Sculpting the Perfect Body Part
Custom Implants, Prostheses, and Surgical Guides Benefit from Advances in 3D Design, Rapid Prototyping, and Rapid Manufacturing Technologies
By Dr. David Chen, Chief Technology Officer
SensAble Technologies, Inc.,Woburn Massachusetts
Replacement parts are not just for cars, washing machines, and toasters. Human body parts are increasingly sculpted in 3D modeling software, then manufactured using the same rapid prototyping and rapid manufacturing techniques associated with consumer goods. These sculptures require the exceptional anatomical detail and accuracy of a master craftsman to provide optimal patient results and efficiently support downstream manufacturability.
Often, when a patient needs a replacement knee, hip, or shoulder, their doctor simply specifies a generic replacement for their size and gender, and then must adjust it to fit their needs--even making last-minute refinements while the patient lies waiting on the operating table. Increasingly, however, there are many instances where custom-designed implants are either preferred as a way to preserve more of the patient's bone structure, or may be the only viable solution. For example, custom implants are imperative when no off-the-shelf parts exist--such as with craniofacial or maxillofacial implants to the head and neck region--or when an off-the-shelf hip, shoulder, or knee replacement just won't fit or fails. War-related trauma, cancer-related bone or tissue loss, accident-caused bone injuries, and even congenital deformities, such as cleft palettes, can require custom implants to restore a patient to normal physical function or appearance.
New sculpting capabilities, materials, and manufacturing methods, in addition to bone reconstruction, can create more realistic and better fitting prosthetic noses, ears, and limbs. Design engineers today would likely be amazed at the plethora of custom body parts that are routinely created, manufactured, and worn by patients around the world to replace injured or disease-affected areas--parts that are designed and manufactured faster, more easily, and more affordably thanks to advances in technology.
The Multi-Step, Delay-Ridden Traditional Process
Creating a custom implant using traditional methods and materials typically requires four steps and about 6 to 8 weeks. The four steps--data import, modeling, prototyping, and production--are outlined below.
Data Import. Upon receiving a request for a custom implant from a surgeon or medical professional, the designer needs to obtain the patient-specific CT or MRI scan data. These files usually reside in DICOM data files that must be converted into standard STL files for use with most 3D modeling and rapid prototyping applications.
Modeling. Working with the medical practitioner to understand the patient requirements and surgical plans, the designer models the prosthetic in a CAD application or by hand using traditional sculpting materials. This is an iterative process that may include several rounds of review and revisions.
Prototyping. Prototyping can be accomplished either via rapid prototyping techniques or by milling, molding, or casting to create a prototype that can be reviewed for changes and revised as needed. Typically, prototypes are created by a prosthetist (also known as an anaplastologist or a prosthodontist) or a biomedical engineer that specializes in creating medical models.
Production. In its final form, the custom implant is produced in titanium or another FDA-approved, bio-compatible material for implanting into the patient's body.
The traditional process is not streamlined, nor easy. Designers must become proficient on many specialized hardware and software systems to accomplish the various steps in this workflow. Such expertise is usually not within the skill sets of prosthetic or medical model-makers who came of age making models in wax, wood, or foam.
Often, the device does not fit directly bone-to-bone, but must be attached to fit within a cavity or to fill a gap atop the bone. This increases the complexity of the manufacturing task because the designer has to create a perfectly fitting part within a very precise margin, perhaps as small as 0.5 millimeter.
Finally, the approval process itself can bloat production cycles and slow the manufacturing of the final device for a patient. Although emailing Adobe Acrobat 3D files of digital models is fast and useful during the design stage, surgeons still need to touch and feel a prototype in order to evaluate the shape and size of the implant, and to help them with surgical planning. When seeing early prototypes, surgeons physically draw with a pen on the early version, drawing areas where they can enhance the curvature of the socket or build up another area. This iterative process can be repeated back and forth until the surgeon is satisfied that the device will meet the patient's needs. While necessary, this process can lengthen the time for delivering custom parts.
Today, digital technologies are allowing the creators of custom body parts and surgical guides to streamline the process to 2 to 4 weeks from concept to delivery, while improving patient fit and often lowering the cost.
Faster, Easier Modeling. Traditional 3D modeling packages were developed for designing cars and aircraft parts, where sleek, geometric, prismatic shapes can be readily calculated by mathematical representations and highly procedural workflows. However, these types of modeling systems are limited when it comes to providing rapid ways to produce the complex, organic shapes of the human body. On the other hand, sculptural CAD approaches, such as the FreeForm® modeling system from SensAble Technologies (Woburn, Mass.), offer product designers a whole new way of working. Modeling with virtual clay, designers quickly and intuitively ideate and create multiple versions of their models by inflating, tugging, smoothing, and carving. The ability to share digital files via annotated 3D PDFs can also speed the design and approval cycles. These solutions allow ready access to the ability to import and export STL files--a file type commonly used for data exchange in working with scan data, rapid prototyping and manufacturing, and milling. FreeForm systems utilize voxel technology (3D pixels that form a type of digital Play-Doh®), which makes modeling complex, organic shapes in FreeForm highly intuitive and much faster than in surface or solid modelers. FreeForm systems also incorporate haptics--force feedback, or sensing abilities--so that medical professionals can use their sense of touch to model with virtual clay as if they were creating a model out of physical clay, foam, or wood. The haptic device (known as the PHANTOM®) makes the modeling process more direct and natural than using a 2D interface to model in 3D. In doing so, it saves design time in the overall workflow.
Rapid Prototyping (RP) and Rapid Manufacturing (RM) Advances. Thanks to new rapid prototyping and bio-compatible manufacturing materials, surgeons and teams can digitally create accurate models or finished, custom body parts faster, using printing or sintering ceramics and titanium. Because they are created from the digital design file (usually STL file format), RP- and RM-made models and parts are often more accurate than hand-made models, and more easily revised and recreated after a surgeon makes modifications to the design. Price/performance advances in RP have also meant that these models cost less than before. The rise of rapid manufacturing technologies means that contract manufacturing is now available for one-off or short run projects, not just mass production. Contract manufacturers can justify the cost of dedicated and often expensive direct fabrication and manufacturing options because they are getting more "custom" work from many sources, including teaching hospitals and pioneering surgeons. Their business focus on custom implants supports the investment in people with specialized skill sets, hardware and software tools, and the latest output methods. As a result, contract manufacturers' staffs can efficiently develop the expertise and processes required for complex implant creation.
Grow-Your-Own Substrates can knit into the patient's existing bone for faster re-growth. For example, craniofacial surgeons today are using sculptural CAD systems to create custom "scaffolds" or partial sections of replacement bone. They then seed them with cells, growth factors, or both, so that the implant ultimately transitions into bone, helping to relieve the stress of implant incompatibility, movement, or extrusion.
Design and Manufacturing Options. Some physicians and medical institutions prefer to outsource both design and manufacturing to specialized vendors who produce custom body parts as needed. Others prefer to own design tools in-house and contract out for the manufacturing. Either way, medical teams can create exactly what is needed for each team of surgeons and for each individual patient.
In acute, life-threatening cases, such as war wounds or other trauma, medical professionals now have the ability to choose design and manufacturing options--design software, materials, and manufacturing location--that optimize speed. By choosing non-proprietary solutions, such as design software that can import and export common file formats, medical teams can maintain the ultimate flexibility to choose the processes that work best for their patient base and their business.
A Custom Hip Revision Cup
Enztec, of Christchurch, New Zealand, a designer and manufacturer of orthopaedic medical devices, needed to provide a custom acetabular (hip) revision cup for a 70-year- old female patient whose existing off-the-shelf hip implant was dislocating into her pelvic cavity. The patient had a portion of her hip intact, but the acetabular implant had been revised before, and although the stem was part of her original implant, it showed some signs of bone resorption. Her doctor needed to create a replacement implant that was slightly bigger than the present, damaged one.
Enztec partnered with a Golden, Colorado contract manufacturing company, Medical Modeling (www.medicalmodeling.com, now part of 3D Systems), to work from CT scan input, to custom-designed implant, through to finished implant creation, using the electron beam melting (EBM) process. Electron beam melting uses a high-power electron beam to melt successive layers of pre-alloyed metal powder, thus forming a solid, metallic part in an additive fashion. It moves a file directly from a CAD environment into a fully, dense titanium or cobalt-chromium part.
The patient had recently obtained a CT scan as part of the diagnostic process. Medical Modeling used a software program to convert the medical imaging files into a standard STL file compatible with the FreeForm 3D modeling system. After the scan was imported into FreeForm, Medical Modeling emailed the design files so that its staff and Enztec could dialogue online with the New Zealand-based surgeon at length about surgical objectives, the patient's challenges, and patient-specific design issues, such as age, weight, and bone condition.
The design needed to enlarge the size of the patient-specific implant compared to the old one that was scanned, since the new implant would be replacing slightly more of the patient's existing bone structure. However, the resulting shape of the new implant was asymmetrical, and not typical of the normal hip bone structure.
Once the surgeon authorized the implant design, Medical Modeling created machine-specific instructions for the EBM manufacturing technique. For single-run parts like custom implants, EBM is cheaper and faster than traditional methods, such as forging or machining, which are better suited for larger volume runs. The result: The patient-specific fit of the new implant allowed the surgery to be completed in half the time--from the anticipated six hours, down to just three hours. Implant design and production also took place in half of the usual time required, just two weeks from CT scan transmission to delivery of the finished implant to the surgeon. Yet costs were comparable to a traditional off-the-shelf solution.
New 3D technologies allow the creation of better fitting implants and give surgeons greater familiarity with the patient's anatomy before surgery. MedCAD (http://medicalcad.com), a Dallas-based company specializing in custom medical development for 3D visualization and modeling, uses FreeForm to work with the University of Texas Southwestern Maxillofacial Department to create facial implants for patients such as children with congenital defects or trauma. In one case, "Jane Doe," a 16-year-old female, had a number of facial defects, including an occlusal cant and complex maxillofacial asymmetry.
After importing the CT data into FreeForm, the surgeon was able to use the 3D anatomical models made from the original patient scan data to create a surgical plan. MedCAD then used FreeForm to digitally sculpt a custom mandibular implant that was perfectly shaped to the new facial design. Seeing the digital 3D models also allowed the surgeon to realize that the bones would collide, and to plan a better surgical approach, virtually. The result of the surgery, and MedCAD's implants, is a new, symmetrical face for the patient that required less surgery time and less risk overall.
David Chen, Ph.D., can be reached at 781-937-8315 or firstname.lastname@example.org.
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