Wear Behavior of Contemporary Direct Resin-Based Restorative Materials

The technique sensitivity and limitations regarding increment thickness associated with conventional composite resins resulted in postoperative complications that impacted their long-term clinical performance, particularly in load-bearing areas. Bulk-fill composite resins, which incorporate nano-sized filler particles, additional photoactive monomers, and potent light activators, are a class of composite resins that allow thicker increments of 4 to 6 mm to be placed.³ Although glass ionomer-based materials were traditionally contraindicated for load-bearing areas, the advantages of being less technique sensitive and not requiring strict isolation made them appealing for hard-to-access cavities. More recently developed resin-modified glass ionomer materials claim to demonstrate improved physical properties and wear resistance.⁴

When direct resin-based restorative materials are utilized in posterior teeth, they face repeated occlusal forces and are at a higher risk of wear. Wear is a complex process, and its manifestation and magnitude can differ for every material. Improvements to laboratory testing protocols and the profession's understanding of the different challenges that these materials face in the patient's mouth have led to the development of better clinical simulation methods. Laboratory thermomechanical fatigue testing provides a close approximation of clinical performance while expediting the outcomes. For example, 3 years of clinical performance can be simulated in a chewing simulator in less than a week. Chewing simulators are machines that provide an environment that emulates the oral environment. During the simulation, test samples of materials are fatigued by repeated vertical and horizontal forces in a water bath that thermally cycles between 5ºC and 55ºC. It has been reported that 10,000 thermocycles in a chewing simulator is equivalent to 1 year of artificial aging.⁵

Valeri and colleagues and Alsahafi and colleagues have studied the wear behavior of multiple direct resin-based restorative materials following a similar protocol.^4,6 Table 1 lists and describes the tested materials. In both studies, a thermomechanical chewing simulator was used to simulate the wear of flat, 3-mm thick samples of these materials for 500,000 cycles. Wear volume was calculated using scans of polyvinyl siloxane (PVS) impressions of each sample following different intervals of thermomechanical fatigue up to 500,000 cycles. In addition, 3D analysis software (Geomagic^® Control X^™, 3D Systems) was used to locate and calculate the volume of wear facets (Figure 1).⁴ The results of the wear volume testing are presented in Figure 2. Natural enamel produced the least amount of wear (0.24 mm³) when compared with resin-modified glass-ionomer materials, bulk-fill composite resins, and a conventional composite resin. These findings do not imply superiority or inferiority of one material over another. When choosing a composite, clinicians should also consider handling properties, translucency, optical properties, wear characteristics, polishability, polish retention, shrinkage, color stability, and more.

Glass-Ionomer Materials

Although conventional resin-modified glass-ionomer restorative materials are not indicated for permanent occlusal restoration, some of those tested demonstrated superior wear resistance when compared with conventional composite resin. A bioactive ionic resin with a reactive glass filler (ACTIVA^™ BioACTIVE-RESTORATIVE^™, Pulpdent)  displayed the highest wear resistance when compared with other resin-based restorative materials. Based on its performance, it is certainly not a standard resin-modified glass ionomer but rather an "enhanced" one. Another resin-modified glass-ionomer material (Ionolux^®, VOCO America) demonstrated approximately one-fold greater wear volume but still less than that of resin composites and a hybrid composite (EQUIA Forte^®, GC America). This material requires no substrate pre-conditioning but is limited to 2-mm thick increments. A glass hybrid material (EQUIA Forte^® HT, GC America), which requires the placement of a light-cured resin coating, resulted in the highest wear volume. According to the manufacturer's data, the resin coating is expected to be worn for 6 to 12 months clinically. During that time, the glass-ionomer polygel matrix matures to increase the material's mechanical properties. The wear volume exhibited by this material during testing can be attributed to the early loss of the resin coating during the chewing simulation, which did not allow for full matrix maturation to occur.⁴

Bulk-Fill Composites

Most of the bulk-fill composite resins that were tested performed similarly. A sonic-activated bulk-fill composite resin that requires the use of a special handpiece to decrease its viscosity during placement (SonicFill^™ 3, Kerr Corporation)⁷ was tested along with two bulk-fill composite resins that contain potent photoinitiators that facilitate the polymerization of thicker increments (Tetric EvoCeram^® Bulk Fill and Tetric^® PowerFill, Ivoclar). The latter of those is further optimized through the incorporation of addition-fragmentation chain transfer reagents. The reagents adjust and control the thermal and mechanical properties of the material, which permits the option of rapid 3-second curing with a high-energy light curing unit. During testing, that material demonstrated less wear resistance when the 3-second cure mode was selected. This supports the general recommendation to lengthen the curing time of resin-based materials to 20 seconds to provide sufficient energy for polymerization to reach its maximum potential. Another bulk-fill composite resin (3M^™ Filtek^™ One Bulk Fill Restorative, 3M Oral Care)  that was tested achieves its depth of polymerization by incorporating a high-molecular-weight aromatic urethane dimethacrylate monomer. This monomer decreases the number of reactive groups in the resin, which results in a reduction in polymerization shrinkage volume and the development of less stress after polymerization. When compared with the bulk-fill composite resins tested, the conventional composite resin (3M^™ Filtek^™ Supreme Ultra, 3M Oral Care)  exhibited the highest wear volume (1.48 mm³).⁶

In vitro thermomechanical fatigue studies are an important aid in understanding the long-term behavior of dental biomaterials.⁸ Fatigue not only affects wear resistance but also impacts the other physical properties of different materials, such as fracture toughness and flexural strength. Although chewing simulators have been studied for their clinical correlation, agreement on a standard protocol is lacking.⁹ Multiple protocols have been reported in the literature, which makes it difficult to compare the outcomes of different studies. Load, horizontal movement, stroke depth, and the type of antagonist used are among the parameters that differ between reported protocols.¹⁰ Therefore, the outcomes of studies involving chewing simulators should be interpreted with caution.

Research shows promising improvement regarding the wear resistance of contemporary direct resin-based restorative materials; however, none of the materials tested in the aforementioned studies demonstrated the wear resistance of natural enamel. Understanding the chemistry of restorative materials is important in using them for their appropriate clinical indications. The selection of restorative materials should be based on the needs of individual cases. The number of steps required, level of technique sensitivity, and need for isolation are important factors to consider when selecting an appropriate material. Furthermore, clinicians should exercise discretion when considering newly introduced direct resin-based restorative materials, especially in the absence of clinical evidence. Newer is not always better.

Tariq A. Alshafhi, BDS, MS
Laboratory Manager
Sulaiman Biomaterials Laboratory
Adams School of Dentistry University of North Carolina at Chapel Hill
Chapel Hill, North Carolina
PhD Student
Oral and Craniofacial Biomedicine
Adams School of Dentistry University of North Carolina at Chapel Hill
Chapel Hill, North Carolina

Taiseer A. Sulaiman, DDS, PHD
Associate Professor Division of Comprehensive Oral Health
Adams School of Dentistry University of North Carolina at Chapel Hill
Chapel Hill, North Carolina