Abstract:
Disclosed is a kit having a set of humeral components used in partial and total joint replacement surgeries. The set of humeral components has a plurality of stems having varying stem diameters and a set of humeral heads having hemispheric surfaces with varying radiuses. Each stem defines a platform having a hole which is a fixed distance from the proximal end of the stem. The distance of the hole from the proximal end of the stem is a function of the stem diameter.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to an apparatus and method for shoulder arthroplasty and, more particularly, to a humeral component and other associated surgical components and instruments for use in shoulder arthroplasty. 
     2. Discussion of the Related Art 
     A natural shoulder joint may undergo degenerative changes due to a variety of etiologies. When these degenerative changes become so far advanced and irreversible, it may ultimately become necessary to replace a natural shoulder joint with a prosthetic shoulder joint. When implantation of such a shoulder joint prosthesis becomes necessary, the natural head portion of the humerus is resected and a cavity is created in the intramedullary canal of the host humerus for accepting a humeral component. The humeral component includes a head portion used to replace the natural head of the humerus. Once the humeral stem component has been implanted, the humeral head is sized and positioned at the scapula head to ensure proper movement. The glenoid may also be resurfaced and shaped to accept a glenoid component. The glenoid component generally includes an articulating surface which is engaged by the head portion of the humeral component. 
     It is generally known in the art to provide a shoulder joint prosthesis having a humeral component, as discussed above. However, the current prior art humeral components along with the associated surgical components and instruments utilized during shoulder arthroplasty suffer from many disadvantages. 
     For example, since the humeral component is subject to various types of loading by the surface of the glenoid component, the humeral component must offer a stable and secure articulating surface. To achieve this, the humeral components provide stems which are inserted into a hole bored into the intermedullary canal. However, such existing stemmed humeral components also exhibit several disadvantages. For example, some of the stemmed humeral components utilize single centered stems of various sizes to stabilize and secure the humeral component to the humerus. While the hemispherical radius of the head can be adjusted to compensate for differences in the stem size, use of such an adjusted head radius does not always provide proper spacing and allow proper joint articulation since the articulation axis of the head does not change with the change in head size. To overcome this, a myriad of designs offer heads having offset pegs and sleeves, thus increasing the complexity of the surgery. Furthermore, the offset head designs require heads having varying hemispherical shape in conjunction with the sleeves to allow for a proper joint functioning. 
     As shown in  FIGS. 1   a  and  1   b , most prior art humeral components rely on stems  100  having a single fixed offset from the proximal end of the stem. The heads  102  to be coupled to the stems  100  are situated so that the articulating surfaces  104  and  104 ′ are parallel. In instances where the head  102  has a large diameter, a portion of the head  102  overlaps the longitudinal axis a significant amount. This is an undesirable condition which leads to improper joint kinematics. The stems  100  may also include extensions which act to anchor the stem within the intermedullary canal once the stem is cemented within the humerus. The proximal end of most humeral components are, thus, generally overlooked to enhance the articulation and are, therefore, generally standardized. 
     What is needed then is a humeral component and associated surgical components for use in a shoulder arthroplasty which does not suffer from the above-mentioned disadvantages. This in turn, will provide a humeral component which is stable and secure, reduces the overall amount of bone tissue required to be removed, reduces the overall surgical time and complexity, increases overall joint articulation, and enhances and increases natural articulation without increasing overall component count. It is, therefore, an object of the present invention to provide such a humeral component and associated surgical components for use in shoulder arthroplasty. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, an apparatus and method for shoulder arthroplasty is disclosed. The apparatus and method employ a humeral component and other associated surgical components for use in shoulder arthroplasty. In this regard, the humeral component is adapted to be implanted into the humerus within the intermedullary cavity and engaged by the bearing surface of a glenoid component. 
     The present invention discloses a kit having a set of humeral components. The set of humeral components has a plurality of stems having varying stem diameters and a set of humeral heads having hemispheric surfaces with varying radiuses. Each stem defines a platform having a hole which is a fixed distance from the proximal end of the stem. The distance of the hole from the proximal end of the stem is a function of the stem diameter. 
     In a further embodiment, the implant includes an articulating head having a first hemispherical radius paired with a first stem. The kit further has a second articulating head having a second hemispherical radius paired with a second stem. Each stem defines a stem center line. The articulating heads are mateable to the stems so that the heads overlap the center line less than a predetermined amount. 
     A further embodiment to the invention is directed toward a modular prosthesis for implant into a humerus of a patient including a proximal head coupling device. The proximal head coupling device is configured to accept heads having an articulating surface. The head is coupled to the proximal head coupling device such that the articulating surface is offset a predetermined distance from a feature of the proximal head coupling device. 
     The present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIGS. 1   a  and  1   b  represent a prior art humeral system; 
         FIGS. 2   a  and  2   b  represent two humeral prosthetics according to the teachings of the present invention with one shown in phantom; 
         FIGS. 3 and 4  represent exploded perspective views of the humeral prosthetic shown in  FIG. 2 ; 
         FIGS. 5 and 6  represent views of the coupling portion of the prosthetic shown in  FIGS. 3 and 4 ; 
         FIG. 7  is a cross sectional view of the prosthetic according to  FIG. 3  shown implanted in a shoulder; 
         FIG. 8  is a cross sectional view of the prosthetic according to  FIG. 4  shown implanted in a shoulder; and 
         FIGS. 9 and 10  represent side views of an alternate embodiment of the present invention. 
     
    
    
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiments concerning an apparatus and method for shoulder arthroplasty is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. Moreover, while the present invention is directed to a shoulder joint, the present invention may be employed in other joints as well, such as a hip joint. 
     With reference to  FIGS. 2   a - 4  represent a kit which contains a pair of modular humeral prosthesis  10  and  50  are shown. Referring now to the first prosthesis  10 , which has a stem portion  12 , and humeral head  14 . When assembled, the humeral head  14  is joined to the stem  12  by means of a Morse Taper  18 . Components of the Morse Taper  18  are formed on the stem  12  and the head  14 . Disposed between a proximal end  16  and a distal end  17  of the stem  12  is a tapered shaft  19  having a first cross-sectional diameter. Disposed on the proximal end  16  of the stem portion  12  is a coupling portion  21  having a platform  24 . 
     The platform  24  has a lower surface  26  which mates with the resected bone surface  28  as shown in  FIG. 2 . The platform  24  further has an annular upper surface  30  which mates with the head  14 . Defined within the platform  24  is the female portion  32  of Morse Taper  18 . The female portion  32  has an axis  34  that is generally perpendicular to the upper surface  30  of the platform  24 . Further, the axis  34  is defined having a fixed distance  38  to the proximal end  16  of the stem portion  12 . The fixed distance  38  is a function of one of the average cross sectional diameter or cross-sectional area of the tapered shaft  19 . 
     Shown disposed above the shaft  12  is a humeral head  14 . The humeral head  14  defines a hemispherical articulating surface  40  and a planar coupling surface  42 . The coupling surface  42  functions to couple to the upper surface  30  of the platform  24 . Defined in the coupling surface  42  is the male portion  43  of the Morse Taper  18 . It is envisioned that either the stem or the head can have either the male or female side of the Morse Taper. Furthermore, other fixation methods such as threaded members can be incorporated. The hole disposed in the stem  12  is threaded to rotatably receive a threaded head portion. The hemispherical surface  40  functions to rotatably mount to the glenoid (not shown). 
     The head component  14  can be comprised of any number of biocompatible materials, such as but not limited to titanium, cobalt chrome, stainless steel, ceramics or any other material that can serve as a bearing surface. The head component  14  can articulate either on the natural glenoid or on a glenoid component (not shown) such as one made of cobalt chrome or any other suitable biocompatible material in order to provide for a metal-metal articulation. 
     The head component  14  is shown as being substantially hemispherical in shape, although the present invention envisions modifications to the shape shown. For example, the head component  14  can be either a full, greater than full, or partial hemisphere. 
     Anatomical differences between patients dictate the proper diameter of the stem  12  be used to properly mate the stem within the intermedullary canal of the humerus. These anatomical differences further dictate the radius of curvature of the hemispherical surface  40 . 
     Reference is now made to the second prosthesis  50  in  FIG. 2 , which represents a second humeral prosthesis  50  according to the teachings of the present invention. Shown is a second stem portion  44  and second head portion  46 . The second stem portion  44  is defined by a second tapered shaft  51 . Further, the stem has a platform  48  defining a head mating surface  50 . Again defined within the head mating surface is the female portion  52  of the Morse Taper  54 . The axis  53  of the Morse Taper  52  is defined at a second fixed distance  56  of the proximal end  58  of second prosthesis  50 . Further, the axis  34  of the Morse Taper  52  is substantially parallel to the axis  39  for the first prosthetic  10 . The fixed distance  56  is a function of the average diameter or the cross sectional area of the stem. 
     Further shown is the second head portion  46  which has a hemispherical surface  59 . The hemispherical surface  59  has a radius smaller than the radius of the first humeral head  14 . 
     The humeral prostheses  10  and  50  shown in  FIG. 2  are depicted sharing a common center line  60  which corresponds to the center line of the intramedullary canal to which the prostheses are being implanted. As can be seen, the center lines of the axis  34  and  53  are displaced from each other a predetermined distance x and intersect the longitudinal axis  60 . The distance x is a function of the difference of the average tapered stem diameters or cross sectional area of stems  19  and  44 . 
     As can further be seen, each head  14  and  46  have a lateral side  62  which are less than 2 millimeters and preferably less than 2 millimeters in the lateral side from the common center line or longitudinal axis  60  of the humeral prostheses  10  and  50 . This is opposed to the distance from the center line of the prior art systems (see  FIG. 1 ) which overlap the center line more than 2 mm. Furthermore, by adjusting the location of the axis  34  for a large headed implant, the radius of curvature will change to provide a better angle of curvature with respect to the inferior side of the glenoid. 
       FIGS. 3 and 4  represent an exploded perspective view of the humeral prosthesis  10  and  50  shown in  FIG. 2 .  FIG. 3  represents a prosthetic having a 6 millimeter diameter stem, while  FIG. 4  represents a prosthetic having a 15 millimeter diameter stem. Shown also are the upper surfaces  30  and  56 , which are planar. It is envisioned that these surfaces can take any shape such as convex or concave. 
     The stem components  10  and  50  can be configured in any number of shapes (e.g., curved, tapered, conical, cylindrical, radial, fluted, and so forth). The surface finish of the stem components  10  and  50  can be smooth, plasma spray, porous coating, threaded, polished, grit blasted, and so forth). The material comprising the stem components  10  and  50  can be any biocompatible material such as but not limited to titanium, cobalt chrome, stainless steel, ceramics, and so forth. 
     The cross-sectional profile of the stem component  12  may comprise many different shapes, and can either be used in a press-fit or cemented application for implantation. 
       FIGS. 5 and 6  best depict the humeral head coupling platforms  24  and  48 . Depicted is the relationship of the coupling member axes  25  and  53  with respect to the humeral prosthetic center lines  60 . In this regard, the coupling mechanism is a first distance along the longitudinal axis from the first proximal end, while the second coupling mechanism is a second distance from the second proximal end. Shown in these views is the clear offset of the axes  25  and  53  from the proximal ends  22  of the stem portions. The difference in fixed distance can also be measured at any fixed point along the stems. 
       FIGS. 7 and 8  represent implanted humeral prostheses  10  and  50 . Shown are the prosthesis  10  and  50  disposed within the intermedullary canal  66  and  68  of the resected humerus  70  and  72 . In each case, the coupled head portions  14  and  46  are shown in articulating contact with a natural glenoid  74 . It is envisioned that the current system can be used with an artificial glenoid in a total joint replacement. 
     By having the lateral side  62  of the head portions  14  and  46  overlap the common center line  60  (see a) by less than 5 millimeters and more preferably less than 2 millimeters, proper joint articulation can be achieved using no adaptive intermediate components between the head and stems. This significantly reduces the complexity of the prosthesis as well as improves the prosthetic life expectancy and for liability. 
       FIGS. 9 and 10  represent an alternate method for coupling the head portions  14  and  46  to the stem portions  10  and  50 . Shown is an intermediary member  82  which is coupled to the stem by way of a threaded post  84  further defined on the intermediary member  82  is the male side  84  of a Morse Taper  86 . The male side  84  is coupled to the female portion  88  of the Morse Taper  86  defined in the head portions  14  and  46 . 
     The humeral prosthesis  10  is shown with stem  12  having longitudinal axis  25 , and proximal and distal ends  16 . The hemispherical head  14  can be coupled to the proximal end having an apparent diameter of greater than about 51 mm where apparent diameter is the intersection of the head within the centerline. The hemispherical head  14  overlaps the longitudinal axis  25  in the lateral direction less than about 5 mm and preferably less than 2 mm, when the distal end  17  of the stem  12  is extending in the inferior position. 
     Further, the humeral components  10  and  50  can have a diameter greater than about 43 mm; wherein said hemispherical head overlaps the longitudinal axis  25  in the lateral direction less than 5 mm and preferably less than 2 mm when the distal end  17  of the stem  12  is extending in the inferior position. 
     The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications, and variations can be made therein without departing from the spirit and scope of the invention.