3D Printing in Pediatric Dentistry: A Review

Introduction

Three–dimensional (3D) printing, commonly known as additive manufacturing is a revolutionary technology that refers to a process of deposition of materials to design 3D objects by utilizing materials like plastic polymers, metal, ceramics, and also living cells. Various applications of additive manufacturing have gained much attention in the medical and dental realms.¹ 3D printing has changed the lives of people in ways we cannot imagine. A blind couple was allowed to feel their baby’s ultrasound with the help of 3D models created using 3D data by Dr. Heron Werner. The process involved in printing living tissues is named 3D bioprinting. Recent advances in the field of 3D bioprinting have given rise to new scenarios in the manufacturing of custom-made, patient-tailored constructs that show a great degree of similarity to the patient’s natural tissue for oral and craniofacial reconstruction. Bioprinting is an evolving field in regenerative medicine² and regenerative endodontics.³

The rationale for 3D printing:

Digitalization of our everyday clinical work can surely help us, especially in this fast-moving modern era. Today’s changing world can be compared to a rat race. Everyone is expected to perform better in their workplace leading to time constraints. The COVID-19 pandemic has also instilled fear in our minds. In such a situation, it would be better to incorporate digital transformation that can help us meet these challenges, as digital processes are often characterized by their increased efficiency, lesser chair side time, greater accuracy, and reproducibility as well as improved material properties and user comforts.⁴

History of 3D printing:

The first 3D printing process, called stereolithography (SLA) was invented by Charles Hull in 1983. Since then, there has been continuous progress in this field. In the year 1986, Carl Deckard invented the selective laser sintering methodology. Wake Forest University (1999) designed the first 3D-printed organ for transplantation. The first inkjet bioprinter was designed in the year 2003. In the year 2009, Organovo Company designed the first 3D-printed blood vessels. The University of Tel Aviv in the year 2019 designed the first 3D-printed heart that contracts with blood vessels. In the year 2020, Fab Rx designed a 3D printer for personalized medicine.5 3D printers were mostly used for rapid prototyping in the 20th century. Though, the technology advanced quickly in the succeeding years. Following the termination of the copyright for the combined deposition modeling procedure in 2009,⁶ 3D printers started advancing into the consumer sector. This was eventually adapted by the dental printing units and turned into smaller, cheaper, ones with the changed application. The array of printable materials extended to include plastics, metal, ceramics, and human tissue.⁴

Additive vs Subtractive technology:

A major benefit of additive technology is that 3D objects to be printed can be planned on computers thus allowing an unlimited variety of silhouettes with varying extents of complexities. A feature that has gained modest consideration is that the mechanical and aesthetic properties can still be changed during the 3D printing process. This is impossible with CAD-CAM, where the physical properties of the prefabricated blocks are adjusted by the production company. This choice to customize, rapid, and effortless availability at a lower cost makes additive manufacturing a foundation stone of digital dentistry.^4,7 Moreover, the additive manufacturing process produces very less wastage of materials when compared to subtractive processes like milling. Contemporary dentistry knows of materials designed to work with CAD-CAM and to substitute for the more traditional precious metal casting alloys, which have been subject to exponential price increases in recent years. This use of technology eases the use of materials, which would otherwise be difficult to work with, and eradicates labor-intensive skilled techniques,⁸ allowing the dental technician to focus on more creative aspects of the manufacturing. Every time a dentist operates to provide restoration, the procedure is unique to that patient. Every single restoration will also have innate complexity requiring the reproduction of complicated geometry with a high level of precision. Even though multi-axis CAD-CAM milling processes will tolerate this to a point, the process is slow as well as wasteful as the object is milled from an intact blank.⁹

3D Printing technology process:

3D printers are simple robotic devices. The CAD software is the “Central Nervous System (CNS)” of this robotic device as it allows virtual designing. The “neurons” that connect the printer and its CNS are the volumetric data in the form of computed tomography (CT), cone beam computed tomography (CBCT), and intraoral or laboratory optical surface scans. ¹⁰

Types of the additive 3D printing process in dentistry:

1. Stereolithography and Digital Light Processing: The oldest and most frequently used technique of 3D printing technique in dentistry is SLA. The principle of SLA is that it is constructed on a layered structure of an entity made of a UV-sensitive liquid monomer which is polymerized and solidified by a laser. Digital-light processing (DLP) is the second method that is generally used. DLP contains a microsystem consisting of a rectangular mirror arrangement called a “digital micromirror device”. The angle of the micromirrors can be individually adjusted which acts as light switches and projects the light from the source as individual pixels onto the projection surface. The advantage of DLP technology over the SLA technique is that every single layer can be cured with a single shot of laser exposure by producing patterned laser light rather than scanning separately one after the other with the laser. A high cost of manufacturing is the disadvantage of DLP.^{7, 9, 11, 13}

2. Photopolymer Jetting and Material Jetting: In the photopolymer jetting and material jetting processes, the object is built up in layers by a print head with several linear nozzles. The principle can be compared to that of a conventional inkjet printer. Instead of ink, a liquid photo monomer is used for photopolymer jetting, and for material jetting wax is used. Afterward, the monomer is cured in layers by UV light or the wax hardens thermally on the building platform. This technique is relatively fast but the high cost of manufacturing is a disadvantage.^{7, 10}

3. Binder Jetting: It is a type of photopolymer jetting where an adhesive is applied to a powdery substrate using pressure nozzles. Additional support structures are not necessary, as the printed entity is completely bounded by a supportive substrate. If metal and glass powders are used, the object can be exposed to a sintering process in which the adhesive is burned out. Due to high adhesive content, the resulting items exhibit high sinter shrinkage and porosity and should be infiltrated subsequently. Due to the complicated geometries in dentistry, the binder jetting process using powder/adhesive is restricted mostly to surgical planning models.^{7, 10}

4. Selective Laser Sintering/Laser Melting: In the laser melting process all powdery materials that can be sintered or melted by laser radiation and solidified after cooling can generally be used. The material used can range from plastics and metallic materials to ceramic materials. In dentistry, these methods are used mainly for metals. The terms ‘‘laser sintering’’ and ‘‘laser melting’’ are understood conflictingly. The two processes are further divided into numerous subgroups, some of which represent the brand names of certain companies (eg; laser CUSING). However, the basic printer building principle is alike. Low cost is the advantage and high maintenance is the disadvantage of laser melting. ^{7, 10, 12, 13}

5. Fused Filament Fabrication: The melt layer process was developed over 20 years ago by the founder of Stratasys (Edina, MN, USA) and protected by the trade name ‘‘fused deposition modeling.’’ The process is called fused deposition modeling. The non-patented term is ‘‘fused filament fabrication (FFF)’’ which works according to the principle of strand extrusion. Thermoplastic materials, like polylactides, acrylonitrile butadiene styrene, and waxes, are provided as semi-finished products in various strand thicknesses to the extruder, where they are melted in the hot end and applied to the building board with the aid of a die at the respective x-y coordinate. Heated construction chambers are used to diminish heat distortion in cases of uneven cooling. After completion of one plane, the next plane (z-axis) is started. Low cost is the advantage and low accuracy is the disadvantage of this method.^{7, 11, 12, 13}

6. Bioprinting: Bioprinting employs the use of biomaterials, cells, or cell factors as a “bio-ink” to fabricate tissue structures. Parameters like biocompatibility, cell viability, and 13 cellular microenvironments of the biomaterial can greatly alter the printed product. The goal of this method is to design 3D artificial tissues that consist of a scaffold, cells, and an environment that is similar to the real environment of the human body. 3D bioprinting attains these three essential constituents as it is an extremely effective and precise method to create artificial tissue in-vitro. Materials used are alginate, fibrin, collagen, PLGA (poly lactic-co-glycolic acid), tricalcium phosphate, chitosan, and hyaluronan. It creates structure with living cells, soft and hard tissue scaffolds, 3- dimensional hydrogels, ceramics, and hydrogels.¹⁴

Subtractive Manufacturing in Dentistry (CAD-CAM)

Dentistry has an extensive association with subtractive manufacturing more usually defined as ‘milling’. CAD-CAM is the elimination of material to shape an object. CAD-CAM for the construction of crown copings is now a synonym with current dental technology. This technology allows the use of materials that would be difficult to work with, and reduces labor-intensive artisanal production procedures, permitting the dental lab technician to improve his hand skills on more innovative features of the manufacturing process.¹⁰

Applications of 3D Printing in Pediatric Dentistry

Laboratory Training of Pedodontists: The education in dentistry has always depended on extracted teeth for preclinical exercises, as these teeth provide semi-realistic clinical scenarios. But there is always a vagueness in the availability of anomalies in those extracted teeth. Construction of 3D dental models for preclinical purposes can also be done with 3D-printing techniques which gives more realistic anatomic structures such as teeth with internal and external resorption defects, open apex, dilacerations, dens in dente and much more by aiding in the development of endodontic skills by giving visual, acoustic, and tactile proprioception during preclinical training.¹⁵

Pediatric Oral Medicine and Radiology: The need for early referral, diagnosis, and management of adenoid hypertrophy in children has been suggested in the literature over the years. The role of a pediatric dentist in identifying a nasopharyngeal obstruction is significant. 3D printing technology combined with CBCT to make 3D printed models can do wonders when identifying a nasopharyngeal obstruction as compared to direct clinical evaluation which can give limited visualization.¹⁶

Pediatric Endodontics and Restorative Dentistry: In-vivo tooth auto-transplantation is a method in which tooth replacement is achieved after traumatic dental injury in children and adolescents. Often pediatric dentist has to wait for a duration of 3 to 4 months to ensure complete healing before reshaping the teeth for aesthetic reasons. Such a time lag can be avoided by using 3D printing technology for designing and fabricating chair-side temporary veneers using a DLP printer (Ray dent RAM500, 16 Ray Medical, Seoul, South Korea). It has been found that values of marginal and internal adaptation were within clinically acceptable ranges. ¹⁷

Tooth anomalies such as dilaceration, pulp stones, and dens in dente which are often difficult to manage during the endodontic procedure can benefit from 3D printing. A translucent tooth model with its complex internal anatomy can be prepared with 3D printing combined with CBCT, along with a customized guide to achieve safe working length thus successfully improving the quality and precision in such anomalous teeth.^{15, 18} Fabrication of 3D printed tooth restoration can be done by using 3D printed materials with uninterrupted selffolding ability, that can move from center to periphery. Hence, avoiding micro leakages with increased aesthetic benefits, good strength, and maximum biocompatibility that eliminates etching and bonding on mechanical retention not chemical. ¹⁹

Guided endodontics is an innovative method of root canal treatment of the teeth with calcified canals, extensive restoration or malpositioning for gaining access using 3D printed templates wherein such templates are designed to target burs in elusive canals thus helping in reducing iatrogenic mishaps and conserving the tooth structure.²⁰ During endodontic surgeries like root-end resection and osteotomy procedures; proper positioning, angulation, and depth of penetration of burs are required for successful cases. Otherwise this can cause deviation in angulation or larger osteotomy diameter leading to iatrogenic errors, increased healing time, and post-operative pain. Surgical stent-like guides that are manufactured using 3D printed technology can minimize such concerns and thus gives more accurate, precise, localized, and less invasive micro surgeries.²¹

Autotransplantation using 3D printing increases the chances of success of the procedure. Computer-aided rapid prototyping (CARP) is used to get a replica of the tooth such that the modification of the recipient bone site is done before extraction without PDL damage from repeatedly pushing in and pulling out. With this recipient, the tooth can be prepared for the crown, and a temporary crown can be placed immediately after placing the tooth in the desired tooth site. This method minimizes extra oral time and the chances of any error during autotransplantation.²² 3D bioprinting can potentially be used to replace pulp tissue. Thus, pulp regeneration can be done by dispensing cells suspended in hydrogel by placing odontoblastic cells in the periphery and fibroblasts in the core with a supportive network of vascular and neural cells.³ Revascularizing, and reinnervating the pulp tissue is a promising track that researchers are still exploring. 3D bioprinting for pulp tissue regeneration sounds feasible, but there is insufficient evidence for this to date. Several studies have thrown light on the possibility of successfully bioprinting 3D 20 blood vessels and even capillaries, however in vivo angiogenesis has not been exhibited in endodontics. ^{22, 23}

Interceptive Orthodontics: Clear removable occlusal splints have been fabricated with 3D printing in early anterior crossbites in young preschool children. This splint relieved the dental phobia of young children as it was comfortable and easier to wear than the conventional appliance as it did not have any wire components. Also, the crossbite in the anterior region was corrected in 6 months which promotes the normal development of the maxilla and mandible and avoids difficulties in the future.²⁴

3D printing has also been used in making band and loop space maintainers, that are printed as a single unit with excellent accuracy, minimizing human errors and breakage of the appliance hence contributing to more successful clinical scenarios. Moreover, the extensive laboratory work of stabilizing, soldering the loop on the band, and polishing it is not required, thereby minimizing the chair side time which is truly important for a pedodontist. When compared to a conventional appliance, a 3D-printed model has a more intricate structure with a higher level of detail.²⁵ Various removable appliances (like Hawley’s retainer), functional appliances, and arch expansion appliances can be fabricated via 3D printing. These factors can dramatically accelerate the professional time of a pedodontist, eliminate physical impressions, and put an end to bulky physical model storage.²⁶

Pediatric Prosthodontics: Removable prosthesis construction with 3D printing has also come under the limelight, especially in adolescent patients with oligodontia. This technology prints the prosthesis as a single unit thus improving the mechanical properties with materials like acrylic resin or Poly(methyl methacrylate) (PMMA). This prosthesis is lighter in weight and has increased fracture resistance when compared to conventional overlay dentures due to the absence of interfaces. Moreover, this denture can be easily trimmed, adjusted, and relined with suitable material in case of tooth movements during growth spurts. The storage of the information as a digital file helps in reprinting the denture as many times as required. ²⁷

Pediatric Oral Surgery: Surgical planning for aggressive cancers of the oral cavity and fractures of the mandible can be successfully done via 3D printing which gives the surgeon a better light on planning the respective surgery. This novel technique of 3D printing has helped surgeons with correct planning of the application of Kirschner wire in adolescent condylar fractures and resecting of tumors of the oral cavity. In addition, 3D printed models are used as a visual aid allowing both the surgeons and the parents to weigh the pros and cons of different treatment modalities and decide on the procedure. It also results in anatomical changes with each modality. ^{28, 29}

The use of a patient-specific digital cap splint for treating pediatric mandibular fractures is another potential application of this innovative technology. The preoperative digital system eliminates the distress and requirement for sedation in the patient during impressions. In addition, the surgical time under general anesthesia is also considerably minimized. Dental contours and morphologies are combined for designing it. The occlusal under the surface of the splint acts as a guiding plane by providing precise positional control for the restoration of the mandibular arch to its preinjury form. The 3D cap-splint is also a perfect fit requiring no intra-operative alterations. Furthermore, the splint is aesthetically better when compared to the traditional acrylic cap splint.³⁰ Another application is assisting in customizing titanium alloy restorations for prefabricated skull defect repair. This application has noteworthy advantages for repairing large skull defects. However, it is crucial to choose appropriate techniques and treat deformities of the head and face with integrated methods and teamwork among multiple departments. ³¹

Limitations of 3D Printing ³²

• High fabricating costs especially in a developing country like India.

• An inherent weakness is built-in due to the staircase effect in the object. This effect is created by successive deposition of material on top of the first layer.

• Requires backing materials, which are tough to eliminate later.

• A few materials used in 3D printing are not autoclavable and sterilizable, thus limiting their use.

• Finishing the final product is laborious.

• It is technique sensitive; therefore, a trained professional is required. • Stereolithography uses only light-curable polymers.

• Depending on materials, supplementary treatment like sintering might be required for added strength.

• Ethical and legal clearance is low.

4D Printing- the future

4D printing is an upcoming technology that has immense possibilities. Skylar Tibbitt’s and his co-workers designed self-folding structures that reshape under certain environmental conditions. They converted the steady 3D printing materials into actively changing objects by this approach. Thus, 4D printing helps in the making of materials that shape-shift over a certain time or space. 4D-printed materials can move in different directions as programmed before they are constructed. Regulating the track of the motion of 4D-printed materials in restorative dentistry can eradicate the use of dental etching and bonding systems as they rely more on retention via mechanical means and not chemical aids.³³

Future applications can include:

1. 4D-printed restorative materials in dentistry that can alter their shape as well as position from the center to the margins in a known time and can prevent fracture or marginal leakage.

2. Designing orthodontic appliances with a controlled, self-folding motion to move the teeth in the required direction and angulation is possible. This amazing technology if made use of, can progress similarly to CAD-CAM and 3D printing and thus change the scope of dentistry.³⁴

Conclusion

Over the past few decades, 3D printing has empowered clinicians, as well as improved diagnostic and surgical skills as it permitted for realistic training, better conception, and surgical planning. In dentistry, this expertise has brought in a revolution as it provided maximum accuracy in a small clinical setup and short chair side time. More explorations are required especially in India so that we can provide a holistic approach to ameliorate the health and wellbeing of our child patients.

Conflict of interest

None.

Supporting files

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