Effect of cavity depth on degree of conversion and microhardness of low-shrinkage resin composites

Abstract

Background

The study aimed to investigate the effect of different cavity depths on the degree of conversion (DC) and surface microhardness (MH) of low-shrinkage resin composites.

Methods

Three low-shrinkage resin composites (Clearfil Majestry Posterior/CMP, Beautifil II LS/BL, Charisma Diamond/CD) and one conventional nanohybrid resin composite (Filtek Z550/FZ) as control were evaluated. For each composite material, a total of 12 disc-shaped samples were prepared using plastic molds with two different depths (2 mm and 4 mm) (n = 6). The same sample preparation protocol was applied for the DC and MH tests, and a total of 96 samples were prepared, 48 for each test. Resin composites filled into molds with the incremental technique were cured with LED light unit only from the top surface. DC was determined by Fourier transform infrared (FTIR) spectroscopy; MH was determined using the Vickers hardness tester. Data were statistically analyzed using Robust and two-way variance analyses (p < 0.05).

Results

For all resin composites, there was no significantly difference in DC and MH between the 2 mm and 4 mm cavity depths. BL showed the highest DC values at both cavity depths, while CD showed the lowest (p < 0.001). At both cavity depths, the highest MH values on bottom-top surfaces ​​were obtained in CMP, while the lowest ​​were in BL. The top surface MH values ​​were higher in all resin composites than the bottom surfaces.

Conclusions

Increasing cavity depth may not negatively affect the DC and MH of low-shrinkage resin composites. Moreover, resin composite material formulations may have a higher potential effect on these parameters.

Clinical trial number

Not applicable.

Background

Resin composite materials are commonly employed in dentistry because of ease of use, high color compatibility with dental tissues, and acceptable physical properties. One of the determining factors in the mechanical properties and hence clinical performance of resin composites is the degree of monomer conversion [1]. The degree of conversion (DC) expresses the percentage of conversion of carbon double bonds to single bonds in the unpolymerized material and is generally in the range of 50 to 80% [2]. DC is affected by some factors such as filler amount, resin type, resin color, spectrum and power of the polymerization light [3]. Low conversion rate of monomers may cause cytotoxicity, decreased hardness, increased wear, marginal deterioration, and microleakage, and consequently secondary caries and pulpal irritation [4, 5]. Most of the resin composites commonly used today consist of methacrylates. In addition to the problems that arise with low conversion rates, polymerization shrinkage and the resulting stresses, especially seen in methacrylate monomers, constitute the basis of the most important problems of resin composites. In order to eliminate these disadvantages, resin composites with low-shrinkage properties have been introduced to clinical use as a result of studies on changing the monomer chemistry. These composites have characteristics that include being BIS-GMA based, having high filler levels, or having high molecular weight. Unlike traditional methacrylate-based composites, a special monomer developed for low-shrinkage resin composite systems is tris-cyclododecane (TCD)-based. This low viscosity monomer reduces the diluent content that causes high shrinkage in conventional composites [6]. One of the restorative materials with low-shrinkage properties, depending on the hybridization of the matrix content, is giomers. Giomers are a restorative material consisting of a hybridized resin matrix and glass ionomer structure, with properties such as low-shrinkage, fluoride release and antibacterial effect [7]. They are similar to conventional glass ionomer restorations and exhibit behavior similar to resin composites, with reported satisfactory clinical performance [8].

A major disadvantage of resin composite restorations is that their DC is proportional to the amount of light to which they are exposed [9]. The depth, size, and location of the cavity all affect the distance of the restoration surface from the light unit, making it a challenging variable to control. As distance increases, light scattering increases, weakening or even blocking the amount of light transmitted into the depth cavity. The reduced light intensity can affect properties such as surface hardness, wear, and color change, potentially leading to the accelerated clinical failure of the restoration [10].Surface hardness is related to the type of matrix and the size, type and degree of curing of the filler [11]. and resin composites showed a gradual decrease in microhardness from the top to the bottom of the composite sample [9]. After curing, some monomers present in the composite material cannot connect with other monomers to form the polymeric network and therefore remain reactive in the resin matrix. The remaining non-reactive monomers act as plasticizers of the resin matrix, which reduces the mechanical properties of the material, especially its hardness [12].

With the changing monomer chemistry, the application of low shrinkage composites that can replace conventional composites, especially in deep cavities, and the possible situations that may occur are a subject that needs to be evaluated. Nowadays, there are a few studies examining the DC for low-shrinkage resin composites, but the number of studies examining the effect of increasing cavity depth on DC and MH is insufficient. Based on this information, the purpose of the this research is to evaluate the effect of cavity depth on the DC and MH of low-shrinkage resin composites in comparsion to a conventional resin composite. The null hypothesis tested was that the cavity depth would not affect the DC and MH of low-shrinkage resin composites.

Methods

Sample size calculation

The power of the sample size was calculated by G*Power software (G*Power Ver. 3.1.9.6, Heinrich-Heine Dusseldorf University, Dusseldorf, Germany) with a 95% confidence interval, an 95% power, and 0.50 effect size values for 48 samples according to one-way ANOVA-type power analysis. Six samples per group were calculated as minimum sample size.

Preparation of samples

The resin composite compositions are provided in Table 1. To simulate different cavity depths, disc-shaped plastic molds with two different depths (6 × 2 mm and 6 × 4 mm) were used. Four different resin composites were tested in the study. A total of 12 samples were prepared for each composite material, six of which were 2 mm thick and six of which were 4 mm thick (n = 6). These samples were produced by a single operator to ensure standardization. The same sample preparation protocol was applied for both tests performed in the study, and a total of 96 samples were produced. Resin composites were placed in plastic molds in 2 mm layers using incremental technique. After covering the samples by using a transparent mylar strip, they were light-cured for 20 s from the top surface with an LED light unit (Valo Cordless, Ultradent, South Jordan, UT, USA) at 1000 mW/cm². (Fig. 1) Aluminum oxide discs (Sof-Lex, 3 M ESPE, USA) were used for polishing the top surfaces. For standardization, polishing process was applied by a same operator with a one direction-low speed hand device, at a uniform, dry, and intermittent pressure of 15,000 rpm for 10 s. After that, the specimens are stored in the absence of moisture and light at 37˚C for 24 h.

Table 1.

Resin composites tested in the study

Material	Manufacturer	Filler content (vol%/wt%)	Matrix composition	Filler composition	Code
Clearfil Majesty Posterior	Kuraray Noritake Dental Inc, Tokyo, Japan	82/92	BIS-GMA, TEGDMA, Hydrophobic aromatic dimethacrylate	Surface treated alumina micro-filler, silanated glass ceramics, silica	CMP
Beautifil II LS	Shofu Co, Kyoto, Japan	68/83	BIS-GMA; Bis-MPEPP, TEGDMA, UDMA	S-PRG, multifunctional Aluminofluoroborosilicate, glass	BL
Charisma Diamond	Heraeus Kulzer, GmbH, Hanau, Germany	64/81	TCD-DI-HEA, UDMA, BIS- GMA, TEGDMA	Barium, aluminum, flour glass and colloidal silica	CD
Filtek Z550	3 M ESPE, St. Paul, MN, USA	68/82	BIS-GMA, UDMA, BIS-EMA, TEGMA, and PEGDMA	Surface-modified zirconia/silica fillers 3,000 nm (3 μm or less), nonagglomerated/non-aggregated surface-modified silica particles 20 nm	FZ

Abbreviations: BIS-EMA: Bisphenol A ethoxylated dimethacrylate; BIS-GMA: Bisphenol A diglycidyl dimethacrylate; Bis-MPEPP: Bisphenole A ethoxylate dimethacrylate; PEGDMA: Polyethylene glycol dimethacrylate; S-PRG: Surface pre-reacted glass; TCD-DI-HEA: 2-Propenoic acid, (octahydro-4,7-methano-1 H-indene-5-diyl) bis(methyleneiminocarbonyloxy-2,1-ethanediyl) ester; TEGDMA: Triethylene glycol dimethacrylate; UDMA: Urethane dimethacrylate

Fig. 1.

Schematic figure of the 2 mm depth (A) and the 4 mm depth (B1-2) sample preparation

Measurement of the degree of conversion (DC)

A Fourier transform infrared (FTIR, Thermo Scientific Nicolet 6700) spectrometer was used to measure the DC. The bottom surface of each sample was scraped using a scalpel and the resulting powder was dissolved in ethanol for analysis. The spectra of uncured and cured composites were recorded in the 4000–400 cm − 1 range with a 4 cm − 1 resolution.

The aliphatic C-C peak for FZ and CMP was measured at 1639 cm-1, while the internal reference aromatic C-C peak was found at 1609 cm-1. For CD and BL, the aliphatic C-C peak was observed at 1638 cm-1, and the internal reference aromatic C-C peak was noted at 1608 cm-1 and 1596 cm-1, respectively. The DC% of every examined specimen was determined used the following formula [13]:

Measurement of the microhardness (MH)

A Vickers microhardness tester (UMT-2, Bruker) was used to measure the MH of top and bottom surfaces of the samples. Three readings were taken one of top and bottom surface by bouncing for 15 s with a 300 g load and average MH was recorded [14].

Statistical analysis

Statistical softwares, JAMOVI (Jamovi v2.28; The Jamovi Project, Sidney, Australia) and Minitab (Minitab Software v.14; State College, PA, USA) was utilized for data analysis. To confirm the normality of the data, Shapiro-Wilk test was implemented. Robust ANOVA and Bonferroni test conducted for multiple comparisons in analyze the DCs. Two-Way ANOVA and the Tukey post hoc test was used for multiple comparison in MH values. The relationship between the DC and MH was examined using Pearson’s and Spearman’s rho correlations. (p < 0.05).

Results

Table 2 shows the median and minimum-maximum values of DC ​​of the bottom surfaces of the resin composites cured at different cavity depths. The Robust ANOVA test results revealed a statistically significant difference in the DC according to composite type (p < 0.001). At both 2 mm and 4 mm depths, the highest DC were observed in the BL, it was statistically insignificant from the FZ. The least was obtanied in the CD, which were significantly different from all other groups (p < 0.001). Additionally, the Robust ANOVA test showed no significant impact of cavity depth on the DC of the resin composites’ bottom surfaces.

Table 2.

Median (minimum-maximum) values of degree of conversion (DC,%) at the bottom surfaces of resin composites

Depth (mm)	Material				p-value
	CMP	BL	CD	FZ
2	82 (62–96)	87 (83–100)	57.5 (51–78)	76 (73–98)	< 0.001
4	65.5 (55–68)	88 (84–100)	62 (52–77)	76 (71–98)

Notes: Robust, ANOVA. (p < 0.05)

Abbreviations: CMP, Clearfil Majestry Posterior; BL, Beatiful II LS; CD, Charisma Diamond; FZ, Filtek Z550

Table 3 summarizes the means and standard deviations of MH for the top and bottom surfaces of the tested resin composites. Two-way ANOVA showed that the composite type had a significant effect on MH (p < 0.001), while cavity depth had statistically insignificant on MH. CMP exhibited the highest MH at 2 mm and 4 mm depths, significantly different from all other groups (p < 0.001). The least was in the BL, which was significantly different from the other groups (p < 0.05). Regardless of cavity depth, the top surfaces had higher MH than the bottom surfaces in all groups.

Table 3.

Mean (standard deviation) values of microhardness (MH) at the top and bottom surfaces of resin composites

Material
Top surface	Depth (mm)	CMP	BL	CD	FZ	p -value
	2	139.17 (16.9)	56.25 (3.86)	100.87 (12.91)	111.45 (12.44)	< 0.001
	4	153.68 (23.96)	56.12 (5.28)	99.93 (16.05)	133.27 (8.31)
Bottom surface		CMP	BL	CD	FZ	< 0.001
	2	111.23 (11.33)	49.33 (4.68)	60.45 (4.85)	88.07 (3.84)
	4	106.57 (19.89)	49.3 (6.46)	66.12 (8.14)	101.18 (23.29)

Notes: Two-Way ANOVA. (p < 0.05)

Abbreviations: CMP, Clearfil Majestry Posterior; BL, Beatiful II LS; CD, Charisma Diamond; FZ, Filtek Z550

Spearman analysis showed no correlation between DC and MH (p > 0.05).

Discussion

One of the determining factors in achieving physical and mechanical performances in resin composites is polymerization kinetics. However, optimum polymerization is crucial to prevent clinical problems that may arise from inadequately polymerization in the base of the cavity [15]. This research analyzed the impact of cavity depth on the DC and MH of low-shrinkage resin composites. The results showed that cavity depth did not significantly affect the DC and MH of low-shrinkage resin composites. As a result, the research’s null hypothesis was accepted.

An ideal resin composite material should have a high degree of monomer conversion. Although there is no definitive literature on the optimum DC value required, it has been reported that the desired value should be above 55% [16]. In the current study, the average DC values of the tested resin composites ranged from 57 to 78%, indicating that the resin composites in this study can provide adequate performance in terms of DC.

The results of the DC test performed with different resin composites showed a statistically significant difference among all resin composites, regardless of cavity depth, and the highest DC was observed in BL. BL used in the study is a giomer-based restorative material containing S-PRG filler that shows low-shrinkage and provides fluoride recharge. Its structure contains BIS-GMA and TEGDMA as monomers and a new monomer, Bis-MPEPP monomer. A previous study showed that low-viscosity monomers used in resin composites increased the resin matrix flow, promoting free radical movement and thus increasing DC [17]. Yu et al. reported that BL promotes higher conversion rates of specific resin blends, leaning toward lower initial viscosity, higher molecular flexibility, and less interfacial scattering of light [18]. According to these results, the low viscosity monomer BIS-MPEPP content of the BL used in the study may have caused increased polymer cross-linking. The low-viscosity TEGDMA used in resin monomers enhances polymerization reactivity by enabling the development of flexible structure [19]. This may have contributed to the higher DC values ​​observed for BL. In addition, the size, shape and distribution of glass-based fillers in resin composites may affect material polymerization [20]. Since BL is an S-PRG-based resin composite, the formation of the fillers in its composition may have affected the DC results.

In this study, the lowest DC among tested resin composites was obtained in CD. According to the manufacturer, CD is a nanohybrid resin composite characterized by the presence of a new TCD-DIHEA monomer with low-shrinkage. There aren’t sufficient investigations in the literature comparing the DC with different restorative materials. Generally, higher molecular weight monomers offer lower mobility, reducing the composite’s final DC [21]. The low DC of CD may be due to the TCD-DIHEA monomer, a new monomer with high molecular weight found in its structure. However, the CD contains large amounts of barium, silica and aluminum particles as inorganic fillers. Darker shades and larger particles in resin composites affect light penetration and reduce DC [22]. Accordingly, the size of the filler particles in CD may have affected DC values.

In the study, CMP showed lower DC values ​​compared to FZ. The lower DC of CMP may be due to the fact that, unlike FZ, it does not contain UDMA and BIS-EMA monomers in its structure. Both of these resin monomers have higher molecular weight and higher double bond concentrations than BIS-GMA [23]. Additionally, the amino groups in the UDMA increases the mobility of radical sites, thereby enhancing polymerization and monomer conversion [24, 25]. However, the filler’s amount, type, size, and shape also affect light scattering efficiency and, therefore, the DC [26]. The increase in the filler ratio negatively affects the polymerization [27]. Accordingly, the high filler ratio of CMP may have influenced the DC values.

For proper photopolymerization of resin composite restorations, the curing light should be placed 0 mm away from the restoration [28]; however, this cannot be achieved in the daily clinical situation. In deep proximal cavities, the photopolymerization distance can exceed 8 mm. Research has indicated that the DC of resin composites is directly proportional to how much light they are subjected to [29, 30]. The DC results for all restorative materials tested in the present study revealed that cavity depth did not affect DC. No study in the literature evaluates the DC of low-shrinkage resin composites at various cavity depths. The obtained result can be attributed to the amount of light radiation reaching the cavity bottom surfaces providing sufficient monomer conversion.

There were significant differences according to the present study in the bottom-top surface MH values among all the tested resin composites at both cavity depths. The variations in matrix formulation chemistry, which is thought to be an effective parameter in influencing the final outcomes, can responsible for this outcome. In the study, CMP showed the highest MH values ​​on both surfaces. Out of all the evaluated restorative materials, CMP has the highest filler ratio. Researches have shown that MH levels and inorganic filler content are positively correlated [11, 31, 32]. In parallel with the results in the literature, the high filler ratio of CMP can explain the high MH values.

In the study, the high DC result in BL was not reflected in the MH results and the lowest MH values ​​were obtained in BL on both surfaces. When examining the literature, previous studies have reported that surface MH increases with increasing conversion [33, 34]. On the contrary, the findings of this investigation contradict those of earlier research [11, 35]. Despite the high DC, the low MH observed in BL may be due to the differences in the S-PRG filler content. Although BL containing S-PRG showed acceptable performance in terms of DC, its low resin content compared to other resin composites may explain its low mechanical performance.

When examining the MH values in the research, it is observed that the bottom surface hardness values at the cavity depths show negligible MH loss compared to the top surface MH values. This finding is consistent with many other studies [36, 37]. Rencz et al. reported in their study that factors such as composite structure, polymerization time, composite thickness or light intensity are effective on durability of resin composites [38]. The energy absorption emitted from the light source decreases with increasing cavity depth [39]. In this study, the decrease in light energy reaching the bottom surface depending on the cavity depth may have affected the different MH results between the surfaces.

The proposed in vitro study has a few limitations. The results obtained are from an in vitro study. In vitro studies cannot fully replicate the oral environment. The presence of saliva, pH changes, and temperature variations in the oral environment can affect the results. Another limitation is that the resin composite samples were not aged. A long aging process can lead to changes in the material properties. Additionally, only 2 mm and 4 mm increment depths were evaluated in this study. However, in clinical practice, posterior restorations may exceed 4 mm in depth. Therefore, this limitation should be considered when interpreting the results. In vivo investigations are necessary to confirm the long-term results and clinical effectiveness of various low-shrinkage resin composites.

Conclusions

Within the limitations of the present study, it can be said that increased cavity depth does not negatively affect the polymerization efficiency and mechanical properties of low-shrinkage resin composites and that matrix formulation differences have a potential effect on resin composites. Considering the lack of a direct relationship between DC and MH, it should be considered that using low-shrinkage resin composites may have negative consequences. Therefore, the properties of newly introduced materials should be accurately evaluated, and evidence-based practices should be applied to improve decision-making.

Abbreviations

BIS-EMA

Bisphenol A ethoxylated dimethacrylate

BIS-GMA

Bisphenol A diglycidyl dimethacrylate

BIS-MPEPP

Bisphenole A ethoxylate dimethacrylate

DC

Degree of conversion

FTIR

Fourier transform infrared

LED

Light-Emitting Diode

MH

Microhardness

S-PRG

Surface pre-reacted glass

TCD

Tris-cyclododecane

TEGDMA

Triethylene glycol dimethacrylate

UDMA

Urethane dimethacrylate

Author contributions

SG and OG conceived the study and contributed to the acquisition of data. SG analysed and interpreted the data and drafted the article. SG and OG critically revised the article for important intellectual content. All members have read, offered feedback, and approved the version of this manuscript to be published.

Funding

This research was supported by Erzincan Binali Yıldırım University Scientific Research Projects with the project number TSA-2023-889.

Data availability

No datasets were generated or analysed during the current study.

Declarations

Ethics approval and consent to participate

Since the study protocol is an in vitro study, it does not require ethical approval.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.