Introduction
Since the beginning of the use of appliances to straighten teeth, metals have been integral to clinical orthodontics. The progression of metallurgy and metal fabrication experienced significant growth during the Industrial Revolution. Pierre Fauchard, the father of dentistry, used a gold bandeau around the dental arch over which teeth were tied with threads for tooth movement. Dr. Edward Hartley Angle (1886–1930) used gold alloys to fabricate innovative fixed appliances. In the early 20th century, materials like 14- to 18-karat gold, iridium-platinum and platinised forms of gold were utilised for crafting brackets, ligatures, wires and bands. Excellent biocompatibility, inertness to the hostile environment in the oral cavity and physical properties of gold could not have been substituted with any other metal or alloy until stainless steel was discovered in the early 1920s.
In August 1913, Harry Brearley, an accomplished metallurgist from England, made a ground breaking discovery that profoundly impacted human life. He identified a type of steel with the unique quality of being impervious to rust, and it soon became known as ‘stainless steel’. His discovery of SS had far-reaching benefits in various industries and has since become ubiquitous in modern-day society, from medical applications to kitchen utensils. Brearley’s discovery of SS has impacted human life and is essential in modern-day society. Despite the availability of stainless steel in the 1930s, its widespread use gained momentum during World War I, revolutionising the metallurgy and fabrication of variety of items using stainless steel. The Germans, British and Americans developed austenitic, martensitic and ferritic stainless steel forms. By the 1960s, stainless steel replaced gold in orthodontic applications almost universally.
Gold wires, brackets and accessories were phased out and replaced with stainless steel. Orthodontists continue using stainless steel wires to fabricate various appliances using orthodontic pliers. The removable appliance consists of multiple components, such as springs and loops, which can be attached to the primary rigid wire or embedded in the acrylic plate. Dr. George B. Crozat (1919) invented a full-wire removable appliance made of precious metal components and integrated it into his treatment philosophy. The Crozat, an all-wire appliance, was later made in SS wires. However, due to the complexity of its design, the skill and time required in its fabrication, and the precision needed for hand soldering, the Crozat appliance became less popular among clinicians and patients. The Crozat appliance gradually faded with the introduction of steel brackets and bands. Machine-fabricated components partially replaced wire bending using pliers and hand tools. Prefabricated stock appliances such as the labial bow, Adams clasps and ball end clasps became available in various sizes. They could be easily adjusted to plaster study models with minor modifications.
The evolution of metallurgy and the possibility of managing the properties of the metals were studied in orthodontics for extended durations of action through spring back and low force levels for physiological tooth movement. In the 1960s, cobalt-chromium alloys emerged, differentiating between wrought and cast alloys. Cobalt, chromium, molybdenum, nickel and iron were constituents of the wrought metal alloys, providing high stiffness. Four available tempers allowed varying formability, beneficial for creating loops, offsets and V-bends in the arch wire.
The machine-made arch forms were available as preformed arches, at least in three shapes: oval, round and V-shaped, and in sizes like small, average and large.
The quest for better orthodontics inducted shape memory properties of nickel-titanium into wires. Nickel-titanium alloy, named ‘Nitinol’ (Naval Ordnance Laboratory), was introduced by Buehler in 1962.
Andreasen first used nickel-titanium (NiTi) alloys in orthodontics in the 1970s. It is the most popular wire for initial levelling and alignment. A few years later, a superior NiTi alloy with excellent spring back, shape memory and super elasticity was introduced by Furukawa Electric Co. Ltd. and became famous as Japanese NiTi. These added properties are the result of variable transition temperature of the alloys. In 1980, Dr. Burstone introduced beta titanium as a novel orthodontic alloy, highlighting its unique properties showing low stiffness yet high spring back, excellent formability and weldability for diverse clinical applications. In 1985, nickel-titanium alloy developed by Dr. Tien Hua Cheng and associates in Beijing, China, was introduced by Dr. Burstone. This Chinese NiTi wire exhibited remarkable spring back characteristics, particularly beneficial in situations requiring low stiffness at large deflections.
The early 1990s witnessed the advent of Neo Sentalloy, a genuinely active martensitic alloy with a shape memory effect (SME) featuring pseudo-elastic and thermoelastic effects during forming and recovery. , In 1994, three varieties of copper NiTi products incorporating chromium were introduced. ,
Most advanced wires could not be shaped into loops or arch forms at room temperature. Therefore, these were available in plain or reverse curve of Spee factory-made under variable temperature conditions (transition temperature).
The robotic wire bending services were made available as commercial support services through Orametrix, part of SureSmile Venture USA. Automation of metal appliances used in orthodontics evolved from preformed wires and robotic wire bending to state-of-the-art 3D printing. Automation in orthodontics and metals began with the availability of preformed stainless steel (SS) steel arch forms, plain NiTi arch forms, reverse curve of Spee (RCS) and, eventually, robotic wire bending.
From producing intricate machine and automobile parts to complex and sophisticated metal castings, the metal industry continues to innovate on both small and large scales.
Innovations in metal printing
Further innovations in metallurgy led to powder forms of metal alloys that could be injected into a 3D printer to create complex internal and external forms that could not be created with casting or milling. Fully melted metal forms could also be injected and printed. The 3D metal printers have been primarily used in industry applications.
Orthodontists inducted the industrial 3D metal printing technology in clinical applications, which were facilitated by the digital revolution of 3D scanning of the dental arches, software that allows onscreen 3D planning of the appliance, facility to export these files of dental models and appliance to a remote location for printing.
The printed appliance has superior contact adaptation to oral tissue and precision of integration with other appliance components, which may require welding the functional components. The 3D printer technology and equipment are essentially grouped into three major types based on the manufacturing technology type or composition of metal power they support. Metal appliance fabrication could be achieved in many ways. Table 96.1 shows the manufacturing techniques, types and composition of alloys of 3D metal fabrication. Table 96.2 shows different techniques used by 3D metal printers.
TABLE 96.1
Classification of 3D metal fabrication
Source: Based on Ahmed N. Direct metal fabrication in rapid prototyping: a review. Manufacturing Processes J. 2019;42:167–191. https://doi.org/10.1016/j.jmapro.2019.05.001.
| S.no. | Manufacturing technique | Manufacturing type | Composition of the metal powder |
|---|---|---|---|
| 1 | Non-melting/partial melting systems are selective laser sintering (SLS) and laser micro sintering (LMS) | Powder in bed technique | Single metal powder |
| 2 | Whole melting systems are SLM, electron beam melting (EBM), laser-engineered net shaping (LENS) and 3D laser cladding. | Power injected through nozzles | Mix of different metals in powder |
| 3 | Pre-alloyed powder |
TABLE 96.2
Technology used by 3D fabrication printers
| S.no. | Technique |
|---|---|
| 1 | Multi-jet fusion (MJF) |
| MJF technology, pioneered by Hewlett Packard, operates using a sweeping arm to deposit a layer of powder. Subsequently, another arm, equipped with inkjets, selectively applies a binder agent over the material. The inkjets also deposit a detailing agent around the binder to ensure precise dimensionality and smooth surfaces. Finally, the layer undergoes exposure to a burst of thermal energy, triggering a reaction among the agents. | |
| 2 | Selective laser sintering (SLS) |
| SLS is a state-of-the-art 3D printing technology that uses a powerful laser to fuse tiny powder particles together, creating a solid object. The laser selectively fuses the powder by scanning cross-sections on the surface of a powder bed. After each cross-section is scanned, the powder bed is lowered by one layer of thickness, adding a fresh layer of material. This process is repeated until the object is fully formed. | |
| 3 | Direct metal laser sintering (DMLS) |
| The process of DMLS is akin to SLS, but with the use of metal powder instead of plastic powder. The excess powder is left in position, creating a supportive structure for the item, and can be utilised again for future prints. DMLS has progressed into a laser melting process as laser power has increased. | |
| 4 | Selective laser melting (SLM) |
| SLM involves fabricating complex 3D metallic parts with precise dimensional accuracies by melting layers one at a time. SLM is similar to SLS regarding the mechanical system, except for its higher energy density level. | |
| 5 | Laser engineering net shaping (LENS) |
| LENS is an additive manufacturing process that uses a high-power laser to create a melt pool on the substrate surface. The process involves melting metal alloy powders through powder-feeding nozzles, which results in rapid melting and solidification of the metallic powder. This allows for the fabrication of parts layer by layer. | |
| 6 | Laser cladding |
| Laser cladding is a technique belonging to the DMLF family, renowned for its ability to create 3D laser cladding. This process involves fusing a metal or alloy onto a substrate’s surface to form a clad. The powder is guided along a predetermined path using a CAD program, while the substrate moves in a traverse direction. | |
| 7 | Directed energy deposition (DED) |
| The technique of DED is commonly employed in the metal industry and for rapid manufacturing purposes. A 3D printing device is typically linked to a multi-axis robotic arm equipped with a nozzle dispensing metal powder or wire onto a surface. An energy source, such as a laser, electron beam or plasma arc, is then utilised to melt the deposited material, ultimately creating a solid object. |
The workflow for 3D metal fabrication in orthodontics
A digital workflow for 3D metal-printed appliances requires a comprehensive diagnosis, treatment plan and appliance selection and design.
An accurate 3D intraoral scan of the dental arch and oral tissues, palate and buccal dental alveolar regions of interest without too much deflection or stretching should make an excellent 3D digital replica substituting dental study model. Accuracy of scanning is fundamental to creating a metal 3D print model with superior accuracy for bondable surfaces and precision of anatomical contact surfaces. After a critical review and having been satisfied with the quality of the scan involving the desired anatomical part and a detailed review, the next step consists of importing the scan file into computer aided design (CAD) software.
The next step involves digital designing the appliance, which will have fully 3D printable metal components and provision for functional components. Technical skills are required to handle the CAD software. Specialised dental technologists housed remotely can coordinate with orthodontists to fulfil the necessary designs through remote collaboration and discussion. As AI evolves, the process of designing parts will disappear, and the orthodontist has to choose the desired appliance from a menu, leaving the design to the computer, saving the orthodontist much time and offering consistent quality.
These 3D fully printable components include saddle molar bands limited to lingual and occlusal embrasure without extending into proximal surfaces of the molars. The components to be designed are a 3D-printed lingual retainer, a transpalatal arch, a space maintainer wire framework, a framework for an appliance such as an expansion appliance, a maxillary distalisation unit or maxillary protraction unit in combination with the miniscrew-supported design of the appliance.
3D printing can integrate treatment with maxillofacial surgery and design appliance frameworks and bone plates using 3D cone beam-computerised tomography (CBCT) data.
The appliance’s function and purpose determine its design. Previously, it was created through wire framework or casting, which needs to be replaced with 3D printing.
The thickness of the molar saddle bands and the surface area covered should be sufficient to withstand the load of the framework and provide sufficient anatomical contact surface area for retention. Occlusal rests should not interfere with occlusal functions. The thickness of the 3D-printed band is maintained at 0.55 mm, with an additional requirement of a luting cement gap of 0.05 mm. The principles of designing cast partial dentures and orthodontic appliance retaining components are similar, with significant additional requirements of a larger contact surface area for cementation for saddle molar bands.
The appliance framework is created using CAD/CAM software. The software has a library of components such as arms, connectors, aligner attachments, spikes for debonding, attachment hooks, buttons, brackets and tubes. Any new design should be saved in the digital library for future use, which will streamline the future design process.
Within the digital design platform, orthodontists are empowered with unique flexibility in the appliance for integration of CBCT data files which are required for incorporating multiple components into an appliance, such as a miniscrew guide.
A provision has been made for adding other preformed elements (such as STL-file components) for which 3D metal printing technology is unavailable yet. The commonly required components are an active transpalatal arch and an expansion screw. These components are provisioned and laser welded later after printing the appliance framework.
Fundamentally, powder chromium cobalt alloy or stainless steel or biocompatible class II alloy is used in 3D printing for dental or orthodontic purposes. The metal components are acceptable in an oral environment and is non-toxic and inert. The chrome cobalt-printed metal components constitute the rigid parts of the appliance. The active components like the Hyrax Expansion screw and or active transpalatal arch would require to be laser welded with the 3D-printed appliance. Remanium wires by Dentaurum (Ispringen, Germany) have been specifically developed to create active components.
The orthodontist conceives the complete appliance design with the expert technician through remote consultation. Once the orthodontist and technician have remotely consulted and agreed upon the full appliance design, it is finalised and saved as a stereolithographic (.STL) file. STL stands for stereolithography, standard triangle language or standard tessellation language. This file format is commonly used in 3D printing and computer-aided design (CAD) to outline the surface geometry of a three-dimensional object without including any information related to colour, texture or additional attributes.
These.STL files are exported to 3D metal printing services that are commercially available. 3D metal printing involves building the metal framework layer by layer using a laser melting process. The preferred metal for this process is a powdered cobalt-chromium alloy or stainless steel. The non-digital components are laser welded onto a perfectly prepared framework, a provision for which is created during CAD-CAM designing process. The most common appliance in use has been 3D-printed maxillary expansion. The jackscrew is laser welded, which can be done without a base model. Graf et al. , introduced the first printed orthodontic expanders using a 3D metal printer and an almost entirely digital process.
The 3D metal-printed appliance is examined for voids or inaccuracies and undergoes electro-polishing. The saddle bands are subjected to surface treatment of the tooth contact surfaces by sandblasting to enhance the bond surface area of the saddle bands.
The appliance undergoes rigorous inspection to ensure no defects or inaccuracies are present before it is released for the patient. After the inspection, electro-polishing is carried out to give the appliance a smooth finish. If necessary, other parts are added to the appliance through laser welding. Finally, the completed appliance undergoes sandblasting of the inner surfaces of saddle bands and attachments to enhance the cement adhesion and bond strength. The appliance is cemented using an orthodontic adhesive such as Transbond XT (3M, Maplewood, Minnesota 55144, USA) or similar.
Fig. 96.1 shows the entire process of scanning, digital design, production and polishing of the appliance. Fig. 96.1 (S) shows a 3D-printed framework, which has a laser-welded rapid maxillary expansion (RME) in the patient’s mouth.
Digital design and metal printing workflow in orthodontics.
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A. Virtual cast in 3Shape appliance designer software (3Shape, Denmark).
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B. Insertion of expansion screw from the STL-file-library.
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C. Design of the bonding site.
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D. Design of the connection of the bonding site to the expansion screw.
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E. Final design with the correct position of the expansion screw.
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F. Control of the laser welding site from the basic Hyrax frame to the expansion screw.
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G. Nesting (positioning) of the designs of the building platform (E-Plus-3D, China).
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H. Building platform left and alloy-feeder right in the EP M150 selective laser melting device (E-Plus-3D, China).
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I. Final selective laser-melted devices with supportive sticks and unbonded alloy powder.
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J. Final selective laser-melted devices with supportive sticks.
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K. Cutting off the printed devices from the building platform.
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L. Final printed devices unpolished with supportive sticks.
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M. Polishing of the supportive sticks.
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N. Loading the devices in the dry polishing machine DLyte-10 (DLyte, Gpainnova, Spain).
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O. Polishing process in the DLyte-10 dry polishing machine.
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P. Final polished device with laser-welded expansion screw.
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Q. Final polished devices with various elements laser welded on them.
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R. Comparison of polished and unpolished devices.
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S. Final polished hyrax bonded on the teeth occlusal view.
Clinical applications of 3D metal-printed appliances
Clinical applications of 3D metal-printed appliances are relatively new and are being innovated by clinicians to manage complex malocclusions. Some of the recently innovated and reported appliances are:
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1.
Transpalatal arch
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2.
Lingual arch
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3.
Space maintainer
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4.
Maxillary expansion devices
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a.
Hyrax (rapid palatal expansion)
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b.
Hyrax–hayrake–blue-grass combination
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c.
Hyrax–Halterman combination
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d.
Miniscrew-assisted rapid palatal expander (MARPE)
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a.
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5.
Herbst with brackets
1. Transpalatal arch
The 3D-printed TPA is similar to the conventional TPA but does not have molar bands. Instead, it has bonded molar saddles on either side that are exclusively limited to the lingual surfaces of molars. This design allows for an immediate start to a fully fixed treatment plan that involves retraction, alignment of the canine, final torque adjustment of the lateral incisor and retention through a bonded retainer. However, the 3D-printed transpalatal arch has a major limitation: being rigid, it cannot perform molar derotation. In cases where derotation force is required on the molar, the entire TPA connector is replaced with a springy wire (e.g. NiTi alloy wire) or laser welding the TPA with Rematitan Dentaurum, Germany and an extended connector for improved force distribution during activation ( Fig. 96.2 ).
