Vacuum Mixing/Bone Cement Cartridge and Kit

FIG. 1 is a schematic, partially sectioned, illustration of a double vacuum-packed cartridge mixer according to the present invention.

FIG. 2 is an isometric view of a bone cement cartridge according to the present invention.

FIG. 3 is a schematic, partially sectioned, illustration of a first embodiment of a releasable hermetic closure for a piston of a bone cement cartridge.

FIG. 4 is a schematic, partially sectioned, illustration of a second embodiment of a releasable hermetic closure of a piston of a bone cement cartridge.

FIGS. 5A and 5B are schematic, partially sectioned, illustrations of a third embodiment of a releasable hermetic closure of a piston of a bone cement cartridge.

FIG. 6 is an illustration of a container for liquid bone cement component.

FIG. 7 is a schematic, partially sectioned, illustration of a liquid bone cement injector according to the presently claimed invention.

FIG. 8 is a sectional view of a bone cement gun containing a bone cement cartridge.

FIGS. 9A and 9B are schematic, partially sectioned illustrations of a pressure gauge for prepressurization of a bone cement cartridge.

FIG. 10 is a sectional view of the application of bone cement to a bone cavity in a bone.

FIG. 11 is a view partially in section illustrating the deposition of bone cement into the femoral canal.

FIG. 12 is a sectional view illustrating pressurization of bone cement in the federal canal.

FIG. 13 is a detailed view of the capped end of the mixer body.

FIG. 14 is a plan view of the underside of the cap fixed on the first open end of the mixer body.

FIG. 15 is a plan view of a protective disc mounted on the underside of the cap fixed to the first open end of the mixer body.

FIG. 16 is a plan view of the mixing element according to the present invention.

FIG. 17 is an illustration of a technique for testing for operability of the bone cement prepared according to the present invention.

It is common practice, in joint surgery today, to anchor components of replacement joints by using a bone cement comprising a two-component resin which polymerizes during the operation at normal temperatures and which, on account of its properties, leads to an interlocking of the prosthesis component in the bony sheath. Because of its physical properties, the bone cement shrinks onto the prosthesis resulting in a closed metal-to-cement contact.

The bone cements commonly used are polymethylmethacrylate consisting of free-flowing, powdery bead polymers which are superficially dissolved by liquid monomers and embedded during the polymerization process. During mixing, the polymer is immersed in the monomers. The polymethylmethacrylate beads are superficially dissolved and embedded in a composite manner. Such a composite structure can be compared with concrete, where air bubbles are likewise included during mixing. Moreover, when the monomer immerses the polymer beads, filling defects remain. These defects are termed "lee phenomena". Furthermore, in the case of bone cements, the monomer liquid evaporates during the exothermic polymerization, whereby further bubbles are formed. The bubbles formed as mentioned above constitute the major portion of the gas enclosures in bone cements.

The chemical reaction of the above-mentioned bone cements is initiated by a starter reaction, wherein, typically, dibenzoyl peroxide is activated by an activator such as p-aminotoluidine and then the radical chain polymerization is started. This polymerization proceeds exothermically. The monomer itself is stabilized by hydroquinone. Some bone cements are further stabilized by chlorophyll with simultaneous coloring. The storability of the monomer liquid can also be stabilized by vitamin C.

In the processing phase, following the mixing phase, the bone cement is applied to the femoral medullary canal or to the bony acetabulum which are both prepared to anchor the cemented prosthesis components; the application of the bone cement is normally performed by hand and sometimes using a syringe.

Using a syringe, the cement anchorage in the bone can be markedly improved. Therefore, so-called "bone cement guns" have been proposed, the principal of which is to impact the cement in the plugged medullary canal to provide transverse anchorage. With bone cement guns, the filling of the bone bedding is performed in different ways. On the one hand, filling is done from above in the downward direction or, on the other hand, it is also performed in the other direction, i.e. upwardly from below, by means of a long nozzle.

With the aforementioned techniques in mind, the present invention provides a two-component bone cement mixing system which allows preparation of the two-component bone cement under vacuum pressure, whereby gaseous inclusions are minimized, and further allows the preparation of the bone cement in a cartridge which may then be directly loaded into a bone cement gun for immediate operative use.

Turning now to FIG. 1 of the drawing, there is generally indicated at 1 a double vacuum-packed cartridge mixer according to the present invention. The cartridge mixer comprises a cartridge member 3 and a mixer member 5 which are detachably connected to each other. The cartridge member 3, as best seen in FIG. 2, comprises a hollow, air-impermeable cartridge body member 9 having a flange 11 at one end thereof along with a collar 13 whereby the cartridge body member may be mounted in a bone cement gun, as best seen in FIG. 8. The wall of the cartridge body member 9 may be reinforced with ribs 15 for added strength.

An air-impermeable piston member 17 is disposed within the cartridge body 9 proximate a first end 19 thereof so as to close the first end of the cartridge body member 9. The piston member 17 is axially slidable within the hollow cartridge body 9.

The mixer member 5 comprises a hollow, air-impermeable mixer body 21 having a first open end 23 and a second open end 25. The second open end 25 of the mixer body member 21 is provided with an internal screwthread (not shown) which is threadingly engageable of a screwthread 27 formed on the exterior of the second open end 7 of cartridge body member 9. By providing an O-ring 29 abutting the reinforcing rib 15' a releasable coaxial hermetic seal of the cartridge body member to the mixer body member may be obtained when the cartridge body member and the mixer body member are threadingly engaged with one another. Of course, any other form of detachable connection capable of forming a hermetic seal may also be utilized, e.g., a snap-fit with O-ring sealing element. In a similar manner, the cap 31 in combination with an O-ring 33 hermetically seals the first end 23 of the mixer body member 21. The cap 31 includes an aperture 35 which is closed by a self-sealing element 37 such as a rubber septum. The cap 31 is also provided with an additional aperture 39 through which the shaft 41 of a mixing element 43 passes. The shaft 41 is axially moveable in the aperture 39 and may also be rotated within the aperture 39. As may best be seen in FIG. 13, the aperture 39 is surrounded by an upstanding, circumferential wall 32 which forms a chamber 34 receivable of a plurality of alternating O-rings and washers (two O-rings 36 and 38 and one washer 40 being illustrated). The chamber 34 is closed by a cap member 42 which is threadingly engageable with the outer circumference of wall 32, the cap member 42 having an aperture 44 therein to allow passage of shaft 41 therethrough. The use of multiple O-rings alternating with washers (to form a packing box) is necessary in order to allow for reciprocal movement of shaft 41 while maintaining vacuum pressure within the mixer. The underside of the cap 31 (as best seen in FIG. 14) has a number of interconnected grooves 48 formed therein, with at least one of the grooves communicating with aperture 35. A porous membrane 47, e.g., a filter paper, is disposed over the underside of cap 31 so as to cover the grooves 48. The porous membrane 47 has an aperture 50 formed therein so as to permit passage of shaft 41 therethrough. The porous membrane 47 is in turn covered by a protective disc 52 (as best seen in FIG. 15). The protective disc 52 has a central aperture 54 to permit passage of shaft 41 therethrough. The protective disc 52 is additionally provided with a plurality of apertures 56 (which may vary in size, shape and pattern). The protective disc 52 protects the porous membrane 47 from being torn by mixing element 43. The function of the grooves 48, the porous membrane 47 and the protective disc 52 will be explained hereinbelow. As best seen in FIG. 16, the mixing element 43 comprises a plate 60 disposed substantially perpendicular to the shaft 41 and conforming substantially to the internal cross section of the cartridge body member and the mixer body member. The mixing element 43 has a number of holes 62, 64 formed therein so as to allow the mixer element to be reciprocated back and forth through a body of polymeric material 46 contained within the cartridge body member 9. The shaft 41 is provided with a manually graspable handle 45 so as to facilitate manipulation of the mixing element 43. In a preferred embodiment, the holes 62, 64 are circular in shape and disposed symmetrically about the plate 60 relative to the shaft 41 (which is concentric with the plate 60). The plate 60 is also circular in shape, and pairs of holes (one larger 64 and one smaller 62) are disposed on diameters of the plate and opposite one another relative to the center of the plate. Most preferably, three pairs of holes are used disposed on diameters of the plate which are offset 60.degree. of arc from the adjacent diameters upon which holes are disposed; and each large hole 64 is immediately adjacent to two small holes 62 on the immediately adjacent diameters upon which holes are disposed. The centers of the smaller holes 62 all fall on the circumference of a first circle concentric with the plate 60; and the centers of the large holes 64 all fall on the circumference of a second circle concentric with the plate 60. Preferably, the diameter of the first circle is larger than the diameter of the second circle.

In use, the cartridge member 3 is loaded with a predetermined amount of a free-flowing, powdery polymeric component 46 having a non-compacted volume, measured in cubic centimeters, represented by Vbc, and the mixer member 5 is then screwed onto the cartridge member so as to form a closed container having a volume, measured in cubic centimeters, represented by Vcm. The ratio Vcm/Vbc is about 1.5 to 3.5, preferably about 2.0, for reasons more fully explained hereinafter. A hypodermic needle connected to a source of vacuum pressure is then inserted through the rubber septum 37 in aperture 35 and a vacuum is drawn within the cartridge mixer. Care must be taken so that the tip of the needle may enter the groove 48 but does pierce the porous membrane 47. This is because the powdery polymeric material would otherwise be sucked up through the needle and block further evacuation of air. The grooves 48, accordingly, allow a uniform application of vacuum across the cross-section of the mixer, while the porous membrane 47 prevents powdery polymeric material from being withdrawn from the mixer along with the air. The protective disc 52, in turn, prevents the mixer element 43 from damaging the porous membrane during handling prior to evacuation. Typically, a vacuum pressure of 30 millimeters of mercury or less is drawn in the cartridge mixer. The vacuum packed cartridge mixer is then vacuum packed within a container 49, such as an air-impermeable plastic bag. This double vacuum packing helps to assure a long and stable shelf life.

In order to provide a hermetic seal of the cartridge mixer, it is necessary that the piston member 17 be hermetically sealed also. However, the piston member 17 must also be axially slidable within the cartridge body member 9 so as to be capable of forcing the fluid bone cement from the cartridge member when the cartridge member is mounted in a bone cement gun. In other words, the piston member is not fitted within the cartridge member in an air tight manner, but rather just sufficiently tight to prevent leakage of the viscous cement mixture thereby. Accordingly, a releasable hermetic seal is necessary for the piston member. In this regard, FIG. 3 illustrates one embodiment of a releasable hermetic seal for the piston member 17 wherein a bead 51 of adhesive seals the piston in place. When the piston is engaged by the plunger of a bone cement gun, the bone cement gun plunger develops sufficient force to break the bead and allow movement of the piston through the cartridge body member 9. In a second embodiment, illustrated in FIG. 4, a thin layer of a resinous material 53 (e.g., polyethylene) is disposed over the piston member 17. In a manner similar to that of embodiment FIG. 3, the thin layer resinous material 53 may be broken by actuation of the plunger of a bone cement gun.

In a further embodiment, illustrated in FIGS. 5A and 5B, a thin diaphragm layer 55, e.g. of polyethylene, which is air-impermeable, is disposed across the first end 19 of cartridge body member 9 as shown in FIG. 5A to form the hermetic seal. When a vacuum is drawn in the interior of the cartridge mixer, the diaphragm layer 55 is bowed at 57 as shown in FIG. 5B. This embodiment not only provides a releasable hermetic seal, which may once again be broken by the actuation of the plunger of a bone cement gun, but also provides an indicator as to the maintenance of vacuum pressure within the cartridge mixer. In this regard, if the diaphragm layer 55 is bowed as shown in FIG. 5B, then this is a clear indication that vacuum pressure has been maintained within the cartridge mixer. If, however, after standing for long periods of time, a user finds the diaphragm layer 55 in the position illustrated in FIG. 5A, then this is a clear warning that vacuum pressure has been lost.

The liquid bone cement component 59 is preferably contained within a glass ampoule 61 due to the noxious and toxic nature of the liquid bone cement component, i.e. a (meth)acrylate monomer. The glass ampoule 61 comprises a body member 63 having a spout 65 and a removable closure 67 which may be broken off at the weakened neck 69 of the ampoule when desired.

An injector 71, as illustrated in FIG. 7, is utilized to transfer monomer to the interior of the cartridge mixer. The injector comprises a hollow injector body member 73 having a first open end 75 and a second open end 77. The injector body member 73 is coaxially receivable of the ampoule 61.

A cap 79 is disposed over the second open end 77 of the injector body 73 for closure thereof. The cap member 79 includes a fluid conduit 81, coaxial with the hollow injector body member 73, for passage of a fluid through the cap member. A hollow needle 83 is hermetically connectable to the fluid conduit 81 so as to allow a fluid medium to pass from the conduit 81 through the hollow needle. A resilient support 85, e.g., of silicone rubber, hermetically, sealingly connects the second end of the hollow injector body and the cap member. Additionally, the resilient support is provided with a fluid passage tapering toward and fluidically connected to the fluid conduit 81 in the cap member 79. When the opened ampoule 61 is inserted into the hollow injector body member 73, and the assembly then inverted to the position of FIG. 7 the resilient support 85 supports the ampoule and the spout portion 65 of the ampoule is received within the fluid passage 87.

It should be noted that temperature control is an important part of the mixing process. For the "dough" type cements in current use, such as Simplex .RTM. and Zimmer-Regular.RTM., it has now been found that chilling the mixing container and the cement components aids in the prevention of excessively fast curing. In particular, when mixing cements under a vacuum pressure of 30 mm Hg or less, it has now been found desirable to chill the mixer and the cement components (powder and liquid) to a temperature below room temperature (20.degree. C.), preferably less than 15.degree. C., most preferably to about 12.degree. C. (+2.degree. C.), prior to bringing the solid and liquid components together in the mixer. This chilling is believed to be necessary due to the fact that, under vacuum pressures of 30 mm Hg or less, oxygen, which normally interacts with monomer free radicals to slow the polymerization of the monomer, is in short supply and will not effectively inhibit the polymerization reaction. In this regard, a thermometer or a temperature indicating strip, e.g., of the liquid crystal type, could be packaged with the mixer so as to allow the user to ascertain whether the mixer and its contents have been sufficiently chilled prior to use.

Prior to insertion of the opened ampoule 61 into the injector body member 73, a polyethylene ball 89 is placed in the ampoule, the polyethylene ball having a specific gravity less than the monomeric material and thus floating on the surface of the monomeric material. When the injector 71 is inverted so as to insert the needle 83 through the rubber septum 37 of the cartridge mixer, so as to inject monomeric material into the vacuum packed polymeric material contained therein, the polyethylene ball 89 will float on the surface of the monomer as illustrated in FIG. 7. As the monomer is drained from the ampoule 61 through fluid passage 87, fluid conduit 81 and hollow needle 83 into the cartridge mixer, by the vacuum pressure maintained in the cartridge mixer, the polyethylene ball will drop downward with the surface 91 of the liquid monomeric material. When the monomeric material is drained from the ampoule the polyethylene ball 89 will then hermetically seal the fluid passage 87 formed in the resilient support 85. This will prevent any air being drawn into the cartridge mixer around the edge of the ampoule 61 and between the ampoule 61 and the hollow body member 73. Thus, personnel transferring monomer to the polymeric material in the cartridge mixer are not required to closely monitor the progress of such transfer as they are with conventional syringe techniques or conventional stop cocks. In other words, when the contents of the ampoule are emptied, the fluid passage 87 is automatically sealed by the polyethylene ball 89 acting as a floating plug. Once transfer of the monomer has been completed, the needle 83 may be withdrawn from septum 37, and mixing of the two-component bone cement may then be conducted within the cartridge mixer. When the hollow needle is withdrawn from the cartridge mixer, air rushing through the hollow needle 83 will violently disengage the polyethylene ball 89 from the fluid passageway 87 in resilient support 85. This violent disengagement will cause the polyethylene ball 89 to bounce around within the empty ampoule 61. The noise so-produced is a further indication that a successful transfer of monomer into the cartridge mixer has been carried out without leakage of air into the cartridge mixer.

In regard to the above, vacuum evacuation of the ampoule is ensured by the provision of a ratio of the interior volume of the cartridge mixer (Vcm), measured in cubic centimeters, to the volume of the first predetermined quantity of a free-flowing, powdery solid bone cement component (Vbc), measured in cubic centimeters, in the range of Vcm/Vbc of about 1.5 to about 3.5, preferably about 2.0. This is because the free-flowing, powdery solid bone cement component has an interstitial volume (free volume between solid particles) which is about 40-50% of its total volume. If the cartridge mixer were filled with the free-flowing powder, then liquid cement component transfer to the cartridge mixer and evaporation of liquid monomer under the low pressures therein would quickly equalize pressures between the cartridge mixer and the ampoule which, in turn, would eliminate the driving force for liquid transfer from the ampoule to the cartridge mixer. This would prevent complete transfer of the ampoule contents to the cartridge mixer, even though the "free volume" of the powder may have been sufficient to contain the liquid volume of the ampoule contents. In order to provide sufficient volume in the cartridge mixer to allow complete transfer of the ampoule contents to the mixer, it has been found desirable to provide a cartridge mixer having an interior volume which is 1.5 to 3.5 times, preferably about 2.0 times, that of the powdery component contained therein. This "excess free volume" allows complete transfer of the required quantity of the liquid component, previously placed in the ampoule, for the quantity of the powdery component, previously placed in the cartridge mixer.

Mixing is readily effected by merely causing reciprocating motion of the mixing element 43 axially within the cartridge mixer. Mixing may be further facilitated by some rotation of the handle 45 during the reciprocating motion of the mixing element 43.

After mixing is completed, rubber septum 37 may then be pierced with a hollow needle so as to release the vacuum pressure within the cartridge mixer. Alternatively, the mixer member 5 may be provided with a vent hole 12 (shown as formed in the mixer body 21, but also could be provided in the cap 31) hermetically sealed with a pressure-sensitive adhesive tape 14; and it would only be necessary to peel away the tape to open vent hole 12 and release the vacuum. The mixer member 5 may then be detached from the cartridge member 3 and a suitable cap 93, as best seen in FIG. 8, may be placed over the second open end 7 of the cartridge body member 9. The cartridge body member 9 may then be fitted in a conventional bone cement gun 95 as illustrated in FIG. 8.

The bone cement may then be utilized in a normal fashion, or, it may be subjected to a prepressurization treatment, which has been found to be effective in certain instances. In this regard, it should be noted that in order to minimize the amount of air entrapped in the viscous mixture as air bubbles, the mixer is maximally evacuated of air. When the liquid monomer component of the bone cement is introduced into the mixing chamber, a small amount of the monomer will evaporate and equilibrate at the partial vapor pressure of the liquid monomer at that temperature. However, turbulence created by the mixer element during mixing, i.e. cavitation, will cause the monomer to boil. With higher vacuums, the mixture "cavitates" more easily. The monomer which boils under these conditions, and hence the bubbles of gaseous monomer formed in viscous mixture will only partially collapse when the vacuum is released, and may require a significant amount of time to collapse fully. Since most bone cements harden within about 8 to 15 minutes of mixing, the monomer bubbles may not have fully collapsed before hardening. Moreover, the heat of polymerization generated during the curing of the bone cement may actually cause the partially collapsed gaseous monomer bubbles to re-expand. Therefore, it has been found to be desirable to pressurize the bone cement shortly after release of the vacuum, but prior to use, hence the name "prepressurization" to accelerate the complete collapse of bubbles of gaseous monomer.

In order to effect the prepressurization treatment, the present invention provides a special pressure gauge cap 97 which may be fitted over the second open end 7 of the cartridge body member 9.

As illustrated in FIG. 9A, the cap 97 comprises a flexible membrane 99 having two protrusions 101, 103 extending substantially perpendicular therefrom; and a flange member 105 which may be screwed onto the end of the cartridge body member. The flange member incorporates an arcuate gauge element 107 having predetermined indicia 109 and 111 formed thereon. When the cartridge body member is mounted in the bone cement gun and subjected to pressure by movement of the plunger of the bone cement gun and concomitant movement of the piston member of the cartridge member, the increase in pressure causes distension of the membrane 99, as best seen in FIG. 9B. The distension of this membrane member causes the protrusions 101 and 103 to effectively bend outwardly away from one another and the degree of this bending is indicative of the internal pressure developed in the cartridge body member. By applying pressure until such time as the indicia 109 and 111 are substantially aligned with the protrusions 101 and 103, respectively, a predetermined pressure may be developed within the cartridge body member. After maintaining the pressure, e.g. 75 psig, for a predetermined period of time, the pressure may be released and the cap 97 removed.

In operation, a closure cap 120 may then be connected onto the open end 7 of the cartridge 9, as illustrated in FIG. 10, and a tip 121 is selected for the particular type of operation and is engaged with the front closure cap 120. (At this point, it is recommended that the first few cc's of the cement be wasted by squeezing the handle of the cement gun. This would allow the surgeon a feel of the flow (viscosity) of the cement from the nozzle of the cement gun prior to injecting it into the patient. If the cement is too stiff, the surgeon should abandon the use of the cement. Alternatively, the temperature of the cement is indicative of the degree of cure and the temperature may be used as the determinative factor in whether or not the cement should be utilized. In this regard, as shown in FIG. 17, a temperature sensitive tape, e.g., of the liquid crystal type, 150 may form the wall of a small container into which a few cc's of cement 152 is injected. Depending upon the type of cement utilized, the tape will have a temperature range in which further utilization of the cement is permissible ("OK") and above a certain temperature further utilization of the cement is not permissible ("ABORT"). One indicia, "OK" or "ABORT", only will be readily visible depending on the temperature of the cement. The engagement between the front closure cap 120 and the cartridge body member may be by means of a snap fit, an internal threading or an external threading. When a prosthesis is to be cemented to a plateau-type surface, holes 122 may be drilled into the bone surface to create a cavity into which bone cement 123 is to be injected, as shown in FIG. 10. An appropriate tip is selected which is configured to interfit with the opening of the bone cavity. The end of the tip is tapered inwardly to terminate in a portion having a diameter less than the diameter of the bone cavity. This allows the tapered portion of the tip to be inserted into the bone cavity and form a seal about the diameter of the bone cavity. Thus, the cement is injected into a closed space so that pressure can build up to force the cement to penetrate the trabeculae of the bone. Cement is also deposited over the remaining bone surface to which the prosthesis is to be attached.

In the case of a long tubular bone canal, some type of plug as shown in FIG. 11 must be lodged in the canal to create a closed space to prevent the extension of bone cement beyond the point where it is useful and to facilitate more complete filling and pressurization of the canal. Various means of plugging the canal have been advocated, including plugs made of natural bone, polyethylene or a bolus of doughy bone cement. Initially, a long straight tip 121b, which has a diameter less than the diameter of the canal to allow the tip to project into the bone canal, is used to deposit bone cement into the long bone canal. This tip 121b does not have a tapered end since the canal is initially just being filled up. Then, a tip such as the femoral canal pressurizer 140 and adapter 141, as shown in FIG. 12, is used to complete the filling of the canal and pressurize the contents of the cavity. The tapered pressurizer seals off the canal thereby enabling the pressure created upon injection of the cement to force the cement to penetrate the bone.

Throughout the specification, reference has been made to the air-impermeability of certain materials of construction which are utilized in the present invention. It should be noted that this terminology does not require perfect air-impermeability, however, it does require a degree of air-impermeability sufficient to prevent any significant loss of vacuum pressure through the material over the expected storage life of the vacuum packed materials, e.g. six months.

The mixing system disclosed in the present invention may be provided in the form of a kit comprising a double vacuum packed cartridge mixer filled with a first predetermined amount of a solid, powdery bone cement component, an ampoule containing a second predetermined amount of a liquid bone cement component, an injector for transfer of the liquid bone cement component to the cartridge mixer and a polyethylene ball for insertion in the ampoule prior to insertion in the injector so as to provide a positive sealing action to prevent air ingress into the cartridge mixer upon passage of the predetermined quantity of monomer thereto.