Apparatus and method for fixing and dispensing bone cement

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DESCRIPTION

Apparatus and procedure for mixing and distributing bone cement.

Background of the invention

This invention relates generally to apparatus for mixing and dispensing bone cement. In particular, this invention concerns improvements to a disposable preassembled kit that includes a set of preparation apparatus used to mix and distribute bone cement. The improvements are aimed at eliminating trapped air in the mixed cement and improving the mixing and distribution characteristics.

In many orthopedic surgical procedures, bone cements are used to secure implants to the bone. Conventional bone cements are generally polymeric materials that are prepared by copolymerizing the components as needed. Bone cement is prepared by copolymerizing a liquid monomer and a powdered copolymer, such as methyl methacrylate and polymethyl methacrylate or methyl styrene methacrylate. During the mixing of the constituent components of the cement, air bubbles can form within the cement. It is thought that to improve the resulting strength of the cement, air bubbles must be evacuated from the mix to ensure a uniform reaction product. Consequently, the mixing of the constituent components is ideally carried out in a vacuum.

Mixing separate constituent components within a mixing cartridge is well known in the art. For example, US patents N^you5,624,184 and 5,586,821, which are assigned to the assignee of the present invention, describe a mixing cartridge according to the preamble of claim 1, comprising two telescopic tubular cylinders. Telescopic cylinders can be placed in a fully extended position in which the bone cement components are mixed under vacuum, in a vacuum release position in which the inner cylinder is partially retracted within the outer cylinder, and in a fully retracted wherein the cartridge is adapted for installation within a bone cement mixing gun. A cap is installed at the proximal end of the outer barrel and includes an opening therethrough which receives the handle of a stirrer. The cap also includes a hole for connection thereto of a vacuum pump that is used to draw a vacuum in the cartridge when it is in the fully extended position. The portion of the stirrer handle that extends into the cartridge includes stirrer blades that extend therefrom to mix the bone cement. The other end of the stirrer handle extending from the lid opening includes a handle thereon for reciprocating movement of the stirrer within the cartridge. The handle includes a tubular outer sleeve and an axially disposed internal rod therein terminating in an integral dumbbell-shaped end plug. The handle sleeve is fragile and has a detachable distal end so that after mixing is complete, the handle can be detached from the cap along with the internal rod. This results in the opening in the cap that retains the distal end of the handle that has the stirrer blades integrally connected thereto. An appropriate cement applicator nozzle can be fitted into the opening (exit port), and the cartridge can be inserted into an applicator gun. The inner cylinder includes a piston plug that is axially movable toward the end of the cartridge cap by actuating the cement applicator gun.

In use, the cartridge described above is initially fully extended and the bone cement constituents are placed therein. The cap is then installed on the outer cylinder and a vacuum pump is attached to the outlet port contained in the cap. A vacuum is made in the cartridge by alternately moving the handle on the vacuum pump. The bone cement components are then mixed under partial vacuum by alternately moving the handle of the stirrer. After mixing is complete, the inner cylinder is partially retracted into the outer cylinder, releasing the seal between them and thereby releasing the vacuum. The cartridge is then placed in its fully retracted position and installed within an applicator gun to inject bone cement into a patient.

It is desirable to prevent trapped air within the bone cement. It is also desirable to further improve the mixing characteristics of the kit described above.

Summary of the invention

The present invention provides an improved agitator that provides improved mixing, especially in an area near the cylindrical walls of the inner and outer cylinders.

In one form thereof, the present invention provides a bone cement mixing apparatus, comprising first and second cylinders telescopically connected to mix and distribute bone cement. The cylinders include respective internal cylinder walls in which one of the internal walls has an internal diameter greater than an internal diameter of the other internal wall. A bone cement stirrer is also provided which has a part that can be inserted into the cylinders and is axially movable within the cylinders. The stirrer comprises a handle and a plurality of paddles are connected to and extend generally radially therefrom to an end portion of the handle. The blades are radially movable and are radially resiliently biased outward in cleaning connection with the inner cylinder walls. In this arrangement, the blades are bent outward to clean the inner walls of both the inner and outer cylinders when the agitator is reciprocated within the telescopically connected cylinders.

An advantage of the improved agitator of the present invention is that it improves mixing around both the inner and outer cylinder walls. This is important because the dry powder component of unmixed bone cement tends to stick to the cylinder walls and does not mix. The cleaning connection provided by the stirrer of the present invention reduces this problem.

Another advantage of the stirrer of the present invention is that it provides an internal and external telescopic cylinder cleaning connection, each of which has a different diameter. Also, resiliently deflected vanes do not "stick" as they move from cylinder to cylinder. On

On the other hand, the resilient vanes clean the inner cylindrical wall of the upper (outer) cylinder, then the vanes are bent inward and clean the inner cylindrical wall of the lower (inner) cylinder. In comparison, a non-resilient vane sized to fit the smaller diameter cylinder cannot provide the larger cylinder cleaning connection in a telescopic cylinder arrangement. Brief description of the drawings

The aforementioned and other features and advantages of this invention, and the manner of achieving them, will become more apparent and the invention itself will be better understood in connection with the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, in the what:

FIG. 1 is a perspective view of a kit including bone cement injection and mixing components in accordance with the exemplary embodiment of the present invention;

Figs. 2 and 2A are side views of the mixing cartridge and the lid connected with a part in cross section to illustrate the mixing of the constituent components;

Figs. 3 and 3A are perspective views of the agitator used with the mixing cartridge;

Figs. 4 and 4A are sectional views of the mixer cartridge illustrating axial movement of a piston plug;

Fig. 5 is an exploded fragmentary sectional view of a lid and stirrer assembly;

Fig. 6 is a fragmentary sectional view of the cap assembly showing bone cement about to be expelled therefrom;

FIG. 6A is a fragmentary sectional view of a prior art cap assembly showing bone cement about to be expelled therefrom;

FIG. 7 is a fragmentary sectional view of the cap assembly showing an annular gap between the cap and the shaker handle that contains air during the initial ejection of bone cement.

Corresponding reference characters indicate corresponding parts throughout the various views. The example set forth herein illustrates an exemplary embodiment of the invention, in one form, and such example should not be construed in any way as limiting the scope of the invention. Detailed description of the invention

FIG. 1 shows the kit 20 kit disposable assembly of an exemplary implementation of the present invention for use in the preparation of bone cement. Kit 20 consists of the following parts: a contoured storage tray 22, vacuum pump 24, connecting tube 26 having a vacuum gauge 28, a mixing cartridge 30, a cartridge cap 32, removable stirrer 34, nozzle cement applicator and cement applicator nozzle plug, generally indicated by 36, and a funnel coupling 38. Preferably, tray 22 is constructed from a molded plastic, having a plurality of contoured recesses 40 within which the various mixing apparatus and dispensing accessories are stored.

Typically, the liquid monomeric components of bone cement are provided in ampoules (not shown) and the powdered copolymers are provided in sealed packages (not shown), although any appropriate container for the components can be provided with the kit. Liquid monomer and powder copolymer are well known in the art and are commercially available from various sources.

Vacuum pump 24 uses a reciprocating piston design, although any suitable vacuum pump can be included as part of kit 20. Vacuum pump 24 includes a cylindrical body 42 and includes a reciprocating handle that actuates an internal piston (not shown). Vacuum pump 24 also includes a check valve (not shown) to allow air to flow through the pump orifice in only one direction. Tube 26 connects pump 24 to cap 32 when cap 32 is connected to mixing cartridge 30. Tube 26 includes vacuum gauge 28, which can be a plastic or rubber knob that collapses under the negative pressure of vacuum. made by pump 20, as shown in Fig. 2A.

Mixing cartridge 30 is preferably constructed of disposable plastic, which allows mixing of the constituent components to be seen. As shown in Figs. 2 and 2A, cartridge 30 includes two tubular cylinders 44 and 46 telescopically connected. The two cylinders, 44 and 46, have open proximal and distal ends. outer cylinder 44 axially receives inner cylinder 46 in a telescopic fashion. That is, the inner cylinder 46 can slide into the outer cylinder 44. The outer diameter of the inner cylinder 46 is concentrically fitted against the inner diameter of the outer cylinder 44. The outer peripheral surface of the inner cylinder 46 is in a tight fit but not sealed. hermetically with the inner peripheral surface of the outer cylinder 44 as a piston. Within the inner cylinder 46, near its distal end, an air-tight piston plug 48 (FIG. 4) is disposed to close the distal end of the inner cylinder 46. The plug 48 is axially slidable within the inner cylinder 46 to ejecting the mixed cement compound from cartridge 30 when the cartridge is connected to a cement applicator gun. That is, when the cement applicator gun is being actuated, the piston plug 48 advances axially from the distal end of the inner cylinder 46 toward its proximal end (toward the cap 32), as shown in Figs. 4 and 4A.

As shown in Figs. 4 and 4A, outer cylinder 44 includes a threaded outer edge 50 that threadedly engages an annular threaded cap edge 52 of cap 32. Although outer cylinder 44 is shown herein to include an outer threaded edge, any connection structure suitable for providing the connection between cap 32 and outer cylinder 44. The outer cylinder 44 also includes an outwardly extending annular end flange 54 near its open distal end. A seal between the inner cylinder 46 and the outer cylinder 44 is achieved by an O-ring 56 fitted within the annular groove 58 of the inner cylinder 46. The lid 32 is sealed on the cylinder 44 by the O-ring 59 fitted within annular groove 61.

Cap 32 includes a vacuum port 60, which can be connected to vacuum pump 24 via tube 26. As shown in Fig. 2, cap 32 also includes a centered cement outlet hole, or aperture. 62. The outlet port 62 includes threads 64 for threaded engagement of various nozzles 36.

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The cover 32 also includes vents 66 which are part of an air exhaust duct described later in this document.

As shown in Figs. 2 and 2A, the mixing system according to the invention includes a stirrer 34 having a handle 70. A handle 72 is connected to one end portion of the handle 70 and a plurality of paddles 74 are connected to the other end portion of the handle 70. paddles 74 include recesses 75 to aid movement of agitator 34 through bone cement, as is known in the art.

The advantages of the agitator shown in the illustrated embodiment can best be appreciated in relation to Figs. 3 and 3A. Paddles 74 extend generally radially from the end portion of handle 70 and are radially movable between a first position that paddles 74 assume when positioned within inner cylinder 46, as shown in FIG. 3A, and a second position. adopted by the vanes when they are removed from the cylinder, as shown in Fig. 3. Comparing Figs. 3 and 3A, it can be understood that the paddles extend from the handle a smaller radial distance when inserted into cylinder 46 than when positioned outside cylinder 46. As further seen in connection with Figs. 3 and 3A, the outer edges 76 of the vanes 74 form a substantially circular shape having a radius that is reduced as the vanes are inserted into the cylinder 46. The blades are formed so that they are radially resiliently biased outward toward the position shown in Fig. 3, so that the blades provide cleaning connection to the inner wall 78 of cylinder 46 when positioned therein. Significantly, the paddles 74 are also biased outward toward the cylinder 44, so that a cleaning connection is provided against the inner walls of the two cylinders 44 and 46 when the agitator is reciprocated therein (Fig. 2) .

The stirrer of the present invention may be formed of high density polyethylene, but it should be understood that one of ordinary skill in the art could substitute other suitable materials for it. The agitator 34 is formed so that the paddles 74 naturally occupy their extended position shown in Fig. 3. By selecting a material composition that has sufficient elastic properties, the vanes can be resiliently bent to form the contracted position shown in Fig. 3A. However, the elastic properties of the paddle material cause the paddles to deflect outwardly so that a force is exerted against the inner wall 78 when positioned within the cylinder 46. Likewise, vanes 74 are biased outwardly to provide a cleaning connection to the inner wall of cylinder 44 when positioned therein. In the illustrated embodiment, it can be appreciated that the paddles 74 need only be capable of withstanding a single use, as the kit 20 is discarded after a single use.

As also shown in Figs. 3 and 3A, the inner cylinder 46 includes a flared portion 80 at its proximal end to facilitate insertion of the paddles 74. Thus, when the paddles are inside the barrel 44 and the stirrer handle 72 is pressed down, the paddles 74 meet flared portion 80 and gradually contract to the position shown in Fig. 3A in which the radial distance blade 74 extending from the longitudinal axis of the handle 70 is substantially equal to the radius of the inner wall of the cylinder 87. The flared portion provides a gradual decrease in radius from the inner wall of the outer cylinder to the smaller diameter inner wall of the inner cylinder, so that the paddles do not jam when the stirrer handle is lowered.

Turning now to Figs. 2 and 2A, the illustrated embodiment incorporates a removable stirrer assembly, essentially the same as that described in US Patent No. 5,624,184. Handle 70 includes a tubular outer sleeve 82 and an inner rod 84 disposed axially within sleeve 82. Sleeve 82 is brittle and has a disposable distal end 86. The distal end of inner rod 84 terminates in an integral end plug in shape. dumbbell 88. The small diameter of the middle segment 90 is positioned proximate an annular notch 81 (Figs. 3 and 3A), or reference line, provided in the sleeve 82 of the handle. Annular groove 81 forms a radius around sleeve 82, around which sleeve 82 breaks when handle 70 is fully extended from cartridge 30. The hollow distal end 86 of the sleeve 82 remains within the outlet port 62 of the cap 32 to provide a conduit for the mixed cementitious compound to be expelled.

Referring to Fig. 4, piston plug 48 includes an integral boss 92 therein that faces toward cap 32. Generally, boss 92 is conical in shape designed to match the shape of cap 32 and resemble largely to the convex or "tangential current" profile of the bone cement, as the bone cement is advanced axially through cartridge 30 by movement of piston plug 48. Thus, although the protrusion 92 is shown in Figure 4 with a frusto-conical shape, it should be understood that the protrusion 92 may have other similar shapes, such as domed, capsule, and the like.

Referring further to FIG. 4, cap 32 includes a gap 94 the shape of which corresponds to shoulder 92. Thus, gap 94 receives shoulder 92 coincidentally when plug 48 is advanced axially toward cap 32 as shown in Fig. 4A. Although the gap 94 is shown in Fig. 4 to be frusto-conical in shape, it should be understood that the gap 94 could be of other similar shapes, such as a dome, bullet-shaped, and the like. The hollow 94 is shaped according to the projection 92. The vanes 74 also comprise a shape that corresponds to the projection 92 and the hollow 94, whereby the cover 32 and the piston plug 48 receive the vanes between them coincidentally. 74 when plug 48 is axially advanced toward cap 32 as shown in FIG. 4A. In the embodiment illustrated in Figs. 4 and 4A, the lid 32 having the recess 94, the piston plug 48, and the stirrer blades 74 all comprise concordant frusto-conical shapes, the operating advantages of which are described later in this document.

The illustrated embodiment also provides an air exhaust duct to vent substantially all of the air contained within the confines of the lid to the environment. As shown in Figs. 5 and 6, vents 66 extend through cover 32 and lead to handle 70. A porous filter cylinder4

ES 2 235 686 T3 dric 102 surrounds the handle 70 and forms a semi-permeable membrane that allows air to pass through it, but not bone cement. Very high molecular weight polyethylene has been found to perform satisfactorily as a material for the porous filter 102. Internally positioned in the cap 32 relative to the porous filter 102 is an O-ring 104 which also surrounds the handle 70. The O-ring 104, preferably made of silicone, seals the handle 70 with an internal surface of the cap 32, thus preventing the passage of air into the cartridge 30 during the expulsion of bone cement, as explained later in this document.

As further shown in Fig. 5, the interior of the lid 32 is widened to provide additional space for the upper filter 106. The filter 106 allows air to be drawn from the system through the vacuum port 60 without removing dry powder or cement. mixed. The upper filter 106 includes a two-step opening 108 that receives the retaining ring 110. As can be understood in connection with Figs. 5 and 6, the upper filter 106 includes the inner annular surfaces 112 and 114 that respectively mate with the outer annular surfaces 116 and 118 of the retaining ring 110. The lower inner portion of the retaining ring 110 includes a frusto-conical portion 120 that mates with the base 122 of blade assembly 74. Cap 32 also includes absorbent filter 124 that absorbs any monomer that is drawn through upper filter 106 before it can move in the direction of the tube.

As shown in Fig. 7, after the stirrer handle 70 is split, the nozzle 36 is screwed onto the cap 32. There is a small angular gap 128 between the stirrer handle 70 and the inner annular surface of the stirrer. cap 32, as shown in Fig. 7. The size of gap 128 shown in Fig. 7 has been exaggerated for illustrative purposes. There is also a small space 130 between the bottom of the nozzle coupling 36 and the end (split) portion of the shaker handle 70. The space 130 allows the passage of bone cement from the outlet opening 62 to the annular gap 128 Thus, referring to Fig. 7, it can be appreciated that annular gap 128 is in fluid communication with outlet port 62 due to the existence of space 130. It can further be appreciated that annular gap 128 is in gaseous communication with vents 66 because porous filter 102 is disposed in the middle of annular gap 128 and vents 66. In other words, porous filter 102 forms a gaseous communication conduit. connecting vents 66 to annular gap 128. Although the porous filter 102 in the illustrated embodiment is a cylindrical element surrounding the handle 70, it should be understood that one of ordinary skill in the art would readily recognize that other forms of a porous filter can be used, provided that one or more porous filters prevent them from bone cement leaks from vents 66.

Figs. 1-7 demonstrate the use of kit 20 of the illustrated embodiment in the preparation of bone cement. Kit 20 is packaged in an outer package (not shown) with funnel coupling 38 connected to cartridge 30. Cartridge 30 is packaged in tray 22 in its extended mixing position; that is, the inner cylinder 46 is fully extended relative to the outer cylinder 44, for the convenience of the user. The constituent components of the cement (not shown) are poured into the cartridge 30 through the funnel coupling 38. Once the correct volumes of the various constituent components of the cement have been poured into the cartridge, the funnel coupling 38 is removed. and is discarded. The lid 32 is then removed from the tray 22 and attached to the cartridge 30. Vacuum pump 24, tube 26, and vacuum gauge 28 are also removed from tray 22 and connected to vacuum port 60 in cap 32. Once cap 32 and vacuum pump 24 are connected to the cartridge 30 as shown in Figs. 2 and 2A, a vacuum is drawn within the cartridge 30 by manually reciprocating the vacuum pump handle 24. The vacuum indicator 28 collapses to indicate that the cartridge has reached the desired vacuum pressure.

Simultaneously with the vacuum being made within the cartridge 30, the inner cylinder 46 is removed from the outer cylinder 44. This is so because, although the inner cylinder 46 fits snugly within the outer cylinder 44, the cartridge 30 does not provide a locking mechanism that keep the inner cylinder 46 in its fully extended position. Thus, when vacuum is drawn within cartridge 30, inner cylinder 46 "seeks its own level" within outer cylinder 44. That is, there is no predetermined position that cylinder 46 must occupy within cylinder 44 when vacuum indicator 28 indicates that the desired level of vacuum within the cartridge has been reached. Such an arrangement has been found to provide an excellent mixing environment for bone cement. When the vacuum gauge 28 collapses to indicate the correct vacuum pressure within the cartridge 30, the inner barrel 46 partially retracts into the barrel 44 to a position where there is enough space to mix the bone cement using the agitator 34, there is still limited space within cartridge 30 for trapped air to occupy. The volume reduction provides less mixing space and therefore reduces the opportunity for air to mix into the cement. Accordingly, the illustrated embodiment achieves a reduction in trapped air within the bone cement.

After the desired vacuum is achieved within cartridge 30, the constituent components of the cement are mixed manually by reciprocating agitator 34 in a manner well known in the art. Advantageously, the self-deflecting action of the paddles 74 against the inner wall 78 of the inner cylinder 46 provides improved mixing around the wall 78 of the inner cylinder 46. Additionally, the vanes provide a self-deflecting action against the inner wall of cylinder 44 as the vanes reciprocate therein. The advantage of the agitator in the illustrated embodiment is that it provides a cleaning connection for both telescopic cylinders, 44 and 46, each of which has a different diameter. Furthermore, the resiliently biased vanes do not "jam" when moving from one cylinder to the other because the cylinder 46 includes a flared end that facilitates entry of the vanes 74, as shown in Fig. 3. In operation, the vanes resilients clean the inner cylindrical wall of cylinder 44; the paddles are then bent inward when the grip is pressed down and cleans the inner cylindrical wall of cylinder 46.

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Upon completion of mixing, the vacuum pump 24 can be removed, thereby releasing the vacuum from within the cartridge 30. The cartridge 30 is then compressed so that the inner cylinder 46 is fully retracted into the outer cylinder 44, as shown. in Fig. 4, for use in a gun to inject cement (not shown). From there, the distal end 86 of the agitator sleeve 82 is split off and discarded. When the agitator 34 is split, the plug 88 is removed from the lid and the outlet port 62 is opened. An appropriate cement applicator nozzle 36 is then placed in the outlet port 62 and the cartridge 30 is inserted into an applicator gun. (not shown). An appropriate bone cement injection gun for use with the illustrated embodiment is shown and described in US Pat. No. 5,638,997, which is owned by the assignee of the present invention and is incorporated by reference herein.

It has been discovered that when the plug of the piston 48 (Fig. 4) advances axially through the internal cylinder 46 pushing with it the bone cement, a convex profile, or "shear flow" develops in the advancement portion of the bone cement, as shown in Fig. 6. The bone cement has a pasty consistency that sticks to the inner cylindrical wall of the cartridge so that there is greater resistance to axial movement around the cylindrical wall than in the middle of the cartridge. The convex profile produced by axially advancing the bone cement can be problematic in a prior art cartridge such as that shown in Fig. 6A. As shown in FIG. 6A, when the apex of the bone cement 96 reaches the exit port 62A, an air-containing gap 98A is formed between the bone cement 96 and the cap 32A. Air within gap 98A can "join" bone cement 96 when bone cement is expelled through exit port 62A, thereby causing unwanted porosity in bone cement. This phenomenon occurs only during the initial expulsion of bone cement 96 and the amount of air involved is limited. However, it is desirable to reduce the size of the gap. As shown in Fig. 6, the illustrated embodiment reduces the size of the gap 98 by use of the specially shaped cap 32, the piston plug 48, and the vanes 74.

When cement applicator nozzle 36 is attached to cap 32 and bone cement advances through nozzle 36, as shown in Fig. 7, the air contained in annular gap 128 must be vented to the environment. As bone cement continues to advance through cartridge and nozzle 36, it also oozes from exit port 62 through gap 130 and into gap 128, thus moving air contained within annular gap 128 through the porous filter 102 and out of the lid through vents 66. Advantageously, porous filter 102 allows air to pass through, but not bone cement. After initial ejection of bone cement from cartridge 30, annular gap 128 is completely filled with bone cement, all of the air previously contained therein having escaped to the environment, as described above. If an air escape passage were not provided, the air from the gap 128 would be trapped and could be drawn into the bone cement that is expelled from the cartridge. O-ring 104 provides a fluid seal between cap 32 and handle 70 so that air cannot enter cartridge 30.

Although this invention has been described as having an exemplary design, the present invention may be further modified within the scope of the appended claims. Furthermore, this application is intended to encompass deviations from the present disclosure such that they fall within known or usual practice in the art to which this invention pertains.