Abstract:
An acetabular component for a prosthetic hip joint generally consists of an acetabular shell and an acetabular liner. The acetabular liner includes a cup portion and at least one constraining portion. The cup portion has a hemispherical or dome-shape that defines a hemispherical cavity for receiving a femoral head of a femoral component. The constraining portions may include extensions for retaining the femoral head in the cavity of the cup portion. The liner further includes cutout portions between the constraining portions. The acetabular liner is rotatable within the acetabular shell to provide a complete range of motion to the femoral component without fear of impingement, subluxation, or dislocation of the femoral head. The constraining portions may include inclined surfaces to induce rotation of the acetabular liner within the acetabular shell.

Description:
BACKGROUND 
   1. Field of the Invention 
   The present invention relates to a constrained liner for a prosthetic hip joint, and, more particularly, to a rotating constrained liner for a prosthetic hip joint. 
   2. Description of the Prior Art 
   Acetabular prostheses generally consist of two separate components, an acetabular shell and an acetabular liner. The shell has a hemispherical shape and is affixed and embedded into a cavity formed in a natural acetabulum of a patient. The liner has a hemispherical shape to mate with an internal cavity of the shell. A low-friction bearing surface is formed along a spherical cavity in the liner and provides an articulation surface for a femoral ball of a hip stem. 
   The shell may be made of a biocompatible metal or metal alloy, and the liner may be made of a polymer, such as ultrahigh molecular weight polyethylene (UHMWPE). Regardless of the materials or geometries, these two components are generally locked together with the liner fitted within the shell and the shell encompassing the external surface of the liner. Once the shell is embedded in bone of the natural acetabulum and the liner has been assembled within the shell, the liner is ready to receive the femoral ball. 
   Hip prostheses can potentially experience impingement, subluxation, and even dislocation after being implanted in the patient. For instance, the spherical femoral ball of the hip stem can become dislocated from the acetabular component. This dislocation can occur from various reasons, such as trauma to the leg or abnormal twisting of the leg. In some instances, an additional surgical procedure is required to remedy dislocation of a prosthetic hip. 
   Due to the potential occurrence of impingement and subluxation, it is desirable to have an acetabular liner that inhibits subluxation and dislocation of the femoral ball from the acetabular component. In some designs, the liner is configured to have more than a hemispherical shape, i.e., the liner encloses and captures more than half of the femoral ball within the spherically shaped cavity of the liner. In some instances, a locking ring is used to lock the femoral ball into the cavity of the acetabular liner. 
   Conventional constrained liners, while providing additional stability to the prosthetic hip joint, inherently reduce the range of motion of the prosthetic hip joint because the femoral neck of the femoral component impinges on the extended portions of the constrained liner which extend beyond the hemispherical shape. 
   Solutions developed to increase the range of motion while still maintaining the advantages of constrained liners, i.e., reduction of impingement, subluxation, and dislocation, is to remove material, e.g., provide two cutouts, from the extended portion of the liner. The cutouts allow the femoral component to move through a range of motion similar to an unconstrained device, yet still maintain the advantages of having a constrained liner because the cutouts effectively leave two constraining portions extending from the liner. The range of motion is only restored to such a state, however, if the femoral component is moving within the cutout area. Therefore, the radial placement, i.e., “clocking,” of the liner is important. 
   SUMMARY 
   The present invention provides a constrained liner for a prosthetic hip joint, and, more particularly, a rotating constrained liner for a prosthetic hip joint. In one embodiment, the rotating constrained liner is adapted to be rotationally connected to an acetabular shell to form an acetabular prosthesis which is inserted into a bone cavity of the natural acetabulum. 
   In one form thereof, the present invention provides a prosthetic joint assembly including a shell; a liner rotatably receivable within the shell and defining perpendicular longitudinal and hemispherical axes including a base portion substantially disposed on a first side of the hemispherical axis; and at least one constraining element projecting from the base portion and substantially disposed on a second side of the hemispherical axis, each constraining element including at least one inclined surface; and a prosthesis including a neck and a head, the head receivable within the liner, whereby contact between the neck and the at least one inclined surface induces rotational movement of the liner within the shell about the longitudinal axis. 
   In another form thereof, the present invention provides an acetabular cup for use with a prosthetic hip joint assembly including a femoral component having a femoral head and a femoral neck including an acetabular shell; and a liner rotatably receivable within the acetabular shell and defining perpendicular longitudinal and hemispherical axes including a base portion substantially disposed on a first side of the hemispherical axis; and at least one constraining element projecting from the base portion and substantially disposed on a second side of the hemispherical axis, each constraining element including at least one inclined surface. 
   In yet another form thereof, the present invention provides an acetabular cup for use with a prosthetic hip joint assembly including a femoral component having a femoral head and a femoral neck including an acetabular shell; and a liner rotatably receivable within the acetabular shell and defining perpendicular longitudinal and hemispherical axes including a base portion substantially disposed on a first side of the hemispherical axis; and means for constraining the femoral head within the liner, the means for constraining including means for inducing rotation of the liner within the acetabular shell about the longitudinal axis upon contact with the femoral neck. 
   In still another form thereof, the present invention provides a prosthetic joint assembly for receiving a prosthesis including a shell; and a liner rotatably receivable within the shell. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above mentioned and other features and objects of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a perspective view of an exemplary rotating constrained liner of the present invention; 
       FIG. 2  is a bottom plan view of the liner of  FIG. 1 ; 
       FIG. 3  is a side plan view of the liner of  FIG. 1  and a fragmentary view of a portion of an acetabular shell, further illustrating impingement by a femoral neck on Zone B of the constraining portion of the liner; 
       FIG. 4  is a side plan view of the liner of  FIG. 1  and a fragmentary view of a portion of an acetabular shell, further illustrating impingement by a femoral neck on Zone A of the constraining portion of the liner; 
       FIG. 5  is a side plan view of the liner of  FIG. 1  and a fragmentary view of a portion of an acetabular shell, further illustrating impingement by a femoral neck on Zone D of the constraining portion of the liner; 
       FIG. 6  is a side plan view of the liner of  FIG. 1  and a fragmentary view of a portion of an acetabular shell, further illustrating impingement by a femoral neck on Zone C of the constraining portion of the liner; 
       FIG. 7  is a cross-sectional view of the liner of  FIG. 1 , taken along line  7 - 7 , further illustrating a femoral head secured within the liner via the constraining portions of the liner; 
       FIG. 8A  is a cross-sectional view of an alternative liner, further illustrating the liner engaged with an acetabular shell; 
       FIG. 8B  is a close-up fragmentary view of a portion of the liner and shell of  FIG. 8A ; and 
       FIG. 9  is a perspective view of the acetabular shell of  FIG. 8A . 
   

   Corresponding reference characters indicate corresponding parts throughout the several views. Although the drawings represent embodiments of the present invention, the drawings are not necessarily to scale and certain features may be exaggerated in order to better illustrate and explain the present invention. The exemplifications set out herein illustrate embodiments of the invention, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
   DETAILED DESCRIPTION 
   The embodiments disclosed below are not intended to be exhaustive or limit the invention to the precise forms disclosed in the following detailed description. Rather, the embodiments are chosen and described so that others skilled in the art may utilize their teachings. 
   Referring now to  FIG. 1 , an exemplary rotating constrained liner  10  is shown including cup portion  11  and a pair of constraining portions  14 . As described below, liner  10  is received within acetabular shell  60  ( FIG. 8A ) of a prosthetic hip joint assembly which additionally includes a femoral prosthesis having a femoral head and a femoral neck. The femoral head of the femoral prosthesis is positioned within liner  10 . Cup portion  11  generally has a partially spherical or dome-shaped body with outer surface  13  and inner surface  12 . Inner surface  12  defines a partial spherical or hemispherical cavity  18  for receiving a femoral head or ball  52  ( FIG. 7 ) of a femoral component  50  ( FIG. 7 ). Inner surface  12  has a concave shape with a smooth articulating wall or surface adapted to articulate with femoral head  52 . Outer surface  13  has a hemispherical or dome shape with a surface that is adapted to engage an inner surface  61  of an acetabular shell  60  (FIGS.  8 A and  9 ). Annular rim  15  extends around an outer perimeter of cup portion  11  along base portion  16 . Base portion  16  includes a distal end with annular platform or ring-shaped surface  17  that provides an entrance way or opening into cavity  18  of cup portion  11 . Two extensions or constraining portions  14  extend outwardly from surface  17 . Preferably, portions  14  are oppositely disposed from one another. Constraining portions  14  each may include Zones A, B, C, and D located on the edge of portion  14  that extends away from cup portion  11 . 
   As shown in  FIGS. 1 and 7 , constraining portions  14  each include inner wall  21  and outer wall  22 . Both walls  21  and  22  circumferentially extend around a circumferential perimeter of cup portion  11 , i.e., around a portion of annular rim  15 . Inner wall  21  may have a smooth surface with a spherical contour and may be arcuately directed toward central longitudinal axis  20 . Similarly, outer wall  22  may have a smooth surface and may be arcuately directed toward central longitudinal axis  20 . 
   Referring now to  FIGS. 1 and 2 , outer wall  22  of each constraining portion  14  extends from Zone B, through Zones A and C, and ends with Zone D. Zones B and D are generally sloped towards Zone E on opposite sides of each constraining portion  14  and generally taper in width from Zone E to Zones A and C, respectively. Zones A and C are generally sloped towards Zones B and D, respectively, and generally slightly taper in width from Zones B and D, respectively, towards apex  23 . In one embodiment, the slope in these zones is constant, gradual, and symmetric with respect to each other, i.e., Zones B and D have an identical slope and Zones A and C have an identical slope. In one embodiment, Zones B and D have a steeper incline or pitch than Zones A and C. In another embodiment, Zones A and C have a steeper incline or pitch than Zones B and D. Zones A, B, C, and D include angled or inclined faces to advantageously induce rotation of liner  10  about central longitudinal axis  20 , as described below. Zones E are substantially flat and disposed around the circumference of annular rim  15  between constraining portions  14 . When femoral neck  51  ( FIG. 7 ) is positioned within Zone E, femoral neck  51  does not impinge on constraining portions  14 . 
   In an exemplary embodiment, constraining portions  14  are opposed to one another, are similarly shaped, and have the same size. Alternatively, constraining portions  14  may be formed in different sizes, e.g., one constraining portion  14  may be larger than the other. In yet another alternative embodiment, constraining portions  14  are not similarly shaped, e.g., one constraining portion  14  may take the general shape as shown in  FIG. 1  and another constraining portion  14  may take the general shape of a rectangle or triangle. 
   As shown in  FIGS. 1 and 2 , constraining portions  14  do not completely circumferentially extend around surface  17 . Instead, two gaps or cutouts  19  are formed between constraining portions  14 . Cutouts  19  are opposed to one another across central transverse axis  25 . 
   Referring now to  FIG. 7 , cup portion  11  has a spherical configuration. Hemispherical axis  30  approximates the hemispherical line through cup portion  11 . Base portion  16  of cup portion  11  may generally be disposed on a first side of hemispherical axis  30 . Constraining portions  14  project from cup portion  11  and are disposed on a second side of hemispherical axis  30  to capture and retain femoral head  52  of femoral component  50 . Inner walls  21  of constraining portions  14  form a partial spherical surface that provides a continuous spherical extension below surface  17  ( FIG. 1 ) and below hemispherical axis  30 . 
   Upon insertion of femoral head  52  in cavity  18  such that outer surface  53  of femoral head  52  can smoothly articulate with inner surface  12  of cup portion  11 , constraining portions  14  radially flex outwardly away from central longitudinal axis  20  to accommodate passage of the diameter of femoral head  52  therebetween. Once femoral head  52  is positioned in cavity  18 , constraining portions  14  resiliently flex back to their original position and capture femoral head  52  within cavity  18 . As such, femoral head  52  is lockingly captured or retained within cavity  18  of cup portion  11 . 
   Upon removal of femoral head  52  from cavity  18  of cup portion  11 , constraining portions  14  radially flex outwardly away from central longitudinal axis  20  to accommodate passage of the diameter of femoral head  52  therebetween. Once femoral head  52  is removed, constraining portions  14  resiliently flex back to their original position. 
   Liner  10  may be made from different biocompatible materials, for example, highly cross-linked UHMWPE, titanium, cobalt chrome alloy, or stainless steel. In an exemplary embodiment, liner  10  is fabricated from a material which allows resilient flexibility of constraining portions  14  for snap-fitting femoral head  52  within cavity  18  in the manner described above. Liner  10  also includes shell/liner interface  40  which allows rotation of liner  10  relative to acetabular shell  60  about central longitudinal axis  20 . 
   As shown in  FIGS. 8A and 8B , liner  10  includes an axial movement retention element, as described below. In one embodiment, the axial movement retention element is protrusion  43  which extends circumferentially around cup portion  11 . Acetabular shell  60 , as shown in  FIGS. 8A ,  8 B, and  9 , includes inner surface  61 , outer surface  62 , cavity  63 , and circumferential edge  64 . Groove  42  is positioned in inner surface  61  and near circumferential edge  64  of acetabular shell  60 . As best shown in  FIG. 8B , in one embodiment, protrusion  43  may include inclined face  44  to facilitate insertion of liner  10  into acetabular shell  60 , as described below. 
   To insert liner  10  into acetabular shell  60 , liner  10  is forced into cavity  63  of acetabular shell  60  with a force sufficient to slightly deform circumferential edge  64  radially outward such that edge  64  moves slightly away from liner  10 . Liner  10  is forced into cavity  63  until protrusion  43  mates with groove  42  in acetabular shell  60 . In an exemplary embodiment, inner surface  61  of acetabular shell  60  contacts outer surface  13  of cup portion  11  of liner  10  upon mating engagement of protrusion  43  with groove  42 . In one embodiment, protrusion  43  includes inclined face  44  to facilitate the initial insertion of liner  10  into cavity  63  of acetabular shell  60 . Inclined face  44  provides a gradual introduction of protrusion  43  into cavity  63  and eases the deformation process of forcing circumferential edge  64  radially outward. In an alternative embodiment, protrusion  43  may take any shape which facilitates insertion of liner  10  into cavity  63  of acetabular shell  60 . The engagement of protrusion  43  in groove  42  allows rotation of liner  10  within acetabular shell  60  with respect to central longitudinal axis  20  while simultaneously preventing relative axial translation between acetabular shell  60  and liner  10  along central longitudinal axis  20 . 
   To reduce the potential wear between liner  10  and acetabular shell  60 , a crosslinked polyethylene bearing may be inserted into groove  42  or a metal-on-metal interface may be used. Alternatively, the axial movement retention element is a locking ring wherein groove  42  may continue to circumferential edge  64  and liner  10  may be inserted into acetabular shell  60  after which a locking ring may be inserted in groove  42  to both axially lock the liner and the acetabular shell and provide a bearing surface formed of crosslinked polyethylene or metal. In another alternative embodiment of the axial movement retention element, shell/liner interface  40  may employ a C-ring configuration to axially lock the liner and the acetabular shell, as fully described in U.S. Pat. No. 5,383,938, the disclosure of which is hereby expressly incorporated herein by reference. 
   In another alternative embodiment, shell/liner interface  40  may include a bayonet lock configuration (not shown) wherein the acetabular shell has a series of radially inwardly-directed protrusions and the liner has a series of cooperating radially outwardly-directed protrusions. The shell protrusions include cutouts between them to accommodate insertion of the liner into the acetabular shell. Once inside the internal cavity of the acetabular shell, the liner is rotated. Such rotation locks the liner to prevent axial displacement along central longitudinal axis  20 . In an exemplary embodiment, the protrusions on the acetabular shell and the liner should be chosen to minimize the possibility of the protrusions on the liner lining up with the cutouts in the acetabular shell which would allow axial translation between the liner and the acetabular shell along central longitudinal axis  20 . 
   Interface  40  may be constructed of any configuration which prevents axial translation of liner  10  and acetabular shell  60  along central longitudinal axis  20  while simultaneously allowing rotational translation of liner  10  and acetabular shell  60  about central longitudinal axis  20 . 
   In operation, as shown in  FIG. 3 , upon femoral neck  51  impinging on Zone B, i.e., femoral neck  51  moving in the general directions of Arrows G or H, a rotation of liner  10  with respect to acetabular shell  60  about central longitudinal axis  20  is induced in the general direction of Arrow AA to force femoral neck  51  to rest in Zone E. Rotation of liner  10  is induced by the inclined sloped surface formed in Zone B. As femoral neck  51  impinges on constraining portion  14  in Zone B, the inclined surface forces liner  10  to rotate in a clockwise direction (clockwise looking towards the bottom of liner  10 ) within acetabular shell  60  and move constraining portion  14  away from femoral neck  51  so that femoral neck  51  rests in Zone E where no impingement of femoral neck  51  on constraining portion  14  occurs. 
   Similarly, as shown in  FIG. 4 , upon femoral neck  51  impinging on Zone A, i.e., femoral neck  51  moving in the general direction of Arrow I, a rotation of liner  10  with respect to acetabular shell  60  about central longitudinal axis  20  is induced in the general direction of Arrow AA to force femoral neck  51  to rest in Zone E. Rotation of liner  10  is induced by the inclined sloped surface formed in Zone A. As femoral neck  51  impinges on constraining portion  14  in Zone A, the inclined surface forces liner  10  to rotate in a clockwise direction within acetabular shell  60  and move constraining portion  14  away from femoral neck  51  so that femoral neck  51  impinges on Zone B. Once femoral neck  51  impinges on Zone B, the action as described above with respect to  FIG. 3  forces femoral neck  51  to rest in Zone E where no impingement of femoral neck  51  on constraining portion  14  occurs. Alternatively, the inclined surface in Zone A may be sufficient to force rotation of liner  10  to where femoral neck  51  rests in Zone E without intermediate impingement on Zone B. 
   As shown in  FIG. 5 , upon femoral neck  51  impinging on Zone D, i.e., femoral neck  51  moving in the general directions of Arrows J or K, a rotation of liner  10  with respect to acetabular shell  60  about central longitudinal axis  20  is induced in the general direction of Arrow BB to force femoral neck  51  to rest in Zone E. Rotation of liner  10  is induced by the inclined sloped surface formed in Zone D. As femoral neck  51  impinges on constraining portion  14  in Zone D, the inclined surface forces liner  10  to rotate in a counterclockwise direction within acetabular shell  60  and move constraining portion  14  away from femoral neck  51  so that femoral neck  51  rests in Zone E where no impingement of femoral neck  51  on constraining portion  14  occurs. 
   Similarly, as shown in  FIG. 6 , upon femoral neck  51  impinging on Zone C, i.e., femoral neck  51  moving in the general direction of Arrow L, a rotation of liner  10  with respect to acetabular shell  60  about central longitudinal axis  20  is induced in the general direction of Arrow BB to force femoral neck  51  to rest in Zone E. Rotation of liner  10  is induced by the inclined sloped surface formed in Zone C. As femoral neck  51  impinges on constraining portion  14  in Zone C, the inclined surface forces liner  10  to rotate in a counterclockwise direction within acetabular shell  60  and move constraining portion  14  away from femoral neck  51  so that femoral neck impinges on Zone D. Once femoral neck  51  impinges on Zone D, the action as described above with respect to  FIG. 5  forces femoral neck  51  to rest in Zone E where no impingement of femoral neck  51  on constraining portion  14  occurs. Alternatively, the inclined surface in Zone C may be sufficient to force rotation of liner  10  to where femoral neck  51  rests in Zone E without intermediate impingement on Zone D. 
   Impingement in Zones A and C, as described above, generally indicate an extreme configuration between shell  60  and femoral component  50 . For example, impingement in Zones A and C may occur when a person crosses their legs or when the person rises from a seated position. Also, impingement in Zones A and C may occur when a person is in the sleep position where the person lays on their side and forms a “FIG.  4 ” with their top leg, i.e., if a person is laying on their left side then the  FIG. 4  is formed with their right leg, thus flexing and internally rotating the hip joint. An extreme configuration may depend on the original respective starting positions of constrained portions  14  and femoral component  50 . 
   In an alternative embodiment, Zones A and B could be a single sloped surface and Zones C and D could be a single sloped surface to facilitate similar movements upon impingement by femoral neck  51 , as described above. 
   Although described above with reference to a prosthetic hip joint assembly, the present invention may be used in a similar manner with a prosthetic shoulder joint assembly. 
   While this invention has been described as having exemplary designs, the present invention may be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains.