Abstract:
An acetabular liner insertion guide ( 10 ) aligns a liner ( 12 ) within an acetabular shell. The liner includes a ring ( 16 ) and a penetrable layer ( 14 ). The ring includes a lip configured to rest on an upper surface of the acetabular shell. The ring is also configured to attach to the liner such that an upper surface of the liner is in a plane that is generally parallel to a plane that includes the upper surface of the acetabular shell. The penetrable layer is configured to receive an impactor and overlie the liner. When the insertion guide is placed on the shell and the impactor impacts the liner, the insertion guide separates from the liner and remains on the impactor.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application 60/892,139 filed Feb. 28, 2007. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Related Art 
         [0003]    The present application relates to prostheses, and more particularly relates to devices for inserting and impacting prostheses. 
         [0004]    2. Related Art 
         [0005]    Shells for hip prostheses may use a hard liner inserted into the shell. When inserting the hard liners, it is important to properly align the liner relative to the shell. Misalignment may create problems with micromotion of the liner relative to the shell. In addition, misalignment may create an uneven force distribution around the liner and may significantly reduce the potential for liner fracture. 
         [0006]    CeramTec has developed an adapter that can be used with ceramic liners to ensure proper alignment with a shell before impacting the liner into the shell. However, this adapter is large and cumbersome. The adapter has three flex prongs at its periphery that grasp the edge of a liner at only a limited number of contact points along its periphery. U.S. Pat. No. 6,468,281 describes this method in more detail. A central thumb-activated plunger is used to impact the liner to the shell. Additionally, U.S. Pat. No. 5,169,399 and U.S. Pat. No. 5,571,111 show other alignment guides of the prior art which lack the inventive features of the present invention. 
       SUMMARY 
       [0007]    In one embodiment, a liner guide comprises a capture portion, a positioning portion, and a passageway. The capture portion is configured to capture a liner. The positioning portion is configured to overlie a shell. The positioning portion is configured to concentrically position and rotationally center the liner within the shell. The passageway is configured to receive an impactor such that when the impactor is received through the passageway and impacted. The capture portion releases the liner and the positioning portion directs the liner into the shell. 
         [0008]    Alternatively, another embodiment includes a shell implanted in an acetabulum. 
         [0009]    Another embodiment includes a capture portion having an upper portion and a lip portion. The liner has a diameter. The upper portion is configured to overlie the liner. The upper portion has a diameter approximately equal to the diameter of the liner. The lip portion is generally perpendicular to the upper portion and extends from the outermost edge of the upper portion. The lip portion exerts a generally radially oriented force against a side of the liner. 
         [0010]    An alternative embodiment includes a capture portion comprising an upper portion configured to overlie the liner. A first lip portion is generally perpendicular to the upper portion and extends from the outermost edge of the upper portion. A second lip portion is generally perpendicular to the upper portion and extends from an innermost edge of the upper portion, the lip portions exerting clamping force against the liner. 
         [0011]    Another embodiment includes a positioning portion that includes a flat flange extending perpendicularly from the capture portion. 
         [0012]    Alternatively, the flat flange extends completely around the capture. 
         [0013]    In another embodiment, the liner includes a passageway that includes a first diameter generally equal to the inner diameter of the liner and projections extending inward from the first diameter such that the innermost portions of the inward projections defines a second diameter less than the first diameter and greater than the diameter of an impactor head. 
         [0014]    Alternatively, the inward projections are intermittently projected from the capture portion. 
         [0015]    Another embodiment includes a method of inserting a liner into a shell. A step captures a liner within a liner guide. A step positions the liner within the shell such that the liner is concentrically and rotationally centered. Another step extends an impactor through the liner guide. A step impacts the impactor. 
         [0016]    An alternative method further comprises the step of retaining the liner guide on the impactor. 
         [0017]    Additionally, the shell may be implanted in an acetabulum. 
         [0018]    Alternatively, the capturing step may comprise exerting a radially force around the liner guide against an outer surface of the liner. 
         [0019]    An alternative method includes a capturing step that comprises exerting a clamping force between an inner and outer surface of the liner. 
         [0020]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the embodiments of the present invention and together with the written description serve to explain the principles, characteristics, and features of the invention. In the drawings: 
           [0022]      FIG. 1  is a perspective view of an embodiment of an insertion liner guide attached to a liner; 
           [0023]      FIG. 2  is a partial perspective exploded view of the insertion liner guide and liner of  FIG. 1 ; 
           [0024]      FIG. 3  is a cross-section view of the insertion liner guide of  FIG. 1 ; 
           [0025]      FIG. 4  is a perspective view of another embodiment of an insertion liner guide; 
           [0026]      FIG. 5  is a top view of the insertion liner guide of  FIG. 4 ; 
           [0027]      FIG. 6  is a top view of another embodiment of an insertion liner guide; 
           [0028]      FIG. 7  is a perspective view of another embodiment of an insertion liner guide over an acetabular shell; 
           [0029]      FIG. 8  is a perspective view of another embodiment of an insertion liner guide coupled to a liner; 
           [0030]      FIG. 9  is a top view of the insertion liner guide of  FIG. 8 ; 
           [0031]      FIG. 10  is an example of implantation of a liner in an acetabulum; 
           [0032]      FIGS. 11-14  are diagrams of an embodiment of the steps for implanting a liner in an acetabular shell; 
           [0033]      FIGS. 15A and 15B  are cross sectional views of an embodiment of an insertion liner guide attached to a liner and overlying a shell; 
           [0034]      FIG. 16  is a top view of additional embodiments of an insertion liner guide; and 
           [0035]      FIGS. 17-20  are cross-sectional partial side views of embodiments of insertion liner guides and liners. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0036]    The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
         [0037]    If a liner of an acetabular shell is inadvertently situated crooked, then there is a risk of fracturing the liner, deforming the liner, deforming the shell, and/or compromising the taper lock fit between the two during impaction. If any one of the abovementioned occurs, it is both time consuming to remove debris from the body cavity (in the case of fracture), and financially costly because a replacement liner implant and shell implant must be purchased and used. The surgical procedure takes longer than normal because the surgeon must ensure that all debris is removed (e.g., if a ceramic liner shatters). Such debris can cause significant “grinding” if any non-removed particles get between the new articulating surfaces or into surrounding bone and soft tissue. 
         [0038]    Additionally, if the integrity of the liner or shell is compromised due to misalignment prior to impaction, the acetabular shell must be completely removed from the acetabulum. In other words, if there is any possibility that the metal taper in the shell may have deformed such that it may prevent a good taper lock, the surgeon will need to replace the shell as a conservative measure. If there is no identical replacement prosthesis readily available, compromises may need to be made. 
         [0039]    The problem with this situation is that if good initial stability has been established, then the removal of the deformed shell from the bone to insert another is more invasive, risks poor stability for the second shell, and would cost double (due to the patient needing a total of two implants instead of one to complete the surgery). 
         [0040]    Therefore, there is a need to ensure proper concentric orientation and axial alignment of a liner prior to impaction, in order to reduce: the cost of non-necessary replacement liners and shells, the risk of a lengthy and complicated procedure, and the risk of potential prosthesis failure in the future due to wear accelerated by residual debris or a dislocation caused by a failed taper lock. 
         [0041]    Turning now to the figures,  FIG. 1  is a perspective view of an embodiment of an insertion liner guide  10  attached to a liner  12 . The acetabular liner inserter guide  10  is used to assure alignment of the liner  12  in an acetabular shell. The guide  10  includes a lid  14  and a liner capture ring  16 . The lid  14  may grab onto an impactor to minimize the risk of accidental implantation of a part of the liner inserter guide  10 . A warning label may be a part of the lid  14  to warn a surgeon that the guide  10  is not meant for implantation. The capture ring  16  attaches to the lid  14  and the capture ring  16 . The lid  14  may be slotted to allow an impactor to penetrate the lid  14  while maintaining the integrity of the lid  14 . 
         [0042]    The ring  16  may be made out of PETG (the same material as implant package trays). Such a material may allow the ring  16  to be disposable. The PETG ring  16  by itself, not attached to a lid  14 , may accidentally be implanted (left in undetected). Therefore, the lid  14  featuring a die-cut “X” through the center is attached to the upper surface of the ring  16 . In one embodiment, the lid  14  may be a Tyvek lid similar to the lids of the PETG implant package trays. The Tyvek lid  14  may be attached to the ring  16  in the typical manner that a Tyvek lid is fixed to a PETG implant package tray, namely a heat seal process. 
         [0043]    The Tyvek lid  14  may be highly visible. For example, the lid  14  may be white. Such highly visible guides reduces the chances of accidental implantation. In addition, the Tyvek lid  14  may include markings such as a company logo or may include important instructions such as “Do Not Implant”, “Disposable Hard Bearing Inserter”, reassembly instructions, etc. Such instructions may be printed on the lid  14  in highly visible hues, both increasing the visibility of the guide  10  and the instructions. The die-cut “X” in the Tyvek lid  14  may hold the disposable ring/lid  16  assembly to the shaft of an impactor instrument so it is extracted from the body simultaneously with the impactor instrument. These simple, disposable guides  10  may be included in the packaging of the liner/bearing surface. In addition, additional guides may be slid into the original packaging to protect the sterile field such as would occur when one lid pre-assembled on the hard bearing is dropped on the floor. 
         [0044]    Turning now to  FIG. 2 ,  FIG. 2  is a partial perspective exploded view of the insertion liner guide  10  and liner  12  of  FIG. 1 . The capture ring  16  of the liner guide  10  may be configured to completely overlie an upper portion of the liner  12 . The capture ring  16 , then, may be placed over the liner  12  and snugly fit to the liner  12  such that the lid  14  may be generally parallel to an upper surface  17  of the liner  12 . 
         [0045]    Turning now to  FIG. 3 ,  FIG. 3  is a cross-section view of the insertion liner guide  10  of  FIG. 1 . The capture ring  16  includes an inner ring surface  18  configured to abut an inner surface  20  of the liner  12  and an outer ring surface  22  configured to abut an outer surface  22  of the liner  12 . The distance between the inner ring surface  18  and the outer ring surface  22  is slightly less than the thickness of the liner  12  so that the ring surfaces  18  and  22  may press against the liner  12  when the guide  10  is placed on the liner  12 . The force from the surfaces  18  and  22  against the liner  12  hold the guide  10  to the liner  12 . 
         [0046]    Turning now to  FIG. 4 ,  FIG. 4  is a perspective view of another embodiment of an insertion liner guide  40 . The guide  40  comprises a ring  42  preferably made from a disposable plastics material and having at least some elastic properties. An inner lip  44  and outer lip  46  of the ring  42  extend generally parallel to each other and generally perpendicular to an upper surface of the ring  42 . An outwardly projecting flange  48  and inward projections  50  are generally perpendicular to the lips  44  and  46  and generally parallel to the upper surface. 
         [0047]    The ring  42  may be placed on the rim of a generally hemispherical acetabular liner, to cover a substantial portion or entire portion of the rim. The outwardly projecting flange  48  is configured to rest upon the liner. The inward projections  50  are configured to allow an impactor to be placed within the guide  40 . The ring  42  is formed so as to be low in profile and have a tactile frictional engagement with the liner. The ring  42  is “snapped” to the liner&#39;s rim and is held in there by frictional forces. 
         [0048]    The lips  44  and  46  are generally cylindrical; however, it may be interrupted or vary in shape so as to exhibit different spring properties and cause higher or lower frictional holding forces between the liner and the alignment guide  40 . During insertion of the liner into the acetabular shell, the radially-outwardly extending flange  48  is configured to come in contact with the rim of an acetabular shell and act as a stop means. The combination of the downwardly-depending lip  46  and the radially-outwardly extending flange  48  provides and maintains a predetermined clearance between the liner and the acetabular shell (i.e., a “standoff”). The combination further serves as a soft tissue barrier which prevents overhanging tissue and bio matter from entering the taper lock interface surfaces. 
         [0049]    The predetermined clearance held is preferably selected to be small enough to ensure that the liner and the acetabular shell are both concentrically and axially aligned, and also selected to ensure minimal relative movement between the liner and acetabular shell. However, the predetermined clearance is also preferably selected to be large enough such that the taper lock between the liner and shell does not fully engage. An impactor device having a shaft may extend through the ring  42  and impact the liner into the acetabular shell. 
         [0050]    In a preferred embodiment, the internal diameter of the ring is provided with inward projections  50  as a means for capturing the ring  42  to the shaft of the impactor device. By capturing the ring  42  to the shaft of the impactor, the ring  42  does not become inadvertently lost inside the body cavity but instead remains fixed to the shaft of the impactor after impaction. The ring  42  is generally flexible enough to release the liner from its elastic grip during impaction and allow forces applied to the impactor to close the predetermined distance between the shell and liner and form a good taper lock. Once a good taper lock between the shell and liner is achieved, the impactor tool may be removed from the body cavity, with the alignment guide  40  still attached thereto. The projections for capturing the ring  42  to the impactor device retains and couples the alignment guide  40  to the shaft of the impactor tool until the alignment guide  40  is manually removed. The alignment guide  40  may be sterilized for a later second use, or may be properly disposed of. 
         [0051]    Turning now to  FIG. 5 ,  FIG. 5  is a top view of the insertion liner guide  40  of  FIG. 4 . The liner guide  40  includes an inner passageway  52  configured to receive an impactor. Projections  54  extend into the passageway  52 . An inner diameter  56  of the passageway is narrower than the impactor. An outer diameter  58  of the passageway  52  is greater than the diameter of the impactor and approximately equal to the inner diameter of the liner. Thus, when the impactor is placed within the guide  40 , the projections  54  extend over the impactor head. 
         [0052]    Turning now to  FIGS. 6 and 7 ,  FIG. 6  is a top view of another embodiment of an insertion liner guide  80 .  FIG. 7  is a perspective view of another embodiment of the insertion liner guide  80  over an acetabular shell  100 . The guide  80  comprises a ring  82  preferably made from a disposable plastics material and having at least some elastic properties. An inner lip  84  and outer lip  86  of the ring  82  extend generally parallel to each other and generally perpendicular to an upper surface of the ring  82 . An outwardly projecting flange  88  and inward projections  90  are generally perpendicular to the lips  44  and  46  and generally parallel to the upper surface. As shown in  FIG. 7 , the flange  88  of the guide  80  overlies the shell  100  to rest the liner within the shell such that a top portion of the shell is parallel with the top portion of the liner. 
         [0053]    Turning now to  FIGS. 8 and 9 ,  FIG. 8  is a perspective view of another embodiment of an insertion liner guide  120  coupled to a liner.  FIG. 9  is a top view of the insertion liner guide  120  of  FIG. 8 . In contrast to the guide  80  of  FIGS. 6 and 7 , projections  124  are recessed within the guide  120 . The recessed projections  124  may provide a lower profile embodiment over a liner  122 . The guide  120  comprises a ring  126  preferably made from a disposable plastics material and having at least some elastic properties. An inner lip  128  and outer lip  1306  of the ring  126  extend generally parallel to each other and generally perpendicular to an upper surface of the ring  82 . 
         [0054]    Turning now to  FIG. 10 ,  FIG. 10  is an example of implantation of a liner  140  in an acetabulum  144 . The liner is impacted into a shell  142  implanted in the acetabulum. An impactor  146  includes an impactor head  148  and an impact face  150 . The impact face  150  receives a blow to transmit the force through the impactor  146  and into the impactor head  148 . 
         [0055]      FIGS. 11-14  are diagrams of an embodiment of the steps for implanting a liner  152  in an acetabular shell  156  implanted in an acetabulum  160 . 
         [0056]    The alignment guide  152  is attached to the rim of a liner  152  (if not already assembled prior). An impactor  162  may be inserted through the center of the alignment guide  154  so that the impactor head  164  is within the liner  152 . Engaging means formed on the internal diameter of the alignment guide allows the temporary passage of the ball/head end  164  of the impactor  162 . 
         [0057]    The engaging means on the inside diameter of the alignment guide  154  holds the liner  152  to the impactor  162  as one piece. The liner  152  is lowered into the acetabular shell  156  via the impactor  162 . Alternatively, the liner  152 /alignment guide  154  combination may be inserted by hand into the shell  156  first, and then once the combination is correctly situated within the acetabular shell  156 , the impactor  162  can then be inserted through the alignment guide to finish assembly (as shown in  FIGS. 11-14 ). 
         [0058]    The outer circumferential downwardly-depending lip of the alignment guide  154  centers the liner on the acetabular shell along the rim of the shell ( FIG. 12 ). The liner is spaced from the shell only so far as to prevent a taper lock between the liner and acetabular shell, and provide an otherwise very close spacing needed for good concentric and axial alignment. The radially-outwardly extending flange serves to form a planar contact surface with the rim of the acetabular shell and improve concentricity and axial alignment. 
         [0059]    A force is applied to the impactor  162 . Flexible portions of the alignment guide  154  having elastic properties which hold itself to the liner start to deform under stress. The engaging means on the inside diameter of the alignment guide  154  may or may not deform slightly under the force applied to the impactor  162 . 
         [0060]    Eventually, the force applied to the liner  154  through the impactor  162  overcomes the frictional holding forces between the alignment guide  154  and the liner. The liner slips out of the elastic holding portion of the alignment guide  154 , and the forces applied to the impactor  162  push the liner into the acetabular shell. 
         [0061]    The liner is “snap-locked” into the acetabular shell with a good taper lock. The energy stored in the alignment guide from flexion and distortion during insertion is released. The alignment guide  154  springs upward, and is guided by the shaft of the impactor  162  ( FIG. 14 ). 
         [0062]    Since the alignment guide  154  is formed with a ring shape, the shaft of the impactor  162  prevents the alignment guide from being displaced from the impactor  162 . This ensures that the small alignment guide  154  is not accidentally left within a small body cavity, which is full of blood. 
         [0063]    The alignment guide  154  finally loses all of the energy retained from elastic deformation during the impaction process. The outer diameter of the ball/head portion  164  of the impactor  162  is greater than the internal diameter of the alignment guide  154 . Therefore, the alignment guide  154  is kept on the shaft of the impactor  162  and is not lost within the body cavity. The engaging means on the internal diameter of the alignment guide  154  preferably comprises spring fingers which flex to allow the insertion of the ball/head portion  164  of the impactor  162 , but prevent the alignment guide  154  from inadvertently separating from the impactor  162  after impaction. 
         [0064]    The alignment guide  154  and impactor  162  may be removed from the body cavity together, leaving the liner  152  properly aligned and fixed within the acetabular shell  156  ( FIG. 14 ). 
         [0065]    The alignment guide  152  may be removed from the impactor  162  by pulling it off of the shaft and over the ball/head portion  164 . The engaging means on the internal diameter of the alignment guide  154  (e.g., spring fingers) flex to allow passage of the larger ball/head portion  164  of the impactor  162 . 
         [0066]    Once removed, the alignment guide  154  is then sterilized or properly disposed of. The liner  152  is correctly installed without worry of misimplantation or compromised taper locks. 
         [0067]    The guides may be pre-assembled with a liner (before or after package sterilization) or not pre-assembled with a liner. For instance, the alignment guide may be packaged with a liner by the liner manufacturer, or may otherwise be packaged in a sterile manner by itself and given its own part number. It would be expected that the alignment guide of the present invention will have many various shapes and geometries to accommodate different liner sizes, and so an assorted collection of alignment guides having different sizes or configurations may be packaged together in a sterile manner. The assorted alignment guides may be individually wrapped and placed into a box, or the may all be placed into a single wrap and then placed into a box. 
         [0068]    Turning now to  FIGS. 15A and 15B ,  FIGS. 15A and 15B  are cross sectional views of an embodiment of an insertion liner guide  180  attached to a liner  184  and overlying a shell  182 . To consolidate the number of alignment guides used for a range of liner sizes, the alignment guide may be made reversible so as to accommodate at least two differently-sized liners and shells. By forming multiple alignment guide sizes into a single “universal” alignment guide, the number of pieces per kit is reduced, the tooling and mould costs are reduced, and the total cost for manufacturing and inventory is reduced due to higher quantities. The exact number of liners that may be used with a single alignment guide is limited only by the size of the incision and the possibility of interference with surrounding soft tissue. 
         [0069]    As shown in  FIG. 15A , a first orientation of the liner guide  180  grips the first liner  184  in a first fold  192 . A flat portion  194  of the liner guide  180  overlies the first shell  182 . An impactor  190  may impact the first liner  184  into the shell  182 . The liner guide  180  may also be used with a second liner  188  and shell  186 . The liner  188  is placed within a second fold  196  (as shown in  FIG. 15B ). The second fold  196  in this embodiment is opposite the first flat  194 , but alternatively may be radially displaced from the first fold  192  and first flat  194 . A second flat  198  overlies the second shell  186 . 
         [0070]    The alignment guide may be formed integrally with sterile packaging (e.g., molded into a container with perforations to remove it therefrom). The method of packaging an alignment guide does not affect the scope of the present invention. The alignment guide of the present invention may be advantageously used in combination with liners made of any material known in the art. For instance, liners made of polyethylene, metals, ceramics, or other conventional materials will work equally well with the present invention. The present invention may be a sterilizable permanent fixture to be included in a surgical tool kit, or it may be a disposable, or semi-permanent item. 
         [0071]    The guide preferably covers a total inner and outer circumferential portion of the rim of the liner for tight fitting. Additional examples of some guides are shown in  FIG. 16 . Liner guides  200 - 210  include inward projections  212 - 222  which are configured to retain the impactor within the ring. These embodiments may include an alignment guide formed as a split ring. An alignment guide having an interrupted engagement surface (such as the projections  214 ,  218 ,  220 , and  222 ) so as to form intermittent contact points around the inner. Similarly, intermittent forms may be placed on the outer rim of any one of the liner and shell (i.e., for material savings or design indicia). The length of the projections may extend fully to the middle as the projections  216  and  222 , have shallow projections like projections  212  and  214 , or have an intermediate length like projections  218  and  220 . Additionally, the projections may include folded portions such as the projections  220 . The folded portions may absorb some of the forces of impaction. 
         [0072]    Other ring geometries such as polygons (e.g., octagon) may be employed so long as the alignment guide is adequately configured for temporary attachment to the rim of a liner and does not interfere with soft tissue and bone. The alignment guide may or may not employ a textured surface or other tactile features such as bumps, ridges, or protrusions to provide additional gripping surfaces and/or to vary the flexibility characteristics of the ring (e.g., circular accordion-type ridges). Such tactile features may also be used to increase or decrease the friction between the liner and alignment guide, and may be practical in compensating for large tolerances in liner dimensions. 
         [0073]    Turning now to  FIGS. 17-20 ,  FIGS. 17-20  are cross-sectional partial side views of embodiments of insertion liner guides and liners. Liner guides  240 ,  264 ,  286 , and  306  are attached to liners  250 ,  262 ,  284  and  304 .  FIG. 17  depicts a profile shape where a radial force  248  pointing inward is applied between the liner guide  240  and liner  250 .  FIG. 18  depicts a profile shape where a pinching force  260  is symmetrically applied between the liner guide  264  and liner  260 .  FIG. 19  depicts a profile shape where a pinching force  282  is asymmetrically applied between the liner guide  286  and liner  284 .  FIG. 20  depicts a profile shape where a pinching force  302  is asymmetrically applied between the liner guide  306  and liner  304 . The lengths  242 ,  252 ,  272 , and  292  of the flanges can effect the amount of flexion of the guides  240 ,  264 ,  286 , and  306  when the liner is impacted. Longer flanges would make the liners  240 ,  264 ,  286 , and  306  flex less. Similarly, the depths  246  and  258  may also affect the amount of flexion in the liners  240  and  264 . In addition, the depths of the guides  240 ,  264 ,  286 , and  306  may also effect the profile of the liners and guides. A deeper depth may also limit the relative position of the impactor to the liner, as deeper liner guides allow for less movement of the impactor head under the liner guide. The widths  254 ,  274  and  294  of the top portions are sized to be generally equal to the width of the liner. The width  244  of the radial force only embodiment of  FIG. 17  is restricted such that the diametrically opposite portion of the liner guide  240  should be a distance generally equal to the diameter of the liner away. 
         [0074]    The asymmetric lengths  278 ,  280  and  298 ,  300  of the asymmetric profiles effect the center of the clamping forces  282  and  302 . The asymmetric forces may effect how stable the liner guide attaches to the liner. For example, a liner guide that does not pop off may be adjusted by adjusting the asymmetry. Alternatively, a liner guide that pops off of the liner before impaction may also be adjusted by adjusting the asymmetry. 
         [0075]    The present invention may be formed as an assembly of two or more separate pieces which are made integral (e.g., using heat fusion or adhesion means). Alternatively, the assembly may be formed of a single unitary material such as a homogeneously-moulded ABS or other cheap, preferably biocompatible plastic. The internal engagement means of the present invention may comprise a flexible inner lip, or any number of flexible finger members so as to retain the ring inserter to the head impactor before, during, and after impaction. 
         [0076]    The present invention may be utilized with any modular portion of a hip or shoulder shell, the portion being of any material or geometry, in situations where proper insertion and alignment are critical. Such modular portions may include but are not limited to: liners, lockrings, and adapters. The present invention may also be used in non-medical applications for joining two cups which may or may not be at least partially spherical. 
         [0077]    While the present invention is particularly useful with ceramic liners, it would be equally advantageous to use it with liners of various materials including cobalt chrome, oxidized zirconium, and others as discussed above. 
         [0078]    Indicia of sorts may be incorporated into the alignment guide at various locations. The indicia may comprise corporate logos, trademarks, sizing info, material info, date of manufacture, instructions, warnings, etc. 
         [0079]    The present invention may be incorporated into trial liners in order to make insertion and trial reduction easier. Such trial liners may be adapted for bipolar, tripolar, or multiple articulating joint prostheses. Trial liners may be formed integrally with the alignment guide of the present invention in the form of a co-moulded flange or the like. Alternatively, the present invention may be made integral with a trial liner via a threaded, snap-fit, or other connection feature known in the art. 
         [0080]    The present invention may further be incorporated into the impactor tool, itself. Such an integration may be made in many different ways. If the alignment guide is made of a more rigid material, ball detents may keep it attached to the impactor and facilitate release during impaction. Alternatively, the impactor tool may be formed integrally with the flexible alignment guide of the present invention in the form of a flange or the like. As an alternative, the present invention may be made integral with the impactor tool via a threaded or other connection feature known in the art. If a normal femoral ball head is used for impaction, such a ball head may incorporate the annular alignment guide of the present invention in a similar manner as discussed above. The ball head may then be attached to an impactor shank via a Morse taper, thread, or the like. 
         [0081]    The present invention ensures that a surgeon has correctly aligned and oriented a liner with an acetabular shell prior to impaction. Correct alignment is critical, because if a liner (in particular, ceramic) is misaligned and is then impacted, fracture is almost inevitable. Cleaning a body cavity of small fragments can be a very stressful and time-consuming process. Furthermore, there is no guarantee that all fragments are completely removed, and any remaining pieces will rapidly grind the prosthesis, surrounding soft tissue, and bone. 
         [0082]    In the case of more robust plastic and metallic liners, misalignment prior to impaction may compromise the designed taper-lock fit between the two components, often requiring the removal of the acetabular shell from the bone, and insertion of a new replacement shell and liner. Any burr or deformation accidentally formed in either part may cause the insert to separate from the shell in-situ and lead to failure. 
         [0083]    As various modifications could be made to the exemplary embodiments, as described above with reference to the corresponding illustrations, without departing from the scope of the invention, it is intended that all matter contained in the foregoing description and shown in the accompanying drawings shall be interpreted as illustrative rather than limiting. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims appended hereto and their equivalents.