The first dental CAD cam was created in 1971 by Dr. Duret. It was designed by Duret. It began to create the functional form of the occlusal surface of the crown with a system that took an optical impression of the abutment tooth in the oral cavity, designed the optimal crown considering functional movement, and manufactured the crown using a milling machine. However, it has not been widely used in the dental field due to lack of digitizing accuracy, computer performance, and materials. And Dr. Moermann developed the CEREC^Ⓡ system, which produces ceramic inlays by measuring and designing the cavity using an intraoral camera in the clinic and then carving the inlay from a ceramic block using a machine right there in the clinic. In addition, as intraoral camera technology is not very developed, a CAD/CAM system has been developed that takes an impression in a general way, creates a working model, and scans the model to produce a restoration [1].

Currently, as CAM equipment becomes widespread, many clinicians are using the CAD/CAM system to manufacture prosthetics. Although it is possible to manufacture cams with a smooth surface using a cutting process, there are disadvan-tages such as unnecessary consumption of materials, complex structures that cannot be manufactured, and the cost of continuous equipment maintenance and significant time loss in the process. In order to compensate for the shortcomings of the grinding processing method, 3D-printers have emerged that require complicated post-processing, but are capable of producing complex structures compared to cutting processing and reduce unnecessary consumption of materials [2].

The 3D-print market is developing rapidly both domestically and internationally. The 3D-printing market has grown more than four times in the past five years. It is expected to grow at an average annual rate of 26%, growing from USD 5.16 billion in 2015 to USD 16.4 billion in 2020. In particular, the 3D-print market in the medical and dental fields will grow from USD 630 million in 2015 to USD 1.2 billion in 2020. It is said to be a field where high growth is expected to be more than twofold [3].

Currently, when manufacturing prosthetics using the CAD/CAM system, the teeth are designed using a CAD program after scanning the model or taking impressions using an intraoral scanner. Even within a CAD program, many internal values can be specified and adjusted. So far, much research has been done on marginal adequacy, which is significantly related to periodontal disease or the lifepan of prosthesis.

Accordingly, this study designed the coping using a CAD program. In addition, the wax coping produced by cutting with a wax milling machine and the resin coping produced by additive processing with a 3D printer were manufactured into metal coping through conventional methods of investing, burnout, and casting. Afterwards, the suitability of the metal coping according to the manufacturing method was tested. We intend to measure and compare it and use it as important data in predicting clinical outcomes.

1. Manufacturing of abutment specimens

The maxillary first molar (Nissin, Japan), which is mainly used in the production of crown restorations and plays an important functional role, was selected as the abutment test specimen model. A mold was made by replicating the abutment test specimen using silicone impression material (Hinrisil KL, HINRICHS-dental, Germany), and die stone (FujiRock, GC, Japan) was injected to create 35 abutment test specimens. Each test specimen was numbers were assigned from 1 to 35 (Figure 1).

Figure 1. 35 abutment specimens.

2. Scan & design

Test specimens of 35 replicated die stone abutment models were scanned with a scanner (E2, 3shape, Denmark) and designed using a dental design program (3shape, Copenhagen K, Denmark) (Figure 2). The minimum thickness of the coping was set to 0.5 mm, the cement gap on the inner side was set to 0.05 mm (50 micrometers), and the distance to the limit line was set to 1.20 mm. Then, a design was made by assigning numbers that matched the 35 abutment test specimens.

Figure 2. Scanning abutment specimens.

3. Manufacturing of coping

Wax coping was produced by cutting a wax block (Casting wax for CAD/CAM, HUGE, China) using a milling machine (K5 VHF, Pearson^TM Dental, Germany) (Figure 3).

Figure 3. Milling machine.

A castable resin pattern was produced using a printer (ASIGA MAX, ASIGA, Australia) (Figure 4). After printing on the printer, it was cleaned with rubbing alcohol and an ultrasonic cleaner according to the manufacturer’s instructions. Then, it was dipped in glycerin and photopolymerized.

Figure 4. 3D printer.

4. Burnout & casting

The copings were prepared by trimming, and an sprue was attached to each coping. To ensure that the experiment was conducted under the same conditions as possible during investing, wax coping produced using milling and resin coping produced using 3D-printing were mixed and attached to the same crucible former. Afterwards, it was investing and burnout using investment (BC-VEST CB-Plus, Bukwang, Korea) according to the W/P ratio instructed by the manufacturer. Metal (T4, TICONIUM, USA) was cast using a high-frequency casting machine (MODULAR 4, NOBILIUM, New York, USA).

5. Adaptation

The investment material attached to the metal coping was removed by sandblasting with 50 micrometer aluminum oxide (Pen Blaster, SEJIN Dental, Korea), and then cut into disks. Using the silicon replica technique, soft silicone was filled on the inside of the metal coping, pressed on the abutment test piece corresponding to each number, waited until it completely hardened, and then only the metal coping was removed.

6. Measurement and analysis

The test specimen was scanned with soft silicone on top, and merged with the previously scanned test specimen. 3 The thickness of soft silicon was measured using the ‘2D cross- section’ function of the shape design program (Figure 5). The measurement area was the four cusps and buccal and lingual margins of the test specimen, which allow comparison of relatively constant positions (Figure 6) [4]. The measurement results were analyzed using the statistical program SPSS 22 (IBM corp, USA).

Figure 5. 2D cross-section function.

Figure 6. Measurement position of abutment test specimen.

The test results for comparing the suitability of metal copings produced using the CAD/CAM system were shown in Table 1. Looking at the average value of the measurement area according to the manufacturing method, there was no difference in the lingual margin, and in all other measurement areas, the suitability of coping using 3D printing was found to be good.

Table 1 . Metal coping fit measurement results (n=35, unit: mm)

Measuring point	Manufacturing method	Mean±SD	Max	Min	p
MB	Wax milling	0.193±.006	0.023	0.182	p<0.05
	3D printing	0.115±.024	0.194	0.013
ML	Wax milling	0.160±.030	0.216	0.095
	3D printing	0.124±.018	0.151	0.087
DB	Wax milling	0.226±.009	0.239	0.203
	3D printing	0.128±.029	0.241	0.089
DL	Wax milling	0.164±.026	0.21	0.109
	3D printing	0.126±.024	0.19	0.094
BM	Wax milling	0.031±.002	0.037	0.027
	3D printing	0.029±.003	0.034	0.025
LM*	Wax milling	0.030±.002	0.035	0.027
	3D printing	0.030±.002	0.034	0.025

*No statistically significant difference.

In the mesiolingual cusp, the difference between coping using wax milling (0.226) and coping using 3D printing (0.128) was the largest at 0.098, and in the mesiolingual cusp, the difference was the smallest at 0.036. The minimum value was 0.013 at the mesiobuccal cusp of coping using 3D printing, and the maximum value was 0.241 at the distaobuccal cusp of coping using wax milling.

In this study, wax coping and resin coping were manufactured using subtractive processing of wax milling and additive manufacturing of 3D printing. Afterwards, metal copings were manufactured using a casting method for each coping, and the inner fit was measured and compared.

Until now, many studies have been conducted on the margins of prosthetics. McLean and von Fraunhofer [5] said that 120 micrometers is the clinically acceptable range for marginal adequacy, Holmes et al. [6] said 50 micrometers, and most clinicians said that 100 micrometers is the ideal clinically acceptable range. It was said that in the case of prosthetics with excellent marginal spacing, the possibility of secondary plaque deposition is lower than in prosthetics with poor margin spacing, which ultimately has a positive effect on the lifespan of the prosthesis [7].

Previous research on the margins has long published many experiments and clinical tolerances, but there are no exact standards. However, in this experiment, the marginal fitness of metal coping using wax milling was measured to be an average of 0.023 mm for the buccal margin (BM) and 0.020 mm for the lingual margin (LM). The marginal fitness of metal coping using 3D printing was measured to be 0.029 mm for BM and 0.021 mm for LM on average. The margins of the metal coping using wax milling were better, and it was measured to meet all clinically acceptable ranges mentioned in previous studies.

The inner gap was measured on average from 0.160 mm to 0.22 6 mm for metal coping using wax milling, and from 0.115 mm to 0.128 mm for metal coping using 3D printing. The inner thickness of traditional all-ceramic pipes is said to be between 123 micrometers and 154 micrometers [8]. In addition, various studies have shown that the highest compressive strength is achieved when the axial surface fit of the inner surface of an all-ceramic is 73 micrometers [9], and that a decrease in holding power occurs only when the thickness of the cement, that is, the gap between the inner surfaces, is more than 140 micrometers [10]. In addition, it was said that there was no change that lowered the holding power even when the inner spacing was 151 micrometers [11]. Since each study has different definitions of where internal fitness is measured and the experimental conditions are all different, I think it is unreasonable to simply compare numbers alone [12].

In the case of metal coping using 3D printing, the inner gap was smaller on average than that of metal coping using wax milling. I think this is because it is manufactured using a 3D printer that builds up layer by layer. No matter how small the thickness between layers is, there are parts that do not require a technician, so I think it came out smaller than the inner gap of metal coping using wax milling. Also, during the cleaning process after producing the coping with a 3D printer using castable resin, light polymerization was performed with glycerin while residual resin remained, which is thought to have affected the inner gap. Therefore, it can be expected that metal coping using 3D printing resulted in a larger standard deviation than metal coping using wax milling. This represents a disadvantage of 3D printers: post-processing is complicated and cumbersome.

Although the design was designed with the same spacing and manufactured by cutting and lamination, the inner spacing gave different results. As mentioned above, 3D printers produce one layer at a time, so gaps between layers occur, and this is thought to be a result of errors in the manufacturing process that can occur during complex post-processing. This will need to be taken into consideration when producing a resin pattern using a 3D printer in the future, and construct data that can be produced.

In this study, wax pattern coping was produced using a wax mill, and resin pattern coping was produced using a 3D printer. Then, it was manufactured with metal coping through casting and the gap between the margins and the inner surface was measured.

The gap at the margin showed excellent results of less than 0.05 mm for both metal coping using wax milling and metal coping using 3D printing, and for the four cusps, the inner surface of metal coping using 3D printing was better than metal coping using wax milling. The gap came out narrower. Except for the lingual margin where there was no difference in mean values, the results of all other measurement areas were statistically significant (p<0.05).

When applied clinically, it is expected to contribute to the completeness of the prosthesis by adjusting the inner spacing and manufacturing by adjusting the amount of inner spacing according to the processing method of cutting and additive manufacturing.

No potential conflict of interest relevant to this article was reported.