By: 11 September 2019
Advances in additive manufacturing technology,  an orthopaedics perspective

Peter Ogrodnik explores 3D technology and how 3D printing may provide a perfect solution for training future surgeons

Engineers, builders and architects have been ‘3D printing’ for millennia. If one recognises that a 3D printer effectively lays one layer on another, and those layers are adhered together, then building a house from bricks is 3D printing on a very large, human scale. To understand the technology it is best to imagine a simple ink-jet printer, but instead of the printing head dispersing ink it is a nozzle that, in the most common embodiment, disperses molten plastic. Also, unlike the ink-jet printer, the head is able to move both across a static bead (x axis) and along the static bed (y axis): called planar motion. The usual process is that the movement of the head in the x-y axes in combination with the release of a specific material builds a layer.

Laser methods (Selective Laser Sintering and Selective Laser Melting) are slightly different in concept. A high-energy laser creates the same profile by locally, and accurately, ‘bonding’ a deposited metal powder layer [1]. This causes the tiny particles to stick together creating said, stipulated profile.

The ability for the head (or the bed/table) to move vertically (z-axis) creates a 3D object by the head printing each layer individually: as illustrated in Figure 1. It does not take much imagination to determine that the accuracy of reproduction depends on the minimum thickness of each layer, the accuracy of positioning of the nozzle, and the width of the nozzle itself. Cheap, off-the-shelf RP machines basically have large nozzles, lay down very thick, wide layers and have poor positioning accuracy: their outputs look right but when measured against the desired outcome are not right. But forget these desktop versions; they are toys in comparison to modern systems that look more like the sort of machine you would find on any shop floor (Figure 2). It is unfair to call these system 3D printers – these are additive manufacturing machines.

Figure 1: Basic principal of a 3D printing system illustrating a polymer-based material being deposited layer by layer. The nozzle moves in the x-y axes. Moving the base in the z-axis after each layer is complete creates depth.

Figure 2: (a) An example of a manufacturing cell comprising Stratasys F900 production grade, 3D printers, building 3D structures in a wide range of engineering grade thermoplastics for the aerospace industry (Image courtesy Stratasys / Tech3D). Sheffield’s Renishaw RenAm 500Q system (b) with sample implants (c), and (d) example batch manufacturing. (Images courtesy Sheffield University AMRC).

The additive manufacturing sector is an industry with room to grow. In 2014, a US government report [2] stated that of all manufacturing processes additive manufacturing only accounted for 0.01 per cent of total output. While this gives one some degree of concern about the traction of this technology, one should state this was circa $246 million worth of shipments – so it is not trivial. Medical / dentistry devices was ranked third out of all industrial sectors accounting for some 15-16 per cent of all activity. In 2018, the additive manufacturing sector has grown to a global market of $9.3 billion with a growth predicted to be above 18 per cent [3].

Historically, maxillofacial departments [4] were early adopters of this technology. They have used 3D printers for some time and have perfected the software, and the art, of converting CT scans to the necessary files required for a successful print run. But these systems are no longer stuck to printing in just one material. The modern 3D printer can print in a variety of materials with a variety of material properties. Many will have seen 3D printed structures having different colours. But Figure 3 illustrates a new innovation: printing 3D structure that has ‘hard’ and ‘soft’ areas at will, at specific locations, and by design. Quite staggeringly, it is perfectly feasible to produce a complete 3D printed assembly of casings, including 3D printed screws, in one run, that is ready for use. Probably the most immediate use for such technology is in the field of instrument trays / cases. It is quite feasible to have a small production run, including all the fittings, 3D printed at costs similar to that of batch production [5]. Furthermore, it is perfectly feasible to produce 3D printed composites where a host matrix is modified with the addition of carbon-fibre elements; greatly increasing the body’s stiffness and strength. Gone are the days when one had to handle 3D printed items with kid gloves, the modern items can now be considered ready for action.

Figure 3: A 3D printed anatomical elements that have different, specific material properties at desired locations: (a) an un-deformed nose, (b) deformed nose, (c) an un-deformed, hollow vessel showing locations of interest, and (d) the vessel deformed. (Images courtesy of Stratasys).

It is common to assume that 3D printing means one is stuck with just one material; this is an incorrect assumption. It is also perfectly feasible to ‘print’ a 3D body using completely different materials in one operation. An area I have been working on is the concept of using 3D printing technology for pre-operative planning. Figure 4 illustrates some models that have been 3D printed, that not only contain the bones, but also highlight vital structures such as tendons, nerves, veins and arteries. It does not take much imagination to understand the power of being able to physically hold and practice a procedure on a realistic model that has been created from the very CT scan one is looking at.

One can imagine printing complex fractures only met rarely by even the most experienced of surgeons. The fracture being surrounded by 3D printed tissues, closely resembling the properties of the real limb and with all the internal structures in place. What an amazing training aid to enable that rare experience to be passed on to the next generation of surgeons. Even in my limited experience of preparing ‘realistic’ orthopaedic training aids – creating a usable model that has a realistic fracture pattern with a realistic ‘feel’ is very hard to do; 3D printing may provide a perfect solution.

Figure 4: Examples of pre-operative models developed from CT scans but illustrating varied levels of complexity all printed in a single process: (a) a simple model of the foot,  (b) a model of the hand illustrating main structures, and (c) a print in process. (Images courtesy Stratasys).

However, one cannot neglect the obvious next step. The level and complexity of modern 3D printing machines means we cannot call them rapid prototyping. As Figure 2 illustrated, they are now additive manufacturing machines. The combination of modern 3D data capture methodologies, sophisticated software and highly sophisticated, repeatable additive manufacturing systems is making bespoke implants a reality rather than a dream.

Figure 2 (b-d) illustrates the Renishaw [6] based system at Sheffield University in their World Class Advanced Manufacturing Research Centre. Here they are working with companies such as Rolls-Royce aero-engines to develop the next generation of additive manufacturing technologies. The benefits of the transfer of these technologies and supporting know-how to orthopaedic implants are obvious.

Mixing this technology with novel materials makes a new generation of resorbable implants [7], and implants with controlled drug delivery a distinct possibility.

Research centres and additive manufacturing centres state that 3D printed alloys have similar properties to that of the original alloy (and some make claims of even greater properties [8], others say the process can leave components with inherent porosity and flaws [1,9]. For many engineers such as I, the trust is beginning to develop; certainly the advent of aerospace adopting these technologies gives one great confidence. But does the auditor examining your Class III design dossier have the same confidence? Do they even know what additive technologies are? This is where you will have to produce robust clinical evaluation reports, validations and verifications. But, will the new Medical Devices Regulations have a detrimental effect on the adoption of these technologies for, say, implants? This is a question for another day, and one to be discussed in a later issue.

Professor Peter Ogrodnik is award leader for the MSc in Medical Engineering Design at Keele University. The contents of this article do not necessarily reflect those of the university, its staff, or its students.


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