ARTICLE FOCUS:
• Designing a suitable prosthesis
• Using DMLS to build the prosthesis
• Looking forward to the next design

Depending on the rock and route, rock climbers use an assortment of gear from carabiners, cams, and chocks to harnesses, helmets, and specialized climbing shoes. While some footwear is stiff, other shoes are extremely flexible. Different shoe styles are designed to work with differing rock faces that can include cracks, small toeholds, or sloping slabs.

The shoes are important for all climbers, but for C.J. Howard, a northern California-based climbing enthusiast and a lower-leg amputee, even more important is the customized prosthetic foot that he designed with climbing partner and environmental/aerospace engineer Mandy Ott.

A life-long athlete, Howard was a distance runner for the University of California, Irvine, cross-country team before a diagnosis of osteosarcoma. The cancer led to amputation of his left leg just below the knee. Howard ran competitively as an amputee, setting world records in several events. Then he met Ott, who introduced him to climbing in 2008.

When he began climbing, he simply had his standard artificial foot fitted to a climbing shoe. Unfortunately, the shoes wore out quickly because of the prosthesis’ generic shape. He and Ott decided to come up with something that would work better and last longer. Howard told Ott that he would like to have a foot that’s “more suited to climbing.”

“So I got out my laptop,” says Ott, “opened an engineering CAD program, and on-the-spot created what he described, which was the shape of an aggressive climbing shoe with a downturned toe—like a banana. I just drew what he wanted. It was simple.”

The next step was to determine the method for manufacturing the new prosthesis. Ott suggested an additive manufacturing technique called direct metal laser sintering (DMLS), which she had encountered in her mechanical engineering work at a major aerospace company.

The technology, which is often used to build one-of-a-kind prototypes, can complete the part in just a few hours. “I never even thought about fabricating it using traditional machining techniques because that process would result in seams in the foot or there might be nuts and bolts sticking out,” says Ott, “and that wouldn’t work well for climbing.”

Ott contacted Morris Technologies (morristtech.com), an Ohio firm that specializes in additive manufacturing and is the largest deployer of DMLS in the world. Morris also had served as a supplier for an earlier project she had worked on. “The rest just fell into place,” she says. “They completed the prosthesis just before Christmas, and when C.J. opened his surprise present and saw the new foot, he had a grin on his face that was priceless.”

Building the prosthesis, one step at a time

Unlike traditional metalforming techniques—such as milling, drilling, sanding, and polishing—that remove, or subtract, material from a solid block, the metal laser-sintering process builds (or “grows”) an object layer-by-layer in an additive manner. DMLS manufactures solid metal parts directly from powdered metals and provides a wide range of part properties, from controlled porosity for venting or filtering to fully dense structures with a higher strength than castings and forgings.

Ott’s digital CAD model of the prosthesis provided the three-dimensional blueprint to guide the process. The Morris engineers chose Ti64, a commercial-grade titanium with superior strength and light weight, which were important properties for the intended use (see the sidebar, “EOS Titanium Ti64”).

Fabricating the approximately 6 × 3 × 2-in., smooth-edged foot took about 40 hours. The CAD file was uploaded to the DMLS machine, where it was converted to a 2D sliced file. Titanium in powder form was deposited on a platform in the build chamber of the system—a unit manufactured by German-based EOS GmbH, developer of the DMLS technology. A focused 200W laser traced the first cross-sectional slice of the foot, melting and hardening the metal powder at high heat into a solid replica of the digital data. Fresh powder was then reapplied, and the next layer of the CAD model was outlined by the laser, fusing it to the first.

The process built one 20-µm layer (about the thickness of five human hairs) at a time. Following the fully automated build cycle, excess powder was removed and the prosthesis was stress relieved in vacuum conditions at 1,202° F + 25° F for about three hours to cure the metal and ensure material strength properties.

To minimize weight, the five-lb. foot was designed to be hollow. The single-piece construction with no seams or fasteners was coated with a rubber used for climbing shoe soles by a separate vendor. A solid titanium rod connected to a socket and Howard’s upper leg.

As designs go, Howard’s climbing prosthesis was fairly simple. Specialized prostheses (replacements) and orthoses (braces) for competitive disabled athletes who run, ski, and cycle can be more complex (see sidebar, “Laser-sintered device enables disabled athletes”). Tim Warden, Morris Technologies’ vice president of sales and marketing, points out that the DMLS process is also perfect for producing medical products with even more critical geometries. This could include orthopedic implants for hips, knees, shoulders, ankles, and even spines, as well as patient-specific surgical instruments. A growing list of materials—including biocompatible plastics and metals—is enabling Morris engineers to consider laser sintering for a number of cutting-edge medical applications.

Putting the best foot forward

Climbing as often as he can, Howard has now had the opportunity to try out his new prosthesis in on a variety of different climbs. In northern California, test sites included the granite of Tahoe’s Lover’s Leap, the single-pitch traditional routes of Phantom Spires, sport climbing at Luther Spires, and the crevice and chimney systems of nearby Sugarloaf. He also has climbed on dome and crack routes in Yosemite using the specially created foot.

“Designing a prosthesis for climbing involves a little bit of trial and error,” says Howard. “Now that I’ve used this one, I know what types of climbing it works really well on and what types it doesn’t.” The first design happened pretty quickly, Ott adds. “Both C.J. and I agree that there are changes that we would like to try out.”

Howard and Ott have considered at least two more artificial foot designs that could be used by disabled climbers, including one shaped like a triangle for pure crack climbing and another with less downturn for more slabby conditions. The different designs can accommodate different climbs, Howard says.

There are some issues still to be addressed with the rubber, which didn’t initially adhere well to the surface of the prosthesis. Warden notes that specialized software can be used with DMLS to create surface features on the titanium that would make it easier for the rubber to adhere. The technology can also build lattice or trabecular structures into prostheses to make them lighter.

Laser-sintered device enables disabled athletes

Competitive sports have seen increased participation in recent years from athletes with physical disabilities. The well-known Paralympics, officially launched in 1960, and the Extremity Games, first held in 2005 for “extreme” sports—such as rock climbing, mountain biking, skateboarding, kayaking, and other events—provide opportunities for disabled athletes to compete on a world stage.

Athletes with amputations often require specialized prostheses (an artificial body part) to participate, while those with impaired function rely on orthoses (braces designed for support). Both types of devices are available in over-the-counter versions. But if customized to the individual, they can, in many cases, enhance performance.

As the result of a spinal cord injury, German professional racing cyclist Michael Teuber is a semi-paraplegic and requires ankle-foot orthoses to race. Currently, he uses a version developed by Ortema (an orthopedic technology firm) and Junior and Tacke (engineering consultants). The orthoses were manufactured of polyamide using plastic laser-sintering technology from Germany-based EOS GmbH.

A computer-aided design (CAD) model was developed from a plaster impression of the cyclist’s leg. The DMLS process built Teuber’s lightweight, durable orthoses, which were tailored to Teuber’s exact anatomy. The orthoses incorporated features beneficial to cycling, such as stability, light weight, and ventilation.

While already a highly successful cyclist with many world records, titles, and victories, Teuber added more to the list with his laser-sintered orthoses, winning a gold medal in the 2008 Beijing Paralympic 24-kilometer individual road time trial.

EOS Titanium Ti64

EOS Titanium Ti64 is a titanium alloy powder that has been optimized especially for processing on EOSINT M systems. The following describes information and data for parts built using EOS Titanium Ti64 powder (EOS art.-no. 9011-0014) on the following system specifications:

• EOSINT M 270 Installation Mode Xtended with PSW 3.4 and default job Ti64_30_030_default.job
• EOSINT M 270 Dual Mode with PSW 3.5 and Original EOS Parameter Set Ti64_Performance 2.0
• EOSINT M 280 with PSW 3.5 and Original EOS Parameter Set Ti64_Speed 1.0

Parts built in EOS Titanium Ti64 have a chemical composition corresponding to ISO 5832-3, ASTM F1472, and ASTM B348. This well-known light alloy is characterized by having excellent mechanical properties and corrosion resistance combined with low specific weight and biocompatibility. This material is ideal for many high-performance engineering applications, including for the production of biomedical implants (note: subject to fulfillment of statutory validation requirements where appropriate). Due to the layerwise building method, the parts have a certain anisotropy, which can be reduced or removed by appropriate heat treatment.

Readers say “Yes” to prosthetics for Olympic athletes

Last month’s MedicalDesign.com reader poll asked: Do advanced carbon-fiber prosthetics, such as those worn by the South African sprinter during the 2012 Olympic Games in London, provide an unfair advantage? The majority—67%— answered no.

This month’s poll asks: Should medical device makers play a greater role in the battle against obesity? Let us know what you think at MedicalDesign.com.