Abstract:
A modular humeral prostheses for replacement of the humeral head of a humerus. The prosthesis generally comprises a stem for insertion in the medullary canal of the humerus, a head for replacing the natural humeral head, and a connecting member for connecting the head to the stem. Specifically, the connecting member of the prosthesis and the engagement of the connecting member to the head provides varying degrees of eccentricity as desired.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to shoulder prostheses. More particularly, the present invention relates to a modular humeral head with eccentric connector that allows the modular humeral head to be connected to a humeral stem with varying degrees of eccentricity.  
         BACKGROUND OF THE INVENTION  
         [0002]    During the procedure of a shoulder replacement operation, at least a portion of the proximal section of the humeral shaft will be replaced by a prosthesis. Early shoulder prostheses attempted to directly replicate the upper portion of the humerus which they were designed to replace. Thus, like the natural upper portion of the humerus, they were typically unitary structures including a stem to be implanted within the humerus and a head to be positioned within the glenoid cavity of the scapula.  
           [0003]    There are several problems with the a conventional unitary shoulder prosthesis. In order to accommodate the varying morphologies of patients, it is necessary to maintain a large inventory of differently configured prostheses. Not only are prostheses with different sizes of heads and stems required but also prostheses with the head and stem configured with varying radial offsets relative to one another. These various configurations are required in each size category.  
           [0004]    To reduce the required inventory, assorted modular prostheses have been devised. A modular prosthesis generally consists of two parts: a stem that is mounted into the medullary canal of the humerus, and a head component connected in some manner to the stem. The head component replaces the bearing surface of the humeral head to allow the movement of the shoulder. Different stem sizes and head sizes in a modular prosthesis provide the surgeon with some degree of inter-operative flexibility which facilitates reconstruction of the original anatomy of the patient. With a range of stem sizes and a range of head sizes available, the surgeon can choose a particular combination to suit the anatomy of each individual without having to have a large inventory of unitary humeral prostheses.  
           [0005]    However, individual patients may also require differing offsets between the axis of the head and the axis of the stem. For example, in one patient the posterior offset may be greater than in another patient. To meet this need, some prior art prostheses provide a plurality of connecting members for connecting the head to the stem with different offsets. Differently configured connecting members are designed to provide varying degrees of eccentricity, or offset. Thus, although prior art modular prostheses have reduced the need of an inventory of differently configured unitary prostheses, an inventory of differently configured connecting members for connecting the humeral head to the stem is still required.  
           [0006]    Accordingly, there is a need for a modular shoulder prosthesis which allows eccentricity adjustment without requiring an inventory of connecting members.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention relates to shoulder prostheses in which a humeral head, chosen to suit a patient, is attached to a stem chosen to suit the resected humerus of the patient, by means of an intermediate connecting member. The intermediate connecting member can be adjusted to vary the eccentricity of the humeral head to a portion of the connecting member attachable to the stem. Two degrees of eccentricity, preferably, are provided.  
           [0008]    The modular humeral prosthesis generally comprises a stem to be fitted to a resected humerus, a head sized and configured to approximate the humeral head, and an intermediate connecting member for connecting the stem to the head. Preferably, a locking piece is provided to secure the intermediate connecting member to the head. The humeral head has a spherical surface formed about an axis on one side and a flat face on the opposite side. A first degree of eccentricity is provided by an opening positioned eccentrically (i.e., not centered) on the flat face of the head for receiving the connecting member. The opening is preferably a threaded, shallow bore.  
           [0009]    The connecting member is formed of a first portion for connection with the head and a second portion for connection with the stem. A second degree of eccentricity is provided by the relationship of the second portion to the first portion. The first portion, for example, may be a shallow, round plate. The second portion, for example, a male connecting piece of substantially smaller diameter than the plate, is located non-coaxially on the plate and extends generally conically therefrom. Preferably, a locking piece is used to secure the connecting member in place in the humeral head. The locking piece, preferably, is a threaded ring that is inserted over the plate of the connecting member and threaded through the bore in the head. In securing the connecting piece to connect the head to the stem, the plate is loosely inserted into the shallow bore on the humeral head. The connecting member is rotated until the desired eccentricity of the male connecting piece to the humeral head is achieved. The locking piece is inserted and threaded in the shallow bore on the humeral head to lock the connecting member in place in the humeral head. The male connecting piece fits into the conical cavity of the stem for, e.g., a Morse taper fit.  
           [0010]    Thus, a single connecting member may provide a great variation of eccentricity of the humeral head to a portion of the connecting member attachable to the stem. This is especially advantageous in that it allows a surgeon to customize the eccentricity of the humeral head to a patient&#39;s specific morphology without requiring an extensive inventory of modular components. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 a cross-section view of one embodiment of the present invention;  
         [0012]    [0012]FIG. 2 is a bottom view of one embodiment of the humeral head of the present invention;  
         [0013]    [0013]FIG. 3 is a cross-sectional view of one embodiment of the humeral head of the invention;  
         [0014]    [0014]FIG. 4 is a bottom view of one embodiment of the connecting member of the present invention;  
         [0015]    [0015]FIG. 5 is a side view of one embodiment of the connecting member of the present invention;  
         [0016]    [0016]FIG. 6 is a side view of one embodiment of the locking piece of the present invention;  
         [0017]    [0017]FIG. 7 is a top view of one embodiment of the locking piece of the present invention;  
         [0018]    [0018]FIG. 8 is a bottom view of one embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    In the drawing, like structures are provided with like reference numerals.  
         [0020]    [0020]FIG. 1 illustrates a cross-sectional view of a first embodiment of the present invention. A shallow bore, generally  18 , is positioned non-coaxially on a flat face of humeral head  10 . In one embodiment, humeral head  10  locks to a connecting member  12  by a locking piece  14 . Connecting member  12  includes a first portion  15  for connection with the head and second portion  16  for connection with the stem. The first portion  15  may be a shallow, generally flat, round plate. Second portion  16  is a male connecting piece, of substantially smaller diameter than the first portion, located eccentrically on the first portion and extending generally conically therefrom. Second portion  16  preferably fits into a conical cavity of the stem for a Morse taper fit. Locking piece  14  locks the connecting member in place in the humeral head. Preferably, locking piece  14  is a threaded ring that is inserted over the first portion of connecting member  12  and threaded through bore  18  in the head. A peripheral shoulder of the locking piece is provided to engage the first portion of the connecting member.  
         [0021]    To secure connecting member  12  to humeral head  10 , the flat round plate is loosely fit into shallow bore  18  of humeral head  10 . A first degree of eccentricity is achieved by rotating connecting member  12  until the desired offset of second portion  16  to humeral head  10  is achieved. To secure connecting member  12  in place in humeral head  10 , locking piece  14  is inserted and threaded in shallow bore  18  on humeral head.  
         [0022]    As seen in FIG. 2, bore  18  is located eccentrically on the flat bottom surface of humeral head  10 ; that is, the axes of the spherical head and the bore are spaced apart. Bore  18  is of relatively large diameter when compared to the second portion of the connecting member. To receive the first portion of the connecting member, bore  18  is relatively shallow. Optionally, bore  18  is threaded to engage with the locking piece to secure the connecting member in place in humeral head  10  at a desired degree of eccentricity.  
         [0023]    [0023]FIG. 3 shows a cross-section view of the humeral head and depicts how bore  18  may be eccentrically located in humeral head  10 . Threads  30  line bore  18  for engaging with the locking member.  
         [0024]    [0024]FIG. 5 illustrates a side view of connecting member  12 . As is shown in FIG. 5, first portion  15  is relatively shallow while second portion  16  is relatively deep. Thus, first portion  15  is configured to fit into the shallow bore of the humeral head. Second portion  16  is preferably configured for a Morse taper fit with the humeral stem. However, any configuration of the second portion that allows the second portion to engage the humeral stem may be used. For example, second portion could be of a dove-tail slot configuration To provide adjustable positioning of connecting member  12  within the bore while preventing undesirable movement, detent  50  is preferably provided in addition to the locking piece. A series of receiving cavities are provided in the bore of the humeral head for engaging detent  50 . Thus, connecting member  12  may be rotatably adjusted in the bore into predetermined positions.  
         [0025]    As seen in FIG. 6, locking piece  14  is of a depth such that both the first portion of the connecting member and locking piece  14  may be inserted into the shallow bore of the humeral head. The outer circumference of locking piece  14  has threads  60  for threadably engaging the bore of the humeral head. Locking piece secures the connecting member in place within the humeral head at a desired eccentricity of the second portion (for engaging the humeral stem) to the humeral head is achieved.  
         [0026]    [0026]FIG. 7 provides a top view of locking piece  14 . Preferably, locking piece  14  is generally configured as a ring. However, any configuration suitable for securing the connecting member in place in the humeral head may be used. Along the inside circumference of locking piece  14  are notches  74  for engaging a tool for rotating the locking piece.  
         [0027]    As seen in FIG. 8, when the components of the invention are fit together, the second portion for engaging the humeral stem may be adjusted to be positioned centrally or eccentrically on the humeral head. Shallow bore  18  is eccentrically located on the flat face of humeral head  10  and receives first portion  15  of connecting member  12 , which is generally shallow and round. Second portion  16 , in this embodiment, is a generally conical piece located eccentrically upon and extending conically from first plate-like portion  15 . By rotating connecting member  12  within bore  18 , second portion  16  may be adjusted to be located at a desired eccentricity to humeral head  10 . Preferably, locking piece  14  is fit over first portion  15  of connecting member  12  and threaded into bore  18 . A tool may engage notches  74  for adjusting locking piece  14 .  
         [0028]    Thus, as is described above, the eccentricity the humeral head to a portion of the connecting member attachable to the stem may be adjusted in two different ways as provided by the connecting member. First, the bore within the humeral head is non-coaxial with the humeral head. Thus, in selecting the placement of the connecting member within the bore, a first degree of eccentricity is achieved. Second, the second portion of the connecting member is non-coaxial with the first portion of the connecting member. Thus, by rotating the connecting member within the bore, a second degree of eccentricity is achieved.  
         [0029]    While various embodiments in accordance with the present invention have been shown and described, it is understood that the invention is not limited thereto, and is susceptible to numerous changes and modifications as known to those skilled in the art. Therefore, this invention is not limited to the details shown and described herein, and includes all such changes and modifications as encompassed by the scope of the appended claims.