Jerry Fireman10.01.08
Hip replacement has been described as the greatest achievement in orthopedic surgery in the 20th century. More than 200,000 total hip replacements are performed each year in the United States, and the total number of full and partial hip, knee and shoulder replacements is estimated at about 1 million per year. Yet no surgery is without risk, as evidenced by the fact that approximately 35,000 hip replacement revisions, or operations to replace or correct existing hip replacements, are required each year.
Orthopedic surgeons, healthcare providers and implant manufacturers are working hard to avoid negative patient outcomes-many of which may be related to the performance of the implant device itself, such as breakage, wear and separation of the implant from the bone. They are making increasing demands for a wide range of improvements on the part of companies that use casting, forging and machining to make implant components. Investment casting is the leading method of producing implant components, so foundries naturally are in the forefront of developments in this field.
Most typically, the casting is made of a cobalt-based alloy such as cobalt-chrome-molybdenum (CoCrMo) because of its excellent mechanical properties, such as high strength and low friction. On the other hand, cobalt-based alloys are not tissue friendly, so bone integration is relatively difficult to achieve. For this reason, it is common to coat cobalt alloy implants with a more tissue-friendly material such as titanium, tantalum, niobium or an alloy consisting of some or all of these materials.
One of the most significant causes of implant failure is the separation of the implant from the bone. Implant manufacturers long have used various methods to impart a textured surface with the goal of promoting the growth of the patient's own natural bone into pores within the surface. The traditional approach to creating such a porous surface involves the metallurgical bonding of a layer of small metal beads or metal mesh to the implant body by plasma spraying or sintering. In the most common process, plasma spraying, the material to be deposited is introduced as a powder into a jet emanating from a plasma torch with a temperature on the order of 10,000 degrees Kelvin. The material is melted and propelled toward a substrate, where the molten droplets solidify and form a deposit. This approach makes it possible to enable the body and surface of the implant to be made from different materials-each of which is optimized for its own special role. On the other hand, it introduces the risk that beads might separate from the implant during the implant procedure and find their way into the articulating surface of the joint, which may lead to premature failure and the need for revision surgery.
Materials suppliers are working hard to tune the properties of the materials that are applied to the surface to improve the bone-integration performance of the implant.
The most critical issue in the past year in this area is more closely matching the properties of bone-a process called bone shielding-according to Dr. Colin McCracken, development manager-Powder Products for Robesonia, PA-based Reading Alloys Inc., an AMETEK company that supplies powdered Ti-6Al-4V alloyused in producing implants, particularly to apply surface textures to castings and forgings.
"We are seeing an increasing demand for powder for use in plasma spraying a surface texture onto cast and forged hip implants," McCracken said. "Implant manufacturers are asking us to more finely tune the properties of our materials in order to provide better implant performance."
When the bone and the surface of the implant to which it has adhered have similar properties, the materials are more likely to deflect together under the application of stress, so loosening is much less likely to occur. The most popular powder for application to the surface of an implant today is Ti-6%Al-4%V, an alloy consisting of 90% titanium, 56% aluminum and 4% vanadium.
The challenge in improving the mechanical properties of Ti-6%Al-4%V is somewhat unique in that in most cases, materials suppliers are asked to improve the mechanical properties of their product, while in this case, they are being asked to reduce the mechanical properties to more closely match those of the bone. What is critical here is reducing the impurities in the powder, particularly oxygen, down to very low levels. This is because increasing the level of oxygen in the powder increases its strength.
"The standard Ti-6Al-4V alloy oxygen levels should be held to 2000 ppm [parts per million] or less," McCracken said. "Through a major effort to tighten up our production process we have developed the ability to produce extra-low interstitial [ELI] powders in the 1,000- to 1,300-ppm range. These powders much more closely match the mechanical properties of bone than the standard material. This product was first introduced on the market two years ago and has rapidly been adopted by many orthopedic implant manufacturers and their suppliers. We are already supplying this low oxygen product in volumes of tens of thousands of pounds per year."
In addition to improving the surface properties of castings, the ELI variant of Ti-6%Al-4%V also has aided a competitive process. The low-oxygen alloy is being used on a small scale to bypass the casting process altogether by producing the entire implant using powder metallurgical methods. The low-oxygen material is important in powder metallurgy because the oxygen content of the powder tends to increase during the sintering process. "By starting with ELI powders, it is possible to hold the finished product to a 2000-ppm oxygen level, which is acceptable for most applications," McCracken said.
A newer approach for applying a surface texture to an investment cast part has taken hold in the past decade or so. It involves applying a textured surface to the wax pattern used to produce the ceramic shell that serves as the mold in the investment casting process. The textured surface normally is created by fixing beads to the wax pattern prior to forming the ceramic shell. The beads are made out of a material that melts along with the wax pattern when metal is poured into the ceramic shell.
By integrating the textured surface with the casting, this approach eliminates a potential point of failure where the beads are bonded to the investment casting. Because of this advantage, this method has become widely used in producing investment castings for orthopedic implants in recent years.
But Dave Beighton, vice president and general manager of Sandvik Medtech Medical Solutions Division, pointed out what he sees as difficulties in this approach. "Surgeons prefer a randomized irregular surface, and the current method of applying beads to the waxed surface does achieve this goal," he said. "However, the difficulty in this approach is that it's impossible to control the exact way in which the beads are distributed over the wax pattern, so every casting is slightly different."
There's little doubt that the precise surface texture that is applied to the implant can have a significant impact on its ability to resist loosening. Surgeons have expressed the desire to evaluate the performance of different surface textures in terms of their ability to promote bone growth. Then, after they have selected an effective texture, they want their suppliers to be able to consistently reproduce this texture time and again.
Beighton said that a new investment casting method his company has developed very recently provides the ability to apply a reproducible textured surface. The key difference in the new method is that wax pattern is produced with an integral textured surface. Beighton said that the company can consistently reproduce an assortment of beaded surface textures time and time again with this new method. He said further that the company is able to produce wax patterns with integral textured surfaces on a production basis so that the throughput of the investment castings produced by this method is substantially improved over the previous approach of applying beads to the wax pattern.
Beighton refused to reveal the details of the process that the company uses to produce wax patterns with an integral textured surface.
"We believe that we have a significant jump on our competition and we hope to preserve it as long as possible," he said. "The technology has moved from the laboratory to the production floor and we are now producing investment castings using this technology."
It's also important to note that the previous method generally is limited to applying a textured surface to external surfaces, because it is so hard to apply beads to internal surfaces, such as the internal profile of the femoral knee. But the new method can apply the surface texture to both internal and external surfaces, which opens up an important new avenue to promote bone integration for more stable implants.
While the most-rapid recent advances have come in the surface texture area, changes also have been percolating in the body of the casting.
The most common hip implant consists of a stem topped by a ball component that is implanted on the top of the femur to replace the degenerated femoral head that surgeons have removed. The ball fits into a socket or cup that is attached to the hip bone.
Over the years, a metal ball and an ultra-high-molecular-weight polyethylene (UHMWPE) cup design has emerged as the preferred design. The main weakness of this approach is that UHMWPE cups typically last for only 10 to 15 years, depending on the age and activity level of the patient. This device lifespan is too short for younger, more active patients who probably would require one or more revision procedures to replace a failed cup.
As a result, interest in metal-on-metal joints has been increasing during the past few years. An earlier generation of metal-on-metal joints had problems with dislocation and cup deformation. But newer designs have a more precise fit that seems to have solved the cup-loosening problem. Likewise, significant improvements in casting processes have helped to address the cup deformation problem. The result has been a significant trend toward the use of metal-on-metal implants with the cups made of CoCrMo investment castings.
Crispin Carney, engineering director for Warsaw, IN-based Symmetry Medical's facility in Sheffield, England, said his company has been working closely with its customers to tailor the properties of CoCrMo investment castings to the requirements of metal-on-metal hip replacement parts.
"When you have two materials as different as metal and polyethylene wearing against each other, one element tends to wear more rapidly than the other," he explained. "On the other hand, two components made of the same material tend to wear at uniform rates, resulting in longer life and less debris released during the life of the implant."
The move toward metal-on-metal implants has focused new attention on the microstructure of the components, with a particular regard to eliminating or minimizing debris.
"Early metal-on-metal implants performed variably, and empirical assessment of high-performing variants suggested a relationship between the microstructure of the casting and longevity," Carney said. "There are several schools of thought on the ideal micro-structure for these types of components in terms of grain size and carbide morphology. Since then, there have been several streams of development to harness the best performing type, which remains inconclusive."
Carney noted that Symmetry could not offer any opinion as to which micro-structure is ideal but said the company is consistently able to provide a wider range of micro-structures by controlling aspects of the casting process.
"Symmetry in the UK has been working on tailoring the micro-structure to suit customers' requirements," Carney said. "This is achieved by adjustments to shell chemistry and construction, alloy chemistry and solidification control."
For example, the surface coating of the shell contains an inoculant that promotes a finer grain structure. The foundry can remove that if the customer requests a coarser grain. The rate of cooling also has a significant effect on grain structure. So, the foundry can remove insulation from the mold to increase the rate of cooling and promote a finer grain structure or add insulation to increase grain size.
Serdar Omur Goren, marketing director of Sayan Orthopedics, an implant manufacturer and purchaser of castings based in Izmir, Turkey, called attention to a stratification in the market for implant castings caused by a trade-off of price against performance. "The first generation of implants was machined from stainless steel," Goren said. "More recently, the implant market has largely migrated from stainless steel to titanium to cobalt chrome."
But Goren pointed out that machined stainless steel still maintains a significant market share in the price-sensitive segments of the market, such as for trauma products. Titanium has a weight advantage, but CoCrMo, on the other hand, is more durable. Since longer life is the most important consideration in most cases, cobalt chrome has replaced titanium in many applications.
A major reason for the dominant share of the implant market held by the investment casting process today is that it is well suited to CoCrMo, the material preferred for most orthopedic implants. CoCrMo can be investment cast in an argon atmosphere, while titanium (in most cases) must be cast in a vacuum furnace, which is more expensive because of the high cost of maintaining a vacuum. Titanium also requires much closer control of process conditions during casting, which means that personnel need to be trained to a higher degree to avoid quality problems. The result is that titanium implants usually are produced by forging rather than casting.
Goren added that there is a lot going on under the surface of a casting process, so it's critical that every part be carefully inspected before it is accepted for use as an orthopedic implant. Sayan requires that its suppliers inspect every part by X-ray to examine its inner structure to detect any voids or other faults that might cause premature failure. The company also requires that the surface be inspected with a penetration test to identify any cracks or other flaws.
"It should be obvious why casting dominates the implant market," Goren concluded. "It can be used to create very complex geometries that possess exceptional dimensional accuracy, provides excellent material properties and is quite economical. But today, casting faces threats to its dominance from newer technologies, such as powdered metallurgy, that appear to offer advantages."
However, this article makes it clear that the entire cast implant supply chain is not standing still but rather working hard to make improvements that will guarantee the future of this ancient technology. Casting has appeared to be on the way out before, but materials suppliers, foundries and implant manufacturers always have worked together to develop innovations that have reestablished casting's dominant position. Barring any major technological breakthroughs, casting should continue to dominate the implant market for many years to come.
Orthopedic surgeons, healthcare providers and implant manufacturers are working hard to avoid negative patient outcomes-many of which may be related to the performance of the implant device itself, such as breakage, wear and separation of the implant from the bone. They are making increasing demands for a wide range of improvements on the part of companies that use casting, forging and machining to make implant components. Investment casting is the leading method of producing implant components, so foundries naturally are in the forefront of developments in this field.
Most typically, the casting is made of a cobalt-based alloy such as cobalt-chrome-molybdenum (CoCrMo) because of its excellent mechanical properties, such as high strength and low friction. On the other hand, cobalt-based alloys are not tissue friendly, so bone integration is relatively difficult to achieve. For this reason, it is common to coat cobalt alloy implants with a more tissue-friendly material such as titanium, tantalum, niobium or an alloy consisting of some or all of these materials.
Promoting Bone Integration
One of the most significant causes of implant failure is the separation of the implant from the bone. Implant manufacturers long have used various methods to impart a textured surface with the goal of promoting the growth of the patient's own natural bone into pores within the surface. The traditional approach to creating such a porous surface involves the metallurgical bonding of a layer of small metal beads or metal mesh to the implant body by plasma spraying or sintering. In the most common process, plasma spraying, the material to be deposited is introduced as a powder into a jet emanating from a plasma torch with a temperature on the order of 10,000 degrees Kelvin. The material is melted and propelled toward a substrate, where the molten droplets solidify and form a deposit. This approach makes it possible to enable the body and surface of the implant to be made from different materials-each of which is optimized for its own special role. On the other hand, it introduces the risk that beads might separate from the implant during the implant procedure and find their way into the articulating surface of the joint, which may lead to premature failure and the need for revision surgery.
Materials suppliers are working hard to tune the properties of the materials that are applied to the surface to improve the bone-integration performance of the implant.
The most critical issue in the past year in this area is more closely matching the properties of bone-a process called bone shielding-according to Dr. Colin McCracken, development manager-Powder Products for Robesonia, PA-based Reading Alloys Inc., an AMETEK company that supplies powdered Ti-6Al-4V alloyused in producing implants, particularly to apply surface textures to castings and forgings.
"We are seeing an increasing demand for powder for use in plasma spraying a surface texture onto cast and forged hip implants," McCracken said. "Implant manufacturers are asking us to more finely tune the properties of our materials in order to provide better implant performance."
Getting the Oxygen Out
When the bone and the surface of the implant to which it has adhered have similar properties, the materials are more likely to deflect together under the application of stress, so loosening is much less likely to occur. The most popular powder for application to the surface of an implant today is Ti-6%Al-4%V, an alloy consisting of 90% titanium, 56% aluminum and 4% vanadium.
The challenge in improving the mechanical properties of Ti-6%Al-4%V is somewhat unique in that in most cases, materials suppliers are asked to improve the mechanical properties of their product, while in this case, they are being asked to reduce the mechanical properties to more closely match those of the bone. What is critical here is reducing the impurities in the powder, particularly oxygen, down to very low levels. This is because increasing the level of oxygen in the powder increases its strength.
"The standard Ti-6Al-4V alloy oxygen levels should be held to 2000 ppm [parts per million] or less," McCracken said. "Through a major effort to tighten up our production process we have developed the ability to produce extra-low interstitial [ELI] powders in the 1,000- to 1,300-ppm range. These powders much more closely match the mechanical properties of bone than the standard material. This product was first introduced on the market two years ago and has rapidly been adopted by many orthopedic implant manufacturers and their suppliers. We are already supplying this low oxygen product in volumes of tens of thousands of pounds per year."
In addition to improving the surface properties of castings, the ELI variant of Ti-6%Al-4%V also has aided a competitive process. The low-oxygen alloy is being used on a small scale to bypass the casting process altogether by producing the entire implant using powder metallurgical methods. The low-oxygen material is important in powder metallurgy because the oxygen content of the powder tends to increase during the sintering process. "By starting with ELI powders, it is possible to hold the finished product to a 2000-ppm oxygen level, which is acceptable for most applications," McCracken said.
Casting in a Textured Surface
A newer approach for applying a surface texture to an investment cast part has taken hold in the past decade or so. It involves applying a textured surface to the wax pattern used to produce the ceramic shell that serves as the mold in the investment casting process. The textured surface normally is created by fixing beads to the wax pattern prior to forming the ceramic shell. The beads are made out of a material that melts along with the wax pattern when metal is poured into the ceramic shell.
By integrating the textured surface with the casting, this approach eliminates a potential point of failure where the beads are bonded to the investment casting. Because of this advantage, this method has become widely used in producing investment castings for orthopedic implants in recent years.
A titanium-coated hip implant. Sources report increasing demand for powder for use in plasma spraying a surface texture onto cast and forged hip implants. Photo courtesy of Reading Alloys. |
But Dave Beighton, vice president and general manager of Sandvik Medtech Medical Solutions Division, pointed out what he sees as difficulties in this approach. "Surgeons prefer a randomized irregular surface, and the current method of applying beads to the waxed surface does achieve this goal," he said. "However, the difficulty in this approach is that it's impossible to control the exact way in which the beads are distributed over the wax pattern, so every casting is slightly different."
There's little doubt that the precise surface texture that is applied to the implant can have a significant impact on its ability to resist loosening. Surgeons have expressed the desire to evaluate the performance of different surface textures in terms of their ability to promote bone growth. Then, after they have selected an effective texture, they want their suppliers to be able to consistently reproduce this texture time and again.
Beighton said that a new investment casting method his company has developed very recently provides the ability to apply a reproducible textured surface. The key difference in the new method is that wax pattern is produced with an integral textured surface. Beighton said that the company can consistently reproduce an assortment of beaded surface textures time and time again with this new method. He said further that the company is able to produce wax patterns with integral textured surfaces on a production basis so that the throughput of the investment castings produced by this method is substantially improved over the previous approach of applying beads to the wax pattern.
Beighton refused to reveal the details of the process that the company uses to produce wax patterns with an integral textured surface.
"We believe that we have a significant jump on our competition and we hope to preserve it as long as possible," he said. "The technology has moved from the laboratory to the production floor and we are now producing investment castings using this technology."
It's also important to note that the previous method generally is limited to applying a textured surface to external surfaces, because it is so hard to apply beads to internal surfaces, such as the internal profile of the femoral knee. But the new method can apply the surface texture to both internal and external surfaces, which opens up an important new avenue to promote bone integration for more stable implants.
Advances in Metal-on-Metal Joints
While the most-rapid recent advances have come in the surface texture area, changes also have been percolating in the body of the casting.
The most common hip implant consists of a stem topped by a ball component that is implanted on the top of the femur to replace the degenerated femoral head that surgeons have removed. The ball fits into a socket or cup that is attached to the hip bone.
Over the years, a metal ball and an ultra-high-molecular-weight polyethylene (UHMWPE) cup design has emerged as the preferred design. The main weakness of this approach is that UHMWPE cups typically last for only 10 to 15 years, depending on the age and activity level of the patient. This device lifespan is too short for younger, more active patients who probably would require one or more revision procedures to replace a failed cup.
As a result, interest in metal-on-metal joints has been increasing during the past few years. An earlier generation of metal-on-metal joints had problems with dislocation and cup deformation. But newer designs have a more precise fit that seems to have solved the cup-loosening problem. Likewise, significant improvements in casting processes have helped to address the cup deformation problem. The result has been a significant trend toward the use of metal-on-metal implants with the cups made of CoCrMo investment castings.
Crispin Carney, engineering director for Warsaw, IN-based Symmetry Medical's facility in Sheffield, England, said his company has been working closely with its customers to tailor the properties of CoCrMo investment castings to the requirements of metal-on-metal hip replacement parts.
"When you have two materials as different as metal and polyethylene wearing against each other, one element tends to wear more rapidly than the other," he explained. "On the other hand, two components made of the same material tend to wear at uniform rates, resulting in longer life and less debris released during the life of the implant."
The move toward metal-on-metal implants has focused new attention on the microstructure of the components, with a particular regard to eliminating or minimizing debris.
"Early metal-on-metal implants performed variably, and empirical assessment of high-performing variants suggested a relationship between the microstructure of the casting and longevity," Carney said. "There are several schools of thought on the ideal micro-structure for these types of components in terms of grain size and carbide morphology. Since then, there have been several streams of development to harness the best performing type, which remains inconclusive."
Carney noted that Symmetry could not offer any opinion as to which micro-structure is ideal but said the company is consistently able to provide a wider range of micro-structures by controlling aspects of the casting process.
"Symmetry in the UK has been working on tailoring the micro-structure to suit customers' requirements," Carney said. "This is achieved by adjustments to shell chemistry and construction, alloy chemistry and solidification control."
For example, the surface coating of the shell contains an inoculant that promotes a finer grain structure. The foundry can remove that if the customer requests a coarser grain. The rate of cooling also has a significant effect on grain structure. So, the foundry can remove insulation from the mold to increase the rate of cooling and promote a finer grain structure or add insulation to increase grain size.
Trading Off: Cost vs. Performance
Serdar Omur Goren, marketing director of Sayan Orthopedics, an implant manufacturer and purchaser of castings based in Izmir, Turkey, called attention to a stratification in the market for implant castings caused by a trade-off of price against performance. "The first generation of implants was machined from stainless steel," Goren said. "More recently, the implant market has largely migrated from stainless steel to titanium to cobalt chrome."
But Goren pointed out that machined stainless steel still maintains a significant market share in the price-sensitive segments of the market, such as for trauma products. Titanium has a weight advantage, but CoCrMo, on the other hand, is more durable. Since longer life is the most important consideration in most cases, cobalt chrome has replaced titanium in many applications.
A major reason for the dominant share of the implant market held by the investment casting process today is that it is well suited to CoCrMo, the material preferred for most orthopedic implants. CoCrMo can be investment cast in an argon atmosphere, while titanium (in most cases) must be cast in a vacuum furnace, which is more expensive because of the high cost of maintaining a vacuum. Titanium also requires much closer control of process conditions during casting, which means that personnel need to be trained to a higher degree to avoid quality problems. The result is that titanium implants usually are produced by forging rather than casting.
Goren added that there is a lot going on under the surface of a casting process, so it's critical that every part be carefully inspected before it is accepted for use as an orthopedic implant. Sayan requires that its suppliers inspect every part by X-ray to examine its inner structure to detect any voids or other faults that might cause premature failure. The company also requires that the surface be inspected with a penetration test to identify any cracks or other flaws.
"It should be obvious why casting dominates the implant market," Goren concluded. "It can be used to create very complex geometries that possess exceptional dimensional accuracy, provides excellent material properties and is quite economical. But today, casting faces threats to its dominance from newer technologies, such as powdered metallurgy, that appear to offer advantages."
However, this article makes it clear that the entire cast implant supply chain is not standing still but rather working hard to make improvements that will guarantee the future of this ancient technology. Casting has appeared to be on the way out before, but materials suppliers, foundries and implant manufacturers always have worked together to develop innovations that have reestablished casting's dominant position. Barring any major technological breakthroughs, casting should continue to dominate the implant market for many years to come.