Effect of elastic gradients on the fracture resistance of tri-layer restorative systems

Abstract

Objectives

To assess the impact of elastic gradients formed among restorative material, cement, and substrate on the fracture resistance of tri-layer restorative systems.

Methods

Four CAD/CAM materials were utilized, two glass-ceramics (IPS e.max CAD, Vita Suprinity) and two resin-ceramic hybrids (Vita Enamic, Lava Ultimate). Their fracture resistance was examined by biaxial flexure (n = 8) and Hertzian indentation (n = 10) tests. Statistical analysis was conducted using ANOVA and Tukey tests (p = 5 %). Finite element analysis (FEA) was employed to simulate the Hertzian indentation test and elucidate the stress-fields formed on the intaglio surface below the loading area.

Results

The biaxial flexural strength (MPa) of glass-ceramics exceeded the hybrid materials (e.max 417^a, Suprinity 230^b, Enamic 138^c, and Lava Ultimate 183^bc). Conversely, the load-bearing capacity (N) of the materials bonded to dentin analog demonstrated the opposite trend, with the hybrid materials achieving superior results (e.max 830 ^C, Suprinity 660^D, Enamic 1822^B, and Lava Ultimate 2593 ^A). The stress-fields observed by FEA were coherent with the experimental results for Hertzian flexural stresses (MPa): e.max 501 ^A, Suprinity 342 ^C, Enamic 406^B, whereas no tensile stress was observed at the intaglio surface of Lava Ultimate.

Significance

Detailed analysis of the fracture resistance of the tri-layer restorative systems showed that the elastic gradients play a more significant role than the flexural strength of the restorative materials. The coherence of the elastic moduli between the restorative material and supporting structures results in reduced tensile stress concentration at the intaglio surface beneath the loading area and enhances the ability to withstand load.