Abstract:
A bone cement delivery system including a mixer, a reservoir for holding bone cement, a cannula that is connected to the reservoir and through which the bone cement is delivered into living tissue. A plunger that is advanced by a drive assembly, pushes the bone cement out of the delivery tube into the cannula for discharge from the cannula. A valve is located upstream of the cannula. A control unit regulates both drive assembly and the setting of the valve. The control unit is configured to, when deactivating the valve so as to stop the advancement of the plunger, close the valve. The closing of the valve reduces the extent to which the bone cement, which is under pressure continues to flow into and be discharged out of the cannula.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. patent application Ser. No. 14/076,536 filed 11 Nov. 2013 now U.S. Pat. No. 9,597,138. Application Ser. No. 14/076,536 is a divisional of U.S. patent application Ser. No. 13/465,615 filed 7 May 2012 now abandoned. Application Ser. No. 13/465,615 is a divisional of U.S. patent application Ser. No. 12/961,216, filed 6 Dec. 2010, now U.S. Pat. No. 8,172,456. Application Ser. No. 12/961,216 is a divisional of U.S. patent application Ser. No. 12/652,295, filed 5 Jan. 2010, now U.S. Pat. No. 7,854,543. Application Ser. No. 12/652,295 is a continuation of U.S. patent application Ser. No. 12/416,171, filed 1 Apr. 2009, now U.S. Pat. No. 7,658,537. Application Ser. No. 12/416,171 is a continuation application of PCT Application No. PCT/US2007/021408, filed 5 Oct. 2007, which claims priority to U.S. Provisional Patent Application Ser. No. 60/828,509, filed 6 Oct. 2006 and U.S. Provisional Patent Application Ser. No. 60/969,173, filed 31 Aug. 2007. Each of the above-listed priority applications are hereby incorporated by reference in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention is generally related to bone cement mixing and delivery systems in which separate components of bone cement are mixed together in a mixer to form a bone cement mixture. The mixture is transferred to a delivery device and then delivered to a target site, such as a vertebral body or other anatomical site. 
       BACKGROUND OF THE INVENTION 
       [0003]    Bone cement mixing and delivery systems are well known for mixing separate components of bone cement together to form a uniform bone cement mixture and then delivering that mixture to a target site. Typically, such systems employ a mixer having a handle for manually mixing the components. Once mixed, the mixture is then manually transferred to a delivery device such as a syringe. The syringe is used to inject the mixture into the target site. Examples of target sites include medullary canals for total hip arthroplasty procedures, vertebral bodies for vertebroplasty or kyphoplasty procedures, and other sites in which bone cement is required. 
         [0004]    Often, the types of bone cements used in these procedures have short working time windows of only a few minutes thereby affecting the amount of time available for mixing and delivering the mixture to the target site. Current systems require a great deal of user interaction in set-up, including manually mixing the bone cement components and manually transferring the mixture to the delivery device. This user interaction delays delivery of the mixture to the target site, while also exhausting the user&#39;s energy. As a result, there is a need for bone cement mixing and delivery systems that are capable of quick set-up, with little user interaction. 
         [0005]    One example of a bone cement mixing and delivery system that attempts to improve set-up time is shown in U.S. Pat. No. 5,571,282 to Earle. Earle discloses a motorized mixer that is used to mix the bone cement components. The mixer mixes the components a pre-selected amount of time, as set by the user. At the end of the pre-selected time, the mixer stops automatically and pressure is applied to the mixture to push the mixture out through a port in the bottom of the mixer to a syringe or a delivery cartridge. 
         [0006]    The release of odors and gases associated with the bone cement components during mixing can also be undesirable. As a result, there is also a need for bone cement mixing and delivery systems that are substantially self-contained such that the odors and gases associated with the components are not substantially released during mixing or transfer. 
         [0007]    One example of a bone cement mixing and delivery system that provides some containment is shown in U.S. Pat. No. 5,193,907 to Faccioli et al. Faccioli et al. discloses an apparatus for mixing and delivering bone cement formed from liquid and powder components. The apparatus comprises a cylindrical body and a plunger slidable within the body. A powder chamber stores the powder component between the plunger and a distal end of the body. A glass ampoule stores the liquid component inside the plunger. To mix the components, a user presses a plug in the plunger&#39;s proximal end to urge a tip of the glass ampoule against a caromed surface (or against a piercing member) to release the liquid component. The liquid component then passes through channels defined in the plunger&#39;s head to the powder chamber. The liquid and powder are mixed by shaking the body to form the bone cement mixture. After mixing, the plunger is pressed to discharge the bone cement mixture out of an exit port in the body and through a flexible conduit to a target site. 
         [0008]    These prior art systems are suitable for reducing set-up times, conserving a user&#39;s energy, and reducing exposure of the user to the bone cement components. However, there is still a need in the art for bone cement mixing and delivery systems that are capable of further reducing set-up time and enabling quick operation to deliver bone cement to a target site. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention provides a bone cement mixing and delivery system. The system comprises a mixer for mixing components to form a bone cement mixture and a delivery device for receiving the bone cement mixture from the mixer and for delivering the mixture to a target site. The mixer includes a housing defining a mixing chamber for receiving the components of bone cement. The delivery device includes a reservoir defining a delivery chamber in communication with the mixing chamber for receiving the mixture from the mixing chamber. The mixer further includes a mixing paddle disposed in the mixing chamber for mixing the components to form the mixture. A mixing shaft engages the mixing paddle. A transfer mechanism transfers the mixture out from the mixing chamber and into the delivery chamber. A motor operatively engages both the mixing shaft and the transfer mechanism. The motor operates to rotate the mixing shaft and mix the components in the mixing chamber for a predetermined mixing time to form the mixture. The motor also operates to actuate the transfer mechanism to automatically transfer the mixture from the mixing chamber to the delivery chamber after the predetermined mixing time has elapsed. 
         [0010]    A method of mixing and transferring the components is also provided. The method includes disposing the components in the mixing chamber of the mixer with the mixing paddle. The motor is started to actuate the mixing shaft and move the mixing paddle in the mixing chamber to mix the components for a predetermined mixing time. After the predetermined mixing time elapses, operation of the motor continues to actuate the transfer mechanism. A predetermined amount of the mixture is automatically transferred from the mixing chamber to the delivery chamber after the predetermined mixing time has elapsed and in response to actuating the transfer mechanism. 
         [0011]    The system and method of the present invention have the advantage of using the same motor to actuate both the mixing paddle and the transfer mechanism to minimize weight, cost, and waste, especially considering that the system is preferably intended for single use. Furthermore, the system and method of the present invention reduce user interaction compared to prior art devices and increases the readiness in which an operator can prepare a batch of bone cement for surgical purposes. This is useful when the bone cement increases in viscosity quickly and has a short working window. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    These and other objects, features and advantages of this invention will be apparent from the following detailed description of the preferred embodiment and accompanying drawings in which: 
           [0013]      FIG. 1  is a top perspective view of a bone cement mixing and delivery system including a mixer and a delivery device; 
           [0014]      FIG. 2  is a side elevational view of the system of  FIG. 1 ; 
           [0015]      FIG. 3  is a top view of the system of  FIG. 1 ; 
           [0016]      FIG. 4  is a partial front perspective view of the system with a casing and middle housing portion removed to show a motor and transfer mechanism of the mixer; 
           [0017]      FIG. 5  is a partial top perspective view of a bottom housing portion of the mixer showing a switch and gears of the transfer mechanism; 
           [0018]      FIG. 6  is a cross-sectional view of the system of  FIG. 1  in a mixing phase; 
           [0019]      FIG. 7  is another cross-sectional view of the system of  FIG. 1  in the mixing phase; 
           [0020]      FIG. 8  is a perspective view of a mixing shaft of the mixer; 
           [0021]      FIG. 9A  is a top perspective view of a mixing paddle of the mixer; 
           [0022]      FIG. 9B  is a top perspective view of the mixing paddle in a flattened state; 
           [0023]      FIG. 10  is a cross-sectional view of the mixing paddle taken generally along the line  10 - 10  in  FIG. 9 ; 
           [0024]      FIG. 11  is a top perspective view of a piston of the mixer; 
           [0025]      FIG. 12  is a bottom perspective view of the piston; 
           [0026]      FIG. 13  is a cross-sectional view of the piston taken generally along the line  13 - 13  in  FIG. 11 ; 
           [0027]      FIG. 14  is a top perspective view of a mixer housing of the mixer; 
           [0028]      FIG. 15  is a bottom perspective view of the mixer housing; 
           [0029]      FIG. 16  is a side elevational view of the mixer housing; 
           [0030]      FIG. 17  is a top perspective view of a transfer disc of the mixer; 
           [0031]      FIG. 18  is a bottom perspective view of the transfer disc; 
           [0032]      FIG. 19  is a cross-sectional view of the transfer disc taken generally along the line  19 - 19  in  FIG. 17 ; 
           [0033]      FIG. 20  is a cross-sectional view of the system of  FIG. 1  in a transfer phase; 
           [0034]      FIG. 21  is another cross-sectional view of the system of  FIG. 1  in the transfer phase; 
           [0035]      FIG. 22  is an exploded view of a base of the mixer; 
           [0036]      FIG. 23  is a top perspective view of the base of the mixer; 
           [0037]      FIG. 24  is a perspective view of a transfer gear; 
           [0038]      FIG. 25  is a perspective view of a driver; 
           [0039]      FIG. 26  is a perspective view of a switch nut; 
           [0040]      FIGS. 27-29  are perspective views of various spur gears; 
           [0041]      FIG. 30  is a top perspective view of a cap of the mixer; 
           [0042]      FIG. 31  is a bottom perspective view of the cap; 
           [0043]      FIG. 32  is a cross-sectional view of the cap taken generally along the line  32 - 32  in  FIG. 30 ; 
           [0044]      FIG. 33  is a top perspective view of a valve ring of the mixer; 
           [0045]      FIG. 34  is a cross-sectional view of the valve ring taken generally along the line  34 - 34  in  FIG. 32 ; 
           [0046]      FIGS. 35A-38B  are top perspective views of alternative mixing paddles in normal and flattened states; 
           [0047]      FIG. 39  is a top perspective view of the delivery device; 
           [0048]      FIG. 40  is an exploded perspective view of the delivery device; 
           [0049]      FIG. 41  is a cross-sectional view of the delivery device; 
           [0050]      FIG. 42  is a top view of a valve housing of the delivery device; 
           [0051]      FIG. 43  is a partial cross-sectional perspective view illustrating an optional clutch mechanism of the delivery device; 
           [0052]      FIG. 44  is a top perspective view of an alternative plunger of the delivery device; 
           [0053]      FIG. 45  is a bottom perspective view of an alternative proximal knob portion of the delivery device; 
           [0054]      FIG. 46  is a top perspective view of the delivery device coupled to an extension tube and an enlarged luer-lock connector; 
           [0055]      FIG. 47  is a cross-sectional view of the extension tube and the enlarged luer-lock connector; 
           [0056]      FIG. 48  is a perspective view of a lock fitting of the extension tube; 
           [0057]      FIG. 49  is an electrical schematic of the mixer; 
           [0058]      FIG. 50  is a top perspective view of a motorized delivery device; 
           [0059]      FIG. 51  is an exploded view of the motorized delivery device; and 
           [0060]      FIG. 52  is a cross-sectional view of the motorized delivery device. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0061]    For the purpose of promoting an understanding of the present invention, references are made in the text hereof to exemplary embodiments of a bone cement mixing and delivery system, only some of which are depicted in the figures. It should nevertheless be understood that no limitations on the scope of the invention are thereby intended. One of ordinary skill in the art will readily appreciate that modifications such as those involving the materials from which the components are made, the size of the components, functional equivalents of the elements, and the inclusion of additional elements do not depart from the spirit and scope of the present invention. Some of these possible modifications are discussed in the following description. Therefore, specific details disclosed herein are not to be interpreted as limiting, but rather as support for the claims and as a representative basis for teaching one skilled in the art to employ the present invention in virtually any appropriately detailed system, structure, or manner. 
         [0062]    As used herein, “distal” refers to the end of the delivery device from which the bone cement mixture is discharged, and “proximal” refers to the end of the delivery device away from the end from which the bone cement mixture is discharged. The terms “substantially” and “approximately,” as used herein, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. 
         [0063]    Referring in more detail to the drawings, a bone cement mixing and delivery system of the present invention is generally shown at  100  in  FIG. 1 . The system  100  includes a mixer  102  to mix separate components of bone cement to form a bone cement mixture and a delivery device  104  to deliver the mixture to a target site. The target site may be an anatomical site such as a vertebral body or the target site may be in or near an implant. 
         [0064]    The system  100  is useful in any procedure in which bone cement or any other mixture is required, particularly when time is a constraint and exposure of the material or its vapors to the user is to be minimized. The system  100  is capable of mixing the components and automatically transferring the mixture to the delivery device  104  upon completion of mixing with no operator interaction. This reduces variability in mixing between users and creates consistency across multiple users. This automatic transfer feature reduces time and energy otherwise spent by a user to manually mix and transfer the mixture to a delivery device such as a conventional syringe. The system  100  also reduces exposure of the user to the bone cement components during mixing and transfer when compared to conventional mixing and delivery devices. 
       I. Mixer 
       [0065]    Referring to  FIGS. 1-3 , the mixer  102  includes a base  106  for supporting the mixer  102  on a surface. The base  106  includes rubber feet  105  for gripping the surface. A casing  107  mounts to the base  106  to cover the base and provide an aesthetically pleasing shape to the mixer  102 . A mixer housing  108  is coupled to the casing  107 . A transfer conduit  110  links the mixer housing  108  to the delivery device  104 . The transfer conduit  110  conveys the mixture from the mixer  102  to the delivery device  104 . A switch cover  112  is pivotally mounted to the casing  107  to protect a switch button  114  (see  FIG. 4 ) used to begin operation of the mixer  102 . Once the switch button  114  is pressed, the bone cement components are mixed together to form the mixture and then, once mixing is complete, the mixture is automatically transferred through the transfer conduit  110  to the delivery device  104 . 
         [0066]    Referring to  FIGS. 4 and 5 , the mixer  102  is shown with the casing  107  removed to expose some of its internal components. As shown, the mixer  102  is battery-powered. Batteries  115  are used to power a motor  150  that drives the mixing and transfer operations of the mixer  102 . In one embodiment, a battery pack  109  of eight batteries  115  is used to power the motor  150 . The motor  150  is preferably a reversible DC motor such as those available from Mabuchi Motor Co. of Matsudo City, Japan. Possible models that could be used include Model Nos. RC-280RA-2865 and RC-280SA-2865. The mixer  102  is preferably disposable such that the motor  150  and batteries  115  are selected for single use. A switch  117  closes a circuit (see  FIG. 49 ) between the batteries  115  and the motor  150  to begin operation of the motor  150 . The switch button  114 , when pressed, trips the switch  117  to close the circuit. Once the mixing and transfer operations are complete, the motor  150  ceases to operate. 
         [0067]    Referring to  FIGS. 6 and 7 , the base  106  of the mixer  102  comprises a bottom housing portion  118  and a middle housing portion  116  secured to the bottom housing portion  118  using conventional fasteners, adhesives, and the like. A mixing shaft  120  is rotatably supported between the housing portions  116 ,  118 . The mixing shaft  120  has a mixing gear  122  with mixing gear teeth  123  at one end. The mixing shaft  120  is rotatably supported in the bottom housing portion  118  by a centering pin  119 . The mixing shaft  120  extends from the mixing gear end to a second end  124  that is connected to a mixing paddle  126 . This connection is preferably releasable, but could include integral or fixed connections. 
         [0068]    Referring to  FIGS. 6-10 , the mixing paddle  126  includes a hub  128  with inner splines  130  that interact with outer splines  132  on the mixing shaft  120  to rotationally lock the mixing shaft  120  to the mixing paddle  126  during the mixing phase (shown in  FIGS. 8 and 10 ). The outer splines  132  extend along the entire length of the mixing shaft  120  from the mixing gear  122 . This rotational locking feature allows the mixing shaft  120  to impart rotational motion to the mixing paddle  126  to adequately mix the bone cement components. When mixing is complete, the rotational lock between the mixing shaft  120  and the hub  128  is removed to prevent further rotation of the mixing paddle  126  in the transfer phase. 
         [0069]    The preferred embodiment of the mixing paddle  126  is shown in  FIGS. 9A, 9B, and 10 . In one embodiment, the mixing paddle  126  is formed of injection molded plastic. In other embodiments, the mixing paddle  126  is formed from a flat piece of plastic or metal material. In these embodiments, the mixing paddle  126  is cut from the flat piece of material and folded/shaped to the configuration shown in  FIG. 9A . The mixing paddle  126  includes a flat base section  222  and a bent flap  220  forming an obtuse angle with the flat base section  222 . The flat base section  222  is fixed to the hub  128  by being integrally molded with the hub  128  or by adhesive or the like. The hub  128  extends downwardly from the flat base section  222 . The bent flap  220  is radially spaced from a center of the hub  128 . As the mixing paddle  126  rotates, the bent flap  220  urges the bone cement components upwardly. A pair of flat arms  224  extends upwardly from the flat base section  222  generally perpendicularly to the flat base section  222 . The flat arms  224  act as mixing vanes to mix the bone cement components. 
         [0070]    A flat connector section  226  extends between and connects the flat arms  224 . The flat connector section  226  forms an obtuse angle A with the flat arms  224 . As a result, when the mixing paddle  126  is urged upwardly in the mixing chamber  138  during the transfer phase (further described below), the flat connector section  226  strikes a top of the mixer housing  108 . As the mixing paddle  126  continues to move upwardly in the mixing chamber  138 , the mixing paddle  126  begins to compress toward a flattened configuration. This includes bending the flat arms  224  downward toward the flat base section  222  about a hinge, then eventually flattening the flat connection section  226  and the bent flap  220  such that they all fall in generally the same plane as the flat base section  222  (see  FIG. 9B ). 
         [0071]    Referring to  FIGS. 6-7 and 11-13 , a piston  134  supports the mixing paddle  126 . More specifically, the hub  128  of the mixing paddle  126  is seated in a bore  136  defined through the piston  134 . An o-ring seals the hub  128  in the bore  136 . The piston  134  is releasably secured in the mixer housing  108 . Another o-ring seals the piston  134  to an interior surface of the mixer housing  108 . The piston  134  includes a pair of flexible tabs  135  that rest beneath a shoulder  137  defined in the interior surface of the mixer housing  108 . The flexible tabs  135  hold the piston  134  in place until such time as the piston  134  is forced upwardly to transfer the mixture to the delivery device  104  in the transfer phase. At that point, the flexible tabs  135  are forced inwardly to allow the piston  134  to move upwardly along the interior surface of the mixer housing  108 . In the mixing phase, however, the piston  134  remains in place and forms a mixing chamber  138  with the mixer housing  108 . 
         [0072]    In one embodiment, the mixer  102  may be shipped with a powder component of the bone cement stored in the mixing chamber  138 . In this embodiment, a cap  140  is releasably coupled to the mixer housing  108  during shipment to keep the powder component in the mixing chamber  138 . More specifically, the cap  140  is secured to a cylindrically-shaped top port  141  of the mixer housing  108 . 
         [0073]    The top port  141  defines a pour opening  143  (see  FIG. 14 ) that enters the mixing chamber  138  through a plurality of web sections  145  that form a web. A plurality of port flanges  147  extends radially outwardly from the top port  141  to engage the cap  140 . The cap  140  includes a plurality of locking tabs  149  that engage the port flanges  147  to lock the cap  140  to the mixer housing  108 . An o-ring seals the cap  140  to the mixer housing  108 . When the system  100  is ready to be used, the user removes the cap  140  to add a liquid component of the bone cement through the pour opening  143  to the powder component already placed in the mixing chamber  138  or also added through the pour opening  143 . Once the components are disposed in the mixing chamber  138 , the mixer  102  is ready for operation. 
         [0074]    The motor  150  operates through a gear arrangement to rotate the mixing shaft  120  during the mixing phase to mix the powder and liquid components. Rotation of the mixing shaft  120  imparts rotation to the mixing paddle  126 , which is disposed in the mixing chamber  138 . The gear arrangement includes a face gear  152  having a set of face gear teeth  154 . A pinion gear  156  (see  FIG. 22 ) is fixed to a shaft of the motor  150  to rotate with the motor  150  during operation. The pinion gear  156  has pinion gear teeth  157  engaging the face gear teeth  154  such that the motor  150  drives the face gear  152  during operation. 
         [0075]    The face gear  152  drives a first spur gear  160 , which drives a second spur gear  166 . More specifically, the face gear  152  has a lower set of gear teeth  154  continuously engaging an upper set of spur gear teeth  162  formed on the first spur gear  160 . A lower set of spur gear teeth  164  formed on the first spur gear  160  continuously engages an upper set of spur gear teeth  168  formed on the second spur gear  166 . The upper set of spur gear teeth  168  engages the mixing gear teeth  123  to rotate the mixing shaft  120  and mixing paddle  126  during the mixing phase. 
         [0076]    The second spur gear  166  drives a third spur gear  167 . In particular, a lower set of spur gear teeth  170  formed on the second spur gear  166  engages a lower set of spur gear teeth  169  formed on the third spur gear  167 . The third spur gear  167  also includes an upper set of spur gear teeth  171  (see  FIG. 7 ). The upper set of spur gear teeth  171  formed on the third spur gear  167  engages a set of transfer gear teeth  176  formed on a transfer gear  172 . As a result, when the motor  150  operates, both the mixing shaft  120  and the transfer gear  172  rotate. Each of the face gear  152  and spur gears  160 ,  166 ,  167  are supported by centering pins captured between the middle housing portion  116  and the bottom housing portion  118 . 
         [0077]    The transfer gear  172  is generally cylindrical and includes a first open end and a second, partially closed, end defining an aperture. The mixing shaft  120  is rotatably supported in the aperture such that rotation of the mixing shaft  120  does not interfere with rotation of the transfer gear  172 . The speed with which the mixing shaft  120  and transfer gear  172  rotate depends on the gear ratios of the gears. In some embodiments, the gear ratios are set such that the transfer gear  172  rotates slower than the mixing shaft  120 . 
         [0078]    The transfer gear  172  forms part of a transfer mechanism of the mixer  102 . The transfer mechanism transfers the mixture out from the mixing chamber  138  and into a delivery chamber of the delivery device  104  after mixing. Transfer threads  178  are defined on an outer surface of the transfer gear  172 . A switch nut  180  is threaded on the outer surface of the transfer gear  172 . The switch nut  180  is fixed from rotation so that as the transfer gear  172  rotates, the switch nut  180  moves along the outer surface of the transfer gear  172 . The switch nut  180  has two projections  182  with a notch  184  defined therebetween. The notch  184  rides along an edge of a printed circuit board  186  fixed to the bottom housing  118  to prevent rotation of the switch nut  180  with the transfer gear  172 . In other words, the edge of the printed circuit board  186  rides in the notch  184  between the projections  182  as the transfer gear  172  rotates thereby preventing the switch nut  180  from rotating. The motor  150 , by way of its rotation of the transfer gear  172 , operatively engages the switch nut  180 . This is best shown in  FIG. 5 . 
         [0079]    During operation, after the switch  117  has been closed, the switch nut  180  rides along the printed circuit board  186  as it further threads onto the transfer gear  172  in one direction until it engages a second switch  190  (see  FIG. 5 ), spaced from the switch  117 . Thus, the switch nut  180  acts as a switch actuator  180 . Other suitable actuators could be employed. The second switch  190 , when tripped by movement of the switch nut  180 , opens the circuit between the batteries  115  and the motor  150  to shut down operation of the motor  150  (see  FIG. 49 ). 
         [0080]    The transfer mechanism further includes a driver  192  that is keyed to the transfer gear  172  to rotate with the transfer gear  172 . Thus, the transfer gear  172  operatively couples the motor  150  to the driver  192 . The driver  192  includes keyways  193  (see  FIG. 22 ), while the transfer gear  172  includes keys  195  (see  FIG. 22 ) slidably disposed in the keyways  193 . In other embodiments, the driver  192  could include the keys  195 , while the transfer gear  172  includes the keyways  193 . Of course, other coupling mechanisms could be used to lock rotation of the transfer gear  172  to the driver  192 . The driver  192  is free to move axially relative to the transfer gear  172 . The driver  192  has driving threads  194  defined on its outer surface. During the mixing phase, the driving threads  194  are rotatably received in a bore  196  of a transfer disc  198 . The transfer disc  198  is coupled to a bottom of the mixer housing  108  and fixed from movement. The transfer disc  198  also forms part of the transfer mechanism and acts as a drive nut  198  for the driver  192 . 
         [0081]    During the mixing phase, the driving threads  194  rotate within the bore  196  of the transfer disc  198  and engage corresponding threads  202  in the bore  196 . Thus, the transfer disc  198  operates as a fixed drive nut.  FIGS. 6 and 7  show the driving threads  194  fully advanced through the bore  196 . This represents the end of the mixing phase. A spring  203  biases the driver  192  upwardly in the cavity of the transfer gear  172  to facilitate engagement with the threads  202 . The time required for the driving threads  194  to fully advance through the bore  196  represents the mixing phase. In other words, a predetermined mixing period is set by the amount of time it takes for the driving threads  194  to fully advance through the transfer disc  198 . Once the driving threads  194  completely pass through the bore  196 , the transfer phase begins. The transfer phase continues for a predetermined transfer period, which is defined between the start of transfer and the actuation of the second switch  190 , which ceases operation of the motor  150 . 
         [0082]    Referring to  FIGS. 20 and 21 , when the driver  192  advances in the transfer phase, it pushes the push cap  200  axially upwardly against the piston  134 , which in turn urges the piston  134  upwardly to move through the mixing chamber  138 . The piston  134  is sealed to the wall of the mixer housing  108  and includes a face that contacts the mixture in the mixing chamber  138 . The mixture is pushed upwardly through an exit port  204  (also referred to as a transfer port  204 ; see  FIG. 21 ) into the transfer conduit  110  and then into the delivery device  104 . For this reason, the piston  134  is also considered part of the transfer mechanism of the mixer  102 . 
         [0083]    As the driver  192  advances in the transfer phase and moves the piston  134  through the mixing chamber  138 , the driver  192 /piston  134  disengages the mixing paddle  126  from the mixing shaft  120 . More specifically, the hub  128  with inner splines  130  is lifted off the outer splines  132  on the mixing shaft  120  to rotationally unlock the mixing shaft  120  from the mixing paddle  126  during the transfer phase. The mixing shaft  120  is held down by the transfer gear  172  while the mixing paddle  126  is disengaged from the mixing shaft  120 . As the piston  134  rises in the mixing chamber  138 , the mixing paddle  126  folds down to a compact size to permit a majority of the mixture to be pressed out of the mixing chamber  138  and into the delivery device  104 . 
         [0084]    The motor  150  operates through the gear arrangement to rotate the mixing shaft  120  and actuate the mixing paddle  126  during the mixing phase to mix the powder and liquid components, while also rotating the transfer gear  172  to actuate the transfer mechanism to automatically transfer the mixture from the mixing chamber  138  to the delivery chamber of the delivery device  104  after the predetermined mixing period has elapsed. In other words, the motor  150  operatively engages both the mixing shaft  120  and the transfer mechanism (including the transfer gear  172 , driver  192 , piston  134 , etc.). The motor  150  continues operation from its start, upon actuation of the switch  117 , until it stops upon actuation of the second switch  190 , during which time the motor  150  operates to mix the components in the mixer  102  and transfer the mixture to the delivery device  104 . In one embodiment, the switch  117  and the second switch  190  are combined into a single switch (not shown) that is closed to start operation of the motor  150  by an actuator, and opened to stop operation of the motor  150 . 
         [0085]    In still other embodiments, the second switch  190  reverses the polarity of the motor  150  and causes the transfer gear  172  to reverse its rotation. Consequently, the switch nut  180  changes direction and rides back along the printed circuit board  186 . In this embodiment, the threads  202  are configured such that during the mixing phase the driving threads  194  cannot engage the threads  202  of the transfer disc  198 . However, when the polarity switch  190  is tripped by the switch nut  180 , the driver  192  reverses its direction of rotation with the transfer gear  172  and engages the threads  202  in a manner that advances the driver  192  axially during the transfer phase. In this embodiment, a third switch (not shown) or other mechanism would be required to be tripped by the switch nut  180  as it travels back along the printed circuit board  186  to stop operation of the motor  150 . 
         [0086]    As shown in  FIGS. 7 and 14-19 , the bottom of the mixer housing  108  includes a flange  173  and a short wall  175  extending downwardly from the flange  173 . A plurality of locking tabs  177  (see  FIG. 15 ) are spaced circumferentially about the short wall  175  and extend radially outwardly from the short wall  175 . During assembly of the mixer  102 , the locking tabs  177  are inserted into openings  179  (see  FIG. 17 ) defined in a top of the transfer disc  198 . The casing  107  is captured between the mixer housing  108  and the transfer disc  198  when this is done (see  FIG. 21 ). The mixer housing  108  is then rotated one-quarter turn such that the locking tabs  177  slide beneath corresponding locking members  183  on the transfer disc  198  until they reach stops  199 . The piston  134  rests on top of the transfer disc  198  and is initially coupled to the transfer disc  198  by the push cap  200 . 
         [0087]      FIG. 22  illustrates an exploded view of the base  106  including the bottom housing portion  118 , the middle housing portion  116 , and the gear arrangement disposed therebetween for converting motor operation into mixing and transfer operations.  FIG. 23  shows the base  106  fully assembled. 
         [0088]      FIGS. 24-29  illustrate perspective views of the transfer gear  172 , the driver  192 , the switch nut  180 , the first spur gear  160 , the second spur gear  166 , and the third spur gear  167 . 
         [0089]    Referring to  FIGS. 30-32 , the cap  140  is shown. The cap  140  includes a top  232 . A cap wall  234  is disposed on the top  232  and extends downwardly from the top  232  to a bottom wall  236 . A gripping flange  238  extends downwardly from the top  232  and is spaced from the cap wall  234 . A plurality of locking tabs  240  are disposed on the gripping flange  238  and extend radially inwardly into a gap between the gripping flange  238  and the cap wall  234 . The locking tabs  240  engage the tabs  147  on the top port  141 . 
         [0090]    Referring to  FIGS. 7, 33, and 34 , a valve  206  is arranged in the exit port  204  to prevent the escape of unmixed components during mixing. Referring to  FIG. 34 , the valve includes a plastic or metal ring  210  having a plurality of apertures  212  for receiving an elastomeric material  213  in a molding process. The material  213  fills in the apertures  212  as shown in  FIG. 34  and includes cross-cut slits  214  that remain closed in the mixing phase, but open up and allow the mixture to flow therethrough into the transfer conduit  110  during the transfer phase. 
       II. Alternative Mixing Paddles 
       [0091]    Alternative embodiments of the mixing paddle  126  are shown in  FIGS. 35A-38B . In  FIGS. 35A and 35B , the mixing paddle  126 ′ is formed of plastic and includes a pair of flat arms  224 ′ extending upwardly from a flat base section  222 ′. A pair of opposed bent flaps  220 ′ form an obtuse angle with the flat base section  222 ′. In this embodiment, the flat arms  224 ′ are opposed from one another on opposite sides of a center of the mixing paddle  126 ′. The flat arms  224 ′ further include bent ends  225 ′ that strike the top of the mixer housing  208  in the transfer phase and bend inwardly to flatten the flat arms  224 ′. 
         [0092]    Referring to  FIGS. 36A and 36B , the mixing paddle  126 ′ is formed of metal such as stainless steel or aluminum. 
         [0093]    In  FIGS. 37A and 37B , a mixing paddle  126 ″ has a pair of opposed arms  224 ″ that are pivotally connected to a flat base section  222 ″ by a pair of pivot pins  229 . 
         [0094]    In  FIGS. 38A and 38B , a mixing paddle  126 ′″ includes a flat base section  222 ′″, a bent flap  220 ′″ forming an obtuse angle with the flat base section  222 ′″, and a single flat arm  224 ′″ extending upwardly generally perpendicularly to the flat base section  222 ′″. An extension  231  extends at an obtuse angle for crossing the mixing chamber  138 . In each of the embodiments of the alternative mixing paddles, the arms  224 ′,  224 ″,  224 ′″ are configured to be supported by the wall of the mixer housing  108  during rotation in the clockwise direction (when viewed from above), but unsupported when rotating in the counterclockwise direction. When unsupported, they are urged into their compressed state. This is useful when the motor  150  changes direction during the transfer phase, as described in the alternative transfer embodiment above. 
         [0095]    The mixer housing  108 , transfer disc  198 , mixing shaft  120 , transfer gear  172 , face gear  152 , spur gears  160 ,  166 ,  167 , switch nut  180 , driver  192 , piston  134 , cap  140 , mixing paddle  126 , bottom housing portion  118 , middle housing portion  116 , casing  107 , and switch cover  112  are preferably formed of a bio-compatible plastic material such as nylon, PBT (polybutylene terephthalate), PC (polycarbonate), ABS (acrylonitrile butadiene styrene), glass-filled nylon, glass-filled polyetherimide, or the like. 
       III. Delivery Device 
       [0096]    Referring to  FIGS. 39-42 , the delivery device  104  is shown. The delivery device  104  comprises a reservoir  302  defining the delivery chamber for receiving the bone cement mixture from the transfer conduit  110  during the transfer phase. The reservoir  302  includes an entry port  314  (or inlet port  314 ) defined in a sidewall of the reservoir  302 . A valve housing  316  (see also  FIG. 42 ) is outfitted with an o-ring  318  and is seated in the entry port  314 . The valve housing includes a plurality of flow paths  319  and a central bore  321 . As shown in  FIG. 41 , a one-way umbrella valve  320  is supported in the central bore  321  of the valve housing  316  such that the bone cement mixture opens the valve  320  to fill the reservoir  302 . The one-way umbrella valve  320  prevents the bone cement mixture from re-entering the mixer  102  during the transfer phase. A handle  304  is mounted about the reservoir  302  for grasping by the user. 
         [0097]    A rotatable fitting  322  is secured in the valve housing  316  during the mixing and delivery phases. To accomplish this, the rotatable fitting  322  fits through an aperture  325  in the handle  304 . The rotatably fitting  322  includes a pair of diametrically opposed locking tabs  306  that engages the handle  304 . The handle  304  includes a plurality of locking flanges  327  spaced circumferentially from one another in the aperture  325 . The locking flanges  327  extend radially inwardly into the aperture  325 . During assembly, the locking tabs  306  pass into the aperture  325  between the locking flanges  327  and are rotated into place with the locking tabs  306  disposed beneath the locking flanges  327 . An annular flange  329  of the rotatable fitting  322  rests on top of the locking flanges  327  when in position (see  FIG. 41 ). 
         [0098]    One end of the transfer conduit  110  fits into the rotatable fitting  322 . A through bore  331  is defined through the rotatable fitting  322  to transfer the bone cement mixture to the reservoir  302  from the transfer conduit  110 . During transfer the bone cement mixture passes through the through bore  331  under pressure thereby opening the one-way umbrella valve  320  and passing through the flow paths  319  (see  FIG. 42 ) into the reservoir  302 . Once transfer is complete, the rotatable fitting  322  is rotated counterclockwise to release the rotatable fitting  322  from the valve housing  316  thereby allowing the user to remove the delivery device  104  from its cradle mounts  333  on the mixer  102  in preparation for delivering the bone cement mixture to the target site. 
         [0099]    A nut  324  is mounted to a proximal end of the reservoir  302 . In particular, the proximal end of the reservoir  302  has a rectangular flange  326  for supporting the nut  324 . The rectangular flange  326  slides into a slot  328  defined in the nut  324 . The nut  324  has a generally box-like shape that is secured between two halves  330 ,  332  of the handle  304 . Each half  330 ,  332  of the handle  304  has a complimentary box-shaped cavity  334  such that the nut  324  fits snugly in the cavities  334  when the halves  330 ,  332  are fixed together. The halves  330 ,  332  may be fixed together by conventional fasteners, adhesives, and the like. 
         [0100]    A plunger  310  drives the mixture through the delivery chamber of the reservoir  302  during delivery. The plunger  310  includes a threaded shaft  336  that engages threads  338  of the nut  324 . A plunger head  344  is snap-fit to the threaded shaft  336  to form a distal end of the plunger  310 . The plunger head  344  is snap-fit to the threaded shaft  336  by inserting a stem  346  of the plunger head  344  into a bore  348  defined through the threaded shaft  336 . Referring to  FIGS. 40 and 41 , the stem  346  has a pair of diametrically opposed detent ramps  354  that slide through the bore  348  in a compressed configuration (by being pressed together via a slot  349  defined through the stem  346 ) until the ramps  354  pass a shoulder  356  in the bore  348 . Once they pass the shoulder  356 , the ramps  354  spring outwardly to engage the shoulder  356  and prevent withdrawal of the plunger head  344 . An o-ring  350  is seated with a dynamic seal  351  in an outer groove defined in the plunger head  344  to seal against an interior of the reservoir  302 . 
         [0101]    A proximal end  311  of the plunger  310  has a generally box-like shape. A knob  312  is mounted about the proximal end  311  of the plunger  310  to facilitate rotation of the plunger  310 . The knob  312  has a proximal knob portion  340  defining a box-shaped cavity  341  for receiving the proximal end  311  of the plunger  310  such that as the user rotates the proximal knob portion  340 , the plunger  310  also rotates. A distal knob portion  342  is fastened to the proximal knob portion  340  using fasteners, adhesives, or the like. The proximal end  311  of the plunger  310  is captured between the proximal  340  and distal  342  knob portions to prevent the proximal end  311  of the plunger  310  from slipping out of the box-shaped cavity  341 . 
         [0000]    IV. Alternative Delivery Device with Clutch 
         [0102]    Referring to  FIGS. 43-45 , an alternative plunger shaft  360  is shown. Referring specifically to  FIG. 44 , a proximal end of the plunger shaft  360  includes a flange  362  and a plurality of projections  364  disposed on the flange  362 . The plurality of projections  364  extend proximally from the flange  362 . The projections  364  are circumferentially spaced from one another about a periphery of the flange  362 . Each of the projections  364  has a vertical surface  366  and an angled surface  368  (forms acute angle with flange  362 ) meeting at a plateau  370  generally parallel to the flange  362 . In the embodiment, a knob  371  is mounted to the proximal end of the plunger shaft  360  to facilitate rotation of the plunger shaft  360 . The knob  371  includes a proximal knob portion  372 . The proximal knob portion  372  includes a top  374  and a plurality of complimentary projections  376  disposed on the top  374  and extending distally from the top  374 . The complimentary projections  376  mate with the projections  364  on the flange  362  by fitting in spaces defined between the projections  364  on the flange  362 . 
         [0103]    Each of the complimentary projections  376  also includes a vertical surface  378  and an angled surface  380  meeting at a plateau  382  generally parallel to the top  374 . A distal knob portion  384  is fastened to the proximal knob portion  372  using fasteners, adhesives, or the like. The proximal end of the plunger shaft  360  is captured between the proximal  372  and distal  384  knob portions. The plunger shaft  360  passes through a bore  385  defined through the distal knob portion  384 . A spring  386  rests on a shoulder  388  defined in the distal knob portion  384  about the bore  385 . The spring  386  acts between the shoulder  388  and the flange  362 . 
         [0104]    The spring  386 , along with the projections  364 ,  376 , form a clutch mechanism. This clutch mechanism can be configured to slip when undesired pressures are reached in the delivery device  104 . During use, when a user is rotating the knob  371 , the projections  376  formed on the proximal knob portion  372  engage the projections  364  formed on the flange  362  of the plunger shaft  360 . In particular, the angled surfaces  368 ,  380  engage one another as the user rotates the knob  371  clockwise. The spring  386  acts to keep the angled surfaces  368 ,  380  in engagement during normal operation. However, when undesired pressures are reached the angled surfaces  368 ,  380  begin to slip and the flange  362  separates from the proximal knob portion  372 . As a result, the projections  364 ,  376  slide out of engagement thereby preventing further advancement of the plunger shaft  360  until pressure is normalized. Different spring constants can be used to alter the pressure at which the clutch mechanism is actuated. Furthermore, the projections  364 ,  376  could be oriented radially, as opposed to axially, such that axial forces supplied by the user does not affect the clutch mechanism&#39;s operation. 
         [0000]    V. Extension Tube with Enlarged Connector 
         [0105]    Referring to  FIG. 46 , an extension tube  400  is shown mounted to the distal end of the reservoir  302 . In one embodiment, the extension tube  400  is automatically primed with bone cement during the transfer phase. In other words, the system  100  is designed for use with specified mixture volumes that fill both the reservoir  302  and the extension tube  400  in the transfer phase. This eliminates the need for the user to prime the extension tube  400  manually. 
         [0106]    Referring to  FIGS. 47 and 48 , the extension tube  400  includes a tube fitting  402  for securing the extension tube  400  to the delivery port  306  of the reservoir  302 . Referring back to  FIG. 39 , the delivery port  306  includes a pair of diametrically opposed projections  404  and the tube fitting  402  includes a pair of diametrically opposed channels  406  for receiving the projections  404  when the tube fitting  402  is axially mounted onto the discharge port  306 . Once the projections  404  bottom-out in the channels  406 , the tube fitting  402  is rotated. The projections  404  then ride in diametrically opposed slots  408  defined through the tube fitting  402 . The tube fitting  402  is then prevented from axially sliding off the delivery port  306 . In other embodiments, the tube fitting  402  is fixed to the delivery port  306  with adhesive, press fit, welding, or the like. 
         [0107]    Referring to  FIG. 47 , an enlarged luer-lock connector  410  is mounted to a distal end of the extension tube  400 . The luer-lock connector  410  comprises a knob  412 , a spindle  414 , and a collar  416 . The collar  416  includes a side port  418  defining a side bore  426 . A main bore  420  is defined through the collar  416  normal to the side port  418 . The distal end of the extension tube  400  fits into the side bore  426  of the side port  418 . The extension tube  400  may be fixed in the side port  418  by press fit, ultrasonic welding, adhesive, or the like. 
         [0108]    The spindle  414  is rotatably supported in the main bore  420  of the collar  416 . A pair of o-rings  415  seals the spindle  414  in the main bore  420 . The spindle  414  includes a through bore  422  and a cross bore  424  aligned with the side bore  426  in the side port  418 . The cross bore  424  is disposed between the o-rings  415 . The knob  412  includes a stem  428  that fits into the through bore  422  in a top of the spindle  414 . The stem  428  is fixed in the through bore  422  by a press-fit, ultrasonic welding, adhesive, or the like. 
         [0109]    The knob  412  further includes a grasping portion  430  shaped for grasping by a hand of the user. The spindle  414  fits inside an annular cavity  432  in the knob  412 . A bottom of the spindle  414  has a connector portion  434 , e.g., a standard luer-lock fitting  434 . The through bore  422  continues through the luer-lock fitting  434 . The luer-lock fitting  434  is configured for attaching to a corresponding luer-lock fitting  436  on a delivery cannula  440 . During use, the user grasps the grasping portion  430  of the knob  412  and rotates the knob  412  and spindle  414  to lock the luer-lock fitting  434  of the spindle  414  on the luer-lock fitting  436  on the delivery cannula  440 . The oversized grasping portion  430  facilitates easier connection of the extension tube  400  to the delivery cannula  440  to deliver the bone cement mixture through the extension tube  400 , the through bore  422 , the delivery cannula  440 , and to the target site. 
         [0110]    The reservoir  302 , rotatable fitting  322 , handle  304 , knob  312 , plunger  310 , nut  324 , valve housing  316 , tube fitting  402 , and enlarged luer-lock connector  410  are preferably formed of a bio-compatible plastic material such as nylon, PBT (polybutylene terephthalate), PC (polycarbonate), ABS (acrylonitrile butadiene styrene), glass-filled nylon, glass-filled polyetherimide, or the like. The umbrella valve  320  is preferably formed of nitrile. 
         [0000]    VI. Alternative Delivery Device with Delivery Motor 
         [0111]    Referring to  FIGS. 50-52 , an alternative delivery device  504  is shown. The delivery device  500  comprises a reservoir  502  defining a delivery chamber for receiving the bone cement mixture from the transfer conduit  110  during the transfer phase. The reservoir  502  threadably engages a cap  505  seated in an end plate  507 . The end plate  507  is supported between and fixed to two side plates  509 . The end plate  507  has a U-shaped cutout portion into which the cap  505  extends. The cutout portion supports the cap  505 . A bottom plate  506  supports and is fixed to the side plates  509 . A middle plate  513  is fixed to the bottom plate  506  and the two side plates  509 . The middle plate  513  is preferably rectangular in shape to prevent rotation of the middle plate  513  between the side plates  509 . A nut  524  is disposed between the middle plate  513  and the cap  505 . The nut  524  is fixed from rotation relative to the plates  507 ,  509 ,  513  by being fixed to the middle plate  513  by adhesive, welding, fasteners, or the like. 
         [0112]    Referring to  FIGS. 51 and 52 , a plunger  510  drives the mixture through the delivery chamber of the reservoir  502  during delivery. The plunger  510  includes a threaded shaft  536  that engages threads (not shown) of the nut  524 . A plunger head  544  is fixed to the threaded shaft  536  to form a distal end of the plunger  510 . An o-ring  550  with a dynamic seal  551  is seated in an outer groove defined in the plunger head  544  to seal against an interior of the reservoir  502 . 
         [0113]    A proximal end  511  of the plunger  510  is slidably disposed in a rotating drive shaft  600 . The drive shaft  600  is hollow and includes a key  602  disposed along its internal surface. The key  602  protrudes radially inwardly. The plunger  510  includes a keyway  604  disposed in an outer surface of the threaded shaft  536 . The key  602  is configured to slide in the keyway  604  as the drive shaft  600  rotates due the fixed nature of the nut  524 . 
         [0114]    Referring to  FIG. 52 , a delivery motor  606  and gear box  608  operate to rotate the drive shaft  600 . The gear box  608  includes a box  610  and a cover  612 . The delivery motor  606  is supported in a mounting sleeve  614  disposed on the cover  612 . A motor shaft  616  penetrates through the cover  612  into the gear box  608 . A pinion gear  616  is fixed to the motor shaft  616  to rotate with the delivery motor  606  during its operation. A series of spur gears  618 ,  620 ,  622 ,  624  are rotatably supported by shafts  626 ,  628 . The shafts  626 ,  628  are fixed to the box  610  and cover  612  for support. 
         [0115]    A proximal end of the drive shaft  600  is rotatably supported in the box  610  by a bushing  630 . A drive gear  632  is fixed to the proximal end of the drive shaft  600  and rotatably supported by a shaft  634 . The shaft  634  is fixed to the cover  612 . The series of spur gears  618 ,  620 ,  622 ,  624  transfer power from the motor shaft  616  to the drive gear  632  during operation. A switch  640  controls operation of the delivery motor  606 . The delivery motor  606  may be powered by a battery pack  607 . After the mixture has been transferred from the mixing chamber  138  to the delivery chamber of the reservoir  502 , as described above, the user can operate the delivery motor  606  to deliver the mixture to the target site. 
       VII. Drool Valve and Viscosity Meter 
       [0116]    Referring back to  FIG. 50 , a drool valve  700  may be positioned at any point along the extension tube  400 , including at the distal end of the extension tube  400 . The drool valve  700  may be a motor-controlled valve or a solenoid valve. The drool valve  700  is controlled by a controller  702 . The controller  702 , in this embodiment, also controls the delivery motor  606  through the switch  640 . The drool valve  700  operates to discontinue flow of the mixture through the extension tube  400  from the delivery device  500  upon actuation of the delivery switch  640  thereby preventing excess mixture from entering the target site. Without the drool valve  700 , when the user actuates the delivery switch  640  to stop operation of the delivery motor  606 , there is still pressure in the extension tube  400  due to the compressible nature of the mixture. This pressure tends to deliver an additional amount of the mixture to the target site after the user desires to stop flow of the mixture. With the drool valve  700 , the amount of the mixture delivered can be better controlled. 
         [0117]    In operation, the user actuates the switch  640  to send power to the drool valve  700  and the delivery motor  606 . This opens the drool valve  700  and starts operation of the delivery motor  606 . Operation of the delivery motor  606  rotates the drive shaft  600  and advances the plunger  510  in the reservoir  502  to begin delivering the mixture from the reservoir  502 , down the extension tube  400 , to the target site. When the user wishes to stop the flow of the mixture, the switch  640  is again actuated to signal the controller  702  that the delivery motor  606  is to be stopped and the drool valve  700  is to be closed. The controller  702  then discontinues power to the delivery motor  606  and the drool valve  700 . 
         [0118]    A viscosity meter  710  monitors current draw on the delivery motor  606  to approximate the viscosity of the mixture in the reservoir  502 . The viscosity meter  710  can be a current meter integrated into the controller  702  to monitor the current draw from the delivery motor  606 . The controller  702  then correlates current draw to viscosity by way of a look-up table using correlation values that can be easily derived. A display  712  then displays the approximate viscosity of the mixture. Of course, the viscosity measurement is an estimate and not an exact measurement of viscosity, but can be useful in determining how much longer the working time window for the particular bone cement being used will remain open. 
         [0119]    While this description is directed to a few particular embodiments, it is understood that those skilled in the art may conceive of modifications and/or variations to the specific embodiments shown and described herein. Any such modifications or variations that fall within the purview of this description are intended to be included herein as well. It is understood that the description herein is intended to be illustrative only and is not intended to be limited.