High-Tech Implants

Recovery rates for American soldiers with head wounds are improving significantly because of new technology to quickly produce custom cranial implants that are more resistant to infection.

More than 70 cranial plates have been produced using the new process at Walter Reed Army Medical Center (WRAMC) in Washington, D.C., with an average surgical time of 90 minutes from first incision to completion of suturing. Previously, the same types of surgeries took from two hours to more than six hours.

"The time savings can be directly attributed to the improved implant design and attached fixation," says Stephen L. Rouse, DDS, a government contractor working in the 3-D Medical Applications Lab at Walter Reed Army Medical Center. "Large implants were previously multi-piece constructs, and were slower to place and fixate."

At the heart of the process is advanced application of 3-D digital scanning and high-powered additive manufacturing equipment that can make highly accurate custom shapes in medical-grade titanium alloys.

The original systems were based on lasers that created 3D shapes in processes called stereolithography (SLA) and selective laser sintering (SLS). Both are owned today by 3D Systems of Rock Hill, SC.

The rapid prototyping market gravitated to inexpensive 3D printers that can quickly make models from a wide range of materials, mostly plastic.

High-End Focus

Higher-end additive manufacturing equipment is increasingly focusing on dental, medical and aerospace applications, where engineers greatly value the ability to create precise, complex, strong parts often not possible with injection molding because of high costs or process limitations.

The work at Walter Reed improves life expectancy and quality of life for wounded soldiers.

One living, breathing example is Paul Statzer, who was a world champion weight lifter in 2000. He was a veteran of the first Gulf War and decided to re-enlist. His unit was sent to Iraq, and in 2005 he was investigating craters created by roadside bombs. An improvised explosive device detonated near Sgt. Statzer, removing much of his skull, his eye and parts of his larynx.

He received emergency treatment in Iraq and Germany, and doctors told his family he might not survive.

W. Lee Warren, the surgeon who did the initial brain surgery on Sgt. Statzer at the 332nd Air Force Theater Hospital in Balad, Iraq, says: "My memory of Paul Statzer was of a man so desperately injured that even while operating on him I gave him no chance for survival."

Following emergency surgery and a transfer to Germany, Sgt. Statzer was taken to Walter Reed where Rouse's lab performed a 3D scan of the cranial opening and made a plastic implant to replace the missing bone. Bone grafts and skin grafts replaced the missing area around his left eye.

After months of rehabilitation, Sgt. Statzer learned to walk again and recovered most of his memory.

Last year, Warren had a tearful reunion with his former patient in Pittsburgh near Sgt. Statzer's home. He recalls the surgery in a recent book: "Called Out: A Brain Surgeon Goes to War."

SLA Master

The standard cranial implant is made from PMMA (polymethylmethacrylate). A stereolithography-produced master is embedded in a two-part flask. The master is then removed and replaced by PMMA resin, which is then processed in a laboratory with pressure and heat.

"In large implant cases, the flask can weigh over 50 pounds, and is limited in contour complexity," says Rouse. Polishing, drilling for fixation and gas sterilization using ethylene oxide (EtO) is done prior to delivery to the operating room.

One significant problem with that approach is that EtO sterilization equipment is not available in many locations, and is being phased out or banned in many hospitals because of environmental issues.

An alternative material used for transplants is PEEK (polyetheretherketone). The great advantage of PEEK is its high-temperature tolerance (its glass transition is 143C). PEEK can be autoclaved in any hospital operating room sterilizer, eliminating the requirement of gas or EtO sterilization.

PEEK implants must be milled to exact shapes because of the expense of tooling. Rouse says that the cost of medical-grade PEEK blocks is very high.

According to industry sources, medical grade PEEK costs close to $400 per pound compared to about $44 per pound for industrial-grade PEEK. A leading medical-grade PEEK producer declined to comment on the cost issue.

The biggest problem, however, is that human tissue does not bond to plastic.

"This results in loss of muscle attachment to the skull replaced by the implant, and the formation of scar tissue which encapsulates the implant provides an area for bacterial growth that cannot be reached by antibiotics," says Rouse. "In patients with history of antibiotic-resistant infections, this can result in the loss of the implant if a recurrence of the infection appears."

The plastic cranial implants at Walter Reed needed to be removed about 12 percent of the time to save the patient's lives.

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.