Effect of increasing tip distances of polywave light curing unit on microhardness profiles of bulk-fill composites containing different photoinitiator systems

Introduction

Resin composites have become the material of choice in restorative dentistry due to their aesthetic appeal, conservative application, and improved adhesive proper­ties [1]. However, challenges, such as polymerization shrinkage, stress development, marginal leakage, and suboptimal mechanical performance, continue to affect their clinical success. Polymerization shrinkage is particularly problematic, as it can lead to adhesive failure, postoperative sensitivity, and decreased restoration longevity [2]. The incremental layering approach, though widely recommended to reduce shrinkage stress, is time-consuming and technique-sensitive, with increas­ed risk of interlayer voids or contamination [3]. Bulk-fill composites were introduced to simplify clinical procedures by enabling the placement of increments up to 4-5 mm. These materials exhibit enhanced translucency, advanced photoinitiators, and polymerization modulators, allowing deeper and more uniform curing while reducing shrinkage-related stress. However, concerns persist regarding their ability to achieve optimal polymerization at greater depths, which may compromise mechanical properties in posterior restorations [4].
Adequate mechanical properties of resin-based composites (RBC) depend upon many factors, including the efficiency of light-curing unit (LCU) and its compatibility with the composite’s photoinitiators and properties of its constituent materials (matrix and reinforcement), their volume fractions, the geometry and orientation of reinforcement, quality of the fiber-matrix interface, and the manufacturing process [5]. Traditional quartz tungsten-halogen units have largely been replaced by light-emitting diode (LED) LCUs, which offer higher efficiency, longer lifespan, and minimal heat generation. Monowave LEDs emit light in the 445-480 nm range and activate camphorquinone, the most common initiator. However, newer composites include alternative initiators, which require shorter wavelengths and necessitate the use of polywave LCUs that emit a broader spectrum to activate all photoinitiators effectively [6].
An additional challenge is the attenuation of light intensity with increasing distance between the curing tip and composite surface [7]. This is particularly relevant in deep cavities or anatomically complex restorations, where light attenuation may result in reduced mecha­nical properties. Clinically, ideal LCU positioning is often compromised by limited access. While some studies have examined LCU effects on composites, few have explored how curing distance impacts microhardness in materials with different photoinitiator systems using polywave LCUs. Monowave LCUs emit light in the blue range (~445-480 nm), efficiently activating camphorquinone (CQ) but not alternative initiators, such as Ivo­cerin (390-445 nm), 2,4,6-trimethylbenzoyl diphenylphosphine oxide (TPO) (380-425 nm), bisacylphosphine oxide (380-450 nm), and monoacylphosphine oxide (380-450 nm). Polywave LCUs, emitting light across a broader spectrum (~380-550 nm), can activate a wider range of photoinitiators, potentially improving polymerization in such composites [8].
Several studies [9, 10] have investigated the effect of different LCUs on mechanical properties of composite surface. However, to our knowledge, no study has been conducted so far that assessed and compared the effect of increasing tip distances of a polywave LCU from bulk-fill composites containing different photoinitiators on the top and bottom surfaces’ microhardness.
The null hypothesis tested was firstly that there will be no effect of the type of bulk-fill composite on microhardness when polymerized with polywave LCU; and secondly, there will be no effect of increasing tip distances of polywave LCU on microhardness.

Objective

The objective of this study was to compare the micro­hardness of bulk-fill composites containing different photoinitiators, and to evaluate and compare the effect of distance of LCUs from the composite on the microhardness.

Material and methods

Sample size was calculated using G*Power v. 3.1 software, with an a error of 0.05, a b power of 0.80, and an N2/N1 ratio of 1. Data were obtained from a previously conducted study by Diab et al. [11], and sample size was calculated as n = 6. To increase the power of study, sample size was kept at n = 10.
Eighty cylindrical specimens (4 mm diameter × 4 mm height) were fabricated from two bulk-fill composites. They were divided into two groups: group SP (n = 40), fabricated with SDR® Plus Bulk Fill Flowable (Dentsply Sirona; Switzerland), containing CQ as photoinitiator, and group TP (n = 40), fabricated with Tetric N-Ceram Bulk Fill (Ivoclar Vivadent; Germany), containing CQ, Ivocerin, and TPO as photoinitiators (Table 1). Samples were divided according to tip-to-composite distances of 0, 2, 4, or 6 mm (n = 10 per subgroup). Teflon ring molds (4 mm diameter × 4 mm height) were gene­rated by sectioning hollow Teflon tubing at equal intervals, and coated black on the outer surface. Each mold was secured onto a Mylar strip using sticky wax and placed on a glass slab. Dimensions were verified with a digital Vernier caliper. To standardize curing distances, PVC tubes of 6 mm, 8 mm, and 10 mm height were prepared to achieve LCU tip distances of 2 mm, 4 mm, and 6 mm, respectively. One end of each tube was sealed with black cardboard, featuring a central 4 mm aperture for consistent LCU positioning (Figure 1). For 0 mm distance, a cover slip was placed on the spe­cimen, and the LCU tip was positioned directly in contact with it. Specimens were cured for 10 seconds using polywave LED unit (Bluephase G4, Ivoclar Vivadent; Germany), with irradiance > 1000 mW/cm2, verified with a radiometer intermittently. According to the manu­facturer, the recommended polymerization time for SDR® Plus Bulk Fill Flowable is 20 seconds, with a light intensity of 550 mW/cm2. However, to standardize curing time across samples, the exposure time was kept at 10 seconds when using a light source with intensity of 1000 mW/cm2, which was considered adequate. After curing, specimens were finished and polished sequentially using Shofu Super-Snap discs (coarse to superfine) at the manufacturer-recommended speeds. Samples were stored in a light-proof incubator at 37°C and 100% humidity for 24 hours.
Surface microhardness was measured on top and bottom surfaces using a Vickers hardness tester (Nexus 4000/60, INNOVATEST; Netherlands), with a 100 g-f load for 10 seconds at four equidistant points, excluding margins, following ISO 4049:2019 guidelines. The mean of the four indentations was recorded as the Vickers hardness number (VHN). A Vickers hardness tester was calibrated prior to use, and all indentations were performed by a single trained operator with the same equipment to minimize variability. Data were collected and subjected to statistical analysis with SPSS version 25.0. Independent t-test was used to compare the microhardness of SP and TP groups at top and bottom surfaces. One-way ANOVA test was employed to compare the micro­hardness of SP and TP groups at different distances at top and bottom surfaces.

Results

In the present study, it was observed that at both, the top and bottom surfaces, the microhardness of TP group was significantly greater than that of the SP group at 0 mm, 2 mm, 4 mm, and 6 mm distances (Table 2, Figure 2).
The results showed that the microhardness decreased as the distance from the light source increased for both the groups at the top and bottom surfaces. For the SP group at 0 mm, the mean microhardness was the highest (48.22 for the top and 47.40 for the bottom), followed by 2 mm (39.38 for the top and 38.86 for the bottom), and 4 mm (33.24 for the top and 32.38 for the bottom), while the least microhardness was seen at 6 mm (29.95 for the top and 27.38 for the bottom) (Table 3, Figure 3). Similarly, for the TP group at 0 mm, the mean microhardness was the highest (53.04 for the top and 51.84 for the bottom), followed by 2 mm (47.80 for the top and 47.64 for the bottom), and 4 mm (41.50 for the top and 40.66 for the bottom), while the least microhardness was seen at 6 mm (35.12 for the top and 34.64 for the bottom), with differences statistically significant at p-value of 0.05 (Table 4, Figure 4). Additionally, it was observed that at 0 mm and 2 mm distances, there was a non-significant difference in the microhardness of SP group when the top and bottom surfaces were compared. However, at 4 mm and 6 mm distances, the microhardness of SP group at the top surface was significantly greater than that of the bottom surface (Table 5, Figure 5). When the effect of the tip distance on VHN values of TP group at each distance were compared between the top and bottom surfaces, there was a non-significant difference in the microhardness values of the top and bottom surfaces at all distances (Table 6, Figure 6).

Discussion

In this study, SDR® Plus Bulk Fill Flowable, a low-viscosity bulk-fill composite containing CQ, and Tetric N-Ceram Bulk Fill (TN), a nanohybrid composite containing CQ, TPO, and Ivocerin, were evaluated. CQ, although widely used, has drawbacks, such as a persistent yellow hue [12] and low quantum efficiency (~7%), which has led to the development of alternative initiators [13]. Ivocerin, a germanium-based initiator, demonstrates high reactivity and a broad absorption range (370-460 nm), with a quantum efficiency of approximately 83% [14]. Microhardness was assessed using the Vickers hardness test, a standard method for evaluating polymerization quality in composites. Hardness is directly related to the degree of conversion, and is considered a clinically relevant indicator of a material’s mechanical performance, especially at depth [4].
Standardized 4 mm thick specimens were prepared, with curing tip distances of 0 mm, 2 mm, 4 mm, and 6 mm, simulating clinical challenges. The 0 mm distance, where the LCU tip is in direct contact with the composite, serves as the ideal baseline with minimal light scattering and maximum energy delivery. The 2 mm distance reflects minor clinical obstructions, such as matrix bands or slight hand movement, commonly encountered in daily practice. The 4 mm gap simulates deeper cavities or steep cuspal inclines, where light attenuation becomes more significant. The 6 mm distance represents extreme scenarios, such as post endodontic restorations or deep class II and III cavities, where the gingival floor lies far from the LCU tip [15, 16]. Assessing microhardness across these distances provides insight into the polymerization efficiency and clinical reliability of bulk-fill composites under varied conditions. The setup ensured consistent irradiation and prevented lateral light curing. The specimens were cured for 10 seconds per manufacturer recommendations, and stored in the dark for 24 hours to complete post cure polymerization.
The results of the present study showed that Tetric N-Ceram Bulk Fill consistently achieved higher microhardness values than SDR® Plus Bulk Fill Flowable at all tested distances and depths. Therefore, the first null hypothesis tested was rejected. This is attributed to Tetric N-Ceram’s higher filler loading and high-reactivity monomers, such as Bis-GMA, contributing to increased stiffness and mechanical strength, in contrast to SDR® Plus Bulk Fill Flowable’s lower filler load and more flexible UDMA-based matrix. These findings are supported by earlier studies highlighting the role of filler content and monomer composition in determining composite hardness [17]. The superior performance of Tetric N-Ceram bulk-fill was also likely influenced by its photoinitiator blend. Polywave LCUs effectively activate Ivocerin and TPO due to their violet light emission, which is outside the range of monowave devices. Santini et al. [18] and Varshney et al. [8] reported significantly improved hardness and polymer conversion in RBCs, with alternative photoinitiators when cured with polywave LCUs. Correa et al. [19] further confirmed that polywave units enhance microhardness in such composites by broadening initiator activation.
Similarly, in the present study, curing distance had a significant inverse effect on hardness in both materials. Increasing tip distance from 0 to 6 mm resulted in a reduction in hardness values. Thus, the second null hypo­thesis tested was also rejected. This is in agreement with prior findings showing that up to 10% of irradiance is lost per millimeter of distance. Nagas et al. [20], Barakah [21], and Shafadilla et al. [22] also reported reduced surface hardness with increased curing distance.
However, Tetric N-Ceram bulk-fill’s microhardness remained relatively stable across all distances, including bottom surfaces, which is notable given that under-curing is a common concern in bulk-fill composite restorations. This can be attributed to Ivocerin’s high quantum efficiency, allowing it to maintain effective polyme­rization even with decreased photon availability at depth. Ilie et al. [23] similarly reported that curing distance did not significantly affected hardness in high-viscosity bulk-fill composites containing Ivocerin.
In contrast, SDR® Plus Bulk Fill Flowable exhibited a significant decline in the bottom surface hardness at 4 mm and 6 mm distances. While CQ was adequately activated at shorter distances, its low quantum efficiency limited polymerization as irradiance dropped with distance [13]. However, both the groups at all curing distances showed the bottom-to-top ratio greater than 80%, suggesting adequate curing at the bottom surface also. The disparity in performance between SDR® Plus Bulk Fill Flowable and Tetric N-Ceram Bulk Fill underscores the importance of photoinitiator selection in achieving depth-cure reliability. These results are consistent with the known limitations of CQ, highlighting the benefits of using composites with advanced photoinitiator systems in challenging clinical settings.
This study has certain limitations; this was an in vitro study and further research with long-term clinical eva­luation are required for final evaluation of the suggested results. Also, only two bulk-fill RBCs were evaluated, thus future studies should incorporate more products, as bulk-fill RBCs are not a homogeneous class of mate­rials. Photopolymerization of bulk-fill RBCs is a complex phenomenon. In addition to LCU positioning and type, a combination of many other factors, such as light beam profile and distribution as well as translucency and depth of cure, may also be evaluated. In this study, only one type of LCU was employed, hence future studies can incorporate different types and brands of LCUs. Although microhardness correlates with the degree of conversion, no direct spectroscopic measurement (e.g., FTIR or Raman analysis) of conversion was performed. Forthcoming studies incorporating direct methods would provide more comprehensive insight into polymerization efficiency.

Conclusions

Within the limitations of this in vitro study, the following conclusions can be drawn. Composite containing a combination of photoinitiators (CQ, TPO, and Ivo­cerin) exhibited significantly higher microhardness values than a composite containing CQ only. Increasing the LCU distance significantly affects microhardness values, showing decrease in microhardness values with increasing distance for the composites tested. The bulk-fill composite containing a combination of photoinitiators maintained consistent microhardness between top and bottom surfaces across all curing distances, highlighting its superior depth of cure and better performance under clinically challenging scenarios. This study assess­ed polymerization indirectly through microhardness testing.

Disclosures

1. Institutional review board statement: Not applicable.
2. Assistance with the article: None.
3. Financial support and sponsorship: None.
4. Conflicts of interest: None.

References