Effect of porcelain and enamel thickness on porcelain veneer failure loads in vitro

https://doi.org/10.1016/j.prosdent.2013.09.025Get rights and content

Statement of problem

Bonded porcelain veneers are widely used esthetic restorations. Although high success and survival rates have been reported, failures occur. Fracture is the most common failure mode. Fractures range from incomplete cracks to the catastrophic. Minimally invasive or thin partial veneers have gained popularity.

Purpose

The aim of this study was to measure the influences of porcelain veneer thickness and enamel substrate thickness on the loads needed to cause the initial fracture and catastrophic failure of porcelain veneers.

Material and methods

Model discoid porcelain veneer specimens of varying thickness were bonded to the flattened facial surfaces of incisors, artificially aged, and loaded to failure with a small sphere. Individual fracture events were identified and analyzed statistically and fractographically.

Results

Fracture events included initial Hertzian cracks, intermediate radial cracks, and catastrophic gross failure. Increased porcelain, enamel, and their combined thickness had like effects in substantially raising resistance to catastrophic failure but also slightly decreased resistance to initial Hertzian cracking. Fractographic and numerical data demonstrated that porcelain and tooth enamel behaved in a remarkably similar manner. As porcelain thickness, enamel thickness, and their combined thickness increased, the loads needed to produce initial fracture and catastrophic failure rose substantially. Porcelain veneers withstood considerable damage before catastrophic failure.

Conclusions

Increased enamel thickness, increased porcelain thickness, and increased combined enamel and porcelain thickness all profoundly raised the failure loads necessary to cause catastrophic failure. Enamel and feldspathic porcelain behaved in a like manner. Surface contact damage occurred initially. Final catastrophic failure followed flexural radial cracking. Bonded porcelain veneers were highly damage tolerant.

Section snippets

Teeth

Maxillary central and lateral incisors with completely intact crowns and roots and free of caries or restorations were selected, cleaned, and stored in 0.01% thymol solution at room temperature. The teeth were embedded in epoxy resin (Fast Cure Epoxy; Extec) within phenolic rings (1¼″ Ring Molds; Extec). The labial surfaces were gently ground under water with 240, 400, and 600 grit silicon carbide (Carbimet Paper Strips; Buehler) until flat areas of enamel more than 6 mm in diameter were

Influence of porcelain veneer thickness on porcelain veneer fracture events

Increasing porcelain thickness tended to slightly decrease the load needed to form initial cone cracks (Fig. 1); the linear equation produced by regression analysis for the influence of porcelain veneer thickness on the initial cone crack fracture event was y=−130x+402 (R2=0.04). Increasing porcelain thickness markedly increased the load needed to produce terminal catastrophic fracture (Fig. 1); the linear equation produced by regression analysis for the influence of porcelain veneer thickness

Discussion

The null hypothesis was rejected; porcelain thickness, enamel thickness, and their combined thickness all influenced the loads needed to produce catastrophic failure.

The results of this study showed that the effects of porcelain and enamel thickness were almost identical, both for the initial fracture events and for the final catastrophic events (Figs. 1, 2). Consistent with this finding, the effects of porcelain and enamel thickness were summative (Fig. 1, Fig. 2, Fig. 3). Qualitative

Conclusions

For a bonded feldspathic porcelain veneer model system:

  • 1.

    Increased enamel thickness, porcelain thickness, and increased combined enamel and porcelain thickness all profoundly raised the loads to catastrophic failure.

  • 2.

    Enamel and feldspathic porcelain behaved in a like manner.

  • 3.

    Initial damage was from surface contact; intermediate radial cracks originating from the inner intaglio surface followed; lastly, catastrophic failure occurred.

  • 4.

    Bonded porcelain veneers were highly damage tolerant.

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