Impact of erosive and abrasive wear on the surface characteristics of hybrid ceramic-polymer dental materials

Abstract

Purpose: To evaluate the surface characteristics of hybrid ceramic-polymer materials before and after exposure to erosive and abrasive media.

Methods: Samples were prepared from Vita Enamic (VE), Cerasmart (CS), VarseoSmile Crown plus (VSC), and VarseoSmile Temp (VST). Surface roughness (SR), scanning electron microscopy (SEM), and surface hardness (SH) analyses were performed before (T0) and after exposing the samples to gastric juice (GJ), toothbrushing (TB), or both (GJ TB) for a simulated period of one (T1) or two years (T2).

Results: At T0, VSC and VST showed higher average (Ra) and maximum (Rz) roughness values, more homogeneous surfaces in SEM micrographs, and lower Vickers numbers (HV) than VE and CS. At T1 and T2, samples showed higher Ra values, greater surface damage on SEM micrographs, and reduced HV. The most pronounced changes were evident for GJ TB samples, followed in order by GJ (within VE and CS) and TB samples (within VSC and VST).

Conclusion: VE and CS showed initially lower SR and higher SH, lower resistance to GJ, and higher resistance to TB than VSC and VST, which would be essential when fabricating restorations for patients who are particularly susceptible to dental erosion and abrasion.

Introduction

Erosive tooth wear presents as slowly progressive loss of dental hard tissues due to their frequent exposure to non-bacterial acidic media. The most common type of dental erosion, dietary erosion, is a consequence of excessive intake of acidic beverages (e.g. carbonated soft drinks, citrus juices, or alcohol) or acidic medicaments (e.g. vitamin C, iron remedies, or acetylsalicylic acid), especially in the form of effervescent tablets [1]. However, an appreciable proportion of patients are also prone to regurgitation erosion by gastric juice from the stomach, which may pass into the oral cavity as a result of gastroesophageal reflux disease (GERD) or nutritive disorders such as bulimia nervosa [2]. Both GERD and bulimia nervosa may result in accumulation of gastric juice in the oral cavity for varying periods, leading to prolonged exposure of oral tissues to acidity, and subsequent dental erosion. It has been reported that 25% of patients with GERD have erosive lesions on their teeth [3]. Moreover, approximately 32% of individuals with GERD or bulimia nervosa perform oral hygiene immediately after regurgitation or vomiting to eliminate any remaining acidic taste or halitosis [4]. Consequently, episodes of extremely low pH followed by toothbrushing abrasion can initiate the loss of tooth substance.

Among several available restorative strategies, rehabilitation of eroded and abraded teeth with indirect restorative materials has become a popular option, due to their better resistance to these processes in comparison to direct restorative materials or human enamel [5]. For this type of restoration, hybrid ceramic-polymer materials may be a viable option. Hybrid ceramic-polymer materials are ideal for computer-aided design/computer-aided manufacturing (CAD-CAM) and three-dimensional (3D) printing systems. Both technologies use digital images of prepared teeth for design and fabrication of restorations. However, the CAD-CAM system relies on subtractive manufacturing of solid materials (blocks and discs) using computerized numerical control (CNC) machines, whereas 3D printing is an additive manufacturing process that uses materials with less inorganic fillers and a flowable consistency [6]. The printing procedure occurs layer-by-layer by sintering of the powder, deposition of a molten thermoplastic material, or light-curing of the resin to create a 3D object [7]. Hybrid ceramic-polymer materials are also classified into two types based on their composition. VITA Enamic (VITA Zahnfabrik, Bad Säckingen, Germany) is a polymer-infiltrated ceramic network (PICN) material composed of porous ceramic infiltrated by a polymer matrix, and is indicated for use with the CAD-CAM system [8]. The other group includes resin nano-ceramic (RNC) materials, such as Cerasmart (GC Europe, Leuven, Belgium), VarseoSmile Crown plus, and VarseoSmile Temp (BEGO, Bremen, Germany), which all comprise ceramic nanoparticles embedded in a polymer matrix [9]. The former is indicated for the CAD-CAM milling process, whereas the latter two are representative materials used for 3D printing.

Hybrid ceramic-polymer materials have been developed to overcome the drawbacks of all-ceramic restorations, such as brittleness with low fracture toughness, and the limitations of dental polymers, including polymerization shrinkage and inferior mechanical properties [10]. Ceramic particles confer high strength and wear resistance on these materials, whereas polymeric parts have higher elasticity, reduced brittleness, and lower wear than all-ceramic materials when used for antagonistic teeth [11]. However, regardless of the above advantages, these materials must maintain their appearance in the oral cavity during years of service, which means they need to be resistant to erosive and abrasive media. In this context, maintenance of surface characteristics is crucial, mainly surface roughness (SR) and surface hardness (SH). One pivotal requirement is resistance to erosive and abrasive attack to ensure the long-term success of dental restorations. In this context, it is necessary to consider that high SR favors biofilm accumulation on restoration surfaces and increases staining and discoloration, whereas changes in SH influence wear resistance and flexural strength [12].

The aim of this study was to evaluate and compare the surface characteristics of hybrid ceramic-polymer materials before and after exposure to erosive and abrasive media. The null hypotheses were as follows: 1. No significant differences in SR and SH would be found among the tested hybrid ceramic-polymer materials before their exposure to erosive and abrasive media. 2. No significant changes in SR and SH would be found among the tested hybrid ceramic-polymer materials after their exposure to erosive and abrasive media.

Materials and Methods

Sample preparation

Four hybrid ceramic-polymer materials (Table 1) were used to produce rectangular samples measuring 5 mm × 5 mm × 2 mm. For measurement of quantitative surface roughness (SR) and surface hardness (SH), a sample size of 180 was calculated to be appropriate using the G*Power 3.1.9.7 program (Heinrich Heine University, Düsseldorf, Germany) with regard to “analysis of variance (ANOVA): fixed effects, special, main effects and interactions” for large effect size 0.4, α 0.05, power 0.95, numerator df 12 and number of groups 36 (4 materials × 3 aging treatments × 3 measurement times). In addition, 72 samples were used for descriptive analysis employing scanning electron microscopy (SEM). Thus, the total sample size for this study was 252 (Fig. 1).

CAD-CAM blocks (18 mm × 14 mm × 12 mm) of Vita Enamic (VE) and Cerasmart (CS) were sectioned perpendicular to their longitudinal axis using a diamond-coated blade (15LC, Buehler, Lake Bluff, IL, USA) mounted on a cutting machine (Isomet 4000, Buehler) at low speed and under constant water irrigation. VarseoSmile Crown plus (VSC) and VarseoSmile Temp (VST) were used to fabricate samples using a 3D printing procedure. First, a Standard Tessellation Language (STL) file was created after scanning one of the previously obtained specimens using a commercial intraoral scanner and CAD software (3Shape, Copenhagen, Denmark). Thereafter, the STL file was transferred to a 3D printer (Varseo XS, BEGO). The printing process was performed according to the following settings: printing orientation 90^o, layer thickness 50 µm, building speed approximately 30 mm/h, and wavelength 405 nm. After printing, the samples were cleaned in an ultrasonic bath (Baku BK-3A, Baku, Guangzhou, PR China) in 96% ethanol for 3 min and then underwent a light-curing process (Otoflash, BEGO) using 2 × 1,500 flashes.

In order to standardize the polishing procedure, all samples were polished with a sequence of 200-, 400-, 600-, 800-, 1,000-, and 1,200-grit silicon carbide papers respectively, attached to a handpiece at 20,000 rpm, for 40 s per side, and under constant water irrigation. Finally, the dimensions of the samples were confirmed using a digital caliper (Lukas Tools Digital Caliper, Vogel, Kevelaer, Germany), and all samples that revealed any visible defects on the surface or did not fulfill the required dimensions were discarded. Following fabrication, the samples were cleaned in an ultrasonic bath in distilled water for 10 min and then in 70% ethanol for another 10 min. The samples were then placed in an incubator for 24 h at 37℃ for 24 h prior to further use.

Table 1 Materials used in the study

Material	Code	Manufacturing	Description	Matrix	Filler	Filler load	Manufacturer
Vita Enamic	VE	CAD-CAM milled	polymer-infiltrated ceramic network material	UDMA, TEGDMA	SiO2 (58%), Al2O3 (20%), Na2O (9%), K2O (4%), B2O3 (0.5%), ZrO2 (< 1%), CaO (< 1%)	86 wt%	VITA Zahnfabrik, Bad Säckingen, Germany
Cerasmart	CS	CAD-CAM milled	resin nano-ceramic material	Bis-MEPP, UDMA, DMA	SiO2 (20 nm), Ba glass (300 nm)	71 wt%	GC Europe, Leuven, Belgium
VarseoSmile Crown plus	VSC	3D printed	ceramic-filled resin material for permanent restorations	Bis-EMA	esterification products of 4.4'-isopropylidenediphenol, ethoxylated and 2-methylprop-2-enoic acid, diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide, methyl benzoylformate, silanized dental glass (0.7 µm)	30-50 wt%	BEGO, Bremen, Germany
VarseoSmile Temp	VST	3D printed	ceramic-filled resin material for temporary restorations	Bis-EMA	esterification products of 4.4'-isopropylidenediphenol, ethoxylated and 2-methylprop-2-enoic acid, diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide, methyl benzoylformate, silanized dental glass (0.7 µm)	30-50 wt%	BEGO

Fig. 1 Number of samples per material/aging treatment subjected to analyses at different measurement times.

Aging treatments

Material samples were then randomly selected for three different aging treatments (Fig. 1 and Table 2). All experimental analyses were performed before (T0) and after aging treatments that simulated one (T1) and two years (T2) of service in the oral cavity.

In the first aging treatment (GJ), the samples were gently placed apart from each other in a Petri dish filled with 10 ml of simulated gastric juice (0.113% hydrochloric acid (HCl) solution in deionized water) and placed in an incubator at 37℃ [13,14]. The pH of the liquid was 1.2, as confirmed using a pH meter (Orion 3-Star, Thermo Fischer Scientific, Waltham, MA, USA). The present research was based on the fact that episodes of regurgitation or vomiting in patients diagnosed with GERD or bulimia nervosa usually occur three times a day, each lasting for approximately 30 s [13]. Therefore, to simulate one year (T1) and two years (T2) in the oral environment the samples were first immersed in the solution for 9 h (T1), and then for another 9 h, or 18 h in total (T2). After the required time had been completed, the samples were removed from the dish, rinsed with distilled water for 10 s, and gently air-dried.

The samples subjected to the second aging treatment (TB) underwent a toothbrushing abrasion procedure [15]. The process was performed using an electrical toothbrush (Oral-B Professional, Braun, Frankfurt, Germany) with medium hardness bristles (Oral-B EB20, Braun). The toothbrush was attached to a special holder that maintained a precise orientation of the toothbrush head parallel to the surface of the tested sample. The sample was simultaneously connected to another holder that was placed in the middle of the plastic container. The plastic container was filled with a custom-made slurry composed of distilled water and toothpaste (Colgate Total, Colgate-Palmolive, Hamburg, Germany) with medium relative dentin abrasivity (RDA = 70) in a ratio of 1:1. During the abrasion process, the pressure of the toothbrush head on the specimen surface was 200 g and the rotation rate was set at 7,500 rpm. It is generally accepted that individuals brush their teeth twice a day for an average of 120 s [15]. However, patients that suffer from GERD or bulimia nervosa regurgitate or vomit on average three times a day, and often brush their teeth immediately afterwards. Therefore, in this study, one more brushing per day was added, assuming three times a day for 120 s each time, or 360 s in total. Moreover, there were four tooth quadrants, with 90 s of brushing per quadrant daily. Also, there were three groups of tooth surfaces to be brushed (vestibular, oral, and occlusal), since proximal surfaces (mesial and distal) are hard to achieve. Therefore, for one group of surfaces (e.g. vestibular) on all teeth in one quadrant, it was considered to take approximately 30 s of brushing daily. Since the toothbrush head length covers approximately two to three teeth simultaneously, it is reasonable to assume that one surface of one tooth is brushed 15 s daily. Therefore, to simulate one year (T1) and two years (T2) in the oral environment, the samples were first brushed for 1 h 30 min (T1), and then for another 1 h 30 min, or 3 h in total (T2). After completion of the required time, the samples were rinsed with distilled water for 10 s and gently air-dried. After each sample, the slurry in the plastic container was renewed and the toothbrush head was changed.

In the third aging treatment (GJ TB), the samples were first exposed to simulated gastric juice and then subjected to toothbrushing abrasion at the same way as described above. Between the two treatments and at the end of toothbrushing abrasion, the samples were rinsed with distilled water for 10 s and gently air-dried.

Table 2 Aging treatments implemented in the study

Aging treatment	Code	Simulation of one year	Simulation of two years
Gastric juice	GJ	immersion in simulated gastric juice for 9 h	immersion in simulated gastric juice for 18 h
Toothbrushing	TB	exposure to toothbrushing abrasion for 1 h 30 min	exposure to toothbrushing abrasion for 3 h
Gastric juice and toothbrushing	GJ TB	immersion in simulated gastric juice for 9 h and exposure to toothbrushing abrasion for 1 h 30 min	immersion in simulated gastric juice for 18 h and exposure to toothbrushing abrasion for 3 h

Surface roughness

SR was analyzed using a contact profilometer (TR200, Beijing Time High Technology, Beijing, PR China) on the same samples for all measurement times (T0, T1, and T2) (Fig. 1). The procedure was performed by moving a diamond needle with a 5-µm radius transversely over the sample surface. The measurements were executed under a constant sampling speed of 0.135 mm/s, 0.02 µm resolution, a cut-off value of 0.25 mm, and a total evaluation length of 1.25 mm. For each sample, the average distance of peaks and valleys from the center line within the measurement length (Ra) and the maximum distance between the highest peak and the deepest valley (Rz) in the measured profile were determined, and the mean value from three measurements in three different directions was calculated.

Scanning electron microscopy

To obtain a closer view of the surface morphology of the tested materials, a scanning electron microscope (JSM-6610 LV, Jeol, Akishima, Tokyo, Japan) was used to investigate randomly selected material samples for each measurement time (T0, T1, and T2) (Fig. 1). The samples were first prepared by sputter-coating with a 20-nm layer of gold for 2 min to ensure electron conductivity and prevent the formation of artifacts on the micrographs. Thereafter, SEM observation was performed on gold-coated discs at an accelerating voltage of 20 kV, with tilt angles ranging from 10° to 45°, at ×1,000 magnification.

Surface hardness

SH measurements were performed on the same samples for all measurement times (T0, T1, and T2) (Fig. 1) using a hardness tester (Wilson VH1102, Buehler) equipped with a pyramid-shaped Vickers indenter. Each sample was fixed using a metal ring on the examination plate, with the tested surface facing the indenter. In the central circular area of each tested sample five indentations were made at five points equidistant from each other, and the average value was obtained. The indentations were performed under a load of 1 kgf (9.8 N) and dwell time of 12 s. The results were expressed as the Vickers hardness number (HV), calculated as HV = 1.854 × (F / d²), where F is the load (in kgf), and d represents the average length of the diagonal formed by the indenter (in mm).

Statistical analysis

The obtained results were analyzed statistically using the SPSS v22.0 statistical software package (SPSS, Chicago, IL, USA) at a significance level of 0.05. The normality of data distribution and the equality of variance were confirmed using the Kolmogorov-Smirnov and Levene tests. Then, three-way repeated measures ANOVA was performed to identify the effects of three fixed factors (materials × aging treatments × measurement times) on the tested parameters (Ra, Rz, and HV). Tukey post hoc test was performed to assess differences in parameters among materials/aging treatments within the same measurement time, whereas changes in parameters among the different measurement times were evaluated using the Bonferroni post hoc test. All data were presented as mean ± standard deviation (SD).

Results

Surface roughness

The results of the SR measurements are presented in Tables 3,4,5. The significance of each fixed factor (material, aging treatment, and measurement time) separately or combined was recorded for both Ra and Rz (P < 0.001) (Table 3).

Before exposure to the different aging treatments (T0), a significant difference was found among the materials in terms of Ra and Rz values (P < 0.001) (Tables 4, 5). VSC and VST samples were significantly rougher than VE and CS samples (P < 0.05), whereas VE revealed significantly higher Ra and Rz values than CS (P < 0.05).

After the samples had been subjected to different aging treatments that simulated one (T1) and two years (T2) of service in the oral cavity, the surfaces of all samples exhibited a significant increase in Ra and Rz compared to the baseline measurements (T0) (P < 0.001). Within each material, the most pronounced changes in Ra and Rz and the highest values of both parameters were recorded for the samples that had first been exposed to GJ and then subjected to the TB procedure (treatment 3), relative to other aging treatments at both T1 and T2. Considering these samples, the Ra and Rz values for VE and CS were more changed and insignificantly higher than those for VSC and VST at both T1 and T2 (P > 0.05), with the exception of the Ra values for VE and VSC at T2, and the Rz values for VE or CS relative to VSC at T1, where the difference was significant (P < 0.05). The samples that were exposed to GJ (treatment 1) showed a greater change in Ra and Rz for VE and CS, and these values were significantly higher than those for VSC and VST at both T1 and T2 (P < 0.05), with the exception of Ra values for CS and VST at T1, and the Rz values for CS and VST at both T1 and T2, where the difference was not significant (P > 0.05). Conversely, the samples subjected to TB (treatment 2) showed a greater change in Ra and Rz for VSC and VST, and both parameters were significantly higher than those for VE and CS at both T1 and T2 (P < 0.001).

Table 3 Three-way ANOVA results for the effect of three fixed factors (material, aging treatment, and measurement time) on the tested parameters

Source	Ra		Rz		HV
	F	P	F	P	F	P
Material	26.66	<0.001	108.21	<0.001	83,948.55	<0.001
Aging treatment	74.68	<0.001	212.22	<0.001	12.03	<0.001
Measurement time	1,850.31	<0.001	11,699.36	<0.001	515.31	<0.001
Material * aging treatment	42.79	<0.001	88.03	<0.001	6.86	<0.001
Material * measurement time	15.35	<0.001	34.64	<0.001	3.36	0.003
Aging treatment * measurement time	21.86	<0.001	55.48	<0.001	8.16	<0.001
Material * aging treatment * measurement time	10.49	<0.001	20.49	<0.001	3.73	<0.001

Table 4 Average surface roughness (Ra) measurements

Material	Aging treatment	Ra (µm) T0 (mean ± SD)	Ra (µm) T1 (mean ± SD)	Ra (µm) T2 (mean ± SD)	Ra Δ
					T0-T1	T0-T2
VE	GJ	0.32 ± 0.01 Bc	0.56 ± 0.07 Ab	0.77 ± 0.04 ABa	0.24 (P < 0.001)	0.45 (P < 0.001)
	TB	0.31 ± 0.03 Bc	0.40 ± 0.05 Cb	0.52 ± 0.02 Fa	0.09 (P < 0.001)	0.21 (P < 0.001)
	GJ TB	0.31 ± 0.02 Bc	0.57 ± 0.04 Ab	0.78 ± 0.03 Aa	0.26 (P < 0.001)	0.47 (P < 0.001)
CS	GJ	0.26 ± 0.02 Cc	0.51 ± 0.08ABb	0.70 ± 0.04 BCa	0.25 (P < 0.001)	0.44 (P < 0.001)
	TB	0.27 ± 0.03 Cc	0.41 ± 0.06 Cb	0.56 ± 0.06 EFa	0.14 (P < 0.001)	0.29 (P < 0.001)
	GJ TB	0.27 ± 0.03 Cc	0.56 ± 0.05 Ab	0.72 ± 0.06ABCa	0.29 (P < 0.001)	0.45 (P < 0.001)
VSC	GJ	0.37 ± 0.03 Ac	0.43 ± 0.06 Cb	0.60 ± 0.09 DEa	0.06 (P = 0.049)	0.23 (P < 0.001)
	TB	0.37 ± 0.04 Ac	0.53 ± 0.09 Ab	0.67 ± 0.06 CDa	0.16 (P < 0.001)	0.30 (P < 0.001)
	GJ TB	0.38 ± 0.03 Ac	0.55 ± 0.05 Ab	0.70 ± 0.08 BCa	0.17 (P < 0.001)	0.32 (P < 0.001)
VST	GJ	0.39 ± 0.03 Ac	0.45 ± 0.06 BCb	0.62 ± 0.07 DEa	0.06 (P = 0.021)	0.23 (P < 0.001)
	TB	0.40 ± 0.03 Ac	0.55 ± 0.05 Ab	0.71 ± 0.05ABCa	0.15 (P < 0.001)	0.31 (P < 0.001)
	GJ TB	0.38 ± 0.02 Ac	0.55 ± 0.07 Ab	0.72 ± 0.07ABCa	0.17 (P < 0.001)	0.34 (P < 0.001)

Different uppercase letters indicate significant difference inside the columns (P < 0.05; Tukey post hoc test), whereas different lowercase letters indicate significant difference inside the rows (P < 0.05; Bonferroni post hoc test).

 

Table 5 Maximum surface roughness (Rz) measurements

Material	Aging treatment	Rz (µm) T0 (mean ± SD)	Rz (µm) T1 (mean ± SD)	Rz (µm) T2 (mean ± SD)	Rz Δ
					T0-T1	T0-T2
VE	GJ	1.59 ± 0.07 Cc	2.94 ± 0.10 Ab	3.91 ± 0.07 Aa	1.35 (P < 0.001)	2.32 (P < 0.001)
	TB	1.58 ± 0.12 Cc	2.30 ± 0.10 Fb	3.27 ± 0.13 Ea	0.72 (P < 0.001)	1.69 (P < 0.001)
	GJ TB	1.56 ± 0.14 Cc	2.99 ± 0.21 Ab	4.05 ± 0.31 Aa	1.43 (P < 0.001)	2.49 (P < 0.001)
CS	GJ	1.42 ± 0.07 Dc	2.59 ± 0.07 Db	3.60 ± 0.12 CDa	1.17 (P < 0.001)	2.18 (P < 0.001)
	TB	1.40 ± 0.14 Dc	2.30 ± 0.18 Fb	3.33 ± 0.17 Ea	0.90 (P < 0.001)	1.93 (P < 0.001)
	GJ TB	1.40 ± 0.11 Dc	2.96 ± 0.19 Ab	3.98 ± 0.04 Aa	1.56 (P < 0.001)	2.58 (P < 0.001)
VSC	GJ	1.74 ± 0.11 Bc	2.42 ± 0.07 EFb	3.40 ± 0.14 Ea	0.68 (P < 0.001)	1.66 (P < 0.001)
	TB	1.73 ± 0.13 Bc	2.70 ± 0.09 CDb	3.73 ± 0.13 BCa	0.97 (P < 0.001)	2.00 (P < 0.001)
	GJ TB	1.72 ± 0.07 Bc	2.77 ± 0.04 BCb	3.87 ± 0.08 ABa	1.05 (P < 0.001)	2.15 (P < 0.001)
VST	GJ	1.89 ± 0.07 Ac	2.57 ± 0.11 DEb	3.42 ± 0.07 DEa	0.68 (P < 0.001)	1.53 (P < 0.001)
	TB	1.95 ± 0.07 Ac	2.91 ± 0.08 ABb	3.89 ± 0.15 ABa	0.96 (P < 0.001)	1.94 (P < 0.001)
	GJ TB	1.96 ± 0.09 Ac	2.95 ± 0.07 Ab	3.90 ± 0.14 ABa	0.99 (P < 0.001)	1.94 (P < 0.001)

Different uppercase letters indicate significant difference inside the columns (P < 0.05; Tukey post hoc test), whereas different lowercase letters indicate significant difference inside the rows (P < 0.05; Bonferroni post hoc test).

 

Scanning electron microscopy

SEM two-dimensional (2D) micrographs of representative samples of each material/aging treatment are presented in Figs. 2,3,4. At the baseline (T0), all samples had more or less heterogeneous surfaces, with a few unidirectional scratches and grooves caused by the polishing procedure.

Following exposure to aging treatments for simulated periods of one (T1) and two years (T2), the Ra and Rz values indicated damage to the surfaces of each tested material as a result of erosive and/or abrasive challenges. The most evident changes in the surface morphology of each material were observed for samples that had been exposed to GJ combined with TB (treatment 3), exhibiting irregularly distributed cracks, pores, and other defects that created a non-uniform surface morphology (Fig. 4). Compared with the baseline (T0), the samples immersed in GJ (treatment 1) revealed more pronounced changes in VE, especially at T2, with vitreous degradation of the ceramic surface creating an etched appearance (Fig. 2). With the same aging treatment, resin-matrix materials infiltrated by ceramic particles, such as CS, VSC, and VST, showed signs of degradation of the organic matrix with consequent exposure of the filler particles. In contrast, VSC and VST samples subjected to TB (treatment 2) showed more evident changes in topography than VE and CS at both T1 and T2 (Fig. 3). As a result of the matrix degradation and filler exposure, cracks and microcavities of differing intensity occurred across all the sample surfaces. Conversely, the surfaces of VE and CS subjected to TB showed relatively minor changes, while in some areas it appeared that the aging treatment had smoothed their surfaces.

Fig. 2 Scanning electron micrographs of representative samples from each material after immersion in gastric juice. Magnification ×1,000.

Fig. 3 Scanning electron micrographs of representative samples from each material after exposure to toothbrushing abrasion. Magnification ×1,000.

Fig. 4 Scanning electron micrographs of representative samples from each material after immersion in gastric juice and exposure to toothbrushing abrasion. Magnification ×1,000.

Surface hardness

The results of the SH measurements are presented in Tables 3 and 6. Three-way ANOVA demonstrated a significant influence of each fixed factor (material, aging treatment, and measurement time) – separately or combined – on HV values (P < 0.05) (Table 3).

Before exposure to aging treatments (T0), VE samples revealed significantly higher HV values than CS (P < 0.001), whereas both VE and CS were harder than VSC and VST (P < 0.001) (Table 6).

Following the different aging treatments that simulated one (T1) and two years (T2) of service in the oral cavity, samples of all the tested materials exhibited a significant decrease in HV relative to the baseline measurements (T0) (P < 0.001). Within each material, the most pronounced changes and the lowest HV values were recorded for the samples that had first been exposed to GJ and then subjected to the TB procedure (treatment 3), relative to the other aging treatments at both T1 and T2. As a result of this treatment, HV values for VE and CS samples were slightly lower, but were still significantly higher than those for VSC and VST at both T1 and T2 (P < 0.05). Furthermore, VE and CS samples that had been exposed to GJ (treatment 1) showed more evident changes in HV, and also maintained significantly higher values in comparison to VSC and VST (P < 0.001). In contrast, VSC and VST samples that had been subjected to TB (treatment 2) showed greater change while maintaining significantly lower HV values than VE and CS (P < 0.001).

Table 6 Surface hardness measurements

Material	Aging treatment	HV T0 (mean ± SD)	HV T1 (mean ± SD)	HV T2 (mean ± SD)	HV Δ
					T0-T1	T0-T2
VE	GJ	249.33 ± 3.90 Ab	237.53 ± 3.78 Ba	234.87 ± 4.70 Ba	11.80 (P < 0.001)	14.46 (P < 0.001)
	TB	247.67 ± 3.78 Ab	243.13 ± 3.46 Aa	239.93 ± 5.97 Aa	4.54 (P = 0.016)	7.74 (P = 0.006)
	GJ TB	249.20 ± 5.14 Ab	235.93 ± 3.30 Ba	233.00 ± 2.78 Ba	13.27 (P < 0.001)	16.20 (P < 0.001)
CS	GJ	80.40 ± 5.04 Bc	72.87 ± 1.81 Cb	65.87 ± 3.25 Da	7.53 (P = 0.001)	14.53 (P < 0.001)
	TB	80.33 ± 3.78 Bb	73.80 ± 3.03 Ca	71.87 ± 4.19 Ca	6.53 (P < 0.001)	8.46 (P < 0.001)
	GJ TB	80.60 ± 3.09 Bc	71.27 ± 5.08 Cb	65.33 ± 3.60 Da	9.33 (P < 0.001)	15.27 (P < 0.001)
VSC	GJ	55.47 ± 4.03 CDEb	51.80 ± 3.69 Db	46.60 ± 4.73 Ea	3.67 (P = 0.150)	8.87 (P < 0.001)
	TB	55.67 ± 5.34 CDc	48.80 ± 2.86 Db	42.07 ± 1.83 Fa	6.87 (P = 0.006)	13.60 (P < 0.001)
	GJ TB	56.73 ± 3.94 Cc	47.53 ± 3.54 DEb	41.87 ± 2.03 Fa	9.20 (P < 0.001)	14.86 (P < 0.001)
VST	GJ	50.87 ± 2.75 Eb	49.73 ± 3.95 Db	39.53 ± 2.47 Fa	1.14 (P > 0.999)	11.34 (P < 0.001)
	TB	51.33 ± 2.92 DEc	44.20 ± 4.14 Eb	39.33 ± 3.18 Fa	7.13 (P < 0.001)	12.00 (P < 0.001)
	GJ TB	52.40 ± 2.56 CDEc	44.07 ± 2.94 Eb	38.67 ± 3.46 Fa	8.33 (P < 0.001)	13.73 (P < 0.001)

Different uppercase letters indicate significant difference inside the columns (P < 0.05; Tukey post hoc test), whereas different lowercase letters indicate significant difference inside the rows (P < 0.05; Bonferroni post hoc test).

Discussion

The aim of the present study was to evaluate and compare the surface characteristics of hybrid ceramic-polymer materials before and after their exposure to erosive and abrasive media. Hybrid ceramic-polymer materials were selected as representative indirect restorative materials that could be potentially employed for rehabilitation of eroded and abraded teeth. Surface roughness (SR) was evaluated using a profilometer that yielded average (Ra) and maximum roughness values (Rz). For better insight into the surface morphology of the tested materials, high-resolution 2D micrographs were obtained using a scanning electron microscope (SEM). Moreover, surface hardness (SH) was determined using a Vickers tester. All analyses were performed before and after aging treatments that simulated exposure to erosive and abrasive media in the oral cavity for one and two years. Immersion in synthetic gastric juice is a commonly used aging treatment that simulates erosive tooth wear initiated by intrinsic factors, whereas laboratory-conducted toothbrushing simulates the usual daily process conducted by most individuals.

Before exposure to aging treatments (T0), although the samples of all tested materials were prepared by the same operator in accordance with the manufacturer’s recommendations, significant differences in their surface characteristics were observed. VSC and VST showed significantly higher Ra and Rz values and lower HV values than VE and CS. One may speculate that the fabrication process is an important factor that would influence the surface characteristics of hybrid ceramic-polymer materials. Specifically, the higher surface roughness and lower surface hardness parameters of the two 3D-printed materials (VSC and VST) relative to the CAD-CAM materials (VE and CS) might be attributable to the fact that the former need to have a flowable consistency and structural stability during printing and storage. In order to maintain a stable liquid consistency, printable material should contain less inorganic filler than the materials available as blocks and discs for CAD-CAM production, thus explaining their poor surface characteristics [16]. Furthermore, the photopolymerization induced by 3D printing devices is never complete, reaching a maximum degree of monomer conversion of slightly less than 85%. Therefore, a substantial amount of unpolymerized monomer in the material might lead to inferior mechanical properties, including surface characteristics [17]. Likewise, the surface characteristics of hybrid ceramic-polymer materials are significantly influenced by the chemical composition of the materials, such as the filler load, size, and type, as well as the structure of the organic matrix [18]. Hence, the significantly higher HV value for VE relative to CS might be related to the former’s higher filler content. Moreover, the most likely reason for the higher HV value for VE relative to the other tested materials is that VE was the only polymer-infiltrated ceramic network material, having a predominance of ceramic particles with excellent mechanical characteristics [19]. Thus, considering the above issues, it is necessary to reject the first null hypothesis that no significant differences in surface roughness and surface hardness would be found among the tested hybrid ceramic-polymer materials before their exposure to erosive and abrasive media.

Following the implemented aging treatments for the simulated one (T1) and two years (T2), the surface roughness of all of the tested materials increased significantly, whereas the surface hardness showed a significant decrease. As expected, within each material, the most pronounced changes in Ra, Rz, and HV values were observed for the samples subjected to both immersion in gastric juice and toothbrushing (treatment 3).

With regard to the effect of gastric juice only (treatment 1), it is known that acidic solutions are capable of initiating hydrolysis of methacrylate ester bonds contained within the resin matrix of polymer-based materials [20]. Inevitably, this leads to degradation of the coupling agent (silane) and consequent detachment of fillers from the resin matrix, which impacts negatively the physical characteristics of the material due to an increase in surface roughness and a decrease in surface hardness [21]. Furthermore, exposure of polymeric components to acidic drinks can increase their diffusion coefficient and water sorption, leading to swelling of the matrix, creating pores and intermolecular spaces within the material from which the inorganic fillers can be released, resulting in mass loss and alteration of surface characteristics [22]. In this connection, only VE and CS samples that had been exposed to gastric juice showed a greater change in surface roughness and surface hardness parameters than VSC and VST, suggesting that these materials may not be optimal for dental restoration in patients with a high degree of dental erosion. This accords with the outcomes of several studies showing that the presence of simulated gastric juice significantly increases the surface roughness [23,24] and decreases the surface hardness of VE and CS [25]. Previous authors have also reported higher surface roughness for VE samples that were exposed to gastric juice [13] or citric acid [26] combined with toothbrushing than for those immersed in distilled water followed by toothbrushing, suggesting that degradation of this material is more severe in an acidic environment. Changes in the surface characteristics of VE and CS after acidic challenge might be due to disruption of the silica phase (broken Si-O-Si bonds) contained in these materials through leaching out of alkaline ions such as Al⁺³, Si⁺², and Zr⁺², consequently leading to surface softening and microdefect formation [13,25]. Another reason is that VE consists of triethylene glycol dimethacrylate (TEGDMA), which is highly susceptible to water sorption during exposure to acid-containing liquids, causing hydrolysis and modification of the mechanical properties of the polymer matrix through expansion, thus reducing friction between the polymer chains [27]. The increase of Ra in VE samples can be attributed to the potential effect of acids on its ceramic part by causing dissolution, leaching out, and loss of glass particles, exposing the boundary around the polymeric phase, which is not similarly affected by acids [28]. In addition, CS has barium-containing filler particles that are soft and susceptible to leaching, thereby reducing the quality of its surface characteristics [29]. Although some studies have reported that gastric juice has no significant effect on the surface characteristics of hybrid CAD-CAM ceramic-polymer materials [30,31], one group has reported a significant decrease in the surface roughness of VE after immersion in simulated gastric juice [25]. However, the range of outcomes in the present and previous studies may be attributable to the various designs that were used to mimic erosion, including measurement instruments and conditions.

Exposing the samples to toothbrushing abrasion (treatment 2) also altered the surface characteristics of the tested materials. It is well established that toothbrushing can cause surface damage to the polymeric part of hybrid ceramic-polymer materials as a result of abrasion of the resin matrix and loss of filler particles, leading to an increase in surface roughness and decrease in surface hardness [32]. In contrast to gastric juice exposure (treatment 1), VSC and VST samples that had been subjected to toothbrushing only (treatment 2) showed more evident changes in surface roughness and surface hardness than VE and CS, suggesting that the former materials may not be the best option for dental restoration in patients with a high risk of tooth abrasion. These results support a previous study demonstrating that the surface roughness of VE remained “unchanged” after toothbrushing [33]. The main reason is that this material contains a ceramic component that confers higher surface hardness, and thus better resistance to abrasion. In contrast, evident changes in surface characteristics of VSC and VST after toothbrushing may be related to the fact that these materials have a lower filler content, leading to decreased surface hardness and higher sensitivity to wear. Considering the above factors, it is also possible to reject the second null hypothesis that no significant changes would be found in the surface roughness and surface hardness of the tested hybrid ceramic-polymer materials after their exposure to erosive and abrasive media.

The present study was unavoidably prone to the inherent limitations of an in vitro investigation in that the results cannot be directly extrapolated to clinical performance, but could help to predict the behavior of the tested hybrid ceramic-polymer materials under similar erosive/abrasive challenges. In addition, as the study design did not consider the buffering and pH-regulating role of saliva, the acidic challenge was probably more hazardous to the surfaces of the tested materials than would be the case in vivo. In summary, although this study has provided valuable insight into the effects of erosive and abrasive media on the surface characteristics of tested materials, much still remains to be understood about the durability and longevity of hybrid ceramic-polymer materials. In the pursuit of optimal treatment options for eroded and abraded teeth, clinicians must consider all factors that influence the surface characteristics of various materials, especially in patients with particular susceptibility to teeth defects. Therefore, to confirm the findings of the present study in the context of the dynamic oral environment and to assess the risks and benefits of using these materials for a wide variety of clinical conditions, further studies are essential.

On the basis of the present findings, it is concluded that there are significant differences in surface roughness and surface hardness among the tested hybrid ceramic-polymer materials, even before their exposure to erosive/abrasive media. VarseoSmile Crown plus and VarseoSmile Temp showed significantly higher surface roughness and lower surface hardness than Vita Enamic and Cerasmart. Furthermore, the tested hybrid ceramic-polymer materials revealed significant changes in surface roughness and surface hardness after exposure to erosive/abrasive media. Vita Enamic and Cerasmart showed lower resistance to simulated gastric juice and higher resistance to toothbrushing than VarseoSmile Crown plus and VarseoSmile Temp.

Abbreviations

2D: two-dimensional; 3D: three-dimensional; ANOVA: analysis of variance; Bis-EMA: bisphenol-A-ethoxylated-glycidyl dimethacrylate; Bis-MEPP: bis-methacryloxyethoxy phenyl propane; CAD-CAM: computer-aided design/computer-aided manufacturing; CNC: computerized numerical control; CS: Cerasmart; DMA: dodecyl dimethacrylate; F: f-statistic; GERD: gastroesophageal reflux disease; GJ: gastric juice; GJ TB: gastric juice and toothbrushing; HCl: hydrochloric acid; HV: Vickers hardness number; P: statistical significance; PICN: polymer-infiltrated ceramic network; Ra: average surface roughness; RDA: relative dentin abrasivity; RNC: resin nano-ceramic; Rz: maximum surface roughness; SD: standard deviation; SEM: scanning electron microscopy; SH: surface hardness; SR: surface roughness; STL: standard tessellation language; T0: baseline measurements; T1: measurements after one simulated year; T2: measurements after two simulated years; TB: toothbrushing; TEGDMA: triethylene glycol dimethacrylate; UDMA: urethane dimethacrylate; VE: Vita Enamic; VSC: VarseoSmile Crown plus; VST: VarseoSmile Temp

Ethical Statements

Not applicable

Conflicts of Interest

The authors declare that they have no conflict of interest.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Author Contributions

SV: conceptualization, methodology, software, formal analysis, investigation, data curation and writing - original draft preparation. MBB: conceptualization, methodology, validation, writing - review and editing, visualization and supervision. MT: formal analysis, investigation, data curation, writing - original draft preparation and visualization. AML: conceptualization, methodology, validation, resources, writing - review and editing, supervision and project administration. AT: methodology, resources, writing - review and editing, visualization, supervision, project administration and funding acquisition. All authors read and approved the final version of the manuscript.

ORCID iD

¹⁾SV: [e-mail], https://orcid.org/0000-0003-2007-8852

²⁾MBB: [e-mail], https://orcid.org/0000-0001-6341-7047

³⁾MT: [e-mail], https://orcid.org/0000-0002-5935-8491

¹⁾AML*: [e-mail], https://orcid.org/0000-0002-1549-7748

¹⁾AT: [e-mail], https://orcid.org/0000-0003-0421-0198

Data Availability Statements

Data generated during the present study are available from the corresponding author on reasonable request.

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