Slide 1

Resin cements continueto evolve with refinements engineered by the manufacturers. One of the big changes in recent years has been the development of all-in-one resin cements, which are said to eliminate the need for a dentin bonding system when placing indirect restorations. These cements are variously la-beled as all-in-one cements, universal cements, self-adhesive cements, or adhesive cements. The classification by the manufacturers is not always clear, because traditional resin cements are now called adhesive cements as well.

Nevertheless, it appears that there are now two groups of resin cements: a group that requires a dentin bonding system before use and a group of cements for which the use of a bonding agent is optional.The bonding mechanism of these resins is based on an acid-base link to the calcium in hydroxyapatite in dentin and enamel while maintaining the physical properties of resin cementan approach that makes the bonding to dentin similar to a resin-modified glass-ionomer cement without the need of the glass-ionomer component.1A list of current self-adhesive resin cements is given inTable 1.

Figure 1New incremental dual dispenser for RelyX Unicem.

PHYSICAL PROPERTIES OF RESIN CEMENTS

There exists an international standard, ISO 4049, that sets forth a group of minimum standards for the physical properties of resin cements,2including the self-adhesive cements. These standards are identical to the ANSI/ADA specification No. 27.3The ISO standard addresses primarily resin-based composites, but includes specific requirements for resin-based luting agents. Even though the ISO standard is a voluntary one, many manufacturers of resin cements are in compliance with the ISO standard and indicate that on the package or the enclosed literature. The ISO standard requires a minimum value for the transverse flexural strength (50 MPa), radiopacity (equivalent to aluminum), and a limit on solubility in water. At this time there is no requirement on adhesive properties; however, most manufacturers report adhesive strength as measured by the shear bond strength test.4In selecting a resin cement, compliance with the ISO standard is a good starting point.

COMPOSITION OF RESIN CEMENTS AND SELF-ADHESIVE RESIN CEMENTS

Resins in this case are defined as blends of polymerizable monomers of methacrylates, dimethacrylates, and polymethacrylates (which have recently been included into the blend as well). For the self-adhesive resin cements, additions to the resin blend consist of highly acidic resin monomers, monomethacrylates, multifunctional methacrylates, and at least one of the new cements includes 4-methacryl-oxyethyl trimellitic acid (4-MET). The inclusion of the acidic monomers creates a self-etching resin, increasing the adhesive properties. This concept of self-etching finds its origin in the development of the self-etching adhesives. Further additives are included to increase the bond strength to metals (eg, 4-MET) and to resins that bond to ceramic interfaces.

The first highly acidic monomer was an acrylic chain on which phosphoric acid groups were grafted. Since then, other acidic monomers have been introduced. The goal is to include the dentin bonding agent and a ceramic bonding system or metal bonding system into the resin cement and have the entire assembly polymerized at once.

Several manufacturers have included a fluoride component as well.5,6This practice is of dubious value, as the acrylate resins are very hydrophobic. The mechanisms of fluoride release and of maintaining low water absorption are mutually exclusive. Cement that meets the ISO standard on water solubility cannot release a clinically effective amount of fluoride. One manufacturer carefully states that its product "contains" fluoride and claims a release of 7 ppm to 9 ppm independent of the exposed surface area.5In the authors opinion, whether a cement contains fluoride or not should not be a factor in the product selection of resin-based cements.For two of the products inTable 1(EMBRACE Wetbond, Pulpdent Corporation, Watertown, MA; and MonoCem Self-Adhesive Resin Cement, Shofu Dental Corporation, Menlo Park, CA), it is recommended that the cement be applied to moist dentin, not dry dentin.5,6It is claimed that the uncured resin is hydrophilic, but that the cured resin is hydrophobic.7The authors hope to investigate this claim and report the findings in the near future.

The viscosity of the resin cements have been significantly reduced over the last few years. The reason for the change is primarily the introduction of new delivery systems (see next section). To achieve these low viscosities, the particle size of the inorganic fillers has been greatly reduced. Most cements now have an average particle size well below 1 m. Submicron or nanofiller particles are becoming a significant component of resin cements and composites.

The filler content of the cement is kept at or below 50% by volume. The second step in reducing the viscosity was obtained by using very low molecular weight monomers as diluents (2-hydroxyethylmethacrylate [HEMA], tetraethyleneglycol dimethacrylate [TEGDMA]).

The result of using these diluents is that the polymerization shrinkage of the cement increases to about 4% by volume (1.2% linearly), which is of little consequence for cement.