3D Printing of Resin Material for Denture Artificial Teeth: Chipping and Indirect Tensile Fracture Resistance

3D printing of denture artificial teeth with resin materials is worthy of study in a novel way. This study evaluated chipping and indirect tensile fracture resistance of 3D printing resin material (Dentca 3D printing denture teeth resin) compared with conventionally prefabricated resin denture teeth (Premium-8, Surpass, SR-Orthosit-PE, and Preference). One hundred tooth specimens were prepared for testing. The 3D printed tooth specimens were printed at a 50 µm layer thickness with methacrylate-based photopolymerized resin by stereolithography 3D printing. Chipping and indirect tensile fracture tests were conducted at a speed of 1 mm/min until fracture. The indirect tensile fracture loads of the 3D printed resin teeth were higher than those of Premium-8, Surpass, and SR-Orthosit-PE, and lower than those of Preference teeth. Regarding chipping resistance, the 3D printed resin teeth were not different from Surpass and SR-Orthosit-PE, and were lower than Premium-8 and Preference teeth. The 3D printed resin teeth exhibited vertical fracture of the loaded cusp without deformation in chipping. The 3D printed resin teeth showed simultaneous fracture of two cusps in indirect tensile fracture, unlike other teeth. The results of this study suggest that 3D printing technology using resin materials provides adequate fracture resistance for denture artificial tooth use.


Introduction
Computer-aided design/computer-aided manufacturing (CAD/CAM) systems are widely used in dentistry, mainly for the fabrication of inlays, crowns, fixed partial dentures, and implant prostheses. Recently, CAD/CAM technology has been applied to fabrication of complete dentures, offering many advantages to dentists and patients over conventional complete dentures [1]. It allows a reduced number of appointments, and spare dentures are more easily available, as digital data are saved [2]. In addition, the laboratory work can be completed more conveniently and cost-effectively than by the traditional methods [3].
Since restorations fabricated and delivered at the dental clinics are not mass-produced but patient-customized, 3D printing manufacturing of denture artificial teeth is worthy, and this technology has received more attention. To confirm the clinical application and its effectivity of 3D printing material, it is necessary to investigate whether it has appropriate physical properties from a clinical point of view. Biomechanical characteristics of 3D printed denture teeth have not been reported yet, comparing those of conventionally manufactured denture teeth. Thus, it is required to evaluate clinical perspective fracture resistance of 3D printed denture teeth compared with conventionally manufactured denture artificial teeth used in dental field. The purpose of the present study was, therefore, to compare 3D printed denture teeth with conventional types of denture teeth, in regard to fracture resistance of chipping fracture and indirect tensile fracture tests. The null hypothesis was that there would be no difference in the fracture resistance among the denture artificial teeth.

Computer-Assisted Designing and 3D Printing for Denture Tooth Specimen Preparation
3D printed denture teeth and four different prefabricated denture teeth were used in this study. The 3D printed resin teeth were made of methacrylate-based photopolymerized resin (Dentca 3D printing denture tooth resin; Dentca TM , Torrance, CA, USA), according to the size and shape of the prefabricated denture maxillary first premolar tooth (Preference; Candulor AG, Glattpark, Switzerland) ( Figure 1). Digital tooth file was uploaded to the 3D-printer software to prepare the pre-processing information for printing. The resin was poured into a 3D-printer (Zenith 3D-printer; Dentis Corp., Daegu, Korea), and teeth were printed layer by layer, also called stereolithography (SLA) technology. After being 3D printed at a 50 µm layer thickness, the teeth were cleaned with isopropanol and cured for a further 40 min by immersion into glycerin in the post-curing oven (UV Honle Sol 500; Honle UV America, Inc., Marlboro, MA, USA) to react the remaining monomers. As conventional types of prefabricated denture teeth, the maxillary first premolar teeth of Premium-8 (Heraeus Kulzer GmbH), Surpass (GC Co.), SR-Orthosit-PE (Ivoclar Vivadent AG), and Preference (Candulor AG) were selected for the current study (Table 1).

Chipping and Indirect Tensile Fracture Testing
Chipping and indirect tensile fracture tests were conducted to evaluate the strength from the clinical aspect of the prepared tooth specimens. Both tests were performed according to previous studies [24,[36][37][38]. For the chipping test, equipment was designed so that the denture tooth specimen was located at a specific position, and did not move when receiving chipping force, prototyped by 3D printing and, finally, manufactured into metal ( Figure 2). A pilot experiment confirmed that the tooth specimen was immobilized during the chipping experiment. The loading rod of the equipment contacting the denture tooth specimen had a hemispherical end that applied a load to the buccal cusp tip of the tooth by point-to-point contact. The bottoms of the denture teeth were ground to ensure a height of 7 mm from the base of the tooth to the buccal cusp and 6 mm to the palatal cusp, in order to apply the force only to the buccal cusp and to prevent the possibility of contact of the palatal cusp while loading (Figure 2a). The denture tooth specimen was secured to the equipment, which was mounted on a universal testing machine (Model 4465; Instron, Canton, MA, USA), and loaded at a speed of 1 mm/min. The load at which the chipping occurred was measured. The test was repeated 10 times for each type of the denture tooth. The method used to measure indirect tensile fracture strength is shown in Figure 3. The prepared denture teeth were embedded with a self-polymerizing resin in cylindrical plastic molds. All embedded teeth in the specimens had a height of 6 mm from the base of the tooth to the buccal and palatal cusps, in order to apply the pressing force to surfaces at an equal level ( Figure 3a). The specimen was positioned and secured to the assembly of the universal testing machine (Model 4465; Instron). A 4 mm diameter round metal bar was connected to the end of the loading rod of the machine, and fixed during loading ( Figure 3b). The round bar was positioned to touch both cusp-slopes of the denture tooth. Load was applied at a speed of 1 mm/min until fracture occurred. The load at which the fracture occurred was measured. Ten samples were tested per each type of the denture tooth.

Qualitative Analysis of the Fracture and Statistical Analysis
After each test, the fractured areas of the specimens were observed with field emission scanning electron microscopy (FESEM) (Hitachi S-4700; Hitachi, Ltd., Tokyo, Japan). The values from the groups were compared using one-way analysis of variance with Tukey's honestly significant difference multiple comparisons test in SPSS (IBM Corp., New York, NY, USA). The significance level was set at p < 0.05.

Results
The results of the chipping test are shown in Table 2. The load-to-chipping fracture values measured in Premium-8 (332.82 ± 54.64 N) and Preference (388.87 ± 55.64 N) teeth were significantly higher than those in the 3D printed resin (89.22 ± 14.87 N), Surpass (89.09 ± 33.75 N), and SR-Orthosit-PE (78.82 ± 25.99 N) teeth (p < 0.05). Preference teeth showed significantly higher chipping fracture resistance than Premium-8 teeth (p < 0.05). The 3D printed resin, Surpass, and SR-Orthosit-PE teeth did not differ significantly from each other. The values from the indirect tensile fracture test are shown in Table 3. The values of the load-to-tensile fracture was 160. 28  Preference teeth. The values of the load-to-tensile fracture were significantly higher in the 3D printed resin and Preference teeth than those in Premium-8, Surpass, and SR-Orthosit-PE teeth (p < 0.05). Preference teeth revealed significantly higher tensile fracture values than the 3D printed resin teeth (p < 0.05). However, the tensile fracture values were not statistically significantly different among Premium-8, Surpass, and SR-Orthosit-PE teeth. The failure patterns differed among the denture tooth types, and the fracture patterns showed a similar pattern within each test group. The fracture patterns after the chipping test consisted of two types (Figure 4). In 3D printed resin, Surpass, and SR-Orthosit-PE teeth, the fracture started from the inner incline of the loaded cusp near the loading point and progressed downward (Figure 4a). For the other, in Premium-8 and Preference teeth, the cusp was depressed and then the crack appeared at the periphery and spread downward (Figure 4b).  Figure 5 shows FESEM images of the fractured denture teeth after the chipping test. Vertical fracture or fracture lines initiating from the buccal cusp were seen in the 3D printed resin, Surpass, and SR-Orthosit-PE teeth, which showed a buccal chipping fracture of the loaded cusp (Figure 5a,e,g). In Premium-8 and Preference teeth, there was initial deformation, such as a reverse cone-shaped depression, after which fracture occurred around the loaded buccal cusp (Figure 5c,d,i,j). The fracture patterns after the indirect tensile fracture test consisted of three types ( Figure 6). The denture tooth types manifested different failure patterns, and each sample of a denture tooth type exhibited a similar fracture pattern after the indirect tensile test. In Premium-8, Surpass, and SR-Orthosit-PE teeth, the fracture went down along the center of the tooth (Figure 6a). In the 3D printed resin teeth, the fracture started from the inner incline of both cusps near the loading point and spread downward toward cervical area (Figure 6b). In Preference teeth, fracture occurred in the middle after the dent of the round bar shape (Figure 6c). FESEM images of the denture teeth after the indirect tensile test are shown in Figure 7. The 3D printed resin teeth showed a fracture in which buccal and palatal cusp of the teeth samples were both broken, rather than a fracture along the central groove of the tooth (Figure 7b). By contrast, a fracture line was seen along the central groove of Premium-8, Surpass, and SR-Orthosit-PE teeth (Figure 7c,e,g). In Preference teeth, deformation, such as a round-shaped depression, occurred initially, after which the tooth fractured along the central groove, similar to the fractures in Premium-8, Surpass, and SR-Orthosit-PE teeth (Figure 7i,j).

Discussion
The null hypothesis was rejected because the results according to the denture tooth types differed significantly. Additionally, fracture modes differed among the denture tooth types, and influenced fracture resistances significantly. Two damage modes were identified: fracture without distinct deformation; and fracture after deformation. Various contact responses to loading indenters have been reported, from the essentially "brittle mode" (crack-dominant) to the essentially "quasi-plastic mode" (deformation-dominant), which represent the microstructure inherent in the material [39].
In the chipping test, Preference and Premium-8 teeth showed high fracture strength. These two types of denture teeth showed deformation, such as cone-type depression, before fracturing. Figure 5 shows that, after the chipping fracture test, visible quasi-plastic deformation occurred with well-developed fractures. In the Premium-8 specimen (Figure 5c,d), a surface depression was seen, and corresponding subsurface cracks were apparent. In the Preference specimen (Figure 5i,j), the quasi-plastic deformation, at the surface and below the surface, became more apparent.
However, the other three types of denture teeth, including the 3D printed resin teeth, did not show this pattern.
In the indirect tensile fracture test, the 3D printed resin and Preference teeth showed high fracture strength, with Preference specimens showing quasi-plastic deformation before the fracture. The 3D printed resin, Premium-8, Surpass, and SR-Orthosit-PE teeth did not show quasi-plastic deformation.
In the indirect tensile test, the fracture pattern of the 3D printed resin teeth was different from other teeth. Simultaneous fracture of buccal and lingual cusps was observed, rather than central line fracture that occurred in other denture tooth types. There was also no quasi-plastic depression pattern before fracture in the indirect tensile fracture test. Nonetheless, the fracture strength of the 3D printed resin teeth was as high as that of the teeth with quasi-plastic depression. This may be because other denture teeth are formed from a combination of different materials, such as enamel material, dentin material, etc., while the 3D printed resin tooth is made entirely of the same material by 3D printing.
In this study, the denture teeth with a quasi-plastic deformation pattern prior to fracture had higher fracture resistance than those without this pattern. The result seems to be due to the deformed shape of the teeth before they broke, which could withstand the load for a longer period of time. In a previous chipping fracture test of conventional denture tooth materials, large deformations were noted in soft materials [40]. In that study, the material with the highest hardness showed the weakest edge-chipping resistance. Bulging deformation and initial tooth split before the main piece chipped off were also exhibited; this was similar to the results of the present study. Characteristically, stiff materials exhibited a more sudden loss of strength than the quasi-plastic materials [39,41].
Ceramic denture teeth are now rarely used; however, denture teeth or bases still chip or fracture [21]. Fracture and separation of the denture teeth are also a frequent complication in other types of removable prostheses, such as implant-supported prostheses [42,43]. Denture tooth fracture is related to various factors. The durability and strength of denture prostheses were influenced by the chemical composition of the teeth and the denture base [44]. In that study, fractures reportedly occurred in the denture tooth itself (cohesive fracture) rather than between the denture base and the tooth (adhesive fracture). Thus, fractures were mostly affected from fracture strength of the denture teeth themselves [22]. Denture teeth are mainly composed of polymethylmethacrylate (PMMA) or urethane dimethacrylate (UDMA) resins. The minor components of each type of denture tooth and the size and amount of the filler vary [45]. There have been few other studies on the biomechanical aspects of 3D printed resin teeth. Dentca 3D printing denture teeth resin is a material developed specially for additive manufacturing. 3D printing of denture teeth is a novel method and uses new materials, therefore, it is necessary to evaluate whether it can be used clinically. In this study, the 3D printed resin denture teeth were compared with prefabricated resin denture teeth currently used in the dental field. The results of this study showed that the 3D printed resin teeth had comparable fracture resistance to some of the conventional prefabricated denture teeth.
One of the goals of material development is to produce materials that are better able to withstand the environmental conditions applied. Effort was carried out in defining a practical and theoretically testing framework [8,46]. Different fracture tests, such as three-point or four-point bending, have been employed, so far. However, it was reported that the traditional fracture criteria without considering T-stress was not accurate for estimating the crack kinking angles [31]. The extended maximum tangential strain criterion, which takes into account the effect of T-strain as well as the singular strain terms, was suggested to obtain more accurate predictions for the mixed fracture mode of dental restorative materials [34,35,47]. To investigate the physical properties of dental restoration, it would be better to measure fracture energy. If 3D printing technology develops and multi-layer artificial teeth can be additively manufactured in the future, further elaborate study is required to carry out through fracture energy analysis. Previous studies have compared the mechanical properties of various types of conventional denture teeth [20,22,36,40,42,48,49]. However, the various tests did not provide a full description of the fracture of denture teeth [50]. The chipping fracture and the indirect tensile tests used in this study were previously used to examine the mechanical strengths of denture teeth. In chipping experiments, the load was applied directly onto the cusp of tooth with an indenter using a universal testing machine, and the load at which chipping occurred was measured and compared [24,36]. Some quantitative indirect tensile fracture strength tests, in which force was applied directly onto buccal and lingual slopes of tooth, have also been described, and fracture strength (N) was recorded as the load at fracture [36][37][38]. Additional material studies are required on the physical properties of dental restorations manufactured by 3D printing with CAD/CAM. Further studies are needed to determine whether mechanical properties of the 3D printed resin teeth can support long-term oral functions to verify the long-term usefulness of the 3D printed teeth. Dynamic cyclic loading with suitable testing devices is also needed to prove the biomechanical stability of the 3D printed resin artificial teeth in the future.

Conclusions
3D printing combined with CAD/CAM methods can automate fabrication of dentures. The 3D printing technology for dentures makes not only denture base but denture teeth. Chipping strength and fracture resistance of the 3D printed resin artificial teeth should be established to address main complications associated with the dentures. The 3D printed resin teeth and four different prefabricated denture teeth were compared through chipping and indirect tensile fracture tests. The prepared denture teeth showed differences in the chipping strength and the indirect tensile fracture resistances, but the 3D printed resin teeth had fracture resistance and biomechanical pattern comparable to the conventionally prefabricated denture teeth. Within the limits of this in vitro study, manufacturing denture teeth with resin materials by 3D printing can be applicable in a dental clinical context.