A Comparative Study on the Flexural strengths of RMGIC and Cention N andEvaluation of their Shear Bond Strengths with Composite: An In Vitro Study

 Introduction

In modern aesthetic dentistry, there is a continual demand for improved, metal-free, adhesive restorative materials. This need has led to the evolution of numerous tooth-colored restorative materials, including composite resins and glass ionomer cements (GICs), which are frequently used for direct restorations in both anterior and posterior teeth. Despite their popularity, composite materials are associated with limitations such as polymerization shrinkage, thermal expansion mismatch, and potential adhesion failures, that may contribute to microleakage and secondary caries. GICs, recognized for their bioactivity, have undergone substantial material evolution. A notable advancement is the resin-modified glass ionomer cement (RMGIC), developed by incorporating hydrophilic resin monomers like HEMA into conventional GICs. RMGICs retain the clinical advantages of traditional GICs while exhibiting enhanced mechanical properties.

Recently, a new material class called “alkasites” has emerged, incorporating alkaline glass fillers within a methacrylate-based resin matrix. Cention N, an example of this novel group, is classified as a bulk-fill alkasite restorative material. It is formulated to release acid-neutralizing ions and demonstrates high flexural strength, making it distinct from conventional restorative materials.¹

According to Prosser et al., flexural testing is an effective method for evaluating a dental material’s tensile properties, as it simultaneously subjects specimens to compressive, tensile, and shear stresses.^2`Additionally, the bond strength between restorative materials and dentin substitutes plays a crucial role in clinical success. Therefore, a comprehensive evaluation of material interfaces is vital before employing newer dentin replacements with composite resins.

This study was undertaken to compare the flexural strength of RMGIC and Cention N and evaluate their shear bond strength when bonded with composite resins.

Materials and Methods

Sample Preparation

This in vitro study assessed the flexural and shear bond strengths of two restorative materials: Cention N (Ivoclar Vivadent) and Resin-Modified Glass Ionomer Cement (RMGIC; GC Gold Label LC, GC Corporation).

Flexural Strength Testing

A total of ten rectangular bar samples were fabricated using stainless steel moulds with dimensions of 25 mm × 2 mm × 2 mm, following ISO 4049 standards.3 The speci-mens were categorized into two groups (n = 5 each):

Group 1: Cention N

Group 2: RMGIC

Each material was mixed as per the manufacturers’ instructions and placed into moulds in a single increment. A polyester strip and glass slide were positioned on top to ensure a flat surface, and polymerization was done using an LED curing unit for 20 seconds per side. The cured specimens were stored in distilled water at 37°C for 24 hours before testing.

Flexural strength (σ) was calculated using the following formula:

σ = (3LF) / (2bh²)

Where:

L = span length (20 mm), F = load at fracture (N), b = specimen width, h = specimen height. 

Shear Bond Strength Testing

Twenty acrylic blocks were prepared, each with a central cylindrical cavity measuring 4 mm in diameter and 2 mm in depth. These were categorized into two groups (n = 10 each):

Group A: Cention N bonded to composite

Group B: RMGIC bonded to composite

After inserting and curing the restorative materials, a 2 mm thick layer of composite resin (Filtek Z350 XT, 3M ESPE) was placed over each sample and cured for 40 seconds. All samples were stored in distilled water at 37°C for 24 hours before testing (Figure 1a).

The shear bond strength was evaluated with a Universal testing machine (UTM) by applying force at the material interface, using a crosshead speed of 0.5 mm/min. Values were recorded in megapascals (MPa) (Figure 1b).

Results

Flexural Strength

Group 1 (Cention N) demonstrated a significantly greater mean flexural strength than Group 2 (RMGIC) (Figure 2).

Cention N (Group 1): 145.2 MPa

RMGIC (Group 2): 56.14 MPa

There was a notable statistically significant difference between the groups (P <0.05).

Shear Bond Strength

Group A (Cention N bonded to composite) demonstrated greater shear bond strength than Group B (RMGIC bonded to composite) (Figure 2).

Group A: 19.2 MPa

Group B: 10.2 MPa

The difference was statistically significant (P <0.05).

Shear Bond Strength Testing

Twenty acrylic blocks were prepared, each with a central cylindrical cavity measuring 4 mm in diameter and 2 mm in depth. These were categorized into two groups (n = 10 each):

Group A: Cention N bonded to composite

Group B: RMGIC bonded to composite

After inserting and curing the restorative materials, a 2 mm thick layer of composite resin (Filtek Z350 XT, 3M ESPE) was placed over each sample and cured for 40 seconds. All samples were stored in distilled water at 37°C for 24 hours before testing (Figure 1a).

The shear bond strength was evaluated with a Universal testing machine (UTM) by applying force at the material interface, using a crosshead speed of 0.5 mm/min. Values were recorded in megapascals (MPa) (Figure 1b).

Results

Flexural Strength

Group 1 (Cention N) demonstrated a significantly greater mean flexural strength than Group 2 (RMGIC) (Figure 2).

Cention N (Group 1): 145.2 MPa

RMGIC (Group 2): 56.14 MPa

There was a notable statistically significant difference between the groups (P <0.05).

Shear Bond Strength

Group A (Cention N bonded to composite) demonstrated greater shear bond strength than Group B (RMGIC bonded to composite) (Figure 2).

Group A: 19.2 MPa

Group B: 10.2 MPa

The difference was statistically significant (P <0.05).

Discussion

In restorative dentistry, the mechanical performance and bonding compatibility of materials are critical factors influencing their clinical success. This study focused on comparing the flexural and shear bond strengths of two commonly used materials Cention N and RMGIC.

Flexural Strength

The results demonstrated that Cention N exhibited significantly greater flexural strength compared to RMGIC. This outcome may be due to the specific composition of Cention N, which includes urethane dimethacrylate (UDMA) as its primary monomer. UDMA-based resins typically form a denser, more cross-linked polymer network, enhancing resistance to fracture. Moreover, Cention N is reinforced with alkaline glass fillers that not only release beneficial ions but also contribute to its mechanical durability (Ivoclar Vivadent, 2016).¹

In contrast, RMGICs, although improved over con-ventional glass ionomers, contain water-sensitive components and less cross-linking, which may explain their reduced strength values. According to ISO 4049 standards, a minimum flexural strength of 80 MPa is required for restorative materials used in stress-bearing areas. Only Cention N met this criterion, making it more suitable for posterior restorations exposed to high occlusal loads.

Shear Bond Strength

The bonding interaction between a restorative material and composite resin plays a crucial role in clinical applications involving layering or core build-up. In this study, Cention N demonstrated comparatively higher shear bond strength when bonded to composite than RMGIC. This could be attributed to the compatibility of its methacrylate-based resin matrix with resin composite, facilitating stronger chemical interactions during polymerization.

RMGICs, however, are primarily acid-base reactive and lack methacrylate groups in sufficient quantity to bond effectively with resin composites. Additionally, the inclusion of HEMA in RMGIC may cause phase separation or water uptake at the interface, reducing bond strength.⁴

Failure mode analysis further supported the mechanical findings. Samples involving Cention N predominantly exhibited cohesive or mixed failures, indicating stronger internal bonding. In contrast, RMGIC specimens primarily exhibited adhesive failure, reflecting weaker interface bonding.

Overall, these findings align with previous studies sug­gesting that newer alkasite-based materials like Cention N offer enhanced mechanical and bonding properties, potentially expanding their clinical applications.⁵

Conclusion

Within the constraints of this in vitro study, Cention N exhibited comparatively greater flexural and shear bond strength than RMGIC. These results indicate that Cention N is a promising alternative for stress-bearing restorations and clinical situations requiring bonding with resin composites. Further clinical studies are recommended to substantiate these findings in vivo.

Conflict of Interest

Nil