A comparative evaluation of retention of record bases fabricated digitally in various types of posterior palatal seal area

Abstract

Objectives

Intraoral scanners used for the fabrication of milled and 3D-printed complete dentures simplify the procedure. However, its effectiveness in recording the functional peripheral seal area needs to be researched. Therefore, this study aims to assess the retention of conventional, milled and 3D-printed denture bases made from conventional impression technique and digital scans in different types of post palatal seal area.

Material and method

Completely edentulous participants were divided into group A and group B based on type of posterior palatal seal area. Each group received three denture bases fabricated using compression molding, milling and 3D printing. The retention was evaluated using a digital dynamometer at 45° and 90° inclination.

Results

The ANOVA test revealed statistically significant difference in the retention of denture bases fabricated using three different techniques. When compared between the three groups at 45 and 90°, statistically significant difference in the retentive values between the control and other two groups were observed.

Conclusion

All three of the fabrication processes have retention that falls within a clinically acceptable range. 3D printed dentures had better adaptation on the PPS area whereas the milled dentures had a better peripheral seal although the conventional process showed highest retention values.

Graphical abstract

1. Introduction

Fabrication of conventional complete dentures is one of the common treatment modality for completely edentulous patients. The complete dentures fabricated should be of good quality to promote patient comfort and function. Retention is a key factor for success of the complete dentures. Adhesion, cohesion, surface tension, salivary film thickness, and air pressure are the physical factors that determine the retention of complete denture.¹ Failures due to lack of retention in maxillary complete dentures is mainly attributed to incomplete recording of the anatomical and physiological landmarks including post palatal seal area (PPS).

The force required to displace a denture is inversely proportional to the thickness of the film of saliva between the tissues and denture. Volumetric shrinkage of acrylic resin during polymerization in conventional techniques may lead to elevation of the denture base away from the posterior part of the palate after polymerization. Studies have shown the greatest distortion in the posterior mid palatal area.^2,3 The dimensional changes that may arise during polymerization of acrylic resin are neutralized by special impression procedures used to determine and establish the post palatal seal.^2,3

The advent of newer digital technologies like computer-aided design/computer-aided manufacturing (subtractive manufacturing) and three-dimensional (3D) printing (additive manufacturing) has led to remarkable advancements in complete denture fabrication.⁴ Complete dentures fabricated digitally are distinguished by simplicity and decreased laboratory steps. It also provides standardized production and superior dimensional precision.

Digital designing confirms a uniform thickness of denture base that can be modified to maintain minimal thickness to ensure patient comfort.^5,6 The currently recommended digital workflow for complete denture fabrication include conventional impression making using an elastomeric material, which is later scanned. However, literature has supported the practicality of the digital workflow, starting with intraoral scanning of edentulous arches to the fabrication of denture that is retentive and functionally effective.7, 8, 9 The intraoral scans prove to have clinical advantages which includes improved patient comfort, avoids gag reflex and allergic response to impression materials. It makes transfer of information to the dental lab technicians and data archiving easier.¹⁰ However, digital scans do have limitations, while they can record the tissues mucostatically, they are unable to capture the functional depth of the mobile tissues and posterior peripheral border in function. The conventional technique of denture fabrication includes border molding with a plastic material and subsequently recording secondary impression with a zinc oxide eugenol-based paste or an elastomer.^10,11

Although the use of intraoral scanner to capture the peripheral seal is challenging, there is no clarity about this contactless recording technique yielding better seal and clinically acceptable results.¹¹ Therefore, this clinical study aimed to assess retention of conventional, milled and 3D printed denture bases made from conventional impression technique and digital scans in different types of post palatal seal area.

2. Methods

The study protocol was approved by the Institutional Ethics Committee (IEC1 403/2023) and CTRI registration (REF/2023/12/076926) was done before commencing the study. The participants visiting the Department of Prosthodontics for complete dentures and willing to participate in the study aged between 50 and 70 years, were enrolled after obtaining written consent.

2.1. Sample description

The sample size estimation was done using the formula $n1=n2=\frac{2\left(Z_{\propto }+Z_{\beta }\right){\sigma }^2}{{\left(\delta \right)}^2}$ where, Z_α = 1.96, α = Type I error at 5 %, Z_β = 1.28 (1-β) = Power at 90 %, σ = std dev and the estimated sample size was n = 20. Based on the type of post palatal seal area patients were divided into 2 groups, group A with type I PPS, and group B with type II PPS, 10 patients in each group. For each patient in Group A and B, different techniques were employed to fabricate maxillary denture bases from which three subgroups were formed, where Subgroup 1- heat cured resin denture bases fabricated by the conventional compression molding technique, Subgroup 2 – milled denture bases from prepolymerized PMMA blocks which were digitally designed from the intra oral scan and Subgroup 3 – 3D printed denture bases which were digitally designed from the intra oral scan.

2.2. Inclusion and exclusion criteria

The inclusion criteria were well-formed, completely edentulous maxillary ridge with a healthy, firm mucoperiosteum, minimal bony or soft tissue undercuts, moderate salivary flow and consistency, Type I and Type II post palatal seal according to House classification of palatal forms.¹² Patients with severe residual ridge resorption, bony undercuts of more than 2 mm in maxilla, flabby ridges, papillary hyperplasia, epulis fissuratum were excluded.

2.3. Fabrication of denture bases

Impression compound (DPI Pinnacle, India) was used for making primary impression, and cast was poured with dental plaster. A special tray was fabricated on the primary cast using autopolymerising acrylic resin and borders were trimmed 2 mm short of the sulcus depth. Border molding was done using low-fusing green stick compound (DPI Pinnacle, India). Zinc oxide eugenol (DPI Impression Paste, India) was used to record the final impression using semi functional technique.¹² The master cast was poured with dental stone. In group 1, a 2-mm layer of modelling wax was adapted on the master cast. The denture base (Trevlon, Dentsply Sirona INC, USA) was fabricated by compression molding technique with curing cycle (74 °C for 8 h). Denture base was trimmed using fine sandpaper fixed to a mandrel and polished with pumice.

In group 2 and 3, intraoral Primescan (Dentsply Sirona INC, USA) was used for digital scanning of the maxillary arch. The manufacturer's recommended scanning path strategy was followed. The denture bases were designed using the software, by setting at 2 mm thickness, with no relief space on the virtual scanned image. In group 2, milled denture bases were milled using a pink prepolymerized PMMA disc (98 mm × 25 mm, Polident, USA) using a 5-axis milling machine (Inlab MC X5, Dentsply Sirona, USA). After milling, the supporting arms were cut using carbide discs and the denture bases was retrieved from the block, and then the polished surface was finished and polished as stated in group 1. In group 3, STL file of the denture base was imported to create the supporting arms, and the denture bases (Lucitone Digital Print TM, Dentsply Sirona INC, USA) were printed in the 3D Printer (Asiga Max UV 3D printer, Australia). After the printing process was finished, the denture bases were taken off the machine's platform and given two ultrasonic bath rinses in 96 % ethanol solution (3-min initial rinse followed by 2-min rinse). The denture bases were then dried and exposed to UV light for 20 min to promote polymerization. The light box produces an output of 43.2 kJ and a blue UV-A wavelength of 315–400 nm. The denture bases were polished and completed in the same way as groups 1.

The denture bases were submerged in water for a whole day to assess retention.^13,14

2.4. Evaluation of retention of denture bases

Using acrylic resin, a loop was created and fastened in the middle of the denture base's polished surface. A digital dynamometer with an accuracy of 0.5 % and a minimum unit of 0.1 N measuring up to 100 N was employed to document the denture bases' retention. The zero button was used to set the display to baseline before each measurement. After inserting the denture base into the patient's mouth and letting it stay there for 5 min to allow it to adjust, the hook was engaged, and a dislodging load was given at 45 and 90° until the denture base was dislodged.

The maximal force required to remove the denture was determined to be the retention force. The measurement was carried out five times at intervals of 5 min, and the average result was noted by a single operator. The data was tabulated, and statistical software (SPSS version 25, IBM) was used to examine the mean and standard deviation of the measured data. ANOVA, or repeated measures analysis of variance, was utilized to compare the groups, and pairwise comparisons using post hoc Bonferroni correction were then conducted at a significant threshold of α = 0.05.

3. Results

3.1. Retention of the denture bases

Descriptive statistics including mean and standard deviation across all groups have been shown in table 1and Fig. 2 which has a significant p value of 6.86e-30∗∗ and 1.21e-28∗∗ at 45 and 90° respectively (see Fig. 1) (see Table 1). The ANOVA test revealed a statistically significant difference in the retention of denture bases fabricated using three different techniques. When compared between the three groups at 45 and 90°, statistically significant differences in the retentive values between the control and the other two groups were observed. However, no statistically significant difference was observed between milled and 3D printed groups as seen in Table 2 and Fig. 2. When a comparison was made between the subgroups, there was a statistically significant difference seen in all three groups, but in subgroup B the difference between 3D printed and milled was less significant as seen in Table 3 and Fig. 2.

Fig. 2.

Graphical representation of the Results.

Fig. 1.

Graphical Abstract.

Table 1.

Comparison across the groups using one way ANOVA at 45° and 90°.

Groups	45°			90°
	Mean	SD	P value	Mean	SD	P value
Control A	37.4	4.812	6.86e-30∗∗	33.8	4.871	1.21e-28∗∗
Milled A	14.6	2.270		12.0	2.981
3D Printed A	11.2	2.347		10.0	2.108
Control B	34.4	3.977		29.8	4.157
Milled B	14.6	2.270		10.8	1.686
3D Printed B	11.2	2.347		9.0	1.054

Table 2.

Comparison between the groups at 45° and at 90°.

Comparison between the groups for 45°
	Control A	Milled A	3D Printed A	Control B	Milled B
Milled A	<2e-16∗∗	–	–	–	–
3D Printed A	<2e-16∗∗	0.30	–	–	–
Control B	0.58	<2e-16∗∗	<2e-16∗∗	–	–
Milled B	<2e-16∗∗	1.00	0.30	<2e-16∗∗	–
3D Printed B	<2e-16∗∗	0.30	1.00	<2e-16∗∗	0.30
Comparison between the groups for 90°
Milled A	<2e-16∗∗	–	–	–	–
3D Printed A	<2e-16∗∗	1.00	–	–	–
Control B	0.088	<2e-16∗∗	<2e-16∗∗	–	–
Milled B	<2e-16∗∗	1.00	1.00	<2e-16∗∗	–
3D Printed B	<2e-16∗∗	0.539	1.00	<2e-16∗∗	1.00

Significance at p < 0.05∗, p < 0.001∗∗.

Table 3.

Comparison between the subgroups at 45° and 90°.

Comparison between the subgroups of A at 45° and 90°
Groups	45°		90°
	Control	Milled	Control	Milled
Milled	2.9e-14∗∗	–	2.5e-13∗∗	–
3D Printed	9.4e-16∗∗	0.095	3.1e-14∗∗	0.64
Comparison between the subgroups of B at 45° and 90°
Milled	4.5e-14∗∗	–	8.5e-15∗∗	–
3D Printed	9.3e-16∗∗	0.049∗	9.0e-16∗∗	0.43

Significance at p < 0.05∗, p < 0.001∗∗.

4. Discussion

Conventional denture fabrication usually requires five clinical steps, which are extremely tedious and taxing on the patient and dentist. If the impressions are digitally recorded, the digital dentures can be fabricated effectively in two clinical steps. A few methods for directly capturing edentulous jaws have been developed, however they do not address the reflections of functional mucosa.^4,15 Moreover, some of these techniques' reproducibility and dependability is in doubt.¹⁵ The primary constraints identified are the impossibility of digitally capturing functional impressions and moderate precision.¹⁶ Using intraoral scans in a digital workflow, without functional border molding⁷ or using a finger to stretch the mucosa and capture its reflections,⁸ a denture with adequate retention was said to be produced.

Nevertheless, the overextension of the plica intermedia and the disregard for the functional movements may be of great concern in this case. Various studies state that the precision of direct digital data shows small variations of about 125 μm.¹⁷ According to Saponaro et al.,¹⁸ the primary disadvantages of CAD/CAM dentures are the occlusal vertical dimension, improper centric relation, and lack of denture retention. Saponaro et al. acknowledged that both the inexperience of dental professionals and the challenge of getting accurate details are to blame for these shortcomings.

To determine the optimal retention of a denture, it is necessary to comprehend the role played by every anatomical region. A satisfactory outcome of treatment is achieved when these criteria are integrated into the prosthesis through appropriate design and technique. According to Jacobson and Kroll, adequate retention, stability, and support are necessary for the effective fabrication of a complete denture.2 Other factors shown to enhance CD retention include the maximal coverage, adaptability, relief in the palatal tissue surface and the PPS design.2 There is a direct correlation between the PPS and retention values, and well-established.

PPS enhances retention quality. Different PPS types contribute to improved retention values: these relate to supero-inferior dimension and anteroposterior dimension of PPS.¹³ The saliva, elasticity, and dynamic movement of the oral mucosa make digital acquisition of the posterior palatal seal more challenging.¹³ As seen in this study 3D printed denture bases showed lesser retentive values in type II PPS.

According to Hamrick¹⁹ an upward extraoral force was more representative of a working scenario when testing the retention of a maxillary denture. Avant²⁰ further added that for applying tipping forces, the anterior ridge serves as the fulcrum. Antolino et al.²¹ demonstrated that applying force to remove the denture at its posterior border was more in line with the denture's dislodging pattern when it was in use. Consequently, the retention was checked by application of forces at 45°and 90°. When comparing the accuracy and repeatability of milled denture bases to traditionally fabricated bases, Goodacre et al.⁸ discovered that the milled denture bases produced superior outcomes. A study by Goodacre et al.⁸ found that in the maxillary complete denture, including the palate, posterior palatal seal area, crest of the alveolar ridge, denture border edge, and 6 mm distant from the denture border, the CAD/CAM milling approach demonstrated less misfit than the injection molding method. The congruence of several CAD/CAM milling systems and the compression molding method in different locations of the maxillary complete denture has been studied by Steinmassl et al.²² Comparatively speaking, compression molding techniques were less congruent with denture-bearing tissues than CAD/CAM milling technologies. Alveolar ridge and palate are the most accurately fit region in traditional and nearly all CAD/CAM systems; the biggest level of misfit was observed in posterior palatal seal area. Similarly, the results of our study as well showed better retention in the 90° as the extent was better reproduced by the milling process.²³ The better retentive properties of milled denture bases, which are probably attributable to a greater fit and higher retention caused by the absence of polymerization shrinkage connected to the milled denture bases.

Hwang et al.²⁴ assessed the adaptability and trueness of milled, printed, and traditional denture bases and found that printed bases produced the best outcomes. It has been observed by that denture bases treated with 3D printed digital light exhibit better surface adaptation (≤100 μm) as compared to milled and pack-and-pressed denture bases which agrees with the results obtained in our study as 3D printed dentures showed better retention when force was applied at 45°.Complete dentures produced using the 3D printing method uses unpolymerized resin; each layer is processed separately, and the process is then meant to be finished with a final light polymerization step. Because the dentures are not fully polymerized before completing this final step, polymerization shrinkage is an eventual phenomenon related to the manufacturing process of 3D printing. When removing the partially polymerized prosthesis from the build platform, deformation may occur. Conversely, there are benefits to 3D printing technology, like reduced raw material waste and inexpensive infrastructure.

In contrast to many studies, Srinivasan et al.²⁵ reported a better precision of denture base fabricated through the conventional method after immersion into saliva. This agrees with the results of our study, as we found that the maximum retention at both angles was found in conventionally fabricated denture bases. This may be attributed to the digital denture bases being fabricated using the intraoral scan without modifications. Goodacre et al.,⁵ also in their proof-of-concept study relined the dentures after fabrication to achieve a good border seal and enhance retention. Most studies compare the accuracy, trueness, retention, and adaptability of models scanned from the casts obtained by final impression using a custom tray. The strength of this study is that the denture bases fabricated using digital scans were used to evaluate the retention in various PPS classifications for both milled and 3D printed techniques.

4.1. Limitations of the study

Digital dentistry was introduced about 30 years ago in dentistry and even today, there is no established protocol for fabrication of removable dentures. This is due to a lack of available literature regarding the same. In the current study, criteria such as surface adaptation and trueness could have been evaluated which could further substantiate the results giving objectivity to the study. Various IOS systems available in the market could have been compared for the accuracy of scanning which could not be done due to lack of resources. The major drawback of this study was that no surface modification was done before processing the denture base or modified techniques to record the PPS. PMMA materials could not be standardized as the techniques used for comparison were different. Further investigation into material-related factors, other factors affecting retention and stability, cost-effectiveness, clinical performance, patient-centred outcomes, and alternative CAD/CAM approaches are necessary to draw further comparisons between conventional and digital methods.

5. Conclusion

Within the limitations of the study, the measures used in the current trial are relative, and when analyzing specific regions of interest individually, it is tough to establish a consistently better method. All three of the processes under investigation appear to have retention that falls within a clinically acceptable range. 3D printed dentures had better adaptation on the PPS area whereas the milled dentures had a better peripheral seal. Both milled and 3Dprinted dentures require relining to achieve better retention. If we can overcome the challenges of recording functional depth, getting a good border seal, and using facial photographs and measurements to establish aesthetics and occlusion complete digitalization of complete dentures fabrication can be achieved within two appointments.

Patient consent

Written consent obtained from patient before commencing the study.

Ethical clearance

Ethical clearance for the study was obtained.

Source of funding: self funded

No funding or grant provided for research.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.