Facts on Resin Cement Use
Technicians need to be aware of bond strength and elastic modulus when selecting material
Gregg A. Helvey, DDS
Do your clients ever ask you, “Which resin cement should I use for all-ceramic restorations?” If so, on what information do you base your reply? Before you answer this question, first look at ADA specification No. 27 for resin-based cements. Even though this specification addresses mostly resin-based composites, it also includes requirements for resin cements.
Specification No. 27 requires a level of radiopacity that is equivalent to the radiopacity of 1-mm thickness of aluminum. The working time of self-cure composites should not be less than 90 seconds. The transverse flexural strength should be a minimum of 50 MPa. Water sorption by composite resins is addressed, as well as solubility, but there is no mention of required bond strength. However, bond-strength parameters are addressed in the International Organization of Standardization (ISO) requirements, but the information from manufacturers is purely on a voluntary basis.
Is Higher Bond Strength Better?
Most advertising information provided by resin cement manufacturers is based on bond strength. Their marketing approach emphasizes that the higher the bond strength, the better the adhesive material. Is that necessarily true?
Heintze1 explained this marketing approach to the dental community in the Journal of Adhesive Dentistry and also discussed the comparison of bond-strength testing by manufacturers. He stated that there was no international consensus on what the most appropriate approach to bond strength is, nor are there adequate parameters to evaluate it. So many factors have an influence on the result that it is impossible to compare material data from one study with those of another. For example, bond-strength testing without dentinal fluid simulation generates higher bond-strength values than testing with dentinal fluid simulation.2 Bond-strength values in human dentin are generally higher than those obtained in bovine dentin.3 The bond-strength values obtained from superficial dentin are higher than those values obtained in deep dentin.4 Caries-affected dentin will have lower bond-strength values than non-affected dentin.5 Data acquired from different studies on bond-strength tests are not comparable.6 Even data collected within the same study group show great variability.1
Effects of Tensile Stress
Considering the large coefficient of variation in bond-strength studies, the criteria for selecting a resin cement should not be based on the bond-strength numbers boasted by manufacturers.
If bond strength is not as critical as one would think, then what is important in selecting a particular resin cement for all-ceramic restorations? Tensile stress is the predominant factor controlling the initial failure of ceramics.7 The critical tensile stress is dependent on the elastic modulus mismatch of the ceramic, cement, and supporting material.7 For example, leucite-reinforced ceramic has an elastic modulus (stiffness) of 69 GPa; the elastic modulus of dentin is approximately 16 GPa. Therefore, the elastic modulus of the resin cement should be as close to the elastic modulus of dentin as possible to minimize the difference.
Hertzian’s cone indentation tests study how a supported ceramic reacts when a load (cone indentation) is applied to the surface. The reaction is dependent upon the elastic modulus of the supporting substrate. If the supporting substrate has a lower elastic modulus (less stiff), then radial (conical) fractures develop more readily on the undersurface of the ceramic, leading to bulk ceramic failures. This is exactly what happens when a stone impacts the outside surface of a car windshield, resulting in a cone-shaped crack or “Hertzian cone crack” on the inside surface.
Habekost et al studied how the mechanical properties of resin cement can influence the fracture resistance of teeth restored with ceramic inlays. Their study consisted of three different cements that had different bond strengths and elastic moduli. In general, they found that the cement with a higher elastic modulus resulted in improved resistance to fracture for ceramic-restored teeth.8 Another way to look at it is to take a pane of glass and place it on a hardwood floor (stiff substrate) and step on it. The chances of cracking the glass are much smaller than if the glass were placed on a carpeted floor (less stiff substrate) and stepped on.
The Final Word
The elastic modulus value is not usually found on the information packet that accompanies the product and must be obtained directly from the manufacturer or the scientific literature. In fact, the author’s experience has been that most sales representatives are not aware of the elastic modulus value of their own product. Through communication with manufacturers, the author compiled a list of the five resin cements with the highest elastic moduli (Table 1).
The next time the question, “Which resin cement should I use?” is posed, the answer can include more than just what the manufacturer is advertising.
References
1. Heintze SD. Systematic reviews: I. The correlation between laboratory tests on marginal quality and bond strength. II. The correlation between marginal quality and clinical outcome. J Adhes Dent. 2007; 9(Suppl I):77-106.
2. Mitchem JC, Gronas DG. Adhesion to dentin with and without smear layer under varying degrees of wetness. J Prosthet Dent. 1991;66(5):619-622.
3. Retief DH. Standardizing laboratory adhesion tests. Am J Dent. 1991;4(5):231-236..
4. Tagami J, Tao l, Pashley DH. Correlation among dentin depth, permeability, and bond strength of adhesive resins. Dent Mater. 1990;6(1):45-50.
5. Nakajima M, Ogata M, Okuda M, et al. Bonding to caries-affected dentin using self-etching primers. Am J Dent. 1999;12(6):309-314.
6. Burke FJT, Hussain A, Fleming G. An analysis of 110 investigations on bond strength to dentin. J Dent Res. 2003;82(Special Issue A): Abstract 715.
7. Dong XD, Darvell BW. Stress distribution and failure mode of dental ceramic structures under Hertzian indentation. Dent Mater. 2003:19(6):542-551.
8. Habekost LV, Camacho GB, Demarco FF, et al. Tensile bond strength and flexural modulus of resin cements—influence on the fracture resistance of teeth restored with ceramic inlays. Oper Dent. 2007;32(5):488-495.
About the Author
Gregg A. Helvey, DDS
Adjunct Associate Professor
Virginia Commonwealth University School of Dentistry
Richmond, Virginia
Private Practice
Middleburg, Virginia
