US 20060285991 A1
An improved method for manufacturing medical implants, and particularly spinal implants, utilizing a metal injection moulding technique (MIM) is provided. The invention is generally directed to the manufacture of implants for complete insertion within the body of a patient. A special set of mechanical, physiological and legal requirements are associated with such medical implants. For example, in contrast with dental implants, such medical implants are not readily observable or removable meaning that they must be significantly more physically resilient. In addition, physiologically, such medical implants must be capable of full integration with the body. Finally, regulatory requirements provide for significantly stricter controls over such fully implanted medical devices.
1. A method for forming a near-net shape medical implant for complete insertion into a patient, comprising the steps of:
mixing titanium powder and an organic binder to form an injection moulding compound;
plasticizing the injection moulding compound by kneading the compound to form an injection moulding feedstock;
moulding the injection moulding feedstock to form a medical implant body, the medical implant body having a weight of no more than 35 kg and a wall thickness of no more than 12 mm,
removing binder from the medical implant body by heating; and
sintering the medical implant body to produce a near-net medical implant wherein the density of the near-net medical implant is within 95% of the theoretical density of the original titanium material.
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The present invention is concerned with a method of manufacture medical implants. It is especially concerned with a method and material suited for some spinal implants.
The traditional process for producing metal objects is to make a casting or machine the piece from a solid block of material. There are three methods: the sand casting method, the lost wax method, and machining. The sand casting method dates back to antiquity and is done by making a model, imprinting the model on two blocks of sand, removing the model, and then pouring the molten metal in between the two sand shapes. When the model has cooled, it is taken from the mould and it is then deburred, polished, and finished. The lost-wax method uses a wax replica of the item to be constructed, and into which the molten metal is poured. This method is more accurate because when the molten metal is introduced into the wax replica, the wax liquefies and is pushed away from the mould and replaced by the molten metal, but because the wax occupies a finite space in the middle of the sand mould creating a more accurate reproduction. Finally, machining requires that a piece be machined from a solid piece of metal stock.
However, in both processes fine details must be either cut or etched onto the surface of the item, and other pieces must be added by braising, welding, or annealing. Moreover, both methods inherently lead to inaccuracies. In the sand mould method, the very fact that sand is used leads to many rough features that require a lot of extensive secondary steps to correct. Meanwhile, in the lost-wax method the differential cooling of the poured metal model leads to shrinking and deformity of the product. In addition, the final product still must undergo deburring, polishing and finishing before use. Finally, both the sand mould and lost-wax methods are inherently expensive because of the large number of individual machining steps necessary and the lack of uniformity in the case pieces.
Medical parts for implantation have traditionally been produced by one of these two casting methods, or by machining the piece from a solid block of metal. However, implantation parts must withstand high strains over a very long life, without the benefit of regular inspection or the hope of easy replacement, and parts made from these casting methods are subject to breakage during use due to defective casting, especially due to the formation of casting pockets or metallurgical impurities, such as sulfide residues in stainless steel casting. In addition, the creation of complicated shapes is difficult in casting due to the problem of running liquid. For example, the shape of a screw or cage component for an implant, is too complicated to be produced exactly as designed.
On the other hand, the additional mechanical processing required by these casting methods, such as cutting and deburring, cause difficulties in processing materials with a high level of hardness, especially titanium materials that have poor machine processability, resulting in products with poor precision. For example, the screw component of an implant must usually be mechanically fabricated because it cannot be produced satisfactorily by casting. Poor precision in the screw component can allow cause a number of mechanical deficiencies and subsequent medical problems.
A solution to the problems associated with both of these traditional casting techniques is metal injection moulding (MIM), which has developed over the last few years as a logical extension of injection moulding of plastics.
In MIM, metal powder is loaded with a polymeric binding, which is removed after moulding. Sintering of the final product occurs at high temperatures and pressures in the mould. Although shrinkage of the final part does occur due to the affects of surface tension, this reaction can be controlled by the environment in the mould, the chemical available, the temperature, and the pressure. Because of this control the surface conditions and the finish of the surface of the product can be precisely arranged. In addition, the outline and accuracy of the moulded part is also more precise. Finally, because the moulding of the individual pieces is more precise, far fewer secondary processing steps are required to complete a finished product.
However, there are some technical limitations in the metal injection moulding field. The technology is very applicable to shapes of high complexity, but the complex shapes are limited in weight to approximately 30 to 35 g, and in wall thickness of up to 10 mm, limiting its adoption in a number of specific fields.
Accordingly, there is a need for an improved manufacturing process for the production of medical implants that are of high metallurgical quality and are cost effective to produce.
The present invention is directed to a method of producing medical implants of elaborate and complicated shapes, such as spinal interbody fusion cages by metal injection moulding.
In one embodiment of the invention, the medical implant is made into a near-net perfect shape, not requiring further machining, by the metal injection moulding method of the current invention.
In another embodiment of the invention, surface features are imprinted onto the surface of the medical implant during the metal injection moulding process.
In still another embodiment of the invention, the medical implants are made of metallic, plastic, or polymeric materials, including titanium-based materials.
In yet another embodiment of the invention, the medical implant has a theoretical density of 95% of the base metal after processing.
In still yet another embodiment of the invention, the medical implant has a wall thickness of up to 12 mm, and preferably of up to 5 mm.
In still yet another embodiment of the invention, the method of manufacture is directed to the production of spinal intervertebral body cages or vertebral alignment screws or their constituent parts, and other spinal implants via a metal injection moulding process.
The invention will be described in conjunction with the following drawings in which like reference numerals designate like elements and wherein:
The current invention is directed to an improved method for manufacturing medical implants, and particularly spinal implants, utilizing a metal injection moulding technique. Although MIM processes have been previously used to manufacture titanium dental implants, a MIM process has never been used to produce parts for full implantation into the body. In this application such fully incorporated implants will be referred to as “medical or surgical implants.” A special set of mechanical, physiological and legal requirements are associated with such medical implants. For example, in contrast with dental implants, such medical implants are not readily observable or removable meaning that they must be significantly more physically resilient and mechanically perfect. In addition, physiologically, such medical implants must be capable of full integration with the body. Finally, the Food and Drug Administration has significantly more stringent controls over such fully implanted medical devices, including 510 k metallurgical testings that are not required for partial implants, such as those used in dental applications.
Albrektsson, et al., in 1981, demonstrated that the prerequisites for proper integration of an implant into the body depend on the following parameters:
For example, historically, implants have been inserted in the body with bone cement, or are completely surrounded by a thin layer of connective tissue. Regardless, living bone tissue is not present at the implant interface in either cemented or non-cemented implants. Instead, the interface layer of conventional medical implants consists of soft tissues regarded by many authorities, i.e. Cook (1967), Southham, et al. (1970), Linder and Lundskog (1975), as less desirable because it results eventually in the loss of fixation of the implant. Indeed, in 1954 Collins stated that although histologically inert, an implanted object never becomes incorporated in the bone. Meanwhile, Southham, et al. in 1970, concluded that when any metallic appliance is implanted in bone, a layer of fibrous tissue will always develop around the appliance, and subsequently, it will never be as secure in the bone as it was at the time of the implantation.
In 1965, a new method of implantation called osseointegration was initiated. Osseointegration comprises a direct, on a light microscope level, contact between living bone and an implant There are many authors who believe that direct contact between the implant and the bone is possible only if the implant is a ceramic, i.e,. Jacobs (1976-1977), Muster, et al. (1978). They report that direct bone to implant contact was achievable only in cases where no metal is in direct contact with the bone. Accordingly, osseointegration was thought to be possible only with ceramic implants or coated metal implants. However, beginning in 1975, there were a growing number of reports that osseointegration could occur with various types of specific metals. The first of the metals reported was titanium, by Branemark in 1969, Linder and Lundskog (1975), Karagianes (1976), Schroeder (1976), Juillerat and Kuffer (1977). However, almost immediately it was found that when exposed to air titanium almost instantaneously becomes coated with an oxide layer, which is a combination of hydrogen and oxygen molecules available in the environment, TiO, TiO2 and Ti. The stable oxide coating is approximately 100 Å (angstrom thickness) and prevents direct contact between the bone and the metal. These discoveries show that titanium as an implant material may be regarded as a ceramic and not as a metal. Indeed, Laing, et al., in 1967, and Brettle concluded that titanium is the most inert metal so far used for implant fabrication, and Solar, et al., in 1979, presented in vitro study of titanium in ringer solution at 37° C. with different measures being taken to imitate in vivo conditions and demonstrated that titanium should tolerate exposure to physiological chloride solutions at body temperature for infinite time with no corrosion. In fact. titanium is the only metal that has been shown to establish a direct bone to implant contact and maintain such connection in man for periods of greater than ten years. There are no other studies that show any osseointegration of greater than five years with any other metallic implant.
Titanium alloy matrix composites have also been developed for specific uses. Titanium carbide and titanium diboride are particulate reinforced titanium alloys. These are now found in commercial applications, particularly in the military and aerospace environment. These monolithic matrix alloys have high yield and tensalic strengths, particularly above 800° F. This strength can be extended to temperatures up to 1,200° C. In the United Kingdom, research and technology has permitted the development of aircraft bolts from titanium metal matrix composites with weight savings of greater than 30% over stainless steel. This composite material combines titanium and silicone carbide. Silicone carbide reinforced titanium composites are now being developed for hypersonic flight by the National Aerospace plane. The manufacturing process begins with the production of a continuous silicone carbide fiber, and then this fiber is mixed with titanium foil. The composite is built up directly by a lay up process. It is believed that these composites may prove suitable for medical implants as well.
In short, titanium is a superior metallic implant material. It permits osseointegration and it is radio-penetrable. In addition, it is does not produce a magnetic field distortion in MRI and imaging technology. In short, to guarantee osseointegration, non-alloyed titanium or titanium composite implants materials should be used.
Implant finish is also an important parameter. Baumhammers, et al., in 1971, compared smooth and sandblasted material, and found the largest surface area of the latter to be beneficial for connective tissue cell attachment. He felt that smooth surfaces should be avoided. This led to the development recently in Canada of titanium foams designed to produce an external crystalline lattice that is not unlike bone. There have been numerous techniques to treat an implant surface to experimentally increase bone bonding capacity. These include iodization or collagenization of the implant surface. There is a technique of coating a metal substrate with a thin layer of glass described by Busceni in 1976. Likewise, other coatings, such as, tantalum, when coated on dental implants to a thickness of 0.05 mm have been found to stiffen the implant by 30%.
Although it is well-known that the material, surface finish, and construction of the implant are directly tied to the bodies ability to effectively integrate the implant, current manufacturing techniques are not well-designed for the production of such implants. As discussed, conventional casting techniques require significant post production processing, which increases the potential oxidation of titanium, alters the physical properties of the material, and makes the production of delicate surface finishes imprecise, time-consuming and expensive. Accordingly, the current invention is directed to the manufacture of medical implants using a metal injection moulding process.
The process of metal injection moulding according to the current invention involves four basic stages:
Prior to these steps, however, the injection mould compound must be formed. This is accomplished by mixing titanium powder and an organic binder. The titanium powder preferably is pure titanium powder produced by a gas-atomizing method. The titanium powder usually comprises carbon and oxygen. The carbon content and the oxygen content of the material have a great effect on the ductility of the material, which will be explained further hereafter. Therefore, the carbon content and the oxygen content of the titanium powder preferably are controlled within certain limits.
In the process to be explained in detail below, some amount of carbon occasionally enters the sintered body from the residue resin during the time of sintering. However, the amount of carbon in the sintered body to be formed during the final stage of the process can be made nearly equal to the amount of carbon contained in the titanium powder. On the other hand, oxygen may enter the sintered body during each of the heating steps explained below. Thus, the oxygen content of the sintered body is determined by adding the amount of entering oxygen to the oxygen contained in the titanium powder. However, the amount of oxygen can be reduced by preventing oxygen from entering into the sintered body during the production process. With the establishment of a production process in which the amount of oxygen entering the body is relatively small, the amount of oxygen in the final sintered body can be sufficiently low if the amount of oxygen initially contained in the original titanium powder is small.
For the organic binder, a binder system having at least two binder components is used. Preferably, a substance whose main components are thermoplastic resin and wax is used with the addition of a plastic agent, a lubricant, a binder removal promotion agent, a mould release agent, and a surfactant, as needed. The amount of organic binder should be determined properly according to the material, but, in general, it is 9%+−0.1% by weight.
In the plastification stage, the feedstock comprising the pre-alloyed metal powders or blended elemental metal powders is mixed with the binder or plasticizer. The binder material usually comprises some 30 to 50% by volume of the feedstock. As discussed above, there are different types of binders in production usage in metal injection moulding. One variant is the Wiech type, which involves the use of waxes and thermoplastic materials. The other, which is older than the Wiech system, was developed by Rivers and utilizes water soluble metal cellulose. Any of these or other suitable binders may be used in the current invention. Once this metal/binder mixture is formed, the mixture goes through a plasticization process involving the bending and kneading of the material. The mixture is then granulated in an extruder to provide the feedstock for moulding.
Next, injection moulding is performed on the compound with an injection moulding machine. Metal moulds for moulding are manufactured to form the desired implant. Although moulding of the feedstock takes place in a standard plastic injection moulding machine. However, in metal injection moulding, closer control of the injection parameters, such as the pressure and temperature of the feedstock barrel, the runners, the gates, and the mould is required. Depending on the type of metal injection moulding system, the feedstock is either maintained at a barrel temperature of 160° C., or at a colder 10° C. in the Rivers system, where one is trying to prevent gelling of the metal cellulose binder. In either system, the goal is to maintain a moulding process time of about twenty seconds per item during which the material is moulded into the desired shape.
Once the object is moulded the binder has to be removed and the residual metal powder sintered. The moulded bodies thus created for the respective purposes are sent to a decompression heating furnace where binder is removed from the bodies. During the binder removal process, heat is applied within a non-oxidation atmosphere or a decompression (i.e., vacuum) atmosphere or both to remove binder. If at least two of the binder components have different melting temperatures, the binder may be removed by raising the temperature of the moulded body above the intermediate temperature at which the binder system flows, which is between the melting point of the various components. Binder will exude from the moulded body wherefrom it can be removed. Since only the binder with the lowest melting temperature is initially removed, the intermediate temperature at which the binder system flows will increase. Therefore, it may be necessary to continually increase the temperature ensuring that it remains below the temperature of the highest melting point of a binder component. Alternatively, a specific solvent (liquid or gas) may be used to remove only a specific component in the binder. For example, in the Wiech system, the waxes and thermoplastic materials require lengthy removal procedures including heating and vaporization of the binder materials. In contrast, in the Rivers system, heating is adequate to complete debinding.
Next, sintering takes place in the vacuum heating furnace. During the sintering process, a non-oxidation atmosphere or a vacuum atmosphere or both can be used. Sintering of the debound parts in a protective atmosphere or vacuum is the final stage of a metal injection moulding process. Because of the fine powders employed in metal injection moulding, significant densification and shrinkage occurs during sintering. For example, the sintered density of metal prototypes are within 95 to 100% of the theoretical density of the metal. As a result of this densification, significant improvements in mechanical properties can be achieved. During the sintering process surface tension physics causes a linear shrinkage in the moulded part in the range of from about 10 to 20%.
In some materials, where the powdered metal is in an oxide form, it is also necessary to run a purge gas to remove these oxygen molecules to produce maximal densification in the sintering process. For example, in titanium different techniques have to be adopted to remove oxygen molecules, which are bound covalently to the metal crystalline lattice structure. For example, oxygen can be driven out of the moulded piece by controlling and regulating different parameters of the metal injection moulding process, including the application of negative pressure and careful control of the temperature of the process. Although the specifics of many of these processes are “trade secrets” that are carefully guarded by the technicians who have developed them, it should be understood that any such process could be used with the MIM process of the current invention. Generally, the oxidation potential can be controlled by purging the mouldings with inert gases, preventing oxidation of the mouldings prior to final formation after shrinkage.
The medical implant material comprising titanium sintered bodies thus created reduce the problems of defective casting and running liquid of the traditional casting method. Hence the delicate and complicated shapes can be easily realized. Furthermore, the desired strength is obtained by providing the binder removal and sintering processes. For example, the formation of fine surface features is not possible with the conventional casting method, which can at best provide gross surface structures. To create fine surface features, conventionally, a further cutting process or bonding to the unit is required after the casting or cutting process. Further, even if such a process is possible, the processing cost becomes extremely high.
As previously discussed, the medical implant materials mentioned above comprising titanium sintered bodies are used in the body to secure the spine, or serve some other load bearing function, requiring a certain level of strength (rigidity). Therefore, hardness and ductility (elongation) need to be controlled during the manufacturing process. The quality of sintered bodies in preferred embodiments of the present invention is determined mainly by the level of ductility obtained in the body. The ductility of the titanium sintered body is determined by carbon content, oxygen content, and the density of the final sintered bodies. Small differences in the amount of carbon and oxygen in the titanium sintered body can change the hardness and ductility of the sintered body. When the amount of carbon increases, TiC (titanium carbide) is produced and ductility declines and hardness increases. Moreover, when the amount of oxygen increases, ductility declines, and hardness increases, due to a rise in the amount of solid solution hardening. The ductility of the material is determined by the amount of elongation tensile that results when subjected to a elongation test. The elongation of materials used for medical implant materials, should be around 2%, preferably 4% or more. If the elongation is below 2%, breakage or chipping occurs during treatment. Therefore, the carbon content and the oxygen content of the raw titanium powder preferably should be held to no more than 0.3% and 0.6% by weight, respectively. Preferably, the total amount of carbon and oxygen should be no more than about 0.5% by weight in the Ti powder. On the other hand, the carbon content and the oxygen content of the titanium sintered body preferably should be held to no more than 0.5% and 0.8% by weight, respectively, and the combined carbon and oxygen content of the sintered body preferably should be held to no more than about 1.0% by weight.
The quality of the medical implant material also depends on the granule size. Specifically, the density of the sintered body falls sharply when the granule diameter exceeds 40 μm. Therefore, the average granule diameter of the powder preferably should be no more than 40 μm in order to obtain a sintered body with a density of 95% or higher. A decline in density causes a sharp expansion in the cavity components due to poor coupling within the sintered body, resulting in a notch effect and in decreased ductility. As explained above, both the carbon content and the oxygen content affect the ductility of the material, and it appears that the ductility is increased by reducing the total carbon and oxygen content.
Accordingly, the present invention allows for the formation of a medical implant with high rigidity, when the carbon content and the oxygen content of the titanium powder are maintained between about 0.05% and about 0.1% by weight and at about 0.1% by weight, respectively, and when the carbon content and the oxygen content of the titanium sintered body are controlled at about 0.15% and about 0.3% by weight, respectively. Preferably, the combined carbon and oxygen of the titanium powder is no more than about 0.5% by weight and the combined carbon and oxygen content of the sintered body is no more than about 1.0% by weight. The above general description of the process will be described in detail hereafter with regard to the following example.
In one embodiment of the invention, titanium powders may be used to form the medical implants. As the organic binder, PBMA (polybutylmethacryrate), EVA (ethylene vinyl acetic acid copolymer), wax, and DBP (dibutylphtarate) may be used. In such an embodiment, the organic binder is mixed and kneaded with the titanium powder using a pressure kneader for 30 minutes at 130° C. to produce an injection moulding compound. Injection moulding is performed on the compound with an injection moulding machine to form a medical implant, having a near-net shape. The conditions for moulding are as follows: moulding temperatures—150° C.; moulding pressure—1,000 kgf/cm2; and metal mould temperatures—20° C.
The moulded bodies are then sent to a decompression heating furnace where the binder is removed. In order to remove the binder from the moulded bodies, the pressure is set at 0.1 torr with a maximum heating temperature of 400° C., the maintenance time being one hour.
Next, sintering takes place in the vacuum heating furnace. Pressure is set at 0.1 to 0.0001 torr with a maximum heating temperature of 1250° C., the maintenance time being three hours to obtain a titanium sintered body. The titanium sintered bodies have a Vickers hardness of 180 to 220 Hv, have sufficient anti-wear characteristics, and have enough hardness to secure a smooth contact with wire.
Density measurement, strength testing, bonding strength testing, and elution testing may be conducted by placing the implant on a steel sheet, and by forcing displacement and deformation from above to determine the existence of breakage.
Using the above method, a medical implant having delicate and complex shapes, which are impossible to produce using conventional casting methods and mechanical processing methods, can be formed with a high level of precision by first injection moulding a compound formed by a mixture of titanium powder and an organic binder, and then by forming the titanium sintered body through a binder removal process and a sintering process. Moreover, the titanium sintered body has sufficient hardness and ductility to insure the required strength for a structural weight-bearing implant, thus realizing a material with higher capabilities than conventional products.
A number of advantages result from the use of MIM in the manufacture of such titanium medical implants. First, using MIM a net or near net-shaped metal and ceramic component with complex geometry, high precision, and superior surface finish can be produced. In such an embodiment, metal injection moulding technology provides the capability to produce or mould a complete part without the need for additional operations, such as machining, reaming, honing, polishing, and straightening. The ability to provide a net shape or near net shape is a significant advantage in producing the complex shapes of most medical implants. In addition, the precision injection moulding according to the current invention provides unique and impressive dimensional accuracy part after part with careful material and processing techniques. Dimensional precision of plus or minus 0.5% is common, and by carefully controlling the MIM process this can be increased to as high as plus or minus 0.05%. Finally, use of MIM in the manufacture of medical implants can reduce manufacturing costs by from between 50 and 80%.
Injection moulding components can also be produced with almost any type of surface finish. For example, in one embodiment, the MIM process of the current invention is used to provide a surface texture suitable to ensure proper incorporation of the implant into the body without further processing. In contrast, conventional manufacturing techniques are too imprecise to accurately render the very small surface features required to ensure proper incorporation of the implant within the body.
The injection moulding process of the current invention may also be used to produce specialized metal alloys that are superior to conventional materials. The injection moulding process of the current invention may also be used to customize the material properties of the implants in order to achieve unique attributes not available with other conventional manufacturing methods. For example, in one embodiment of the invention the sintering step is performed at temperatures that are nearly the same as the melting point of the metal. In such a process, the metal particles fuse together due to the physics of surface tension, the binder compounds are extracted, the metal particles establish their initial bond, the internal porosity of the parts begin to reduce, and the powder particles are literally collapsed into themselves. Externally the injection moulded component shrinks in size. The temperature of the sintering cycle is then increased to nearly the melting point of the metal and then cooled. Classic crystallization of the metal alloy occurs during this final phase of the sintering cycle, resulting in a moulded piece with a crystalline uniformity impossible to achieve with conventional casting processes.
For example, in conventional casting technologies, such as machining from a solid block of metal, a crystalline gradient is often produced in the metal. These differentials occur because of alterations to the crystalline structure of the metal near where the metal was cut. This in turn can lead to variations in the mechanical properties of the moulded pieces. In addition, the heating that occurs in the adjacent portions of the crystalline lattice to the machine surface alters the overall consistency of the machined piece, and multiple machinings, which are common, can produce a complex product with a greatly distorted crystalline structure.
In summary, metal injection moulding technology produces net-shape parts, which have theoretical densities of greater than 95%, leading to moulded parts with strength and modulus values equal to and greater than wrought metal. Accordingly, parts manufactured in accordance with the MIM process of the current invention have mechanical properties that are equal to or surpass those made using other metal forming processes, which include investment casting, forging, machining, and welding. Metal injection moulding of titanium through special techniques can also produce implants whose mechanical properties surpass those of other metal foreign processes, including investment casting, forging, and machining. In this technology it is possible to get rid of the titanium oxide coating. Multiple surface treatments can also be performed, which improve fixation and, therefore, intrinsic stability of the implant.
Although the above-discussion has focused on the MIM process, the current invention is also directed to orthopaedic devices suitable for the inventive metal injection moulding process. For example, although the technology is very applicable to shapes of high complexity, these complex shapes are limited in weight to approximately 30 to 35 g, and in wall thickness of up to 12 mm, and preferably of up to 5 mm. Accordingly, the MIM technique of the current invention may be used for the fabrication of, for example, many pieces of a vertebral alignment screw including the universal coupling mechanisms, the internal saddles, and the cap nut. In addition, intervertebral disc cages would be particularly suitable for manufacture by the inventive metal injection moulding technology. Similarly, total hips, total knees, acetabular femoral tibial components, plates for prosthetic replacement of cranial deficiencies, etc. will all be well executed by this metal injection moulding technique at decreased expense.
While this invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention as set forth herein are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention as defined in the following claims.