Patent Publication Number: US-2004059429-A1

Title: Mechanically attached elastomeric cover for prosthesis

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
     [0001] This application claims priority under 35 USC §119 to U.S. Provisional Patent Application Serial No. 60/411,932, filed on Sep. 20, 2002, U.S. Provisional Patent Application Serial No. 60/426,345, filed on Nov. 15, 2002, and U.S. Provisional Patent Application Serial No. 60/462,327, filed on Apr. 14, 2003, all of which are incorporated herein by reference. 
    
    
     
       FIELD OF THE INVENTION  
       [0002] The present invention relates generally to implantable prostheses including an elastomeric contact surface and methods of making same, and particularly to prostheses having an elastomeric cover mechanically attached thereto.  
       BACKGROUND  
       [0003] Implantable prostheses can be used in surgical procedures to replace joints or parts of joints. For example, an artificial ball can be used to replace the humeral head of a shoulder joint in arthroplasty surgery. The prosthesis may include a soft-cushioned ball (or “head”), typically constructed of a metal ball covered by an elastomeric shell, which replaces the naturally articulating surface of the shoulder joint.  
       [0004] Failure of soft-cushioned heads may arise from dislodgement of the elastomeric shell from the underlying hard ball or from tearing of the elastomeric shell, which can cause total failure of the prosthesis.  
       [0005] An elastomeric shell is typically bonded to a hard ball (which may in some cases be porous), by means of glue, cement or in situ hardening of the elastomer. Available glues and cements have typically failed after a time shorter than the expected useful life of the elastomeric shell or the hard ball, causing dislodgement and shortening the useful life of the implant. In situ hardening on the hard ball has also led to the same result.  
       [0006] The bonded attachment of the elastomeric shell to the hard ball may also lead to tearing of the elastomeric shell while it is still adhered to the hard ball. The tearing may result from the bonded attachment that leaves no possibility for translational motion of any portion of the elastomeric shell with respect to the surface of the hard ball. As a result, uneven pressure on the elastomeric shell can cause tearing and needlessly shorten the useful life of the implant.  
       SUMMARY  
       [0007] In a first general aspect, a prosthetic orthopedic implant assembly includes an implant body and an elastomeric cover connected to said implant body by a mechanical fastener.  
       [0008] Implementations may include one or more of the following features. The mechanical fastener may include a tongue-and-groove connection between the implant body and said elastomeric cover. The tongue-and-groove connection may include at least one indentation formed on an exterior surface of the implant body and at least one protrusion formed on an interior surface of the elastomeric cover, where the at least one protrusion is received in said at least one indentation. The at least one indentation includes at least a partial annular groove formed on the exterior surface of the implant body.  
       [0009] Further advantages are apparent from the description and the drawings. The implementations mentioned herein can be used either individually or collectively in arbitrary combination. The implementations are not to be understood as an exhaustive enumeration, but rather have exemplary character for describing the invention. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0010] The invention is herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the present invention only  
     [0011] In the drawings:  
     [0012]FIG. 1 is a cross-sectional view of a prosthetic orthopedic implant assembly;  
     [0013]FIG. 1A is a cross sectional view of a portion of the implant of FIG. 1, showing a fluid sealing structure of the implant assembly;  
     [0014]FIG. 2A is a bottom view of another prosthetic orthopedic implant assembly;  
     [0015]FIG. 2B is a cross-sectional view of the prosthetic orthopedic implant assembly of FIG. 2A, taken along lines A-A in FIG. 2A;  
     [0016]FIG. 3 is a cross-sectional view of yet another prosthetic orthopedic implant assembly; and  
     [0017]FIG. 4 is a cross-sectional view of still another prosthetic orthopedic implant assembly. 
    
    
     DESCRIPTION OF EMBODIMENTS  
     [0018] As is described more in detail hereinbelow, an elastomeric cover is placed onto an implant body and fixed in place by a mechanical fastener.  
     [0019] Referring now to FIG. 1, a prosthetic orthopedic implant assembly  10  includes an implant body  22 , which may have any degree of rigidity, having an exterior surface  23 , and an elastomeric cover  20  connected to the implant body  22  by a mechanical fastener. Elastomeric cover  20  may serve to reduce contact stress between implant body  22  and a complementary articulating surface  40 . Implant body  22  can be shaped like a ball, and may be useful in a ball and socket joint. A “ball” refers to at least a portion of a three-dimensional object having a convexly curved contour, such as, but not limited to, a sphere, capable of articulation within a “socket” (i.e., a portion of a three-dimensional shape having a concavely curved contour). The ball may have a radius r, whose size may vary depending on where the ball is used and/or according to the patient&#39;s size. For example, the radius of the ball used in an adult patient shoulder may be greater than that used in a hip of the same adult patient. Children may require smaller values of r for a given joint than adults. Further, the portion of the sphere used in the ball also varies depending on where it is to be used. For example 220-280 degrees of a full sphere may be used in a hip while 20-90 degrees of a full sphere may be used in a shoulder.  
     [0020] The material hardness of the elastomeric cover  20  may be between about 60 Shore A to about 65 Shore A. More particularly, the material hardness may be greater than about 60 Shore A, greater than about 65 Shore A, greater than about 70 Shore A, greater than about 75 Shore A, greater than about 80 Shore A, greater than about 85 Shore A, greater than about 90 Shore A, or greater than about 95 Shore A. The material hardness may also be less than about 105 Shore A, less than about 95 Shore A, less than about 90 Shore A, less than about 85 Shore, less than about 80 Short A, less than about 75 Shore A, less than about 70 Shore A, or less than about 65 Shore A. The elastomeric cover  20  may have an elastic modulus of between about 10 to about 150 MPa. More particularly, the material may have an elastic modulus of greater than about 10 MPa, greater than about 30 MPa, greater than about 50 MPa, greater than about 70 MPa, greater than about 90 MPa, greater than about 110 MPa, greater than about 130 MPa, and may have an elastic modulus of less than about 150 MPa, less than about 130 MPa, less than about 110 MPa, less than about 90 MPa, less than about 70 MPa, less than about 50 MPa, or less than about 30 MPa. The outside surface of the elastomeric cover  20  may have a smooth surface or it may have a non-smooth surface, such as some modifications to a generally smooth surface to improve its lubrication quality when in use, e.g., grooves or a slightly rough texture. Scoring the surface slightly with shallow grooves may facilitate lubrication by providing passageways for retaining synovial fluid on the surface.  
     [0021] The implant body  22  can include a lower end  29  at least partially surrounded by a ridge  33 , which may help prevent elastomeric cover  20  from becoming detached from exterior surface  23  during articulation within complementary articulating surface  40 .  
     [0022] Still referring to FIG. 1, the mechanical fastener that fastens elastomeric cover  20  to implant body  22  is a tongue-and-groove connection between implant body  22  and elastomeric cover  20 . The tongue-and-groove connection may include at least one indentation  36  on exterior surface  23  and at least one protrusion  38  on an interior surface  19  of elastomeric cover  20 . The one or more protrusions  38  are designed, constructed and arranged to be received in the one or more indentations  36  in a sufficiently snug manner so as to mechanically secure cover  20  to curved surface  23  of implant body  22  during articulation within complementary articulating surface  40 . The at least one indentation  36  may be formed as an annular groove about the exterior surface  23 , or a partial annular groove. Alternately or additionally, the one or more indentations  36  may be on an interior surface  19  of elastomeric cover  20 , and the one or more protrusions  38  may be on curved surface  23  of implant body  22 .  
     [0023] Referring now to FIG. 1A, a fluid sealing structure  44  of the implant assembly  10  includes a tooth (e.g., at least partial annular in shape) protruding from indentation  36  that sealingly presses against protrusion  38 . Fluid sealing structure  44  prevents ingress of fluid between elastomeric cover  20  and implant body  22 .  
     [0024] Referring now to FIGS. 2A and 2B, a prosthetic orthopedic implant assembly  30  includes an elastomeric cover  20  that can include a lip  35  that faces radially inwards and which may be formed with an at least partial annular groove  37 . Elastomeric cover  20  fits over the exterior surface  23  of implant body  22  and may be fastened thereto with a mechanical fastener, such as but not limited to, a retaining element, e.g., a retaining ring  24 . (The retaining element may have other shapes than a ring shape, such as but not limited to, rectangular. The terms retaining element and retaining ring will be used interchangeably, it being understood that ring does not limit the shape of the retaining element.) Retaining ring  24  may be secured to a non-articulating (bottom) surface  34  by means of one or more fasteners  25 , such as but not limited to, screws, rivets, bolts and the like. Additionally or alternatively, retaining ring  24  may be secured to non-articulating (bottom) surface  34  by welding, bonding or other joining methods. Non-articulating (bottom) surface  34  may be a flat surface. Retaining ring  24  may be formed with an at least partial annular ridge  27  that mates with the at least partially annular groove  37 . (Alternatively, groove  37  may be formed in implant body  22 .)  
     [0025] During articulation of the joint, elastomeric cover  20  articulates (moves) with respect to complementary articulating surface  40 . The attachment of retaining ring  24  to lip  35  and implant body  22  may prevent elastomeric cover  20  from becoming detached from surface  23  of implant body  22  during articulation within complementary articulating surface  40 .  
     [0026] Retaining ring  24  may be constructed of any material, such as the same or different material of which implant body  22  is constructed. Typically implant body  22  is constructed of a ceramic or metallic material, although other materials may be employed as well. Retaining ring  24  may be constructed of a material with the same, less or greater hardness (or other properties) than implant body  22  or elastomeric cover  20 .  
     [0027] Retaining ring  24  may permit a slight amount of local relative translational or rotational motion between elastomeric cover  20  and implant body  22 , such as a result of friction with surrounding tissue (e.g., the socket wall) or when pressure is applied to a particular point on the elastomeric cover  20 . This slight local relative movement can reduce the possibility of tearing the elastomeric cover  20 .  
     [0028] Referring now to FIG. 3, implant assembly  50  may include an implant body  52 , which may have any degree of rigidity, having an exterior (top, in the sense of the drawing) surface  53 , and an elastomeric cover  54  mechanically connectable to the implant body  52 . Implant body  52  may include a plate portion  56  having one or more mounting members  58  protruding from an interior (bottom) surface  60  thereof. Mounting members  58  may be in the form of round pegs for installation in other prosthetic elements or in bone structure, for example. Mounting members  58  may alternatively or additionally include threaded fasteners, threaded inserts, spikes and the like.  
     [0029] The exterior surface  53  of plate portion  56  may include one or more fasteners  62  for fastening elastomeric cover  54  to implant body  52 . Fastener  62  may include an at least partial annular lip  64  generally parallel (although not necessarily parallel) to plate portion  56  and spaced therefrom by an at least partial annular spacer ring  66 . Elastomeric cover  54  may include an inwardly facing lip  68  that snugly fits into the gap between lip  64  and plate portion  56 , thereby mechanically fastening elastomeric cover  54  to implant body  52 .  
     [0030] The at least partially annular lip  64 , plate portion  56  and the at least partial spacer ring  66  may be formed of one integral piece. Optionally, the at least partial annular lip  64 , plate portion  56  and the at least partial spacer ring  66  may be formed of separate pieces, and the elastomeric cover  54  may include a partition wall  70  that snugly fits into the gap between the individual plate portions  56 .  
     [0031] Optionally, implant assembly  50  may include an inner core  72  inwards of elastomeric cover  54 , which is held in place by the mechanical attachment of elastomeric cover  54  to implant body  52 . Inner core  72  may be constructed of a material with different or same properties as elastomeric cover  54 . For example, inner core  72  may be made of a material that is more compliant and resilient than elastomeric cover  54 , which may impart increased compliancy to implant assembly  50 .  
     [0032] Referring now to FIG. 4, implant assembly  80  may include an implant body  82 , which may have any degree of rigidity, having an exterior (top, in the sense of the drawing) surface  83 , and an elastomeric cover  84  mechanically connectable to the implant body  82 . Implant body  82  is generally shaped like a ball, and may be useful in a ball and socket joint.  
     [0033] The mechanical fastener may be a threaded fastener (also referred to as a threaded connection) that fastens elastomeric cover  84  to implant body  82 . The threaded connection may include a threaded ring  86  that mates with complementary formed threads  88  on a stem  89  of implant body  82 . The threaded ring  86  may include internal threads  87  whereas the complementary formed threads  88  are external threads. Alternatively, the threaded ring  86  may include external threads whereas the complementary formed threads  88  may be internal threads.  
     [0034] As similarly shown in FIG. 1, the elastomeric cover  84  may have one or more protrusions  85 , which may be formed as an at least partial ring about elastomeric cover  84 . The threaded ring  86  may be tightened against protrusion  85  to secure elastomeric cover  84  against implant body  82 . The threaded ring  86  may have a chamfer  90  that abuts against protrusion  85 . As similarly shown in FIG. 1A, fluid sealing structure  92  may be provided, such as a tooth (e.g., at least partial annular in shape) protruding from implant body  82  that sealingly presses against protrusion  85 . Fluid sealing structure  92  may help prevent ingress of fluid between elastomeric cover  84  and implant body  82 .  
     Other Embodiments  
     [0035] A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made. Accordingly, other embodiments are within the scope of the following claims.