The Influence of the Additive Techniques and Aging on Color Stability, Translucency, and Surface Texture of 3D-Printed Resins for Dental Restorations

ABSTRACT

Aim:

We investigated the impact of additive manufacturing (AM) techniques, specifically stereolithography (SLA) and digital light processing (DLP), on the optical properties and surface characteristics of resin-based materials used in restorative dentistry. With the increasing adoption of AM in dental applications, understanding the performance of these techniques under challenging conditions is essential.

Methods:

Sixty resin specimens of Senertek P-Crown V2 were fabricated using both SLA and DLP technologies. We aimed to evaluate color stability, translucency, and texture of the surface following thermocycling and acidic aging treatments. Statistical analysis was assessed using nonparametric tests, including the Mann–Whitney U, Kruskal–Wallis, and independent sample t-tests, with the significance level set at P < 0.05.

Results:

DLP samples demonstrated significantly greater color changes (ΔE values of 4.64 under thermocycling and 5.79 under acidic conditions) compared to SLA samples, which remained below the threshold of ΔE < 3.3, which is clinically acceptable in either condition. In addition, translucency values indicated inferior performance of DLP materials, especially after exposure to acidic environments. Surface roughness was also markedly higher in DLP.

Conclusion:

SLA demonstrated superior color stability and surface quality compared to DLP. Optimizing the printing parameters and addressing unreacted monomers are essential for enhancing the resilience of dental repairs in challenging environments.

INTRODUCTION

Additive manufacturing (AM) has revolutionized restorative dentistry by enabling precise and cost-effective fabrication of complex geometries, thereby improving clinical outcomes and patient comfort.[1] The integration of biocompatible materials with 3D-printing technologies has significantly expanded dental applications.[2] AM, or 3D-printing, is an alternative to subtractive manufacturing, in which structures are built layer by layer from digital data using resins, thermoplastics, waxes, metals, and ceramics. This technique reduces material waste by more than 40% and achieves more details than grinding burs without increasing production time.[3,4]

In dentistry, the digital light processing (DLP) and stereolithography (SLA) are two of the most widely used 3D-printing techniques. Together, they rely on the polymerization of photosensitive resins to create restorations.[5] Despite the many advantages of AM, challenges remain, including anisotropy and little filler content, which negatively impact the mechanical characteristics of resin-based structures.[6,7] While much of the existing research has focused on strength, accuracy, and durability, the optical properties of 3D-printed restorations remain underexplored.[8]

Properties such as translucency, color stability, and surface texture are critical for both esthetic and functional success. However, these characteristics can degrade over time due to various aging processes, particularly thermocycling and exposure to an acidic environment.[9,10] Therefore, selecting appropriate materials is essential to ensure the continuing success of dental repairs.[11,12]

The durability of dental repair is influenced by mechanical and chemical factors, including occlusal stress and acid exposure from gastroesophageal reflux disease or dietary sources.[13,14] We assessed the effects of SLA and DLP techniques, along with aging in acidic conditions and thermocycling, on the color stability, translucency, and texture of the surface of 3D-printed nano resin-based material. The null theory states that neither manufacturing techniques (SLA and DLP) nor aging affects these properties.

MATERIALS AND METHODS

Sample Size

A G-Power statistical software (version 3.1.9.7) was used to determine the size of the sample. An alpha level of 0.05, beta of 0.1 (power = 80%), and an effect size (d) of 0.3 were detected, resulting in a total model size of 60 specimens (n = 10 per group) to evaluate differences in color stability, translucency, and surface roughness. Sixty rectangular bars (20 × 12 × 1.5 ± 0.01 mm) were fabricated using Senertek P-Crown V2 resin (Libral Traders, India) that contains a nanoceramic filler, which contributes to its mechanical properties. The filler is composed of a silica-based material, with an approximate percentage of 40% by weight. This type of filler enhances the overall strength and wear resistance of the resin, which are crucial for long-term performance in dental applications. The bar geometry was designed using AutoCAD and exported as an STL file. This file was then processed for 3D printing using Anycubic Photon Mono 2 (SLA) and Anycubic Photon D2 (DLP) printers. Slicing software converted the STL file into G-code, which was used by the machines for precise fabrication, producing 30 bars per technique.[15]

Printing and Post-processing of Samples

Both SLA and DLP technologies achieved a resolution of 50 µm, ensuring fine detailing. SLA printing required 38 min, whereas DLP printing was completed in 32 min. Post-processing involved the removal of uncured resin using ethanol, followed by ultraviolet (UV) polymerization at 385 nm for 8 min in a WANHAO BOXMAN-1 UV curing oven. Samples were completely immersed in glycerin to shield them from oxygen, thereby enhancing polymerization and mechanical properties.[16] Further post-processing steps included sanding, polishing, high-pressure air cleaning, and surface finishing to standardize the samples. Thickness verification was conducted using a digital caliper to ensure uniformity within ±0.01 mm.[17] All samples were incubated at 37 ± 1°C for 24 h in distilled water to complete polymerization before aging procedures.[18]

Aging Procedures and Grouping

To evaluate long-term stability, the samples were divided into three groups, as shown in Figure 1.

Figure 1.

Schematic diagram of the study design

• Control Group: 20 rectangular bars (10 from SLA, 10 from DLP) were tested without any aging treatment.

• Thermocycling Group: 20 bars (10 per technique) underwent 1000 thermocycles in a Dorsal thermocycling apparatus (Tehran, Iran). Each cycle alternated from 5°C to 55°C, with a 30 s dwell time in respectively immersion, simulating approximately 1 year of intraoral use.[18]

• Acid Exposure Group: 20 bars (10 per technique) were immersed in separate volumetric flasks containing 15 mL of hydrochloric acid (HCl, pH = 2) to simulate gastric acid exposure. The flasks were incubated at 37°C for 91 h in a 40 L digital incubator to replicate 1 year of acid exposure. Exposure time was calculated based on the assumption that bulimic patients purge three times daily, with each purge exposing teeth to gastric acid for approximately 5 min, totaling 15 min of exposure daily.[19]

After aging, entirely samples were washed thoroughly with distilled water, air-dried, and stored in a desiccator to remove residual moisture before testing. For each printing technique, color change (∆E), translucency parameter (TP), and surface roughness were evaluated in 30 rectangular bars: 10 control, 10 thermocycled, and 10 acid-immersed.

Evaluation Procedures

Using the CIELAB system, color values (L*, a*, b*) were documented with a VITA Easyshade^® V spectrophotometer. To minimize edge loss, glycerin was applied between the specimen and backing due to its similar refractive index to porcelain.[20] Each specimen was removed from the storage medium, gently dried, measured three times, and recalibrated after every two readings.[21]

• A. Color Difference (ΔE)

The color measurement (ΔE) was calculated according to the following formula:

Samples were divided into ΔE 1 (thermocycling vs. control) and ΔE 2 (acid vs. control) groups. The CIELAB scale was used to analyze L* (lightness), a* (red-green), and b* (blue-yellow) differences in both SLA and DLP groups. A ΔE of ≤1.2 specifies no significant color alteration, values between 1.2 and 2.7 are considered well-thought-out within a clinically suitable threshold, while ΔE >2.7 is considered clinically undesirable.[22] Values of ≤1 are not distinguishable to the humanoid eye, while values between 1 and 3.3 are observable by skilled dentists and remain within clinical acceptability. ΔE values of >3.3 are identified by untrained observers, such as patients, and are, therefore, deemed unsatisfactory.[23]

• B. Measurement of Translucency Parameter

The color of all samples was measured using the VITA Easyshade^® V spectrophotometer. Measurements were conducted compared to dark and white backgrounds. The TP value was obtained by manipulating the color change among the samples on the white and dark backgrounds using the following formula:[24]

where B refers to the color coordinates against a black background and W against a white background. L represents lightness, a represents the red-green axis, and b represents the blue-yellow axis. All readings were reserved at the midpoint of each specimen, and the values were averaged. Color-coordinate measurements were achieved inside a traditional colorimetry container for standardization.

• C. Surface Roughness Testing

Thirty samples were arbitrarily designated from the individual printing technique for surface roughness measurements. Three randomized parts (10 μm × 10 μm) were certain on the polished surface of each sample and analyzed using an atomic force microscope (AFM; FlexAFM, Nanosurf AG, Liestal, Switzerland). Additionally, three nonadjacent typical areas measuring 20 × 20 μm² will be located haphazardly designated per sample for 3D scanning to distinguish the surface irregularity index. The acquired raw 3D pictures were then managed using datum modification (surface undulation exclusion), automatic noise elimination, and filter handing out to obtain a magnified 400-fold 3D morphology copy of the surface. The Sa value representing the total average alteration in height from the mean surface was calculated using specialized software (MountainsMap^® for Profilometr). This software automatically calculated the mean surface unevenness and mean surface area for each group. These values were analyzed statistically.[25]

Regarding statistical investigation, data using Statistical Package for the Social Sciences software (Version 27.0; IBM Corp., Armonk, NY, USA), precise evaluations were made consuming each test (Mann–Whitney U, Kruskal–Wallis, and independent sample t-tests) as follows: For ΔE, L*, a*, b* values, TP, and Sa, the Mann–Whitney U test was practical to comparen variations between SLA and DLP samples. The Kruskal–Walli’s test was used to assess multiple groups within each printing method, and the independent sample t-tests were used to compare the alterations between the two printing methods for surface roughness and TP. The justification for using nonparametric tests for color data (ΔE, L*, a*, b*) was based on the tilted delivery of the data, as proved by normality tests. Parametric tests, like the t-tests, were suitable for translucency (TP) and surface roughness (Sa) due to their normal spreading. Following the Kruskal–Walli’s test, Dunn’s post hoc test was applied to regulate specific pairwise comparisons between groups. This test was necessary to correct for multiple contrasts and to guarantee a more accurate explanation of the data. A 95% confidence level was applied at P ≤ 0.05 significantly statistic.

RESULTS

The influence of production techniques and aged protocols on the color stability of 3D-printed Senertek P-Crown V2 resin material was evaluated. Tables 1 and 2 present the descriptive statistics, including median and range values for color change (ΔE) and Lab Color Assessment (L*a*b* Color Evaluation), to assess the influence of control, thermocycling, and acidic aging conditions on both SLA and DLP technologies.

Table 1.

Independent samples Mann–Whitney U test showed a relation between the 3D-printing techniques and color change values

Test			Median	Range	Value-test	P-value
Color changes test (∆E)	∆E1	SLA	3.31	10.40	19.00	0.019*
		DLP	4.64	4.13
	∆E2	SLA	3.16	11.04	23.00	0.041*
		DLP	5.79	8.15

∆E1 (thermocycling – control); ∆E2 (acidic – control)

SLA = stereolithography, DLP = digital light processing

Table 2.

Kruskal–walli’s test (H) and Dunn test showed a relation between the 3D-printing techniques and lab color assessment values

Test				Median	Range	Test value	P-value	Compare (Dunn test)
Lab color assessment	SLA	Control	a*	4.85	3.50	25.806	0.000**	C
			b*	40.65	6.30			B
			L*	86.35	7.90			A
		Thermocycling	a*	6.90	2.30	25.818	0.000**	C
			b*	41.15	6.60			B
			L*	83.70	6.80			A
		Acidic	a*	5.05	3.80	25.841	0.000**	C
			b*	36.45	4.30			B
			L*	82.08	6.80			A
	DLP	Control	a*	0.45	0.80	25.829	0.000**	C
			b*	36.35	4.10			B
			L*	83.75	13.90			A
		Thermocycling	a*	0.50	0.50	25.876	0.000**	C
			b*	37.05	5.50			B
			L*	84.40	13.90			A
		Acidic	a*	1.45	1.20	25.841	0.000**	C
			b*	35.60	7.6			B
			L*	83.70	9.30			A

Lab color assessment (a^*, b^*, L^*)

As shown in Table 1, color changes—ΔE1 (thermocycling vs. control) and ΔE2 (acidic vs. control)—were significantly higher in DLP samples (ΔE1 = 4.64, ΔE2 = 5.79) compared to SLA samples. Although both SLA and DLP samples had no statical significant differences in both ΔE1and ΔE2 states, the DLP group exceeded the clinically acceptable threshold of ΔE2 < 3.3 under both aging conditions [Figure 2].

Figure 2.

Stacked line graph of the relation between the color difference thresholds (∆E1: thermocycling – control; ∆E2: acidic – control) and 3D-printing techniques (SLA = stereolithography, DLP = digital light processing)

Table 2 illustrates the lab color values for individual group (control, thermocycling, and acidic). A statistically important variance was observed in the lightness (L*) values among SLA and DLP samples. SLA samples had higher L* values (86.35, 83.70, 82.08) compared to DLP samples (83.75, 84.40, 83.70). The a* values were lower in DLP samples (0.45, 0.50, 1.45) than in SLA samples (4.85, 6.90, 5.05).

Table 3 shows that only the acidic aging group of the DLP samples exhibited a statistically significant difference in translucency (TP), with a value of 60.6897, compared to SLA. Regarding the surface roughness (Sa), DLP samples demonstrated statistically significantly higher values compared to SLA in both the thermocycling and acidic groups (22.5296, 307.2650), respectively.

Table 3.

Independent samples t-test showed a relation between the 3D-printing techniques and environmental aging. In relation to their translucency and surface roughness values

Test			N	Mean	Std. deviation	Std. error mean	T-test	P-value
Translucency test (TP)	Control	SLA	9	10.1700	2.40593	0.80198	1.68	0.112
		DLP	9	8.3511	2.18144	0.72715
	Thermocycling	SLA	9	12.7948	4.64702	1.54901	0.1	0.922
		DLP	9	13.0432	5.82408	1.94136
	Acidic	SLA	9	8.7314	1.40494	0.46831	5.867	0.000**
		DLP	9	60.6897	26.52935	8.84312
Surface roughness (Sa (	Control	SLA	10	4.8090	0.66166	0.20924	1.792	0.09
		DLP	10	5.3585	0.70850	0.22405
	Thermocycling	SLA	10	7.1051	1.71642	0.54278	7.947	0.001**
		DLP	10	22.5296	5.89325	1.86361
	Acidic	SLA	10	23.8149	7.28209	2.30280	4.789	0.000**
		DLP	10	307.2650	187.03097	59.14439

Independent samples t-test

SLA = stereolithography, DLP = digital light processing

Figure 3 shows the representative three-dimensional external geomorphology of the three assemblies for each printing technology. The superficial morphology for control [Figure 3: A1, A2], thermocycling [Figure 3: B1, B2], and acidic groups [Figure 3: C1, C2] displayed additional irregular superficial, categorized by extra noticeable crests and valleys and more height variances. The overall morphology remained relatively varied, with scratches becoming more pronounced in both the SLA and DLP 3D-printed samples, which displayed more irregular defect surfaces and development of minor overhangs and pits in certain zones for acidic groups [Figure 3: C1, C2].

Figure 3.

(A1–C2) 3D graphic representation of surface morphology with uneven surface, peaks, valleys, and greater height difference of their surfaces, especially in acidic groups, for both stereolithography and digital light processing 3D-printed samples, with more irregular defect surfaces and the appearance of minor lumps and mine in definite zones for (C1, C2)

DISCUSSION

In conservative dentistry, a major advancement includes the use of hybrid resin nanoceramic materials, which syndicate the esthetic potentials of resin composites with the power of ceramics. With the introduction of digital fabrication, 3D-printing knowledges, such as SLA and DLP, have become broadly used for constructing these restorations. Although such techniques have proved high precision and efficiency, a remaining concern is related to their long-term esthetic stability, particularly in terms of color and translucency changes over time.[16]

While several studies have examined discoloration and surface alterations in 3D-printed materials, limited research has compared the impact of different fabrication techniques on color stability.[26] Since dental restorations are frequently exposed to variations in the oral environment, understanding the aging behavior of SLA and DLP-produced resin-ceramic hybrid materials is essential. We aimed to focus this gap by evaluating the surface texture, color, and translucency changes after prolonged immersion in commonly consumed beverages, simulating 1 year of clinical aging. Even with restrictions of the aging practice in fully duplicating the compound oral environment, ended a year, the enhanced aging practice used in this study may not exactly replicate the ordinary wear and tear in the oral hollow, particularly regarding influences such as chewing forces, temperature fluctuations, and contact with saliva and food acids. A more nuanced recreation of these environmental issues might offer more dependable data on the long-term concert of the materials.[27]

Using AFM and spectrophotometric analysis, this study provides an objective assessment of the esthetic durability of these materials, offering insights into their longstanding medical concert. Color stability in 3D-printed dental restorations is essential for maintaining aesthetics. Environmental factors, such as thermocycling and acidic beverages, can cause discoloration, along with intraoral factors, such as temperature changes. However, there has been limited research on the properties of superficial grounding procedures on the color constancy and geometry of printed objects. Research shows that unpolished surfaces stain more, varnish provides temporary protection but may peel, and polished surfaces offer better stability.[28] The extent of hardening of the photopolymerizable materials varies reliant on the device employed. SLA uses a focused laser beam to cure resins, which can result in more complete and uniform curing throughout the printed part, potentially leading to better color stability. A more complete cure means fewer unreacted monomers, which are often susceptible to staining and degradation that can cause color changes,[29] while in DLP, a projector is used to cure an entire layer simultaneously. This can sometimes lead to variations in the light intensity across the layer and potentially less penetration of the curing light, especially in thicker sections. Nonuniform curing can lead to areas with more unreacted monomers and, therefore, lower color stability.[30] At the same time, during the curing process, oxygen can inhibit the polymerization reaction, especially at the surface of the resin. This can lead to the formation of a sticky layer that is more prone to staining. SLA printers with point-by-point curing might be less susceptible to oxygen inhibition, making them more reliable for long-term applications.[31]

In this in vitro study, outcomes exhibited that using the DLP 3D-printing technique, the mean ΔE values (color difference thresholds) were higher than the clinically acceptable values (<3.3) for all tested samples subjected to aging conditions (thermocycling and acidic beverages), as shown in [Figure 2], compared to those produced using the SLA technique. This might be linked to the differences in the production technology of 3D-printed materials, which may have influenced the color stability observed in this study. Additionally, unreacted residual monomers, which are addressed through post-polymerization, may also contribute to color changes.[27] Among the techniques tested, DLP showed the highest mean ΔE values after aging under both thermocycling and acidic conditions. However, the nanoceramic resin group demonstrated statistically significant color changes, possibly due to their microstructural properties and water absorption capacity. Though water absorption alone cannot completely explain the low color stability, previous research has pointed out that different techniques can exhibit varying behaviors even when utilizing the same 3D-printing nanoceramic material.[32]

The significant increase in L* (lightness) values in samples produced by SLA and DLP 3D-printing techniques indicates that environmental factors, such as thermocycling and acidic conditions, greatly affect color scales. Thermocycling can alter the structure and surface roughness of materials, enhance light reflection, and contribute to higher L* values. Similarly, exposure to acidic conditions can lead to chemical changes in dyes and polymers, making certain colors more pronounced and increasing their lightness. This underscores the importance of environmental stability and durability when selecting materials for 3D-printing applications.[33]

The TP was measured, which is the most commonly employed parameter and defined as the alteration in color of the same material with an even thickness all over a black and a white background.[34] Aging in acidic environments significantly affected the TP value of the DLP samples compared to SLA in relation to their color stability. Faster curing time in the DLP process might lead to inadequate cross-linking in certain sections, triggering a drop in translucency. The unreacted monomers in DLP samples can also affect the optical possessions, resulting in a cloudier appearance under acidic environments, which could be due to the dissolution or deprivation of polymer chains under long-term contact with oral environments.[35] At the same time, structural changes in the unreacted monomers and volumetric shrinkage lead to increased surface porosity and enhanced susceptibility to discoloration. Additionally, nonuniform polymerization due to variations in light intensity contributes to these problems. These findings emphasize the need for optimized curing protocols and protective measures to maintain aesthetics under challenging conditions.[29,30]

Studies have shown that the materials used in DLP 3D printing undergo significant surface quality degradation owing to exposure to acidic and thermal environments over long periods, such as a full year of laboratory simulation. This degradation results in an important rise in the surface irregularity, which affects the functional and esthetic appearance. This is due to the chemical reactions in which acids break down the unpolymerized or cured resin material, contributing to structural weakness. Heat exposure also leads to thermal degradation, which causes uneven material deformation. Volumetric changes and loss of structural integrity contribute to the creation of micropores on the surface, which increase its roughness, leading to the accumulation of stains and bacterial plaques that put oral health at risk. Understanding these effects reinforces the need to develop more resistant materials and apply preventive treatments to expand the durability as well as performance of dental materials under harsh environmental conditions.[36] Finally, dissimilarities in curing efficacy, oxygen reserve, and the integral features of each production method at the microscopic level should be considered in the experimental conclusions. In SLA production, oxygen hangup near the surface mains to imperfect polymerization, which can cause abridged strength and changed visual assets. In contrast, DLP printing has an additional even curative apparatus, but dissimilarities in contact time, light concentration, and layer depth can lead to variations in polymerization, which strength elucidate the practical variances in translucency and outward unevenness.[31]

To conclude, SLA demonstrated superior color stability and surface quality compared to DLP. Optimizing the printing parameters and addressing unreacted monomers are essential for enhancing the resilience of dental repairs in challenging environments.

LIST OF ABBREVIATIONS

• AM additive manufacturing

• DLP digital light processing

• SLA stereolithography

CONFLICTS OF INTEREST

There are no conflicts of interest.

AUTHOR CONTRIBUTIONS

All four authors contributed to the work, including the conception and design of the study, literature review, data collection and interpretation, manuscript drafting, critical revision for important intellectual content, and final approval of the version to be published.

ETHICAL POLICY AND INSTITUTIONAL REVIEW BOARD STATEMENT

This study did not involve human or animal participation. The study was performed solely on laboratory models; therefore, informed consent was not required.

DECLARATION OF PATIENT CONSENT

Not applicable.

DATA AVAILABILITY STATEMENT

The data supporting the findings of this study are available from the corresponding author upon reasonable and justified request.

Funding Statement

Nil.