A Question of Quality: Measuring Strength and Durability in 3D-printed Lower-limb Prostheses

[caption id="attachment_252081" align="alignright" width="309"] Photograph courtesy of UNYQ.[/caption] A lot depends on the strength, stamina, and reliability of weight bearing lower-limb prostheses, with both safety and patient satisfaction in the balance. When concerned critics consider the possible catastrophic failure of such a device, the adjective does not exaggerate the potential consequences of such an unexpected collapse. Natural concern for their patients’ safety guides prosthetists in choosing lower-limb devices that have demonstrated their durability and stood the test of time, but devising safe methods of testing and proving the trustworthiness of new materials and prosthetic designs—such as 3D-printed lower-limb sockets—presents a challenge to conscientious innovators. Tony Gutierrez, CP, national clinical specialist, Bionic Prosthetics and Orthotics, headquartered in Indiana, notes that standards are necessary as a reference point to provide liability protection for clinicians and technicians interested in providing lower-limb devices created through additive fabrication. “They want a standard,” says Gutierrez. “We have a standard within our practice because it seems acceptable, and the failure rate we have is minimal. We know when a socket may break, and we can design some reinforcement into the socket to prevent that. It’s not very often we hear about a 3D socket breaking. “When we’re doing diagnostic sockets, we limit them to in-clinic supervised fittings, so we have the patient in a safe space. They’re utilizing parallel bars for support while walking. We are taking standard precautions that we would take even with a standard fabricated test socket. We know that our machine’s output is consistent. And we’ve shown that over time, with that added consistency of additive manufacturing over traditional fabrication, we’re able to prevent the mistakes that we see in standard fabrication where sometimes the vacuum form is uneven in some areas, or too thick in one area and too thin in another area. “Medicare has no standards that define what a check socket is, but it’s a clear thermoplastic socket that’s used to verify fittings. No ISO [International Organization for Standardization] or other standard is required, so we are routinely seeing Medicare, Medicaid, private pay, and VA patients at all of our clinics and using this technology, and there’s no issue with reimbursement.” Joe Johnson, CP, owner of Quorum Prosthetics, Colorado, also deplores the absence of much-needed prosthetic standards. [caption id="attachment_252083" align="alignright" width="300"] Photograph courtesy of Quorum.[/caption] “When we make a Quorum socket,” he explains, “we do wet lamination, and the practitioners have in mind a layout of how we want that socket created, so we hand it off to our technicians, providing them a recipe for carbon fiber. It’s up to the technician to decipher that recipe and create that socket for us. We depend on that technician to interpret our note and take us to the final product as we envisioned it. That’s wet lamination. “In 3D printing, it’s much more precise because we have layer lines of one micron each in 3D printing, which Hewlett Packard has used for applications other than O&P for years. If we ever had a failure in a 3D-printed socket, we could identify that exact layer line and find out what happened. Not only is that socket engineered to a precise standard, but through the use of finite element analysis, engineers could trace and correct the flaw—we can do that with our sockets before they’re even printed.” Finite element analysis (FEA) is the simulation of a physical phenomenon using a numerical mathematic technique called the finite element method (FEM). This process is at the core of mechanical engineering, and is often used in illustrating three-dimensional structural failures in bridges and buildings; it’s also a key principle used in developing simulation software , and is usually integrated into computer-aided design (CAD) software, making it much easier for engineers to go from design to running complex structural analysis. “There’s going to be a standard,” Johnson predicts. “We’re doing a white paper, and we’re sending the abstract to AOPA [the American Orthotic and Prosthetic Association] as a guideline for developing that standard—the first standardization, really, for all prosthetics, but specifically for 3D Multi Jet Fusion-printed sockets. I’ve adopted it in our practice; I’ve done hundreds and hundreds of these sockets with no failures.” As someone with limb loss, he reports testing his own products by walking millions of steps, and notes that his last socket had almost 4 million steps but felt brand new. “I think we know what we’re doing,” he concludes. “But I understand people’s concerns, so I’ve got patent-pending carbon-fiber reinforcement that I can insert into the print process if somebody’s concerned.” Whose Standards? ISO is an independent, nongovernmental international organization with a membership of 168 national standards bodies. Through its members, it brings together experts to share knowledge and develop voluntary, consensus-based, market-relevant international standards that support innovation and provide solutions to global challenges. Its 140-page guideline for lower-limb prostheses is available at ISO 10328:2016 - Prosthetics — Structural testing of lower-limb prostheses — Requirements and test methods . The site explains that “ISO 10328:2016 specifies procedures for static and cyclic strength tests on lower-limb prostheses, which typically produce compound loadings by the application of a single test force. The compound loads in the test sample relate to the peak values of the components of loading, which normally occur at different instants during the stance phase of walking.” Some parts of the world place a higher value on observing the ISO recommendation than others: Eythor Bender, vice president, UNYQ, based in California and Seville, Spain, confirms that the additive manufactured socket his company developed and tested in 2018 (before COVID forced them to delay its launch) was successfully tested using the ISO 10328 protocol. “We used their protocol for cyclic testing of two million cycles,” he notes. “The good news is that there has been more work done on creating a standard in terms of 3D printing. We were able to prove at least that this socket held up very well at two million cycles. And now,” he adds, “we have also tested our 3D-printed foot for up to three million cycles.” The durability of 3D-printed devices, he agrees, is best demonstrated by continued testing in daily use. “Our popular prosthetic covers, for example, are currently being tested in the market by many people, because they’ve been around for eight years, and we’ve had people wearing them for years. The colors might be a little faded, but the durability has improved tremendously, especially since 2017 and 2018, when there was an influx of new 3D printers and materials that really helped to improve the quality.” Sean McClure, BME, ME, R&D engineer and director, Quorum Prosthetics, agrees that Quorum is actively pursuing standards development through their white-paper study and testing of Multi Jet Fusion-fabricated sockets. They hope their test data will provide helpful evidence to AOPA in its continuing efforts to develop an appropriate standard for Multi Jet Fusion-printed sockets. “We did an ultimate strength test, loading a 3D-printed socket versus a carbon-laminated socket, loosely based on ISO standard 10328. We loaded it in a heel strike or toe-off position but modified it to make it a bit more extreme—so it doesn’t exactly fit ISO standard 10328, which focuses primarily on the prosthetic components unrelated to the socket. “Our conclusions were that our 10328-inspired approach may have deviated from the true standard, but still provided us with a ballpark idea of where it needs to go, and in what direction to take future testing. It will at least give us the data for a really rough generalized justification that these 3D-printed sockets will hold up in a close comparison to the carbon fiber socket that has been traditionally regarded as a benchmark.” McClure notes that they are developing additional test parameters in partnership with HP and other companies doing socket testing. “We’re also putting it into finite element analysis to simulate the forces of walking or extreme cases, so that we can predict how the 3D-printed sockets are going to react under certain loading. “That’s really only possible by creating a digital CAD model of the socket, which has never been done before, since it’s all made by hand. It will require a combination of using engineering techniques on custom-made sockets, and then being able to 3D print certain designs that may hold up better. We’re in the process of discovering what those designs look like in order for these sockets to be safe—and thus something that practitioners can confidently trust.” Comparing Materials Gutierrez is also pursuing collaborative partnerships in pursuit of definitive 3D-printed sockets. A relationship with Filament Innovations and their partner, Mitsubishi Chemicals, is leading the way in the development of a filament that can be utilized in a definitive aspect, he notes. “We haven’t been able to print anything that we’d be able to call definitive yet. But we’re utilizing a copoly-based filament that we actually tested in partnership with a university in Chicago: the Center for Lower Extremity Ambulatory Research (CLEAR) at Rosalind Franklin University.” The mechanical testing study looked at 3D-printed PETG sockets, traditional PETG sockets, carbon fiber laminated sockets, printed copoly sockets, and printed polycarbonate sockets. An ultimate force test was performed on each socket, essentially trying to bend it until it broke, based on specific loading criteria outlined in the study, which has been utilized in other socket testing studies. “We did see that carbon was the strongest, as we anticipated,” Gutierrez reports. “We also saw that the 3D PETG sockets compared to the traditional PETG sockets were very close in performance. And in fact, the 3D-printed sockets performed more consistently compared to the standard fabricated sockets because of more consistency in the fabrication method. The printer does the same thing every time, whereas human error in traditional fabrication can lead to some inconsistencies and waste,” Gutierrez says. “Even the failure results were good news, confirming consistency; we knew they were going to fail, but we also knew how they were going to fail: more and more consistently than a standard. One of the more promising results of that study as well as the results on the copoly socket demonstrated that when carbon fiber breaks, it’s a catastrophic break; it will be strong until it breaks. The copoly will break eventually, but it revealed a “bend-before-it-breaks” reaction in the study that can serve as a warning. “ Since there are not a lot of studies out there on the strength of prosthetic sockets, this study allowed us to show, through some definitive numbers, why printing could be beneficial in a practice and how strong the material is, despite the popular misconception that it’s too weak, and it’s going to break. We showed that it’s equivalent to what we’re using traditionally, in some cases.” There’s a lot yet to learn, granted; but what they’re testing and investigating is providing a lot of promising new knowledge, experts agree. “I once had a small business, too,” empathizes Dustin Kloempken, 3D printing application engineer, HP. “To change your business over to a completely different workflow is risky for anyone to do. Questions remain—such as how strong do 3D-printed sockets have to be—and who decides? “But additive fabrication is not something that we need to be afraid of; we just need to tread lightly, investigate thoroughly, and make sure that ultimately, at the end of the day, we’re taking good care of our patients.” Judith Philipps Otto is a freelance writer who has assisted with marketing and public relations for various clients in the O&P profession. She has been a newspaper writer and editor and has won national and international awards as a broadcast writer-producer.

A Question of Quality: Measuring Strength and Durability in 3D-printed Lower-limb Prostheses

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