Resin Elasticity and the Strengthening of all-Ceramic Restorations
Resin luting of all ceramic restorations results in increased performance; however, the strengthening mechanism and the role of the mechanical properties of the resin are not fully understood. The hypothesis tested is that ceramic strength enhancement is dependent on the elastic modulus of the resin. Three point flexural moduli of a flowable, luting, and hybrid composite resin were characterized. Two hundred forty porcelain disks were air abraded. One group acted as the control, and 3 additional groups were coated with 120 + 20 micrometers of each resin prior to bi-axial flexural testing. All resins significantly increased in mean strength, and the associated strength increase was related to the elastic modulus of the resin (R2 = 0.9885), so the hypothesis was accepted. The combination of Poisson constraint and the creation of a resin-inter-penetrating layer sensitive to the elastic modulus of the resin may provide an explanation of the strengthening mechanism.
It has been more than 32 years since Rochette described bonding a porcelain restoration treated with silane using an etch-and-rinse resin adhesive technique.1 Calamia and Simonsen further refined our understanding with their research of the adhesion of porcelain to resin that led to the clinical treatment with bonded porcelain veneers.2,3 The initial response to bonded porcelain veneers was met with skepticism that thin, custom-fabricated porcelain laminate (usually 0.7 mm to 0.3 mm in thickness) would not fracture or debond from teeth. The author’s reported research with Cerinate porcelain (Den-Mat, Santa Maria, CA) bonded to enamel to change the esthetic appearance of teeth demonstrated success of up to 20 years (average restoration of 15.2 years).4 The author has recalled some of these patients with veneers that are esthetically and clinically acceptable for up to 23 years. While the discussion of why porcelain veneers do not fracture and shatter in the mouth has been described by Nathanson5 and Marquis,6 this research paper has tested and defined why ceramics bonded with flowable resins are successful. The researchers and authors of this paper describe in detail using a biaxial-flexural modulus testing technique that clearly demonstrates a strengthening of silica-based ceramics. Their testing methodology was chosen to replicate the failure mechanism identified by an axisymmetric finite-element analysis of molar crowns. The failure due to fracture typically occurs on the inner surface of the restoration. As practitioners are placing increas-ing numbers of all-ceramic restorations as veneers, inlays, onlays, crowns, and fixed partial dentures, it is reassuring to know that what has been demonstrated in the laboratory actually translates to the clinical success seen with resin-adhesive bonded ceramic restorations for our patients.
1. Rochette AL. A ceramic restoration bonded by etched enamel and resin for fractured incisors. J Prosthet Dent. 1975;33(3):287-293.
2. Simonsen RJ, Calamia JR. Tensile bond strength of etched porcelain. J Dent Res. 1983;61(special issue):297. Abstract no. 1154.
3. Calamia JR, Simonsen RJ. Effects of coupling agents on bond strength of etched porcelain. J Dent Res. 1984;63(special issue):179. Abstract no. 79.
4. Strassler HE. Long term clinical evaluation Cerinate etched porcelain veneers. J Dent Res. 2005;84(special issue). Abstract no. 432.
5. Nathanson D. Dental porcelain technology. In: Garber DA, Goldstein RE, Feinman RA. Porcelain Laminate Veneers. Chicago, IL; Quintessence Publishing; 1988:24-35.
6. Marquis PM. The influence of cements on the mechanical performance of dental ceramics. Bioceram. 1992;5:317-324.
Howard E. Strassler, DMD