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Orthopedics

As life expectancies increase, many of the aging baby boomers who receive their first knee, shoulder, and other implants today are likely to require a second and third replacement implant in the future—a key trend behind the $22 billion orthopedic implant market and its projected 8.9% annual growth rate.1 Surgeons strive to preserve as much of the patient’s own bone structure as possible during the initial implant surgery so that if future replacements are needed, enough structures remain intact to support them.

The replacement-for-the-replacement trend is increasing the number of instances in which off-the-shelf implants cannot meet patient needs. Off-the-shelf implants are often too large, don’t fit well into the remaining bone structure, or have a strong likelihood to chafe already infected or inflamed tissue.

For years, replacement hips, knees, and sections of spinal columns have come in a small range of sizes, and more recently, in modular designs that surgeons can order for use together. Surgeons relied on them because they were easy to order, affordable, and covered by insurance. Patient-specific implants were reserved for the most complex of cases and sometimes required between six and eight weeks of lead time. Clinicians tried to limit these efforts, in part because manufacturing processes were too costly and labor-intensive to economically support custom projects, which often received little or no additional reimbursement beyond conventional procedures.

Thanks to a confluence of digital hardware and software advances, tech-savvy physicians, and new manufacturing techniques, patient-specific implants now are becoming more common and are readily designed. Patient-specific implants are also developing into a viable business for orthopedic firms, which can command a premium for custom implants. However, there is still a need for improvements in the overall reimbursement process.

The same sculptural computer-aided design (CAD) technology used for making product prototypes, toys, and collectibles is now helping to fuel a shift toward custom-designed implants, including hips, leg bones, and shoulders as well as restorative muscle implants. Such implants fit better and are significantly faster and less expensive to design and manufacture than just a few years ago.

Digital Advances

Four major factors are propelling greater ease in creating patient-specific implants. These factors include the digital transition, tech-savvy clinicians, sculptural CAD software, and the advent of rapid manufacturing.

The Digital Transition. The technology to create digital 3-D patient models has existed for some time, but it is really taking off now because the entire digital work flow—from data acquisition to modeling and custom manufacturing—is vastly easier. A patient’s x-ray or CT scans are more readily converted into the stereolithography (STL) file type for import into 3-D modeling software.

These digital modeling packages quickly handle computationally complex tasks such as creating mirror images of body parts or using a normal section of bone to digitally recreate its missing counterpart. Using actual patient data to design these models on a computer delivers far greater accuracy than hand drawings that approximate the patient’s bone geometry.

In addition, a cottage industry of medical modeling specialists has arisen so that physicians can more easily source patient-specific implants. They can get everything from data acquisition to 3-D modeling to implant creation and delivery from a single source.

Tech-Savvy Physicians and Technicians. Tech-savvy users are comfortable with digital technologies and, in fact, often see such technologies as superior ways to create better-fitting replacement body parts. For them, these tools can translate to reduced surgical time when the patient is open and exposed to infection.

Sculptural CAD Software. Traditional 3-D CAD modeling packages were created for designing cars and aircraft, for which geometric shapes could be readily extruded mathematically. However, with complex, organic shapes—like the bones of the human body—the time and effort required to create such models skyrockets. So-called sculptural CAD programs, such as the FreeForm modeling system from SensAble Technologies (Woburn, MA), have become more popular among the medical modeling sector because the underlying voxel technology of sculptural CAD more quickly and easily handles the intricate organic shapes of patient-specific implants.

The Rapid Manufacturing Revolution. Improvements in scanners, 3-D printers, rapid prototyping, and additive manufacturing techniques have led to more design and manufacturing options. Titanium remains the metal of choice for implants, and most custom implants are produced using traditional milling processes. However, newer biocompatible materials are appearing more frequently, and rapid manufacturing techniques such as electron-beam melting (EBM) show particular promise.

EBM uses a high-power electron beam to melt successive layers of pre-alloyed metal powder, forming solid, metallic parts in an additive fashion. EBM enables moving a file directly from a CAD environment into a fully dense titanium or cobalt-chromium part.

The process can reduce the need to use other near-net-shape fabrication processes such as machining, forging, and casting as well as their associated long lead times. Research has validated EBM-produced Ti6Al4V components for use compatible with the demanding specifications of the orthopedic industry. It is particularly well-suited for the following:

Four examples show the range of design challenges and manufacturing options typically faced today.

Mid-Shaft Tibia Implant

Biomet Inc., a manufacturer of products for joint replacement of the hip, knee, shoulder, elbow, and other small joints, has a Patient-Matched Implants (PMI) business unit with 30 professionals who develop more than 1000 customized highly durable joint replacement components annually.

In early 2009, Biomet’s PMI team designed a replacement for a mid-shaft tibia implant to replace a failed allograft component, a donor bone segment. The tibia is the larger of the two bones in the human leg beneath the knee. The previous allograft tibia implant had been attached to the patient’s intact bone by a trauma nail; however, an infection in that area caused the allograft construct to fail.

Due to the need to remove additional intact bone in support of another implant, the patient had only 3 cm of remaining intact tibia proximally (the end closer to the knee joint) and 5 cm distally (the end closest to the ankle)—a very small amount of intact bone into which to anchor an implant with screws.

Biomet’s challenge was to digitally define the precise dimensions of the replacement tibia implant and to design a platform for the base and the side of the tibia onto which the implant would rest next to the intact bone (see Figure 1). This was not an easy task when viewing a 2-D image of the complex 3-D shape and form of a bone.

After importing digital imaging and communications in medicine (DICOM) files from the patient’s CT scans, and using software to convert them into STL files, Biomet’s designer used the sculptural CAD system to determine the exact size and shape required for the replacement implant. Since the patient’s bone was already cut, Biomet’s team was able to gain a cross-sectional view of its geometry.

The sculptural CAD software also allowed the user to establish a plane at the level at which the designer wanted to acquire a cross-section. The software allows use of various software commands to extract curves where the plane intersects the evolving implant, allowing the designer to see within and behind the implant to verify its shape and design.

By manipulating complex, organic 3-D shapes, Biomet’s team was able to create a perfect-fitting tibia implant construct from design to manufacture in just four weeks, reducing the typical six- to eight-week process of moving from design concept to use by at least 50%. Because the sculptural CAD system retained all images as STL files, the file was made into an Adobe Acrobat 3-D file and e-mailed to the physician.

Custom Glenoid Socket in Shoulder

Biomet’s patient-matched implant team also recently created an implant for a patient’s glenoid, the dish-shaped portion of the shoulder blade (scapula) where the long bone of the upper arm (humerus) meets the shoulder. The patient’s glenoid was worn away, and there was insufficient bone left from previous operations to utilize a standard off-the-shelf component.

Using the sculptural CAD system, Biomet’s designer imported the patient’s CT scan, positioned the model of the glenoid, custom-shaped a portion of clay to fill the space between the glenoid component and the bone, then subtracted the difference in the computer to obtain an image of the patient-matched implant that is required for a perfect fit. Because sculptural CAD systems are based on voxels (think 3-D pixels) and not on geometric constructs, such subtraction can take place in seconds compared with hours that a computer needs to calculate unusual curves and angles.

Sculptural CAD software also enabled Biomet designers to determine attachment sites for screws, bones, and other fixative devices used for implants much faster than by using traditional methods. The implant must be able to be securely attached to healthy bone. Sculptural CAD allowed Biomet to define the size and trajectory of screws in minutes, indicating to surgeons exactly where a screw enters the intact bone, the depth and the angle of entry, and possible locations for other screws or fixatives (see Figure 2 on p. 27). Typical CAD environments often require spending hours trying to define the position of screws. But in sculptural CAD, the designer can rotate, translate, and position the screw in limitless positions. The designer simply e-mails an Acrobat 3-D file to the physician for review and approval.

Calf Muscle Implant

Custom body parts aren’t restricted to replacement bones; they can also include implants that achieve the same subtle shape of musculature. For example, a 30-something female dancer was born with an underdeveloped calf muscle, and an off-the-shelf silicone implant was causing her great pain and exacerbating the atrophy of the existing muscles.

MedCAD, a Dallas-based medical modeling service bureau, and AART, a full-service implant provider in Reno, NV, designed a custom reconstructive calf implant that was approximately 25% smaller than the failed off-the-shelf calf implant and curved to perfectly match the patient’s other leg (see Figure 3 on p. 28). The implant did not operate mechanically like a muscle, but added appropriate volume and conformed perfectly to the patient’s existing leg shape. It provided greater stability and visual symmetry. After surgery, for the first time in her life, the patient danced on matching legs, and without pain.

Pectoral Muscle and Matched Breast Implant

Custom reconstructive muscle implants can also better replace off-the-shelf implants that are used to correct disfigurement that results from asymmetry. For example, Poland’s syndrome is a condition in which underdeveloped chest and chest muscles leave female patients with the appearance of having at best only one breast, and at worst, a sunken depression next to the normal breast. A physician recently treated a 50-something physically fit woman with Poland’s syndrome who had previously received off-the-shelf breast implants with unsatisfactory results.

Using sculptural CAD to achieve the perfect fit, MedCAD created a replacement pectoral muscle plate and a matched breast reconstruction. The firm designed a custom pectus excavatum (breast muscle) implant to first address the chest wall deformity, mirroring the intact breast on the other side and added mass that filled in the missing muscle tissue areas. The surgeon attached the implant to her intact muscle wall and used it as a solid foundation for addressing the missing, undeveloped pectoral musculature (see Figure 4 on p. 28). Using sculptural CAD technology, MedCAD also estimated and specified the best-fitting, off-the-shelf breast implant to match the opposing side.

Conclusion

The world of medicine has advanced dramatically since New York City’s Public Health Officer Hermann M. Biggs, MD, famously uttered, “The human body is the only machine for which there are no spare parts.” Now, off-the-shelf replacement body parts are commonplace. Patient-specific implants offer surgeons a superior option, not just for treating special cases, but also for replacing the replacements. These advances have begun to change the landscape for custom implant manufacturing.

David Chen, PhD, is chief technology officer of SensAble Technologies Inc. (Woburn, MA).