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High-tech implants resist infection
Doug Smock, Contributing Editor, Materials & Assembly, DesignNews
7/31/2012 8:19 AM EDT
Brainstorm implant
New Approach
Rouse and the surgical teams at Walter Reed brainstormed the implant problem.
Requirements from surgeons included:
"The only material available to us for this purpose is titanium alloy, with its proven biocompatibility, strength and most importantly, its ability to promote fibrovascular ingrowth," says Rouse. "The only manufacturing method capable of producing such a complex geometric structure is additive-based."
Walter Reed officials selected a new technology developed in Sweden by a company called Arcam using the electron beam melting technique (EBM).
Electron Beam
In the EBM process, fully dense metal parts are built up layer-by-layer as metal powder is melted by a powerful electron beam. Each layer is melted to the exact geometry defined by a 3D CAD model.
The build takes place in a vacuum at elevated temperatures, resulting in stress-relieved parts with material properties better than cast and comparable to wrought material, according to Magnus René, CEO of Arcam.
The vacuum system is designed to provide a base pressure of 1x10-4 or better throughout the entire build cycle. The EBM machine produces precise titanium mesh shapes that allow bone ingrowth and prevent fluid pooling under the implant that can lead to infection.
Arcam's EBM technology is also used to make off-the-shelf orthopedic implants. René says that pores can be engineered to improve bone fixation. The goal is to improve bone ingrowth compared to current technologies of coating cobalt-chrome implants with titanium beads or other materials.
The new implant technology is working well at WRAMC.
Removal rates in the past three years have dropped to 4 percent. "None have had to be removed following the healing process," says Rouse.
Other technologies such as Selective Laser Melting (SLM by MTT Technologies, Staffordshire, UK), and Direct Metal Laser Sintering (DMLS by EOS, Munich, Germany), were not available in the U.S. when Walter Reed began its testing.
The implants are designed after segmenting CT scan data using Mimics software from Materialise that allows engineers to bridge 2D data to 3D. 3Matic, also from Materialise, and/or FreeForm Modeling Plus, from Sensable, are used for the actual implant design.
Sections can be built in both mesh and solid, and the implant design includes the fixation plates. A skull model is created using a stereolithography machine and is sent with the completed implant to the surgeon for approval.
Seven Day Limit
Speed is a critical factor.
"Our goal is to keep the entire process, from CT scan to delivery of the finished implant, to seven days," says Rouse. "In most cases, we are successful. Some of the issues, or problems that we have experienced include intermittent build failures, machine availability and shipping delivery delays."
One problem Rouse has faced is lack of space for a program that is rapidly growing due to its huge success.
His five additive manufacturing machines are scattered across the Walter Reed campus where space permits. That changes soon under the Base Realignment Program (BRAC), which is designed to make more efficient use of military assets.
Rouse's group will soon move to a medical campus in Bethesda, MD, about six miles from WRAMC, which is located near the outer border of Washington, D.C. The new campus will be called the Walter Reed National Military Medical Center at Bethesda.
His equipment and three-person staff will be in one location at the new medical center. That equipment includes an SLA 7000, SLA 500, Z Corp. 650, Z Corp. 450, and an Arcam A-1. On order is a Connex 500 from Objet Corp.
The equipment serves a myriad of roles, ranging from pre- and post-surgical medical models to custom cranial implants, subperiosteal dental implants, facial bone implants and custom fixation devices.
Use of the additive manufacturing machinery is not necessarily simple, unlike milling machines, which can be left unattended.
They require monitoring to make sure there has been no warpage of the part. It's also important to ensure that the part is being produced to specification
Rouse says the Ti6Al4V powder must be monitored for oxygen content. The final part must be tested to ensure that metallurgy is within specifications. Vigilant preventive maintenance is required to make sure machines are available when needed. Units like the EBM machine are too expensive to have backups on hand.
"The bottom line for this entire process is the ability to build an implant that is designed for a specific patient's needs, that reduces the operating room time requirement significantly, and provides better outcomes with more resistance to infection," says Rouse.
New Approach
Rouse and the surgical teams at Walter Reed brainstormed the implant problem.
Requirements from surgeons included:
- Standard available non-gas sterilization methods;
- Porosity in the implant to reduce trapped fluid pooling underneath;
- Material compatibility with tissue ingrowth to reduce the free space for infection;
- Ability to conform to complex contours and thickness changes, regardless of location; and
- The implant needed to be visible on radiographs without causing radiographic artifacts and be safe to use MRI (magnetic resonance imaging).
"The only material available to us for this purpose is titanium alloy, with its proven biocompatibility, strength and most importantly, its ability to promote fibrovascular ingrowth," says Rouse. "The only manufacturing method capable of producing such a complex geometric structure is additive-based."
Walter Reed officials selected a new technology developed in Sweden by a company called Arcam using the electron beam melting technique (EBM).
Electron Beam
In the EBM process, fully dense metal parts are built up layer-by-layer as metal powder is melted by a powerful electron beam. Each layer is melted to the exact geometry defined by a 3D CAD model.
The build takes place in a vacuum at elevated temperatures, resulting in stress-relieved parts with material properties better than cast and comparable to wrought material, according to Magnus René, CEO of Arcam.
The vacuum system is designed to provide a base pressure of 1x10-4 or better throughout the entire build cycle. The EBM machine produces precise titanium mesh shapes that allow bone ingrowth and prevent fluid pooling under the implant that can lead to infection.
Arcam's EBM technology is also used to make off-the-shelf orthopedic implants. René says that pores can be engineered to improve bone fixation. The goal is to improve bone ingrowth compared to current technologies of coating cobalt-chrome implants with titanium beads or other materials.
The new implant technology is working well at WRAMC.
Removal rates in the past three years have dropped to 4 percent. "None have had to be removed following the healing process," says Rouse.
Other technologies such as Selective Laser Melting (SLM by MTT Technologies, Staffordshire, UK), and Direct Metal Laser Sintering (DMLS by EOS, Munich, Germany), were not available in the U.S. when Walter Reed began its testing.
The implants are designed after segmenting CT scan data using Mimics software from Materialise that allows engineers to bridge 2D data to 3D. 3Matic, also from Materialise, and/or FreeForm Modeling Plus, from Sensable, are used for the actual implant design.
Sections can be built in both mesh and solid, and the implant design includes the fixation plates. A skull model is created using a stereolithography machine and is sent with the completed implant to the surgeon for approval.
Seven Day Limit
Speed is a critical factor.
"Our goal is to keep the entire process, from CT scan to delivery of the finished implant, to seven days," says Rouse. "In most cases, we are successful. Some of the issues, or problems that we have experienced include intermittent build failures, machine availability and shipping delivery delays."
One problem Rouse has faced is lack of space for a program that is rapidly growing due to its huge success.
His five additive manufacturing machines are scattered across the Walter Reed campus where space permits. That changes soon under the Base Realignment Program (BRAC), which is designed to make more efficient use of military assets.
Rouse's group will soon move to a medical campus in Bethesda, MD, about six miles from WRAMC, which is located near the outer border of Washington, D.C. The new campus will be called the Walter Reed National Military Medical Center at Bethesda.
His equipment and three-person staff will be in one location at the new medical center. That equipment includes an SLA 7000, SLA 500, Z Corp. 650, Z Corp. 450, and an Arcam A-1. On order is a Connex 500 from Objet Corp.
The equipment serves a myriad of roles, ranging from pre- and post-surgical medical models to custom cranial implants, subperiosteal dental implants, facial bone implants and custom fixation devices.
Use of the additive manufacturing machinery is not necessarily simple, unlike milling machines, which can be left unattended.
They require monitoring to make sure there has been no warpage of the part. It's also important to ensure that the part is being produced to specification
Rouse says the Ti6Al4V powder must be monitored for oxygen content. The final part must be tested to ensure that metallurgy is within specifications. Vigilant preventive maintenance is required to make sure machines are available when needed. Units like the EBM machine are too expensive to have backups on hand.
"The bottom line for this entire process is the ability to build an implant that is designed for a specific patient's needs, that reduces the operating room time requirement significantly, and provides better outcomes with more resistance to infection," says Rouse.
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