one aspect of the present invention relates to injectable compositions that cure in situ and are used in treating and repairing skeletal defects , such as through pvp and / or pkp procedures , to repair damaged vertebrae . other applications for a composition according to the invention include the filling of bone - voids and repair of intervertebral discs . one preferred composition according to the invention comprises two flowable components including : ( 1 ) one or more polysiloxanes and one or more catalysts ( previously defined as “ component a ”); and ( 2 ) one or more polysiloxanes and one or more cross - linking agents ( previously defined as “ component b ”). the polysiloxanes may be reinforced with an additive such as amorphous silica . the catalyst is most preferably platinum and the cross - linking agent is preferably trimethyl methyl - hydrodimethyl siloxane , although any suitable catalyst ( s ) or cross - linking agent ( s ) may be used . the preferred embodiment utilizes a platinum catalyst , which yields benefits over other catalysts , such as tin . the platinum catalyst ( 1 ) enables the composition to have a longer shelf life , ( 2 ) is less toxic in the body than some other catalysts ( such as tin ), and ( 3 ) results in the final composition being less brittle . in preferred compositions , component a and component b are mixed in equal parts ( i . e ., a 1 : 1 weight ratio ), although any suitable ratio may be used ) to form a flowable , viscous composition that is then injected into the body where it cures into a silicone elastomer . the compositions of the present invention are fully cured in situ ( after injection into a damaged vertebra , other bone or body structure ) in about 3 to 15 minutes , and more preferably in about 3 to 8 minutes , most preferably in about 3 minutes , at body temperature . the viscosity of a composition immediately after components a and b are mixed is preferably about 100 , 000 cps , but can vary within a preferred range of about 100 , 000 cps to 180 , 000 cps , although the composition may have any suitable viscosity that enables the composition to be injected into the body and sufficiently fill voids . the compositions of the present invention optionally comprise a compound to provide radiopacity . the radiopaque additive may be added to either or both components , either prior to or as they are mixed to form the composition that is injected into a vertebral body or other skeletal body . preferably , the radiopaque additive comprises between about 10 % and 40 % by weight , and most preferably about 15 % by weight ( also referred to as “ w / w ”), of the entire composition after mixing , although any suitable amount may be used and can vary depending upon the type of additive and the application for which the composition will be used . suitable radiopaque materials include those selected from one or more of the group consisting of silver powder , barium sulfate , bismuth trioxide , zirconium dioxide , tantalum or titanium powders or fibers , calcium sulfate , calcium phosphate , hydroxyapatite , tri - calcium phosphate , and other medically appropriate opacifier additives . in preferred embodiments , barium sulfate is added at about 15 % by weight to both components a and b to provide the desired radiopacity . the addition of a radiopaque material to the compositions allows the injection of the mixed composition into the disc to be monitored with standard fluoroscopic visualization techniques . one advantage of materials of the present invention is that because the compositions are naturally radiopaque the percentage of radiopaque additive needed to provide the desired degree of radiopacity is less ( often about half ) than that required in pmma based bone fillers . the catalyst for initiating cross linking and curing is preferably platinum metal in a weight percent of the total composition of between 0 . 0005 % and 0 . 010 %, and most preferably greater than 0 . 001 %, although any suitable weight percentage will suffice as long as the resulting composition yields one or more of the benefits set forth herein . the concentration of catalyst may also be expressed in units of parts per million ( ppm ). for example , in one embodiment , the concentration of the platinum catalyst is preferably 15 ppm in component a prior to mixing and subsequently 7 . 5 ppm in the mixed composition . in alternate embodiments , other catalysts may be used , such as tin . a preferred composition for the composition according to the invention ( example 1 ) is formed by mixing together the following component a and component b . component a : from 50 % to 75 % vinyldimethyl terminated dimethylpolysiloxane / trimethylsiloxy terminated polydimethyl siloxane ( in preferably a 1 : 1 ratio by weight ), from 10 % to 30 % amorphous silica , 10 % to 40 % radiopaque additive , and greater than 0 . 001 % pt catalyst . [ note : all percentages are weight %.] component b : from 50 % to 75 % vinyldimethyl terminated dimethylpolysiloxane / trimethylsiloxy terminated polydimethyl siloxane ( in preferably a 1 : 1 ratio by weight ), from 10 % to 30 % amorphous silica , 10 % to 40 % radiopaque additive , and from 0 . 5 % to 5 % trimethyl methyl - hydro dimethyl siloxane ( cross linking agent ). [ note : all percentages are weight %.] component a and component b may be intermixed in ratios depending on the desired consistency of the cured composition , for instance , component a and component b may be intermixed in the ratios of 0 . 1 : 1 to 1 : 0 . 1 by weight . another preferred embodiment of a composition ( example 2 ) is formulated with the following ranges of components : component a : 64 % vinyldimethyl terminated dimethylpolysiloxane / trimethylsiloxy terminated polydimethyl siloxane ( in preferably a 1 : 1 ratio by weight ), 21 % amorphous silica , 15 % barium sulfate powder , and greater than 0 . 001 % pt catalyst . [ note : all percentages are weight %.] component b : 63 % vinyldimethy terminated dimethylpolysiloxane / trimethylsiloxy terminated polydimethyl siloxane ( in preferably a 1 : 1 ratio by weight ), 21 % amorphous silica , 15 % barium sulfate powder 1 % trimethyl methyl - hydro dimethyl siloxane ( cross linking agent ). [ note : all percentages are weight %.] component a and component b may be intermixed in ratios depending on the desired consistency of the cured composition , for instance , component a and component b may be intermixed in the ratio of about 1 : 1 by weight . each of the above preferred compositions is a silicone elastomer composition , and after curing has about 60 - 85 % by weight poly dimethly siloxane , about 0 . 5 %- 5 % cross linking agent , about 10 to 20 % dilutent , and about 10 to 20 % radiopaque material , all based on the weight of the entire composition . compositions preferably have a working time ( i . e ., from the mixing of component a and component b to the time the composition completely cures ) of between 2 - 20 minutes , and preferably between 3 - 15 minutes , more preferably between 3 and 8 minutes and most preferably about 3 minutes . the hardness ( durometer ) of a composition of the present invention can be altered depending upon the application for which it will be used . factors such as the density and hardness of the bone of the affected area where the composition is to be applied may be taken into consideration when determining the durometer . typical compositions of the present invention have a shore a durometer value in the range of 15 to 90 , or in the range of 20 to 70 , or in the range of 20 to 60 . a composition having a durometer value below 15 shore a does not have the desired properties for repair of vertebrae or bones , but may be used for intervertebral discs . disc augmentation may be performed using the composition and system of the present invention when the composition is of an appropriate durometer in the range of 15 to 90 . embodiments of compositions of the present invention have been characterized by the following tests . table 1 summarizes biocompatibility evaluation data for compositions of the present invention formed using component a and component b as described above . testing was carried out using standard procedures known to those skilled in the relevant art . the gel time for a preferred composition according to example 2 , above , was measured over a time period of 1 - 5 minutes at a range of room temperatures between 15 . 6 ° c ./ 60 ° f . and 35 ° c ./ 95 ° f . as a point of comparison , gel time for a silicone elastomer corresponds to the dough time for a pmma polymer . the setting time for the composition was measured over a time period of 5 - 15 minutes at temperatures between 60 ° f . and 85 ° f ., and at approximately 3 minutes at 95 ° f . ( in order to provide a conservative estimate of setting time at body temperature ). the results are summarized in the following table : in this test according to section 7 . 8 . 5 of astm f451 - 99 , a composition formed of component a and component b was injected into a cylindrical mold with four 1 mm - diameter holes in the bottom , and allowed to cure under a 1 lb . load . the composition viscosity was about 168 , 000 cps when injected . after curing , the material was removed from the mold and the sample was evaluated to see how well the material flowed into the holes at the bottom of the mold . this testing demonstrated that the composition completely filled in the holes , indicating excellent intrusion properties ( and thus ability to fill voids in vertebrae or other body structures ). the weight average molecular weight of the base polymer in component a and component b , vinyldimethyl terminated dimethyl polysiloxane / trimethylsiloxy terminated polydimethyl siloxane ( in a 1 : 1 ratio in this preferred embodiment ), was determined to be about 71 , 000 and the number average molecular weight about 29 , 000 . the weight average molecular weight of the cross linking agent in component b , trimethyl methyl - hydro dimethyl siloxane , was determined to be about 10 , 569 and the number average molecular weight about 5 , 043 . the temperature of two samples of the preferred composition formed from component a and component b at room temperature ( 23 . 9 ° c ./ 75 ° f .) were monitored throughout the curing process . the maximum temperature reached was 26 . 2 ° c ./ 79 . 2 , and the maximum total temperature increase was 2 . 0 ° c ./ 3 . 6 ° f . the results are summarized in the table below . one beneficial performance characteristic for a composition used in pvp or pkp is the ability to sustain compressive loading . in mechanical testing and / or cadaver testing an embodiment of the composition described herein demonstrated compressive load carrying properties that are equivalent to bone . the force needed in axial loading to cause deflection was also similar to bone . testing also showed no significant deterioration of the mechanical properties of the composition under accelerated aging conditions . another aspect of the invention relates to devices for mixing and dispensing a composition according to the invention for use in medical procedures . once the mixing process has been completed in the helical element the composition continues to flow through a nozzle ( or mixing element ) and line where it is immediately injected into the affected vertebral body or other area being treated using standard percutaneous techniques known to those skilled in the art . a mixing and dispensing device ( 10 ) according to one embodiment of the present invention is shown in fig1 . the pre - filled dispensing system ( 10 ) comprises a pre - filled cartridge ( 12 ) containing , in the illustrated embodiment , two side - by - side , pre - filled chambers ( 14 ) and ( 16 ), one of which ( 14 ) contains component a comprising a catalyst and the other of which ( 16 ) contains component b comprising a cross - linking agent . the total volume of material in each pre - filled cartridge is preferably between about 20 and 70 ml , and more preferably between about 35 to 40 ml . the respective mixing and dispensing cartridges keeps component a and component b separate until time of use , at which time the contents of the chamber containing component a and the contents of the chamber containing component b are delivered into the mixing element ( 20 ). in the illustrated embodiment , the two respective chambers are of equal volume and a plunger ( not shown ) is used to apply equal pressure to each chamber through application of a squeezing force preferably applied by the user &# 39 ; s hand to the dispenser trigger / handle assembly ( 18 ) at the bottom of dispensing system ( 10 ). in this embodiment , as pressure is applied equal amounts of component a and component b are pushed into and through the mixing element ( 20 ) and mixed together to form a liquid composition for injection into a treatment space such as a volume defined within a bony structure ( such as the interior of a vertebra ) in the body . fig2 shows a close - up of a static mixing element ( 20 ) of a dispensing system according to the invention . although this preferred embodiment shows a static , helical mixing element ( 20 ), any suitable mixing element may be used . the static mixing element ( 20 ) has a helical geometry ( 22 ) disposed in a cavity inside the element that mixes component a ( including the catalyst ) and component b ( including the cross linking agent ) in the desired ratio to form a composition , which is then immediately injected at the time of pvp , pkp or other procedure into the body using the same pressure that pushed component a and component b out of their respective chambers and through the mixing element ( 20 ). the connecting end of the static mixing element ( 20 ) is attached to the output end of pre - filled cartridge ( 12 ) by rotatably engaging a locking bayonet fitting ( 24 ). the dispensing system ( 10 ) and mixing element ( 20 ) ensure that components a and b are mixed to the proper injection consistency before being dispensed from the dispensing system ( 10 ) and into the body . thus , by using a pre - filled cartridge that communicates with a mixing element , and by pushing predetermined amounts of component a and component b through the mixing element , a relatively uniform and constant composition is achieved . fig3 shows a device application configuration ( 30 ) according to one embodiment of the present invention . as described above in relation to fig1 , squeezing pressure applied to the dispenser trigger / handle assembly ( 18 ) advances material from the pre - filled cartridge ( 12 ) in a desired ratio ( such as 1 : 1 ), into the static mixing element ( 20 ). attached to the end of the static mix element ( 20 ) by luer - lock adapters ( 36 ) is extension tubing ( 40 ). the other end of the extension tubing is attached via a luer - lock adapter ( 36 ) to an appropriately sized trocar ( 34 ) that is inserted into a vertebral body such as the model vertebra ( 32 ). after combining the two components in the desired ratio through the static mix element ( 20 ), the composition is immediately dispensed into the extension tubing ( 40 ) and injected into the vertebral body ( 32 ) via trocar ( 34 ) to fill voids and repair defects . once injected the composition polymerizes ( i . e ., fully cures ) in situ . the positioning of the trocar ( 34 ) in the vertebral body and filing of vertebral body with material can be monitored using fluoroscopic or other radiological guidance techniques . the luer - lock adapters ( 36 ) are preferable high - flow , wide inner - diameter , low pressure fittings that provide improved flow of the material with reduced back pressure on the system to lessen the squeezing pressure exerted by the user on the dispenser trigger / handle assembly ( 18 ). a mixing and dispensing device of the invention is preferably formed of any suitable materials that can be sterilized and / or sanitized . for example , the cartridge , pistons , plunger , and static mix element can be formed of a polymer such as polypropylene , while the o - ring of the pistons may be formed of viton . the dispenser can be made of a material such as glass - filled nylon . fig4 shows a pre - filled cartridge ( 12 ) in sterile packing ( 46 ). the preferred packaging for the pre - filled cartridge ( 12 ) is a double peel pouch of clear mylar ® ( mylar is a registered trademark of dupont for its polyester film material ) with a tyvek ® ( tyvek is a registered trademark of dupont for its spunbonded polyolefin material ) breather . other suitable materials from other manufacturers may also be used . fig5 shows a delivery system packaging ( 50 ) comprising a double blister package of clear petg ( polyethylene terepthalate glycol ) with tyvek lid ( 58 ) and containing dispenser trigger / handle assembly ( 18 ) for the dispenser , static mix elements ( 20 ) and extension tubing ( 40 ). the pre - filled cartridge dispensing systems of the present invention are preferably provided as single use kits . fig6 shows a device kit ( 60 ) according to one aspect of the present invention . the sterile device kit ( 60 ) comprises three components in ( preferably ) sterile packaging : ( 1 ) a pre - filled cartridge ( 12 ) containing a partitioned component a chamber and a component b chamber ; ( 2 ) delivery system package ( 50 ) comprised of a dispenser trigger / handle assembly ( 18 ), one or more static mix elements ( 20 ), one or more extension tube assemblies ( 40 ), and preferably , eight extension tube assemblies ( 40 ); and ( 3 ) one or more trocars ( 54 ). the components of the kit are packaged in materials that are capable of withstanding the stress associated with distribution , shipping and sterilization , and are capable of maintaining a sterile barrier for a shelf life of at least two years . in one embodiment , each pre - filled cartridge of the dispensing system contains two chambers , which may be cylindrical in shape and are preferably located side by side . one cylinder is filled with component a and the other with component b . after the cartridge is filled and sealed , each pre - filled cartridge and its contents ( component a and component b , respectively ) are sterilized via a dry heat process at 121 ° c . for 12 hours . the degree of sterilization is performed preferably to obtain at least a 10 − 6 sterility assurance level (“ sal ”). the sterilized cartridges are then packaged as shown in fig4 and assembled into dispensing device kit ( 60 ) according to the invention . in the illustrated embodiments , each cartridge body and the outside surface of the inner cylinders may be sterilized via ethylene oxide while the contents of the cylinders may be sterilized via a dry heat process . the sterilized , sealed cartridge is then placed into a kit ( 60 ) with the dispensing handle / trigger assembly ( 18 ) and mix nozzle ( 20 ) and other components . the external surfaces of the delivery system packaging , cartridge , dispenser handle , static mix elements , trocars , and other components of the device kit may be sterilized with an ethylene oxide sterilization process ( eto ) and then the kit is assembled in the packaging described above . the closed system of the kits provides for virtually eliminating the risk of changing the composition prior to injection . fig7 shows the front perspective view of a portion ( 70 ) of a human spinal column . the spinal column portion ( 70 ) comprises a series of vertebrae ( 71 ) and ( 72 ) between which is disposed an intervertebral disc ( 73 ). intervertebral discs generally function to maintain the proper spacing and alignment between vertebrae while also absorbing and distributing loads imposed upon the spinal column . bone structures ( 73 ) and ( 74 ) protrude from the vertebrae forming a canal ( 75 ) that provides a protective structure around spinal cord ( 76 ). fig8 ( a ) and ( b ) illustrate respective perspective and axial views of portions of a vertebra undergoing an orthopedic procedure in accordance with embodiments of the present invention . such orthopedic procedures may be implemented to stabilize a vertebra having , for example , vertebral fractures ( 81 ). in fig8 ( a ), a needle or cannula ( 80 ) such as may be used in association with the trocar ( 54 ) mentioned above , is inserted into the damaged vertebra ( 71 ). a composition consistent with embodiments of the present invention is injected into the vertebral cavity ( 81 ) using a dispensing device coupled to the needle or cannula ( 80 ). as described above in regards to fig3 , such coupling mechanism may include extension tubing ( 40 ). once injected , the composition flows to fill the cavity ( 81 ) as shown in fig8 ( b ). the dispensed composition then cures in situ to form a solid but flexible material ( 82 ), which stabilizes fractures or other damage to the vertebra . while particular embodiments of the present invention have been described , those skilled in the art will recognize changes and modifications that can be made without departing from the spirit and scope of the teachings of this invention . the specification and examples are thus exemplary only , with the true scope and spirit of the invention being set forth in the following claims and legal equivalents thereof .
compositions for use in treating and repairing orthopedic defects are provided . the compositions are especially useful for performing percutaneous vertebroplasty , percutaneous kyphoplasty and other procedures . when cured , the compositions are relatively solid but compressible , and prior to curing are viscous liquids with a low enough viscosity to fill voids in cavities such as voids within fractured vertebrae . optionally , the properties of a composition according to the invention can be adjusted taking into account the hardness and density of the orthopedic structure to be repaired . also disclosed are devices and methods for using the compositions , and a kit containing components for forming and utilizing the composition .