The world of 3D printing has opened up new possibilities in every area of healthcare, including that of lower extremity medicine. 3D printing—sometimes called additive manufacturing—has already led to major changes in healthcare, but experts say it’s important to remember that clinical utility should drive this area of technological innovation rather than the other way around.
“Many three-D printed products on the marketplace are designed for the sole purpose of being three-D printed, as opposed to being designed for the functional needs of the human body,” said Andrew Pedtke, MD, a San Francisco-based orthopedic surgeon and founder of a company focused on 3D printing of prosthetic devices. “Cheap, fast, and sleek manufacturing is irrelevant if not coupled with purposeful design and function.”
That purposeful design and function can be found through numerous lower extremity applications that, in addition to prostheses, include foot orthoses, ankle foot orthoses, orthopedic implants, fracture casting, and footwear.
Additive manufacturing processes have the potential to greatly enhance the clinical effectiveness of foot orthoses, which traditionally have been manufactured using subtractive processes akin to chiseling away a block of marble to create a sculpture.
Personalized insoles have long been used to enhance exercise, especially the biomechanics and comfort of running. But 3D printing can take things further with embedded sensors that track everything from pressure points to temperature. Athletes will also benefit from the use of additive technology to fuse different materials within a single orthotic device.
“Three-D printing can generate a dynamic structure that creates a varying degree of flexibility and rigidity in the insert itself based on the user,” said Wenjay Sung, DPM, a foot and ankle surgeon and consultant podiatrist for the Los Angeles Dodgers. “For athletes, this means lighter, stronger, and more durable inserts that take less space in cleats or athletic wear. Also, the decreased volume and weight of the product will lessen the burden on the athlete’s feet.”
The ability to combine materials of different rigidities and densities in a single insole will also potentially benefit patients with diabetes and other conditions associated with variable pressure and shear patterns in different regions of the foot.
Ankle foot orthoses
Because customization of ankle foot orthoses (AFOs) involves even more variables than customization of foot orthoses, mass customization of AFOs stands to benefit even more from 3D printing technologies. A preliminary study from KH Kempen University in Geel, Belgium, and another from the University of Texas at Austin, have found that traditionally manufactured passive-dynamic AFOs and those manufactured using the additive process of selective laser sintering have similar effects on gait in patients with foot drop and other lower limb impairments.
Not only can 3D printing potentially improve customization of orthoses, it might also allow for much more affordable pieces. San Francisco-based Not Impossible Labs has been a champion of such efforts, and a 2014 collaboration between its engineers and robotics enthusiasts from Granada Hills High School in California resulted in a 3D-printed, low-cost exoskeleton with robotic leg elements designed to enhance gait in children with cerebral palsy and other conditions.
Outrageously fashion-forward footwear has grabbed most of the 3D printing headlines in the mainstream media, but using additive technologies to create custom footwear isn’t just about fashion.
Enterprising companies are currently beta-testing shoe manufacturing processes that utilize 3D printing to customize shoe insoles without the need for a conventional footwear last. Many of the same potential benefits of 3D printing for foot orthoses, of course, also apply to footwear. The use of additive technology for mass customization also has the potential to improve fitting issues for people with wide feet or foot deformities, or those whose feet are two different sizes. And additive footwear manufacturing also requires fewer seams than conventional methods, which is a unique potential advantage for patients with diabetic neuropathy.
Currently, 3D printing of orthopedic implants is primarily focused on upper extremity applications that do not involve weight bearing, but lower extremity implants are already in the works. The design complexities that are possible with 3D printing can lead to implants that more closely resemble the bony structures they are replacing, which could lead to better outcomes.
“Many orthopedic implants are much stiffer than the bone that they will interact with or replace, which causes problems and often leads to premature implant loosening,” said Ola L. A. Harrysson, PhD, the codirector of Center for Additive Manufacturing and Logistics at North Carolina State University in Raleigh. “With three-D printing we can design very complex mesh structures where we can control the properties so that we can mimic the bone much better. This research will allow us to replace damaged bone with a metal implant that will act with the bone and like the bone.”
Customization has long been a part of the prosthetics field with regard to both fit and fashion, and 3D printing has the potential to take those concepts to new levels. Amputees who love to express their individuality with 2D-printed graphics related to favorite sports teams and other interests will have the option to upgrade to prosthetic panels featuring 3D renderings of tattoo designs, lace, or other patterns.
Additive manufacturing also has the potential to facilitate the mass customization of one-of-a-kind, task-specific prosthetic designs, such as a prosthetic foot in a cupped shape for rock climbers.
Researchers from the University of Akron in Ohio used additive manufacturing to create a prosthetic socket with a helical cooling channel within the socket wall to address the skin issues that result from residual limb sweating. In a 2015 study, the prototype socket effectively created a temperature differential across the socket wall.
Clinical relevance is key, Pedtke emphasized.
“We must understand that the human body is dynamic and built for homeostasis,” Pedtke said. “Just like people reject medications if taken inappropriately, an amputee will reject an ill-fitting prosthetic socket.”
Fracture casting differs from many other lower extremity applications for 3D printing in that the conventional plaster-casting method it aims to replace is essentially an additive process. The advantage of 3D printing here is its ability to create complex shapes; early designs feature a latticed plastic material that is breathable as well as lightweight and washable.
In one prototype design, the material is typically printed in a single cylindrical sheet with built-in fasteners on its edges that snap closed after the cast has been positioned. Another prototype cast comes in two pieces with a secondary locking mechanism.
As with foot orthoses, the ability to use 3D printing to embed electronics in a device could take fracture healing to the next level. One early 3D casting concept allows for the delivery of ultrasonic bone stimulation through a port in the cast, which could help speed healing. A similar Spanish design features a sensor that offers two-way transmission—it delivers electrostimulation to the injured site and also communicates patient data to the clinician.
Though 3D-printed cast prototypes have so far been limited to the upper extremities, experts are optimistic that the technology can evolve to allow for weight bearing.
Shannon Lee is a freelance writer based in Pennsylvania.