Composite bone constructs and methods

Abstract

Embodiments of the present technology include a graft for administration at a treatment site of a patient. The graft may include a human cadaveric bone material bonded together with a polymeric binder. The human cadaveric bone material may include demineralized bone particles. The demineralized bone particles may have an average diameter less than 1.1 mm, less than 750 μm, less than 500 μm, or less than 250 μm. The human cadaveric bone material may include non-demineralized bone, cancellous bone, and/or cortical bone in embodiments. In some embodiments, bone from animals other than humans may be used, and the patient may be an animal other than a human.

Claims

1. A graft for administration at a treatment site of a patient, the graft consisting of: a human cadaveric bone material; and a plurality of polymethyl methacrylate binder particles, wherein the human cadaveric bone material is immobilized in the plurality of polymethyl methacrylate binder particles, wherein the human cadaveric bone material is present in an amount that is 50 weight percent of the graft, or less, and wherein the graft has a yield strength that is at least 13,430 N/cm.sup.2 and no greater than 15,090 N/cm.sup.2.

2. The graft of claim 1, wherein the human cadaveric bone material comprises demineralized bone particles.

3. The graft of claim 2, wherein the demineralized bone particles have an average diameter less than 1.1 mm.

4. The graft of claim 2, wherein the demineralized bone particles have an average diameter less than 250 μm.

5. The graft of claim 1, wherein the graft has a compressive modulus that is at least 7.5 Gpa and no greater than 8.2 GPa.

6. The graft of claim 1, wherein the graft has a surface area, and at least 50% of the surface area is the human cadaveric bone material.

7. The graft of claim 1, wherein the human cadaveric bone material is present in an amount within a range from 20 weight percent of the graft to 50 weight percent of the graft.

8. The graft of claim 1, wherein the human cadaveric bone material is present in an amount within a range from 20 weight percent of the graft to 40 weight percent of the graft.

9. The graft of claim 1, wherein the human cadaveric bone material is present in an amount that is 30 weight percent of the graft.

10. The graft of claim 1, wherein the human cadaveric bone material comprises demineralized bone material.

11. A graft for administration at a treatment site of a patient, the graft comprising: a human cadaveric bone material; an amount of muscle fibers; and a polymeric binder, wherein the human cadaveric bone material and the amount of muscle fibers are immobilized in the polymeric binder.

12. The graft of claim 1, wherein the graft has a surface area, wherein a percentage of the surface area is the human cadaveric bone material, wherein the human cadaveric bone material is present in an amount that is a weight percent of the graft, wherein the percentage of the surface area that is human cadaveric bone is 55.4 percent and the weight percent is 50 weight percent.

13. A graft for administration at a treatment site of a patient, the graft consisting of: a plurality of human cadaveric bone particles; and a plurality of polymethyl methacrylate binder particles, wherein the plurality of human cadaveric bone particles are immobilized in the plurality of polymethyl methacrylate binder particles.

14. The graft of claim 13, wherein the plurality of human cadaveric bone particles and the plurality of polymethyl methacrylate binder particles are densified.

15. The graft of claim 13, the plurality of polymethyl methacrylate binder particles are present as a densified powder.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a block flow diagram of a method of making a bone composite according to embodiments.

(2) FIG. 2 shows a block flow diagram of a method of making a bone composite according to embodiments.

(3) FIG. 3 shows a system 300 for making bone composites according to embodiments.

(4) FIG. 4 is a graph of load versus displacement for a bone composite according to embodiments.

(5) FIG. 5 is a microscope image of a slice of a bone composite according to embodiments.

DETAILED DESCRIPTION

(6) Embodiments of the present technology may bond together bone particles with a polymeric binder to form a bone graft or composite for implantation in a patient. Embodiments include grafts and methods of making composites that would use otherwise discarded bone particles. Embodiments may involve grafts or composites with consistent properties similar to natural bone. What is more, processing parameters and composition parameters may be adjusted to affect the properties of the graft or composite.

(7) Conventional technologies do not provide the advantages of embodiments of the present technology. Simply mixing polymeric binder and bone particles may not provide a bone graft with consistent and desirable properties. For example, mixing a polymeric binder and bone particle may produce a mixture with properties similar to bread dough. This mixture may have voids and air pockets within the mixture, and the mixture may not harden to form a composite with sufficient mechanical integrity to replace or supplement natural bone. Other conventional technologies may include synthetic composites, which may not have the flexibility of natural bone and may disintegrate under stress that normal bone can withstand. Furthermore, synthetic composites, without natural bone present on its surface, may not facilitate the growth of natural bone on the composite and may not integrate in a patient's body as well as embodiments of the present technology.

(8) Turning to the figures, FIG. 1 shows a block flow diagram of a method 100 of making a bone composite according to embodiments. Method 100 may include adding a liquid with a plurality of monomers to a plurality of human cadaveric bone particles to form a wet mixture 102. Mixing the liquid and bone particles together may form a wet mixture that has a soft, doughy consistency. Method 100 may also include compacting the wet mixture to form a compacted wet mixture 104. Compacting the wet mixture may include packing the wet mixture into a container, such as a centrifuge tube. Compacting the wet mixture may include spincasting or centrifuging the wet mixture, which may compact the particles and/or incorporate monomers in the voids between the particles. The centrifugation forces used can reach 100,000×g.

(9) Method 100 may further include polymerizing the plurality of monomers 106 to form a solidified polymer. Also, method 100 may include immobilizing the plurality of human cadaveric bone particles 108 in the solidified polymer to form the bone composite. Without the polymer to act as a binder, the human cadaveric bone particles may otherwise not be immobilized or stick together. The binder may or may not physically or covalently bond with the bone particles. The bone composite may be any graft described herein. Additionally, the monomer, bone particle, and polymer may be any monomer, bone particle, and polymer described herein.

(10) Turning to FIG. 2, this figure shows a block flow diagram of a method 200 of making a bone composite. Method 200 may include densifying a plurality of human cadaveric bone particles 202. Also, method 200 may include adding a liquid comprising monomers to the plurality of human cadaveric bone particles 204. The method may further include polymerizing the monomers 206 to form a solidified polymer. Additionally, method 200 may include immobilizing the plurality of human cadaveric bone particles 208 in the solidified polymer to form the bone composite. The bone composite may be any graft described herein. Additionally, the monomer, bone particle, and polymer may be any monomer, bone particle, and polymer described herein.

(11) Methods and bone grafts may exclude cross-linking polymers. Cross-linking polymers may often involve cross-linking agents. Cross-linking agents may not be desired in the final bone graft and may be difficult to remove if used to cross link polymers.

(12) Method 200 may differ from method 100 in the order the liquid is added and the particles are densified or compacted. In method 200, the liquid may be added after initially densifying the plurality of human cadaveric bone particles, while in method 100, the liquid may be added before compacting the particles. Method 200 may also include further densification after liquid is added. Method 200 may be controllable and repeatable and may not form an intermediate mixture with too low a density.

(13) Bone grafts and methods of forming bone composites and may involve different ratios of bone and binder. Different ratios may affect the strength and appearance of the final product. Bone may be stronger than binder. A higher ratio of bone may result in a stronger composite, at least initially. The ratio may eventually become too large, with not enough binder to bond the bone material together.

(14) Likewise, different ratios of polymer to monomer may be used in methods of making bone composites. The ratio of the polymer powder, such as PMMA, to liquid monomer, such as MMA, may range from 0:100 to 75:25. The ratio may affect the strength and synthetic content of the final product. Without any polymer particles initially with the bone particles, the liquid monomer may be able to more effectively eliminate space in the bone particles. However, the lack of polymers in the bone particles may reduce the amount of polymerization of the monomer. Thus, decreasing the ratio of polymer to monomer may then decrease the spaces or voids between bone particles but may also decrease the amount of polymerization. This trade off would affect the strength of the final composite. Ratios may also impact setting time, hardening time, and temperature during the process. The polymerization may be exothermic, and hence the degree of polymerization may affect the heat generated in the process.

(15) The setting and hardening times may be affected by the temperature of the materials and the ambient environment. The liquid monomer, the bone particles, and/or the polymer particles may be cooled in order to increase the setting and hardening times.

(16) Turning to FIG. 3, system 300 may be used to make bone composites by method 200. System 300 may include a reservoir 302. Reservoir 302 may be a cylinder or part of a syringe. Reservoir 302 may hold a plurality of particles 304. Particles 304 may include bone particles and optionally polymer particles. A translatable partition 306 may move along the reservoir and against particles 304, resulting in decreased voids or smaller voids between particles 304. Translatable partition 306 may be a plunger of a syringe. Translatable partition 306 may compact or densify particles 304. Translatable partition 306 may define an aperture 308, with one end of aperture 308 open to particles 304. The other end of aperture 308 may be open to a fluid source 310. In this manner, a fluid from fluid source 310 may be delivered to particles 304. Fluid source 310 may include monomers and/or a solvent. Aperture 308 may also be defined by plastic, metal, or other tubing. The end of reservoir 302 opposite aperture 308 may include a channel 312. Channel 312 may be defined by plastic, metal, or other tubing. Channel 312 may be attached to a pump 314. Pump 314 may create a vacuum or negative pressure in reservoir 302, specifically in the area that contains particles 304. This negative pressure may densify particles 304 and/or draw fluid from fluid source 310, through aperture 308, and into voids in particles 304. The size of channel 312 or a filter in reservoir 302 may prevent particles 304 from exiting reservoir 302 or channel 312 and into pump 314.

(17) System 300 and method 200 may be combined with a method similar to method 100. For example, system 300 may be included in a centrifuge. Centrifuging may densify particles 304. Densifying by centrifuging may occur before or during the addition of fluid to particles 304.

(18) FIG. 4 shows a graph of load versus displacement for a bone composite formed by mixing 40% ground bone matrix (GBM) with a polymethyl methacrylate binder. GBM is bone that has not been demineralized. The bone composite was formed with a spincasting method, which included centrifuging bone particles, polymer particles, and liquid monomer. A 1 cm.sup.3 cube is subjected to compression testing to obtain the yield force (N) versus displacement (mm).

(19) Turning to FIG. 5, a picture 500 is shown of the surface of a 0.5 mm thick slice from a composite of 50 weight percent demineralized bone particles and 50 weight percent PMMA binder. Example demineralized bone particles are distinguishable as amorphous structures 502, 504, and 506. FIG. 5 shows over 55% of the surface area as demineralized bone particles. The more surface area that is demineralized bone, the more likely the bone composite may have superior results in the body of a human patient. Exposed bone material in a bone composite may allow for bone growth or may allow for increased compatibility in the body.

(20) Bone Composites Via Spincasting (Spincast Human Bone Composites)

(21) In some embodiments, the purpose of the development is to make a cadaveric human bone based structural graft that incorporates using a synthetic binder using centrifugation to create a strong long lasting bone based graft.

(22) The development takes human cadaveric bone particles of specific sizes and combines with a synthetic binder. The ratio of each of the components is optimized to give the required strength of natural human bone, while minimizing the concentration of binder. The mixed components are then subjected to centrifugation forces to compact the particles with incorporation of the binder in the voids between the particles. The centrifugation forces used can reach 100,000×g.

(23) To make human cadaveric bone particles into strong lasting implantable grafts using centrifugation.

(24) The mode of operation is a structural graft that can withstand forces exerted by the human body. The content of the graft makes the graft mostly natural human bone which in theory can lead to better incorporation of the foreign graft into the host.

(25) Bone particles do not stick together without the aid of a medium. To make a composite of bone that can withstand high levels of pressure and stress, the medium has to be substantially strong.

(26) One such medium is cyano acrylate (Super Glue®) which is currently used as surgical glue in humans that the body dissolves over a time. In this method the medium is used at its lowest possible concentration to yield the strength similar to human bone.

(27) Other similar embodiments may include applying vacuum or chemically modifying the tissue to make it bind together. The method of using centrifugation (spincasting) to make the structure makes the application different.

(28) Human cadaveric bone is harvested and then section cleaned of soft tissue and bodily fluids. It is next ground into particles that are less than 1.1 mm. The particles are then cleaned and demineralized via a proprietary method. The ground bone is then vacuum dried. From here the bone is used as is (Demineralized Bone Matrix-DBM) or is further ground to less the 250 microns to make a powder (Demineralized Bone Powder-DBP). The binder component is currently cyano acrylate (Super Glue®) and could be replaced with a number of other biocompatible synthetic glues.

(29) The components, DBP, DBM, and binder are weighted out and combined then mixed and placed in a centrifuge tube. The tubes are then centrifuged at high rpm to compress the particles and to extrude any excess fluid to the top. The centrifugation is run long enough to allow for the glue to set and then allowed to cure completely. Next the contents are extracted. The solidified plug is then machined via CNC into its final configuration.

(30) There are other glues that can be used as a binder.

(31) Currently initial prototypes of compressed structures have been made with bone and cyano based glue and currently testing compression strength of different combinations. Prototypes have also been machined to evaluate how the formulation behaves during shaping.

(32) Polylactic Acid (PLA) Coated Bone Particles

(33) In further embodiments, the purpose of development is to make a bone based powder that is coated with PLA to give the particles a binder for adhesion.

(34) The development takes bone particles of any size and coats the outer surface either partially or completely. With the addition of PLA to the surface of the particle, it now has a glue/binder that can be heated to cause the PLA to melt and form a solid.

(35) To make bone particles moldable into strong long lasting configurations.

(36) The PLA coated particles can be molded into a shape and then the mold heated to melt the PLA and get a solid, slowly dissolved (months) structure that can support substantial forces and stress.

(37) Bone particles do not stick together without the aid of a medium. To make a composite of bone that can withstand high levels of pressure and stress the medium has to be substantially strong. One such medium is PLA (polylactic acid) and it is used as a human implant material that the body dissolves over a long period of time. In this method the PLA is dissolved and then combined with bone particles and subsequently treated to cause the PLA to solidify out of solution onto the surface of the bone particles/powder.

(38) The product is made by obtaining clean and sterile bone particles/powder and then dissolving amorphous PLA in glacial acetic acid and combining the bone and the PLA solution to a dough consistency and no excess fluid. The dough mixture is then mixed with sodium bicarbonate to homogeneity. The mixture is then exposed to water for injection (WFI) to cause a twofold reaction, one, the acetic acid combines with the sodium bicarbonate and forms CO.sub.2 gas, water, and Na ions, therefore neutralizing the acetic acid. Second, the introduction of water causes the PLA to precipitate on the surface of the bone particles. The mixture is allowed to convert the acetic acid to gas and water and then the mixture is washed followed by lyophilization.

(39) Structural Bone Graft Composites

(40) In yet further embodiments, the purposed of the development is to make a cadaveric human bone based structural graft that incorporates using human cadaveric fibers, human cadaveric proteins, and a synthetic binder using isostatic pressure to create a strong long lasting bone based graft.

(41) The development takes human cadaveric bone particles of specific sizes and combines with human cadaveric collagen fibers and a synthetic binder. The ratios of each of the components are optimized to give the required strength of natural human bone, while minimizing the concentration of binder. The mixed components are then subjected to an initial compaction to reduce the volume and then subjected to isostatic pressures (up to 66,000 psi), followed by a curing at elevated temperature for a specified time and then cooled. After cooling the graft can be shaped mechanically into its final configuration.

(42) To make human cadaveric bone particles into strong long lasting configurations using isostatic pressing.

(43) The mode of operation is as a structural graft that can withstand forces exerted by the human body. The content of the graft makes the graft mostly natural human tissue which in theory can lead to better incorporation of the foreign graft into the host.

(44) Bone particles do not stick together without the aid of a medium. To make a composite of bone that can withstand high levels of pressure and stress the medium has to be substantially strong. One such medium is PLA (polylactic acid) which is currently used as a human implant material that the body dissolves over a long period of time. In this method the medium is used at its lowest possible concentration to yield the strength similar to human bone.

(45) Human cadaveric bone is harvested and then section cleaned of soft tissue and bodily fluids. It is next, ground into particles that are less than 1.1 mm. The particles are the cleaned and demineralized via a proprietary method. The ground bone is then vacuum dried. From here the bone is used as is (Demineralized Bone Matrix-DBM) or is further ground to less than 250 microns to make a powder (Demineralized Bone Powder-DBP). Fibers for this project were obtained from a variety of sources by ultimately used muscle fibers because muscle is abundant during a recovery of other tissues. The muscle is cut into ˜3-8 cm.sup.3 cubes and homogenized with saline solution. The homogenized muscles is then freeze dried. Once dry the muscle is pulverized and sieved over a 1 mm sieve. The retained fibers are collected. The fibers >1 mm are then treated with 3-6% hydrogen peroxide and washed with saline. The remaining fibers are then vacuum dried. Once dried they are untangled and used as is. The binder component is currently polylactic acid and could be replaced with a number of other biocompatible synthetic plastics or glues.

(46) The four components, DBP, DBM, fibers, and binder are weighed out and combined then mixed and placed in a cylindrical mold. A plunger is inserted and then pressed using a hydraulic press to 15,000 to 18,000 psi for approximately 5 minutes, next the pressed plug is removed from the mold and placed in a foam mold and sealed. The sealed mold is then inserted into the isostatic press and subjected to up to 66,000 psi for a specific time. Once the time has elapsed the mold is pulled out the isostatic press and extracted from the mold. The pressed plug is then placed in a compression device and pressed and heated for a specific amount of time at a specific elevated temperature (>50° C.). After the heating the compression device is removed from the oven and cooled to room temperature and the plug is extracted. The plug is then shaped into a final configuration using a CNC system with specific tooling.

(47) Currently initial prototypes of compressed structures have been made with the four components and currently testing compression strength of different combination. Prototypes have also been machined to evaluate how the formation behaves during shaping.

(48) Embodiments of the present technology can be better understood by the following examples.

EXAMPLE 1

(49) To determine mechanical properties of bone composites, a 1 cm.sup.3 cube is machined from a casted bone composite. The cube is compressed and the yield force is measured. With returning reference to FIG. 4, the results for two different samples of a composite of 40 weight percent ground bone matrix are shown. The results for one sample are indicated by the solid line, and the results for another sample are indicated by the dashed line. The composite was made from spincasting a mixture of PMMA particles, bone particles, and MMA liquid monomer initially at 10° C.

(50) The maximum yield strength can be determined from a local maximum point where the yield force stops increasing with displacement resulting from compression. With both these samples, the yield strength reaches a local maximum at around 2 mm of displacement. The compression modulus is determined by the slope of the curve in the linear portion of the graph before the local maximum. This example showed a repeatable and quantifiable method for measuring the mechanical properties of a bone composite.

EXAMPLE 2

(51) The mechanical properties of several bone composites and other materials were tested in a manner similar to what was used in Example 1. Samples of demineralized bone material with a PMMA binder formed by spincasting were tested, including bone composites with 20%, 30%, 40%, 50%, and 66% demineralized bone material (DBM) by weight. Additionally, polyether ether ketone (PEEK), a thermoplastic polymer sometimes used as a bone graft or implant, is tested. Navicular bone, a bone in the foot that may be a candidate for a bone graft or implant, was also tested for comparison. The resulting compressive modulus (GPa) and yield strength (N) of these tests is shown in Table 1. In addition, the published result of cortical bone by D. T. Reilly and A. H. Burstein, “The Elastic and Ultimate Properties of Compact Bone Tissue,” J. Biomech., 8:393-405, 1975, was listed for comparison.

(52) TABLE-US-00001 TABLE 1 Compressive Modulus and Yield Strength of Bone Materials Compressive Yield Material Modulus (GPa) Strength (N) 20% DBM 7.5 14,020 30% DBM 7.8 15,090 40% DBM 8.2 14,655 50% DBM 8.0 13,430 66% DBM 7.2 10,787 PEEK 3.5 9,500 Navicular 4.1 11,586 Cortical bone 17.9 20,500

(53) As shown in Table 1, the demineralized bone material composites range in compressive modulus and yield strength. The bone composites have a higher compressive modulus and yield strength than PEEK. PEEK, while sometimes used as a bone graft or implant, may lack the flexibility of bone and may crush at certain pressures. The relatively lower properties in Table 1 provide quantified data behind the observations of PEEK. Navicular bone has a compressive modulus and yield strength greater than PEEK, indicating a higher flexibility than PEEK, but lower than the DBM bone composites tested. Cortical bone has a higher compressive modulus and yield strength than any bone composite or bone material tested.

(54) This example shows that the compressive modulus and yield strength of a bone composite could be adjusted at least by varying the concentration of the DBM. This example also shows that the DBM bone composites can have similar mechanical properties to human bones and have superior mechanical properties compared to other materials used for bone implants.

EXAMPLE 3

(55) The amount of surface area of a bone composite that is bone was analyzed. DBM particles were dyed with Coomassie Blue stain. After the DBM particles were dried, they were mixed with PMMA powder, and a 50 weight % DBM bone composite was made using a spincasting method. A 0.5 mm thick slice of the bone composite was shaved off and analyzed with a light microscope at 10× magnification. The resulting picture is shown in previously described FIG. 5.

(56) The DBM particles are distinguishable as amorphous structures with blue outlines. Examples of DBM particles in FIG. 5 include particles 502, 504, and 506. Particles sizes of DBM in FIG. 5 range from 100 μm to 800 μm. FIG. 5 was analyzed for the amount of surface area taken by DBM. The DBM surface area was calculated to be 55.4%, and the binder surface area was calculated to be 44.6%.

(57) This example shows that greater than 50% of the surface area of a composite may be bone material. This percentage may allow enough bone at the surface of the composite to integrate with native bone in the body of a patient.

(58) In the preceding description, for the purposes of explanation, numerous details have been set forth in order to provide an understanding of various embodiments of the present technology. It will be apparent to one skilled in the art, however, that certain embodiments may be practiced without some of these details, or with additional details.

(59) Having described several embodiments, it will be recognized by those of skill in the art that various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the invention. Additionally, a number of well-known processes and elements have not been described in order to avoid unnecessarily obscuring the present invention. Additionally, details of any specific embodiment may not always be present in variations of that embodiment or may be added to other embodiments.

(60) Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limits of that range is also specifically disclosed. Each smaller range between any stated value or intervening value in a stated range and any other stated or intervening value in that stated range is encompassed. The upper and lower limits of these smaller ranges may independently be included or excluded in the range, and each range where either, neither, or both limits are included in the smaller ranges is also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included.

(61) As used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a method” includes a plurality of such methods and reference to “the monomer” includes reference to one or more monomers and equivalents thereof known to those skilled in the art, and so forth. The invention has now been described in detail for the purposes of clarity and understanding. However, it will be appreciated that certain changes and modifications may be practice within the scope of the appended claims.