Abstract:
A device for mixing bone cement material, the device comprising a mixing body having a chamber, and wherein the mixing body has an open end; a lid arranged to be fitted to the open end; means for mixing bone cement material in the chamber, the means for mixing comprising a rod extending through the lid and into the chamber, wherein the rod has a frangible portion; said lid comprising a cap moveable to a first position covering the frangible portion of the rod, and to a second position revealing the frangible portion.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an apparatus for mixing and delivering bone cement or the like. 
       BACKGROUND OF THE INVENTION 
       [0002]    Orthopaedic bone cement is used throughout the world to secure hip, knee and other metallic prostheses in an appropriate medical position. Also, bone cement can be used to replace and/or repair damaged bone, such as a bone void filling and spinal column with degenerative intervertebral discs by permanently stabilising adjacent vertabrae by fusion. 
         [0003]    Bone cement for both joint surgery and bone void filling is generally provided as two or more components, often a powder and a liquid, that, when thoroughly mixed together, form a paste or cement. Thorough mixing of the components is necessary to avoid brittle or “hot” spots. After mixing, the cement generally has to be used quickly before it sets hard. 
         [0004]    Bone void filling is used in the repair of osteoporotic bone, typically after fracture, by stabilising the bone and allowing bone growth. In some cases bone growth stimulants are added to the materials to hasten healing. Calcium based sulphates and phosphates materials are typically used. The principles of mixing apply equally to both PMMA bone cement and bone void filling cements. 
         [0005]    Bone cement is produced by thoroughly mixing together two components, usually methylmethacrylate monomer liquid and polymethylmethacrylate (PMMA) powder. This type of material is typically used in joint surgery including hips, knees and small joints such as the shoulder, hand and wrist, and foot and ankle. 
         [0006]    The mixing is usually carried out using a simple bowl and spatula. The liquid and powder components are put into the bowl and the surgeon or assistant uses a pestle or spatula to thoroughly mix the components. The surgeon then removes the required amount of cement and manipulates it by hand before inserting it into a preformed cavity or applying it to a resected bony surface where the prosthesis is to be positioned. Cement may be applied by hand or may be put into a syringe and applied separately. However, this simple mixing has two major drawbacks. 
         [0007]    Firstly, free methylmethacrylate fumes are emitted from the mixture. It is desirable to remove these fumes, or prevent them from escaping into the atmosphere, since they have an unpleasant odour and may be harmful to operating room personnel. The fumes are known to cause nausea and giddiness and are generally objectionable, particularly to the nurses who carry out the mixing. There has also been concern that long term exposure to these fumes results in a more serious health risk. Current employment law relating to occupational health mean medical staff must now be protected against the exposure to hazardous substances. 
         [0008]    Secondly, a very high mixing efficiency is required to produce a homogenous cement material. During the mixing process, air is naturally introduced into the mixture since air is inherently existent within the powder and also in and around the mixing vessel. Air bubbles are also produced by the “boiling off” of monomer which occurs during the mixing process. The introduction of air produces a weak cement and, since the joint must usually support a heavy load, it is important to reduce the amount of air in the mixture as much as possible in order to improve the mechanical strength of the cement material. 
         [0009]    In order to eliminate as much air as possible from the mixture, desirable for most types of cement, mixing is now preferably carried out in a closed vessel, most preferably under vacuum. This considerably reduces the amount of air in the mixture. Mixing in a conventional bowl and spatula system can produce a product with a porosity value of 20% to 25%. In a vacuum mix, the porosity value is often reduced to levels below 5%. 
         [0010]    As mentioned above, calcium phosphates and sulphates can be used as another type of bone cement material, which does not necessarily need to be mixed in a vacuum. Conventionally, this type of bone growth stimulant is prepared in paste form and is delivered to an application site. 
         [0011]    Several devices for mixing the cement are available. Some of these are in the form of hand-held mixing bowls as mentioned above. WO 93/10892 describes an improved bowl mixer. The substances to be mixed are placed by means of a rotating paddle extending into the bowl to which a vacuum is applied. The substances are mixed by means of a rotating paddle extending into the bowl which is rotated manually by means of a handle extending through the lid of the bowl. In some applications, an example of which is disclosed in WO 93/10892, bowl mixing is favoured. Many surgeons prefer to “hand pack” the cement. Bowl mixing tends to be preferred by nurses who are used to the convenience of mixing in this vessel; a bowl is easier to use and it is important that nurses feel confident since timing is very crucial and the mixture must be “right first time”. Many surgeons also tend to prefer bowl mixers because they can easily take samples of the cement from the bowl at any time to determine the progress of polymerization as it is crucial that the mixture does not begin to set before it is applied. 
         [0012]    However, in some applications, it is preferable or necessary to apply the mixed cement to the bone by means of a syringe. Indeed some surgeons, particularly in Europe, prefer syringe-type application to “hand packing”. If the cement is mixed in a bowl, it must then be transferred to a dispensing syringe which can be messy and time consuming and may expose the mixture to more air entrapment. This problem has been overcome by combining a mixing chamber with a syringe. Although advantages can be obtained with a simple closed mixing chamber/syringe combination, creating a vacuum can provide additional advantages. For example, EP 0178658 discloses a device for mixing bone cement comprising a mixing container connected to a feed device. A vacuum source may be connected to the feed device for mixing the substances under vacuum. This device has proved to be a very efficient mixing and transfer system and eliminates the need to transfer the mixed cement from the mixing bowl to a syringe. 
         [0013]    However, the mixing paddle of EP 0178658 is rotated by a rotary electric drive motor. This makes the device costly and space consuming and requires specialist and time-consuming installation. The device is not easily portable and its use is, therefore, not particularly flexible. 
         [0014]    U.S. Pat. No. 4,758,096 and U.S. Pat. No. 3,606,094 also disclose bone cement mixers in which the cement is mixed in the dispensing vessel. In U.S. Pat. No. 4,758,096, the mixing is effected manually by means of a “masher” plate-type agitator. The masher plate is attached to a shaft attached to a handle. The agitator is movable in the chamber both axially and rotatably to permit mixing of the cement by the user moving the handle vertically and rotatably. 
         [0015]    In the device of U.S. Pat. No. 3,606,094, the mixing element comprises an elongate conduit having paddle projections. The paddles are rotated by a rotating motor or by hand. 
         [0016]    EP 0744991 discloses a bone cement mixer in which the mixing chamber forms the body of a dispensing syringe wherein a nozzle can be attached to one end of the mixing chamber so as to dispense the bone cement. 
         [0017]    GB 2411849 discloses an apparatus for mixing bone cement and discharging the mixed bone cement from a mixing container into a discharging device such as a syringe or syringes. 
         [0018]    Another product on the market that provides a mixing and dispensing device is the HiVac(™) 7 (provided by Summit Medical Ltd., see http://www.summit-medical.co.uk/product/hivac7.html). This device allows for mixing, e.g. biologics, in a mixing chamber. The HiVac(™) 7 provides a mixing rod having a diameter of 8 mm. Once the mixing phase is complete, a cap can be lifted from one end to reveal a “luer” connection to which a nozzle can be attached. The mixing chamber then also acts as a dispensing chamber and the bone cement can be dispensed through the nozzle. 
         [0019]    The bone cement mixing and dispensing devices discussed above have the advantage of providing an apparatus that both mixes and dispenses bone cement material. 
         [0020]    Generally, these mixing apparatuses discussed are made to hold a standard volume of cement of 40 cc or more in the mixing and dispensing chambers. Sometimes, however, there is a need for only a small amount (e.g. 10 cc) of cement, for example for smaller joints such as ankles etc., where a volume of 0.5-20 cc, for example, is needed; or for a combination of calcium based sulphates and phosphates. The bulky nature of chambers having, say, 40 cc volume is not easily manoeuvred in a smaller target site (for example, ankle joints etc.). Also, the cement components are expensive and use of a greater volume than needed is wasteful and costly. Simply reducing the size of the apparatuses discussed above causes the components therein to become weak and to easily break. For example, simply reducing the size of the components shown in the HiVac(™) 7, and particularly the rod, makes these components weak and prone to breaking. There is therefore a need for a device that can cope and assist in treatment at smaller target sites. 
         [0021]    Further, optimum mixing occurs in the above devices when the chambers are mostly full—e.g. ˜80% full. When a chamber is ˜80% full, this allows for a mixer paddle or disc to push through the material and have the resistance from the bone cement material, caused by the material being compressed against the chamber bottom to push the material through the paddle or disc. As the paddle or disc is then returned through the chamber, the material is again pushed through the paddle or disc by resistance. However, if the devices discussed above were to be used for treatment in smaller target sites, it would be necessary to use a smaller amount of bone cement material that would be less than the volume of the chambers discussed above. Simply providing a smaller amount of bone cement material into a chamber with, say, 40 cc volume, or more, has disadvantages. For example, if the chamber were only ˜40% full, the material simply clings to the paddle or disc until the paddle or disc hits the bottom of the chamber where it would be pushed through the paddle. Before mixing the material again, the paddle or disc has to travel ˜60% of the chamber, which does not provide efficient and optimum mixing. Therefore, there is a need for a device that provides adjustable volumes in the chamber to allow for a range of volume of bone cement material. 
         [0022]    The present invention aims to overcome the above-mentioned problems. Of course, several types of bone cement material have been discussed above, but the present invention is not so limited to these types of bone cement material. The present invention aims to provide a device for known types of bone cement material and bone cement material that may be developed in the future. 
       SUMMARY OF THE INVENTION 
       [0023]    In one aspect of the present invention, there is provided a device for mixing bone cement material, the device comprising:
   a mixing body having a chamber, and wherein the mixing body has an open end;   a lid arranged to be fitted to the open end;   means for mixing bone cement material in the chamber, the means for mixing comprising a rod extending through the lid and into the chamber, wherein the rod has a frangible portion;   said lid comprising a cap moveable to a first position covering the frangible portion of the rod, and to a second position revealing the frangible portion.   
 
         [0028]    In a preferred embodiment, the mixing rod can be moved vertically and/or rotatably in the chamber to assist in mixing the bone cement material. In a preferred form, the means for mixing the bone cement material comprises a ‘mashing’ or paddle portion to assist in the mixing of the bone cement, in the chamber. The ‘mashing’ or paddle portion can take the form of a ‘masher’ paddle or a flat-type paddle, and, most preferably, a paddle portion extending radially from the rod. 
         [0029]    In a preferred embodiment, a lid is provided at an open end of the mixing body. The lid preferably has a moveable cap that can move to a first position to cover a frangible portion of the rod. When the cap is covering the frangible portion of the rod, the rod is advantageously reinforced against snapping whilst the user is mixing the bone cement in the chamber. 
         [0030]    In a further embodiment of the present invention, there is provided means for securing the cap in the first position. This allows for the cap to be secured in a position that covers the frangible portion of the rod so as to reinforce the rod during the mixing phase. In a preferred embodiment, the means for securing the cap in a first position is a removable clip. 
         [0031]    In preferred embodiments of the present invention, the mixing body chamber and plunger are preferably cylindrical, and most preferably circular cylindrical. 
         [0032]    In a preferred form of the present invention, the lid comprises an inner cap and a movable outer cap. In a preferred embodiment, the inner cap is attached to the open end of the chamber and the outer cap is movable relative to the inner cap. This allows for the open end of the chamber to remain closed during the mixing phase, but to also allow for the rod to be reinforced by the outer cap when this is moved into the first position. 
         [0033]    In a preferred form, the frangible portion is a weakened or snap portion, preferably formed by a slight indent or notch provided in the rod. This allows for the user to snap the rod after the mixing phase. 
         [0034]    The device is preferably used for mixing and dispensing bone cement material. In a further embodiment of the present application, the mixing body preferably has a second open end. In a preferred form, the mixing body has means for dispensing the cement located at the second open end to assist in dispensing the bone cement material after the mixing phase. Preferably, the means for dispensing the bone cement material is a plunger movable by the application of force through the chamber. 
         [0035]    Preferably, the inner cap has a delivery port that allows for bone cement material to be dispensed after the bone cement material has been mixed. In a preferred embodiment, the outer cap has a protrusion extending towards the delivery port of the inner cap so that it can be introduced into the delivery port. Advantageously, the protrusion blocks the delivery port such that no bone cement material can escape. In a preferred form, the delivery port of the inner cap includes a luer port so as to receive a nozzle for dispensing the bone cement material. 
         [0036]    Preferably, the mixing or paddle portion of the rod is shaped so as to be engaged by the protrusion of the outer cap such that, when the outer cap is moved down to fit over the inner cap, the protrusion pushes the paddle portion to the side of the chamber. Advantageously, this ensures that the paddle portion is not occluding the delivery port of the inner cap. 
         [0037]    In another aspect of the present invention, there is provided a device for mixing bone cement material, the device comprising:
   a mixing body having a chamber, and wherein the mixing body has a first end and a second end;   a lid arranged to be fitted to the first end;   means for mixing bone cement material in the chamber, the means for mixing comprising a rod extending through the lid and into the chamber;   means for dispensing bone cement material from said mixing body attached to a second end of the mixing body, wherein the means for dispensing the bone cement material is a plunger movable by the application of force through the chamber, said plunger provided to set an adjustable volume in the chamber prior to mixing;   said device further comprising:   means for positioning the plunger between the first and the second end of the mixing body to define a volume of the chamber; and
 
means for maintaining the position of the plunger so as to maintain the volume of the chamber during mixing.
   
 
         [0044]    The means for positioning the plunger between the first and the second end of the mixing body advantageously allows the user to define a volume of the chamber of the mixing body. Therefore, in this embodiment of the present invention, the user can adjust the volume of the chamber for an amount of bone cement material that is necessary for a particular type of surgery. 
         [0045]    There is also provided means for maintaining the position of the plunger so as to maintain the volume of the chamber during mixing. Advantageously, this allows the user to first adjust the amount of volume in the chamber and to then set the volume within the chamber so that the user can then mix the bone cement material for a particular volume. 
         [0046]    Preferably, the means for positioning the plunger and the means for maintaining the position of the plunger comprises a screw thread arranged on an outer surface of the plunger, and a nut having a screw thread to be received by the screw thread arranged on the outer surface of the plunger, the nut further having a recess having a screw thread to be received by a screw thread located at the second end of the mixing body. This allows for the user to first position the plunger within the chamber and then to securely screw a nut to the mixing chamber such that the plunger is maintained in a position between the first end and the second end of the mixing body during the filling and mixing phases. This then advantageously allows for the user to adjust the volume for a particular amount of bone cement material that is necessary for a particular type of surgery. 
         [0047]    The features of the second aspect, relating to the adjustable mixing volume, are also advantageous for the first aspect of the invention. 
         [0048]    In a preferred embodiment, for both aspects, there is also provided a base that is adapted to receive the plunger to stabilise the device during a mixing or filling phase. 
         [0049]    Preferably, the mixing rod has a handle or knob located on the end so that the user can easily move the rod vertically and/or rotatably. 
         [0050]    In a preferred embodiment of the present invention, the volume of the chamber is between 0-15 cc. 
         [0051]    Preferably, the outer cap can be lifted a distance away from the inner cap of between 2-3 mm. 
         [0052]    In a preferred form, the diameter of the rod is 4 mm. 
         [0053]    In a further embodiment of the present invention, there may be provided a vacuum port located on the lid so as to receive a vacuum pump in the chamber. The vacuum pump advantageously creates a vacuum in the chamber. 
         [0054]    In another aspect of the present invention, there is provided a method of mixing bone cement material, said method having the steps of:
   providing bone cement components in a mixing chamber of a mixing device;   fitting a lid to an open end of the mixing chamber to close the chamber;   mixing bone cement material using a rod extending through the lid and into the chamber of the mixing device, wherein the rod has a frangible portion; said method characterised by   positioning the lid in a first position so as to reinforce the frangible portion from breakage during mixing;   positioning the lid in a second position so as to reveal the frangible portion; and
 
snapping the rod at the frangible portion.
   
 
         [0060]    In this embodiment, the method of mixing the bone cement material allows the user to mix the bone cement material and also allows for the frangible portion of the rod to be reinforced during mixing. 
         [0061]    In a preferred form, the steps of positioning the lid to reinforce the frangible portion comprises providing an outer cap and an inner cap on the lid and preferably moving the outer cap away from the inner cap to a first position. This advantageously allows for the outer cap to cover the frangible portion during mixing. 
         [0062]    Preferably, the outer cap can be removed after the mixing phase to reveal a delivery port located on the inner cap so that the user can dispense the bone cement material. In a preferred embodiment, the delivery port includes a luer port that can receive a nozzle so as to assist in the dispensing of the bone cement material. 
         [0063]    In a further aspect of the present invention, there is provided a method of mixing bone cement material, said method having the steps of:
   setting a volume of a chamber of a mixing device with a plunger, said plunger movable through the chamber with the application of force between a first and a second end of the mixing device;   maintaining the position of the plunger so as to maintain the volume of the chamber during mixing;   mixing the bone cement material in the chamber of the mixing device.   
 
         [0067]    In this embodiment, the method allows the user to adjust the volume of the chamber of the mixing body. This advantageous when an amount of bone cement material different to the total volume of the chamber is needed for a particular type of surgery. The volume can then be adjusted by the user and maintained for use during the filling and mixing phase. 
         [0068]    In a preferred form, the steps of setting the volume and maintaining the set volume comprises providing a screw thread arranged on an outer surface of the plunger and providing a nut having a screw thread to be received by the screw thread arranged on the outer surface of the plunger, the nut further having a recess having a screw thread to be received by a screw thread located at the second end of the mixing body. This advantageously allows the user to screw the nut onto the second end of the mixing body. 
         [0069]    In one embodiment of the present invention, and when the bone cement material is ready to be dispensed, the user can unscrew the nut from the second end of the mixing body and the plunger can be axially moved through the chamber to push the bone cement material out through a delivery port located on the lid. 
         [0070]    In an alternative embodiment of the present invention, and when bone cement material is ready to be dispensed, the user can rotate a second end of the plunger through the screw thread provided on the nut to axially move the plunger through the chamber. This advantageously provides a greater force when dispensing the bone cement material through a delivery port and is particularly advantageous for viscous bone cement material. 
         [0071]    In another embodiment of the present invention, a vacuum can be created in the chamber by attaching a vacuum pump to a vacuum port on the lid. 
         [0072]    Exemplary embodiments of the present invention will now be described, by way of example only, with reference to the figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0073]      FIG. 1  shows a bone cement mixing and dispensing device in accordance with an embodiment of the present invention. 
           [0074]      FIG. 2  shows an enlarged portion of the bone cement mixing and dispensing device shown in  FIG. 1  prior to delivery of the material to be mixed. 
           [0075]      FIG. 3  shows an enlarged portion of the bone cement mixing and dispensing device shown in  FIG. 1 , in use, prior to delivery of the material to be mixed. 
           [0076]      FIG. 4  shows the bone cement mixing and dispensing device of  FIG. 1  pre-mixing or during the mixing phase. 
           [0077]      FIG. 5  shows the bone cement mixing and dispensing device of  FIG. 1  prior the mixing phase. 
           [0078]      FIG. 6  shows a bone cement mixing and dispensing apparatus in accordance with another embodiment of the present invention. 
           [0079]      FIG. 7  shows a cross-sectional view of a cap in an open position in accordance with an embodiment of the present invention. 
           [0080]      FIG. 7 a    shows a cross-sectional view of a cap in a closed position in accordance with an embodiment of the present invention. 
           [0081]      FIG. 7 b    shows a cross-sectional view of the cap as shown in  FIG. 6   a.    
           [0082]      FIG. 8  shows a plan view of the cap shown in  FIGS. 6 a    and  6   b.    
           [0083]      FIG. 9  shows a cap in accordance with another embodiment of the present invention. 
           [0084]      FIG. 10  shows the bone cement mixing and dispensing device of  FIG. 1  with a nozzle attached thereto. 
           [0085]      FIG. 11  shows the bone cement mixing and dispensing device of  FIG. 1  during the dispensing phase. 
           [0086]      FIG. 11 a    shows an alternative embodiment of the bone cement mixing and dispensing device of  FIG. 1  during the dispensing phase. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0087]      FIG. 1  shows a complete assembly of a bone cement mixing and delivery device  1  in accordance with an embodiment of the present invention. The bone cement and delivery device  1  can be used to mix and deliver bone cement. As can be seen in  FIG. 1 , there is provided a mixing/dispensing body  2  having a chamber  23  for receiving bone cement material (not shown). The mixing/dispensing body  2  has a first open end  21  and a second open end  22 . The mixing/dispensing body  2  is preferably cylindrical, and more preferably circular cylindrical. However, the mixing/dispensing body  2  may take other forms. 
         [0088]    As shown in  FIG. 1 , there may also be provided an inner cap  411  and an outer cap  41  (described in more detail below) located at, and releasably fastened to, the first open end  21  of the mixing/dispensing body  2 . 
         [0089]    Also, there may be provided a plunger  3  having a first end  31  and a second end  32 , as shown in  FIG. 1 . The first end  31  of the plunger  3  may include a piston  35  that can be introduced into the mixing/dispensing body  2  at the second open end  22 . The piston  35  is preferably flush with an inner surface of the mixing/dispensing body  2 . The plunger  3  may have a handle or knob on the second end  32  to assist the user in using the plunger  3 . The plunger  3  is preferably cylindrical, and more preferably circular cylindrical. However, the plunger  3  may take other forms. 
         [0090]    As shown in  FIG. 1 , there may also be provided a rod  42  to assist in mixing the bone cement material. The rod  42  may movably extend through openings (not shown) of the inner cap  411  and outer cap  41 . The rod  42  has a first end  43  and a second end  44 . At the second end  44  of the rod  42 , in the mixing/dispensing chamber  23 , is provided a ‘mashing’ or paddle portion  44  to assist in mixing the bone cement material. In a preferred embodiment, the diameter of the rod is between 2-5 mm, and is preferably 4 mm in diameter. 
         [0091]    A base  34  may be provided, as shown in  FIG. 1 . The base  34  may be adapted to receive the second end  32  of the plunger  3  to stabilise the mixing/dispensing device  1  during a mixing or filling phase. 
         [0092]    As can be seen in  FIGS. 1, 2, and 3 , the first end  31  of the plunger  3  can be received by the second open end  22  of the mixing/dispensing body  2 . In this arrangement, the piston  35  is, therefore, introduced into the mixing/dispensing body  2  and is flush with an inner surface of the mixing/dispensing body  2 . The plunger  3  has, on its outer surface, a screw thread extending from the first end  31  to the second end  32 . A nut  33  is located on the screw thread of the plunger  3 . The nut  33  includes a recess  331  having a screw thread to fasten to a screw thread located on the outer surface of the second end  22  of the mixing/dispensing body  2 . In this arrangement, the piston  35  can be introduced into the second open end  22  of the mixing/dispensing body  2  and can be moved, in a longitudinal direction (shown by the arrow in  FIG. 3 ), through the mixing/dispensing body  2  to provide a specific volume required in a mixing/dispensing chamber  23 . The piston  35  can then be held in place by screwing the nut  33  on to the screw thread located on the outer surface of the second end  22  of the mixing/dispensing body  2 . This then provides a volume within the mixing/dispensing chamber  23 . In a preferred embodiment, the volume of the chamber can be between 0-15 cc. 
         [0093]    Once the volume has been set by the user with the technique discussed above, bone cement material is introduced into the first open end  21  of the mixing/dispensing body  2 , e.g. through a funnel (not shown). 
         [0094]    Either before or after the bone cement material has been introduced into the mixing/dispensing chamber  23 , the second end  32  of the plunger may be positioned, and held, in a recess of a base  34 . The base  34  assists in keeping the bone cement mixing and delivery device  1  still during the filling and/or mixing phase. 
         [0095]    Referring now to  FIGS. 1 and 4 , the lid  4  is now described. After the bone cement material has been introduced into the mixing/dispensing chamber  23 , the lid  4  is fitted over the first open end of the mixing/dispensing body  2 . The lid has an outer cap  41  and an inner cap  411 . The inner cap  411  may have a recess having a screw thread thereon which can mate with a screw thread located on the outer surface of the first open end  21  of the mixing/dispensing body  2 . The outer cap  41  can fit over the inner cap  411  and is axially movable relative to the inner cap  411 . The lid  4  can be attached to the mixing/dispensing body  2  by mating the screw thread of the inner cap  411  with a screw thread located on the outer surface of the first end  21  of the mixing/dispensing body  2 . Other means could be provided to fasten the lid to the chamber. 
         [0096]    The outer cap  41  and the inner cap  411  include an opening (not shown) through which a rod  42  moveably extends. At the second end of the rod  42 , in the chamber, is provided a ‘mashing’ or paddle portion  44  to assist in mixing the bone cement material in the mixing/dispensing chamber  23 . The rod  42  extends from the paddle portion  44  and through the opening of the outer cap  41  and inner cap  411  to a first end  43 . The ‘mashing’ or paddle portion  44  could be radially extending arms (not shown) from the rod  42  or a ‘masher’ paddle, or a flat-type paddle portion. Other types of ‘mashing’ or paddle portions could be used. 
         [0097]    In use, the outer cap  41  is lifted a distance away from the inner cap  411  and the first end  21  of the mixing/dispensing body  2 . In a preferred embodiment, the outer cap  41  is lifted away by a distance of between 2-3 mm. In order to maintain the distance between the outer cap  41  and the inner cap  411 , a clip  50  may be attached around the circumference of the inner cap  411  and/or the outer cap  41 . The clip  50  therefore holds the outer cap  41  in place during the mixing phase. The outer cap  41  may also include a reinforcing portion  46  that protrudes from the hole in which the rod  42  extends through. The reinforcing portion  46  provides support to the rod  42  during the mixing phase so that the rod  42  does not snap or break. 
         [0098]    The mixing phase is achieved by a user axially and/or rotatably moving the rod  42  so that the paddle portion  44  mixes the bone cement material in the mixing/dispensing chamber  23 . The first end  43  of the rod  42  may be provided with a handle or knob  43  to assist the user in mixing the bone cement material. 
         [0099]    As is shown in  FIG. 5 , when the clip  50  is removed from the outer cap  41  and/or inner cap  411 , the outer cap  41  is axially moveable relative to the inner cap  411  and the mixing/dispensing body  2 . 
         [0100]      FIG. 6  shows a complete assembly of a mixing device  1  in accordance with another embodiment of the present invention. The mixing device  1  of  FIG. 6  is similar to that of  FIG. 1  except that a vacuum port  400  may be included in the outer cap  41  and inner cap  411  so as to be in communication with the chamber  23 . A vacuum pump (not shown) may then be received by the vacuum port  400  so as to create a vacuum in the chamber  23 . 
         [0101]      FIGS. 7, 7   a  and  7   b  show the inner cap  411 , outer cap  41  and rod  42  in more detail. As can be seen in these figures, the inner cap  411  has a side wall  61  extending from an upper wall  63 . On the inner surface of the side wall  61 , there is provided a screw thread to engage and mate with a screw thread located on the outer surface of the first end  21  of the mixing/dispensing body  2 . The upper wall  63  of the inner cap  411  also may include a delivery port  62  extending therefrom towards the outer cap  41  and a circumferential wall  65  located around the delivery port  62  providing a gap  66  therebetween. 
         [0102]    The outer cap  41  has a side wall  71  to fit over the side wall  61  of the inner cap  411 . The outer cap  41  has an upper wall  73  and a circumferential wall  74  extending therefrom so as to fit over the delivery port  62  and circumferential wall  65  of the inner cap  411 . The circumferential wall  74  of the outer cap  41  is closed by an upper surface  77 . A protrusion  72  extends from the upper surface  77  towards the delivery port  62  of the inner cap  411  so that the protrusion  72  can be introduced into the delivery port  62 . A circumferential wall  75  also extends from the upper surface  77  towards the gap  66  provided between the circumferential wall  65  and the delivery port  62  of the inner cap  411 . The circumferential wall  65  includes, on its surface, a luer thread for connection to a luer fitting during the delivery phase. 
         [0103]    As shown in  FIG. 7 , the rod  42  extends through holes  64  and  80  of the inner cap  411  and outer cap  41  respectively. The rod includes a weakened or snap portion  80 , which can be formed for example by a slight indent or notch in the material of the rod  42 . The outer cap  41  also includes a reinforcing portion  46  which extends from the upper surface  73  of the outer cap  41 . 
         [0104]    A method of using the outer cap  41  during and after mixing is now described with reference to  FIGS. 7, 7   a  and  7   b.    
         [0105]    During mixing, the outer cap  41  is raised from the inner cap  411  so as to cover the snap portion  80 . The outer cap  41  may then be held in position by a clip  50  (as shown in  FIG. 4 ). The rod  42  can then be moved vertically and/or rotatably by the user to mix the bone cement material in the mixing/dispensing chamber  23  (as shown in  FIG. 1 ). The snap portion is protected against breakage by the outer cap  41 . Other means for holding the outer cap  41  in position may also be used. 
         [0106]    After the mixing phase, the outer cap  41  is then moved down toward/over the inner cap  411 . The protrusion  72  of the outer cap  41  naturally also moves down and engages the paddle portion  44  of the rod  42  and pushes this to the side of the mixing/dispensing chamber  23  to ensure that the paddle portion  44  is not occluding the delivery port  62 . In a preferred embodiment, the protrusion  72  is in the form of a spike. The paddle portion  44  may be shaped so as to be engaged by the protrusion  72  to assist in moving the paddle portion  44  to the side of the mixing/dispensing chamber  23 . 
         [0107]    As can be seen in  FIG. 7 b   , the paddle portion  44  of the rod  42  is moved to the side of the mixing/dispensing chamber  23  such that the paddle portion  44  does not occlude the delivery port  62 . 
         [0108]      FIG. 8  shows the outer cap  41  when it is moved down towards, and is fitted over, the inner cap  411 . The reinforcing portion  46  has a gap G through which the snap portion  80  is revealed. The user can then “snap” the rod  42  at the snap portion  80  to remove a portion of the rod  42 . The paddle portion  44  then remains in hole  64  of the inner cap  411 . Due to the paddle portion  44  of rod  42  remaining in hole  64  of the inner cap  411 , the paddle portion  44  blocks hole  64  of the inner cap  411  to maintain a closed chamber (as shown in  FIGS. 9 and 10 ). 
         [0109]      FIG. 9  shows that the outer cap  41  can be removed from the inner cap  411  entirely, after the rod  42  has been snapped. As can be seen in this figure, paddle portion  44  remains in hole  64  of the inner cap  411 . The outer cap  41  is removed to reveal the delivery port  62 . 
         [0110]      FIG. 10  shows that a luer fitting or nozzle  100  can then be connected to the luer thread located on the surface of the circumferential wall  65  of the inner cap  411 . The nozzle  100  is then in fluid communication with the mixing/dispensing chamber  23 . As mentioned above, paddle portion  44  remains in hole  64  of the inner cap  411 . 
         [0111]      FIGS. 11 and 11   a  show the mixing/dispensing device  1  during the dispensing phase. Once the outer cap  41  has been removed and a nozzle  100  connected so as to be in fluid communication with the mixing/dispensing chamber  23 , the bone cement is ready to be delivered to a target site of a patient. 
         [0112]      FIG. 11  shows the mixing/dispensing device  1  in one embodiment of the present invention. When the bone cement is ready to be deployed from the mixing/dispensing chamber  23 , the user can unscrew the nut  33  from the second end  22  of the mixing/dispensing body  2 . The nut  33  can be moved along the screw thread provided on the plunger  3 . After unscrewing the nut  33 , the plunger  3  can then be axially moved so as to axially move the piston  35 . The piston  35  then delivers bone cement through the delivery port  62  and, for example, through a nozzle  100  to the target site. 
         [0113]      FIG. 11 a    shows the mixing/dispensing device  1  in an alternative embodiment of the present invention. When the bone cement is ready to be deployed from the mixing/dispensing chamber  23 , the user can rotate the second end  32  of the plunger  3 . The second end  32  of the plunger  3  can have a handle or knob  321  attached thereto to assist the user in rotating the plunger  3  through the screw thread provided on the nut  33  to axially move the piston  35 . This is particularly useful for viscous bone cement material. Rotating the plunger  3  through the screw thread provided on the nut  33  causes the bone cement to be deployed through the delivery port  62  and, for example, through a nozzle  100  attached thereto. 
         [0114]    Although the invention has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims.