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Effect of post-rinsing time and method on accuracy of denture base manufactured with stereolithography

PURPOSE This in vitro study investigates the effect of different post-rinsing times and methods on the trueness and precision of denture base resin manufactured through stereolithography. MATERIALS AND METHODS Ninety clear photopolymer resin specimens were fabricated and divided into nine groups (n = 10) based on rinsing times and methods. All specimens were rinsed with 99% isopropanol alcohol for 5, 10, and 15 min using three methods-automated, ultrasonic cleaning, and hand washing. The specimens were polymerized for 30 min at 40℃. For trueness, the scanned intaglio surface of each SLA denture base was superimposed on the original standard tessellation language (STL) file using best-fit alignment (n = 10). For precision, the scanned intaglio surface of the STL file in each specimen group was superimposed across each specimen (n = 45). The root mean square error (RMSE) was measured, and the data were analyzed statistically through one-way ANOVA and Tukey test (α < .05). RESULTS The 10-min automated group exhibited the lowest RMSE. For trueness, this was significantly different from specimens in the 5-min hand-washed group (P < .05). For precision, this was significantly different from those of other groups (P < .05), except for the 15-min automated and 15-min ultrasonic groups. The color map results indicated that the 10-min automated method exhibited the most uniform distribution of the intaglio surface adaptation. CONCLUSION The optimal postprocessing rinsing times and methods for achieving clear photopolymer resin were found to be the automated method with rinsing times of 10 and 15 min, and the ultrasonic method with a rinsing time of 15 min.


INTRODUCTION
The application of computer-aided design and computer-aided manufacturing (CAD-CAM) has significantly impacted restorative dentistry, including the fabrication of complete dentures (CDs). [1][2][3][4] CAD-CAM prostheses can be fabricated using either additive manufacturing (AM) or subtractive manufacturing (SM). [5][6][7][8] AM methods implement the fabrication of 3D objects in a layer-by-layer process, 9 which offers high-resolution printing of complex geometries, including surgical or occlusal devices, dental casts, dentures, and maxillofacial prostheses, while reducing waste material and time consumption. [10][11][12] Stereolithography (SLA) is an alternative vat polymerization method based on the processing of ultraviolet (UV) light-activated photosensitive liquid polymers. 13,14 The manufacturing of a vat polymerized product is divided into data processing, manufacturing, and postprocessing. 15 Data processing involves standard tessellation language (STL) file slicing of the virtual design of the dental device and subsequent modification of 3D printing parameters. Manufacturing is the process of dental device printing via vat polymerization in a layer-by-layer manner. 16,17 The postprocessing steps consist of the post-rinsing and post-polymerizing processes. 18 During post-rinsing, the specimen is washed with a solvent to remove any superficial uncured resin, such as isopropanol alcohol (IPA) or tripropylene glycol monomethyl ether (TPM). The post-polymerizing process includes monomer-polymer conversion. These steps are essential for completing the final polymerization and can affect the accuracy of the interim prosthesis. 19 However, the recommendations for optimal solvent, rinsing times, and methods for washing printed SLA objects remain unclear.
Accuracy is classified into trueness and precision (ISO 5725-1). 20  Ammoun et al. 25 found that hand washing with ultrasonics appeared to be consistently better than the automated method. Mostafavi et al . 26 reported that washing the specimen with the TPM solvent group obtained higher trueness and precision values compared to the IPA solvent group. Lee et al . 27 reported that interim crowns rinsed with IPA for 10 minutes exhibited high accuracy. Xu et al . 28 reported no apparent surface alterations that could be detected on the devices post rinsed with IPA for less than 1 h. Mayer et al. 29 reported that specimens treated with IPA exhibited lower fracture loads than those cleaned with centrifugal force or Yellow Magic. In addition to the type of resin, printing parameters, and light intensity, the accuracy of the AM methods may also be influenced by the degree of residual resin removal with different post rinsing times and methods of the postprocessing step.
Even though each manufacturer should provide guidelines for their specific materials and printers, dental literature analyzing the influence of rinsing times and rinsing methods on manufacturing accuracy is nonexistent. Furthermore, there are no in vitro studies on the use of SLA to manufacture denture bases under varying post rinsing times and methods.
Thus, this study aimed to investigate the effects of different post rinsing times and methods on the trueness and precision of SLA denture base resin. The null hypothesis is that there will be no significant difference in trueness and precision under different post rinsing times and methods. Figure 1 illustrates the study design for measuring the accuracy of denture base resin fabricated using SLA for varying rinsing times and methods. The geometry of the specimen was designed to simulate a maxillary complete denture using a CAD software program (Geomagic Freeform; 3D Systems, Rock Hill, SC, USA) https://doi.org/10.4047/jap.2022.14.1.45 (Fig. 2). The original STL file of the denture base was imported into a 3D printing software (PreForm software; Formlabs, Somerville, MA, USA). A photopolymer resin (Clear resin; Formlabs, Somerville, MA, USA) was used in this study. Ninety specimens with orien-tation at an angle of 45 degrees (Fig. 3) and a thickness of 1.5 mm at a 100-μm layer height were fabricated using a 3D printer (Form 2; Formlabs, Somerville, MA, USA). The details of the resin and 3D printer have been described in a previous study. 24 To standardize
The color map deviations for both trueness and precision are depicted in Figure 7 and Figure 8. In terms of trueness, the color map data showed that the deviation patterns ranged from light green to green at the palatal surface for all specimens except for those under the 5U group, which exhibited a partially negative deviation (blue) (Fig. 7). The left and right buccal slopes indicated a partial normal deviation to positive deviation, ranging from partial green to yellow and red in all groups except the 10A group. This group exhibited a green intaglio surface at the palatal surface, palatal slope, alveolar ridge, and buccal slope. In    Positive deviation displayed with yellow to red, and negative deviation with blue to navy blue. Numbers 5, 10, and 15 denote rinsing rimes of 5, 10, and 15 min, respectively. Letters A, U, and H stand for automated, ultrasonic, and hand washing, respectively. contrast, the right and left alveolar ridges and palatal slopes ranged from green to light blue in all groups.
The line angle between the palatal slope and the alveolar ridge displayed yellow to red on the right side in all groups, except 10A group. Moreover, a complex deviation pattern was observed in the 5U group. In terms of precision, it was observed that deviations in the intaglio surface were generally within acceptable ranges (normal deviation; green) for all groups (Fig. 8). therefore, the null hypothesis was rejected.

DISCUSSION
In terms of trueness, the RMSE of the 10A group was significantly lower than that of the 5H group, wherein the highest RMSE was obtained. The rinsing time of 5 min exhibited a higher distortion compared to other rinsing times because of the inability to clean the unpolymerized resin on the intaglio surface during rinsing. 27 However, there was no significant difference between the RMSE values obtained from the three methods when the rinsing time increased. There seems to be a tendency of the RMSE of trueness to decrease with increasing rinsing time for the ultrasonic and hand-washing methods. This result was consistent with those of previous studies, 27,28 which reported that prolonging the post rinsing time may result in a better outcome in the removal of any residual monomers. Nevertheless, this may also adversely affect the mechanical strength. 37,38 For precision, the post rinsing times and methods influenced the RMSE. Comparing the rinsing methods, the RMSE of the 10A group was significantly lower than those of the 10U and 10H groups. In addition, the RMSE of the 5H group was significantly lower than those of the 5A and 5U groups. However, there was no significant difference between the RMSE values obtained for the three rinsing methods of 15 min. The RMSE of precision for the ultrasonic method tended to decrease significantly with increasing rinsing time.
Because the specimens were placed in a glass container with 99%IPA in an ultrasonic baht. Therefore, the ultrasonic vibration force could not be transferred directly to the specimen. The RMSE of precision for the hand-washing method also tended to decrease but was not significantly different when the rinsing time increased. This may be because there was no movement of the IPA solvent in this method.
The results of this study indicate that the automat- In the color map deviation pattern for trueness, a positive deviation (yellow to red) was presented for the left and right buccal slopes in all groups. However, the 10A group exhibited the most uniform color of the intaglio surface (Fig. 7). The deviation pattern indicates a normal distribution, particularly at the palatal surfaces. This is because the presence of complex surfaces and the implementation of complicated slicing techniques along the z-direction, e.g., curves, grooves, and angles, may result in a higher distortion. 16,[34][35] Moreover, the observed positive deviations on the left side were higher than those on the right side of the specimen, which can be explained by the shorter supports on the left side and the greater number of support structures than those on the right side (Fig. 3). The support structures in these areas may result in excessive polymerization during the processing of the subsequent layer of the SLA system. Negative deviations were observed in the right palatal slope in all groups except the 10A group. This can be explained by the shrinkage in the z-direction due to the slicing of the photopolymerized resin layer-by-layer and longer support in this area. 7,8,30 Positive deviations of trueness were obtained in many groups at line angles between the alveolar ridge, palatal slope, and buccal slope (Fig. 7) owing to the inability of rinsing to wash away the residual resin, especially in the hand-washed groups. In this method, the specimen was only shaken in the IPA solvent for a short time Thus, an average value of less than 300 μm deviation for denture base adaptation is assumed to be clinically acceptable. 9,14,32 Based on the results of this study, the RMSE values of trueness ranged from 71 to 86 μm, while the precision RMSE ranged from 40 to 66 μm, below the limit of 300 μm, indicating that the accuracy of SLA 3D-printed dentures is within the clinically accepted range.
The limitations of this study include the in vitro design, excluding variations in intraoral conditions. The limited rinsing solvents were compared, and the standardized design geometry of the specimens was designed to have less complex morphologies than those of the complete maxillary dentures. Moreover, clear photopolymer resin was selected for this study because it was an in vitro study. We also conducted material property tests of the same materials and designs. Therefore, we believe it is important to verify the accuracy using standardized materials and designs. However, since there exist various types of photopolymer resins other than the ones used in this study, it is necessary to investigate the most accurate denture with various types of 3D printers, photopolymer resins, or a combination of these. Consequently, these cannot be generalized for other materials.
Moreover, different prosthesis designs may also have some effects on the postprocessing step. Therefore, future studies should investigate the effects of rinsing time on the accuracy using various rinsing solvents for different material types and prostheses.

CONCLUSION
Within the limitations of this study, the optimal postprocessing rinsing times and methods for achieving