3D printing is revolutionizing the healthcare market, bringing new tools to all medical specialties. This technology was first introduced mainly in traumatology and maxillofacial surgery, where bone models were printed to help learn about and understand complex fractures, deformities, or scoliosis. Since the anatomy of each patient is different, 3D printing is essential to manufacturing individualized solutions.
As expected, the idea of printing implants and custom surgical tools soon emerged. With this development two questions arise: how advanced is this technology today, and how far could we go with it?
Surgical guides, personalized instruments
One of the advances with the greatest impact on orthopedic and maxillofacial surgery is surgical guides – tools provided with channels or tubes that guide traditional surgical instruments on a path planned by radiological software. These guides are designed and personalized to adapt to the anatomical contour of the patient.
The field where they are most developed is dentistry, although they are also being implemented in traumatology, maxillofacial surgery, and oncology.
Choosing the material is not an easy task. Guides, like any other surgical instrument, must be printed using an inexpensive material with extensive proven medical application and with good mechanical properties so it will not deform during use.
The most widespread material choice is polylactic acid (PLA), the material used in absorbable sutures. For example, Polysorb Braided Absorbable Sutures use a synthetic polyester composed of glycolide and lactide. Also, PLA printing can be done by extrusion, an inexpensive, reliable printing system.
Not only can implants be printed with inert materials such as poly-ether-ketone-ketone (PEKK) or carbon fiber reinforced filament, but also, in recent years, with fully bio-compatible materials, which throughout time the body replaces with its own tissue. Even full or partially functional human tissues have been printed.
The biocompatible materials used for 3D printing, bio-inks, allow printing of a scaffold that is colonized by the patient’s own cells. Therefore it's vital to consider the parameters of porosity and pore-interconnectivity to allow cell movement into the scaffold. These are two characteristics in which 3D printing is more advantageous than other production techniques.
Bio-inks are usually natural polymers such as hyaluronic acid, elastin, or spider-derived silk proteins. Other materials that can be used to fabricate bone scaffolds include salts – such as tricalcium phosphate – and synthetic polymers such as amphiphilic block copolymers, the latter of which stand out for having the ability to self-assemble. Due to their hydrophilic spherical structures on the outside with hydrophobic interiors, synthetic polymers allow the transport of some drugs or other liposoluble substances in an aqueous medium, including polyethylene glycol (PEG), polyN-isopropylacrylamide (PNIPA), and polyphosphazenes.
How difficult is it to print a functional organ or tissue? Take the example of a knee meniscus, which has a merely mechanical function. A meniscus can be imagined as a cushion between the femur and the crescent-shaped tibia that serves as a stop, stabilizes the joint, and acts as a shock absorber.
The first question is: how do we adapt the shape of the meniscus to each patient? Luckily, under normal conditions, the meniscus presents a reasonable bilateral symmetry, meaning it would be enough to copy the one from the other knee.
The process may seem simple, but in practice is much more complicated. First, a magnetic resonance imaging (MRI) scan must be carried out using radiological visualization software. Then, the meniscus must be selected and separated from the rest of the structures so that it can be exported in STL format for printing. Some software performs tissue segmentation automatically, with the newest using algorithms based on fractals or deep learning.
The second consideration is the material. The supporting structure – the extracellular matrix – forms part of the tissue and in the meniscus is composed mainly of collagen. Some companies, such as Advanced Biomatrix, commercialize this material in the form of bio-inks, although synthetic absorbable polymers such as polycaprolactone (PCL) can also be used. The final product must offer the same mechanical properties as the natural one, so the polymer fibers chosen must have the same placement as in reality.
Overall, 3D printing is an effective tool in the medical field for printing anatomical models, surgical guides, and personalized implants. Despite the rapid implementation of this technology in the healthcare sector, printing a functional tissue is not a trivial process. That is why today, though the market is directing its efforts in this direction, bio-printing has not yet reached operating rooms, excluding exceptions restricted to research. If companies, universities, and doctors can work together as a team, considering the benefit to the patient, this dream should come to fruition in the next few years.