The traditional lost-wax process remains the major approach used for fabricating dental prostheses, such as full metal crowns, porcelain fused metal (PFM) crowns, pressed ceramic crowns, and removable partial denture (RPD) frameworks. However, the fabrication of wax patterns, which is the most critical and labor-intensive step in the creation of these prostheses, continues to largely depend on the skilled labor of individuals. Therefore, it is difficult to effectively ensure accuracy during this step^[1].

Due to technological advances, the digitized and automated processing of dental prostheses is becoming increasingly popular^[2]. CAD/CAM milling systems have been successfully used in the production of fixed prostheses in clinical settings, and their use has increased remarkably over the prior two decades^[3-5]. Rapid prototyping techniques, such as three-dimensional printing (3DP), have been used in the automated production of dental wax patterns to optimize the quality of wax patterns and achieve high production rates^[6,7]. Certain commercial digital dental wax-up systems using 3DP have been developed for and successfully applied in the fabrication of wax patterns for fixed prostheses^[8]. However, according to the literature, the application of 3DP in the fabrication of wax patterns for removable prostheses remains rare. Moreover, there is a lack of data regarding the accuracy of wax patterns fabricated using 3DP.

Accordingly, the aims of this study were to assess the accuracy of the fit of maxillary complete denture base plates fabricated using a 3DP technique and to evaluate the feasibility of using this technique in the production of wax patterns for removable prostheses.

Materials and Methods

A silicon mold was created from a standard edentulous maxillary die (dental study model 402U, GC, Japan), and ten corresponding plaster casts were poured in type IV dental stone (Die-Keen, Heraeus-Kulzer, Germany).

All of these master casts were randomly divided into two groups (n=5): (1) wax edentulous maxillary denture base plates manufactured using the 3DP technique and (2) wax edentulous maxillary denture base plates manufactured by hand.

Fabrication of the edentulous maxillary denture base plates

Five edentulous maxillary master casts were separately scanned with an in vitro scanner (D700 3D Scanner, 3Shape, Denmark) to obtain data for individual casts. Only data for the basal seat and retainer parts were retained. These data were reconstructed into 3D images and stored in STL (standard triangulation language) format (Figure 1). Using CAD software (3Shape Dental System 2012, 3Shape, Denmark), each digital denture base plate was designed using the following sequence of steps: block out undercuts, outline the peripheral extent of the denture base, design mesh, apply the finishing line, design the plate thickness, and design spruing. Using the digitized base plates, a 3D wax printer for dentures (ProJet DP3000, 3D Systems, USA) was used to fabricate 5-mm wax denture base plates. These wax denture plates were printed using a 3DP machine (Figure 2).

Figure 1: CAD model of edentulous maxillary denture base plate

Figure 2: Wax pattern of edentulous maxillary denture base plate

Another five wax denture base plates were appropriately fabricated using the conventional method by an experienced technician.

All of the wax patterns were embedded, cast (Co-Cr-Mo alloy, Shanghai Changping Dental Alloy Co., Ltd., China), sandblasted, trimmed and polished in accordance with the manufacturer’s instructions. Each base plate was checked and adapted to an individual plaster cast for testing (Figures 3, 4).

Figure 3: Base plate fabricated applying 3DP technique

Figure 4: Base of complete fabricated by conventional method

Evaluating the fit of edentulous maxillary denture base plates

To evaluate the fit of the maxillary complete denture base plates created using the 3DP technique and by hand, the final ten plates were attached to the corresponding master casts. All measurements were obtained by a single examiner.

In accordance with a prior report^[9], a blade was used to section each metal base plate cast set into three parts: the canine, first molar, and posterior palate seal regions (Figure 5). The gap between the inner surface of the maxillary denture base (the palatal surface) and the plaster cast’s surface (the tissue surface) was measured for these three sections at three points (Figure 6): the midline (C, determined using the line from the labial frenum to the incisive papilla to the midpalatine raphe to the posterior border of the cast) and the midpoints of the lines from the midline to the left and right residual ridge crests (A, B). Measurements were obtained using a stereomicroscope (SteREO Discovery V12 stereomicroscope, Carl Zeiss, Germany) at a magnification of 50×. The cross-sections were adjusted horizontally to obtain an orientation parallel to the microscope’s plate and to achieve a vertical observation angle. The gap was measured using a digital measuring device (SPOT Advanced, Carl Zeiss) that was checked and calibrated at regular intervals (Figure 7a, 7b). These procedures were performed by a single trained investigator who was not involved in the treatments. Data were recorded and regarded as the adaptation values for each section.

Figure 5: Transverse cuts through the stone cast-metal base set

Figure 6: Example of a region with locations of the measurement points

Figure 7a, 7b: Different measurements under a stereomicroscope

Statistical analysis

The study data were analyzed using t-tests. Statistical analyses were performed using statistical software (SAS 9.13, SAS Institute, Cary, NC, USA). A threshold for significance of p < 0.05 was established.

Results

Table 1 presents the mean and standard deviation values for the denture base plates of each cross-section obtained using the 3DP and conventional methods. When the 3 sections were compared individually (canine: 3DP=214.75 ± 85.63 μm, CM=116.62 ± 19.10 μm; molar: 3DP=149.34 ± 53.09 μm, CM=140.35 ± 83.53 μm; seal area: 3DP=235.68 ± 105.80 μm, CM=173.94 ± 115.17 μm), there were no statistically significant differences between the 3DP and conventional methods (p > 0.05).

Table 1: Means,standard deviation (SD)values (μm) for the gap widths between casts and denture base plates concerning to cut region for two group (p>0.05)

A comparison of the mean gap widths for the conventional method (143.64 ± 86.96 μm) and the 3DP technique (199.72 ± 97.66 μm) revealed no significant differences in the adaptation of the base plate (p>0.05).

Regardless of whether denture base plates were fabricated using 3DP (canine: 214.75 ± 85.63 μm, molar: 149.34 ± 53.09 μm, seal area: 235.68 ± 105.80 μm) or the conventional method (canine: 116.62 ± 19.10 μm; molar: 140.35 ± 83.53 μm; seal area: 173.94 ± 115.17 μm), there were no significant adaptation differences (p>0.05) for any of the 3 sections in each group.

Discussion

In the present study, the wax patterns of maxillary complete denture bases for lost-wax casting were fabricated using a 3DP wax-up system. This additive technique allowed for the automated fabrication of dental wax-ups at a high production rate using a CAD/CAM milling system. Although the traditional lost-wax process is still required after 3DP is used to fabricate a wax pattern, the 3DP-assisted approach is more affordable than selective laser melting or other direct sintering-based manufacturing processes.

Fit, which refers to the space between the inner surface of the denture base and the cast model, is one of the most important criteria for evaluating prostheses and directly influences the retention of maxillary complete dentures. This study used the classical denture base fit evaluation method described by Antony and Peyton^[10]. To thoroughly evaluate fit quality, the gap between the inner surface of the maxillary denture base and the surface of the plaster cast was measured at three points in three sections (the canine, first molar, and posterior palate seal regions). Consani et al. presented base adaptation evaluations using the microwave-disinfected traditional flask closure (TFC) and restriction system flask closure (RSFC) methods. The denture base fit values obtained using the TFC and RSFC methods were 220 ± 50 μm and 170 ± 40 μm, respectively^[9]. Wu et al. conducted a virtual adaptation test to assess the quality of the fit between a LRF-fabricated Ti base plate and original edentulous cast and found that the deviation was 340 μm^[11]. In addition, reports have indicated that the mucosa’s displacement capability is approximately 0.14–0.34 mm^[12]. The data from the current study indicated that the mean adaptation values in both the 3DP group (199.72 ± 97.66 μm) and the conventional group (143.64 ± 86.96 μm) was similar to the adaptation values obtained in prior research and may be acceptable in clinical contexts. Furthermore, the study results demonstrated that there were no statistically significant differences between the 3DP group and the conventional group either for the three individual sections or overall, suggesting that the 3DP technique is clinically acceptable for the production of wax patterns for complete denture bases.

A new 3DP system was compared with the conventional method and used in the present study. This system has an accuracy of 0.025–0.05 mm, receives digitized data files (STL), and creates solid, dissolvable 3D wax patterns via an additive, layer-by-layer process, with a resolution of 16 μm per layer. These steps ensured the accuracy of the 3DP fabricated wax pattern. In contrast to conventionally produced patterns, 3DP-assisted wax patterns were invested and cast without support from a duplicate model to compensate for shrinkage during casting. However, 3DP wax has a greater elastic modulus than normal wax. Thus, during investment, deformation and fracture may be reduced due to the rigid character of the wax.

Although the sample size of this study was small, this trial validated a CAD/CAM/3DP method that represents a potential alternative for the fabrication of denture base wax-ups. Because the present study was restricted solely to lab research, future studies should focus on in vivo clinical evaluation and examine a larger sample.

Conclusion

In this in vitro study, wax patterns of complete maxillary denture base plates were successfully designed and printed using the 3DP technique. With respect to accuracy, comparable fit qualities were obtained using the 3DP and conventional methods. The adaptation of denture bases manufactured using the 3DP technique may be acceptable in clinical contexts. Although areas for improvement remain, the 3DP technique has potential for the design and manufacture of denture frameworks. Continued advancements in 3DP techniques will increase the efficiency of these techniques for clinical applications.

Acknowledgement: We acknowledge the work was supported by Natural Science Foundation of Shanghai Municipality grant: 09ZR1416600

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