Abstract:
A humeral prosthesis includes a first body having a first body articulating surface defining a generally circular outer periphery, and a first centerline axis. The first body having a support surface opposite to the first body articulating surface, the first body is hollow and configured to receive a head of the humerus, and a stem extending away from the support surface, the stem defining a second centerline axis which is coincident with the first centerline axis. The prosthesis further includes a second body in the form of a flange which extends laterally from a portion of the circular outer periphery, the first body articulating surface and a surface of the second body defining a boundary portion between them which is generally smooth and continuous, wherein the second body is located superiorly and the surface of the second body which bounds the first body articulating surface is a second articulating surface.

Description:
[0001]    This application is a continuation of co-pending application Ser. No. 14/449,284, filed on Aug. 1, 2014, which is a continuation of application Ser. No. 13/874,958, filed on May 1, 2013, now U.S. Pat. No. 8,814,943 which issued Aug. 26, 2014, which is a divisional of application Ser. No. 12/421,335, filed on Apr. 9, 2009, now U.S. Pat. No. 8,444,646, which issued on May 21, 2013, which is a divisional of application Ser. No. 10/403,708, filed on Mar. 31, 2003, now U.S. Pat. No. 7,517,364, which issued on Apr. 14, 2009. The disclosures of which are hereby totally incorporated by reference in their entirety. 
         [0002]    Cross reference is made to the following applications: U.S. patent application Ser. No. 10/403,707, filed Mar. 31, 2003, now U.S. Pat. No. 7,527,631, which issued on May 5, 2009 entitled “ARTHROPLASTY SIZING GAGE”, U.S. patent application Ser. No. 10/403,750, filed Mar. 31, 2003, now abandoned, entitled “ARTICULATING SURFACE REPLACEMENT PROSTHESIS”, U.S. patent application Ser. No. 10/403,577, filed Mar. 31, 2003, now abandoned, entitled “MODULAR ARTICULATING SURFACE REPLACEMENT PROSTHESIS”, U.S. patent application Ser. No. 10/403,710, filed Mar. 31, 2003, now U.S. Pat. No. 8,366,713, which issued on Feb. 5, 2013, entitled “ARTHROPLASTY INSTRUMENT AND ASSOCIATED METHOD”, and U.S. patent application Ser. No. 10/403,364, filed Mar. 31, 2003, now U.S. Pat. No. 7,338,498, which issued on Mar. 4, 2008, entitled “PROSTHETIC IMPLANT, TRIAL AND ASSOCIATED METHOD” each of which are incorporated herein by reference. 
     
    
     TECHNICAL FIELD OF THE INVENTION 
       [0003]    The present invention relates generally to the field of orthopaedics, and more particularly, to an implant for use in arthroplasty. 
       BACKGROUND OF THE INVENTION 
       [0004]    The invention relates to implantable articles and methods for implanting such articles. More particularly, the invention relates to a bone prosthesis and a method for implanting the same. 
         [0005]    There are known to exist many designs for and methods of implanting implantable articles, such as bone prostheses. Such bone prostheses include components of artificial joints, such as elbows, hips, knees and shoulders. 
         [0006]    Early designs of implantable articles relied upon the use of cements to anchor the implant. However, the current trend is to use cements to a lesser extent because of their tendency to lose adhesive properties over time and the possibility that cement contributes to wear debris within a joint. 
         [0007]    Recently, implantable bone prostheses have been designed such that they encourage the growth of hard bone tissue around the implant. Such implants are often implanted without cement and the bone grows around surface irregularities, for example, porous structures on the implant. 
         [0008]    One such implantable prosthesis is a shoulder prosthesis. During the lifetime of a patient it may be necessary to perform a total shoulder replacement procedure on a patient as a result of, for example, disease or trauma, for example, disease from osteoarthritis or rheumatoid arthritis. Currently, most implantable shoulder prostheses are total shoulder prostheses. In a total shoulder replacement procedure, a humeral component having a head portion is utilized to replace the natural head portion of the upper arm bone or humerus. The humeral component typically has an elongated intramedullary stem, which is utilized to secure the humeral component to the patient&#39;s humerus. In such a total shoulder replacement procedure, the natural glenoid surface of the scapula is resurfaced or otherwise replaced with a glenoid component that provides a bearing surface for the head portion of the humeral component. 
         [0009]    With the average age of patients requiring shoulder arthroplasty decreasing, device manufacturers are developing bone sparing implants for the initial treatment of degenerative arthritis. Surface replacement prostheses are being developed to replace the articulating surface of the proximal humerus with a minimal bone resection and minimal disruption of the metaphysis and diaphysis. Current designs utilize a semi-spherical articular dome with a small stem for rotational stability. The under surface of the articular head is also semi-spherical and mates with the spherically machined humeral head. 
         [0010]    The need for a shoulder replacement procedure may be created by the presence of one of a number of conditions. One such condition is the deterioration of the patient&#39;s rotator cuff. Specifically, an intact rotator cuff stabilizes the humeral head in the glenoid fossa of a scapula during abduction of the arm. While it is stabilized in such a manner abduction of the arm causes the humeral head to translate only a short distance in the superior direction (e.g. a few millimeters), whereby a space is maintained between the humeral head and the acromion. However, for patients with rotator cuff arthropathy, significantly greater humeral excursion is observed. 
         [0011]    Referring to  FIG. 2 , a healthy long bone in the form of humerus  1  is shown. The humerus  1  includes a healthy humeral head  2 . 
         [0012]    Referring now to  FIG. 3 , a diseased humerus  3  is shown. The diseased humerus  3  includes a diseased or flattened humeral head  4 . Whereas the healthy humeral head  2  of the healthy humerus  1  of  FIG. 2  has a generally hemispherical shape, the flattened humeral head  4  is quite flat and only slightly domed. 
         [0013]    Referring now to  FIGS. 4 ,  5  and  6 , a prior art prosthesis  5  is shown. Referring first to  FIG. 4 , the prosthesis  5  is shown installed on the diseased humerus  3 . The prosthesis  5  is positioned over flattened head or bony defect  4 . The prosthesis  5  includes a hollow generally hemispherical cup  6 . Extending distally from the interior of the cup  6  is a generally conically shaped stem  15  that anchors the prosthesis  5  into the humerus  3 . 
         [0014]    Referring now to  FIGS. 5 and 6 , the prosthesis  5  is shown implanted in a shoulder joint. As shown in  FIG. 5 , the humerus  3  is shown in a position in which the arm is resting against the patient&#39;s torso. Articulating surface of the cup  6  of the prosthesis  5  is shown in contact with the scapula  7 , the clavicle  8 , and the acromion  9 . As can be seen in  FIG. 5 , in this downward position of the humerus  3  the prosthesis  5  provides the articulating surface of cup  6  in contact with the acromion  9 , the clavicle  8 , and the scapula  7  to provide for an acceptable artificial joint in this position. 
         [0015]    However, referring to  FIG. 6 , the humerus  3  is shown abducted in the direction of arrow  10  such that the long bone or humeral centerline  11  is at an angle α of about fifteen (15) degrees with the vertical centerline  12 . As can be seen in  FIG. 6 , on a slight abduction of fifteen degrees, the acromion  9  is positioned outside the articulating surface  6  of the prosthesis  5  causing the acromion  9  to impinge upon the humerus  3  causing great pain to the patient and severely limited motion of the humerus  3 . 
         [0016]    In particular, hyper-translation of the humeral head in the superior direction is observed in patients with massive rotator cuff deficiency, thereby resulting in articulation between the superior surface of the humeral head and both the inferior surface of the acromion and the acromioclavicular joint during abduction of the patient&#39;s arm. Such articulation between these components accelerates humeral articular destruction and the erosion of the acromion and acromioclavicular joint. Moreover, such bone-to-bone contact is extremely painful for the patient, thereby significantly limiting the patient&#39;s range of motion. In short, patients with massive rotator cuff tear and associated glenohumeral arthritis, as is seen in cuff tear arthropathy, may experience severe shoulder pain, as well as reduced function of the shoulder. 
         [0017]    In order to treat patients suffering from cuff tear arthropathy, a number of prostheses and techniques utilizing existing prostheses have heretofore been designed. For example, surgeons heretofore utilized a relatively large humeral head prosthesis in an attempt to completely fill the shoulder joint space. It was believed that such use of a large prosthesis would increase the efficiency of the deltoid muscle, thereby improving motion of the shoulder. However, clinical experience has shown that such use of a large humeral head prosthesis (overstuffs) the shoulder joint thereby increasing soft tissue tension, reducing joint range of motion, and increasing shoulder pain. Moreover, such use of an oversized prosthetic head fails to resurface the area of the greater tubercle of the humerus, thereby allowing for bone-to-bone contact between the greater tubercle and the acromion during abduction of the patient&#39;s arm. 
         [0018]    A number of humeral head bipolar prostheses have also been utilized in an attempt to address the problems associated with cuff tear arthropathy. It was believed that the relatively unstrained motion of the bipolar head would improve shoulder motion. However, heretofore designed bipolar prosthetic heads include relatively large offsets, thereby overstuffing the shoulder joint in a similar manner as described above. Moreover, scar tissue may form around the bipolar head thereby (freezing) the dual articulating motion of the prosthesis that has been known to create a large hemi arthroplasty that likewise overstuffs the shoulder joint. In addition, such bipolar prosthetic heads do not cover the articulating surface between the greater tubercle and the acromion, thereby creating painful bone-to-bone contact between them. 
         [0019]    Yet further, a number of techniques have heretofore been designed in which the relatively rough surface of the greater tubercle is resurfaced with an osteotome or high speed burr. Although this approach results in a smoother tubercle contact surface, relatively painful bone-to-bone articulating contact still occurs, thereby reducing the patient&#39;s range of motion. 
         [0020]    More recently, the assignee of the applicant of the present invention has invented a method and apparatus for performing a shoulder replacement procedure in a treatment of a cuff tear arthroplasty which has been filed in the U.S. Patent and Trademark Office under U.S. application Ser. No. 09/767,473 filed Jan. 23, 2001, hereby incorporated in its entireties by reference in this application. This application provides for a method and apparatus for treating cuff tear arthroplasty utilizing a total shoulder replacement prosthesis. This prosthesis includes an artificial head as well as a stem that extends into a rimmed medullary canal. Such a prosthesis is limited to use with a total shoulder prosthesis and is not suitable for use with bone sparing implants for the initial treatment of the degenerative arthritis. 
         [0021]    What is needed, therefore, is a method and apparatus for performing bone sparing arthroplasty shoulder replacement surgery utilizing bone sparing implants for the initial treatment of degenerative arthritis, which will be useful in the treatment of cuff tear arthroplasty, which overcomes one or more of the aforementioned drawbacks. What is particularly-needed is a method and apparatus for performing a bone sparing implant shoulder procedure that eliminates painful articulation between the great tubercle of the humerus and the acromion. 
       SUMMARY OF THE INVENTION 
       [0022]    The present invention provides for an extended articulation resurfacing shoulder that provides a low-friction prosthetic bearing surface for articulation between the greater tuberosity and the acromion. Such a prosthesis is utilized with a bone sparing minimal resection of a portion of the humeral head. 
         [0023]    The present invention provides for an extended articulation resurfacing shoulder with superior/lateral flange for extended articulation into the coracoacromial arch. 
         [0024]    According to one embodiment of the present invention, a prosthesis for use in performing joint arthroplasty is provided. The prosthesis is to be fitted to a long bone. The prosthesis includes a first body having a first body articulating surface defining a generally circular outer periphery of the first body articulating surface. The first body has a support surface opposed to first body articulating surface. The support surface is adapted to receive the head of the long bone. The prosthesis also includes a second body operably associated with the first body. The second body has a second body articulating surface extending from a portion of the circular outer periphery of the first body articulating surface. 
         [0025]    According to another embodiment of the present invention, a tool kit for preparing a humerus to receive a prosthesis is provided. The prosthesis has a first body having a first articulating surface and an opposed first support surface and has a second body having a second articulating surface and an opposed second support surface. The kit is used to prepare the humerus to receive the prosthesis. The tool kit includes a reamer for preparing a first prepared surface on the humerus. The first prepared surface receives the first support surface. The tool kit also includes a bone cutting tool for preparing a second prepared surface on the humerus. The second prepared surface receives the second support surface. 
         [0026]    According to a further embodiment of the present invention, a method for performing shoulder arthroplasty for an indication of rotator cuff tear arthropathy is provided. The method includes the step of providing a prosthesis with a first body having a first articulating surface and an opposed first support surface and with a second body having a second articulating surface and an opposed second support surface. The method also includes the step of providing a tool kit for preparing a humerus for receiving the prosthesis. The method includes the step of preparing a first prepared surface for cooperation with the first support surface with the tool kit. The method further includes the step of preparing a second prepared surface for cooperation with the second support surface with the tool kit. The method also includes the step of implanting the prosthesis onto the first prepared surface and the second prepared surface. The method also includes the step of providing an instrument for preparing a surface on a long bone, providing a plurality of trials, each of said trials being adapted to mate with the surface, selecting one of the plurality of trials, performing a trial reduction on said one of said plurality of trials, determining if said one of said plurality of trials is satisfactory, performing additional trial reductions as required, selecting one of a plurality of joint prostheses corresponding to one of said plurality of trials based upon the trial reductions, and implanting the selected one prosthesis onto the long bone. 
         [0027]    The technical advantage of the present invention includes the ability to provide a low friction bearing surface between the greater tuberosity and the acromion. For example, according to one aspect of the present invention, a superior/lateral flange extends from a periphery of the hemispherical body of the prosthesis, which flange provides for extended articulation in the coracoacromial arch. Thus, the present invention provides a low friction bearing surface between the greater tuberosity and the acromion. 
         [0028]    According to another embodiment, a humeral prosthesis includes a first body having a first body articulating surface defining a generally circular outer periphery, and a first centerline axis. The first body having a support surface opposite to the first body articulating surface, the first body is hollow and configured to receive a head of the humerus, and a stem extending away from the support surface, the stem defining a second centerline axis which is coincident with the first centerline axis. The prosthesis further includes a second body in the form of a flange which extends laterally from a portion of the circular outer periphery, the first body articulating surface and a surface of the second body defining a boundary portion between them which is generally smooth and continuous, wherein the second body is located superiorly and the surface of the second body which bounds the first body articulating surface is a second articulating surface. 
         [0029]    In yet another embodiment, a kit for use in resurfacing a humeral head includes a humeral prosthesis includes a first body having a first body articulating surface defining a generally circular outer periphery, and a first centerline axis. The first body having a support surface opposite to the first body articulating surface, the first body is hollow and configured to receive a head of the humerus, and a stem extending away from the support surface, the stem defining a second centerline axis which is coincident with the first centerline axis. The prosthesis further includes a second body in the form of a flange which extends laterally from a portion of the circular outer periphery, the first body articulating surface and a surface of the second body defining a boundary portion between them which is generally smooth and continuous, wherein the second body is located superiorly and the surface of the second body which bounds the first body articulating surface is a second articulating surface. The kit further includes a reamer for preparing a first prepared surface on the humerus for receiving the support surface on the first body which is opposite to the first body articulating surface, and a bone cutting tool for preparing a second prepared surface on the humerus for receiving a second support surface which is provided by the second body opposite to the second body articulating surface. 
         [0030]    The technical advantages of the present invention further include the ability to provide for an effective remedy for rotator cuff tear arthropathy as part of a bone saving surgical procedure. For example, according to one aspect of the present invention, a prosthesis is provided which includes a generally hollow hemispherical body which mates with a slightly resected humeral head. Thus, the present invention provides for a surgical procedure with minimal bone loss. 
         [0031]    Other technical advantages of the present invention will be readily apparent to one skilled in the art from the following figures, descriptions and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]    For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in connection with the accompanying drawings, in which: 
           [0033]      FIG. 1  is a plan view of an extended head humeral prosthesis according to the present invention shown in position implanted on a diseased humerus; 
           [0034]      FIG. 2  is a plan view of a healthy humerus; 
           [0035]      FIG. 3  is a plan view of a diseased humerus; 
           [0036]      FIG. 4  is a plan view of a prior art humeral prosthesis; 
           [0037]      FIG. 5  is a plan view of a prior art humeral prosthesis shown in position implanted on a diseased humerus with the humerus shown in the retracted position; 
           [0038]      FIG. 6  is a plan view of a prior art humeral prosthesis shown in position implanted on a diseased humerus with the humerus shown in the extended position with the humerus impinged on the distal acromion; 
           [0039]      FIG. 7  is an enlarged plan view of the extended head humeral prosthesis of  FIG. 1 ; 
           [0040]      FIG. 8  is a plan view of the extended head humeral prosthesis of  FIGS. 1 and 7  shown in position implanted on a diseased humerus and in cooperation with the glenoid cavity shown in the retracted position; 
           [0041]      FIG. 9  is a plan view of the extended head humeral prosthesis of  FIGS. 1 and 7  shown in position implanted on a diseased humerus and in cooperation with the glenoid cavity shown in the abducted position free from impingement in the glenoid cavity; 
           [0042]      FIG. 10  is an enlarged plan view of the extended head humeral prosthesis of  FIGS. 1 and 7 ; 
           [0043]      FIG. 11  is a cross sectional view of the extended head humeral prosthesis of  FIG. 10 ; 
           [0044]      FIG. 12  is an auxiliary view of the extended head humeral prosthesis of  FIG. 10  along the lines  12 - 12  in the direction of the arrows; 
           [0045]      FIG. 13  is a plan view of a reamer assembly to prepare a humeral head for the extended head humeral prosthesis of  FIGS. 1 and 7 ; 
           [0046]      FIG. 14  is a plan view, partially in cross section of a cutting guide for use with a cutter to prepare a humeral head for the extended head humeral prosthesis of  FIGS. 1 and 7 ; 
           [0047]      FIG. 15  is a top view of the cutting guide of  FIG. 14 ; 
           [0048]      FIG. 16  is a plan view of a tool kit for preparing a humeral head for the extended head humeral prosthesis of  FIGS. 1 and 7 ; 
           [0049]      FIG. 17  is a plan view of an end mill for preparing a humeral head for the extended head humeral prosthesis of  FIGS. 1 and 7 ; 
           [0050]      FIG. 18  is a plan view of another cutting guide for use with a cutter to prepare a humeral head for the extended head humeral prosthesis of  FIGS. 1 and 7 ; 
           [0051]      FIG. 19  is a top view of the cutting guide of  FIG. 18 ; 
           [0052]      FIG. 20  is a plan view of another tool kit for preparing a humeral head for the extended head humeral prosthesis of  FIGS. 1 and 7 ; 
           [0053]      FIG. 21  is a plan view of another embodiment of an extended head humeral prosthesis according to the present invention; 
           [0054]      FIG. 22  is a cross sectional view of the extended head humeral prosthesis of  FIG. 21 ; 
           [0055]      FIG. 23  is an auxiliary view of the extended head humeral prosthesis of  FIG. 21  along the lines  23 - 23  in the direction of the arrows; 
           [0056]      FIG. 24  is a plan view of a tool kit for preparing a humeral head for the extended head humeral prosthesis of  FIGS. 21-23 ; and 
           [0057]      FIG. 25  is a process flow chart for a method of performing shoulder arthroplasty surgery according to another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0058]    Embodiments of the present invention and the advantages thereof are best understood by referring to the following descriptions and drawings, wherein like numerals are used for like and corresponding parts of the drawings. 
         [0059]    Referring now to  FIG. 7 , an embodiment of the present invention is shown as prosthesis  20 . The prosthesis  20  is used in performing bone preserving joint arthroplasty. The prosthesis  20  is to be fitted to the head of a long bone. For example, the long bone as shown in  FIG. 7  is in the form of humerus  3 . The prosthesis  20  includes a first body  22  having an articulating surface  24  defining a generally circular outer periphery  26 . The first body  22  also includes a second surface  28  opposed to the articulating surface  24 . The second surface  28  is adapted to receive the head of the humerus  3 . 
         [0060]    The prosthesis  20  further includes a second body  30  operably associated with the first body  22 . The second body  30  has a second body articulating surface  32  extending from a portion  34  of the circular periphery  26  of the first articulating surface  24 . The second body articulating surface  32  is adapted to prevent impingement of the acromion  9  (see  FIG. 1 ) with the humerus  3  when the humerus  3  is in the abducted position (see  FIG. 8 ). 
         [0061]    While the prosthesis of the present invention may be secured to the humerus by securing of the second surface  28  to the humerus and bony ingrowth there between, preferably and as shown in  FIG. 7 , the prosthesis  20  further includes a stem  36  operably associated with the first body  22  or the second body  30 . As shown in  FIG. 7 , the stem  36  is associated with the first body  22 . The stem  36  is adapted to assist in securing the prosthesis  20  to the humerus  3 . As shown in  FIG. 7 , the prosthesis  20  is an integral or one-piece item. It should be appreciated that the prosthesis  20  may be modular. For example, the stem  36  may be a separate component from the first body  22  or the second body  30 . Likewise, the second body  30  may be a separate component from the first body  22 . 
         [0062]    As shown in  FIG. 7 , the first body  22  may have any shape capable for articulating motion with the glenoid cavity (see  FIG. 8 ); however, preferably the articulating surface  24  is generally convex. For example, and as shown in  FIG. 7 , the articulating surface  24  may be generally hemispherical. Likewise, the second surface  28  may likewise be generally hemispherical with the first body  22  being generally a hollow hemisphere. Similarly, the second body articulating surface  32  is preferably convex and may have a generally spherical shape. Likewise, the second body second surface  38  may likewise be generally spherical. The second body  30  may thus also be generally a sector of a hollow sphere. 
         [0063]    To provide for smooth motion of the humerus  3  through the abduction of the humerus  3  with respect to glenoid cavity  14  (see  FIG. 8 ) boundary portion  40  of the prosthesis  20  located between the second articulating surface  32  and the first articulating surface  24  where the second articulating surface  32  extends from the first articulating surface  24  is generally smooth and continuous. 
         [0064]    Referring now to  FIG. 1 , the prosthesis  20  of the present invention is shown installed in the humerus  3  with the adjacent bones of the clavicle  8 , scapula  7  and acromion  9  shown in position. As shown in  FIG. 1 , the humerus  3  is abducted into an angle β with respect to vertical erect body reference line  42 . As can be seen in  FIG. 1 , the acromion  9  extends past the boundary portion  40  separating the first body  22  of the prosthesis  20  from the second body  30  of the prosthesis  20 . Thus, the second body articulating surface  32  is utilized to prevent the acromion  9  from impinging on the humerus  3 . 
         [0065]    Referring now to  FIGS. 8 and 9 , the humerus  3  is again shown in position with the adjacent skeletal structure. In  FIG. 8 , the long bone or humerus  3  is shown in position when the upper arm is in position against the torso. At the humeral position as shown in  FIG. 8 , the angle β between the vertical centerline  42  and humeral centerline  44  is represented by an angle ββ of approximately 15 degrees. In this position of the humerus  3 , it can be seen that the acromion  9  is in contact with the first articulating surface  24  at acromion distal edge or outer edge  13 . 
         [0066]    Referring now to  FIG. 9 , the humerus  3  is shown being abducted in the direction of arrow  46  with the respective angle β equaling approximately 60 degrees. In this abducted position, the outer edge  13  of the acromion  9  is in a position such that the second articulating surface  32  of the second body  30  is used to provide a smooth articulating surface for the outer edge  13  of the acromion  9 . 
         [0067]    Referring now to  FIG. 10 , the prosthesis  20  is shown in greater detail. While the stem  36  of the prosthesis  20  may have any suitable shape capable of providing support for the prosthesis  20  in the humerus  3 , preferably, and as shown in  FIG. 10 , the stem  36  has a general cylindrical shape. The first body  22  as shown in  FIG. 10  may be in the form of a hollow hemisphere having a first longitudinal body centerline  48 . The stem  36  has a stem longitudinal centerline  50 . As shown in  FIG. 10 , the prosthesis  20  may be such that the first longitudinal body centerline  48  and the stem longitudinal centerline  50  are coincident. Such a configuration provides for a preferred central location of the stem  36 . 
         [0068]    Applicants have found that since the diseased humerus may become flattened around the humeral head, to provide adequate support to the prosthesis  20  in a diseased humerus, the support surface opposed to the articulating surface  24  of the prosthesis  20  may include at least part of the second surface  28  to include a support surface  52  opposed to the first articulating surface  24 . Preferably, for simplicity, the support surface may be generally planar. 
         [0069]    To assist the prosthesis  20  in its strength and stability in the humerus  3  and to promote the bony ingrowth around the prosthesis  20 , the prosthesis  20  may include a porous coating  53  secured to, for example, the second surface  28  of the first body  22 , the second surface  38  of the second body, the planar portion  52  of the second surface  28 , as well as on the periphery of the stem  36 . Any commercially available porous coating will assist in the bony ingrowth of the prosthesis  20  to the humerus  3 . One particular porous coating is provided by the assignee of the instant application under the trade name POROCOAT® Porous coating may be more fully understood by reference to U.S. Pat. No. 3,855,638 to Pilliar, hereby incorporated in its entireties by reference. 
         [0070]    As shown in  FIG. 10 , the circular outer periphery  26  of the articulating surface  24  of the first body  22  defines a first plane  54 . Similarly, the second articulating surface  32  of the second body  30  defines a second surface periphery  56 . The second surface periphery  56  defines a second plane  58 . The first plane  54  and the second plane  58  are non-coincident. The first plane  54  and the second plane  58 , therefore, define an angle αα there between. As shown in  FIG. 10 , the angle αα is obtuse. While the angle αα may approach 180 degrees, the angle aa may likewise have a range of about 120 to 180 degrees. As shown in  FIG. 20 , the angle aa may be around 138 degrees. 
         [0071]    Referring now to  FIGS. 11 and 12 , the prosthesis  20  is shown in greater detail. As shown in  FIG. 11 , the stem  36  may be generally cylindrical having stem diameter SD and a length SL. The stem  36  may be tapered and defined by an angle ω. The angle ω may be around 5 to 30 degrees. 
         [0072]    The planar portion  52  of the prosthesis  20  may have any reasonable location with respect to the articulating surface  24  of the prosthesis  20 . The proper position of the planar portion  52  will depend on the flattening of the humeral head and how much corresponding amount of resection may be required to the humeral head. The position of the planar portion  52  with respect to the articulating surface  24  may be defined by a flat dimension FD. 
         [0073]    The hemispherical body  22  and second body  30  of the prosthesis  20  may be defined with respect to a prosthetic center point  60 . The articulating surface  24  may be defined by a radius R 1  extending from center point  60  the articulating surface  24 . The second surface  28  may be defined by a radius R 2  extending from the prosthetic center point  60  to the second surface  28 . Similarly, the second articulating surface  32  may be defined by radius R 4  from the prosthetic center point  60  to the second articulating surface  32 . Similarly, the second surface of the second body  38  may be defined by radius R 3  from the prosthetic center point  60  to the second surface  28 . For simplicity, the radii R 1  and R 4  may be identical and for simplicity the radii R 2  and R 3  may be identical. 
         [0074]    Referring now to  FIG. 12 , the underside of the prosthesis  20  is shown. 
         [0075]    The prosthesis  20  may have any size compatible with the humerus. Preferably, and as shown in  FIGS. 7 through 12 , the prosthesis  20  preferably blends with the periphery of the humerus. Thus, the size of the prosthesis  20  is governed generally by the size of the anatomical humerus. 
         [0076]    The prosthesis  20  may be made of any suitable durable material that is compatible with the human anatomy. For example, the prosthesis  20  may be made of a ceramic, a plastic or a metal. If made of a metal, the prosthesis  20  may be made, for example, of a cobalt chromium alloy, a titanium alloy, or a stainless steel alloy. 
         [0077]    Referring now to  FIG. 16 , an instrument in the form of, for example, a reamer assembly  62  is shown for use in reaming, for example, the articulating surface  24  of the prosthesis  20  of  FIG. 7 . 
         [0078]    As shown in  FIG. 13 , the reamer assembly  62  includes a tool driver  64 . The tool driver  64  includes a body  66  having a cutting tool adaptor  68  and a drive adapter  70 . The reamer assembly  62  also includes a reamer  72 . The reamer  72  is connected to the tool driver  64  by the cutting tool adapter  68 . The reamer  72  may be a hemispherical grater type reamer. 
         [0079]    Referring now to  FIGS. 14 and 15 , a bone cutting tool guide  74  is shown. The tool guide  74  may include a body  76  having a cylindrical bore  78  for receiving a guide pin  80  and an elongated slot  82  for receiving a cutting tool in the form of osteotome  84 . The tool guide  74  is used to prepare surface  88  of humerus  3 . 
         [0080]    Referring now to  FIG. 16 , a tool kit  86  is shown for preparing a humerus to receive a prosthesis, for example, prosthesis  20  of  FIGS. 7 through 12 . 
         [0081]    As shown in  FIGS. 7 through 12 , the prosthesis  20  includes the first body  22  having the first articulating surface  24  and an opposed first surface support surface  28 . The prosthesis  20  also includes the second body  30  that has the second articulating surface  32  and an opposed second support surface  38 . 
         [0082]    Referring again to  FIG. 16 , the kit  86  is used for preparing the humerus  3  to receive the first support surface  28  and the second support surface  38 . The tool kit  86  of  FIG. 16  includes the reamer  72  and may include the tool driver  64  to form the reamer assembly  62 . The tool kit  86  further includes a bone cutting tool  84  for preparing a second prepared surface on the humerus  3  for receiving the second support surface  38  of, for example, the prosthesis  20 . 
         [0083]    As shown in  FIG. 15 , the bone cutting tool  84  is used to prepare second prepared surface  88  on the humerus  3 . As shown in  FIG. 15 , the second prepared surface  88  represents the inside surface of a cylinder. Such a shape on the humerus is necessary to accommodate the inner edge  90  of the second body  30  of the prosthesis  20 . (See  FIG. 11 ). 
         [0084]    Preferably and as shown in  FIG. 16 , the kit  86  further includes the bone cutting tool guide  74  for guiding the osteotome  84  through the elongated slot  82  of the bone cutting tool guide  74 . The osteotome  84  may be struck with mallet  96 . 
         [0085]    Referring now to  FIG. 10 , it should be appreciated that if the angle as approaches 180 degrees, it may be possible to provide for the reaming of the humerus with a reamer assembly  62  (see  FIG. 13 ) and merely pivot the reamer assembly  62  to form a contour for both the second surfaces  28  and  38 . 
         [0086]    It should be appreciated, however, as the angle αα moves from 180 degrees to something less, for example, 140 degrees the inner edge  90  of second body  30  of prosthesis  20  (see  FIG. 11 ) moves closer to the stem centerline  50  such that a generally hemispherical grater-type reamer such as that of  FIG. 13  may not be appropriate to prepare the hemispherical head of the humerus. Therefore, the guide  74  of the kit  86  provides for a surface  88  that is generally cylindrical to permit the corner  90  of the prosthesis  20  to be fully seated into the humerus. 
         [0087]    It should be appreciated that the bone cutting tool necessary to prepare the humerus may include a drill, a reamer, a broach, a saw or an osteotome. 
         [0088]    Referring now to  FIGS. 17 through 20 , an alternate tool kit  186  for preparing a humerus is shown. Referring first to  FIG. 17 , end mill  192  is shown. End mill  192  may be any commercially available milling cutter capable of end cutting. 
         [0089]    Referring now to  FIGS. 18 and 19 , a bone cutting tool guide  174  is shown for use with the end mill  192  of  FIG. 17 . The guide  174  includes a body  176 . The body  176  includes an opening  178  for cooperation with location pin  180 . The location pin  180  is used to orient the guide  174  with respect to the humerus  3 . The guide  174  further defines a plurality of slots  182  formed in the body  176 . 
         [0090]    Referring to  FIG. 19 , the slots  182  extend an angle θθ about the periphery of concave inner surface  194  of the guide  174 . The slots  182  are aligned somewhat parallel to each other and slightly spaced apart. The slots  182  are close enough to minimize the amount of material not covered by the slots but far enough apart to provide enough strength to the guide  174 , particularly around the slots  182 . 
         [0091]    Referring now to  FIG. 20 , the kit  186  is shown. The kit  186  includes reamer assembly  162  for preparing the first prepared surface on the humerus. The reamer assembly  162  is similar to reamer assembly  62  of  FIG. 13  and includes a tool driver  164  similar to the tool driver  64  of  FIG. 13  as well as a grater type hemispherical reamer  172  similar to the reamer  72  of the reamer assembly  62  of  FIG. 13 . The kit  20  further includes end mill  192  as well as an osteotome  184  similar to osteotome  84  of the kit  86  of  FIG. 16 . The osteotome  184  serves to remove the material remaining after the end mill  192  removes materials in alignment with the slots  182  of the guide  174 . Preferably and as shown in  FIG. 20 , the kit  186  may further include the guide  174 . Further, the kit  20  may include a surgical mallet  196 . The surgical mallet  196  is used to strike the osteotome  184 . 
         [0092]    Referring now to  FIGS. 21 ,  22  and  23 , an alternate embodiment of the present invention is shown as prosthesis  120 . Prosthesis  120  is similar to prosthesis  20  of  FIGS. 7 through 12  except that prosthesis  120  is designed to take into consideration the limit in installing the prosthesis onto the prepared humeral head. Mainly, the prosthesis  120  takes into consideration the inability to clear the edge  90  of the prosthesis  20  when utilizing a spherically shaped humeral head. 
         [0093]    Referring now to  FIG. 21 , the prosthesis  120  includes a first body  122  having a hemispherical articulating surface  124  and an opposed concave arcuate surface  128 . The first body  122  also has a planar portion  152  opposing the articulating surface  124 . The prosthesis  120  further includes a tapered cylindrical stem  136  extending inwardly from the planar portion  152  of the first body  122 . The prosthesis  120  further includes a second body  130  extending from circular outer periphery  126  of the first body  122 . A boundary portion  140  is located between the first body  122  and the second body  130  and is preferably smooth and continuous. Unlike the prosthesis  20 , the second body  130  of the prosthesis  120  has an opposed surface in the form of a cylindrical inner periphery  138 . The cylindrical periphery  138  is designed to matingly fit with the cylindrical surface prepared on the humeral head. 
         [0094]    Referring now to  FIG. 22 , the cylindrical periphery  138  is shown in greater detail. The cylindrical periphery  138  is defined by a radius R c  extending from stem centerline  150 . 
         [0095]    Referring now to  FIG. 23 , another view of the cylindrical periphery  135  of the prosthesis  120  is shown. The cylindrical periphery  135  is again defined by a radius R c  from stem centerline  150 . 
         [0096]    Referring now to  FIG. 24 , an alternate tool kit is shown as tool kit  286 . The tool kit  286  is used to prepare a humerus for the prosthesis of the present invention. The tool kit  286  includes a reamer assembly  262  similar to the reamer assembly  162  of the kit  186  of  FIG. 20 . The tool kit  286  further includes a cylindrical end cutting reamer  292  having a cylindrical cutting surface  297  and a guiding surface  298 . The kit  286  further includes a guide  274  for guiding the cylindrical reamer  292  in a proper path to properly prepare the humerus. The guide  274  includes a body  276  defining an opening  278  for cooperation with location pin  280 , which is utilized to properly position the guide  274  on the humerus. The body  276  of the guide  274  further includes an arcuate elongated slot  282  that cooperates with the guiding surface  298  of the reamer  292  to guide the reamer along a proper path. The arcuate slot  282  is defined by an included angle θθθ. 
         [0097]    Referring now  FIG. 25 , an alternate embodiment of the present invention is shown as surgical method  300 . The method  300  includes the first step  302  of providing a prosthesis with a first body having a first articulating surface and an opposed first support surface and with a second body having a second articulating surface and an opposed second support surface. The method  300  further includes a second step  304  providing a tool kit for preparing a humerus for receiving the prosthesis. The method  300  further includes a third step  306  of preparing a first prepared surface for cooperation with the first support surface with the tool kit. The method  300  also includes a fourth step  308  of preparing a second prepared surface for cooperation with the second support surface with the tool kit. The method  300  further includes a fifth step  310  of implanting the prosthesis onto the first prepared surface and the second prepared surface. 
         [0098]    Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made therein without departing from the spirit and scope of the present invention as defined by the appended claims.