The first time that Dr Peter Liacouras saw one of his custom-built, 3D-printed prosthetic attachments being used in the real world, he almost accosted an ex-serviceman at his local ice rink.

“I was preparing for my game and they have, at the same rink, a Wounded Warriors hockey skate,” he recalls. It was while idly gazing at the ex-servicemen on the ice – whose legs had been amputated below the knee – that the engineer noticed that one of the players was sporting a set of custom-built skates that he’d recently designed. Liacouras couldn’t contain himself, and began exclaiming at the top of his voice that the prosthetic attachment was his. It took a bit of further explanation to dissuade the serviceman that he wasn’t being accused of theft.

“It was like he was skating like he used to,” says Liacouras. “It was absolutely incredible to see.”

As the director of services at the 3D Medical Applications Center (3D MAC) in Walter Reed National Military Medical Center, US, it’s Liacouras’s job to oversee the printing of prosthetic attachments for applications that may seem mundane or esoteric yet make a vast difference to the quality of life for wounded US military personnel. Provided to servicemen and women who have lost hands, feet or entire limbs, these tools enable them to resume daily activities that they might not otherwise have the chance to pursue with standard prostheses. “That’s easily the most rewarding [aspect]: letting these guys accomplish not just the complex, but also the simple tasks that we take for granted,” affirms Liacouras.

Printing power

Liacouras has been designing and 3D-printing prosthetics at Walter Reed for more than 12 years. His fascination with the medical potential of additive manufacturing began at Virginia Commonwealth University, where he studied in an orthopaedic research lab. It was there that Liacouras first became acquainted with the world of limb mechanics.

“What I did was use a digital reconstruction of the ankle joint, [and] set up three-dimensional contacts, constraints, and represented ligaments and springs,” he explains. “Forces and torques were then applied to the digital model in the same fashion as previously performed cadaver experiments on a mechanical testing machine. We then compared the motion of the bones and length of the ligaments to the previous studies.”

That’s easily the most rewarding [aspect]: letting these guys accomplish not just the complex, but also the simple tasks that we take for granted.

This was all ample preparation for Liacouras’s work at 3D MAC, which he joined in 2006. Founded four years prior, the facility initially housed a large-frame vat photopolymerisation (or sterolithography) machine, in addition to a material jetting machine. Since then, the equipment roster has significantly expanded to include ten printers of various types, as well as computer-aided design (CAD) software, 3D scanners, a cranial-photogrammetry system and medical segmentation software, all of which is operated by a skeleton crew of six. “Four of us are probably the main worker bees, so we have to do everything,” adds Liacouras.

When the doctor first arrived at the department, most of the work 3D MAC was doing revolved around the threedimensional reconstruction of medical models. The US was fighting major conflicts in Iraq and Afghanistan, and there was growing demand among army surgeons for better tools to prepare for complex operations. “We saw a lot more conflictrelated injuries [and] crazy orthopaedic fractures,” he states.

In 2011, the National Navy Medical Centre and Walter Reed Army Medical Centre merged to form the Walter Reed National Medical Centre. All of the additive manufacturing capacity of the former was transferred to 3D MAC’s control, and internal interest in the department’s work spiked. The following year, Liacouras and his team completed their first prosthetic attachment.

“That was the shorty feet,” he highlights. “This one specific patient just wanted to hang out at the pool on his honeymoon, without getting his very expensive full prosthetic legs wet.”

Resembling cast-iron Brillo pads with metal nodules at their centre, these prosthetics may not have looked aesthetically pleasing, but they were much less cumbersome than full prostheses and were waterproof too. Liacouras estimates that 3D MAC has produced more than 80 pairs of shorty feet since 2012. This surge in popularity is attributable to their ability to allow recipients to reacquire mobility quickly across a range of situations.

He adds that the prosthetics are used for training, with patients starting out low before being brought higher over time. Shorty feet are also useful “if they want to get up in the middle of the night to use the restroom [or] if they want to play with their kids on the ground.” A modified version for use in the gym, called the ‘mechanic’s foot’, is also available.

Unique uses

Some of the prosthetic attachments 3D MAC prints are a little more esoteric. One user, for example, was provided with a specialised holder for wine glasses, which allowed the recipient to hold them with his prosthetic arm without crushing the stems. Liacouras has also overseen work on attachments for mobile phones, mechanical toothbrushes, deodorant and even a specially adapted fishing rod.

“I think the funniest project we ever manufactured something for was a crab claw and hammer,” Liacouras smiles. The recipient “wanted to eat his crabs and couldn’t successfully do it with his prosthetic hand.”

No matter the purpose, however, each design brings its own challenges. A case in point was an instrument 3D MAC produced that helped disabled veterans to lift weights. It’s proved to be a popular addition to the department’s portfolio, but is one with specifications that require careful consideration. Despite this, Liacouras estimates that his team has made up to a dozen variations of the device, either for quick disconnection from the prosthesis or to be customised to specific sockets.

“You have to look at the proper biomechanics,” he says. “For the weightlifting adaptor, the human hand’s only so big. You can’t have a patient do pull-ups and bench-presses, and [their] prosthetic hook be 6–7in long when the human hand probably has a difference of 4in [when] you’re going to be doing pull-ups and bench-presses.”

The choice of material is also a major issue, not least because there’s a limited number that can actually be used in 3D printing. Liacouras and his colleagues usually rely on titanium and ABS – a material named for its constituent plastic polymers, acrylonitrile, butadiene and styrene – to print prostheses; 3D MAC is looking to add in nylon at a later stage.

“But then there’s other challenges where, okay, maybe the 3D-printed material isn’t what they want,” he explains. “Maybe they [the hospital] want something soft and flexible. So, then we use the 3D printer to make a mould and fill [it] with silicon. So, it really is a challenge per case, and it is very interesting because sometimes we don’t get it completely right the first time.”

Every patient, on their residual limb, has a different centre of gravity… So we have to look at that, especially if we’re making a design that we want to be transferable to more than just that initial patient.

3D MAC’s work also doesn’t end once the design has been printed. Feedback from users is integral for determining where the products need to be improved, and how. The mechanic’s foot – which, according to Liacouras, resembles a bread bowl – has gone through several iterations.

“We went through a few curvature iterations on that piece, and a few design changes and mounting considerations so that they could be adaptable not just for the first patient, [but also] for the next patients,” he stresses. “Every patient, on their residual limb, has a different centre of gravity… So we have to look at that, especially if we’re making a design that we want to be transferable to more than just that initial patient.”

Model patients

One of the core services 3D MAC has provided throughout its existence has been the creation of custom surgical models, including skull plates, implants for dental surgeries or anatomical models to better prepare surgeons for complex operations. One of the most recent models the department created was a printed, rubberised eyeball, designed to feel as close as possible to the real thing.

“We’ve done a few eye simulation models,” Liacouras highlights. “We’re working on some intestinal models [from] ultrasound simulation models… Sometimes these take a little longer to develop [and] sometimes they [use a] very simple concept, and you’re wondering why nothing [similar] exists out there, or why it’s so expensive.”

Any hospital associated with the US Department of Defense can apply for a surgical model or a newly printed prosthetic for one of their patients. “They might draw something up,” says Liacouras. “We scan a lot of partial amputee casts that they make. They’ll bring it to us, we’ll scan it and make something custom [from] that scan. So, it’s really a collaborative effort between the engineers and the providers here on multiple different levels.”

What is most important during this entire process is that doctors and engineers understand what they want from the other. It’s so easy, adds Liacouras, for the vagaries of technical language to get in the way of that.

“Sometimes [doctors] go ahead and explain an aneurysm in very complex terms, and I have to say, ‘Okay, stop for a second. Explain what this means to me. What am I supposed to be seeing on this scan?’” he outlines. Furthermore, “If you start going into a lot of engineering terms with, say, the forces of movement [or] torque, they might understand the basic concepts, but then you say something like beta transformation matrix, global registration or point registration, and they’re not going to understand these terms.”

Ultimately, this is a minor worry for Liacouras. Any confusion as to the purpose/look of a surgical model or prosthesis is usually ironed out by the time 3D MAC boots up and goes to print. And, in any case, doctors and engineers have the same goal in mind: improving the lot of the patient.

“That’s what I think keeps us all here,” says Liacouras. It’s “serving these [ex-soldiers] who put their lives on the line for us, and being able to give back something.”