustrate stems  18  with different lengths L. As can be seen the angle of the taper of the outer surface  34  may depend on the relative widths W at the ends of the stem  18  as well as the length L of the stem  18 . 
       FIGS. 7 and 8  illustrate a knee replacement  110  with a stem  118 . All of the components of the knee replacement  110  are the same as the knee replacement  10  except for the stem  18 . The only difference between the stem  18  and the stem  118  is that the entire outer surface  34  of the stem  18  is tapered. In contrast, the outer surface  134  of the stem  118  has a tapered section  150  and a non-tapered section  152 . 
     The stem  118  may be used when the patient has severe bone loss and a more substantial mechanical fixation is needed. In such a circumstance, the stem  118  may be inserted further into the patient&#39;s bone canal than the stem  18 . Typically bone canals are tapered at the end but become non-tapered in deeper sections. Thus, the tapered section  150  and the non-tapered section  152  may be designed to substantially match and/or complement the tapered and non-tapered structure of the patient&#39;s bone canal. 
     It is contemplated that the tapered section  150  and the non-tapered section  152  may be modular components to be assembled to each other. It is further contemplated that the tapered section  150  and the non-tapered section  152  may be formed as one piece. Also, the non-tapered section  152  may be a fixation bolt similar to the fixation bolt  20 . Although the non-tapered section  152  is not tapered, the non-tapered section  152  may still vary in shape, size, length, surface finish, material and may include a tuning fork/clothes peg end or other modified end to decrease terminal stiffness. 
     Similar to the outer surface  34 , the outer surface  134  may have a grit blasted finish to provide rotational support. The grit blasted finish may also prevent bony ingrowth, which may facilitate easy removal of the stem  118 . It is further contemplated that the finish may have additional or alternative features to add texture to the outer surface  134  and to roughen the outer surface  134 . The outer surface  134  may also have projections similar to the projections  36   
     It is contemplated that the stems  18  (or  118 ) may be modular components assembled to the rest of the knee replacement  10  (or  110 ) or may be formed as one piece with the rest of the knee replacement  10  (or  110 ) (i.e., the stem  18  (or  118 ) may be formed as one piece with the tibial component  14  or the femoral component  12 ). When the knee replacement  10  (or  110 ) includes two stems  18  (or  118 ), one or both stems  18  (or  118 ) may be formed integrally with the components of the knee replacement  10 . For example, one stem  18  (or  118 ) may be formed integrally with the tibial component  14  with the other stem  18  (or  118 ) being secured to the femoral component  12  by way of the fixation bolt  20 . In another example using two stems  18  (or  118 ), one stem  18  (or  118 ) may be secured to the tibial component  14 , while the other stem  18  (or  118 ) may be formed integrally with the femoral component  12 . In yet another example using two stems  18  (or  118 ), one stem  18  (or  118 ) may be formed integrally with the tibial component  14  and the other stem  18  (or  118 ) may be formed integrally with the femoral component  12 . 
       FIGS. 1-8  show knee replacements that utilize a fixation bolt that may be longer than the stem to secure the stem to the tibial and/or femoral component.  FIG. 9  illustrates a stem  218  that utilizes a relatively smaller fixation bolt to secure the stem  218  to the tibial and/or femoral component. The fixation bolt  220  may be shorter than the length of the stem  218 . Thus, unlike the fixation bolt  20 ,  120  (whose head may remain outside the central bore  30 ,  130 ), the head of the fixation bolt  220  may remain within a central bore  230  of the stem  218 . 
     The stem  218  may be similar to the stem  18 . For example, the stem  218  may include an internal receiving space  232  (similar to the internal receiving space  32 ) that receives the stem engagement portion  22  of the femoral component  12  and/or the stem engagement portion  24  of tibial component  14 . The stem  218  may have an outer surface  234  similar to the outer surface  34 . For example, the outer surface  234  may be tapered and may have a surface finish and/or projections similar to the surface finish and/or projections of the outer surface  34 . 
     The stem  218  may also include a seat  238 . Similar to the seat  38 ,  138 , the seat  238  may include an opening for receiving the fixation bolt  220 . While the opening in the seat  38 ,  138  may have the same diameter as the central bore  30 ,  130 , the opening in the seat  238  may have a smaller diameter than the diameter of the central bore  230 . In addition, the diameter of the head of the fixation bolt  220  may be smaller than the diameter of the central bore  230  but larger than the diameter of the opening in the seat  238 . This configuration may allow the fixation bolt  220  to exert a compressive force against the seat  238  when securing the stem  218  to the tibial or femoral component. In contrast, the fixation bolt  20  may exert a compressive force against a distal end of the stem  18  (the end of the stem  18  furthest from the tibial component  14  or the femoral component  12 ). It is contemplated that in all embodiments, the bore  30 ,  130 ,  230  may be threaded and that the force securing the stem to the tibial component  14  or the femoral component  12  may originate from the interaction between the fixation bolt and the threading inside the central bore. 
     Once the fixation bolt  220  is secured within the central bore  230 , the distal end of the central bore  230  (the end furthest from the seat  238 ) may remain open or may be closed off by way of a protective covering. The protective covering may be made of any biocompatible material and may vary in shape, size, length, taper, surface finish, material. The protective covering may include a tuning fork/clothes peg end or other modified end to decrease terminal stiffness. In addition, the central bore  230  may be filled by a filler material. 
       FIGS. 10A-10D  illustrate a stem  318  similar to the stem  218 . However, instead of a central bore centered on the longitudinal axis of the stem, the stem  318  includes an offset bore  330  that is offset from the longitudinal axis of the stem  318 . The offset bore  330  may also be part of the stem  18  and the stem  118 . Stems  18  and  118  may also be asymmetric. 
     It should be understood that the location of the offset bore  330  may be the only difference between the stem  318  and the stem  218 . In particular, the stem  318  may include an internal receiving space  332  (similar to the internal receiving space  232 ) that receives the stem engagement portion  22  of the femoral component  12  and/or the stem engagement portion  24  of tibial component  14 . The stem  318  may have an outer surface  334  similar to the outer surface  234 . For example, the outer surface  334  may be tapered and may have a surface finish and/or projections  336  similar to the surface finish and/or projections of the outer surface  234 . 
     Similar to the seat  238 , the seat  338  may include an opening for receiving the fixation bolt  320  that may have a smaller diameter than the diameter of the offset bore  330 . In addition, the diameter of the head of the fixation bolt  320  may be smaller than the diameter of the offset bore  330  but larger than the diameter of the opening in the seat  338 . This configuration may allow the fixation bolt  320  to exert a compressive force against the seat  338  when securing the stem  318  to the tibial or femoral component. It is contemplated that the offset bore  330  may be threaded and that the force securing the stem to the tibial component  14  or the femoral component  12  may originate from the interaction between the fixation bolt  320  and the threading inside the offset bore  330 . 
     Once the fixation bolt  320  is secured within the offset bore  330 , the distal end of the offset bore  330  (the end furthest from the seat  338 ) may remain open or may be closed off by way of a protective covering. The protective covering may be made of any biocompatible material. In addition, the offset bore  330  may be filled by a filler material. 
     The stem  318  may also include gaps  340  similar to the gaps  40  of the stem  18 . The gaps  340  may be positioned to receive the engagement portion  24 . Although illustrated as relatively wide, the gaps  340  may be narrowed to substantially match the thickness (i.e., within manufacturing tolerances) of the engagement portion  24  so that the gaps  340  cooperate with the engagement portion  24  to resist rotational movement of the stem  318  relative to the tibial component  14 . 
       FIGS. 11 and 12  illustrate another exemplary tibial component  414  of a knee replacement  410 . Similar to the tibial component  14 , the tibial component  414  may be secured to a bearing insert  416  (which may be similar to the bearing insert  16 ). The tibial component  414  may include a stem connection portion  423  that is configured to connect a stem  418  to the tibial component  414 . 
     The stem connection portion  423  may include an engagement portion  424 , a bore  426  and lateral openings  428 . The engagement portion  424  may engage a surface of the stem  418 . Similar to the engagement portion  24 , it is contemplated that the engagement portion  424  may interact with the stem  418  to resist rotational movement of the stem  418  relative to the tibial component  414 . At least part of the engagement portion  424  may be rib-shaped (or any other shape) and may provide structural support to the rest of the tibial component  414 . 
     The bore  426  may receive an internal tapered structure  429  within an interior of the stem  418  to secure the stem  18  to the tibial component. The interior of the bore  426  may be tapered to match the tapered shape of the internal tapered structure  429 . In addition, the bore  426  may be received within an internal receiving space  431  in the stem  418 . 
     The lateral openings  428  may be sized and located to receive the set screws  433  in the side of the stem  418 . When the bore  426  is received within the internal receiving space  431 , the lateral openings  428  align with the set screws  433  and receive the set screws  433  to secure the stem  418  to the stem connection portion  423 . The bore  426 , the lateral openings  428 , the internal tapered structure  429  and the set screws  433  may work together to fixedly secure the stem  418  to the underside of the tibial component  414  and resist any movement of the stem  418  relative to the tibial component  414 . 
     It is also contemplated that the lateral openings  428  may be replaced with biased projections (not shown) that are biased in a radially outward direction. The set screws  433  may be replaced with openings or indentations (not shown) that may receive the biased projections. 
     The stem  418  may include an outer surface  434  similar to the outer surface  34 . For example, the outer surface  434  may be tapered and may have a surface finish and/or projections  436  similar to the surface finish and/or projections of the outer surface  34 . 
       FIG. 13  shows how the stem  418  may be attached to an exemplary femoral component  412  of the knee replacement  410 . Similar to the femoral component  12 , an upper portion of the femoral component  412  may include an engagement portion  422  that connects the femoral component  412  to the stem  418 . 
     Similar to the connection portion  423 , the stem engagement portion  422  may include the bore  426  and the lateral openings  428 . Thus, the stem  418  may connect to the femoral component  412  in a similar way as the stem  418  connects to the tibial component  414 . 
     It is also contemplated that the lateral openings  428  may be replaced with biased projections (not shown) that are biased in a radially outward direction. The set screws  433  may be replaced with openings or indentations (not shown) that may receive the biased projections. 
     In some situations, the patient&#39;s bone canal may have a shape that widely varies from the shape of the stem due to, for example, excessive bone loss. In such situations, the stem might not sufficiently anchor the knee replacement device to the patient&#39;s bones.  FIGS. 14A-14C  illustrate another exemplary stem  518  that may be used when the shape of the bone canal varies widely from the shape of the stem. 
     The stem  518  is similar to all of the previously disclosed stems. For example, the stem  518  includes a central bore  530 , an internal receiving portion  532 , an outer surface  534  and at least one projection  536 . Although illustrated as being centered, the central bore  530  may be offset. The internal receiving portion  532  may include a seat  538  and gaps  540 . In addition, the stem  518  may include a tapered projection (lobe)  550  mounted to the outer surface  534 . 
     The tapered projection  550  may provide additional surface area for a better fit within the patient&#39;s bone canal. The tapered projection  550  may have any shape. For example, the tapered projection  550  may have a partially frusto-conical shape (i.e., the cross-sectional shape of the tapered projection  550  may be an incomplete circle). As illustrated in  FIG. 14C , the tapered projection  550  may have a partially conical shape (i.e., the cross-sectional shape of the tapered projection  550  may be an incomplete circle). It should be understood that the cross-sectional shape of the tapered projection is not limited to incomplete circles or rounded shapes. 
     The tapered projection  550  may be tapered in the same direction as the outer surface  534 . That is, the taper of the tapered projection  550  may be in a direction substantially parallel to the longitudinal axis of the stem  18 . In addition, the taper of the tapered projection  550  may be the same as or different from the taper of the outer surface  534 . 
     The tapered projection  550  may be made of the same material as the outer surface  534  or any other bio-compatible material. In addition, the surface of the tapered projection  554  may be a mesh or solid material. Furthermore, the tapered projection  550  may be hollow or solid-filled. Also, the surface of the tapered projection  550  may be stiff or flexible. 
     It is contemplated that the surface of the tapered projection  554  may be roughened, polished or coated to add texture to the tapered projection  554  to provide some rotational support. For example, the tapered projection  554  have a grit blasted finish and/or a coating that includes (but is not limited to) hydroxyapatite or similar elements or any other biocompatible material. The grit blasted finish and/or coating may prevent bony ingrowth (or ongrowth), which may facilitate easy removal of the stem  518 . It is further contemplated that instead of a grit blasted finish or in addition to the grit blasted finish, other textured features may be added to the tapered projection  554  to increase the surface roughness of the tapered projection  554 . 
     The tapered projection  550  may be removably or permanently mounted to the outer surface  534  by any means. For example, the tapered projection may be mounted by way of adhesive, chemical bond, or mechanical fastener. It is contemplated that the tapered projection  550  may be formed integrally with the outer surface  534 . 
       FIGS. 14A and 14B  show the tapered projection  550  being positioned between neighboring projections  536 . It is also contemplated that the tapered projection  550  may straddle on projection  536 . It is further contemplated that the tapered projection  550  may extend all of the way between neighboring projections  536  or only part of the way between projections  536 . In addition, the tapered projection  550  may extend the entire axial length of the stem  518  or only part of the axial length of the stem  18 . 
     It should be understood that although only one tapered projection  550  is illustrated in  FIGS. 14A-14C , the stem  518  may include more than one tapered projection  550 . In addition, the tapered projection  550  may be different from the projections  536 . For example, the projections  536  may be non-tapered (i.e., the both ends of the projections  536  may project the same distance from the outer surface  534 ). In addition, the tapered projection may be larger than the projections  536 . Also, additional projections  536  may be positioned on the tapered projection  550 . 
       FIGS. 15A-18B  illustrate exemplary knee replacement devices that are formed unitarily. The knee replacement device  610  may include a tibial component  614 , a bearing insert  616  and a stem  618 . The knee replacement device  610  may also (or alternatively) include a femoral component with a stem  618 . The stem  618  may include projections  636  and may be hollow and open at the bottom. In addition, the tibial component  614  may include engagement portions  624 . 
     The knee replacement device  710  may be similar to the knee replacement device  610  except that the stem  718  may be closed instead of open at the bottom. Thus, the knee replacement device  710  may include a tibial component  714 , a bearing insert  716  and engagement portions  724 . It should be understood that the knee replacement device  710  may also (or alternatively) include a femoral component with a stem  718 . In addition, the projections  736  may extend all the way around the bottom of the stem  718 . 
     The knee replacement device  810  may be similar to the knee replacement devices  610  and  710  except that the tibial component  814  may lack engagement portions. Thus, the knee replacement device  810  may include a bearing insert  816  and a stem  818 . It should be understood that the knee replacement device  810  may also (or alternatively) include a femoral component with a stem  818 . In addition, the projections  836  may extend all the way around the bottom of the stem  818 . 
     The knee replacement device  910  may be similar to the knee replacement devices  610 ,  710  and  810 . Thus, the knee replacement device  910  may include a tibial component  914 , a bearing insert  916  and a stem  918 . It should be understood that the knee replacement device  910  may also (or alternatively) include a femoral component with a stem  918 . In addition, the projections  936  may extend all the way around the bottom of the stem  918 . Furthermore, the tibial component  914  may include pegs  940  that extend downward away from the tibial component  914 . 
     The pegs  940  may provide additional fixation to the patient&#39;s bone including increased rotational and translational stability. The pegs  940  may also contribute to increased implant longevity and durability. The pegs  940  can be made of any biocompatible material and may be configured to promote bony ingrowth. Additionally, the pegs  940  may be configured to facilitate easier removal from the patient&#39;s bones. For example, the pegs  940  may be configured to be easily cut or divided (precut). The pegs  940  can vary in shape, size, location, length, material and surface finish. It is contemplated that the pegs  940  may unitarily formed with the tibial component  914  or may be attached to the tibial component  914  by varies means (e.g., chemical bond, adhesive, mechanical fixation, etc.). 
     It should be understood that in configurations utilizing two stems, one or both stems may be the stem  18 . Alternatively, one or both stems may be the stem  118 ,  218 ,  318 ,  418 ,  518 ,  618 ,  718 ,  818  or  918 . 
     It should be understood that the taper of the outer surface  34  (or  134 ,  234 ,  334 ,  444  or  534 ) may substantially match the taper of the patient&#39;s bone canal when the voids between the bone canal walls and the outer surface  34  (or  134 ,  234 ,  334 ,  444  or  534 ) are minimized. The tapers do not have to exactly match. 
     It should be understood that the dimensions of the outer surface  34  (or  134 ,  234 ,  334 ,  444  or  534 ) may substantially match the dimensions of the patient&#39;s bone canal when a difference between the dimensions is within manufacturing tolerances. The dimensions do not have to exactly match. 
     It should be understood that the shape of the outer surface  34  (or  134 ,  234 ,  334 ,  444 , or  534 ) may substantially match the shape of the bone canal when both are tapered or both are not tapered and the differences in the respective cross-sectional shapes minimize the voids between the bone canal walls and the outer surface  34  (or  134 ,  234 ,  334 ,  444  or  534 ). The shapes do not have to exactly match. 
     In addition, it is contemplated that any of the above disclosed stems could be non-modular cone shaped portions permanently attached to the rest of the implant. It is further contemplated that any of the above stems could be formed as one piece with other components of the knee replacement implant or all of the components of the knee replacement implant. In the non-modular, permanently fixed configurations, different knee replacement implants can have different sized cone shaped portions. Such knee replacement implants may be used as routine primary implants for average replacements or as primary and secondary implants for more complicated replacements. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.