Abstract:
A plug for stopping the flow of bone cement in a channel within bone includes an elastomeric inflatable structure protected from bone fragments and sharp edges by a shield extending around the inflatable structure. The shield may be formed as two or more coaxial structures having flexible members extending between opposite ends, with flexible members from an inner such structure curving outward to lie between adjacent flexible members from an outer such structure as the inflatable structure is inflated. The core is also removably connected to an insertion tool, with passageways in the insertion tool and in the core being used to insert a fluid into the inflatable structure. A valve extends as a sleeve around the core, and over the passageway, allowing fluid to flow into the inflatable structure, but preventing it from flowing out of the inflatable structure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This is a continuation-in-part of application Ser. No. 09/590,039, filed Jun. 8, 2000, for which a Notice of Allowance has been received.  
         [0002]    This is also related to application Ser. No. 09/730,972, filed Dec. 6, 2000, which is incorporated herein by reference. 
     
    
     
       BACKGROUND INFORMATION  
         [0003]    1. Field of Invention  
           [0004]    This invention relates to medical apparatus for use in the implantation of a joint prosthesis to the end of a bone and, more particularly, to a plug for stopping the flow of bone cement, used in the attachment of a prosthesis, at a predetermined point within the intramedullary bone channel.  
           [0005]    2. Description of the Related Art  
           [0006]    A number of U.S. Patents describe apparatus for plugging the open end of an intramedullary bone canal to restrict the flow of bone cement during the insertion of bone cement, particularly during the pressurized injection of such a cement during an operative procedure involving the fixation of the stem of an artificial joint prosthesis to the end of a bone such as in the fixation of a hip joint prosthesis to the proximal femur.  
           [0007]    For example, U.S. Pat. No. 4,697,584 describes an inflatable bone plug which is inflated with a fluid capable of escaping from the plug within a relatively short period of time after the bone cement holding the prosthesis in place has hardened to avoid possible weakening of the cortical bone surrounding the inflated plug. This invention also relates to an inflatable bone plug of the above type which is preferably constructed from a silicone elastomer and is pressurized with carbon dioxide gas. However, for substantial inflation to occur in the desired manner, the bone plug is made of an elastomeric material. Such materials, which have elastic properties allowing substantial elongation, are subject to cutting and puncturing due to bone splinters and sharp edges in a channel cut into bone, as the plug is moved into position within the channel. Furthermore, the use of an internal valve associated with a needle for injecting a fluid complicates the manufacture of the inflatable bone plug.  
           [0008]    U.S. Pat. No. 5,849,014 describes a cement restrictor system including an inflatable body, a conduit having first and second ends that defines a fluid passage to and from the inflatable body, and a shield releasably securable to the conduit. In an exemplary method of making a cement plug with the system an obstruction, such as the shield, is placed in a medullary canal of a long bone beyond the isthmus of the long bone. The obstruction is held in place with the inflatable body. A predetermined quantity of bone cement is poured into the medullary canal and localized by the obstruction. The bone cement is allowed to harden; and the conduit and inflatable body are removed from the bone. Again, the inflatable body is elastomeric and is not protected from cutting or puncturing by bone splinters and sharp edges as the restrictor system is moved into place within a bone channel, with the inflatable body preceding the shield. Also, the separate formation of a bone plug from cement and subsequent removal of the conduit and inflatable body to allow insertion of the prosthesis may lengthen the time required for a hip replacement procedure.  
           [0009]    U.S. Pat. No. 5,935,169 describes a bone cement plug including a core having a substantially cylindrically-shaped base portion defining a threaded bore therein extending axially and distally from a proximal end of the base portion; a first leg portion depending from and extending distally from the base portion; and a second leg portion depending from and extending distally from the base portion and opposed to the first leg portion; the base portion threaded bore being adapted to receive an expander screw to wedge apart the first and second leg portions, whereby to expand the core widthwise to secure the plug in the bone canal; and the expander screw, the screw comprising a generally cylindrically-shaped body having a tapered distal end, and a proximal end in which is disposed a threaded bore, external threads disposed on the body, and an annular flange extending outwardly from the proximal end of the body, the screw being threadedly engageable with the core threaded bore for advancement of the screw into the plug for the wedging apart of the first and second legs. A method is also disclosed for using the bone cement plug to compact bone cement into a bone canal during total joint replacement surgeries. However, this type of bone cement plug, having a finite number, such as two, mechanically coupled legs to expand, cannot have the flexibility of a system using an inflatable plug in terms of forming a proper seal within a bone channel which may not be round and smooth, and which varies locally in radius.  
           [0010]    U.S. Pat. No. 5,997,580 describes a cement restrictor including a member or body that is expandable or transitionable from a first diameter to a second diameter. The cement restrictor includes a single or multiple finned body having a first stable state and a second stable state. In the first stable state, the cement restrictor is narrower than in the second stable state. While the cement restrictor is readily transitionable from the first stable state to the second stable state, the transition can be irreversible. An illustrative embodiment of the cement restrictor includes a body having a first end and a second end. Bistable fins extend radially from the body and are irreversibly movable from a first stable state to a second stable state. The fins are concave with respect to the first end of the body in the first stable state and convex with respect to the first end of the body in the second stable state. The diameter of each fin is larger in the second stable state than in the first stable state. Other embodiments of inventive cement restrictors are shown that include shape memory material that changes shape or dimension(s) in response to temperature and/or stress. However, the expansion of the cement restrictor is limited to the transition between the first stable state and the second stable state, together with elastic and plastic deformation of the material. This method thus does not offer the kind of flexibility of a system with an inflatable body in expansion to meet varying conditions within the bone channel. Furthermore, the time required to apply liquids at different temperatures to make the transitionable body perform as desired may increase the time required for hip replacement surgery.  
           [0011]    A number of patents describe bone plugs including a central core from which a number of disks extend as fins at spaced locations. Examples of such devices are found in U.S. Pat. Nos. 5,383,932, 5,662,657, 5,766,178, 5,782,917, 5,861,043, 5,879,403. Such systems allow only deformation of the individual disks to compensate for changes in the shape of the bone channel, such as out-of-round conditions and changes in hole diameter. Therefore, such systems cannot compensate for such conditions to the extent possible with systems including inflatable bodies.  
         SUMMARY OF THE INVENTION  
         [0012]    It is therefore a first objective of the present invention to provide a bone plug having an elastomeric inflatable structure which is protected from puncture by bone splinters and sharp bone edges by a flexible but tough shield extending around the inflatable structure as it is inserted within a bone channel.  
           [0013]    It is a second objective of the present invention to provide a bone plug having a shield structure which can expand greatly without significantly stretching the material from which it is made.  
           [0014]    It is a third objective of the present invention to provide a bone plug having an elastomeric inflatable structure which is easily manufactured.  
           [0015]    It is a fourth objective of the present invention to provide a bone plug and an associated insertion tool which are easily disconnected when the bone plug has been inserted into a proper position.  
           [0016]    According to a first aspect of the present invention, there is provided apparatus for plugging a channel within a bone to stop the flow of bone cement through the channel. The apparatus includes a bone plug and an elongated insertion tool. The bone plug, which is insertable within the channel, includes an inflatable structure, a core attached to the inflatable structure, having a distal portion extending beyond a distal end of the inflatable structure, a shield extending from the distal portion of the core outwardly around and along the inflatable structure, and a valve admitting a fluid into the inflatable structure and preventing a flow of the fluid from the inflatable structure. The core includes a core passageway for the fluid injected into the inflatable. The shield, which is substantially more resistant to damage from sharp objects than the inflatable structure, includes a number of coaxial shield structures, each of which includes a distal end portion, a proximal end portion, and a number of flexible beam portions extending longitudinally along the inflatable structure between the distal end portion and the proximal end portion. The flexible beam portions of the coaxial shield structures curve outwardly with inflation of the inflatable structure, with central portions of the flexible beam portions within each of the coaxial shield structures separating from one another, and with the central portions of the flexible beam portions of one of the coaxial shield structures extending outward between adjacent central portions of another of the coaxial shield structures. The elongated insertion tool, which is removably connected to the core, includes a tool passageway for the fluid injected into the core passageway. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a longitudinal cross-sectional view of a bone plug made in accordance with a first embodiment of the present invention with an inflatable sleeve of the bone plug in a fully deflated condition;  
         [0018]    [0018]FIG. 2 is a longitudinal cross-sectional view of the bone plug of FIG. 1, with the inflatable sleeve in a fully inflated condition;  
         [0019]    [0019]FIG. 3 is a side elevation of a shield of the bone plug of FIG. 1 in an undeflected condition;  
         [0020]    [0020]FIG. 4 is a proximal end elevation of the shield of FIG. 3 in an undeflected condition;  
         [0021]    [0021]FIG. 5 is a side elevation of the shield of FIG. 3 in a fully compressed condition;  
         [0022]    [0022]FIG. 6 is a proximal end elevation of the shield of FIG. 3 in a fully compressed condition;  
         [0023]    [0023]FIG. 7 is a side elevation of the shield of FIG. 3 in a fully extended condition;  
         [0024]    [0024]FIG. 8 is a proximal end elevation of the shield of FIG. 3 in a fully extended condition;  
         [0025]    [0025]FIG. 9 is a longitudinal cross-sectional elevation of an insertion device used to insert the bone plug of FIG. 1 into a previously-prepared channel within a bone;  
         [0026]    [0026]FIG. 10 is a fragmentary side elevation of the insertion device of FIG. 9;  
         [0027]    [0027]FIG. 11 is a fragmentary longitudinal cross-sectional elevation of an alternative insertion device used to insert the bone plug of FIG. 1 into a previously-prepared channel within a bone;  
         [0028]    [0028]FIG. 12 is a longitudinal cross-sectional elevation of a femur including a previously-prepared channel, showing the insertion and inflation of the bone plug of FIG. 1 with the insertion device of FIG. 9;  
         [0029]    [0029]FIG. 13 is a longitudinal cross-sectional view of a bone plug made in accordance with a second embodiment of the present invention with an inflatable sleeve of the bone plug in a fully deflated condition;  
         [0030]    [0030]FIG. 14 is a longitudinal cross-sectional view of the bone plug of FIG. 13, with the inflatable sleeve in a fully inflated condition.  
         [0031]    [0031]FIG. 15 is a side elevation of a bone plug made in accordance with a third embodiment of the present invention in a fully deflated condition;  
         [0032]    [0032]FIG. 16 is a distal end elevation of the bone plug of FIG. 15 in a fully deflated condition;  
         [0033]    [0033]FIG. 17 is a transverse cross-sectional view of the bone plug of FIG. 15 in a fully deflated condition, taken as indicated by section lines XVII-XVII therein;  
         [0034]    [0034]FIG. 18 is a longitudinal cross-sectional view of the bone plug of FIG. 15 in a fully deflated condition, taken as indicated by section lines XVIII-XVIII in FIG. 16;  
         [0035]    [0035]FIG. 19 is a fragmentary longitudinal elevation of a central portion of the bone plug of FIG. 15, showing a first version of a retaining band to hole an inflatable structure in place on a core therein;  
         [0036]    [0036]FIG. 20 is a fragmentary longitudinal elevation of a central portion of the bone plug of FIG. 15, showing a second version of a retaining band to hole an inflatable structure in place on a core therein;  
         [0037]    [0037]FIG. 21 is a distal end elevation of the bone plug in FIG. 15 in a fully inflated condition;  
         [0038]    [0038]FIG. 22 is a longitudinal cross-sectional view of the bone plug of FIG. 15, in a fully inflated condition, taken as indicated by section lines XXII-XXII in FIG. 21;  
         [0039]    [0039]FIG. 23 is longitudinal cross-sectional view of a bone plug made in accordance with a fourth embodiment of the present invention, shown in a fully deflated condition;  
         [0040]    [0040]FIG. 24 is a transverse cross-sectional view of the bone plug of FIG. 23, shown in a fully deflated condition, taken as indicated by section lines XXIV-XXIV therein;  
         [0041]    [0041]FIG. 25 is a distal end view of the bone plug of FIG. 25, shown in a fully inflated condition;  
         [0042]    [0042]FIG. 26 is a fragmentary longitudinal elevation of a central portion of the bone plug of FIG. 15, showing a retaining band to hole an inflatable structure in place on a core therein; and  
         [0043]    [0043]FIG. 27 is a longitudinal cross-sectional view of a bone plug having a shield attached to a core therein at one end thereof.  
     
    
     DESCRIPTION OF THE INVENTION  
       [0044]    [0044]FIG. 1 is a longitudinal cross-sectional view of a bone plug, generally indicated as  10 , made in accordance with a first embodiment of the present invention. The bone plug  10  includes a generally cylindrical core  12  having a tapered proximal end  13  which is removably attached to a mating tapered hole  14  at a distal end  15  of an insertion device, generally indicated as  16 . The core  12  includes an “L”-shaped internal passage  18 , extending between the tapered proximal end  13  and a cylindrical surface  20  extending under an elastomeric valve sleeve  22 . The core  12  is preferably composed of a molded thermoplastic material such as acetal or high-density polyethylene. A generally cylindrical inflatable elastomeric sleeve  23  extends along a central portion  24  of the core  12 , being held in place by internal ridges  26 , which extend within grooves  28  in the core  12 . The grooves  28  extend around the circumference of the core  12 . The internal ridges  26  may also be attached by means of an adhesive within the grooves  28 . The bone plug  10  also includes a shield  30  fastened, preferably by ultrasonic welding or by means of an adhesive, to a cylindrical distal end  32  of the core  12 . A conical portion  34  of the shield  30  is expandable and compressible from the undeflected condition in which it is shown in FIG. 1.  
         [0045]    Preferably, a specific provision is made to make the bone plug  10  visible during X-ray examination. For example, a stainless-steel ring  36  is included as an insert in the mold making the core  12 . Alternately, a radiopaque coating may be applied to a portion of the core  12 , or radiopaque fillers may be molded into the core  12 . During installation of the bone plug  10  into a cannel within bone, X-ray examination may be used to determine if the correct placement of the plug  10  has been achieved.  
         [0046]    [0046]FIG. 2 is a longitudinal cross-sectional view of the bone plug  10  with the inflatable elastomeric sleeve  23  in a fully inflated condition. The inflation of the elastomeric sleeve  23  within the conical portion  34  of the shield  30  causes this portion  34  to expand as shown in FIG. 2. When the bone plug  10  is inserted within a bone channel during actual use, the bone channel limits the expansion of the conical portion  32  of the shield  30 , causing its deflection into a partially cylindrical shape. At any level of inflation, the shield  30  extends around the inflatable elastomeric sleeve  23 , protecting it from bone splinters or other sharp edges of a channel within the bone.  
         [0047]    After the bone plug  10  is inserted into a bone channel, the inflatable elastomeric sleeve  23  is filled with a biocompatible fluid  38 , such as saline solution, causing the sleeve  23  to extend from the uninflated condition of FIG. 1 to the inflated condition of FIG. 2. This inflation is accomplished by injecting the fluid  38  through the internal passage  18 , so that pressure of the fluid  38  spreading between the cylindrical surface  20  of the core  12  and an internal surface  40  of the elastomeric valve sleeve  22  causes the expansion of the valve sleeve  22 . This expansion allows the release of the fluid  38  past the ends  42  of the valve sleeve  22 . When the pressure causing the injection of the fluid  38  through the internal passage  18  is stopped, the valve sleeve  22  closes, reducing the stresses previously causing its expansion, and preventing flow of the fluid  38  in either direction past the valve sleeve ends  42 . In this way, the valve sleeve  22  acts as an internal check valve.  
         [0048]    Thus, the inflatable elastomeric sleeve  23 , together with the core  12 , provide an inflatable structure which can be easily molded using conventional techniques, being removable from a central die or mandrel. The valve sleeve  22  is also easily molded. There is no need to form an enclosed structure with an internal valve, in the manner of the prior art device described in U.S. Pat. No. 4,697,584.  
         [0049]    [0049]FIGS. 3 and 4 show the shield  30  in an undeflected condition (i.e. in the shape resulting from the manufacturing process, such as molding, with which it is made). FIG. 3 is a side elevation of the shield  30 , while FIG. 4 is a proximal end elevation thereof.  
         [0050]    Referring to FIGS. 1, 3, and  4 , The shield  30  includes a distal cylindrical portion  44 , which is attached to the distal end  32  of the core  12  (shown in FIG. 1) and a pleated conical portion  34 , which extends outward in the undeflected condition at an included angle of, for example, 28 degrees. At the transition  46  between the cylindrical portion  44  and the conical portion  34 , the pleats  48  extend inward, into a reduced-diameter cylindrical portion  50  of the core. The width of the individual pleats  48  increases linearly toward the proximal end  52  of the shield  30 , providing more material to allow the conical portion  34  to be expanded. For example, each individual pleat  48  has a width of 0.76 mm (0.03 in.) at the transition  46 , increasing to a width of 2.5 mm (0.10 in.) at the proximal end  52 . The shield  30  is preferably molded from a flexible but tough material, such as polypropylene or a polyester resin, having a wall thickness of 0.25 mm (0.01 in.).  
         [0051]    [0051]FIGS. 5 and 6 show the shield  30  in a fully compressed condition, with FIG. 5 being a side elevation and with FIG. 6 being a proximal end elevation. In the fully compressed condition, the pleated conical portion  34  extends outward at an included angle of, for example, 5 degrees. The shield  20  is compressed by the application of external forces, such as the forces produced as the bone plug  10  is slid into a gradually narrowing bone canal.  
         [0052]    [0052]FIGS. 7 and 8 show the shield  30  in a fully extended condition, with FIG. 7 being a side elevation and with FIG. 8 being a proximal end elevation. In the fully extended condition, the pleated conical portion  34  outward at an included angle of, for example, 70 degrees. At the proximal end  52 , the pleats  48  are essentially unfolded. The shield  20  is extended by the application of a force from within the pleated conical portion  34 , as the inflatable elastomeric sleeve  23  is inflated.  
         [0053]    Thus, the pleated conical shape of the proximal portion  34  allows a tough but flexible material to be used to form the shield  30 . For the inflatable sleeve  23 , an elastomeric material is used to allow the stretching necessary to accommodate the desired change in shape. Elastomeric materials are by nature relatively easily torn by sharp objects, such as bone splinters and other sharp edges which may protrude from the previously-prepared hole in a bone. The shield  30  thus provides substantial protection for the inflatable sleeve  23 , being made of a material which is much more resistant to piercing or tearing by splinters and sharp edges.  
         [0054]    [0054]FIG. 9 is a longitudinal cross-sectional elevation of the insertion device  16  used to insert the bone plug  10  of FIG. 1 into a previously-prepared channel within a bone. Referring to FIGS. 1 and 9, the distal end  15  of the insertion device  16  includes a tapered hole  14  for removably receiving the tapered end  13  of the bone plug  10 . The proximal end  54  of the insertion device  16  includes a tapered hole  56 , for removably receiving the tapered end of a conventional syringe, and a cylindrical knob  58  to facilitate handling the insertion device  16 . The knob  58  may have a grooved or knurled outer surface. The insertion device  16 , which is preferably molded from a thermoplastic resin, also includes a longitudinally extending hole  60 , through which the fluid  38  is injected into the bone plug  10 .  
         [0055]    [0055]FIG. 10 is a fragmentary side elevation of the insertion device  16  of FIG. 9. Referring to FIGS. 9 and 10, the outer cylindrical surface  62  of the insertion device  16  includes a number of spaced-apart ribs  64 , which are used to determine how far the insertion device  16  is inserted into a channel within bone. Preferably, the first rib  64 , closest to the distal end  15 , is 120 mm (4.73 in.) from the distal end  15 , and sixteen additional ribs  64  extend from this rib  64  toward the proximal end  54  of the insertion device  16 , being spaced along the length of the insertion device  16  at center-to-center distances of 10 mm (0.394 in.).  
         [0056]    [0056]FIG. 11 is a fragmentary longitudinal cross-sectional elevation of an alternative version  66  of an insertion device. This alternative version  66  is similar to the insertion device  16 , except that the distal end  68  is extended to include a pair of tabs  70  engaging an internally threaded section  72  of an alternative version  74  of the bone plug, and the proximal end  76  is similarly extended to include a pair of tabs  78  engaging a conventional internally threaded section  80  of a syringe  82 . This alternative construction may be applied to either end of the insertion device, or to both ends, as shown in FIG. 11.  
         [0057]    [0057]FIG. 12 is a longitudinal cross-sectional elevation of a femur  84 , including a previously-prepared channel  86 , showing the insertion and inflation of the bone plug  10  of FIG. 1 with the insertion device  16  of FIG. 9. Referring to FIGS. 1 and 12, before this process is begun, the bone plug  10  is fastened to the distal end  15  (shown in FIG. 9) of the insertion device  16 , with the bone plug  10  and the insertion device  16  being twisted relative to one another so that the tapered proximal end  13  of the bone plug  10  firmly engages the tapered hole  14  within the insertion device  16 . Similarly, the syringe  82 , filled with the fluid  38  used for inflating the inflatable elastomeric sleeve  23 , is attached to the proximal end  54  of the insertion device  16 , being twisted relative to the insertion device  16  so that a conventional tapered end  88  firmly engages the tapered hole  56  within the insertion tool  16 .  
         [0058]    Next, the insertion tool  16  is used to push the bone plug  10  into place, with the ribs  64  providing a visual indication of the depth to which the bone plug  10  is inserted. When the desired depth is reached, the plunger  90  of the syringe  82  depressed to inject the fluid  38  into the inflatable elastomeric sleeve  23  of the bone plug  10 . Within the bone plug  10 , the resulting inflation of the sleeve  23  pushes the pleated conical portion  34  of the shield  30  outward, to be held against the bone channel  86 . A distal part  92  of the pleated conical portion  34  is forced into an essentially cylindrical configuration by contact with the channel  86 , as flexure of the pleated conical portion occurs around the inflated elastomeric sleeve  23 . After the bone plug  10  has been inflated in place, as shown in FIG. 12, the insertion tool  16  is twisted to unlock its connection with the bone plug  10 , and the insertion tool  16  is removed. Then the prosthesis (not shown) is placed within the upper portion  94  of the bone channel  86 , and the remaining space within this upper portion  94  is filled with bone cement. The presence of the bone plug  10  prevents the migration of this bone cement into the lower portion  96  of the bone channel  86 .  
         [0059]    The process of curing bone cement is exothermic, causing the local temperature to raise. In certain cases, this temperature increase may cause the formation of steam from the fluid  38  within the inflatable elastomeric sleeve  23 . This sleeve  23  is preferably made to rupture in this event, so that extreme increases in pressure will not be transferred to surrounding bone. When the temperature has reached a such a high level, the surrounding bone cement has sufficiently hardened to make continued pressure within the inflatable sleeve  23  unnecessary to prevent leakage of cement around the plug  10 .  
         [0060]    [0060]FIGS. 13 and 14 are longitudinal cross-sectional views of a second embodiment  100  of a bone plug built in accordance with the present invention. FIG. 13 shows the bone plug  100  with the inflatable elastomeric sleeve  23  in a deflated condition, while FIG. 14 shows the bone plug  100  with the sleeve in a fully deflated condition. This second embodiment  100  is similar to the first embodiment  10 , with like parts being accorded like reference numerals, except that, in the second embodiment  100 , the shield  30  having a single pleated conical section  34  is replaced by an alternative shield  102  having a distal pleated conical section  104  and a proximal pleated conical section  106 . These pleated conical sections  104 ,  106  meet at their maximum openings, with the entire shield  102 , including the conical sections  104 ,  106 , preferably being made as a single part, together with a distal cylindrical section  108  and a proximal conical section  110 . A forming process such as blow molding can be used to make the use of an internal die unnecessary.  
         [0061]    During inflation of the elastomeric sleeve  23 , flexure occurs in the region  112  of the shield  102 , but there is no need to form the proximal half portion of the pleated section into a cylindrical shape, as discussed above in reference to FIG. 12. The distal cylindrical section  108  of the shield  100  is attached to the distal cylindrical portion  32  of the core  12 . The proximal cylindrical section  110  of the shield  100  is allowed to slide along the insertion tool  16  with inflation of the elastomeric sleeve  23 .  
         [0062]    FIGS.  15 - 18  show a third embodiment  120  of a bone plug made in accordance with the present invention, in a fully deflated condition, with FIG. 15 being a side elevation thereof, with FIG. 16 being a distal end elevation thereof, with FIG. 17 being a transverse cross-sectional view thereof, taken as indicated by section lines XVII-XVII in FIG. 15, and with FIG. 18 being a longitudinal cross-sectional elevation thereof, taken as indicated by section lines XVIII-XVIII in FIG. 16.  
         [0063]    The third embodiment bone plug  120  includes a central portion, generally indicated as  122 , that is similar to a corresponding portion of the first embodiment bone plug  10 , together with a shield, generally indicated as  124 , comprising an outer shield structure  126  and an inner shield structure  128 . Each of the shield structures  126 ,  128 , which are coaxial with one another, includes a cylindrical end portion  130  at each end and a number of flexible beam portions  132  extending between the end portions  130 . As particularly shown in FIG. 18, the outer shield structure  126  and the inner shield structure  128  each include six flexible beam portions  132  that are evenly spaced at sixty-degree angles around the structure  126 ,  128 . The coaxial shield structures  126 ,  128  are further oriented so that the center  134  of each of the flexible beam portions  132  in the inner coaxial shield structure  128  lies directly inward from a gap  136  between adjacent flexible beam portions  132  in the outer coaxial shield structure  126 . (While the coaxial shield structures  126 ,  128  are shown in cross-section in FIG. 18, cross-hatching that would otherwise obscure the relationships among the flexible beam portions  132  is not shown.) Preferably, the adjacent end portions  130  of the coaxial shield structures  126 ,  128  are fastened to one another to maintain this relationship. For example, these coaxial shield structures  126 ,  128  are composed of a molded thermoplastic resin, such as a high density polyethylene, being fastened together by ultrasonic welding or by an application of an adhesive between their end portions  130 .  
         [0064]    In the example of FIGS.  15 - 18 , the shield  124  is slidably mounted on the central portion  122 . Flanges  138  extend inward at each end of the shield  124  to restrain the sliding movement of the shield  124  on the central portion  122 .  
         [0065]    The central portion  122  includes a core  140  and a number of features that, being similar or identical to features of the first embodiment  10 , described above in reference to FIG. 1, are accorded like reference numbers. For example, the core  140  includes an “L”-shaped internal passage  30  extending between its proximal end  142  and a recessed surface  20  underlying a valve sleeve  22 . The core  140  also includes a groove  28  near each end, which is used to hold an inflatable elastomeric sleeve  23  in place to extend around the core  140  and over the valve sleeve  22 .  
         [0066]    In FIGS. 15 and 18, the bone plug  120  is shown attached to the distal end  144  of an insertion tool  146 , which is similar to the insertion toll  16 , described above in reference to FIGS.  9 - 11 , except that an internally threaded hole  148  is provided therein for removably engaging a threaded proximal end portion  150  of the core  130 .  
         [0067]    [0067]FIGS. 19 and 20 are fragmentary longitudinal elevations of the central portion  122  of the bone plug  120 , showing alternative versions of a retaining band holding the inflatable elastomeric sleeve  23  in place at each of the grooves  28  (shown in FIG. 18) within the core  140 . In the example of FIG. 19, a retaining band  152  comprises several turns of wire tightly wound around the elastomeric sleeve  23  at the groove  28 . In the example of FIG. 20, a retaining band  154  comprises a loop of wire with its ends  156  twisted to hold the elastomeric sleeve  23  at the groove  28 .  
         [0068]    [0068]FIGS. 21 and 22 show the bone plug  120  in a fully inflated condition, with FIG. 21 being a distal end view thereof, and with FIG. 22 being a longitudinal cross-sectional view thereof, taken as indicated by section lines XXII-XXII in FIG. 21.  
         [0069]    The bone plug  120 , in its deflated condition, is made ready for insertion by attachment to the distal end  144  of the insertion tool  146  by screwing the threaded proximal end portion  150  of the core  130  into place within the threaded hole  148  of the insertion tool  146 . Next, in the general manner described above for the insertion of the bone plug  10  with the insertion tool  16 , in reference to FIG. 12, the insertion tool  146  is used to insert the bone plug  120  into a prepared hole in bone to a desired level and to fill the inflatable elastomeric sleeve  23  with a biocompatable fluid  38 . The valve sleeve  22  also functions as described above in reference to FIG. 2, allowing the fluid  38  to move past its ends  42  to fill the inflatable elastomeric sleeve  23  before preventing the return of the fluid  38  into the passageway  18 .  
         [0070]    As the inflatable elastomeric sleeve  23  is thus filled, the flexible beam portions  132  of the coaxial shield structure  126 ,  128  each bow radially outward. Central portions  158  of the flexible beam portions  132  of the outer coaxial shield structure  126  thus move apart from one another, as do the central portions  160  of the flexible beam portions  132  of the inner coaxial shield structure  128 , so that the central portions  160  move outward into position between the adjacent central portions  158 . In this way, both the central portions  158  and the central portions  160  bow into contact with the bone channel  86  (shown in FIG. 12).  
         [0071]    After the bone plug  120  is filled in this way, the insertion tool  146  is unscrewed from the bone plug  120  and removed, with the bone plug  120  remaining in place through pressure against the bone channel  86 , and with the fluid  38  being held within the bone plug  120  by the valve sleeve  22 .  
         [0072]    [0072]FIGS. 23 and 24 show a fourth embodiment  170  of a bone plug made in accordance with the invention un a fully deflated condition, with FIG. 23 being a longitudinal cross-sectional view thereof, and with FIG. 24 being a transverse cross-sectional view thereof, taken as indicated by section lines XXIV-XXIV in FIG. 23. This fourth embodiment bone plug  170  is similar to the third embodiment bone plug  120 , described above in reference to FIGS.  15 - 22 , except for modifications made to provide for the expansion of the bone plug  170  to fill a larger channel in a bone. Specifically, the inflatable elastomeric sleeve  172  is constructed to inflate to a larger diameter, and the shield  174  includes three coaxial shield structures, an outer coaxial shield structure  176 , an intermediate coaxial shield structure  178 , and in inner coaxial shield structure  178 .  
         [0073]    In the example of FIG. 24, the central portion  181  of each of the shield structures  176 ,  178 ,  180  is divided into six flexible beam portions  182 , with each ov these beam portions extending around the bone plug  170 , in the direction of arrow  184 , through a sixty-degree angle. In each of the underlying shield structures  178 ,  180 , each edge  186  of a flexible beam portion  182  is displaced from a corresponding edge  186  of an outwardly adjacent shield structure  176 ,  178  through a twenty-degree angle in the direction of arrow  184 . (In FIG. 24, cross-hatching is omitted from the coaxial shield structures  176 ,  178 ,  180  to avoid obscuring the geometrical relationships among these structures.)  
         [0074]    In general, this pattern is achieved in a bone plug having a first number of coaxial shield structures by dividing each of these coaxial shield structures into a number of flexible beam portions, each of which extends around the bone plug through a first angle, and by configuring the coaxial shield structures relative to one another so that, within each of the coaxial shield structures extending below the outermost of the coaxial shield structures, each of the beam portions has an edge displaced from a corresponding edge of a beam portion on an outwardly adjacent coaxial shield structure.  
         [0075]    [0075]FIG. 25 is a distal end view of the fourth embodiment bone plug  170  in its fully inflated condition. Central portions  181  of each of the flexible beam portions are bowed radially outward, with central portions  181  of the flexible beam portions  182  within the outer coaxial shield structure  176  separating from one another to admit central portions  181  of flexible beam portions  182  within both the intermediate coaxial shield structure  178  and the inner coaxial shield structure  180  to move into place between these central portions  181  of the outer coaxial shield structure  176 .  
         [0076]    [0076]FIG. 26 is a fragmentary longitudinal elevation of the bone plug  170  with the shield  174  removed to show a restraining band  188  used to hold the inflatable elastomeric sleeve  172  in place within each groove  188  in the underlying core  190  (shown in FIG. 23), and to prevent an escape of fluid from the inflated elastomeric sleeve  172 . A depression  194  within the restraining band  188 , which may be continuous around the band  188  or restricted to certain locations around the band  188 , can be formed after the band  188  is moved into place.  
         [0077]    In the example of FIG. 23, the shield  174  is slidably mounted on the inflatable elastomeric sleeve  172 , with inward extending flanges  196  of the shield  174  restraining movement of the shield  174  along the sleeve  172 . These flanges  196  may be formed as part of a process of joining the ends of the coaxial shield structures  176 ,  178 ,  180  so that the angular relationships among these structures, described above in reference to FIGS. 24 and 25 are maintained.  
         [0078]    [0078]FIG. 27 is a longitudinal cross-sectional view of a bone plug  200  having a shield  202  attached at a first end  204  to an underlying core  206 . The remaining portion  208  of the shield  202  is slidable on the underlying inflatable elastomeric sleeve  172 . Other aspects of this alternative bone plug  200  are similar to those of the bone plug  170 , described above in reference to FIGS.  23 - 25 .  
         [0079]    While the present invention has been described in its preferred forms or embodiments with some degree of particularity, it is understood that this description has been given only by way of example and that numerous changes in the details of construction, fabrication, and use, including changes in the combination and arrangement of parts, may be made without departing from the spirit and scope of the invention as described in the appended claims. For example, it is understood that the retaining band structures described above in reference to FIGS. 19, 20, and  26 , and the use of screw treads for removable attachment to an insertion tool, may be applied to the first and second embodiments of the invention, as described in reference to FIGS. 1 and 13.