Abstract:
The present invention discloses a method of hardening a bone cement under an exerted pressure in a bone cavity including preparing a cement paste; injecting said cement paste into a pocket placed in a bone cavity; and allowing the cement paste to harden in the pocket. Preferably, the pocket is opened after the cement paste is set, and the opened pocket is removed from the bone cavity.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a continuation-in-part application of U.S. patent application Ser. No. 10/393,044, filed Mar. 21, 2003. The above-listed application is commonly assigned with the present invention and the entire contents of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     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 
     A 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. 
     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 
     A primary objective of the present invention is to provide a method for forming a hardened cement in a bone cavity, which are free of the aforesaid prior art drawbacks. 
     The method disclosed in the present invention comprises the following steps: 
     a) preparing a cement paste from a powder and a liquid, so that said cement paste is injectable through a syringe; 
     b) disposing a pocket in a bone cavity; said pocket is made from a material penetrable to said liquid but substantially impenetrable to the powder of said cement paste; 
     c) injecting said cement paste into said pocket; 
     d) applying a pressure unto said cement paste before said cement paste is substantially hardened, causing a portion of said liquid to be squeezed out of said pocket, so that the powder/liquid ratio of said cement paste in said pocket is increased; and 
     e) allowing said cement paste to harden in said pocket. 
     Preferably, said method further comprises: 
     f) opening said pocket; and 
     g) separating the resulting opened pocket from the hardened cement. 
     More preferably, said injecting in step c) and said opening in step f) is carried out with a means which is able to be operated outside said bone cavity, and the resulting opened pocket is attached to said means. Further, said separating in step g) is preferably carried out by removing the resulting opened pocket from said bone cavity with the hardened cement remaining in said bone cavity. 
     Preferably, said pocket is made from a fiber cloth or a polymer foil. More preferably, and said fiber cloth is biodegradable, and said method further comprises leaving the hardened cement resulting from step e) together with the pocket in said bone cavity. 
     Preferably, said pocket is made from a fiber cloth or a polymer foil, wherein said opening in step f) comprises cutting at least a portion of said pocket with a blade or a thin wire. 
     Preferably, wherein said pocket is made from a fiber cloth, wherein said opening in step f) comprises loosening or unthreading at least a portion of said fiber cloth. 
     Preferably, said means comprises a syringe tube having an injection end; a mounting mechanism at said injection end for mounting said pocket to said injection end; 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. More preferably, said means further comprises a thin tube on said outer surface of said syringe along said longitudinal direction of said syringe; and said blade slidably received in said thin tube, said blade 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 by said retractable blade, and that said protruding retractable blade can be retracted by pulling the rod. 
     Alternatively, said means comprises a syringe having an injection end; a mounting mechanism at said injection end for mounting said pocket to said injection end; a thin tube on an outer surface of said syringe along a longitudinal direction of said syringe; and said blade slidably received in said thin tube, said blade 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 by said retractable blade, and that said protruding retractable blade can be retracted by pulling the rod. 
     Preferably, said mounting mechanism of said means comprises an annular groove formed on said outer surface of said syringe and an elastic ring having a shape and size corresponding to those of said annular groove, so that a neck of said pocket can be clamped by said elastic ring received in said annular groove of said syringe after said injection end of said syringe being inserted into an opening of said pocket. 
     Preferably, said pressure in step d) is of about 0.1–200 MPa. More preferably, said pressure is of about 0.5–50 MPa. Said pressure is measured in-situ with a pressure sensor. 
     Preferably, said cement paste comprises a calcium phosphate-based cement, a calcium sulfate-based cement, or a bioactive glass-based cement. 
     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”. 
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 1E  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. 
         FIG. 2  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. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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. 1A and 1B . The device contains a syringe  10  having a substantially cylindrical tube  11  and a plug  12  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 near an injection end of the syringe, and a ring  22  adapted to elastically grip the annular groove  21 . A pocket  30 , preferably an inflatable and expandable or non-expandable pocket made of a fiber cloth is used, is mounted to the injection end of the syringe by inserting the injection end into an opening of the pocket  30 , so that a neck  31  of the pocket  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 pocket  30  and clamping it at the annular groove  21  on the cylindrical tube  11 . 
     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. As shown in  FIG. 1B , 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 pocket  30  by pushing the plug  12  in the tube  11  toward the injection end of the syringe, so that the pocket  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 pocket  30 . The CPC paste in the pocket  30  is maintained under the pressure exerted by the plug  12  while setting, 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 pocket  30  after the CPC paste is hardened in the pocket  30 , as shown in  FIGS. 1C–1D . 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 pocket  30  is cut open, as shown in  FIG. 1D . Finally the hardened CPC is left in the bone cavity by retreating the device together with the opened pocket  30  from the patient, as shown in  FIG. 1E . 
     The cutting of the pocket  30  can be carried out by a different cutting structure.  FIG. 2  shows a modified device of the present invention based on the design shown in  FIGS. 1A–1E , 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 blade  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 pocket  30  with the retractable blade  72 , and retract it once again. 
     It is apparent that the cutting structure shown in  FIG. 2  can be incorporated to the device shown in  FIGS. 1A–1E  to assure a successful cutting of the pocket. 
     PREPARATIVE EXAMPLE 
     Preparation of Non-Dispersive TTCP/DCPA-Based CPC Powder (Abbreviated as ND-CPC) 
     A Ca 4 (PO 4 ) 2 O (TTCP) powder was prepared by mixing Ca 2 P 2 O 7  powder with CaCO 3  powder uniformly in ethanol for 24 hours followed by heating to dry. The mixing ratio of Ca 2 P 2 O 7  powder to CaCO 3  powder was 1:1.27 (weight ratio) and the powder mixture was heated to 1400° C. to allow two powders to react to form TTCP. 
     The TTCP powder prepared was sieved and blended with dried CaHPO 4  (DCPA) powder in a ball mill for 12 hours. The blending ratio of the TTCP powder to the DCPA powder was 1:1 (molar ratio). The resultant powder mixture was added to a 25 mM diluted solution of phosphate to obtain a powder/solution mixture having a concentration of 3 g powder mixture per 1 ml solution while stirring. The resulting powder/solution mixture was formed into pellets, and the pellets were heated in an oven at 50° C. for 10 minutes. The pellets were then uniformly ground in a mechanical mill for 20 minutes to obtain the non-dispersive TTCP/DCPA-based CPC powder (ND-CPC). The particles of this ND-CPC powder have whisker on the surfaces thereof. 
     EXAMPLE 
     To a setting solution of 1M phosphoric acid solution (pH=5.89) the ND-CPC powder from PREPARATIVE EXAMPLE was added in a liquid/powder ratio (L/P ratio) of 0.4, i.e. 4 ml liquid/10 g powder, while stirring. The resulting paste was filled into a cylindrical steel mold having a length of 12 mm and a diameter of 6 mm, and was compressed with a gradually increased pressure until a maximum pressure was reached. The maximum pressure was maintained for one minute, and then the compressed CPC block was removed from the mold. At the 15 th  minute following the mixing of the liquid and powder, the compressed CPC block was immersed in a Hanks&#39; solution for 1 day. 
     The CPC paste in the mold was compressed with a maximum pressure listed in Table 1. In the course of the compression the compression speeds were about 5 mm/min during 0˜104.1 MPa; 3 mm/min during 104.1˜138.8 MPa; 1 mm/min during 138.8˜159.6 MPa: and 0.5 mm/min during 159.6˜166.6 MPa. The liquid leaked from the mold during compression was measured, and the liquid/powder ratio was re-calculated as shown in Table 1. 
     Each test group has five specimens, the compressive strength of which was measured by using a AGS-500D mechanical tester (Shimadzu Co., Ltd., Kyoto, Japan) immediately following the removal thereof from the Hanks&#39; solution without drying. The measured wet specimen compressive strength is listed Table 1. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Pressure for 
                   
                   
                   
               
               
                 compressing the 
                 L/P ratio (after a 
               
               
                 CPC paste in 
                 portion of liquid 
                 Compressive 
                 Standard 
               
               
                 mold (MPa) 
                 removed) 
                 strength (MPa) 
                 deviation (MPa) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1.4 
                 0.25 
                 26.4 
                 1.4 
               
               
                 34.7 
                 0.185 
                 75.3 
                 3.9 
               
               
                 69.4 
                 0.172 
                 100.4 
                 6.8 
               
               
                 156.2 
                 0.161 
                 138.0 
                 8.2 
               
               
                 166.6 
                 0.141 
                 149.2 
                 12.9 
               
               
                   
               
             
          
         
       
     
     The data in Table 1 show that the compressive strength of the CPC block increases as the liquid/powder ratio decreases during molding. 
     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.