Patent Application
20220008627
The present disclosure generally relates to antibacterial biomedical implants, and in particular to materials, apparatuses and methods for improving the antibacterial characteristics of an intervertebral spinal implant.
Polymeric materials are poor at both osteointegration and microbial resistance. Prior work to overcome this involves inclusion of either an antimicrobial or osteogenic materials onto or into the polymeric material. In these cases, hydroxyapatite is typically cited as the material that improves osteoconduction, whereas the antimicrobial compound is typically silver or an antibiotic. However, there is a need for the improvement of the osteogenic and anti-infective properties of the polymeric material using one material.
Craniomaxillofacial (CMF) implants are used for head and facial reconstruction due to trauma, infection, cancer, and congenital and developmental deformities. To be effective, prosthetic devices must be biocompatible, infection resistant, strong, durable, thermally insulating, shape stable, osteoconductive, low cost, and readily available. Current CMF implants lack bioactivity, infection control, osseous integration, mechanical stability, or radiographic imaging. There is thus a need for medical imaging compatible 3D printed biomaterials for CMF osteoplasty that can be personalized, promote integration, and prevent infection.
It is with these observations in mind, among others, that various aspects of the present disclosure were conceived and developed.
A need exists for an improved biomedical implant with antibacterial properties. Accordingly, one embodiment of the present disclosure may include a method for improving the antibacterial characteristics of a biomedical implant. The method may include the steps of providing a biomedical implant and loading the biomedical implant with about 10% to about 20% of a powder, wherein the powder comprises a silicon nitride material. The method may further include increasing the surface roughness of at least a portion of the biomedical implant to a roughness profile having an arithmetic average of at least about 500 nm Ra to improve the antibacterial characteristics of the biomedical implant by at least one of micromachining, grinding, polishing, laser etching, laser texturing, sand- or other abrasive-blasting, chemical etching, thermal etching, and plasma etching. The silicon nitride material may be selected from the group consisting of α-Si3N4, β-Si3N4, β-SiYAlON, and combinations thereof. The biomedical implant may be a hip implant, intervertebral spinal implant, bone screw, or a craniomaxillofacial implant. In one example, the biomedical implant may be a silicon nitride coating on a titanium femoral stem of a hip implant. Examples of intervertebral spinal implants may include cervical or lumbar devices. Similarly, examples of craniomaxillofacial implants may include burr plugs, cranial, temporal, maxilla, or zycomatic, and mandible plates and screws, and temporal mandibular joints. The biomedical implant may be a bone screw. The biomedical implant may include poly-ether-ether-ketone (PEEK) or titanium, and α- or β-Si3N4 powder, titanium or PEEK and β-SiYAlON powder, poly-ether-ketone-ketone (PEKK), and α- or β-Si3N4 powder, or PEKK and β-SiYAlON powder.
In other embodiments, the method may further include applying a coating of silicon nitride to the biomedical implant. The step of increasing a surface roughness of at least a portion of the biomedical implant may be performed either prior to or after the step of applying a coating to the biomedical implant, and the step of increasing a surface roughness of at least a portion of the biomedical implant may include increasing a surface roughness of at least a portion of the coating. The step of increasing a surface roughness of at least a portion of the biomedical implant to a roughness profile having an arithmetic average of at least about 1,250 nm Ra or between about 2,000 nm Ra and about 5,000 nm Ra.
Another implementation of the present disclosure may take the form of a biomedical implant with improved antibacterial characteristics. The biomedical implant may include a polymeric or metallic substrate material; and about 10% volume percent (vol. %) to about 20 vol. % of a powder, wherein the powder comprises a silicon nitride material. At least a portion of the implant may have an increased surface roughness profile having an arithmetic average of at least about 500 nm Ra created by at least one of micromachining, grinding, polishing, laser etching, laser texturing, sand- or other abrasive-blasting, chemical etching, thermal etching, and plasma etching. The silicon nitride material may be selected from the group consisting of α-Si3N4, β-Si3N4, β-SiYAlON, and combinations thereof. The substrate material may include poly-ether-ether-ketone (PEEK), poly-ether-ketone-ketone (PEKK), or titanium. The biomedical implant may be an intervertebral spinal implant, a hip implant, a bone screw, or a craniomaxillofacial implant. The biomedical implant may include a hip implant with a silicon nitride coating on a femoral stem of the hip implant. The biomedical implant may further include a silicon nitride coating on the biomedical implant.
Further provided herein is a method for forming a biomedical implant. In some embodiments, the method includes the steps of: dispersing silicon nitride powder within a poly-ether-ether-ketone (PEEK) or poly-ether-ketone-ketone (PEKK) substrate material to form a composite material; and forming the composite material into a biomedical implant. The biomedical implant has improved antibacterial characteristics and/or improved bone-forming characteristics as compared to a monolithic PEEK or PEKK implant.
Also provided herein is a biomedical implant that includes a poly-ether-ether-ketone (PEEK) or a poly-ether-ketone-ketone (PEKK) substrate material; and a powder comprising α-Si3N4, β-Si3N4, β-SiYAlON, or combinations thereof. In various embodiments, the powder is dispersed within the substrate material, forming a composite material.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
The written disclosure herein describes illustrative embodiments that are non-limiting and non-exhaustive. Reference is made to certain of such illustrative embodiments that are depicted in the figures, in which:
Embodiments described herein may be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. It will be readily understood that the components of the present disclosure, as generally described and illustrated in the drawings herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the apparatus is not intended to limit the scope of the disclosure, but is merely representative of possible embodiments of the disclosure. In some cases, well-known structures, materials, or operations are not shown or described in detail.
As used herein, the terms “comprising,” “having,” and “including” are used in their open, non-limiting sense. The terms “a,” “an,” and “the” are understood to encompass the plural as well as the singular. Thus, the term “a mixture thereof” also relates to “mixtures thereof.”
As used herein, “about” refers to numeric values, including whole numbers, fractions, percentages, etc., whether or not explicitly indicated. The term “about” generally refers to a range of numerical values, for instance, ±0.5-1%, ±1-5% or ±5-10% of the recited value, that one would consider equivalent to the recited value, for example, having the same function or result.
As used herein, the term “silicon nitride” includes Si3N4, alpha-(α) or beta-phase (β) Si3N4, SiYAlON, SiYON, SiAlON, or combinations of these phases or materials.
Various embodiments of apparatus, methods, and systems are disclosed herein that relate to biomedical implants having antibacterial characteristics and materials and methods for improving the antibacterial function and/or characteristics of such implants. In preferred embodiments, silicon nitride ceramics, composite polymer silicon nitride, or metal silicon nitride implants are provided that may be, in some embodiments, treated so as to improve upon their antibacterial characteristics and/or other desirable characteristics. For example, embodiments and implementations disclosed herein may result in improved inhibition of bacteria adsorption and biofilm formation, improved protein adsorption, and/or enhanced osteoconductive and osteointegration characteristics. Such embodiments may comprise a silicon nitride ceramic or doped silicon nitride ceramic substrate. Alternatively, such embodiments may comprise a silicon nitride or doped silicon nitride coating on a substrate of a different material. In other embodiments, the implant may be a composite, made up of a silicon nitride material and a polymer, or a silicon nitride material and a metal. In still other embodiments, one or more portions or regions of an implant may include a silicon nitride material and/or a silicon nitride coating, and other portions or regions may include other biomedical materials.
The terms used in this specification generally have their ordinary meanings in the art, within the context of the disclosure, and in the specific context where each term is used. Alternative language and synonyms may be used for any one or more of the terms discussed herein, and no special significance should be placed upon whether or not a term is elaborated or discussed herein. In some cases, synonyms for certain terms are provided. A recital of one or more synonyms does not exclude the use of other synonyms. The use of examples anywhere in this specification including examples of any terms discussed herein is illustrative only, and is not intended to further limit the scope and meaning of the disclosure or of any example term.
As another alternative, silicon nitride or other similar ceramic materials may be incorporated into other materials used to form biomedical implants, including but not limited to hip implants, intervertebral spinal cervical or lumbar implants, or craniomaxillofacial implants. For example, silicon nitride may be used as a filler or otherwise incorporated into polymers or biodegradable polymers, such as poly-ether-ether-ketone (PEEK), poly(methyl methacrylate), poly(ethylene terephthalate), poly(dimethylsiloxane), poly(tetrafluoroethylene), polyacrylic acids, polylactic acids, polycarbonates, polyethylene, and/or polyurethane, in their porous scaffolds or bulk structures. In some embodiments, the silicon nitride filler may be β-silicon nitride and may be present in the biomedical implant in amounts ranging from about 1 vol. % to about 99 vol. %. For example, a β-silicon powder may be incorporated into a PEEK biomedical implant in an amount from about 10 vol. % to about 20 vol. %. Silicon nitride may also be used as a filler otherwise incorporated into other materials used to form other biomedical implants, such as metals, including titanium, silver, nitinol, copper, cobalt/chromium, and related alloys, for example. As still another alternative, silicon nitride may be used as a filler or otherwise incorporated into other materials, such as other oxide ceramics and cermets.
In embodiments including one or more coatings, the coating(s) can be applied by any number of methods such as chemical vapor deposition (CVD), physical vapor deposition (PVD), plasma spraying, electro-deposition or electrophoretic deposition, slurry coating and high-temperature diffusion, or any other application method known by those skilled in the art. In some embodiments, the coating thickness can range from between about 5 nanometers up to about 5 millimeters. In other such embodiments, the coating thickness may be between about 1 micrometer and about 125 micrometers. The coating may adhere to the surface of the implant but need not necessarily be hermetic.
Silicon nitride ceramics have tremendous flexural strength and fracture toughness. In some embodiments, such ceramics have been found to have a flexural strength greater than about 700 Mega-Pascal (MPa). Indeed, in some embodiments, the flexural strength of such ceramics have been measured at greater than about 800 MPa, greater than about 900 MPa, or about 1,000 MPa. The fracture toughness of silicon nitride ceramics in some embodiments exceeds about 7 Mega-Pascal root meter (MPa·m1/2). Indeed, the fracture toughness of such materials in some embodiments is about 7-10 MPa·m1/2.
Examples of suitable silicon nitride materials are described in, for example, U.S. Pat. No. 6,881,229, titled “Metal-Ceramic Composite Articulation,” which is incorporated by reference herein. In some embodiments, dopants such as alumina (Al2O3), yttria (Y2O3), magnesium oxide (MgO), and strontium oxide (SrO), can be processed to form a doped composition of silicon nitride. In embodiments comprising a doped silicon nitride or another similar ceramic material, the dopant amount may be optimized to achieve the highest density, mechanical, and/or antibacterial properties. In further embodiments, the biocompatible ceramic may have a flexural strength greater than about 900 MPa, and a toughness greater than about 9 MPa·m1/2. Flexural strength can be measured on standard 3-point bend specimens per American Society for Testing of Metals (ASTM) protocol method C-1161, and fracture toughness can be measured using single edge notched beam specimens per ASTM protocol method E399. In some embodiments, powders of silicon nitride may be used to form the ceramic implants, either alone or in combination with one or more of the dopants referenced above.
Other examples of suitable silicon nitride materials are described in U.S. Pat. No. 7,666,229 titled “Ceramic-Ceramic Articulation Surface Implants,” which is hereby incorporated by reference. Still other examples of suitable silicon nitride materials are described in U.S. Pat. No. 7,695,521 titled “Hip Prosthesis with Monoblock Ceramic Acetabular Cup,” which is also hereby incorporated by reference.
Silicon nitride has been discovered to have unexpected antibacterial properties and increased bone formation properties. Indeed, as discussed in greater detail below, it has been recently demonstrated that the adhesion and growth of bacteria on silicon nitride materials is substantially reduced with respect to other common spinal implant materials, such as titanium and polyetheretherketone (PEEK). As discussed in greater detail below, compared to medical grade titanium and PEEK, silicon nitride significantly inhibits in vitro and in vivo bacteria colonization, and bio-film formation. Silicon nitride also exhibits a much lower live count and live to dead ratio for bacteria during studies.
It has also been demonstrated that silicon nitride materials provide significantly greater adsorption of vitronectin and fibronectin, which proteins are known to decrease bacteria function, than titanium, PEEK, and PEKK. It is thought that these properties will be very useful in biomedical implants of all types by significantly reducing the possibility of infection. This may be accomplished by, for example, preventing or disrupting bacterial formation on/in the implant and/or killing bacteria that have been transferred to the implant.
Without being limited by theory, it is thought that the higher adsorption of proteins that characterizes silicon nitride may facilitate the inhibition of bacteria growth and promote stem cell differentiation to osteoblasts. This preferential adsorption may be a cause for silicon nitride's ability to decrease bacteria function. Again, without being limited by theory, the mechanisms for the enhanced antibacterial characteristics of silicon nitride may be a combination of its features. For example, its hydrophilic surface may lead to preferential adsorption of proteins that are responsible for reduced bacteria function. This effect may be enhanced by increasing the surface texture or roughness of a silicon nitride based implant or silicon nitride based coating on an implant made up of a different material. Because of these characteristics, silicon nitride also exhibits enhanced in vivo osteoconduction and osteointegration when compared with titanium, PEEK, or PEKK alone.
As discussed above, using a silicon nitride coating or filler on one or more regions of an implant's surface may be used, in some embodiments and implementations, to inhibit bacterial adhesion, while increasing/fostering adsorption of proteins necessary for healing and bone reformation. This same effect may, in other embodiments, be accomplished using monolithic silicon nitride as an implant.
In such embodiments, the surface of the ceramic implant may be engineered to provide for an increased degree of micro-roughness and surface texture to enhance these desirable properties. For example, in some embodiments, the micro-roughness—i.e., the texture of the surface in between the peaks and valleys typically measured by Ra values—may also, or alternatively, be increased by suitable texturing. In some implementations, the micro-roughness of the implant and/or coating may be increased by micromachining, grinding, polishing, laser etching or texturing, sand- or other abrasive-blasting, chemical, thermal or plasma etching, and the like. Micro-roughness may be measured by measuring the height of surface asperities using cut-off limits on a profilometer. This method may be used to selectively assess the roughness of a surface between the peaks and valleys. Alternatively, or additionally, the skewness and/or kurtosis could be measured. These measurements consider the deviation of the surface from what might be expected of a normal Gaussian distribution of surface roughness. Such surface engineering may also be performed on a silicon nitride coating, rather than on a monolithic silicon nitride or silicon nitride composite implant.
In some embodiments, the density of the silicon nitride material, or doped silicon nitride material, may vary throughout the implant, or throughout the portion of the implant made up of silicon nitride. For example, in spinal implant embodiments, the outermost layer, or a portion of the outermost layer, may be more porous, or less dense, than the core or center of the implant. This may allow for bone to grow into or otherwise fuse with a less dense portion of the implant, and the denser portion of the implant can be wear-resistant and may have a higher strength and/or toughness, for example.
In certain embodiments, one or more inner portions of the implant may have a relatively low porosity or non-porous ceramic, and thus exhibit high density and high structural integrity generally consistent with, and generally mimicking the characteristics of, natural cortical bone. And, by contrast, one or more of the surface coatings, layers, or linings formed at an outer surface of the implant can exhibit a comparatively greater or higher porosity that is generally consistent with and generally mimics the characteristics of natural cancellous bone. As a result, the higher porosity surface region(s), coating(s), or lining(s) can provide an effective bone ingrowth surface for achieving secure and stable bone ingrowth affixation of the ceramic portion of the implant (which, in some embodiments, comprises the entire implant) between a patient's vertebrae or another suitable location within the human body.
In some embodiments, the antibacterial behavior of other implant materials, such as polymeric, metallic, or ceramics, may be improved through the application of silicon nitride as an adherent coating. This coating may, in some implementations, be roughened or textured to provide for increased surface area of the silicon nitride material/coating. In other embodiments, monolithic silicon nitride implantable devices may be provided which may be subjected to similar surface engineering.
The surface roughness values disclosed herein may be calculated using the arithmetic average of the roughness profile (Ra). Polished silicon nitride surfaces may have a roughness of 20 nm Ra or less. However, as discussed in greater detail below, counterintuitively, the antibacterial properties of certain embodiments may be improved by roughening, rather than polishing, all or one or more portions of the surface of a silicon nitride ceramic or another similar ceramic implant. In some embodiments, a relatively rough surface may be created as part of the process of creating the material, such as during a firing stage, without further roughening or other surface engineering. However, in other embodiments, as discussed in greater detail below, the surface may be roughened to further increase the roughness beyond what would occur as a result of standard firing/curing alone. Thus, in some embodiments, the surface roughness may be greater than about 1,250 nm Ra. In some such embodiments, the surface roughness may be greater than about 1,500 nm Ra. In some such embodiments, the surface roughness may be greater than about 2,000 nm Ra. In some such embodiments, the surface roughness may be greater than about 3,000 nm Ra. In other embodiments, the surface roughness may be between about 500 nm Ra and about 5,000 nm Ra. In some such embodiments, the surface roughness may be between about 1,500 nm Ra and about 5,000 nm Ra. In some such embodiments, the surface roughness may be between about 2,000 nm Ra and about 5,000 nm Ra. In some such embodiments, the surface roughness may be between about 3,000 nm Ra and about 5,000 nm Ra.
In certain embodiments, metallic, polymeric, or ceramic implant substrates may be filled with a silicon nitride powder to form a composite. Non-limiting examples of filler silicon nitride powders include α-Si3N4, β-Si3N4, and β-SiYAlON powders. Non-limiting examples of metallic or polymeric biomedical implant substrates that may be filled with silicon nitride powder include poly-ether-ether-ketone (PEEK), poly-ether-ketone ketone (PEKK), poly(methylmethacrylate), poly(ethyleneterephthalate), poly(dimethylsiloxane), poly(tetrafluoroethylene), polyacrylic acids, polylactic acids, polycarbonates, polyethylene, polyurethane, titanium, Silver, Nitinol, Copper, and/or related alloys. In various embodiments, a PEEK implant may be filled with α-Si3N4, β-Si3N4, or β-SiYAlON powders. In various other embodiments, a PEKK implant may be filled with α-Si3N4, β-Si3N4, or β-SiYAlON powders. The percentage of silicon nitride powder in the implant may range from about 1 weight percent (wt. %) to about 99 wt. %. In various aspects, the silicon nitride in the implant may range from about 1 wt. % to about 5 wt. %, from about 5 wt. % to about 15 wt. %, from about 10 wt. % to about 20 wt. %, from about 15 wt. % to about 25 wt. %, from about 20 wt. % to about 30 wt. %, from about 25 wt. % to about 35 wt. %, from about 30 wt. % to about 50 wt. %, from about 40 wt. % to about 60 wt. %, from about 50 wt. % to about 70 wt. %, from about 60 wt. % to about 80 wt. %, from about 70 wt. % to about 90 wt. %, and from about 80 wt. % to about 99 wt. %. In one embodiment, a PEEK implant may include up to about 30.4 wt. % β-Si3N4 or β-SiYAlON. In another embodiment, a PEKK implant may include up to about 30.4 wt. % β-Si3N4 or β-SiYAlON.
In still other embodiments, the percentage of silicon nitride powder in the implant may range from about 1 vol. % to about 99 vol. %. In various aspects, the silicon nitride in the implant may range from about 1 vol. % to about 5 vol. %, from about 5 vol. % to about 15 vol. %, from about 10 vol. % to about 20 vol. %, from about 15 vol. % to about 25 vol. %, from about 20 vol. % to about 30 vol. %, from about 25 vol. % to about 35 vol. %, from about 30 vol. % to about 50 vol. %, from about 40 vol. % to about 60 vol. %, from about 50 vol. % to about 70 vol. %, from about 60 vol. % to about 80 vol. %, from about 70 vol. % to about 90 vol. %, and from about 80 vol. % to about 99 vol. %. In one embodiment, a PEEK implant may include up to about 15 vol. % β-Si3N4 or β-SiYAlON. In another embodiment, a PEKK implant may include up to about 15 vol. % β-Si3N4 or β-SiYAlON.
In some additional embodiments, the implant may be 3D printed. A 3D printed implant of the present disclosure may include PEEK or PEKK and silicon nitride, or combinations thereof. In some aspects, the method of 3D printing used may include fused filament fabrication or selective laser sintering. In some examples, the 3D printed implant may include PEEK and silicon nitride. In other examples, the 3D printed implant may include PEKK and silicon nitride. In some embodiments, the printing speed may be between about 1500 mm/min to about 3000 mm/min. In some additional embodiments, the bed temperature of the 3D printer may be about 300° C. In still further embodiments, the print temperature may be about 400° C.
In some embodiments, the 3D printed implant may be a cervical spinal cage.
In some embodiments, the cross-section (e.g. as seen in
In some embodiments, when the 3D printed cervical spinal cage 600 includes a diamond lattice structure 602, the diamond lattice structure 602 may have a unit cell with a length of about 3 mm. In some embodiments, the diamond lattice structure may comprise pores and struts. In some aspects, the diameter of the pores may be about 0.2 to 1.0 mm. In some examples, the diameter of the pores may be about 0.2 mm, 0.25 mm, 0.3 mm, 0.35 mm, 0.4 mm, 0.45 mm, 0.5 mm, 0.55 mm, 0.6 mm, 0.65 mm, 0.7 mm, 0.75 mm, 0.8 mm, 0.85 mm, 0.9 mm, 0.95 mm, or about 1.0 mm. In some additional aspects, the length of the struts may be about 0.5 to 1.5 mm. In some examples, the length of the struts may be about 0.5 mm, 0.55 mm, 0.6 mm, 0.65 mm, 0.7 mm, 0.75 mm, 0.8 mm, 0.85 mm, 0.9 mm, 0.95 mm, 1 mm, 1.05 mm, 1.1 mm, 1.15 mm, 1.2 mm, 1.25 mm, 1.3 mm, 1.35 mm, 1.4 mm, 1.45 mm, or about 1.5 mm.
In some aspects when the 3D printed cervical spinal cage 600 includes teeth 604 (see
In an embodiment, the SiYAlON and ß-Si3N4 materials may have added aluminum-oxide and yttrium-oxide. Without being limited to a particular theory, the functional surface chemistry of the implant may be enhanced by the additions of these oxide dopants.
In some embodiments, a polymeric implant filled with silicon nitride powder may improve the osteoconductivity and antibacterial activity of the implant compared to the implant without the silicon nitride filler. For example, a PEEK or PEKK implant filled with α-Si3N4, β-Si3N4, or β-SiYAlON may improve osteoconductivity and antibacterial characteristics of the implant compared to a monolithic PEEK or PEKK implant. In an embodiment, the surface of the implant filled with silicon nitride powder may be further modified with a surface roughness and may or may not further include a silicon nitride coating. Some of the methods disclosed herein may therefore provide for engineering of the surface roughness of silicon nitride ceramic filled implants in order to improve their antibacterial performance.
Without being limited to a particular theory, the addition of a relatively low fraction of α-Si3N4, β-Si3N4, or β-SiYAlON or a suitable mixture thereof may enhance in vitro osteoconductivity and antibacterial resistance of PEEK or PEKK. The silicon nitride filled PEEK or PEKK implants have substantially better results than other substrates without a silicon nitride filler. It was unexpected that a PEEK implant filled with α-Si3N4 exhibited an increased osteoconductivity and reduced antibacterial resistance while β-Si3N4 or β-SiYAlON had both an increased osteoconductivity and antibacterial resistance.
In some embodiments, metallic, polymeric, or ceramic substrates may be pre-engineered with a surface texture onto which a silicon nitride coating may be applied. This texture can range from as low as about 5 nanometers up to about 5,000 nanometers or more in average surface roughness (Ra). Alternatively, as another embodiment, the surface texture of the silicon nitride coating itself can be increased, exclusive of the surface roughness of the substrate, to obtain a similar Ra range and resulting antibacterial effect. Some of the methods disclosed herein may therefore provide for engineering of the surface roughness of monolithic silicon nitride ceramic implants in order to improve their antibacterial performance, and other methods disclosed herein may provide for engineering the surface roughness of layers or coatings applied to substrates made up of any other suitable material available for use in biomedical implants. Of course, in some implementations, surface engineering may be applied to both the substrate and the coating.
Increasing the surface roughness of the ceramic or ceramic filled implant can be accomplished using any number of known methods by those skilled in the art, including micromachining, grinding, polishing, laser etching or texturing, sand or other abrasive blasting, chemical etching, thermal etching, plasma etching, and the like.
In some embodiments of the present disclosure, a porous PEEK surface embedded with silicon nitride may be formed by light pressing on a PEEK surface sprinkled with a mixture of coarse NaCl and fine silicon nitride grains, followed by leaching of the NaCl grains by distilled water at room temperature, as described in
The inventive techniques disclosed herein, including but not limited to the silicon nitride coatings and roughened surface finishes, may be applied to any number and type of biomedical components including, without limitation, spinal implants, orthopedic screws, plates, wires, and other fixation devices, articulation devices in the spine, hip, knee, shoulder, ankle and phalanges, catheters, implants for facial or other reconstructive plastic surgery such as craniomaxillofacial implants, middle ear implants, dental devices, and the like. In aspects wherein the implant is a spinal cage, the implant may have properties that meet or exceed the minimum performance metrics outlined in ASTM F2077.
As illustrated in the Examples presented below, in comparison with titanium and poly-ether-ether-ketone (PEEK), silicon nitride significantly inhibits in vitro and in vivo bio-film formation and bacterial colonization, and shows much lower bacteria live/dead ratios for bacteria, including but not limited to Staphylococcus epidermidis (Staph. Epi.), Staphylococcus aureus (Staph. aureus), Enterococcus, Pseudomonas aeruginosa (Pseudo. aeruginosa), and Escherichia Coli (E. Coli). Silicon nitride also demonstrates significantly higher in vitro adsorption of three proteins (Fibronectin, Vitronectin, and Laminin) which can displace or inhibit bacteria growth and promote stem cell differentiation to osteoblasts.
In a clinical setting, bacteria are an ever-present menace, particularly when associated with surgical intervention and the introduction of foreign material into the human body, such as orthopedic, cardiac or dental endoprostheses. Microorganisms introduced during surgery tend to initially populate the sterile surfaces of implants. Bacterial adhesion to the biomaterial surface is the essential step in the development of an infection. The human body's defensive mechanisms are triggered if the implant is excessively colonized by bacteria. Chronic infections arise when the bacterial colony reaches a critical size and overcomes the local host defenses. When this occurs, the body tends to encapsulate the infection and reject the implant. Consequently, patients typically must undergo re-operation, removal of the implant, treatment of the infection, and replacement of the implant. Deep wound infections associated with common orthopedic surgeries can cost up to $100,000 or more for corrective treatment. The reduction in quality of life and the associated cost of treating infections represents a significant burden for present day medical care.
Various embodiments and implementations disclosed herein will therefore provide materials and methods that resist bacterial adhesion, colonization, and growth, which, as discussed above, often lead to chronic infections. The embodiments and implementations disclosed herein may also provide for enhanced in vivo osteointegration and increased bone growth in comparison to other common implants, such as those made up of only titanium and PEEK.
Factors influencing bacteria adhesion to biomaterial surfaces may include chemical composition, surface charge, hydrophobicity, and surface roughness or physical characteristics of the surface and/or coating of an implant. There are marked differences in the surface chemistry of metallic, polymeric, and ceramic implants. Metals typically have a thin protective oxide layer on their surfaces (typically less than about 25 nm in thickness). Polymers may also have oxide surfaces, but the oxides are typically part of longer chain carboxyl or hydroxyl groups. Both metallic and polymeric surfaces are often low in hardness, and therefore are easily abraded and highly sensitive to chemical attack and dissolution. Ceramics, such as silicon nitride, may also have oxide surfaces. However, unlike their metal counterparts, they are highly resistant to chemical and abrasive action.
Metallic and polymeric devices are also typically hydrophobic. Consequently, bacteria do not have to displace aqueous bodily fluids in order to adhere to the implant's surface. By contrast, ceramics, and silicon nitride in particular, are known to be hydrophilic. For instance, sessile water drop studies demonstrate that silicon nitride has higher wettability than either medical grade titanium or PEEK. This higher wettability is thought to be directly attributable to the hydrophilic surface of silicon nitride.
In order for bacteria to adhere to a hydrophilic surface, it must first displace the water that is present on the surface. Therefore, hydrophilic surfaces typically inhibit bacterial adhesion more effectively than do hydrophobic surfaces. It has also been shown that implant surface finish and texture play important roles in bacteria colonization and growth. Irregularities on the surface of typical polymeric or metallic implants tend to promote bacterial adhesion, whereas smooth surfaces tend to inhibit attachment and bio-film formation. This is true because rough surfaces have greater surface area and include depressions that provide favorable sites for colonization.
Counterintuitively, however, certain ceramic materials, including in particular silicon nitride, not only provide desirable antibacterial properties, but they also provide further enhanced antibacterial properties with increased, rather than decreased, surface roughness. In other words, silicon nitride surfaces of higher roughness appear to be more resistant to bacterial adhesion than smooth surfaces. This is precisely the opposite of what is observed for many other implant materials, such as titanium and PEEK. As referenced above and as discussed in greater detail below, compared to medical grade titanium and PEEK, silicon nitride has been shown to significantly inhibit in vitro bacteria colonization and bio-film formation, and it shows a much lower live count and a live to dead ratio for bacteria during studies. However, in studies between different types of silicon nitride, rough silicon nitride surfaces have been shown to be more effective in inhibiting bacterial colonization (rather than less effective as with most common implant materials) than polished silicon nitride (although both were much more effective in doing so than either titanium or PEEK).
Various embodiments and implementations will be further understood by the following Examples:
In a first working example, the abilities of biomedical implant materials to inhibit bacterial colonization were tested. The study included silicon nitride materials, biomedical grade 4 titanium, and PEEK. Four types of bacteria were included in the study: Staphylococcus epidermidis, Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, and Enterococcus.
Implant samples in the study were sterilized by UV light exposure for 24 hours and surface roughness was characterized using scanning electron microscopy. Bacteria were then inoculated on the surfaces of the samples and incubated for 4, 24, 48, and 72 hours.
Two methods were used to determine bacteria function at the end of each time period: (1) Crystal violet staining; and (2) Live/dead assay. Bacteria were also visually counted using a fluorescence microscope with image analysis software. The experiments were completed in triplicate and repeated three times. Appropriate statistical analyses were then compiled using Student t-tests.
For all bacteria, and all incubation times, the silicon nitride samples demonstrated lower bio-film formation, fewer live bacteria, and smaller live to dead bacteria ratios when compared with medical grade titanium and PEEK. Rough silicon nitride surfaces were even more effective in inhibiting bacterial colonization than polished surfaces. In addition, silicon nitride implants with polished or rough surfaces were both significantly better in inhibition of bacterial colonization than either titanium or PEEK.
Bio-film formation was also much higher for titanium and PEEK than for silicon nitride. For example, bio-film formation for Staphylococcus aureus on titanium was three times higher than polished silicon nitride after 72 hours of incubation and more than eight times higher than PEEK after 72 hours of incubation. And the results were even better using relatively rough silicon nitride having a surface roughness of about 1,250 nm Ra. Bio-film formation for Staphylococcus aureus on this rougher silicon nitride was less than half of that for the polished silicon nitride after 72 hours.
Live bacteria counts followed similar patterns. Live bacteria counts after 72 hours of incubation were between 1.5× and 30× higher for titanium and PEEK when compared with silicon nitride. And, again, rough silicon nitride outperformed polished silicon nitride. For example, for Pseudomonas aeruginosa, live bacteria count after 72 hours for rough silicon nitride (again, about 1,250 nm Ra) was about one-fifth of that for polished silicon nitride.
Live/dead bacteria ratios were similarly lowest for silicon nitride, and generally lower for rough silicon nitride than for polished silicon nitride. For example, live/dead ratios after 72 hours of incubation for E. coli on polished silicon nitride were over three times as high as titanium and about twice as high as PEEK. For rough silicon nitride, live/dead ratios were about six times as high for titanium and nearly three times as high for PEEK.
In this study, the ability of biomedical implant materials to adsorb common bone-forming proteins was tested. As with Example 1, rough silicon nitride, polished silicon nitride, medical grade titanium, and PEEK were tested. The proteins tested were fibronectin, vitronectin, and laminin. Enzyme-linked immunosorbent assays (ELISA) were performed for 20 minutes, 1 hour, and 4 hours. Fibronectin, vitronectin, or laminin were directly linked with primary rabbit anti-bovine fibronectin, anti-vitronectin, and anti-laminin, respectively. The amount of each protein adsorbed to the surfaces was measured with an ABTS substrate kit. Light absorbance at 405 nm on a spectro-photometer was analyzed with computer software. ELISA was performed in duplicate and repeated three different times per substrate.
For all incubation times, silicon nitride exhibited significantly greater adsorption of fibronectin and vitronectin when compared with titanium and PEEK. Silicon nitride also showed greater adsorption of laminin at 1 and 4 hours incubation in comparison to titanium and PEEK. Rough silicon nitride surfaces (approximately 1,250 nm Ra) were more effective in adsorption of proteins than polished silicon nitride surfaces. However, both silicon nitride surfaces were generally better than either titanium or PEEK, particularly for fibronectin and vitronectin. Without being limited by theory, it is thought that preferred adsorption of these proteins onto silicon nitride is a probable explanation for its improved bacterial resistance.
In this study, in vivo bone formation, inflammation, and infection of various implant materials were studied using a Wistar rat calvaria model. The study considered the strength of bone attachment to these materials. Rough silicon nitride, medical grade titanium, and PEEK were used in the study.
The study was conducted by implanting sterilized samples into the calvaria of two-year old Wistar rats using standard techniques. Another group of samples was inoculated apriori with Staphylococcus epidermidis and implanted into a second group of similar Wistar rats.
The animals were sacrificed at 3, 7, 14, and 90 days. Histology was quantified for the number of macrophages, bacteria, and bio-film proteins surrounding each of the implant materials. In addition, push-out tests were performed to determine bone attachment results and performance.
After 3 days using the non-inoculated samples, the titanium and PEEK implants were unstable, and thus no histology was able to be performed. The silicon nitride implants (surface roughness of approximately 1,250 nm Ra) exhibited about 3-5% bone-implant interface, as measured using microscopic linear analysis, and about 16-19% new bone growth in the surgical area, as measured using microscopic areal analysis, after 3 days.
After 7 days using the non-inoculated samples, the titanium and PEEK implants were unstable, and thus no histology was able to be performed. The silicon nitride implants, by contrast, exhibited about 19-21% bone-implant interface and about 28-32% new bone growth in the surgical area after 7 days.
After 14 days using the non-inoculated samples, the titanium implant exhibited about 7% bone-implant interface and about 11% new bone growth in the surgical area. The PEEK implant exhibited about 2% bone-implant interface and about 14% new bone growth in the surgical area. The silicon nitride implants, by contrast, exhibited about 23-38% bone-implant interface and about 49-51% new bone growth in the surgical area after 14 days.
After 90 days without inoculation, the titanium and PEEK implants exhibited about 19% and 8% bone-implant interface, respectively, and about 36% and 24% new bone growth, respectively. The silicon nitride implants again performed much better. These implants exhibited a bone-implant interface of about 52-65% and new bone growth of about 66-71%.
With the inoculated samples, all implants were too unstable to perform histology at 3 and 7 days. After 14 days, the titanium implant exhibited only about 1% bone-implant interface, 75% bacteria-implant interface (measured using microscopic linear analysis), about 9% new bone growth in the surgical area, and about 45% bacterial growth in the surgical area. PEEK exhibited essentially no bone-implant interface, about 2% new bone growth, and about 25% bacterial growth. The bacteria-implant interface with PEEK was unclear. The inoculated silicon nitride implants exhibited a bone-implant interface of about 3-13% after 14 days. New bone growth with the silicon nitride implants was about 25-28%, and bacterial growth was about 11-15%.
After 90 days, the inoculated titanium implant exhibited about 9% bone-implant interface, about 67% bacteria-implant interface, about 26% new bone growth, and about 21% bacterial growth. The PEEK implant exhibited about 5% bone-implant interface, about 95% bacteria-implant interface, about 21% new bone growth, and about 88% bacterial growth. The inoculated silicon nitride implants exhibited a bone-implant interface of about 21-25% after 90 days. New bone growth with the silicon nitride implants was about 39-42%, and there was no measurable bacterial-implant interface or bacterial growth after 90 days. In fact, there were no bacteria detected on the silicon nitride implants after 90 days.
Push-out strengths were also substantially better with the silicon nitride implants than with either the titanium or PEEK implants after all implantation times were measured, both with and without inoculation. After 90 days implantation without inoculation, push-out strengths for the silicon nitride implants were more than twice as high as titanium and more than two-and-a-half times as high as PEEK. With inoculation, silicon nitride push-out strengths were even better compared to titanium and PEEK for all implantation times. Silicon nitride push-out strengths were more than five times those of either titanium or PEEK. These results demonstrate substantial bone attachment for silicon nitride when compared to titanium and PEEK.
Push out strengths were measured by taking a sectioned portion of the calvaria including the implant and cementing the calvaria to wood blocks over a support plate. A load was then applied to the implant and the force required to dislodge the implant from the calvaria was measured.
The histology results further confirm the tested push-out strengths. As discussed above, significantly greater new bone growth was observed in the calvaria defect area for silicon nitride when compared with titanium and PEEK at all implantation times and under all inoculation conditions.
In this study, in vitro assessment of osteoconductivity of various implant materials were studied using a SaOS-2 cell line. The study considered the SaOS-2 cell proliferation on these materials. Silicon nitride filled PEEK (i.e., PEEK filled with 15 vol. % α-Si3N4, β-Si3N4, and β-SiYAlON powders) and monolithic PEEK substrate materials were used in the study.
The study was conducted by seeding SaOS-2 cells onto squares (5×105 cells/ml) of each substrate material using standard techniques. After 24 hours, the cells were stained with Blue Hoechst 33342 and counted by fluorescence spectroscopy. Cell seeding was completed after 7 days. The cells were evaluated and counted by fluorescence spectroscopy and the substrate materials were evaluated using laser microscopy, Raman spectroscopy, and scanning electron microscopy (SEM).
All PEEK composites loaded with 15% Si3N4 (α- or β) or β-SiYAlON have shown a greatly improved SaOS-2 cell proliferation as compared with monolithic PEEK. All PEEK composites loaded with 15 vol. % Si3N4 (α- or β) or β-SiYAlON have shown significantly improved osteoconductivity with SaOS-2 cell line as compared with monolithic PEEK. The above results were confirmed by several different analytical tools and statistically validated.
In this study, in vitro assessment of antibacterial activity of various implant materials were studied using Staphylococcus epidermidis. Staphylococcus epidermidis (S. epidermis) is an important opportunistic pathogen colonizing on human skin inducing high probability of orthopedic device contamination during insertion. Costs related to vascular catheter-related bloodstream infections caused by S. Epidermidis are about $2 billion per year in US alone. Treatment with antibiotics is complicated by its capability of immune evasion, with high risk of chronic diseases.
The study considered the S. epidermis viability on these materials. Silicon nitride filled PEEK (i.e., PEEK filled with 15 vol. % α-Si3N4, β-Si3N4, and 13-SiYAlON powders) and monolithic PEEK substrate materials were used in the study. S. epidermis was cultured (1×107 CFU/ml) and then set in the samples of substrate materials in BHI Agar (1×108/ml). After 24 hours, the bacteria and samples were assessed by Microbial Viability Assay (WST) and fluorescence spectroscopy by adding DAPI and CFDA and measuring concentration through absorbance at 450 nm.
PEEK composites loaded with 15 vol. % β-Si3N4 or β-SiYAlON have shown a greatly improved antibacterial resistance as compared with monolithic PEEK. The PEEK composite with 15 vol. % α-Si3N4 did not exhibit the same degree of antibacterial behavior as the other PEEK composites. The above results go clearly beyond a simple rule-of-mixture improvement and show how a relatively low fraction of β-Si3N4 phase could at least lead to 100% improved antibacterial resistance as compared to monolithic PEEK.
The results in each of the Examples discussed above suggest that, compared to medical grade titanium and PEEK, silicon nitride results in a substantially better inhibition of in vitro bacterial colonization and bio-film formation, and results in a much lower live to dead ratio for all studied bacteria at all incubation periods. Silicon nitride also demonstrates significantly higher in vitro adsorption of three proteins which may inhibit bacteria growth and promote stem cell differentiation to osteoblasts. This preferential adsorption correlates with, and may be a causative factor in, silicon nitride's ability to decrease bacterial function. Silicon nitride also exhibits enhanced in vivo osteogenesis and osteointegration and demonstrates significant resistance to bacteria compared to monolithic titanium and PEEK.
The studies discussed in the Examples also tend to suggest that roughened silicon nitride implants generally outperform polished silicon nitride in terms of antibacterial function and/or bone growth and integration. These results suggest not only that monolithic silicon nitride implants and/or or other similar ceramic implants may be surface roughened in order to improve antibacterial function, but also that silicon nitride coatings may be applied to other implants (both silicon nitride and non-silicon nitride, such as metals, polymers, and/or other ceramics). Such coatings may be surface roughened to further improve antibacterial function and provide other desirable characteristics, as discussed above. Preliminary research also tends to indicate that increasing the surface roughness beyond the levels used in the Examples—i.e. about 1,250 nm Ra—may further increase the antibacterial function of the material. For example, in some such embodiments, the surface roughness may be greater than about 1,500 nm Ra. In some such embodiments, the surface roughness may be greater than about 2,000 nm Ra. In some such embodiments, the surface roughness may be greater than about 3,000 nm Ra. In other embodiments, the surface roughness may be between about 500 nm Ra and about 5,000 nm Ra. In some such embodiments, the surface roughness may be between about 1,500 nm Ra and about 5,000 nm Ra. In some such embodiments, the surface roughness may be between about 2,000 nm Ra and about 5,000 nm Ra. In some such embodiments, the surface roughness may be between about 3,000 nm Ra and about 5,000 nm Ra.
Some alternative ceramic materials, such as alumina and zirconia (ZrO2) for example, have certain properties that are similar to those of silicon nitride. As such, it is thought that these ceramic materials, or other similar materials, may exhibit similar antibacterial and osteogenic effects. It is thought that those of ordinary skill in the art, after having had the benefit of this disclosure, may be able to identify such alternative materials. It is also thought that these ceramic materials, or other similar materials, may exhibit improvement in antibacterial function with increased surface roughness, as is the case with silicon nitride ceramics.
Additional embodiments and implementations will be further understood by the following drawings.
For example, because PEEK, which is very common in spinal implants, performs very poorly in a bacterial environment, silicon nitride ceramic coatings or layers (or another similar material) may be applied to a PEEK spinal implant to improve the antibacterial function of the implant and/or to provide other advantages as discussed in greater detail above. The coating(s) may be applied by any suitable methodology known to those of ordinary skill in the art, such as chemical vapor deposition (CVD), physical vapor deposition (PVD), plasma spraying, electro-deposition or electrophoretic deposition, slurry coating and/or high-temperature diffusion.
To further enhance the antibacterial characteristics of the implant, the coating 220, or one or more portions of the coating 220, may be surface roughened, as illustrated in
The principles, materials, and methods described herein may also be applied to other biomedical implants. For example,
One or more coatings 320 may be applied to the femoral stem 330 of hip implant 300, as shown in
In order to further enhance the antibacterial properties of the implant 300, one or more surfaces/portions of the implant 300 may be roughened and/or textured. For example, as shown in
Still other alternative embodiments are depicted in
In other embodiments, a bone screw 500, or any of the other embodiments disclosed herein, may be comprised of another suitable material, such as titanium. In such embodiments, a silicon nitride coating may be applied to the implant rather than forming the entire implant from a silicon nitride material. As disclosed above, the coating and/or the undersurface of the coating (i.e., the surface of the original implant itself) may be roughened or textured to further improve antibacterial and other characteristics.
In still other embodiments, a bone screw 500, or any of the other embodiments disclosed herein, may comprise a biomedical material, such as a metal, ceramic, or polymer that includes a silicon nitride filler to form a composite, or that otherwise incorporate a silicon nitride material into the material used to form the implant. For example, silicon nitride may be used as a filler or otherwise incorporated into polymers, such as poly-ether-ether-ketone (PEEK), poly(methylmethacrylate), poly(ethyleneterephthalate), poly(dimethylsiloxane), poly(tetrafluoroethylene), polyethylene, and/or polyurethane. Silicon nitride may also be used as a filler or otherwise incorporated into other materials used to form other biomedical implants, such as metals, including titanium, silver, nitinol, copper, and related alloys, for example. As still another alternative, silicon nitride may be used as a filler or otherwise incorporated into other materials, such as oxide ceramics and cermets. By incorporating silicon nitride into other materials, it is expected that some of the antibacterial advantages and/or other advantageous properties described herein may be realized. Silicon nitride may also be incorporated into another materials to form a composite or used as part of one or more of the coatings described herein to increase antibacterial function.
It will be understood by those having skill in the art that changes may be made to the details of the above-described embodiments without departing from the underlying principles presented herein. For example, any suitable combination of various embodiments, or the features thereof, is contemplated.
Any methods disclosed herein comprise one or more steps or actions for performing the described method. The method steps and/or actions may be interchanged with one another. In other words, unless a specific order of steps or actions is required for proper operation of the embodiment, the order and/or use of specific steps and/or actions may be modified.
In this study, the response of KUSA-A1 mesenchymal progenitor cells to PEEK/Si3N4 composites was investigated to verify if the effects previously observed for monolithic ceramics can be observed also in polymer-matrix composites, namely upregulation of cell differentiation, inhibition of osteoclastogenesis and stimulation of the bone tissue production in vitro.
Samples were prepared as cylindrical discs with dimensions of ø12.7 mm and a thickness of 1 mm. Ti-alloy (ASTM F136-13, Ti6Al4V-ELI, Grade 23, Vincent Metals, Bloomington, Minn., USA) and PEEK (ASTM D6262, Ketron®PEEK 1000, Quadrant EPP USA, Inc., Reading, Pa., USA distributed by McMaster-Carr, Santa Fe Springs, Calif., USA) discs were cut from commercially-sourced rod stok. Si3N4 discs were fabricated and were composed of 100% β-phase grains infiltrated with a minority amorphous Si—Y—Al—O—N phase. The composite material consisted of 15% (by volume) β-Si3N4 powder, fabricated from the same spray-dried feedstock and subjected to the same thermal cycling as the monolithic Si3N4 samples, dispersed within a PEEK (KetaSpire®, Solvay Specialty Polymers USA, LLC, Alpharetta, Ga., USA) matrix. The composite was compounded and then extruded into rod stock using conventional techniques (Foster Corporation, Putnam, Conn., USA).
The surface morphology of the samples was analyzed using a confocal scanning laser microscope (Laser Microscope 3D and Profile measurements, Keyence, VKx200 Series, Osaka, Japan). All images were collected at various magnifications ranging from 10× to 150×. Roughness values were measured on 25 randomized 100×100 mm square areas.
A field-emission-gun scanning electron microscope (JSM 700 1F Scanning Electron Microscope, JEOL, Tokyo, Japan) was used to obtain high magnification images of the morphology of the different samples following KUSA-A1 cell exposure. The instrument was also equipped with an Electron Dispersive X-ray Diffraction (EDS) probe. All images were collected at an acceleration voltage of 10 kV and magnifications between 250× and 20000×. Samples were sputter-coated (Cressington, Watford, UK) with a thin (20-30 Å) platinum layer before observation.
Fourier Transformed Infra-Red Spectroscopy (FT-IR) spectra were collected using a FT-IR Spectrometer (FT/IR-4000 JASCO, Tokyo, Japan) equipped with a Michelson 28 degree interferometer (corner-cube mirrors type). The aperture size was 200×200 mm2 and the acquisition time was set to 30 seconds, for all samples. The instrument was operated using a dedicated software (Spectra Manager, JASCO, Tokyo, Japan). Signal averaging and deconvolution were performed using ORIGIN (OriginLab Corporation, Northampton, United States), and statistical analyses were obtained using “R”.
KUSA-A1 cells (JCRB, Osaka, Japan) were first cultured and incubated in medium consisting of 4.5 g/L of glucose DMEM (D-glucose, L-glutamine, phenol red, and sodium pyruvate, Nacalai tesque, Kyoto, Japan) supplemented with 10% fetal bovine serum.
The various samples were previously sterilized upon exposure to ethanol and put in the 24-well plate one by one. The cultured cells were then deposited on the samples in the well at the seeding concentration of 105 cells/well. Cells were concentrated into 50 mL of solution and mildly deposited on the samples, then incubated for an hour. 1 mL of culture medium was added to each well.
An osteogenic medium was used after 24 hours. The medium consisted of Dulbecco's modified Eagle medium (DMEM) supplemented with nominal amounts of the following constituents: 50 mg/mL of ascorbic acid, 10 mM b-glycerol phosphate, 100 mM hydrocortisone, and 10% fetal bovine serum. All samples were incubated at 37° C. up to 10 days. The medium was changed a total of three times.
The cytotoxicity of the substrates was observed and compared analyzing the samples (N=3) using colorimetric assay based on water-soluble tetrazolium (Cell counting Kit-8, Dojindo, Kumamoto, Japan) which is based on the employment of a colori-metric indicator (WST-8). This later, upon reduction in the presence of an electron mediator, produced a water-soluble formazan dye which was directly proportional to the number of living cells. The resulted solutions were analyzed using micro-plate readers (EMax, Molecular Devices, Sunnyvale, Calif., USA) upon collecting the OD value.
To mark the osteogenic differentiation, the membrane-bound enzyme ALP was detected on KUSA-A1 cells exposed to the different substrates (N=3) for 10 days in osteogenic medium. The ALP activity was monitored upon staining with the TRAP/ALP Stain Kit (Wako, Osaka, Japan) according to the manufacturer's instructions. ALP activity was assessed through direct pixel counting on optical micrographs using specific Image software (Rasband, W. S., ImageJ, National Institutes of Health, Bethesda, Md., USA).
The statistical significance of all performed experiments was checked by two-way analysis of variance (ANOVA). A p value <0.01 was considered statistically significant.
The values of WST as measured after 0 and 24 hours of in vitro testing are presented in
ALP-stained micrographs on silicon nitride, silicon nitride-reinforced PEEK, titanium alloy, and PEEK substrates are shown in
Laser microscope images obtained after 10 days indicate the presence of bone tissue formed on all samples. To correctly estimate the amount of bone tissue, the topographical and optical images were combined together to identify only the white areas (white is the natural color of hydroxyapatite) protruding from the surface. The results are marked in red in a few representative 10× images on
The specific volume of bone tissue per square micron of surface area, measured from the laser microscope images by dividing the protruding bone tissue volume by the total investigated area, is given in
ALP staining (
PEEK, even if considered a bio-inert material, was outperformed even by titanium when considering the ALP osteogenic differentiation enzyme (
Results on PEEK-Si3N4 composites, on the other hand, suggest that the differentiation activity of PEEK cannot be improved just by adding limited amounts of reinforcing ceramic, as it seems to be regulated by the polymeric matrix. This hypothesis is supported by previous literature on mesenchymal stem cells differentiation on PEEK, when compared to osteoblasts.
The limited amount of cellular differentiation observed on Si3N4-reinforced PEEK still results in a total amount of bone tissue formed per square unit of surface (
From the results of
The FTIR spectroscopic analysis of
Thus, this study confirmed that Si3N4 positively interacts with mesenchymal stem cells, resulting in the formation of higher amounts of bone tissue with better degrees of mineralization. Monolithic PEEK samples showed poor osteogenic differentiation and low amounts of bone tissue formed after 10 days of cell exposure in vitro. Silicon nitride reinforced PEEK showed higher amounts of bone tissue formed.
In this study, the surface of a PEEK monolith was characterized, wherein the PEEK monolith was modified by heat-pressing a mixture of particulate NaCl and Si3N4 particles onto the PEEK. After water dissolution of the NaCl, a porous surface was left, with embedded Si3N4 particles.
Two sets of 12.7×3 mm discs (n=12 each) were prepared from commercially available bulk PEEK used to make spinal fusion cages (ASTM D6262, Ketron®PEEK 1000, Quadrant EPP USA, Inc. Reading Pa., USA; distributed by McMaster-Carr, Santa Fe Springs, Calif., USA). Of 24 disks, eight were smooth control surfaces, and the other 16 underwent experimental treatments.
A roughened PEEK surface with coarse open pores (
The surface roughness of experimental groups was compared with a laser microscope coupled to 3-D imaging analysis software (VK-X200K series, Keyence, Osaka, Japan).
Human osteosarcoma (SaOS-2) cells were monitored with respect to cell adhesion and osteoconductivity tests. The SaOS-2 cells were cultured in an osteo-blast inducer medium consisting of 4.5 g/L glucose Dulbecco's modified Eagle medium (DMEM) (D-glucose, L-glutamine, phenol red, and sodium pyruvate) supplemented with 10% fetal bovine serum (FBS). Cell proliferation occurred on Petri dishes for 24 hours at 37° C. After achieving a final concentration of 5×105 cells/mL, the cultured cells were deposited on the PEEK surfaces. Cell seeding in osteo-genic medium (i.e. consisting of DMEM supplemented with 50 μg/mL ascorbic acid, 10×10−3 M β-glycerol phosphate, 100×10−3 M hydrocortisone, and 10% FBS) was followed by incubation for 7 days at 37° C. The medium was changed twice during the incubation period. Cell adhesion tests were repeated three times (n=3) for each sample; mean values were then plotted for comparison.
SaOS-2 cells were stained for fluorescence microscopy with Hoechst 33 342 (blue; nuclei) for 1 hour and then washed three times with 1 mL Tris Buffered Saline with Tween 20 (TBST) solution. Cell counts were performed in a fluorescence microscope (BZ-X700; Keyence, Osaka, Japan). The colorimetric assay of LDH Cytotoxicity test (LDH assay kit-WST; Dojindo, Kumamoto, Japan) was applied to quantitatively measure lactate dehydrogenase (LDH) released into the media from damaged SaOS-2 cells as a biomarker for cellular cytotoxicity and cytolysis.
For osteoconductivity testing, cells were seeded in DMEM osteogenic medium supplemented with 50 μg/mL ascorbic acid, 10×10−3 M β-glycerol phosphate, 100×10-3 M hydrocortisone, and 10% FBS. The test samples were then incubated for 7 days at 37° C. All experiments were repeated in triplicate (n=3); surfaces were examined with a Schottky-emission scanning electron microscope (SEM, Hitachi S-4300SE/N, Tokyo, Japan) at 15 kV and equipped with an energy dispersive x-ray spectrometer (EDX).
Gram-positive Staphylococcus epidermidis (S. epidermidis; 14990ATCC) were cultured at the Kyoto Prefectural University of Medicine in a brain heart infusion (BHI) agar culture medium with an initial concentration of 1.8×1010 CFU/mL. Bacterial concentration was diluted to 1×108 CFU/mL using a phosphate buffered saline solution to obtain physiological ionic strength. The bacterial suspension was then transferred in 100 μL aliquots onto Petri dishes containing the substrate samples embedded in BHI medium. An incubation time of 24 hours at 37° C. was set under aerobic conditions followed by biological testing.
Bacterial metabolic activity was identified with the WST colorimetric assay (Microbial Viability Assay Kit-WST, Dojindo, Kumamoto, Japan), which employs the WST-8 indicator producing a water-soluble formazan dye upon reduction mediated by electrons. The amount of the formazan dye correlates with the number of live microorganisms. Solutions were analyzed using microplate readers (EMax, Molecular Devices, Sunnyvale, Calif., USA) upon collecting optical density values.
Fourier transform infrared spectroscopy (FTIR) spectra were collected in time-lapse fashion after the osteoconductive tests. FTIR spectra were collected by means a high sensitivity spectroscope (Spectrum100FT-IR/Spotlight400; PerkinElmer Inc. Waltham, Mass., USA). The spectral resolution of this equipment was 0.4 cm−1. Average FTIR spectra targeting both bony hydroxyapatite and collagen grown by the SaOS-2 cells on the surface of different substrates were computed for each substrate from five independent measurements performed on n=4 samples. Pre-processing of raw data included baseline subtraction, smoothing, normalization, and fitting of the raw spectra using commercially available software (Origin 8.5, OriginLab Co., Northampton, Mass., USA).
Experimental data were analyzed with respect to their statistical meaning by computing their mean value±one standard deviation and using one-way ANOVA for surface topography and two-way ANOVA with Tukey's post hoc analysis for biological assays; p≤0.01 was considered statistically significant and highlighted with two asterisks.
The results of topographic characterizations of the PEEK surfaces are shown in
The presence of β-Si3N4 particles in PEEK was confirmed by FTIR spectroscopy.
The LDH plot was normalized to a control value obtained on SaOS-2 cells freely proliferating in substrate-free environment. The LDH value ˜1, for PEEK functionalized with NaCl—Si3N4 indicates a slight increase in substrate toxicity. In contrast, LDH values <1 were found for both smooth and NaCl-roughened PEEK with no statistical difference between these groups (labeled as n.s.). These data, and cell counts suggest that SaOS-2 cells were hardly stimulated to proliferate on PEEK surfaces alone, regardless of surface roughness. On the other hand, the higher amount of LDH detected on samples containing the NaCl—Si3N4 mixture is likely related to the higher amount of cells as compared to the samples functionalized with NaCl only.
Higher amounts of bony apatite were found for NaCl-roughened PEEK, while the open pores in the NaCl—Si3N4 PEEK were completely filled with bony apatite (cf
A comparison can also be made using EDX elemental data giving the Ca/P mass ratio, the latter data being shown in
The bacterial proliferation experiments were repeated after preliminary autoclaving both surface-treated porous PEEK samples for 1 hour at 121° C. The CFU count was then carried out in order to clarify the nature of the improved bacteriostasis of both the porous PEEK samples versus the smooth PEEK (
Tables 1 and 2 list, according to EDX analyses, the atomic fractions of the elements found on the NaCl-PEEK and NaCl—Si3N4 PEEK surfaces, respectively, before and after autoclaving. The results suggest that the as-prepared functionalized PEEK surfaces contained a non-negligible fraction of residual CI, which was removed upon autoclaving. Based on the data in
Osteointegration was less in NaCl-PEEK in the present study, probably because residual CI discouraged cell proliferation (cf
Similarly, it should be appreciated that in the above description of embodiments, various features are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure. This method of disclosure, however, is not to be interpreted as reflecting an intention that any claim require more features than those expressly recited in that claim. Rather, inventive aspects lie in a combination of fewer than all features of any single foregoing disclosed embodiment. It will be apparent to those having skill in the art that changes may be made to the details of the above-described embodiments without departing from the underlying principles set forth herein. The scope of the present invention should, therefore, be determined only by the following claims.
This application is a Continuation-In-Part to U.S. patent application Ser. No. 17/029,534 filed Sep. 23, 2020, which is a continuation application of Ser. No. 15/470,637 filed Mar. 27, 2017 that is a Continuation-In-Part to U.S. Non-Provisional application Ser. No. 13/890,876, filed on May 9, 2013, which claims benefit to U.S. Provisional Patent Application No. 61/644,906 filed May 9, 2012, which are herein incorporated herein by reference in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61644906 | May 2012 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 15470637 | Mar 2017 | US |
| Child | 17029534 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 17029534 | Sep 2020 | US |
| Child | 17384236 | US | |
| Parent | 13890876 | May 2013 | US |
| Child | 15470637 | US |
