From Three-Dimensional (3D)- to 6D-Printing Technology in Orthopedics: Science Fiction or Scientific Reality?

Over the past three decades, additive manufacturing has changed from an innovative technology to an increasingly accessible tool in all aspects of different medical practices, including orthopedics. Although 3D-printing technology offers a relatively inexpensive, rapid and less risky route of manufacturing, it is still quite limited for the fabrication of more complex objects. Over the last few years, stable 3D-printed objects have been converted to smart objects or implants using novel 4D-printing systems. Four-dimensional printing is an advanced process that creates the final object by adding smart materials. Human bones are curved along their axes, a morphological characteristic that augments the mechanical strain caused by external forces. Instead of the three axes used in 4D printing, 5D-printing technology uses five axes, creating curved and more complex objects. Nowadays, 6D-printing technology marries the concepts of 4D- and 5D-printing technology to produce objects that change shape over time in response to external stimuli. In future research, it is obvious that printing technology will include a combination of multi-dimensional printing technology and smart materials. Multi-dimensional additive manufacturing technology will drive the printing dimension to higher levels of structural freedom and printing efficacy, offering promising properties for various orthopedic applications.