Abstract:
A binary attachment mechanism for a modular prosthesis comprises a body and a stem. The body has a top surface, a bottom surface, an internal surface bounding a bore extending between the top and bottom surface. The stem has a protrusion having an external surface adapted to be received in the bore of the body. Sliding the protrusion into the bore causes the external surface of the protrusion to form discrete, spaced apart, releasable connections with the internal surface of the body.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    Not applicable.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. The Field of the Invention  
           [0003]    The present invention relates generally to modular orthopedic prostheses and, more specifically, to attachment mechanisms for securing components of a modular orthopedic prosthesis.  
           [0004]    2. The Relevant Technology  
           [0005]    Modular orthopedic prostheses offer many advantages to the user. By selecting independent modular components to construct a complete prosthesis, custom fitting of a patient&#39;s specific anatomy or specific bony condition can be accomplished.  
           [0006]    Several attachment mechanisms are known in the art for connecting the components of a modular prosthesis. Generally, any two modular components are connected by one contiguous interface. Even three-piece modular connections typically rely on only one contiguous connection interface between any two modular components.  
           [0007]    Because of the high physiological loads borne by the skeletal structure, orthopedic prostheses are subject to high bending, shear, and torsional loads. Where a single contiguous connection is used to connect components of a modular prosthesis, the applied loads can be localized, thereby increasing the failure at that point. It would therefore be an improvement in the art to provide modular orthopedic prostheses that can better withstand the mechanical service loads by better distributing the loads acting upon the prosthesis.  
           [0008]    Furthermore, one of the advantages of modular orthopedic prostheses is the capacity to select, at the time of surgery, a desired orientation between modular components. Many modular connections known in the art do not facilitate a state of partial assembly that closely replicates the final longitudinal configuration of the prosthesis, where, in the state of partial assembly, the modular components can be freely rotated with respect to each other. It would therefore be another improvement in the art to provide modular prostheses that would accommodate a state of partial assembly that closely replicates the longitudinal configuration of the prosthesis while permitting free relative rotation between the modular components.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    Various embodiments of the present invention will now be discussed with reference to the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope.  
         [0010]    [0010]FIG. 1A is a cross sectional view of a binary attachment mechanism in a disassembled state.  
         [0011]    [0011]FIG. 1B is the binary attachment mechanism shown in FIG. 1A in a partially assembled state.  
         [0012]    [0012]FIG. 1C is the binary attachment mechanism shown in FIG. 1A in a fully assembled state.  
         [0013]    [0013]FIG. 2 is a cross sectional view of an alternate embodiment of an assembled binary attachment mechanism.  
         [0014]    [0014]FIG. 3 is a cross sectional view of another alternate embodiment of an assembled binary attachment mechanism in a disassembled state.  
         [0015]    [0015]FIG. 4 is a cross sectional view of yet another alternate embodiment of an assembled binary attachment mechanism.  
         [0016]    [0016]FIG. 5A is a cross sectional view of still another alternate embodiment of a binary attachment mechanism in a partially assembled state.  
         [0017]    [0017]FIG. 5B is the binary attachment mechanism shown in FIG. 5A in a fully assembled state.  
         [0018]    [0018]FIG. 6 is a cross sectional view of a modular hip implant having components connected together by a binary attachment mechanism.  
         [0019]    [0019]FIG. 7 is a cross sectional view of a modular tibial knee implant having components connected together by a binary attachment mechanism.  
         [0020]    [0020]FIG. 8 is a cross sectional view of a modular intramedullary rod having components connected together by a binary attachment mechanism.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    Referring to one or more of the preferred embodiments of the present invention as depicted in FIGS.  1 - 8 , there are two components, a body  3  and a stem  4 , adapted to connect to each other to form a binary, or two-piece, modular prosthesis assembly. Body  3  and stem  4  may be made from any suitable biocompatible material that can withstand the physiological loads during the lifetime of the implant. Preferentially, body  3  and stem  4  would be made from biocompatible metals, such as titanium alloys, zirconium alloys, cobalt chromium alloys, and stainless steels.  
         [0022]    Body  3  has a bore  2  bounded by an internal surface extending between a top end  24  and a bottom end  28 . The internal surface of bore  2  has an upper socket wall  21  extending from top end  24  to a transition surface  23 . The internal surface of bore  2  further has a lower socket wall  20  extending from transition surface  23  to bottom end  28 . Alternatively, lower socket wall  20  may extend from upper transition surface  23  to a lower transition surface  22  as shown in FIGS.  2 - 5 . In the preferred embodiment, socket wall  21  defines a diameter that is smaller than a diameter defined by socket wall  20  as shown in FIGS.  1 - 4 . Alternatively, the diameter of socket wall  21  is the same as the diameter of socket wall  20  as depicted in FIG. 5. Additionally, bore  2  may include an access hole  26  extending from top end  24  to a shoulder  27  and, correspondingly, upper socket  21  may extend from the shoulder  27  to the upper transition surface  23  as depicted in FIGS. 2 and 7.  
         [0023]    The upper and lower transition surfaces,  23  and  22 , help guide protrusion  1  into bore  2 . Transition surfaces  23  and  22  can be in the form of an internal chamfer as depicted in FIGS. 2 and 4, or in the form of a shoulder as depicted in FIGS. 1 and 5.  
         [0024]    Stem  4  has a protrusion  1  which is the upper end of stem  4 , and protrusion  1  is adapted to slide into the bore  2 . Protrusion  1  has a free end  14  and an external surface  19  descending longitudinally downward from free end  14 . The external surface  19  is comprised of upper surface  11  and lower surface  10 . Alternatively, the external surface  19  of protrusion  1  may include upper transition surface  13  and lower transition surface  12  as depicted in FIG. 1. Furthermore, protrusion  1  includes a female thread  15  extending down from free end  14  to facilitate assembly of body  3  to stem  4 .  
         [0025]    The upper and lower transition surfaces,  13  and  12 , help guide protrusion  1  into bore  2 . Transition surfaces  13  and  12  can be in the form of an internal chamfer as depicted in FIG. 1, or in the form of a shoulder as depicted in FIG. 3.  
         [0026]    To assemble the stem  4  to the body  3 , protrusion  1  is slid partially into the bore  2  as depicted in FIG. 1B. As depicted in FIGS.  1 - 4 , upper surface  11  is sized to slide freely past lower socket wall  20 . With the components  3  and  4  partially assembled, upper surface  11  acts like a trunnion constrained by lower socket wall  20  to define an axis of rotation, permitting the body  3  and the stem  4  to be placed into a desired rotational orientation with respect to each other before final assembly. A threaded fastener  16  is provided as a tool to draw the stem  4  towards the body  3 , thereby drawing the protrusion  1  into the bore  2  to cause the upper surface  11  and lower surface  10  to form simultaneous, discrete, and releasable connections with the upper socket wall  21  and lower socket wall  20 , respectively. The upper surface  11  and upper socket wall  21  define a first connection length  31 , and the lower surface  10  and the lower socket wall  20  define a second connection length  33 . Connection length  31  and connection length  33  are spaced apart by distance  32 .  
         [0027]    The releasable connections may be in the form of a press fit or a self-locking taper. Both the press fit and the self-locking taper provide for frictional biasing between the external surface  19  of the stem and the internal surface of the body. The frictional biasing provides a releasable connection that relies on a recoverable elastic deformation of the mating internal and external surfaces.  
         [0028]    In one embodiment the distance  32  between the releasable connections is generally greater than sum of the connection lengths  31  and  33 , and preferably the distance between the releasable connections is at least greater than the shortest of the connection lengths  31  and  33 . Other distances can also be used. For example, the distance  32  between the connections can be in a range between about 5 mm to about 50 mm or can simply be larger than 5 mm, 10 mm, or 15 mm. By increasing the distance  32  between the connections, reaction forces and stresses associated with the connections are decreased when bending loads act upon the assembled body  3  and stem  4 . Decreased reaction forces and stresses provide for higher performance assemblies that can carry higher bending loads and reduce fretting caused by cyclic loads. Furthermore, the higher performance assembly can enable smaller sizes that sufficiently withstand physiological loads.  
         [0029]    To enable releasable press fit connections in one embodiment, the amount of interference between the surfaces  10  and  11  and the socket walls  20  and  21 , respectively, is less than the radial yield strain of the chosen material, and preferably less than 75% of the radial yield strain. To ensure that a press fit is achieved, the interference between the surfaces  10  and  11  and the socket walls  20  and  21 , respectively, is typically at least 10% of the radial yield strain and preferably greater than 25% of the radial yield strain. For example, provided that the upper surface  11  of stem  4  defines a diameter of 0.500 inch, and provided that the stem  4  and body  3  are made from titanium alloy with 6% vanadium and 4% aluminum, then the yield strain would be approximately 0.0035 inch. Therefore, the preferred interference would be greater than 0.0009 inch and less than 0.0027 inch.  
         [0030]    The connection lengths  31  and  32  should be of sufficient length to produce a connection strength that can withstand physiological loads, yet the connection lengths  31  and  32  must remain short enough to that assembly loads are not excessive. In one embodiment the connection length is in a range between about 0.020 inch and 0.500 inch, and preferably between about 0.040 inch and about 0.100 inch, although other ranges can also be used.  
         [0031]    A self-locking taper may be used in combination with a press fit to form the releasable connections. The self-locking taper may be present at the upper surface  11 B and upper socket  21 B as depicted in FIG. 3A, or the self-locking taper may be present at the lower surface  10 B and lower socket  20 B as depicted in FIG. 4A. Generally speaking, the self-locking taper would have an included angle between 2° and 8°, and preferably the self-locking taper would have an included angle between 3° and 6°. Other angles can also be used.  
         [0032]    An alternate embodiment of the present invention is depicted in FIGS. 5A and 5B. The protrusion includes an undercut  17  positioned between the upper surface  11  and the lower surface  10 . Furthermore, upper surface  11  and lower surface  10  are nominally the same size, and, correspondingly, upper socket wall  21  and lower socket wall  22  are nominally the same size. Where both connections are in the form of a press fit, and where the interference associated with the press fit is nominally the same for both connections, then a certain force would be required to move upper surface  11  to a position above lower socket wall  22 . When upper surface  11  is located above lower socket wall  20  and below upper socket wall  11 , undercut  17  is adapted to provide clearance around lower socket wall  20 . In this arrangement, stem  4  is prevented from inadvertently moving out of body  3 , yet stem  4  is free to rotate with respect to body  3 , thereby allowing the user to create a desired rotation between body  3  and stem  4 . Once the desired rotation is achieved, body  3  can be assembled to stem  4  in the manner previously described.  
         [0033]    Depicted in FIG. 6 is a modular femoral hip implant, wherein a neck  41  is analogous to the body  3  shown in FIGS.  1 - 5 , and a stem  42  is analogous to the stem  4  shown in FIGS.  1 - 5 . The neck  41  is designed to fit into a proximal femur that has a resected femoral head. The stem  42  is designed to fit into the intramedullary canal of the femur. The neck  41  has bore  2  and the stem  42  has protrusion  1 . Frustoconical surface  43  is adapted to carry a spherical ball (not shown) adapted to articulate with a prosthetic or natural acetabulum (not shown). It is appreciated that any of the embodiments depicted in FIGS.  1 - 5  can be substituted to permit secure attachment between neck  41  and stem  42 .  
         [0034]    Depicted in FIG. 7 is a modular tibial knee implant, wherein a plate  51  is analogous to the body  3  shown in FIGS.  1 - 5 , and a stem  52  is analogous to the stem  4  shown in FIGS.  1 - 5 . The plate  51  is designed to fit onto a proximal tibia that has its upper most surface resected. The stem  52  is designed to fit into the intramedullary canal of the tibia. The plate  51  has bore  2  and the stem  52  has protrusion  1 . It is appreciated that any of the embodiments depicted in FIGS.  1 - 5  can be substituted to permit secure attachment between plate  51  and stem  52 .  
         [0035]    Depicted in FIG. 8 is a modular intramedullary rod for stabilizing fractures of long bones. The proximal module  61  is analogous to the body  3  shown in FIGS.  1 - 5 , and a distal module  62  is analogous to the stem  4  shown in FIGS.  1 - 5 . The proximal module  61  and distal module  62  are designed to fit within the intramedullary canal of a long bone, such as a femur, tibia, or humerus. Both the proximal module  61  and the distal module  62  have holes to accommodate interlocking bone screws. If desired, the relative rotational position between the holes  63  in the proximal and distal modules  61  and  62  can be selected at the time of surgery to better align with bone fragments. It is appreciated that any of the embodiments depicted in FIGS.  1 - 5  can be substituted to permit secure attachment between proximal module  61  and distal module  62 .  
         [0036]    The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.