Three-dimensional printing is the process of creating a 3D model of any shape from a digital model by using selective addition of material. Three-dimensional printing or rapid prototyping started in 1981, when Hideo Kodama prepared a 3D print prototype, using ultraviolet light in the process of layering of special material to construct a three-dimensional plastic model.
The term stereolithography (SLA) was used for the first time by Chuck Hull in his patent submission in 1984 (U.S. patent No. 4575330). The SLA technology is based on layer-by-layer laser beam curing of photosensitive polymers. When the layer of resin is completely polymerized, the lifting platform moves a one-layer thickness in the vertical direction and cures the next layer and then the process is repeated many thousands of times to form a 3D physical object. The precision of printing in dentistry ranges approximately from 25 to 100 µm and printing time depends mainly on the number of layers [1
Three-dimensional printing in dentistry and orthodontics is based mainly on intraoral scanners which contain a handheld camera, computer and software. All devices prepare a reconstruction of the dental arch with three-dimensional geometry [2
]. The 3D printing result is a digital format, standard tessellation language (STL), which can be used in systems applicable in clinical practice: CEREC Omnicam (Dentsply Sirona), CEREC Bluecam (Dentsply Sirona), Planmeca Planscan (Planmeca USA), Cadent iTero (Align Technology), Carestream 3500 (Carestream Dental), 3Shape Trios 3 (3Shape North America) and 3Shape D800 model scanner (3Shape North America) [3
]. The main advantages of those systems are in the data collection and treatment which is based on a virtual model and following 3D printing [4
]. Trueness and precision of scanners depend also on the substrate itself and it was proved that dentin as substrate gives the most accurate scans in contrast to enamel [5
]. In addition, data acquisition using an intraoral 3D scanner has shown that object rotation is an important factor which can increase or reduce distances from 0.9% to 1.4% [6
For dentistry and orthodontics, four types of printers are the most popular: fused-deposition modeling (FDM), polyjet technology (PJ), SLA and selective laser sintering (SLS). These methods are not the only methods, where additive manufacturing plays important role. The cement-based approaches studied by Gbureck [7
] with his Würzburg team aimed to develop a dual-setting biocement system based on a brushite-forming calcium phosphate cement and a tetraethyl orthosilicate based silica gel, which may be beneficial mainly in clinical applications, where we expect continuous releasing of medicaments (for example vancomycin) or support of osseointegration of implants of substances used to cure bone defects for example in maxilla-facial surgery.; De Wild with his team [8
] published in 2014 research, where selective-laser-melted NiTi parts were confirmed as possible way of manufacturing of medical implants and exhibits ultra-high mechanical damping properties. The additive manufacturing using ceramic [9
] and metallic materials allows to use hard materials for teeth replacement now, too.
Fused-deposition modeling (FDM) is a method of 3D printing where the final object is composed of thin layers of material which come to the printer in the form of filaments of thermoplastic material [10
]. The head of the printer moves usually in one plane to create one layer of the object and is controlled digitally. As a result of this, we can usually see the layers on the surface.
Polyjet (PJ) is another kind of 3D printing technology, which uses drops of liquid photopolymer which solidify after exposure to UV light. This produces objects with a very smooth surface and thin layers of material [11
Stereolithography (SLA) technology uses light curing of monomers, which causes polymerization and creation of a 3D object. The advantage of this method is speed of printing; the products often require machining after the priming time [10
Selective laser sintering (SLS) uses high-power lasers to sinter powdered material to create the printed object. This can cause changes in the inner structure of the object as well as the surface.
These layers accumulate on the build tray until the part is complete. The transformation deviations during scanning from 56 positions showed that the most precise systems for surface analysis are SLA and FDM laser prints [12
]. The data acquisition [13
] and printing accuracy of printers depends on layer thickness which is not greater than 50 µm. The average absolute deviations for orthodontic applications must be smaller than 0.05 mm [14
For clinical practice, the optimal size of models is equally important [15
] as precise reproducibility of the surface and the shape of the dental arch in cross-section. The aim of our study was to evaluate this 3D SLA model precision in comparison with models from FDM, PJ and SLS. Our contribution aims also to evaluate the several main indications of those methods in clinical practice.
4. Discussion and Conclusions
Three-dimensional scanning methods associated with computational methods and modern printing technologies have many applications in stomatology and in the clinical practice now. These methods enable the examination of results of complex dental operations and corrections of dental arches as well. Associated problems are related to the accuracy of sensor systems, materials used for three-dimensional printers and the surface analysis of resulting physical models. From practical point of view, it is known that conventional impression materials provide highly accurate impressions based on highly precise plaster casts. Disadvantages of these traditional methods are the possible loss of quality due to, e.g., abrasion, attrition and transport and storage problems. On the other hand, the digital impression process using intraoral [6
] scan can be resulted in loss of information. The printing is more precise, prepare long term stable material, but the time of printing is longer.
Our approach to the study used profilometer to determine surface structure of the models and electron microscope to find the inner structure. Similar approach was recently chosen by Cruz at al. to analyze surface of custom/made titanium meshes used in maxilla/facial surgery [21
]. This way seems to be an alternative to micro CT scans with its pros and cons. As disadvantage we can find the necessity to destroy the examined object to characterize the inner structure. It is well known, that profilometers are great for surface analysis of 2D object, but may result in some errors in curvy objects [22
]. Micro CT scans can provide information about inner and outer structure from one scan, was confirmed as useful tool in reconstruction of fine anatomic structures as for example middle ear [23
], but as disadvantage we see that the method is based on X-ray, which means may have limited use in in vivo studies, other limitation can be also small sample size, high cost and need of specialists in the field to obtain reliable scans. Kulcyzk et al. [24
] published recently research comparing different methods (CT scans with standard and high resolution, optical 3D scans, micro-CT scans) of 3D data acquisition for tooth replication. Methods were examined on a dry human mandible and showed differences in quality of obtained 3D models, which may also affect 3D printing, if we use these data as source for printing. In that study authors found optical scanning better for more detailed replica than other methods mentioned.
Some studies compared various types of 3D printing with different results—surgical templates printed by the light-cured method were statistically less incongruent than those made by an FDM printer in a study by Sommacal et al. in 2015 [25
]. The accuracy of models obtained by intraoral scanning is very high and shows a high level of precision even in more complicated procedures such as dental implantology and prosthodontics [26
]. A study by Jang et al. from 2020 showed lower accuracy of fixed dental prostheses fabricated on a 3D printed model than on a conventional cast, but still sufficient for clinical practice [27
]. Although many studies have been devoted to the accuracy of models in various clinical applications, with various results, it is not only the metric quality and stability that are important attributes of the 3D printed model: in prosthodontics, esthetic dentistry and orthodontics, the surface of the model is equally important. This also gives information about individual characteristics such as the presence and shape of mamelons, individual curves and the shape of teeth and incisal edge as well as cusps.
Specific studies are devoted to profilometer-based approach [28
] and advanced micro computed tomography [29
], owing to the fact that hard X-ray allow for the isotropic spatial resolution in three-dimensional space. These methods can improve possibilities of the assessment of implants and surrounding bones, to determine mineral concentration in the teeth and to estimate the thickness of enamel. In this way these methods can contribute to the appropriate use of three-dimensional models.
Intraoral scanning and 3D printing can be the only method of choice when we must take care of patients with serious diseases or handicaps where is not possible to use conventional impressions and models for dental or orthodontic treatment [30
]. Wesemann et al. advise intraoral scanning in orthodontics for full arch scans but find it more time-consuming than conventional impression-taking [31
]. Our case report demonstrates treatment options for a patient with a rare disease in the orofacial region with a population prevalence of less than 1:2000 inhabitants. Therapy of these diseases is mostly long term and quality cooperation between parents and medical specialists across many different disciplines is necessary during the therapy [32
]. It does not mean that the child patient cannot live a fully-fledged life with smooth integration into society in adulthood. It is necessary to take into account the limited possibility of a medically compromised patient’s cooperation when devising a global therapeutic plan. Use is made of interdisciplinary cooperation with a specialized department which can give medical treatment in general anesthesia, sedation, etc. Many specialists take part in the treatment, from a geneticist to an orthodontist to a maxillofacial surgeon. The case report describes progress of the treatment of a medically compromised patient with Kabuki syndrome in our clinic. Orthodontic treatment was finished after one year and three months, with a recommendation to maintain proper hygiene by periodic check-ups of dental hygiene. The stability of therapy was supported with intraoral scans, 3D print models and special splints.