3D-printed Sockets Safe?
By Keith Loria
There are approximately 2 million people living with an amputation in the United States, a number that is expected to increase to 3.6 million by 2050 according to recent studies.
In the poster, “Clinical Trial Examining Safety and Feasibility of Definitive 3D-Printed Transtibial Sockets,” 1 Jared Howell, MS, CPO, FAAOP, director of orthotics and prosthetics at Baylor College of Medicine was part of a research team that attempted to validate the use of 3D-printed devices for orthotics and prosthetics care.
“While people have been fitting these devices and even making them in their own labs, there has been very little formal validation of the materials, the process, and the outcomes associated with 3D-printed prostheses,” he said. “If 3D-printed devices are to be successful, we need to be very deliberate in our approach and validation.”
In the study, the researchers looked at the long-term impact of 3D-printed devices for definitive use in orthotics and prosthetics, with subjects tested and fit with custom-made fused deposition modeling printed sockets made from polylactic acid (PLA), dictated by the Institutional Review Board committee as there was sufficient use of this polymer in other body interface devices.
“Subjects underwent a variety of tests prior to fitting to form the baseline data. Subjects were scanned and device models modified by an experienced clinician. These devices were then printed with connectors and all feet and pylons attached,” Howell said. “The devices were fit and validated using the same metrics used at visit one in standard of care. The subject was then seen at 2 weeks, and 6 weeks for follow-up and testing using the same battery of outcome measures to assess clinical success.”
Jared Howell, MS, CPO, FAAOP
The results revealed that 3D-printed devices tend to perform equivalently to standard of care devices. Additionally, there were no adverse or abnormal outcomes, and functional outcomes tended to produce similar results.
“One of the results that was surprising is many of the patients perceived that the 3D-printed prosthesis was lighter than their carbon version, even though the weights were virtually identical across subjects,” Howell said. “It is hypothesized by the team that this resulted from the texture of the print against the liner.”
Howell hopes people are willing to consider use of these devices in the future, but also exercise caution in the belief that PLA and other printed materials have the same strength to weight or strength to thickness ratios as carbon devices.
“This is simply not true. I hope that they understand that science and testing is required to validate and prove efficacy of new material processing devices before they are transitioned as the de facto process or technique,” he said. “I hope it peaks interest in 3D printing as an option and leads readers to explore and understand specific terminology related to the printing movement.”
Examining Differences: Carbon Fiber vs Fiberglass
Are there functional and clinical differences between carbon fiber prosthetic feet and fiberglass prosthetic feet in patients with lower limb amputations? A team at UT Southwestern Medical Center set out to find answers.
The discussion stemmed from the clinical questions that come up while treating patients, said David G. Wilson, MPO, CPO, LPO, FAAOP, assistant professor of the schools’ prosthetics-orthotics program, so the team looked to see if there was a difference noted between carbon fiber and fiberglass feet in the literature.
“We have our clinical experience, but wanted to know what the science said,” Wilson said. “When helping a patient select the best prosthetic foot for their needs, we often rely on anecdotal evidence. We were curious if there was more quantitative evidence in the literature that could be used to assist in making clinical decisions between carbon fiber and fiberglass prosthetic feet.”
Leslie Gray, MEd, CPO, LPO, FAAOP
In the poster, “The Clinical Differences Between Carbon Fiber and Fiberglass Composite Prosthetic Feet,” 2 a team of researchers went looking for answers.
The search strategy involved looking through databases PubMed, OVID, ScienceDirect, Google Scholar and Researchgate as well as oandp.org, with the search terms a combination of the following: “Dynamic elastic response feet,” “Carbon fiber,” “Fiberglass,” “Prosthetic feet,” “lower limb,” and “glass composite.”
The inclusion criteria were literature written in English published between 2000 and present day, which was primary peer reviewed; the exclusion criteria included articles from a secondary sources/reviews and product specific advertisements.
“We attempted to find all literature on the subject, and it was a challenge due to fiberglass feet being on the commercial market for a relatively short period of time,” said Leslie Gray, MEd, CPO, LPO, FAAOP, assistant professor and program director at the UT Southwestern School of Health Professions’ prosthetics-orthotics program.
“[Our review] showed that there is a gap in the scientific knowledge in this area,” she said, noting the project highlighted the opportunity for more research on this topic.
“There are some differences in stiffness that were found; however it’s a challenge to make widespread claims for patient selection at this time,” Gray said. “We assumed the literature would be limited, but we were surprised just how little research has been done directly comparing the different composite feet.” In fact, the team was only able to identify 5 peer-reviewed articles.
“This is a clinical question that is asked repeatedly, and patients have questions about the new feet on the market.” said Gray. “In order to provide the best evidence-based care, the field must understand what the literature says, even if there isn’t much evidence available in the literature yet.”
Socket by Filament Fabrication
The advent of 3D scanners, which can digitally capture complex residual limb geometries allowing for lower cost and higher degrees of customization, is driving renewed interest in socket usage overall.
While a research student at Rice University, Nicolette Chamberlain-Simon, now a research assistant at Northwestern University Prosthetics-Orthotics Center, was part of a research study whose goal was to use additive manufacturing to create a transtibial prosthesis with comparable strength to existing prostheses.
“My role was to develop the socket, while two others were working on the foot and the structural component similar to a pylon,” she said. “Ultimately we wanted to show that additive manufacturing can create definitive prosthetic components at a reasonable cost.”
Nicolette Chamberlain-Simon, MS candidate
A poster of the study, entitled “A Preliminary Investigation of Continuous Filament Fabrication for the Development of a Composite Prosthetic Socket,” 3 revealed just that. The study analyzed both the design process and the material properties of a composite socket. The socket was designed for a transtibial K3 prosthetic user under 100 kg.
“For those who have started incorporating 3D printing into an O&P practice, they often use it as a creative resource for prototypes rather than a method to create end-use components,” Chamberlain-Simon said. “Most of the research we’ve seen on this subject use PLA or Acrylonitrile Butadiene Styrene, which are very cheap, brittle plastics not suited for most O&P applications. I want people to see the potential of 3D printing when the right materials, the right printer, and the right mesh-editing softwares are used. I also want practitioners to see that 3D printing is not just for low-income countries.”
The study showed that the tools to create definitive sockets with additive manufacturing exist, but current printers need to be modified to make clinical production more feasible.
“It was surprising how little carbon fiber we had to use to significantly overpass the properties of copolymer,” Chamberlain-Simon said. “Most of the carbon fiber regions of the socket are only 5% carbon fiber by volume.”
It must be noted that a huge limitation of the study was that while the printer used had all the tools to create a definitive composite socket, the print bed was too short to print most sockets. Ideally this technology can be incorporated into larger print volumes so that any socket can be printed.
“For now, I’m working with a group to use the same technology to develop composite prosthetic feet using nylon and fiberglass,” Chamberlain-Simon said. “They are more appropriate for the size of the current print bed. We’ve already demonstrated that this foot is considered dynamic, but we need to perform static and cyclic tests to make sure it’s safe to use long-term.”
Semi-flexible Sockets Can Result in More Comfortable Experience
A lack of studies and progression of socket design led researchers at Loma Linda University to explore and illustrate how semi-flexible sockets can maintain rigidity and resist progressive shear forces from daily activity—making a prosthetic system a more comfortable experience for the users.
Travis Dubuque, MS, resident in Orthotics and Prosthetics and a member of the study team, said the team noticed that most studies evaluating forces applied to a residual limb are liner related. In other words, the only ideal comfort that an amputee could rely on would be from soft materials or gel liners applied to the limb itself.
“We thought it would be great to branch away from the norms of comfort and create a study where comfort could be accommodated from the socket itself,” he said. “From my years as a prosthetic technician, I was always told to make our sockets strong. With strength, comes a very rigid socket that may almost be indestructible for some time. However, a rigid socket applies more force to the limb during certain phases of gait. We were hoping to create a foundation for socket design that could be extremely light and flexible enough to not only distract a tremendous amount of force applied to the patient’s limb, but to also be able to disperse forces being applied to the socket.”
In the poster, “Reduction of Forces Using Semi-flexible Sockets on Transtibial Amputees,”4 the Loma Linda University study team found that it is plausible to create a thin, semi-flexible socket that could be almost half the weight of normal prosthetic sockets and could handle the day-to-day activities of a prosthetic user.
“In our study, we asked our patients to take our socket home for a week at a time and to solely use our socket,” Dubuque explained. “We had three athletes in our study that were above average ambulators, one of which was an avid soccer player who decided to play a game while wearing our socket. He described our socket as the most comfortable socket he had ever worn and was surprised at how strong the socket still was.”
That player described that when he would pivot or come to an abrupt stop, he would not feel the normal pressures that his current rigid socket would give, according to Dubuque. Lastly, he stated that the socket was the closest thing to having his limb back because of the weight reduction and comfort of the flexibility in the socket.
“We did not re-invent the wheel. However, our purpose was to shine light on socket design and acknowledge that comfort can come from the prosthetic socket and distribute those unwanted pressures that could be troublesome,” Dubuque said. “Health of the patient’s limb was considered to limit ulcers, pressure sores, and overall skin breakdown. We hoped to create a foundation to push our peers to research better application for socket comfort.”
“We acknowledge that what we did is not groundbreaking. However, we hope that our research shines light for new ideas in socket design technology and patient acceptance for prosthetic use,” Dubuque said. “By allowing us to progress a design for socket comfort, we also hope to expand the lifetime of costly prosthetic gel liners and soft materials used in our patient’s daily application.”
Keith Loria is a freelance writer in Washington, DC.
- Le DM, Arredondo CN, Soyars CR, Howell J. Clinical Trial Examining Safety and Feasibility of Definitive 3D-Printed Transtibial Sockets. Poster presented at American Association of Orthotists & Prosthetists; March 6-9, 2019; Orlando, Florida.
- Hatler KR. Gray L, Wilson DG. “The Clinical Differences Between Carbon Fiber and Fiberglass Composite Prosthetic Feet” Poster presented at American Association of Orthotists & Prosthetists; March 6-9, 2019; Orlando, Florida.
- Chamberlain-Simon N, Araya-Calvo M, Lopez-Gomez I, Sanchez-Brenes O. A preliminary investigation of continuous filament fabrication for the development of a composite prosthetic socket. Poster presented at American Association of Orthotists & Prosthetists; March 6-9, 2019; Orlando, Florida.
- Dubuque T, Ortiz M, Sabate J, Stephens K, Smith I, Bains G, Becerra B, Kingsley A. Reduction of Forces Using Semi-flexible Sockets on Transtibial Amputees. Poster presented at American Association of Orthotists & Prosthetists; March 6-9, 2019; Orlando, Florida.