Abstract:
A device for injecting a cement paste into a bone cavity and forming a hardened cement in-situ under pressure includes a syringe having an injection end; a pocket having an inlet and a body for containing the paste entering the inlet; and a mounting mechanism at the injection end for connecting the inlet of the pocket to the injection end so that the body of the pocket can contain the paste through the syringe. The device further includes a leaking mechanism provided at the injection end of the syringe for allowing liquid contained in the paste inside the pocket to be expelled from the pocket under pressure, and an opening mechanism which can be operated at a location away from the injection end to open the body of the pocket.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention is related to a technique of forming a set or hardened bone cement in bone cavity, and in particular to a technique of hardening a bone cement under an exerted pressure in a bone cavity.  
       BACKGROUND OF THE INVENTION  
       [0002]     Calcium phosphate cement (abbreviated as CPC) has been widely used as an implant or filling material in dental and bone prosthesis, and its technical details can be found in many patents, for examples U.S. Pat. Nos. 4,959,104; 5,092,888; 5,180,426; 5,262,166; 5,336,264; 5,525,148; 5,053,212; 5,149,368; 5,342,441; 5,503,164; 5,542,973; 5,545,254; 5,695,729 and 5,814,681. Similar to CPC, calcium sulfate and bioactive glass have also been suggested or used as an implant or filling material in dental and bone prosthesis.  
         [0003]     Heretofore the conventional method of forming a set or hardened bone cement in bone cavity involves directly injecting a cement paste into bone cavity, which suffers the followings drawbacks among others: 
        (1) While the liquid-powder ratio of the cement paste is too high, the strength of the hardened cement becomes too low, that can cause the cement to more easily disperse/disintegrate;     (2) While the liquid-powder ratio of the cement paste is too low, the viscosity of the paste becomes too high, the working and setting times become too short, and the paste is hard to inject through a syringe;     (3) Dispersed cement particles in body fluid/blood, especially before being fully set, can penetrate into surrounding tissue, that can cause serious hazard during or after surgery.        
 
       SUMMARY OF THE INVENTION  
       [0007]     A primary objective of the present invention is to provide a method and a device for forming a hardened cement in a bone cavity, which are free of the aforesaid prior art drawbacks.  
         [0008]     The present invention provides a device for forming a hardened cement in a bone cavity comprising a syringe having an injection end; a pocket having an inlet and a body for containing a paste entering said inlet; and a mounting mechanism at said injection end for connecting said inlet of said pocket to said injection end so that said body of said pocket can contain said paste through said syringe; characterized in that the device further comprises a leaking mechanism provided at said injection end of said syringe for allowing liquid contained in the paste inside said pocket to be expelled from said pocket under pressure, and an opening mechanism which can be operated at a location away from said injection end to open said body of said pocket.  
         [0009]     Preferably, said syringe comprises a tube and an injector plug movably received in said tube, wherein said tube is provided with one or more longitudinal grooves on an outside surface thereof at an injection end thereof; and said pocket comprises a neck defining said inlet, wherein said injection end of said tube is inserted into the inlet of said pocket and said neck of said pocket is fastened to the outside surface of the tube by said mounting mechanism, so that said one or more longitudinal grooves form one or more gaps between the neck of said pocket and the outside surface of said tube of said syringe, and said leaking mechanism comprises said one or more gaps. Alternatively, said one or more longitudinal grooves are provided on said injector plug or on an inside surface of said tube at an injection end thereof, so that said one or more longitudinal grooves form one or more gaps between the injector plug and the inside surface of said tube of said syringe, when the injector plug reaches said injection end of said tube, and said leaking mechanism comprises said one or more gaps. Preferably, the device of the present invention further comprises a hole provided on said tube, said hole being adapted to be connected to a vacuum facility, so that said liquid expelled from said pocket can be suck out via said hole when said vacuum facility is driven.  
         [0010]     Preferably, said pocket is made of an elastic polymeric material, so that the body of said pocket can be dilated with the paste inside said pocket under pressure; and said opening mechanism comprises an electrically conductive wire of high electric resistance, said wire having two ends being adapted to connect to a positive electrode and a negative electrode of a power supply, respectively, and one or more points between said two ends of said wire being attached to the body of said pocket, so that the dilated pocket body will rupture due to melting or weakening of the elastic polymeric material caused by a heat generated at the attached wire, when the ends of said wire are connected to the positive electrode and the negative electrode of the power supply.  
         [0011]     Preferably, said pocket is made of an elastic polymeric material, and said opening mechanism comprises a first set of wire holders on an outer surface of said syringe, which are spaced apart along a longitudinal direction of said syringe; a second set of wire holders on said outer surface of said syringe, which are spaced apart along said longitudinal direction of said syringe, wherein an imaginary plane formed by said first set of wire holders and said second set of wire holders divides the syringe into halves; and said thin wire which is slidably received said first set of wire holders and said second set of wire holders with a portion thereof passing across said injection end of said syringe, thereby said pocket can be cut to rupture with a sliding movement of said thin wire.  
         [0012]     Preferably, said pocket is made of an elastic polymeric material, and said opening mechanism comprises a thin tube on an outer surface of said syringe along a longitudinal direction of said syringe; and a knife slidably received in said thin tube, said knife having a retractable blade and a rod connected to said retractable blade at one end thereof, so that said retractable blade received in said tube is able to protrude from said injection end of said syringe by pushing the rod, and thus said pocket can be cut to rupture by said retractable blade, and that said protruding retractable blade can be retracted by pulling the rod.  
         [0013]     The present invention solves the aforesaid prior art drawbacks, because the cement paste set within the closed pocket without contacting directly body fluid/blood, and pressure can be applied/developed within the pocket, which will increase largely the strength of the cement, reduces the risk of cement dispersion/disintegration, and also avoid “cement paste leaking”.  
         [0014]     Further, the present invention has an advantage of being easy to keep a powder/liquid ratio of the cement paste accurate by monitoring the pressure build-up within the pocket, that is important to cement properties such as setting time and strength. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]      FIG. 1 , FIGS.  3  to  6  are cross-sectional views of a device for forming a hardened cement in a bone cavity constructed according to a first preferred embodiment of the present invention, and together show a process flow diagram of the method of the present invention.  
         [0016]      FIG. 2   a  is a lateral cross-sectional view of the plug  12  depicted in  FIG. 1 .  
         [0017]      FIG. 2   b  is a lateral cross-sectional view of a tube  11  suitable for use in the device of the present invention, wherein grooves  14  are provided on an inside surface thereof.  
         [0018]      FIG. 2   c  is a lateral cross-sectional view of a tube  11  suitable for use in the device of the present invention, wherein grooves  14  are provided on an outside surface thereof.  
         [0019]     FIGS.  7  is a cross-sectional view of a device for forming a hardened cement in a bone cavity constructed according to a second preferred embodiment of the present invention, and together show a process flow diagram of the method of the present invention.  
         [0020]      FIG. 8  is a cross-sectional view of a device for forming a hardened cement in a bone cavity constructed according to a third preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]     A device for forming a hardened cement in a bone cavity constructed according to a first preferred embodiment of the present invention is shown in FIGS.  1  to  6 , which will elaborated as follows:  
         [0000]     A. System Description  
         [0022]     The bone cement delivery tool described below consists of the following major components, as shown in  FIG. 1 : 
        1) A reservoir  100  which can contain the cement paste to be delivered into the bone cavity;     2) An injector plug  12  which can be used to push the cement paste through a slender cylindrical tube  11 ;     3) The cylindrical tube  11  which serves as the conduit for cement injection. The tube  11  and the injector plug  12  form a syringe  10 . The tube can be drilled with a side hole  13  to access to a vacuum facility;     4) A rubber-type balloon  30  attached to the proximal end (injection end) of the tube  11 . This balloon  30  can hold the cement paste against the wall of the bone cavity;     5) An electric wire  50  of high electric resistance. The two points  51  at the middle section of this electric wire is embedded in the balloon  50  whereas the two ends are connected to a power supply  200  having a switch.        
 
         [0028]     The rubber-type balloon  30  should have appropriate elastic property which allows the balloon expand 3-5 times during volume dilation. As cement paste enters inside the balloon  30 , the balloon can hold the cement non-permeably while fitting tightly against the bone cavity wall. The balloon thickness should be selected with sufficient strength when expanded to its intended dilated volume. This dilated balloon should develop appropriate tension to facilitate its rupture and shrinkage as described below. However, the developed tension should not be too large so as to avoid premature balloon rupture. The balloon  30  can be manufactured using those techniques available in the design of intra-vascular balloon catheter devices, for instance, the technique of solution casting of polyurethane or other polymers.  
         [0029]     On the outer surface of the balloon  30 , there is the naked high-resistance electric wire  50  attached. The length of between the points  51  of the wire  50  should be calculated using the maximally dilated balloon configuration. More than two attachment points can be used to fix the wire  50  onto the balloon  30 . The balloon  30  can be formed by casting and curing a liquid polyurethane on a mold. Two or more than two points of the electric wire  50  can be embedded in the polyurethane before the solidification of the polyurethane, or glued to the surface of the balloon by an adhesive after the solidification of the polyurethane.  
         [0030]     As the power supply  200  is turned on, the temperature of the electric wire  50  will be developed and heat released to melt the balloon material in contact with the electric wire  50 . The dilated balloon  30  will soon be ruptured and shrunk back to its original zero-stress state. This rupturing constitutes automatically an extraction function of the balloon  30 . The higher the tension in the balloon, the more effective the rupture and back extraction of the balloon. However, care must be exercised to avoid excessive tension developed which may promote undesired premature balloon rupture caused by contacting with the rough surface of the bone cavity during its cement delivery period.  
         [0031]     The pressurization of the delivered cement has two major functions that characterize the present method. The first is the function of expanding the collapsed bone structure to some desired shape and size. Through the fluid motion of the cement paste, pressure can be transmitted to make the cement fill tightly within the bone cavity and in the same time push the bone structure restoring back to its original shape and size. Secondly, during the pressurization of the cement, water content of the cement can be squeezed out of the balloon  30  via a groove system made on the wall of the injector plug  12  or on the cylindrical tube  11  as shown in  FIGS. 2   a ,  2   b  and  2   c , wherein longitudinal grooves  14  are formed on the injector rod  12 , inside surface of the cylindrical tube  11 , and outside surface of the cylindrical tube  11 , respectively. For the design in  FIGS. 2   a  and  2   b , water can be forced out using the tube as a drainage duct, wherein a vacuum facility can be connected to the hole  13  to suck out the water. For the design depicted in  FIG. 2   c , however, water can be drained along the grooves  14  carved on the outer surface of the tube  11  and absorbed by the human body. Pressurization and water extraction will help the solidification of the cement, which is critical for the structure and strength development as the cement is dried within the balloon  30 .  
         [0000]     B. Delivery and Formation of Cement  
         [0032]     1) Before delivering an appropriate amount of cement paste into a bone cavity using the long slender cylindrical tube  11  and an injector plug  12 , the balloon  30  with the attached electric wire  50  is connected to the injection end of the cylindrical tube  11  by a mounting mechanism  20  having an annular groove  21  provided on an outer surface of the tube  11 , and a ring  22  adapted to elastically grip the annular groove  21 . The injection end of the cylindrical tube  11  is inserted into an opening of the balloon  30 , so that a neck  31  of the balloon  30  covers the annular groove  21 ; and putting the ring  22 , which is a closed ring or a C-shaped ring, on the neck  31  of the balloon  30  and clamping it at the annular groove  21  on the cylindrical tube  11 , as shown in  FIG. 3 ;  
         [0033]     2) Expand the balloon  30  by pushing the injector plug  12  to compress the cement paste with a pre-calibrated pressure until the damaged bone is expanded to the desired shape and size as shown in  FIG. 4 , wherein liquid expelled from the cement paste via the grooves  14  is suck out using the tube  11  as a drainage duct by connecting a vacuum facility to the hole  13 ;  
         [0034]     3) Pressurize the cement paste for a period of time until the cement is dried and hardened in the balloon  30 ;  
         [0035]     4) Electrify the high-resistance electric wire  50  to rupture the balloon  30 , as shown in  FIG. 5 ;  
         [0036]     5) Extract the ruptured balloon  30  while holding the injector plug against the cavity outlet until the balloon  30  clears the exit; and  
         [0037]     6) Withdraw the whole delivery system out of the patient body as shown in  FIG. 6 .  
         [0038]     A device for forming a hardened cement in a bone cavity constructed according to a second preferred embodiment of the present invention is shown in  FIG. 7 , in which parts having a similar function to parts shown in  FIG. 1  have been given similar reference numerals. The device contains a syringe  10  having a substantially cylindrical tube  11  and a plug identical to the one used in the first embodiment (not shown in  FIG. 8 ) slidably received in the tube  11 ; and a mounting mechanism  20  having an annular groove  21  provided on an outer surface of the tube  11 , and a ring  22  adapted to elastically grip the annular groove  21 . A balloon  30  is mounted to the injection end of the syringe with the ring  22 .  
         [0039]     The device further contains a first set of wire holders  40  on the outer surface of the cylindrical tube  11  and along the longitudinal direction thereof; and a second set of wire holders  41  on the outer surface of the cylindrical tube  11 , which are symmetrical to the first set of wire holders  40 ; and a thin wire  50 ′ slidably received in the first and second sets of wire holders  40  and  41 . The wire holders  40  and  41  are blocks each having a through hole, through which the thin wire  50 ′ is passed and guided longitudinally. Preferably, the device further has two tension-adjustable rollers  42  and  43 , on which the ends of the thin wire  50  are wound, so that the thin wire  50 ′ is maintained in the first set and second set of wire holders  40  and  41  under a controlled tension. The device of the present invention is now ready to be used. The injection end of the syringe  10  is inserted into a bone cavity through an incision cut and a hole drilled by the operator. A cement paste, preferably a CPC paste giving a setting time less than 20 minutes, more preferably less than  10  minutes, is injected into the balloon  30  by pushing the plug in the tube  11  toward the injection end of the syringe, so that the balloon  30  is inflated and the portion of the thin wire  50 ′ passing across the injection end is pushed, and thus the thin wire  50 ′ is un-wound from one or both of the rollers  42  and  43  until all the CPC paste is injected into the balloon  30 . The CPC paste in the balloon  30  is maintained under the pressure exerted by the plug while setting with a reduced liquid/solid ratio due to leakage of liquid via meshes of the fiber cloth of the balloon  30 , and preferably the pressure is about 1-5000 psi, and more preferably 10-1000 psi. The thin wire  50 ′ is pulled forward and backward alternatively at its ends under tension to cut the balloon  30  after the CPC paste is hardened in the balloon  30 . One end of the thin wire  50 ′ is released from the roller  42  by continuously pulling the thin wire  50 ′ with the roller  43 , after the balloon  30  is cut open. Finally the hardened CPC is left in the bone cavity by retreating the device together with the opened balloon  30  from the patient.  
         [0040]     The cutting of the balloon  30  can be carried out by a different cutting structure.  FIG. 8  shows a modified device of the present invention based on the design shown in  FIG. 7A , wherein like elements or parts are represented by like numerals. A thin tube  60  is provided on the outer surface and along a longitudinal direction of the cylindrical tube  11  of the syringe  10 . A knife  70  having a rod  71  and a retractable blade  72  is slidably received in the thin tube  60  by inserting the rod  71  into the thin tube  60  from the end near the injection end of the syringe  10  until the retractable blade  72  enters the thin tube  60 . The retractable blade  72  is preferably made of metal and is elastic, so that it resumes its shape after being pushed to protrude from the thin tube  60 . The operator can grip the rod  71  from the other end of the thin tube  60  to push the retractable blade  72  to protrude from the thin tube  60 , cut the balloon  30  with the retractable blade  72 , and retract it once again.  
         [0041]     It is apparent that the cutting structure shown in  FIG. 8  can be incorporated to the device shown in  FIG. 7  to assure a successful cutting of the balloon.  
         [0042]     Although the present invention has been described with reference to specific details of certain embodiments thereof, it is not intended that such details should be regarded as limitations upon the scope of the invention except as and to the extent that they are included in the accompanying claims. Many modifications and variations are possible in light of the above disclosure.