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3D Printing Orthotic and Prosthetic Devices

By Arif Sirinterlikci Department of Engineering, Robert Morris University
By Isaac Swink Department of Engineering, Robert Morris University

Additive is making affordable customized devices

3D printing has become the medium of the new technological revolution as its applications diversify from printing food to weapons, from clothing to industrial products. It is also finding more uses in the medical space, including Orthotics and Prosthetics (O&P).

According to Constantinos Mavroidis, director of the Biomedical Mechatronics Laboratory at Northeastern University (Boston), for 3D printing to be used on a greater scale in O&P, development and manufacturing times and their associated costs need to be reduced. Making customized O&P relies on some form of medical imaging or 3D scanning data while canceling the need for plaster casts. Customization is the key for better O&P devices, however it increases the costs. In contrast, 3D printing allows digitization of O&P device designs and their manufacturing process, enabling quick reprints and changes in the designs leading to some savings.

The concept is currently being applied to ankle and foot orthoses (AFOs) and prosthetic sockets. The cost of the 3D printers has been decreasing, but an industrial printer with good accuracy and repeatability can still cost a few hundred thousand dollars, not to mention the material costs when needed for metal alloy implants. However, in the case of O&P devices, lower-end Fused Deposition Modelers (FDMs) can be helpful in printing AFOs or low-cost prosthetic sockets. As low-cost prosthetic sockets are being developed for third-world countries, scientists are also investigating concepts such as the Variable Impedance Prosthetic socket (VIPr) for Transtibial Amputees. VIPr is being developed to achieve lower interface peak pressures over bony protuberances and include 3D-printed elements made from rubber-like PolyJet Polymers, Vero White Plus and Tango Black Plus. MRI technology and Finite Element Method are also used to understand the compliance levels of the device.

Novel Devices

Potential O&P devices are not limited to AFOs or prosthetic sockets. Exoskeletons, fairings, and medical casts along with others are also being considered.

Medical exoskeletons promise a great future even though they are not as impressive as Iron Man’s exo-suit. Body-powered or electrically actuated exoskeletons designed to assist humans with neuromuscular diseases is another potential application for 3D printing.

Bespoke Fairings are customized coverings that surround an existing prosthetic leg to give the impression of an original leg. They are created from a 3D scan to capture contours of the exterior of an existent leg and used mainly to boost the morale of the prosthetic user.

Osteoid medical cast: Deniz Karasahin, an industrial designer, developed the Osteoid, a functional medical cast attachable as a bone stimulator. This 3D-printed cast has the ability to deliver low-intensity pulsed ultrasound (LIPUS) which can reportedly reduce the healing process time for the bone by up to 38%. Non-union fractures showing no sign of healing after three to six months have also been healed up to 80% by using this device. A 3D scan of the patient is modified using the modeling software. The modified CAD geometry is then employed in 3D printing with the FDM Process in making the Acrylonitrile Butadiene Styrene (ABS) cast. The software is used to design a locking mechanism and then generate the holes with an algorithm. The cast is made from two pieces assembled together like a jigsaw puzzle with a central hole passing through the edges of the pieces where a flexible pin is inserted to firmly connect them together.

Another trend is to incorporate sensors to these 3D-printed O&P devices. Mavroidis and his colleagues have been working on fabricating smart orthoses containing sensors printed along with them. These sensors are expected to determine the structural state of the device or keep track of the patient’s gait.

Materials and Processes

Material selection in 3D printing has been improving greatly due to heavy industrial focus, and with some help from the Rep-Rap movement. Many different composites are being developed. Robert Morris University (RMU) is active in this regard, with its work in Fused Deposition Modeling (FDM) and Stereolithography (SLA) processes. Ease of recycling is another advantage for material development where an RMU team is mechanically alloying leftover FDM ABS material with other ingredients for various purposes. The concept of natural fibers is also being employed and tested including wood particulates and bamboo fibers and may be applicable to low-cost O&P devices.

Medical implants have been fabricated dominantly from metallic compounds such as stainless steel or titanium alloys. However, polymer materials are being developed for implant purposes. Polyetherketoneketone (PEKK) is a semicrystalline thermoplastic material. Its structure gives PEKK a high heat resistance, chemical resistance, and the ability to withstand high mechanical loads. Oxford Performance Materials (OPM; South Windsor, CT) developed two PEKK composites, OXPEKK-MG (medical grade) and OXPEKK-IG (implant grade). OPM materials come in the form of amorphous and crystalline injection grade unfilled pellets, composite pellets, crystalline extrusion rods, mono or multifilament films. Filler materials include metallic compounds and carbon. OPM was granted the first patent for a 3D-printed polymer implant and has the ability to create a wide range of implants including cranial maxillofacial, upper extremity, and small bone implants for hands and feet. Selective Laser Sintering (SLS) process (labeled as OsteoFab by OPM) and the newly developed PEKK materials allow fabrication of these custom polymer implants for patients within short turnaround times.

The benefits of customization and characteristics of the PEKK composites result in a rough structure with strong osteo-integration ability. SLS process is desirable for polymers because the part can be built without fully melting the substrate while the mechanical properties of the parent material are still maintained at the grain interfaces. Other than sterilization, no major operations, including finishing or polishing, are needed for postprocessing.

General characteristics of OXPEKK biomedical polymers include:

  • Radiolucence
  • Osteoconductivity
  • Density and stiffness similar to bone
  • Excellent abrasion resistance
  • Minimization of detrimental stress shielding
  • Compatibility with common sterilization methods
  • Twice the compressive strength of Polyetheretherketone (PEEK)
  • Chemically inert and nonabsorbable

OXPEKK-IG materials were designed to meet the FDA and the European Union requirements for use in Long Term Human Implantable Medical Devices. OXPEKK-IG had been tested according to ISO 10993 (Biological Evaluation of Medical Devices) through a successful 52-week biocompatibility test. In 2013, OPM received its first 510(k) clearance for the OsteoFab Patient Specific Cranial Device, a 3D-printed device made from the OXPEKK-IG. OPM’s South Windsor facility was also certified to ISO 9001 and ISO 13485 quality systems and with 100% dimensional inspection certification.

Future Work

The role of 3D printing in the medical field will grow greatly and will include the O&P devices mentioned in this article and more. 3D printing is poised to change the medical field due to its quicker turnaround time compared to its competitors, fabrication ability of customized devices which closely replicate desired body characteristics, and the potential that encompasses bone implants, exoskeletons, dental prosthetics, and even bioficial organs. Recent developments of 3D-printed full jaw and hip replacements are signs of what is to come. Even the technology of the hobbyist 3D printing space will impact the medical applications and especially O&P with its low-cost processes and materials, allowing developers to experiment freely. After overcoming the cost barrier, the technology will require integration of imaging technologies with 3D printers.

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