Patent Publication Number: US-6699290-B1

Title: Modular elbow

Description:
This application is a continuation-in-part of U.S. patent application Ser. No. 09/222,601 filed Dec. 29, 1998, now U.S. Pat. No. 6,290,725 which is a continuation-in-part of U.S. patent application Ser. No. 08/963,138, filed Nov. 3, 1997 now U.S. Pat. No. 6,027,534. 
    
    
     BACKGROUND AND SUMMARY OF THE INVENTION 
     The present invention relates to artificial joints and, in particular, to a modular elbow prosthesis. 
     In the human elbow, three degrees of freedom are present. These are flexion-extension, varus-valgus (carrying angle) and axial rotation. Various elbow prostheses have been constructed as a replacement for the natural human elbow. The two basic types of elbow prosthesis known in the prior art are constrained and unconstrained. In constrained prosthesis, the prosthetic joint is held together mechanically, by components of the prosthesis. Such devices are shown, for example, in U.S. Pat. No. 5,376,121 to Huene et al., U.S. Pat. No. 3,708,805 to Scales, et al., U.S. Pat. No. 3,939,496 to Ling, et al., and U.S. Pat. No. 4,224, 695 to Grundei, et al. In an unconstrained device, the prosthetic device is held together by the patient&#39;s natural soft tissues. Such a device is shown in U.S. Pat. No. 4,293,963 to Gold, et al. In each of these devices, one portion of the prosthesis is implanted in the humerus of the patient and the other portion is implanted in the ulna. The two portions then mate in some manner to allow articulation of the joint. In the &#39;695 patent to Grundei, et al., an additional portion of the prosthesis is implanted in the radius of the patient. 
     A surgeon may not always know prior to beginning an operation whether a patient would be better served by a constrained or unconstrained elbow prosthesis. Thus, it would be desirable to provide an elbow prosthesis that may be utilized in either the constrained or unconstrained manner. 
     It may also be necessary to convert an unconstrained elbow prosthesis to a constrained one, or vice versa, after implantation and use for a period of time. In order to do so, it is typically necessary to remove the portion of the prosthesis implanted in the humerus and ulna and to replace the entire prosthesis with either the constrained or unconstrained variety. 
     The present invention provides an elbow prosthesis that can be utilized in either a constrained or unconstrained fashion. The elbow prosthesis of the present invention can be converted from a constrained to an unconstrained prosthesis and from an unconstrained to a constrained prosthesis after implantation in a patient&#39;s body. Certain embodiments of the present invention also provide an elbow prosthesis that allows for three degrees of freedom: flexion-extension, varus-valgus (carrying angle) and axial rotation. 
     These features are attained by the provision of a modular prosthetic joint having a first stem, a second stem and three bearing components. The first stem has a first end and a second end and a body connected to it. A slot is formed in the body. The first bearing component has a flange configured to mate with the slot. A pair of arms extend from one end of the second stem. The second bearing component is adapted to fit between the arms of the second stem and configured to mate with the bearing surface of the first bearing component. The third bearing component is interchangeable with the first and second bearing components and is adapted to fit between the arms of the second stem. The third bearing component also includes a flange configured to mate with the slot. 
     According to one embodiment of the invention, the prosthesis includes an opening in each of the arms, an opening in the second bearing component and a pin adapted to extend through the openings in the arms and second bearing component. 
     In another embodiment, the prosthesis includes an opening in each of the arms, an opening in the third bearing component and a pin adapted to extend through the openings in the arms and the third bearing component. 
     In one embodiment of the invention, the bearing surface of the first bearing component is concave and the second bearing component includes a convex surface. 
     According to another embodiment of the invention, a modular prosthetic elbow includes an ulnar component having a stem with a first end adapted to fit within the medullary canal of a human ulna and a second end, a humeral component having a stem with a first end adapted to fit within the medullary canal of a human humerus, a first bearing component adapted to engage the ulnar component, a second bearing component adapted to engage the humeral component and mate with the first bearing component so as to be held in place by the soft tissues of the elbow, and a third bearing component interchangeable with the first and second bearing components, the third bearing component adapted to engage the ulnar component and be held in place by the humeral component. 
     In one embodiment, the ulnar component includes a slot and the first bearing component includes a flange configured to mate with the slot. The humeral component includes a pair of arms and the second bearing component is adapted to fit between the arms. An opening is formed in each of the arms and in the second bearing component and a pin is adapted to extend through the openings in the arms and the second bearing component. 
     In one embodiment, the humeral component includes a pair of arms and the third bearing component is adapted to fit between the arms. An opening is formed in each of the arms and in the third bearing component. A pin is adapted to extend through the openings in the arms and the third bearing component. 
     In another embodiment, a slot is formed in the ulnar component and a flange is connected to the third bearing component and configured to engage the slot. In another embodiment, a slot is formed in the ulnar component, a flange is connected to the first bearing component and configured to engage the slot, and another flange is formed on the third bearing component and configured to engage the slot. 
     In another embodiment of the invention, a prosthetic modular elbow includes an ulnar component having a proximal end and a distal end and a humeral component having a proximal end and a distal end. A first bearing mount is formed on the humeral component for engagement with at least two of a plurality of bearing components. A second bearing mount is located on the ulnar component for engagement with at least two of a plurality of bearing components. In one embodiment, the first bearing mount includes a pair of spaced apart arms extending from the humeral component. The arms may be located at the distal end of the humeral component. In another embodiment of the invention, the second bearing mount is located at the proximal end of the ulnar component and may include a slot formed in a portion of the ulnar component. In one embodiment of the invention, the first and second bearing mounts are configured to simultaneously engage one of the plurality of bearing components. 
     According to another embodiment of the present invention, a modular elbow prosthesis includes a humeral component with a bearing mount and an ulnar component with a bearing mount. First and second bearing components are configured to engage the humeral and ulnar bearing mounts respectively so as to form an unconstrained prosthesis. A third bearing component, interchangeable with the first and second bearing components, is configured to engage the humeral and ulnar components so as to form a constrained prosthetic elbow. The humeral bearing mount may include a pair of spaced apart arms. The ulnar bearing mount may include a slot. The third bearing component may include a cylindrical body with an opening therein and may have a flange attached thereto. The flange is configured to mate with the ulnar bearing mount. The first bearing component may include a flange configured to mate with the ulnar bearing mount. The second bearing component may include an opening. 
     In another embodiment of the present invention, a modular prosthetic elbow includes a humeral component, an ulnar component, a first bearing for joining the humeral and ulnar components to form a constrained prosthetic elbow and a set of bearings interchangeable with the first bearing for providing pivotal movement of the ulnar component relative to the humeral component in an unconstrained manner. 
     A method according to the present invention includes the steps of implanting a first component of a prosthesis in the ulna, implanting a second component of the prosthesis in the humerus and selecting from a group of three bearing components two bearing components, one to be joined to the first component of the prosthesis and a second to be joined to the second component of the prosthesis, or selecting the remaining bearing component and securing it to the first and second components of the prosthesis. 
     In one embodiment, the method further comprises the step of joining the first two bearing components to the first and second components so as to form an unconstrained prosthetic elbow. In another embodiment, the method further includes the step of joining the third bearing component to the first and second component so as to form a constrained prosthetic elbow. 
     In another embodiment of the invention, a prosthetic joint includes a first stem having a first end and a second end, a body connected to the first stem, a slot formed in the body, a first bearing component having a spherical bearing surface, a flange connected to the first bearing component and configured to mate with the slot, a second stem having a first end and a second end, a pair of arms extending from one end of the second stem and a second bearing component adapted to fit between the arms of the second stem and configured to mate with the bearing surface of the first bearing component. The second bearing component may include a spherical bearing surface. The prosthetic joint may include an opening in each of the arms, an opening in the second bearing component and a pin and sleeve adapted to extend through the openings in the arms and second bearing component. A ridge may be formed on a portion of the first stem and/or a portion of the body. A recessed area may be provided in the second bearing component for engaging at least one of the arms. An ear may be provided on the second stem. A nubbin may be located on the first bearing component and an opening may be provided in the body for engaging the nubbin. 
     In another embodiment of the invention, a prosthetic elbow includes an ulnar component having a stem with a first end adapted to fit within the medullary canal of an ulna and a second end, a first bearing component adapted to engage the ulnar component, a humeral component having a stem with a first end adapted to fit within the medullary canal of a humerus and a second bearing component having a spherical surface, the second bearing component being adapted to engage the, humeral component and mate with the first bearing component so as to be held in place by the soft tissues of the elbow. 
     In another embodiment of the invention, an elbow prosthesis includes a humeral component having a bearing mount, an ulnar component having a bearing mount and first and second bearing components, each including a spherical bearing surface, the first bearing component configured to engage the ulnar bearing mount and the second bearing component configured to engage the humeral bearing mount. 
     In another embodiment of the invention, a prosthetic elbow includes a humeral component, an ulnar component and a plurality of bearings for providing axial rotation of the ulnar component relative to the humeral component. At least one of the bearings may include a spherical bearing surface. The prosthetic elbow may provide a total of two or three degrees of freedom. 
     In another embodiment of the invention, an elbow prosthesis includes an ulnar component, a humeral component and means for providing axial rotation of the ulnar component with respect to the humeral component. The means for providing axial rotation may include a spherical bearing surface. 
     Another embodiment of the invention provides an elbow prosthesis including an ulnar component, a humeral component and means for providing at least three degrees of freedom between the ulnar and humeral components. 
     In another embodiment of the invention, a prosthetic elbow includes an ulnar component having a stem with a first end adapted to fit within the medullary canal of an ulna and a second end. A first bearing component is adapted to engage the ulnar component. A humeral component has a pair of arms and a stem with a first end adapted to fit within the medullary canal of a humerus. A second bearing component is adapted to engage the humeral component and mate with the first bearing component. The second bearing component has a pair of slots that engage the arms and form a taper lock therewith. 
     In yet another embodiment of the invention, an elbow prosthesis includes a humeral component having a bearing mount, an ulnar component having a bearing mount, and a bearing component configured to form a taper lock with the bearing mount on the ulnar component and to engage the bearing mount on the humeral component. 
     According to another embodiment of the invention, a modular elbow includes an ulnar component having a slot, a first bearing component having a flange configured to mate with the slot so as to form a taper lock, a humeral component having a pair of arms, and a second bearing component adapted to fit between the arms of the humeral component and configured to surround a portion of the first bearing component. 
    
    
     Other features of the present invention will become apparent from the following detailed description of the preferred embodiments and the accompanying drawings. 
       
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view of a modular elbow according to the present invention for use in the unconstrained configuration. 
     FIG. 2 is a longitudinal cross-sectional view showing the modular elbow of FIG. 1 implanted in the arm of a person. 
     FIG. 3 is a cross-sectional view taken along line  3 — 3  in FIG.  2 . 
     FIG. 4 is an exploded perspective view of the modular elbow according to the present invention for use in the constrained mode. 
     FIG. 5 is a longitudinal cross-sectional view showing the modular elbow of FIG. 4 implanted in the arm of a person. 
     FIG. 6 is a plan view of an alternative second bearing component that forms a component of a modular elbow according to the present invention. 
     FIG. 7 is a plan view of the bearing component of FIG. 6 attached to a humeral component that is a component of modular elbow according to the present invention. 
     FIG. 8 is an-exploded view of another embodiment of a modular elbow according to the present invention for use in the unconstrained configuration. 
     FIG. 9 is a partial cut-away, side elevational view of a sleeve that forms a component of the modular elbow shown in FIG.  8 . 
     FIG. 10 is a side elevational view of a pin that forms a component of the modular elbow shown in FIG.  8 . 
     FIG. 11 is a detail of the barb on the pin shown on FIG.  10 . 
     FIG. 12 shows the modular elbow of FIG. 8 implanted in a patient and illustrates one degree of freedom of motion permitted by the elbow. 
     FIG. 13 is a partial cut-away view showing the modular elbow of FIG. 8 in a neutral position. 
     FIG. 14 is a partial cut-away view illustrating a second degree of freedom of motion permitted by the modular elbow of FIG.  8 . 
     FIGS. 15-17 are partial cut-away views illustrating the manner in which the modular elbow of FIG. 8 permits a third degree of freedom of motion. 
     FIG. 18 is an exploded view of another embodiment of a modular elbow according to the present invention for use in the constrained configuration. 
     FIG. 19 is an exploded view of another embodiment of a modular elbow according to the present invention for use in an unconstrained configuration. 
     FIG. 20 is an exploded view of another embodiment of a modular elbow according to the present invention for use in the constrained configuration. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view of a modular elbow according to the present invention for use in the unconstrained mode. The modular elbow prosthesis comprises ulnar component  10 , first bearing component  20 , humeral component  30 , second bearing component  40 , and pin  50 . 
     Ulnar component  10  comprises a stem  11  having a first or distal end  12  and a second or proximal end  13 . A portion of stem  11  adjacent second end  13  may be coated with a material  14  to allow bone ingrowth between the stem and ulna. Any of various known bone ingrowth coatings, such as cobalt-chromium or titanium alloys, may be used. A generally rectangular body  15  is secured to second end  13  and has a slot  16  formed therein. Body  15  and slot  16  form a mount for a plurality of bearing components, as described below. Stem  11  extends from body  15  at an angle thereto. A raised stop member  17  is formed at the juncture of stem  11  and body  15 . An opening  18  is formed in body  15  and is configured to receive a nubbin on first bearing component  20  or third bearing component  60 , as described below. Body  15  further includes a sloped, recessed area or notch  18   a  utilized for guiding the nubbin into opening  18 , as described below. Ulnar component  10  can be made from any surgical alloy, such as cobalt-chrome or titanium. Portions of ulnar component  10  other than or in addition to stem  11  may include a bone ingrowth coating. For example, bone ingrowth coating may be applied to underside  19  of body  15 . 
     First bearing component  20  generally comprises a base  21  with a stop member  22  extending therefrom at a generally ninety degree angle. A flange  23  is formed on base  21  and is configured to mate with slot  16  in body  15  of ulnar component  10 , as described below. A nubbin  23   a  extends from flange  23 . Bearing component  20  further includes a concave bearing surface  24 . Bearing surface  24  mates with second bearing component  40  as described below, when the modular elbow prosthesis is used in the unconstrained mode. 
     Humeral component  30  includes a stem  31  having a first segment  32  with a first or proximal end  33  and a second segment  34  with a second or distal end  35 . Second segment  34  is integral with first segment  32  and widens with distance from first end  33 . A pair of spaced apart arms  36  extend from second end  35  at an angle to second segment  34  of stem  31 . Each arm  36  includes an opening  37 . Arms  36  and openings  37  form a mount for a plurality of bearing components, as described below. A locking ring  38  is disposed about one opening  37 . Humeral component  30  may also be made of a surgical alloy, such as cobalt-chrome. 
     Second bearing component  40  is a generally cylindrical member that flares outward at first end  41  and second end  42 . Middle segment  43  of second bearing component  40  has a convex outer surface  44 . The outer surface of second bearing component  40  mates with bearing surface  24  of first bearing component  20  when the modular elbow prosthesis is used in the unconstrained mode, as described below. Second bearing component  40  also includes a central opening or bore  45 . 
     Pin  50  is a generally cylindrical member having a main body portion  51 , a first end  52  and a second end  53 . First end  52  includes a slot  52   a  to engage with locking ring  38  and secure pin  50  in place. Second end  53  includes an enlarged head or flange  54 . Head  54  has a larger diameter than openings  37  in arms  36  of humeral component  30 . A central opening or bore  55  extends through pin  50 . 
     To use the elbow prosthesis of the present invention in the unconstrained mode (FIGS.  2  and  3 ), ulnar component  10  is implanted in an ulna such that stem  11  is located in the intramedullary canal as shown in FIG.  2 . In a similar fashion, humeral component  30  is inserted in the humerus such that stem  31  is located in the intramedullary canal as shown in FIG.  2 . Ulnar component  10  and humeral component  30  can be fixed to the ulna and humerus, respectively, with or without bone cement. First bearing component  20  is positioned such that flange  23  is adjacent slot  16  in body  15  of ulnar component  10 . First bearing component  20  is then slid toward stop member  17 . As this occurs, locking nubbin  23   a  engages notch  18   a . As first bearing component  20  is moved into further engagement with ulnar component  10 , locking nubbin  23   a  rides upward along notch  18   a  and is compressed by it. When first bearing component  20  is fully seated in ulnar component  10 , locking nubbin  23   a  is located over opening  18  and decompresses. Locking nubbin  23   a  thus extends into opening  18   a  and prevents first bearing component  20  from pulling back out of slot  16  in body  15 . Second bearing component  40  is positioned between arms  36  of humeral component  30  such that opening  45  is aligned with opening  37 . Pin  50  is then inserted through an opening  37  in one arm  36 , through opening  45  in second bearing component  40  and through the remaining opening  37  in the other arm  36  such that locking ring  38  engages slot  52   a . Second bearing component  40  is then placed adjacent first bearing component  20  such that their concave and convex surfaces mate. First and second bearing components  20  are held in position by the soft tissues of the elbow. In this manner, bearing components  20  and  40  articulate about their bearing surfaces and permit movement of the lower arm. 
     FIG. 4 shows an exploded perspective view of the components of the modular elbow prosthesis of the present invention for use in the constrained mode. In this mode, ulnar component  10 , humeral component  30  and pin  50  of the unconstrained configuration are utilized in conjunction with a third bearing component  60 . Third bearing component  60  includes a generally cylindrical member  61  having a first end  62 , a second end  63  and a bore or opening  64  extending therethrough. Cylindrical member  61  is connected to base  65 . A flange  66  is connected to base  65  opposite cylindrical body  61 . Flange  66  is configured to mate with slot  16  of body  15  in the same way as previously described for first bearing component  20 . A locking nubbin  66   a  is formed on flange  66 . First bearing component  20 , second bearing component  40  and third bearing component  60  may all be made from surgical metal or plastic, such as ultra-high molecular weight polyethylene. 
     To implant the modular elbow of the present invention in the constrained mode, ulnar component  10  is implanted in the ulna such that stem  11  is located in the intramedullary canal as shown in FIG.  5 . Similarly, stem  31  of humeral component  30  is positioned in the intramedullary canal of the humerus as shown. Third bearing component  60  is then secured to body  15  of ulnar component  10  by sliding flange  66  in groove  16  of body  15  until locking nubbin  66 a engages opening  18 . Cylindrical body  61  of third bearing component  60  is then positioned between arms  36  of humeral component  30  such that opening  64  is aligned with openings  37  in arms  36 . Pin  50  is then inserted through one opening  37  in an arm  36 , through opening  64  in third bearing component  60  and through opening  37  in the remaining arm  36  such that locking ring  38  engages slot  52   a . In this manner, third bearing component  60  can articulate about pin  50  between arms  36 . Third bearing component  60  is held in place by pin  50  and arms  36 , rather than the soft tissues of the elbow. 
     Thus, with the present invention, a modular prosthesis kit is provided that allows for intra-operative implantation of a constrained or unconstrained elbow prosthesis. Additionally, if it is necessary to convert an unconstrained prosthesis to a constrained configuration, with the present invention, the change can be made after implantation and without removing the ulnar and humeral components. To do so, the arm is surgically opened and pin  50  and second bearing component  40  are removed from arms  36  on the humeral component. First bearing component  20  is removed from ulnar component  10  by compressing locking nubbin  23   a  and sliding flange  23  out of slot  16 . Third bearing component  60  is then secured to ulnar component  10  by inserting flange  66  into slot  16 . Cylindrical body  61  is then positioned between arms  36  such that opening  64  is aligned with openings  37  in arms  36 . A pin  50  is then inserted through the openings in arms  36  and third bearing component  60  and locked in place with lock ring  38 . In this manner, a surgeon is provided with a modular elbow prosthesis kit that allows the surgeon to decide, after surgery has begun, whether to utilize a constrained or unconstrained prosthesis. Furthermore, the present invention provides a prosthesis that can be converted from a constrained to an unconstrained configuration, or vice versa, without removing the ulnar and humeral components from the patient. 
     FIGS. 6 and 7 show an alternative embodiment of the second bearing component attached to humeral component  30 . Second bearing component  140  includes a generally cylindrical member having a first and  141  and a second and  142 . The middle segment  143  of second bearing component  140  includes a convex outer surface  144 . The outer surface of second bearing component  140  mates with bearing surface  24  of first bearing component  20  when the modular elbow prosthesis is used in the unconstrained mode. Bearing component  140  also includes a central opening or bore  145  extending therethrough. Adjacent second end  142  is an enlarged body or head  146 . A pair of slots or openings  147  are formed in bearing component  140 . 
     To secure bearing component  140  to humeral component  30 , arms  36  are inserted into slots  147  such that openings  37  therein align with opening or bore  145  in bearing component  140 . Openings  37  may be threaded to receive screws  150  to secure bearing component  140  to humeral component  30 . Alternatively, a pin and locking ring arrangement, as described above, may be used. Additional methods of securing bearing component  140  to humeral component  30  may also be used. 
     FIG. 8 is an exploded perspective view of another embodiment of a modular elbow according to the present invention for use in the unconstrained mode. The modular elbow prosthesis comprises ulnar component  210 , first bearing component  220 , humeral component  230 , second bearing component  240 , pin  250  and sleeve  260 . 
     Ulnar component  210  comprises a stem  211  having a first or distal end  212  and a second or proximal end  213 . A portion of stem  211  adjacent second end  213  may be coated with a material  214  to allow bone ingrowth between the stem and ulna. Any of various known bone ingrowth coatings, such as cobalt-chromium or titanium alloys, may be used. A generally rectangular body  215  is secured to second end  213  and has a slot  216  formed therein. Body  215  and slot  216  form a mount for at least one bearing component, as described below. Stem  211  extends from body  215  at an angle thereto. A raised stop member  217  is formed at the juncture of stem  211  and body  215 . An opening  218  is formed in body  215  and is configured to receive a nubbin on first bearing component  220 , as described below. Body  215  further includes a sloped, recessed area or notch  218   a  utilized for guiding the nubbin into opening  218 , as described below. Ulnar component  210  also includes a ridge  219   a  on underside  219  of body  215  and stem  211 . Ridge  219   a  helps stabilize ulnar component  210  in the intermedullary canal of the ulna by preventing rotation. Ulnar component  210  can be made from any surgical alloy, such as cobalt-chrome or titanium. Portions of ulnar component  210  other than or in addition to stem  211  may include a bone ingrowth coating. For example, bone ingrowth coating may be applied to underside  219  of body  215 . 
     First bearing component  220  generally comprises a base  221  with a stop member  222  extending therefrom at a generally ninety degree angle. A flange  223  is formed on base  221  and is configured to mate with slot  216  in body  215  of ulnar component  210 , as described below. A nubbin  223   a  extends from flange  223 . Bearing component  220  further includes a concave, spherical bearing surface  224 . Bearing surface  224  mates with second bearing component  240  as described below, when the modular elbow prosthesis is used in the unconstrained mode. 
     Humeral component  230  includes a stem  231  having a first segment  232  with a first or proximal end  233  and a second segment  234  with a second or distal end  235 . Second segment  234  is integral with first segment  232  and widens with distance from first end  233 . A pair of ears  235   a  is provided adjacent distal end  235  of humeral component  230 . Ears  235   a  help to stabilize humeral component  230  in the intermedullary canal of the humerus by preventing rotation. A pair of spaced apart arms  236  extend from second end  235  at an angle to second segment  234  of stem  231 . Each arm  236  includes an opening  237 . Note that in the embodiment shown, arms  236  are keyed about openings  237  by providing a recessed area  237   a . Arms  236  and openings  237  form a mount for at least one bearing component, as described below. Humeral component  230  may also be made of a surgical alloy, such as cobalt-chrome. 
     Second bearing component  240  includes a first end  241 , a second end  242  and a middle segment  243 . Ends  241  and  242  are keyed with recessed areas  241   a  and  242   a  generally corresponding in shape to arms  236 . This prevents second bearing component  240  from rotating with respect to arms  236 . Middle segment  243  of second bearing component  240  has a convex, spherical bearing surface  244 . Bearing surface  244  of second bearing component  240  mates with bearing surface  224  first bearing component  220  when the modular elbow prosthesis is used in the unconstrained mode, as described below. Second bearing component  240  also includes a central opening or bore  245 . 
     Pin  250  is a generally cylindrical member having a main body portion  251 , a first end  252  and a second end. Second end  253  includes an enlarged head or flange  254 . Head  254  has a larger diameter than openings  237  in arms  236  of humeral component  230  and is shaped to generally correspond to the shape of areas  237   a . A barb  255  is located on body portion  251 . Barb  255  includes a first, angled segment  255   a , a second segment  255   b  generally parallel to body portion  251 , a third, angled segment  255   c  and a fourth, angled segment  255   d . Barb  255  secures pin  250  to sleeve  260  as described below. Pin  250  is preferably made from a metal material. However, other materials can also be utilized. 
     Sleeve  260  is a generally cylindrical member having a bore  261 , an interior wall  262 , an exterior wall  263 , an enlarged head  264  and an edge  265 . Head  264  has a larger diameter than openings  237  in arms  236  of humeral component  230  and is shaped to generally correspond to the shape of areas  237   a . Bore  261  is roughly the diameter of body portion  251  and is smaller in diameter than second segment  255   b . Sleeve  260  is preferably made from ultra high molecular weight polyethylene. However, other materials can also be utilized. 
     To use the elbow prosthesis of the present invention in the unconstrained mode, ulnar component  210  is implanted in an ulna such that stem  211  is located in the intramedullary canal. In a similar fashion, humeral component  230  is inserted in the humerus such that stem  231  is located in the intramedullary canal. Ulnar component  210  and humeral component  230  can be fixed to the ulna and humerus, respectively, with or without bone cement. First bearing component  220  is positioned such that flange  223  is adjacent slot  216  in body  215  of ulnar component  210 . First bearing component  220  is then slid toward stop member  217 . As this occurs, locking nubbin  223   a  engages notch  218   a . As first bearing component  220  is moved into further engagement with ulnar component  210 , locking nubbin  223   a  rides upward along notch  218   a  and is compressed by it. When first bearing component  220  is fully seated in ulnar component  210 , locking nubbin  223   a  is located over opening  218   a  and decompresses. Locking nubbin  223   a  thus extends into opening  218   a  and prevents first bearing component  220  from pulling back out of slot  216  in body  215 . Second bearing component  240  is positioned between arms  236  of humeral component  230  such that opening  245  is aligned with opening  237  and arms  236  extend into recessed areas  241   a  and  242   a.    
     Sleeve  260  is then inserted through an opening  237  in one arm  236 , through opening  245  in second bearing component  240  until head  264  is located in recessed area  237   a . Pin  250  is then inserted through opening  237  in the other arm  236  and into bore  261  of sleeve until head  254  is located in recessed area  237   a . As pin  250  is inserted into bore  261 , edge  265  rides along first segment  255   a  and expands or deforms bore  261  so that barb  255  can be fully located within bore  261 . If an attempt is made to-remove pin  250  from sleeve  260 , barb  250  will cut into or otherwise engage interior wall  262 , thereby resisting removal. 
     Second bearing component  240  is then placed adjacent first bearing component  220  such that their concave and convex surfaces mate. First and second bearing components  220  are held in position by the soft tissues of the elbow. In this manner, bearing components  220  and  240  articulate about their bearing surfaces and permit movement of the lower arm. 
     As shown in FIG. 12, bearing components  220  and  240  articulate to permit movement similar to the flexion-extension type, thus providing a first degree of freedom of motion. 
     As shown in FIGS. 13 and 14, this embodiment of the invention also permits lateral movement between the bearing components. FIG. 13 shows the elbow in a neutral position, i.e., bearing component  220  is centered with respect to bearing component  240 . FIG. 14 illustrates the position of bearings  220  and  240  when the prosthesis moves as indicated by the arrows in FIG.  14 . As can be seen, bearing components  220  and  240  shift with respect to one another while still maintaining contact. Thus, the prosthesis presents a second degree of freedom of motion. 
     Additionally, bearing components  220  and  240  permit axial rotation, as shown in FIGS. 15 through 17, thereby providing a third degree of freedom of motion. FIG. 15 shows the relative position of bearing components  220  and  240  in the neutral position. FIGS. 16 and 17 indicate that as the joint is subject to axial rotation, as indicated by the arrows, the bearing components  220  and  240  shift as indicated. However, because bearing surfaces  224  and  244  are spherical, they maintain contact even during axial rotation. 
     As with the embodiment of FIGS. 1-5, the embodiment of FIG. 8 could also be converted for use in the constrained mode. To do so, bearing components  220  and  240  would be removed and replaced with a bearing component similar to third bearing component  60 . 
     FIG. 18 shows another embodiment of a modular elbow according to the present invention for use in the constrained configuration. In this embodiment, nubbin  66   a  has been removed from flange  66  on bearing component  60 . Notch  18   a  and opening  18  have been eliminated from ulnar component  10 . In this embodiment, flange  366  and slot  316  are configured so as to form a taper lock, commonly referred to as a Morse taper. The taper lock configuration is self-locking and designed to reduce micro motion between flange  366  and slot  316 . While the taper lock configuration may include any one of a variety of acceptable taper angles, an angle of less than ten degrees is preferred. Thus, when flange  366  is fully seated within slot  316 , bearing component  60  is secured to ulnar component  10 . 
     FIG. 19 shows yet another embodiment of a prosthetic elbow for use in the unconstrained configuration. This embodiment is similar to that shown in FIG.  8 . In FIG. 19, a numeral “ 4 ” has been substituted at the beginning of the reference numbers for the numeral “2” to show corresponding elements with FIG.  8 . In this embodiment, arms  436  of humeral component  430  engage recessed areas  441   a  and  442   a  and form a taper lock therewith similar to the taper lock described above. Thus, by fully engaging arms  436  and recessed areas or slots  441   a  and  442   a , bearing component  44   b  is secured to humeral component  430 . Note that this embodiment also utilizes a pin  450  and a sleeve  460  to further secure bearing component  440  to humeral component  430 . The use of pin  450  and sleeve  460  provides a redundant locking function in the event recessed areas  441   a  disengage from arms  436 . Note also that in this embodiment, pin  450  has been provided with an additional barb  455   a  for securing pin  450  within sleeve  460 . Note also that notch  418   a  and opening  418  in ulnar component  410  and nubbin  466   a  (not shown) could be eliminated from bearing component  420  as shown in the embodiment of FIG.  18 . If this is done, flange  423  and slot  416  in ulnar component  410  would be configured so as to form a taper lock to secure those components in a manner similar to that described above. 
     FIG. 20 shows yet another embodiment of a modular elbow for use in the constrained configuration. This embodiment is similar to that shown in FIG.  19  and corresponding numbers indicate corresponding parts. In this embodiment, bearing  420  and bearing  440  have been replaced by bearings  500  and  600 . Bearing  500  includes a body  501  having an opening  502  extending therethrough. A flange  503  is formed on the bottom of body  501 . Ulnar component  410  had been modified to remove opening  418  and notch  418   a . Slot  516  in ulnar component  410  and flange  503  on bearing component  500  are configured to form a taper lock similar to the taper lock formed between flange  366  and slot  316  (FIG.  18 ). Thus, fully inserting flange  503  in slot  516  secures bearing  500  to ulnar component  410 . 
     Bearing  600  includes a pair of arms  601  extending from a base  602 . A pair of openings  603  extend through arms  601 . Bearing component  600  is configured and sized such that it can fit between arms  436  of humeral component  430 . A projection  604  is formed on base  602  of bearing component  600 . Projection  604  engages a corresponding slot or opening (not shown) in second end  35  of humeral component  410  so as to stabilize bearing component  600  between arms  436 . Projection  604  and the opening in second end  435  of humeral component  430  can be configured to form a taper lock to assist in securing bearing component  600  to humeral component  430 . 
     In use, bearing component  600  is positioned between arms  436  such that openings  603  align with openings  437 . Bearing component  500  is secured to ulnar component  410  through the taper lock connection of flange  503  in slot  516 . Arms  601  of bearing component  600  are then positioned about body  501  such that opening  603   s  are aligned with opening  502 . Pin  450  and sleeve  460  are then used to secure bearing component  600  and bearing component  500  in place. 
     Although the present invention has been shown and described in detail the same is to be taken by way of example only and not by way of limitation. Numerous changes can be made to the embodiments shown without departing from the scope of the, invention. For example, the shapes of the various component can be changed, so long as the principals of the operation are maintained. Also, either the pin and locking ring arrangement of FIG. 1 or the pin and sleeve arrangement of FIG. 8 can be used with either set of ulnar and humeral components and bearings. Accordingly, the invention is to be limited only by the terms of the claims appended hereto.