By Shalmali Pal

In the past two decades, additive manufacturing—commonly known as 3D printing—has evolved from a bleeding-edge, high-priced prototype with limited compositional materials to moderately priced systems with a wealth of manufacturing possibilities. Manufacturers of foot orthoses and ankle foot orthoses (AFOs) are taking advantage of this new technology, particularly as 3D printing’s price point of entry has decreased while its capabilities have increased.

Now biomechanics researchers are beginning to test the effects of new orthotic designs that take advantage of 3D printing capabilities, and to compare additive devices with their subtractive counterparts.^{1, 2, 3, 4, 5, 6, 7}

Subtractive techniques for creating foot orthoses and AFOs are limited due to the conventional technologies used in their manufacture, particularly when it comes to customizing or innovating new features during fabrication.

On the other hand, 3D printing offers design freedom and the ability to develop customized, innovative orthotic designs that offer a vast range of prescription options to clinicians, according to one study.

Scott Telfer, PhD, a biomechanics researcher at Glasgow Caledonian University in Scotland, and colleagues examined this issue in a 2012 single-subject study⁶ that evaluated 3D printing of foot orthoses and AFOs. They wrote the technology “may allow novel personalized orthotic devices to be produced which are beyond the current state of the art.”

He and his team used 3D printing technology to create a three-quarter length foot orthosis with adjustable metatarsal support elements (threaded to be adjustable with a screwdriver) to relieve pressure at the metatarsal heads, as well as an AFO designed to have adjustable stiffness levels in the sagittal plane. The foot orthosis was based on a direct scan of the participant’s foot, while the AFO was based on a 3D surface scan of a plaster cast.

Additive manufacturing allowed the AFO’s four components—a shank section, strut, foot section, and slider—to be composited separately and then assembled. Device stiffness was controlled by adjusting the pressure in two gas springs attached to the posterior aspect. The geometry of the strut section was designed to allow for adjustment of the foot to shank angle while maintaining the stiffness required to withstand the forces generated during gait. Both of these adjustability features would be difficult to create using traditional manufacturing methods, the authors wrote.

A man aged 29 years tested both devices at Glasgow Caledonian University motion analysis lab. As expected, the foot orthosis could be adjusted to significantly reduce peak plantar pressures under the second and third metatarsal heads during gait—reductions similar to those associated with conventionally manufactured foot orthoses in an earlier study.⁸ However, peak pressure reduction under the first metatarsal head was not statistically significant, and the peak pressures under the fourth and fifth metatarsal heads unexpectedly increased, leading the authors to suggest that some aspects of the device design need to be reevaluated.

The AFO tests confirmed that adjusting the device’s stiffness was associated with significantly different levels of plantar flexion during early stance. The authors believe that it may be possible to use this type of device to allow patients to tailor the support needed for different activities.

Telfer told LER that “in the 2012 paper, the material used for both foot orthoses and AFOs [Nylon 12] was somewhere between polypropylene and carbon fiber in terms of stiffness. It’s possible that this affected the results; however, when walking with devices, we weren’t able to tell the difference between the printed foot orthoses and standard ones.”

In the three years since the study came out, Telfer noted that “along with new printers, new materials seem to be released every week, so even with low-cost machines, it’s now possible to get a closer match to a standard range of materials than when we carried out the study. Beyond this, several printers now allow the actual properties of the material used to be programmed in at the design stage, and it’s now even feasible to use this in order to print a single device in one build that has regions with different material properties.”

A 2014 study took a comparative approach, using conventional and 3D printing methods to create foot molds, from which otherwise identical orthoses were made. Colin Dombroski, PhD, clinical and lab placement instructor in the Diploma of Pedorthics Program at the University of Western Ontario in London, Canada, and colleagues compared the arch height index (AHI) of custom foot orthoses manufactured using either additive or subtractive methods. They found that AHI was similar for the two devices.⁷

A 3D scan was taken of the participant’s right foot and ankle, and a foot mold was created of acrylonitrile butadiene styrene filament, layered at 2 microns. A second foot impression was acquired using traditional plaster casting, and a positive foot mold was made of gauging plaster.

Custom-made orthoses were fabricated from both molds. The researchers then assessed AHI during gait in a healthy young adult man while he wore shoes only, followed by wearing shoes with each type of orthosis. Although the findings indicated minimal differences in AHI (within .8 mm) among the control (shod) and the two orthotic conditions, the researchers noted the sample size of one prevented statistical analysis and called for further research with additional participants.

Dombroski’s group pointed out that “if proven successful in the future, this model could be a low cost method of custom foot orthotic manufacturing, thus controlling the ultimate cost to the clinician and end user.”

Wenjay Sung, DPM, a foot and ankle surgeon in Los Angeles and consultant podiatrist for the Los Angeles Dodgers, said he believes that research such as Telfer’s and Dombroski’s can help encourage the acceptance of 3D printed orthoses by practitioners and patients.

“These papers are good examples of momentum building,” Sung told LER.

“The feasibility of fabrication with 3D printed structures has already been proven in many industries including aerospace and medicine,” he added. “Now that the fabrication is not such a big challenge, the anatomical design-to-fabrication steps are currently the greatest opportunity to explore different methodologies.”

Future research will not necessarily be limited to the laboratory, Sung said, since the open nature of the 3D printing community will facilitate organic efforts to explore ways the technology will fit into current practice.

Shalmali Pal is a freelance writer based in Tucson, AZ.

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