Patient-Specific Implants for Soldiers

New Technologies Speed Treatment,
Improve Patient Outcomes.

by David Chen, PhD

Recent cases at military medical facilities such as Walter Reed Army Medical Center and Wilford Hall Medical Center at Lackland Air Force Base showcase the potential of these advances for improved patient care— when medical institutions unite the latest computing systems, biomedical engineering, and surgical know-how to benefit complex surgical cases.

Combat-related medicine often involves specialized care for patients with far more complex reconstruction needs than seen among a civilian medical population, particularly in the head and face. For example, improvised explosive device (IED) wounds commonly shatter large sections of the cranium— a region that, if left untreated, leaves the patient with a misshapen appearance or a lack of a hard bony structure protecting the brain. Heavy falling objects can shatter jaw bones or eye sockets, creating intense pain during activities of daily living such as eating, reading or speaking. Unlike hips, knees and shoulders, the complex nature of these bones produce limited choices in off-theshelf customizable cranium and mandibular implants.

Restoring these injured soldiers to normal function can be challenging. Restoring their outward appearance to their pre-injured look, as much as feasible, can be even more difficult in many cases. Establishing the exact curvature of the forehead or skull pre-injury, or the shape of an absent or crushed bone section, can be difficult. In addition, cases often require complex surgical approaches—or multiple surgeries.

Pre-planning the locations for the incisions or visualizing the location of bone in relation to blood vessels and nerves, can greatly expedite actual operating theater performance, when the patient is open—literally— and exposed to infection, blood loss or respiration complications from anesthesia.

THE RAPID REVOLUTION

That’s exactly where medical modeling of anatomically correct body parts can help. Walter Reed Army Medical Center has operated a 3-D Medical Applications Center since 2004, and Wilford Hall Medical Center opened its Almquist Stereolithography Lab in 1998—the only facility in the Department of Defense at the time.

These facilities maintain scanners, 3-D modeling systems, and 3-D printers to create patient-specific models of replacement parts that provide the perfect match needed to repair the injured area, and fit with the patient’s intact bone structure.

Models can be reviewed and revised, as physical prototypes or even as 3-D PDF electronic files that can be e-mailed among colleagues—until they represent the perfect fit with intact bone structure. Once finalized, models are then manufactured via the latest techniques in polymethyl methacrylate (a shatterproof, FDA-approved plastic known as PMMA) or titanium, for implantation.

Patient-specific implants allow the surgeon to preserve as much of the patient’s own bone structure as possible and repair the full damaged area with smooth edges that facilitate soft-tissue acceptance of the implant, as well as provide a pleasing appearance for the patient.

Medical models can also allow physicians to more fully visualize the defect area, consider the tools they will need during surgeries, visualize areas of bone, vessels, tissue and nerves, and even perform surgical incisions on the model itself before the patient’s surgery.

GREATER SPEED, MORE STREAMLINED WORKFLOW

It’s useful to look back just a few short years ago to compare how far and how fast the technology has advanced. Three years ago, Walter Reed’s team needed three to four weeks to create a patient-specific implant model. Today, the process can take place in a matter of days; in emergencies, the 3-D medical applications team has designed craniofacial models in 24 hours, and completed the entire finished implant—including sterilization— in a week. Time savings translate directly into time saved for soldiers—often in pain—to receive implants faster, with better fit, and higher likelihood of long-term acceptance.

Rather than one major breakthrough, the advances result from a long march of progress in computing and manufacturing techniques, including:

• The Digital Transition—Traditionally, medical teams hand-carved wax, clay or gypsum models of patient-specific implants—a time-consuming, manual process requiring approximating the design of the replacement bone section by viewing and interpreting X-ray and CT scans, and waiting time while the model was delivered via mail or courier to the surgeon. If changes needed to be made to the model, surgeons would literally mark up the model with a pen, attempting to indicate cut lines and depth—then return it to the design team. Obviously the process was rife with delays and potential inaccuracies in bone thickness, shape and curvature.

• Beyond the process inefficiencies, inaccurate models also have potential risks to the patient. Once the actual surgery begins, any ill-fitting areas on the final PMMA or titanium implant, or difficulties with surgical guides needed to access the target area, require the surgeon to halt the operation and address them— not an optimal situation.

• Today teams are sculpting the same complex models digitally—with greater accuracy and repeatability. Instead of visually approximating, technicians can scan an intact bone, and then mirror it in the computer—for example, scanning the left mandible if the right was damaged. Or, instead of altering bone thickness by carving away 1 millimeter by hand in a proscribed section—and hoping the carving is exact—the computer can make the precise changes required and also refine the edges for a gradual taper that can help assure a comfortable fit.

• Mainstream acceptance of theseapproaches—Advanced equipment has come down in price to the point that medical centers can afford to locate them on site, broadening access to them. Magnetic resonance imaging (MRI) and computed tomography (CT) scanning are available at Landstuhl Regional Medical Center, as well as at Walter Reed and Wilford Hall—and the ability to electronically transfer files between facilities means that work can begin more quickly after the soldier’s data is captured. In addition, the concept of surgical planning has become a mainstream philosophy, particularly among surgeons fresh from medical school or post-graduate studies, as a common-sense way to ready them to provide the best possible outcome for the patient.

• New 3-D modeling technologies—Traditional 3-D CAD modeling packages were created for designing cars and aircraft, where geometric shapes could be readily calculated mathematically. However, with complex, organic shapes—like the bones of the human body—the time required to create such models skyrockets. Creating smooth edges that match the thickness and curvature of the intact bone structures can require special modeling features.

• Sculptural CAD solutions, such as the FreeForm modeling system from SensAble Technologies, have grown popular in the last few years because they are better suited to handling the organic shapes of patient-specific implants. Military maxillofacial experts have reported that by using a sculptural CAD system, they have decreased the time it takes to design a digital 3-D model of an implant or prosthetic body part from a full day or more, down to two to five hours. That doesn’t even factor in the additional time-savings from the scanning and 3-D printer aspects of the process.

• MedCAD, a Dallas-based company specializing in custom implant development, has helped military medical centers such as Wilford Hall. Recently MedCAD assisted with a case where a team started with a CT scan in the morning, through collaboration over the Web. The patient’s surgeons directed a MedCAD technician in the design of a mandibular implant in a few hours. A 3-D PDF of the final implant model was delivered to the surgeon by the end of the day.

• Interoperability between scanning, digital modeling and rapid manufacturing technologies—As more patient data is captured digitally, the industry is moving to more interoperability between file formats, either directly or via programs like MIMICS or 3Matic (Materialise, Leuven, Belgium), which are used to convert medical scan data into STL files that can then be imported into a 3-D modeling package like FreeForm.

• New manufacturing processes—In particular, the rapid manufacturing technique of electron beam melting (EBM) is becoming the technique of choice for titanium patient-specific implants. EBM uses a high-power electron beam to melt successive layers of metal from powdered raw material, thus forming a solid, metallic part in an additive fashion. In addition to supporting extremely complex, organic shapes, EBM is cheaper and faster than traditional methods such as forging or machining, which are better suited for larger volume runs.

CRANIOPLASTY, ORBITAL IMPLANTS

In 2008 alone, the 3-D Applications Lab at Walter Reed Army Medical Center handled over 200 cases for military, retired military and dependents. Between 60 and 75 percent of cases are for trauma-related care, with the rest relating to diseases ranging from cancer to scoliosis.

Many of Walter Reed’s cases involve cranial implants. Figure 1 shows a typical cranioplasty implant for a section of skull incorporating a portion of the parietal bone. In this case, the right side of the soldier’s cranium was intact; so, technicians took a CT image of that side, converted the CT scan to an STL file, imported it into a sculptural CAD system, and then mirrored it in the computer to create the initial 3-D model from which they designed the implant. In cases where both sides of a bone are absent, technicians can begin with a similar image from another patient or construct implant manually on the computer using CAD software.

Using a sculptural CAD system to create a 3-D model of the implant is similar to physically modeling, only the designer models with virtual clay and has the digital precision and flexibility of a computer. In this case, the technician created a patch designed to replace the missing bone, aligned it with the patient’s available anatomy, and then adjusted and smoothed out the shape and edges of the patch to match the contours and thickness of the patient’s skull and facial structure.

After adding fixation sites—tiny tap holes for 4–5 mm screws to affix the implant to the patient’s available bone—the resulting digital model of the implant was electronically sent to a 3-D printer, where a hard plastic prototype was created. Then, the final implant was created by an outside contractor using the EBM method.

Figure 2 shows an orbital implant that the WRAMC team created for a soldier who sustained severe blunt trauma from a gunshot wound to the head region. Unlike cranioplasties, where the skull section has one concave shape, an orbital implant can have several different concavities, and stretches out side to side with unique geometry, depending on the individual.

The orbital bones also extend back into the head, further complicating the task of designing a replacement implant that will fit and can be attached with tiny screws into the existing facial bone structure. Because the eye area is small and surrounded by soft tissue, the implant needed extremely fine edges to facilitate soft-tissue acceptance. It also required very small attachment sites.

In this orbital implant case, Walter Reed lab technicians relied heavily on features provided by their sculptural CAD system. Beginning with a CT scan of the soldier’s intact left orbital bone structure, engineers designed a digital model of the implant by sculpting virtual clay to match the unique surface features and complicated geometries of the patient’s damaged orbital area. Special care was required to design the implant so that it could allow sturdy attachment sites matched to available facial bones.

After using the digital model to create a physical prototype so the on-base surgeon could review and suggest revisions to the design, Walter Reed lab staff sent the digital model to a company possessing the EBM technology for manufacturing.

MANDIBLE IMPLANTS

MedCAD has worked under contract to Wilford Hall Medical Center at Lackland Air Force Base on cases of soldiers injured in Middle East combat. For example, one case involved a 24-year-old male member of the United States Army and veteran of combat in Iraq. He suffered severe facial trauma, losing almost his entire lower mandible.

In the field, the medical unit was able to fashion an emergency replacement with a U-shaped titanium bar attached to the remaining rami. Using the FreeForm sculptural CAD system, MedCAD was able to create an entire replacement jaw, even taking into account the quickly developed improvisation of an implant that the patient already had in place. One of the key benefits of the FreeForm system is the ability to use the patient’s anatomy when creating custom medical implants, yet freely modify the implant with extreme precision for a perfect fit.

Wilford Hall’s Almquist Stereolithography Lab also created a model of a jaw rebuild case for a soldier whose jaw was shattered by an IED. For years he suffered debilitating jaw pain and was unable to chew. Doctors worked to repair his jaw but they were unable to get enough blood flow to the jaw bone to keep it alive.

At the surgeon’s request, Wilford Hall staff created a model of how his jaw should look for use in surgical planning. The lab’s technicians first used images from CT scans to create a 3-D graphic. They then used sculptural CAD to mirror image the patient’s mandible and created a corrected anatomical jaw. Technicians used virtual clay to adjust and smooth out the edges of the jaw so it would fit perfectly into the area of intact bone. A computer-guided laser used the graphic to create an epoxy photopolymer resin model of how his jaw should look. The surgeon was able to hold a model, and look at a film, then look at the patient to visualize what was missing to better anticipate what was needed for his surgery.

With the help of the model created by the stereolithography lab, the surgeon was able to successfully use a bone, artery and vein from the patient’s leg to repair his jaw. Today the patient still has a way to go on his road to recovery, but he is able to eat, swallow and talk again. The surgeon was convinced that she achieved the quality of reconstruction and returned this patient to normal occlusion thanks to the aid of the patient-specific models.

TAKING ADVANTAGE

Whether treating trauma or disease and creating actual implants, or for surgical visualization and planning, sculptural CAD modeling solutions hold particular promise for military medicine and its population of individuals that are more likely to require complex, patient-specific implants.

As military medical centers take advantage of more affordable imaging and rapid manufacturing technologies, as well as streamlined digital workflows, they are providing better treatment, faster and improving the quality of patient outcomes for soldiers who deserve the best we have to offer. ♦
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David Chen, Ph.D., is chief technology officer of SensAble Technologies Inc. He has led the development of software for transforming medical data into formats for visualization, surgical simulation and biometric qualification. He also has researched the areas of interactive computer graphics and animation, virtual environment techniques, robotic control of jointed figures and forcebased finite element models of skeletal muscle in the study of biomechanics.