Automated Bone Cement Mixing Apparatus

This invention relates to an apparatus and method for mixing bone cement for use in surgical procedures such as hip replacements and, more particularly, to an automated mixer with a disposable mixing bowl for mixing the bone cement.

Damage to the joints of the human body, due to arthritis, other degenerative diseases or trauma, often results in the need to replace the joint with a prosthetic device. In the case of hip replacement surgery such devices include a cup which fits into the hip socket, and replaces the acetabulum, and a ball attached to a stem which fits into the femur and replaces the head of the femur. A cement is used to secure the prosthetic devices to the bone.

Currently, the cement used for such replacement is polymethylmethacrylate (PMMA), which requires the use of a catalyst for hardening of the cement. The catalyst, which is supplied in liquid form, must be mixed thoroughly with the methyl methacrylate monomer, and other dry components of the cement, to ensure a complete and even hardening of the cement. Preferably the cement is mixed under a vacuum to prevent air bubbles from being trapped in the cement. Air bubbles in the final cement mix may result in a cement with an undesirably low tensile strength. Typically, mixing of the cement components for between 0.5 to 2 minutes is required to obtain a cement mixture of the desired consistency.

Currently, devices available for mixing of bone cement require hand mixing. This procedure is very laborious and requires a sustained physical effort to complete the mixing process. If the mixing process is not adequate or if the cement hardens before it is delivered, the cement must be discarded and an additional batch prepared. If the process has to be repeated, the surgical procedure, which is under way by the time the cement mixing is commenced, is delayed.

The bone cement is, and should remain sterile, therefore, a "sterile" person is required to perform the mixing and handling of all the sterile components of the cement preparation. With bone cement mixers currently available, a "non-sterile" person is also required, in addition to the "sterile" person, to assist in the set-up of the cement mixing apparatus and for handling of all nonsterile components involved in the mixing procedure. At the end of the mixing procedure an additional "sterile" person is often required to assist in the transfer of the cement into cartridges for delivery of the bone cement to the patient. Therefore, preparation of the cement requires at least two and often as many as three people.

The components of the pre-cured cement, methyl methacrylate monomer and catalyst, are irritants and potentially toxic. Therefore, inhalation of the cement components, particularly the monomer, is preferably avoided. Bone cement mixers currently available are not sealed throughout the mixing process and require that the components be manually dispensed into the bone cement mixer, thus exposing the operator to their fumes. These fumes may then be inhaled, for an extended period of time, by all personnel in the vicinity of the cement preparation, making cement preparation irritating and potentially hazardous to their health.

After the cement is mixed, it is poured into a cartridge for dispensing to the site for cementing. At this time, all instruments which have come in contact with the cement must be thoroughly cleaned or discarded. An additional problem with transferring the cement is the possibility of folding in air pockets that could negatively affect the cement/bone interface and reduce the cement's bonding strength.

It is desirable that an apparatus be provided which requires only a single operator to complete all the steps necessary to set-up the apparatus and to mix the cement. It is also desirable that the apparatus isolates all the fumes associated with the cement preparation to thus reduce exposure of hospital workers to these toxic compounds. It is also desirable that such an apparatus be motor driven to avoid the physical endurance required to mix cement and also to ensure reproducible mixing which reduces the need to dispose of cement batches that are unacceptable. It is also desirable that all parts of the device which come in contact with the cement are disposable so that clean-up is minimized.

A motorized mixer for mixing bone cement for use in attachment of prosthetics and method for using the mixer is described. The mixer comprises a mixing chamber, a liner in the mixing chamber, an impeller within the liner, a motor for driving the impeller and programmable means for regulating the operations of the motor.

In operation dry components of the bone cement are placed in the mixer and vials containing catalyst for the bone cement are placed in vial cavities within the mixer. The mixer is sealed and the vials are broken to release the catalyst into the dry components of the bone cement. The mixer is then started. The mixer runs under the control of a preprogrammed mixing sequence. At the end of the mixing sequence the mixed cement is collected.

In one preferred embodiment, the liner is provided in two parts: a mixing bowl for initially receiving the dry components of the bone cement and a basket for initially receiving vials of the liquid bone cement catalyst. The basket nests within the mixing bowl. While the liner is provided in two parts, the two are preferably permanently attached to one another with the impeller in place. The bone cement powder is placed into the mixing bowl with the use of a funnel. Once the bone cement components have been added to the liner, the lid of the mixer housing is closed, breaking the vials of bone cement catalyst to release their contents through the basket and into the mixing bowl. A preprogrammed mixing sequence is then initiated to start the impeller and mix the bone cement.

In a preferred embodiment of the present invention, the preprogrammed mixing sequence comprises applying a vacuum to the interior of the mixing bowl, starting a motor to drive the impeller within the mixing bowl, mixing the bone cement for a preselected time period, re-applying a vacuum to the mixing bowl to evacuate fumes liberated by the bone cement and applying pressure to the interior of the mixing bowl to discharge the cement from the mixer.

In another preferred embodiment of the present invention, a discharge tube is provided in communication with the mixing bowl for discharging the mixed bone cement to a bone cement cartridge for use with a bone cement gun or syringe for use in applying the bone cement to a patient during surgery. This preferred embodiment also includes a U-shaped opening and a U-shaped trap door between the mixing bowl and the discharge tube that further assist in the rapid withdrawal of bone cement from the mixing bowl. The U-shaped opening is shaped to straddle an impeller bearing provided on the bottom of the mixing bowl.

Also in the preferred embodiment, once the mixing of the bone cement is completed, the vacuum to the mixing bowl is broken and a vacuum is applied to the discharge tube. This also promotes the rapid filling of a bone cement cartridge. This configuration makes it easier to discharge cement to the cartridge because no air pressure is in the cartridge to counteract the pressure forcing the cement into the cartridge. It also helps prevent the introduction of air into the cement during the discharge of cement from the mixer into the cartridge. The application of a vacuum to the discharge tube further assists in opening the trap door and the large area of the U-shaped opening in the mixing bowl still further promotes the rapid discharge of the mixed bone cement through the discharge tube.

Features, aspects and advantages of the invention will be more fully understood when considered with respect to the following detailed description, appended claims and accompanying drawings where:

FIG. 1 is a perspective view of one embodiment of a bone cement mixer;

FIG. 2 is a perspective view of another embodiment of a bone cement mixer;

FIG. 3 is a top view of a one embodiment of a bone cement mixer liner;

FIG. 4 is a front view of the bone cement mixer liner taken along the line 4--4 of FIG. 3;

FIG. 5 is a side view of the bone cement mixer liner taken along the line 5--5 of FIG. 3;

FIG. 6 is a top view, partly in section, of another embodiment of a bone cement mixer liner;

FIG. 7 is a front sectional view of the bone cement mixer liner taken along the line 7--7 of FIG. 6;

FIG. 8 is a side sectional view of the bone cement mixer liner taken along the line 8--8 of FIG. 6;

FIG. 9 is a front view of one embodiment of an impeller for use in the present invention;

FIG. 10 is a top view of the impeller illustrated in FIG. 9;

FIG. 11 is a front view, partly in section, of another embodiment of an impeller for use in the present invention in position for mixing;

FIG. 12 is a front view of, partly in section, a collapsible paddle and a plunger of FIG. 11 in position for evacuation;

FIG. 13 is a side sectional view of one embodiment of a port in the liner, shown in the closed position;

FIG. 14 is a side sectional view of the port of FIG. 13, shown in the open position;

FIG. 15 is a side sectional view of another embodiment of a port in the liner, shown in the closed position;

FIG. 16 is a side sectional view of the port of FIG. 15, shown in the open position;

FIG. 17 is a side sectional view of another embodiment of a port in the liner, shown in the closed position;

FIG. 18 is a side sectional view of the port of FIG. 17 in the open position;

FIG. 19 is a top view of one embodiment of an adapter for cement dispensing cartridges;

FIG. 20 is a side view of the adapter taken along line 20--20 of FIG. 19;

FIG. 21 is a side view of another embodiment of an adapter for cement dispensing cartridges;

FIG. 22 is a front view of the adapter for cement dispensing cartridges of FIG. 21;

FIG. 23 is a front elevation of a key pad;

FIG. 24 is a perspective view of another embodiment of an impeller for use in the present invention;

FIG. 25 is a front view, partly in section, of the impeller illustrated in FIG. 24;

FIG. 26 is a front sectional view of a "one way" gas flow stopper located in the shaft of the impeller illustrated in FIG. 24;

FIG. 27 is a front sectional view of the bottom end of the impeller shaft of FIG. 24 contacting the bottom of a mixing chamber to which is attached a bone cement gun cartridge;

FIG. 28 is an exploded perspective view of another embodiment of the liner of the present invention;

FIG. 29 is a top view of the mixing bowl of FIG. 28;

FIG. 30 is a front sectional view of the mixing bowl of FIGS. 28 and 29;

FIG. 31 is a side sectional view of the mixing bowl of FIGS. 28-30;

FIG. 32 is a perspective view of a trap door for use with the mixing bowl of FIGS. 28-31;

FIG. 33 is a top view of the trap door of FIG. 32; and

FIG. 34 is a schematic diagram illustrating the use of the liner embodied in FIGS. 28-33.

The present invention relates to a bone cement mixer 10, one embodiment of which is illustrated in FIG. 1. A second embodiment of a bone cement mixer 10 is illustrated in FIG. 2 (the same numbers are used for similar parts throughout this description). The bone cement mixer of the present invention has the advantage of being motorized and automated to minimize or eliminate contact, by the operator and others in the surgery, with the components of the cement and fumes generated by the components of the cement.

In general, the bone cement mixer of the present invention comprises a housing 12 which is non-disposable and houses other non-disposable parts of the mixer such as motors, batteries, programmable memory mechanism and a vacuum-pressure pump (not shown). The non-disposable components are sterilized by autoclaving for use. The non-disposable parts of the mixer do not come in direct contact with the cement.

The components which come in contact with the cement are disposable and are supplied as a cement mixing kit, which comprises a liner 30, which fits into the housing and comprises the necessary mixing impeller 90 for mixing the cement. Other disposable components of the kit include a cement injector gun or adapters into which the mixed cement is collected at the end of the mixing procedure. The disposable components of the mixer are supplied in kit form which are supplied as pre-sterilized. The individual components of the bone cement mixer are described in detail below.

In operation, liner 30 (see FIGS. 3-8, described in detail below) is placed in the mixer housing 12 and an injector gun or adapters (not shown) are placed on the cement outlet of the mixer. Components of the cement, such as the powder which includes the methyl methacrylate monomer are then placed in the liner. Vials containing catalyst, for the hardening of the cement, are placed in vial cavities 32 located in the liner. The bone cement mixer is then programmed for the desired mixing time. Lid 14 of the housing is then closed to seal the components of the cement within the mixer. Closing the lid also results in breaking of the vials thus releasing the catalyst into the other cement components (see below for a detailed description). A vacuum is applied to the interior of the liner to prevent the formation of air bubbles in the cement as it is mixed and also to remove fumes generated from the components of the cement. A vacuum may also be applied to a cement gun or various cartridges for dispensing the cement, positioned below the mixing chamber, to prevent the formation of air bubbles in the cement while it is discharged from the mixing chamber to the cement gun. After the bone cement mixer has been evacuated the mixer is started.

At the end of the preselected mixing time, the mixer stops automatically and, in one embodiment of the present invention, sounds an alarm to alert the operator. A vacuum is again applied to the chamber to evacuate fumes which have built up during the mixing procedure. When the mixing is completed, the vacuum is released and pressure is applied to the chamber to evacuate the cement from the mixer by pushing the cement through a port located at the bottom of the mixer into the cement gun or various cartridges for dispensing the cement. Prior application of a vacuum to the cement gun will assist in evacuation of the cement from the mixer because no positive air pressure in the cement gun will be present to counteract the pressure being used to discharge the cement. The disposable portions of the mixer, i.e., the liner, can then be removed from the mixer housing and discarded after use. The components of the mixer and its operation are described in detail below.

The mixer housing 12 (FIGS. 1 and 2) comprises a pair of lids 14 which are attached to a housing body by a hinge (not shown) or other suitable means of attachment. The lids each house a motor (not shown) that supplies the power for mixing. Motors suitable for use in the present invention are motors such as conventional gear motors, pancake motors or the like. The motors are preferably reversible so that mixing can be performed by mixing in one direction for a period of time and then in the opposite direction, if desired, to ensure complete mixing of the cement. Motors suitable for use in the present invention are gear motors such as those sold by Globe of Dothan, AL, Model number 407A6008-2, which is a 12 V, 120 rpm, reversible motor.

In one embodiment of the present invention the motor is controlled by a current sensing mechanism. When the current required to operate the mixing motor exceeds a preset level, the device automatically terminates the mixing cycle, sounds an alarm and begins evacuating the cement from the mixer. This mechanism, therefore, indirectly senses the viscosity of the cement to ensure the cement is evacuated before it becomes too viscous to flow from the mixer and also prevents over-mixing of the cement which would result in the cement hardening too quickly.

In a preferred embodiment, the motorized mixer of the present invention is programmable so that different cement consistencies and/or compositions can be mixed effectively. For example, cement for tibial/acetabulum replacement surgery is generally preferred at a thick or viscous consistency and mixing of the cement components for about two minutes generates a suitable cement for this application. Cement for femoral application is generally preferred at a thinner or less viscous consistency than tibial/acetabulum cement, because it has to be placed within the femur. Mixing of the cement components for about one minute generates a suitable cement for this application. Moreover, some bone cements require a "stop period" during mixing so that the cement may "set up" properly. These mix requirements could be placed in the "permanent" memory of the mixer. Alternatively, a "custom" mix program could also be programmed by the user. Suitable programmable memory mechanisms are well known in the art and suitable devices for use in the present invention are devices such as Model No. 8748 sold by NEC or other similar devices sold by other manufactures of programmable controllers. The programmable controllers are included within the housing. In another embodiment of the present invention the "program" is "hard wired" rather than incorporated into a programmable memory mechanisms.

Located on the front of the mixer housing is a key pad 16 (see FIG. 23). The key pad may include a display which displays mixing time elapsed and the level of vacuum or pressure within the chamber. Also, buttons which allow for programming such functions as mixing, pausing the mixing, applying and releasing vacuum and selecting a program option may be included. Additional buttons may be included which allow the selection of a preprogrammed or custom mix protocol and re-setting the mixer program. The key pad may also include LED's to indicate the phase of the mixing cycle the mixer is currently executing and segmental LED's may be included to indicate the mixing time elapsed and the vacuum level within the chamber. LED's suitable for use in the present invention are those made by Kingbrite or the "mil-spec" high temperature resistant LED's made by Hewlett-Packard. Key pads suitable for use in the present invention include membrane key pads which have an overlay such as a multi-layer polycarbonate, such as those sold by 3M or a silicone rubber.

When a custom mix is chosen the program parameters may also be specified using the key pad. In one embodiment the key pad for custom mixing times includes a group of switches which are identified as numbers 0-9 for specifying mixing times. Included in the group of switches are switches to enter the program. Also located on the key pad is an alarm to indicate that a mix procedure has been completed. Lights can also be included to indicate that power is being supplied to the mixer.

In one embodiment of the present invention power for the mixer is supplied by batteries which are included within the housing. Batteries suitable for use in the present invention are rechargeable nickel-cadmium batteries (12 V, 2.8 Amp). These batteries will typically run the mixer for three mixing cycles prior to requiring recharging. In an alternative embodiment of the present invention the mixer can be connected to A.C. power by use of a D.C. transformer. Such transformers are well known in the art. When batteries are used as a source of power, a "low battery" indicator light is also included on the key pad.

Also included within the housing is a vacuum-pressure pump for evacuating the interior of the liner during mixing. Pumps suitable for use in the present invention include diaphragm pumps such as those made by ASF Inc. of Narcross, Ga., which draw about 70 cm of mercury in vacuum mode and generate 100 psi in pressure mode of operation, used in combination with solenoid valves such as those sold by Humphrey Valve Co. of Kalamazoo, Mich. and vacuum transducers, made by Omega of New York, N.Y., which provide a sensing mechanism to control and maintain the desired pressure level. The solenoid valves are chosen to operate in a range of about 0-76 cm of mercury in vacuum mode and up to 120 psi in pressure mode of operation.

To sterilize the mixer for use the housing is autoclaved. Since the housing includes the vacuum-pressure pump, the program controller, batteries and motors within the housing, which are not readily removable, the housing also includes shielding for these components to prevent them from being exposed to the high temperatures experienced during autoclaving. Suitable shielding is provided by encasing the heat sensitive components in protective "cans" made of materials such as glass-filled FORTRON grade of polyphenyline sulfide, such as that sold by Hoechst Celanese of Chatham, N.J. (which is suitable for use at temperatures up to about 280.degree. C.) EKTAR, a 15% glass filled thermoplastic co-polymer resin sold by Eastman Performance Plastics of Kingsport, Tenn. (which is suitable for use at temperatures up to about 255.degree. C.) or ULTEM sold by General Electric.

On the underside of each lid 14 is a seal 17 which seals against the housing when the lid is closed. Also located on the underside of each lid is a drive socket 18, which connects the motor to a paddle or impeller shaft 92 of the liner. The end of the shaft has, in a preferred embodiment, a pentagonal cross section to prevent slippage between the shaft and drive socket. While a pentagonal shape is preferred, it is understood that other shapes such as triangular, square and hexagonal would also be suitable.

Also located on the underside of the lid are blades 20, so that when the lid is closed, the blades extend into vial cavities 32 of the liner to break and thereby open vials contained in the vial cavities.

The housing body comprises a pair of chambers into which liners are placed (the chambers are shown with the liners in place). Each of the chambers are open at their bottom (not shown) to allow the liner to extend to the exterior of the housing and for the attachment of cement collection adapters. O-rings are located around these opening to seal the liner in place.

The housing body is preferably mounted on a stand 22 projecting upward from a broad horizontal base 24 that provides support for the housing. The housing is preferably made from a high temperature plastic material such as ULTEM, made by General Electric, or VECTRA made by American Celanese Corp. However, areas that may come in contact with the cement, such as the area directly under the mixing chambers, are preferably made from, or coated with, a material such as stainless steel which can be easily cleaned to remove any cement that may be spilled during the preparation of the cement.

In one embodiment of the present invention (FIG. 1), a vacuum-pressure port 26 is also included in the lid. The vacuum port is connected to the vacuum pump, located in the housing of the mixer, by pressure resistant or vacuum tubing. The vacuum may also be connected to a "house" vacuum line as well as, or in place of, the vacuum pump. In another embodiment (FIG. 2), multiple vacuum-pressure holes 48 are located around the periphery of the liner. These holes communicate with a manifold in the liner that is connected to the vacuum pump and/or house vacuum line. In another embodiment vacuum/pressure holes 31 (FIG. 3) are provided in the liner at the ends of the vial cavities. In this embodiment the holes attach to a manifold in the housing, and communicate with the chamber of the liner via cavities placed in the lid (not shown).

Liner 30 of the present invention, illustrated in FIGS. 3-8, is made from materials such as polyethylene, and can be discarded after use.

In general, the liner comprises a cup shaped chamber 36 which forms a well of generally circular cross section, into which the components of the cement are placed for mixing. Located at the top of the liner, and at one or both sides, are vial cavities 32 into which are placed vials which contain the liquid catalyst for the cement. At the bottom of the liner is a port 130 for collecting the cement at the end of the mixing procedure. An impeller 90 is provided within the liner for mixing the cement.

In one embodiment of the present invention (see FIGS. 6-8) ribs 50 are placed along the outside of the liner and at its bottom that act to strengthen the liner and inhibit its collapse under the desired operating vacuum. In this embodiment of the present invention four ribs are placed around the outside of the liner and are equally spaced from each other. Matching grooves can be placed in the walls of the housing chambers, into which the liners are dropped, so that the ribs can act to align the liner within the housing. However, the ribs may result in stress risers with extended use or at high vacuum/pressure.

In one embodiment of the present invention the liner is evacuated via vacuum port 26 located in lid 14. In another embodiment (see FIGS. 6-8) a port 46 is located in the liner for connecting the liner to the vacuum-pressure pump. A series of holes 48 are located in the liner, in close proximity to the vacuum port connector and open to a manifold that is connected to the vacuum port. These holes allow the interior of the liner to be evacuated and vented as needed.

In another embodiment of the present invention (FIGS. 25-27) the liner has an additional hole 326 in the center of its bottom that is aligned with the bottom end of a hollow impeller shaft 302 that fits snugly into the bottom of the liner. Liner hole 326 is aligned with hole 306 in the bottom of shaft 302 exposing conduit 320 to the interior of attached cement cartridge 310 so that the air can be evacuated from the cartridge before the cement is transferred after mixing.

Vial cavities 32 in one embodiment of the present invention (FIGS. 3-5) comprise a shallow section 38 and a deep section 40 which are connected by an angled floor 42. In operation, vials containing catalyst, are placed in the vial cavity in a horizontal position, resting at one end on the floor of the shallow section and at the other end on a shelf 44 that is at the same level as the floor of the shallow section and above the floor of the deep section. A single vial is placed in one vial cavity when a single batch of cement is to be mixed or, in those embodiments with two vial cavities per cup shaped chamber, a vial is placed in each of the two vial cavities when a double batch of cement is to be mixed. The vial cavities are located in a position on the liner so that, when the lid of the mixer is closed, the blades penetrate into the vial cavities, thus rupturing the neck of the vials in the vial cavities. In the embodiments of FIGS. 3-5, the rupturing also results in the vial being upended into the deep section of the vial cavity and ensures complete emptying of the contents of the vial.

Catalyst is commercially available in different sized vials, which have different lengths. Therefore, it is preferable that the vial cavity is able to accommodate the different sized vials. In the case of the long vial, the vial cavity is of a dimension such that a long vials will fit snugly into the vial cavity. When a shorter vial is used, the vial is held by the close fit of the liner side walls 54 (FIG. 4).

In another embodiment, a mechanism (see FIGS. 6-8) which comprises a plunger 56 is used to hold the small vial in place. The plunger extends along the length of the vial cavity and through an end 57 to the exterior of the vial cavity. The plunger is attached to a spring 58. When the vial cavity is empty, the plunger contacts the end wall 60 of the cavity and the spring is relaxed. When the plunger is pulled away from the end of the vial cavity, the spring is compressed and a small vial can be placed in the vial cavity. The plunger, with the force applied by the spring, holds the vial in place and at the end of the vial cavity. When a large vial is used, the plunger is drawn back along the full length of the vial cavity, to allow the large vial to be placed in the vial cavity.

In another embodiment of the present invention (not shown), a tension spring is used to hold the vial in the vial cavity. In this case a shorter plunger is attached to a tension spring. The spring is attached to end wall 60 of the interior of the vial cavity. When the plunger is drawn back to accommodate a vial, the spring is stretched and applies a force to the plunger which in turn secures the vial in place.

When the lid is shut, the vial is ruptured and the liquid contents of the vial are released and flow through a duct, into the liner chamber where they are mixed with the other components of the cement.

In each embodiment of the present invention, the vial cavities are connected to the liner chamber by a duct 52. In a preferred embodiment of the present invention, a screen is located in the duct to prevent any glass slivers, that may result from the rupturing of the vial, from entering the cement mixture.

In the interior of the liner is a mixing impeller. In one embodiment of the present invention (see FIGS. 9 and 10) the impeller comprises two blades 94 extending from opposite sides of a shaft 96. The blades are bent at an angle of about 45.degree. so that the extremities of the two blades are approximately parallel to each other (see FIG. 10). When viewed from the side (see FIG. 9) the blades form a roughly semicircular shape to conform to the shape of the interior of the liner. In one embodiment of the present invention the blades include "cut-out" sections 98 which reduce the amount of blade which comes in contact with the cement thus reducing cement loss.

At the top end of the shaft is a cross-shaped member 100 which centers the shaft, and the blades, when the impeller is placed in the liner. In a preferred embodiment of the present invention the top sides of the cross-shaped member are rounded to inhibit cement powder collecting on its tops when the cement powder is introduced into the liner. On the top of the shaft in the center of the cross-shaped member is shaft 92 which connects to the drive of the motor when the lid is closed. The end of the shaft comprises a generally pentagonal shaped projection for mating with the pentagonal shaped cavity of the socket.

In another embodiment of the present invention (see FIGS. 6-8), two pairs of arms 102 of an impeller bearing extend across the interior of the liner. Each of the arms is placed at 90.degree. to each adjacent arm and is attached at one end to a centrally located cylindrical bearing 103. The arms are slanted downward to form a "V" shape. The outer ends, and the upper part of the "V", attach to the liner body. The powder components of the cement are introduced into the liner by pouring them around the arms and into the well of the chamber of the liner. The downward slant of the arms prevents the powder from collecting on the arms.

At the intersection of the arms and extending upwardly through cylindrical bearing 103, in the center of the liner, is a shaft 92 which connects to the drive of the motor when the lid is closed. As in the previous embodiments, the end of the shaft comprises a generally pentagonal shaped projection for mating with the pentagonal shaped cavity of the socket. It is desirable that the end of the shaft does not contain any cavity or other structure that could collect powder as the powder in introduced into the liner. When the lid is closed the shaft connects with the socket and thus the drive of the motor. The matching pentagonal shapes of the shaft and the socket prevent the drive shaft slipping relative to the socket. Again, while a pentagonal shape is preferred, it is understood that other shapes such as triangular, square and hexagonal would also be suitable.

At a lower end of the mixer shaft 96 are paddles 94. When the mixer shaft is turned by the action of the motor the paddles move through the cement components thereby mixing them.

In another embodiment of the present invention (see FIGS. 24-27) two pairs of arms 301 of an impeller extend radially outward from opposite sides of the impeller shaft across the interior of the liner and then turn downward as shown in FIG. 24 eventually tapering to a narrower distance from the shaft at the bottom to fit the small end of the "bowl-like" liner. Each of the arms 301 then extend inward to reattach to shaft 302 at 180 degrees to each other. Vertical members 312 extend parallel to the impeller shaft on each side to extend between the upper and lower radially extended portions of the arms 301. These members help to stiffen the impeller blades. When viewed from the side, as shown in FIG. 25, the blades form a geometric pattern with minimal surface area to which cement can cling after mixing. In a preferred embodiment of the present invention, the top of the arms are rounded to prevent the powdered components of the cement from collecting on the thin top part of the blade arms when the cement is added to the liner.

Impeller shaft 302 has a hollow interior to form a central conduit 320 running from the top of the shaft to its bottom. At the top end of shaft 302 is a cross-shaped fitting 303 that attaches to the drive of the motor when the lid is closed enabling the attached motor to turn the impeller and mix the cement. The radial prongs 304 of the cross-shaped fitting 303 are open at the top and the ends exposing the interior conduit 320 to the interior of the mixer. The interior conduit has a wider diameter at its upper end 313 than at its lower end 314 (see FIG. 25). An inwardly extending lip 315 separates the wider diameter section from the lower diameter section.

Shaft hole 306 at the bottom end of shaft 302 (see FIG. 27) aligns with hole 326 in the bottom of the liner exposing it to the portable cartridge 310 (which in turn will be filled with mixed cement, removed from the mixer apparatus and attached to a bone cement gun). The outside diameter of bottom tip 325 of shaft 302 is less than the outside diameter of the rest of shaft 302 (see FIG. 27) and is hemispherical in shape. This allows the impeller shaft 302 to fit into a hemispherical depression or centering bearing at the bottom hole of the liner. The tight fit between the bottom tip 325 of impeller shaft 302 and the bottom of the liner (see FIG. 27) provides a sufficient seal to prevent the cement from leaking through prematurely.

At the top of shaft 302, positioned inside the interior conduit 320 and supported by lip 315, is a conventional "one-way" check valve 305 (see FIG. 26) that allows gas to flow up and out of conduit 320 into the mixing chamber, but prevents gas from flowing down conduit 320. The "one-way" check valve 305 consists of a plunger 307 attached to spring shaft 321 connecting to stopper 309. Ring 311 rests on edge 322 and spring lip 323. In its closed position spring 308 pushes on plunger 307 which pulls stopper 309 down onto ring 311 creating a seal. When pressure, greater than the force of spring 308 is applied to plunger 307, the plunger causes spring 308 to contract and stopper 309 is lifted off ring 311 by spring shaft 321 allowing gas to pass through valve 305.

Applying a vacuum to the mixing chamber reduces the gas pressure in the mixing chamber and the resultant greater gas pressure inside conduit 320 (exposed to cartridge 310) pushes on plunger 307 overcoming the force of spring 308. This lifts stopper 309 off ring 311 exposing conduit 320 and cartridge 310 to a vacuum source which evacuates the gas inside cartridge 310. Subsequently, as the gas pressure inside the conduit 320 and cartridge 310 approaches that of the mixing chamber, it is no longer sufficient to over come the force of spring 308. Spring 308 extends and causes stopper 309 to seal up against ring 311 and the opening is closed. Likewise, when positive air pressure is introduced inside the mixing chamber to help in the transfer of the cement to cartridge 310, stopper 309 is pushed down against ring 311 and no air is allowed to flow down conduit 320. This preserves the vacuum in cartridge 310. It is the combination of vacated cartridge 310 and the positive pressure in the interior of the mixer that enhances delivery of the cement from the mixer into cartridge 310 after mixing is complete. Additionally, this reduces the likelihood that air pockets will be folded into the cement during the transfer.

In another embodiment of the present invention (see FIGS. 11 and 12) a liner comprises a paddle which is collapsible. The paddle 94 is of a fan-like construction. In this design the collapsing of the blade is desirable since a plunger 104 is used to force the cement out of the liner when the mixing procedure is complete. In this embodiment of the present invention the plunger is moved to the bottom of the liner by the action of a spring 106. The spring is coiled during the mixing procedure and is released when the mixing is completed.

It should be noted that at all times during the preparation of the cement, i.e., from the time the lid is closed, the chamber of the mixer is sealed and fumes from the contents of the chamber are unable to escape. Also, during the mixing procedure a vacuum is applied which, on one hand prevents the formation of air bubbles in the cement and on the other hand removes fumes from the chamber, preferably through the house vacuum line. Thus the room in which the cement mixing procedure is performed remains relatively free of the irritating and potentially toxic fumes which are normally associated with the preparation of cement. It is only at the completion of the mixing procedure that the chamber is opened, at its bottom, to allow the cement to flow from the chamber of the liner into a cartridge.

The vacuum applied to the interior of the liner during mixing is preferably about 55 cm of mercury. If a stronger vacuum is used the components of the cement may "boil" or evaporate and the liner may implode. A weaker vacuum may be insufficient to "degas" the cement during mixing.

At the completion of the mixing procedure a vacuum is again drawn in the interior of the chamber to evacuate any fumes that have been generated during the mixing cycle. The vacuum is released and positive pressure is applied to the interior of the liner. The cement is then allowed to flow from the liner through a port at the bottom of the liner. As noted above, in one possible alternative embodiment, a plunger is used instead of or in addition to the positive air pressure to dispense the cement through the bottom port. The cement is collected into cartridges or syringes for dispensing the cement.

In yet another alternative embodiment, at the end of the mixing cycling when the impeller 300 with the hollow shaft is being used as shown in FIGS. 24-27, cartridge 310 attached to the bottom of the mixing apparatus will be vacated at the same time as the interior of the mixer. The hollow shaft 306 of impeller 300 is then sealed off from the interior of the mixer by valve 305 before the vacuum is "released", preserving the vacuum in the cartridge 310. This vacuum then aids in the transferring of "bubble free" cement from the mixer to the cartridge 310.

In one embodiment of the present invention, the port 130 (as illustrated in FIGS. 13 and 14) for removing the cement from the liner comprises a generally cylindrically shaped member 132. At one end of the cylinder is an aperture 134 comprised of multiple openings which mates with cup shaped chamber 36 of the liner. In one embodiment of the present invention the cylindrically shaped member is attached to the liner by a "bayonet" type fit 139 (see FIGS. 13 and 14). In another embodiment the cylindrically shaped member is attached to the liner by welding.

The aperture 134 encloses an open grid. In the center of the grid is an attachment point 138, to which is attached a membrane 140, on the under side of the grid. When a vacuum is drawn in the chamber (see FIG. 13) the membrane is drawn flat against the aperture forming a seal. At the end of the mixing cycle, when pressure is applied to the chamber, the membrane is deflected away from the aperture allowing cement to flow out through the aperture (see FIG. 14).

At the opposite end of cylinder 132 is a second aperture 142. At the base of the second aperture is a attachment means which comprises two parallel slots 144 which run across the interior of the cylinder. The slots allow attachment of a detachable cement collection means 146 to be attached to the liner for collection of the cement after mixing. The collection means comprises flanges 149 which slide into slots 144. The collection means is held in place and sealed against the cylinder 132 by O-ring 147.

In another embodiment of the present invention, port 130 (see FIGS. 15 and 16) comprises a slide mechanism 153, and attachment means for the attachment of a cement collection device to the liner is provided similar to that described above. Flanges on the top of a cartridge adapter slide into slots (as illustrated in FIGS. 13 and 14) attached to the bottom of the mixer housing. As an adapter is pushed into place, it abuts lever 152 and slide 60 is pushed away from a liner plug 154 located at the bottom of the mixer chamber, aligning aperture 155 with the liner plug.

The port is in a closed position, as shown in FIG. 15, during the mixing procedure and is held in the closed position by slide 153. During mixing the port retains the components of the cement within the liner chamber. At the end of the mixing process, a cartridge is slid into place, pushing the slide away from the plug, the vacuum is "released" and positive pressure is applied to the liner chamber to thereby open the plug and force the cement to flow into cartridges for dispensing.

In another embodiment of the present invention, illustrated in FIGS. 17 and 18, the bottom of the mixer comprises a slot which is offset, by walls 157, from the main body of the mixer housing. The slot is similar to that described above. In this design the liner, which fits into the mixing chamber, has a bell shaped port 156 at its bottom end. At the top of the port is a plug 158 which is contiguous with the floor of the liner, when in the closed position. In the closed position the components of the cement are retained in the liner and there is no "dead space" into which powder or liquid components could become trapped and thereby, not mixed. The plug is held in place by the vacuum which is applied during the mixing process and by a "crush rib" 160 on a hinge 162 which attaches the plug to the liner. If the alternative embodiment in which the hollow shaft 306 of impeller 300, as shown in FIGS. 24-26, is used to evacuate a cartridge 310 attached to the bottom side of the mixer, the plug on the bottom side of the mixer will be held in place solely by a "crush rib" 160 on a hinge 162 attached to the plug of the liner.

When the mixing of the cement is complete the vacuum is released and positive pressure is applied to the inside of the liner. This positive pressure forces the plug out of the liner and allows the cement to flow from the liner and into cartridges attached to the bottom of the mixer. The cement is collected in the cartridges which can then be fit into a cement injection gun for dispensing as required.

Dispensing of the cement is performed by placing the cement into a cartridge that fits onto a cement injection gun, such guns being well known in the art. The cement injection gun is then used to deliver the cement to the surgical site as required. Many different designs of cement injection guns, and cartridges designed to fit them, are available. The cartridge designs differ depending on the manufacturer of the cartridge and the use for which the cartridge was designed. Some cartridges have screw tops while others have handles and flanged tops and many of the available cartridges have different diameters. Some cartridges have long nozzles, such as those designed to deliver cement to the femur, while others have relatively short nozzles. In one embodiment of the present invention, an adapter 190, which will fit all, or most of the available cartridge designs, is used to attach the cartridge to the mixer.

In one embodiment of the present invention, shown in FIGS. 19 and 20, cartridge holder or adapter 190 comprises a handle 194 attached at a top end to a top brace 196. The top brace comprises a central bore 198 through which the cement can flow from the mixer into a cartridge held in place by the adapter. The central bore is formed by a tube. On an upper side of the outer diameter of the tube are attached flanges 192 which attach the cartridge adapter to the mixer by sliding into slots 210 (illustrated in FIGS. 21 and 22) on the bottom of the mixer housing. The outer surface of the tube is stepped 200 to form different sized diameters. Each of the different diameters is chosen to fit an internal diameter of a tubular cartridge. Thus the cartridge is attached to the top brace by pushing the cartridge over the appropriately sized diameter step thus forming a friction fit which holds the cartridge in place. The step also centers the cartridge below the bore so that cement can be delivered into the cartridge.

A bottom brace 202 comprises a ring 204, the internal diameter of which is stepped 205 to accommodate the outside diameter of the various tubular cartridges that are commercially available. A central bore 206 in the ring allows different types of cartridges, such as those with long nozzles, to be fitted into the bottom brace. A thimble shaped member 207 is attached to the outer perimeter of the ring. The ring is attached to the handle by forcing the thimble shaped member over a projection 208 attached to the base of the handle. When the bottom brace is attached to the handle, a cartridge is held securely in place.

In another embodiment, an adapter is designed for use with cartridges which have flanges at the top of their tubular structure as shown in FIGS. 21 and 22. Adapters for use with such cartridges comprise a flange capture member 210 or slot which is the same as that described above. Generally commercially available cartridges which have a flanged top 192 are relatively small and in use it is desirable to have two such cartridges filled with cement at the same time. Therefore, it is desirable that the cartridge adapter have places for two cartridges.

In this embodiment of the present invention, the adapter handle 214 is attached to a holder which comprises two slots 216 on its lower side, adjacent to each other, into which are slid the flanges 212 of the cartridges 218. At a top side of the adapter are flanges for attaching to the bottom of the mixer, as described above. The adapter preferably comprises a means for diverting the cement, as it flows from the mixer, to each of the cartridges. The adapter comprises a lever 220 attached to a handle 222. When one cartridge is being filled the lever is in a first position which allows the cement to flow into a first cartridge. When the first cartridge is full the handle is moved so that the lever allows the cement to flow into a second cartridge.

In another embodiment the adapter of FIGS. 20 and 21 may include an "angled" top brace so that, when installed on the mixer, the adapter is angled away from the perpendicular plane to allow for clearance of large adapters and to allow easy fitting and removal of the adapters.

In operation, the housing 12 is sterilized by autoclaving and then connected to house vacuum line in the operating room. A pre-sterilized bone cement kit is opened and liner 30 is placed in the mixer housing 12 and an injector gun or adapters (not shown) are placed on the cement outlet of the liner. Components of the cement, such as the powder which includes the methyl methacrylate monomer are then placed in the liner and vials containing catalyst, for the hardening of the cement, are placed in vial cavities 32 located in the liner. A programmed specifying the desired mixing and holding times, sequence of vacuum pump and mixer operation or other desired parameters are input via the key pad. Lid 14 of the housing is then closed to seal the components of the cement within the mixer.

Closing the lid breaks the vials, releases the catalyst into the other cement components and initiated the preprogrammed functions. A vacuum is applied to the interior of the liner to prevent the formation of air bubbles in the cement as it is mixed and also to remove fumes generated from the components of the cement. After the bone cement mixer has been evacuated the mixer is started.

At the end of the preselected mixing time, the mixer stops automatically and, in one embodiment of the present invention, sounds an alarm to alert the operator. A vacuum is again applied to the chamber to evacuate fumes which have built up during the mixing procedure. When the mixing is completed, the vacuum is released and pressure is applied to the chamber to evacuate the cement from the mixer by pushing the cement through a port located at the bottom of the mixer into a cement gun or various cartridges for dispensing the cement.

In a preferred embodiment of the present invention, the liner, alone or along with the cartridge adapter, are supplied in pre-sterilized packets. Therefore, the packets only need to be aseptically opened and the liners placed in the mixer chamber, for the bone cement mixer to be ready for use. Such procedures can easily be performed by a "sterile" person and an additional "nonsterile" person is not required for the routine operation of the mixer.

At the end of the mixing procedure the cartridge adapter packet is opened and a sterile cartridge is placed in the adapter, which is then attached to the mixer. The cement then flows into the cartridge without any intervention by the operator. Again this procedure is easily performed by a "sterile" person without the assistance of a "non-sterile" or an additional "sterile" person. Therefore, the operation of the mixer is easily carried out by a single person, reducing the need for additional staff in the cement mixing procedure.

According to yet another embodiment of the invention as illustrated in FIGS. 28 to 34, a two-part liner is provided for insertion into a mixer housing. Referring to FIG. 28, a liner 401 is illustrated comprising a basket 402 which nests into a mixing bowl 403. Such a configuration is useful in accommodating a greater volume of bone cement than the previously described embodiments. While not illustrated, a mixer housing similar to those of the previous embodiments is provided for receiving the liner.

The basket of this embodiment includes three partition walls 404 which define four vial cavities 406 for receiving four vials of bone cement catalyst. Consequently, four batches of bone cement can be mixed together at once. Of course, if a smaller batch is desired, one, two or three batches can be prepared at once. Opposite the partitions is a shelf 407 upon which the end of the vials may rest. Between the shelf and the vial cavities, a trough 408 is provided in the basket for collecting the liquid catalyst from the vials. The liquid catalyst collected in the trough is directed into the mixing bowl through a pair of catalyst ports 409. As set forth above, the catalyst ports preferably include screens to prevent pieces of broken glass from the vials from being mixed into the bone cement. Of course, while a pair of catalyst ports are illustrated, one, two, three or more catalyst ports can be provided.

A hollow cylindrical impeller shaft sleeve 411 is provided extending up through the basket such that the basket can be nested into the mixing bowl with the impeller shaft extending up through the impeller shaft sleeve. The impeller shaft sleeve can also act as a bearing for the impeller shaft. In the preferred embodiment, the basket and mixing bowl are permanently attached to one another with the impeller in place. Preferably, the two are sonically welded to one another.

Referring to FIGS. 28 to 31, a cup-shaped mixing chamber 414 is provided in the mixing bowl for receiving the powdered bone cement. The mixing bowl also includes a ledge 416 for receiving the basket. In using the present liner, the liner is first placed in the mixer housing and the powdered bone cement is added to the mixing bowl through top opening 420 in the liner. Up to four vials of catalyst are then placed in the vial cavities of the basket with their necks resting on the shelf in the basket. As described above for the other embodiments, when the lid of the housing is closed the necks of the vials are ruptured releasing the catalyst into the powdered bone cement. The impeller is also started as the lid is closed so as to thoroughly mix the ingredients.

The mixing bowl of this embodiment also includes a U-shaped cement port 417 which simplifies the withdrawal of the mixed cement from the mixing bowl as will be described in further detail below. A vacuum port 418 is provided so that a vacuum can be applied to the mixing bowl to prevent any bubbles from forming in the bone cement as it is being mixed and to withdraw fumes that are generated during the mixing. A first vacuum conduit 419 connects the mixing bowl vacuum port to a source of a vacuum.

A discharge tube 421 is provided for filling bone cement cartridges such as are useful for applying the bone cement during a surgical procedure. An impeller shaft hole 422 is provided at the bottom of the mixing bowl to provide a bearing surface for the end of the impeller shaft as it rotates in the mixing bowl to stir the bone cement mixture. A pair of hinge brackets 423 with hinge apertures 424 are provided beneath the cement port of the mixing bowl in the discharge tube. Also beneath the cement port are a pair of discharge tube vacuum ports 426. These discharge tube vacuum ports connect to a vacuum source by a second vacuum conduit 427.

Referring to FIGS. 32 and 33, a U-shaped trap door 431 is provided for sealing the U-shaped cement port of the mixing bowl. An upper surface 432 of the trap door is contoured to mate with the cement port. A pair of hinge pins 433 on hinge arms 434 cooperate with the hinge apertures of the hinge brackets so that the trap door can swing down opening the cement port once the cement has been sufficiently mixed with the catalyst. The trap door is designed to form a friction seal within the U-shaped cement port so that the trap door is normally in a closed position.

Referring again to the procedure for mixing bone cement using the present embodiment, first one or more bone cement cartridges are put in place at the discharge tube. The proper amount of bone cement powder is placed in the mixing bowl of the liner with the use of a funnel and the correct number of bone cement catalyst vials are placed in the basket of the liner. A mixing program is selected and the lid of the housing is closed to simultaneously rupture the vials and begin rotation of the impeller. At the same time, a vacuum is drawn on the mixing bowl through the first vacuum conduit. Once the mixing is complete, the vacuum at the first vacuum conduit is broken and a vacuum is drawn on the second vacuum conduit. The vacuum in the second vacuum conduit along with the weight of the bone cement in the mixing bowl causes the trap door to rapidly swing open and release the contents of the mixing bowl through the discharge tube and into one or more bone cement cartridges. The rapid discharge is promoted by both the shape of the trap door which is shaped to straddle the impeller shaft hole in the mixing bowl and provide a large flow area and the pressure differential between the mixing bowl and the discharge tube. By rapidly discharging the contents of the mixer into the bone cement cartridges, the formation of air bubbles in the cement is reduced and the ability to use the bone cement at its optimum state of cure is ensured.

Referring to FIG. 34, a schematic diagram illustrates the vacuum system of this embodiment. According to this diagram a mixing bowl 403 having a cup-shaped mixing chamber 414 is provided with an impeller 441 having an impeller shaft 442 extending into the mixing chamber for mixing the powdered bone cement with the liquid catalyst. A U-shaped trap door 431 is provided at the bottom of the mixing chamber straddling the impeller hole 422.

A first vacuum port 418 through the mixing bowl communicates with a first vacuum conduit 419 for drawing a vacuum during the mixing procedure. A second vacuum port 426 through the discharge tube 421 communicates with a second vacuum conduit 427 provided for helping to draw the mixed bone cement into the bone cement cartridge.

A vacuum pump 443 is provided for drawing the necessary vacuum. A first solenoid valve 444 and a second solenoid valve 446 are provided on the first and second vacuum conduits, respectively. During the mixing operation, the first solenoid valve is opened while the second solenoid valve is closed. This permits the vacuum pump to draw a vacuum on the mixing chamber during the mixing operation. Once mixing is complete, the first solenoid valve is closed and the second solenoid valve is opened in order to draw a vacuum in the discharge tube to assist in filling the bone cement cartridge and further to assist in rapidly opening the trap door to discharge the contents of the mixing bowl.

In the preferred embodiment a pair of vacuum pumps are provided. A first vacuum pump provides a high volumetric flow rate for when the mixing operation is first initiated so that the air in the mixing chamber can be rapidly withdrawn. Once a desired vacuum is reached, the vacuum is maintained by a second vacuum pump which operates at a lower volumetric flow rate to withdraw any fumes that may form during mixing. Generally, only the first higher volume vacuum pump communicates with the second vacuum conduit as it is desired to have a high vacuum in the discharge tube to rapidly open the trap door and rapidly discharge the bone cement once the mixing operation has been completed. The use of two vacuum pumps is useful for conserving battery power during operation. Of course, a single multiple speed vacuum pump can also be used with a similar result.

In yet another preferred embodiment, the first vacuum conduit can also communicate with a source of pressurized gas so that when the mixing is completed, the mixing bowl can be pressurized to assist in rapidly opening the trap door and discharging the contents of the mixing bowl through the discharge tube. By maintaining a vacuum in the discharge port, if any air bridges the discharge port, it will be withdrawn from the discharge tube rather than forming air bubbles in the bone cement.

The above descriptions of exemplary embodiments of a mixing apparatus are for illustrative purposes. Variations will be apparent to those skilled in the art. For example, an injection gun designed specifically for use with the bone cement mixer could be used in place of the cartridge adapters described. Therefore, the present invention is not intended to be limited to the particular embodiments described above. The present invention may also be practiced in the absence of any element not specifically disclosed. The scope of the invention is defined by the following claims.