This clinical study was performed in accordance with the approval of Kyungpook National University Dental Hospital Institutional Review Board (approval number: KNUDH-2021-11-02-01) (Fig. 1). The informed consent was obtained from all subjects. Of the patients who visited the Department of Prosthodontics, Kyungpook National University Dental Hospital and need single crown treatment, those who wanted to participate in the present study were recruited. The selection criteria are as follows: (1) patients who need single restoration treatment in mandible molar teeth; (2) those without decay or periodontal abnormalities in the abutment tooth; (3) those without any occlusal problems. In addition, the following patients were excluded since there was a possible impact on wear or possibility of a side effect according to the experiment: (1) patients with the restoration of antagonist teeth; (2) those with a possible crack tooth; and (3) those with bruxism or clenching habit. This study recruited a total of six patients, considering a 20% dropout rate. One participant was dropped out because of frequent fracture of the temporary tooth.
On Day 1 of visit, the mandible molar teeth were prepared by one skilled technician, using a diamond bur (102R bur, Shofu, Kyoto, Japan). In tooth preparation, the margin shape was formed as a supragingival chamfer margin, and the occlusal surface was excised at 1.5 mm min thickness. Then, one skilled operator took a virtual working cast of the abutment tooth, proximal teeth, and antagonist teeth, and took an occlusal scan, using an intraoral scanner (CS3600; Carestream Dental, Rochester, NY, USA).
Immediately after tooth preparation, interim crowns were fabricated, using self-cured resin (Unifast III; GC Corporation, Tokyo, Japan) with the conventional direct method on the chairside (Table 1). In all groups, the occlusal adjustment process and the final polishing process were performed in the same process by a skilled clinician. The interim crowns were polished with a silicon carbide paper of 600- and 1200-grit grain sizes on a rotary machine with water cooling. Whether the occlusal point and height were appropriate was checked with a 21-um-thick check bite (Check-Film II - Red/Black; Caicedo Group Inc, USA) and an 8-µm-thick shimstock (ARI SHIMSTOCK; TAEKWANG, Seoul, Korea). Immediately before cementation after the final polishing process, the interim crown’s surface was extraorally scanned, using an intraoral scanner (CS3600, Carestream Dental, Rochester, NY, USA). The patients were asked to use temporary cementation (Temp-bond NE, Kerr, Orange, CA, USA) for one week, and to be careful about sticky or hard foods that might cause fall or fracture of the interim crown.
Table 1
Group | Product name | Method | Manufacturer | LOT Number |
Conventional group | UNIFAST III | self-cured | GC corporation, Tokyo, Japan | 2004171 |
3D printing group | RAYDENT C&B | DLP 3D printing | Ray Co., Ltd., Hwaseong-si, Korea | RCB209082B |
The acquired virtual working cast was exported in the STL format, using the intraoral scanner. In addition, with the CAD software (3Shape Dental Designer; 3Shape A/S), interim crowns were designed in the following condition: 60-µm cement space. After the CAD process, the virtual interim crowns were exported with STL, and in the 3D printer software (Megagen, Daegu, Korea), the build angles for printing were set to 0, 45, and 90 degrees (Fig. 2). 3D printing support was set to the software-recommended value. In addition, crowns were printed, using a DLP 3D printer (MEG-PRINTER 3D II; Megagen, Daegu, Korea), under the following conditions (Table 1): 50-µm XY resolution, 50-µm layer thickness. A resin (Raydent C&B; Ray Co., LTD., Hwaseong-si, Korea) was selected as the 3D printing material. The printed interim crowns went through the washing and post-curing process according to the manufacturer’s recommendations and were stored in distilled water at 37 degrees until cementation.
The patients revisited the hospital one week after the first interim crown use. The existing interim crown was removed, and the surface was extraorally scanned with an intraoral scanner (CS3600, Carestream Dental, Rochester, NY, USA).
In addition, the patients were provided with a new interim crown randomly selected from the printing angles of 0, 45, and 90 degrees. They went through the relining, occlusal adjustment, and polishing steps in the oral cavity. For relining, a self-cured resin (Unifast III, GC Korea, Seoul, Korea) was employed. Before use, the interim crown’s surface was scanned. The patients lived with the new interim crown for one week. In addition, in the next hospital visit, the existing interim crown’s surface was scanned, and one of the remaining interim crowns was provided in random order.
Repeating these processes, the patients used four crowns (conventional method, n = 1; 3D printing method, n = 3) respectively for one week in one month. The interim crowns’ surface before and after use was scanned. Lastly, the technician took the final impression and set the final prosthesis.
Using the 3D inspection software (Geomagic Control X, 3Dsystems, Cary, NC, USA), STL file changes of the interim crowns before and after wear were imported (Fig. 3). The interim crowns’ volume and height were observed. The interim crown before wear was designated as a reference, and the best-fit alignment of those after wear was made based on the interim crown’s outer surface area, excluding the occlusal surface area before wear (Fig. 3). To verify the coincidence of the outer surface area in which interim crowns overlapped before and after wear, it was evaluated in 3D inspection software (Geomagic Control X, 3Dsystems, Cary, NC, USA), and it was found that the two models’ overlapping areas had very high coincidence (3.60 ± 0.80 µm). Through the volume comparison before and after wear, the volume loss was calculated. In addition, the data pointers’ interval before and after wear was calculated through the root mean square (RMS) value (Fig. 3), and the formula is as follows.
$$RMS=\frac{1}{\sqrt{n}}\bullet \sqrt{{\sum }_{i=1}^{n}{\left({X}_{1,i}-{X}_{2,i}\right)}^{2}}$$
,
\({X}_{1,i}\) means the interim crown’s point cloud before wear, \({X}_{2,i}\); the point cloud after wear, which means the three-dimensional position of the ith measurement point. n means the number of all point clouds evaluated.
The RMS value shows how different the deviation between two different data sets is from 0. Thus, a low RMS value shows a high degree of three-dimensional agreement of the overlapped data. In addition, a 3D comparison was shown with a color difference map, and the range of − 100 µm and the tolerance range of − 10 µm were designated (green).
Statistical analysis used in this study was conducted, using SPSS Statistics (IBM Co., Armonk, NY, USA). First of all, through the Shapiro-Wilk test, the normal data distribution was investigated. In addition, since normal distribution is formed, each group’s wear volume was compared with a one-way ANOVA test, and as ex-post analysis, using the Tukey HSD test (α = 0.05).