Fatigue Properties of Weakened and Non-weakened Roots Restored with CAD-CAM Milled Fiber Post, Prefabricated Fiber Post, or Cast Metal Post.

STATEMENT OF PROBLEM: Customized glass fiber posts using CAD-CAM technology have been suggested for restoring endodontically treated teeth. However, how weakened or non-weakened roots restored with anatomical CAD-CAM posts behave under cyclic fatigue is not clear. OBJECTIVE: To evaluate the load-bearing capacity under fatigue (fatigue failure load [FFL], the number of cycles for failure [CFF], and survival probabilities) and fracture pattern of weakened and non-weakened roots restored with CAD-CAM fiber post and cores, metal cast-post-core, and prefabricated fiber post and resin core. METHODS AND MATERIALS: A total of 60 crack-free bovine incisor roots (13 mm in length) with standard geometry were obtained and randomly allocated considering the factor "root condition" in two levels (weakened and non-weakened). Thus, half of the roots were weakened to obtain a wall thickness of 0.5 mm. After that, the endodontic treatment was executed, all roots embedded with acrylic resin and the specimens randomly allocated (n=10) considering the factor "post system" in three levels (CAD-CAM: CAD-CAM milled glass-fiber post and core; MBC: metallic-based post and core; and FRC: prefabricated glass-fiber post and composite resin core). The posts were luted with a dual-cure self-adhesive luting agent. Then, all teeth received a metallic crown. An initial load of 100 N at 20 Hz for 5000 cycles was applied for the step-stress fatigue test, followed by incremental steps of 50 N for 20,000 cycles each step, up to failure. A fracture pattern analysis was performed. RESULTS: CAD-CAM fiber post (FFL: 865 N; CFF: 311,000 cycles) presented similar fatigue performance (p>0.05) to FRC (FFL: 925 N; CFF: 335,000 cycles), with 100% of repairable fractures for non-weakened roots; however, both groups presented worse performance than MBC (p<0.05; FFL: 1265 N; CFF: 471,000 cycles) which led to 100% of catastrophic failures. No statistical difference was found in fatigue performance among the three systems for weakened roots (p>0.05; FFL: 1035-1170 N; CFF: 379,000-433,000 cycles), with a high rate of catastrophic failures. CONCLUSIONS: CAD-CAM fiber post presented similar fatigue performance to MBC and FRC approaches when restoring weakened roots. CADCAM was similar to FRC when restoring non-weakened roots, while MBC enhanced fatigue properties in this scenario.

Strengthening of Porcelain Provided by Resin Cements and Flowable Composites.

This study evaluated the effect of mechanical properties of resin-based luting agents on the strength of resin-coated porcelain. The luting agents tested were two flowable resin composites (Filtek Z350 Flow and Tetric-N Flow), a light-cured resin cement (Variolink Veneer [VV]), and a dual-cured resin cement (Variolink II) in either light-cured (base paste) or dual-cured (base + catalyst pastes [VD]) mode. Flexural strength (σf) and modulus of elasticity (Ef) of the luting agents were measured in three-point bending mode (n=5). Porcelain discs (Vita VM7) were tested either untreated (control) or acid etched, silanized, and coated with the luting agents. Biaxial flexural strength (σbf) of the porcelain discs was tested using a ball-on-ring setup (n=30). The σbf of the resin-coated specimens was calculated at z-axial positions for multilayer specimens in the ball-on-ring test: position z = 0 (ceramic surface at the bonded interface) and position z = -t2 (luting agent surface above ring). The σf and Ef data were subjected to analysis of variance and the Student-Newman-Keuls test (α=0.05). A Weibull analysis was performed for σbf data. Weibull modulus (m) and characteristic strength (σ0) were calculated. Linear regression analyses investigated the relationship between mechanical properties of the luting agents and the strengthening of porcelain. VD had higher and VV had lower mechanical strength than the other materials. At z = 0, all resin-coated groups had higher σbf than the control group. No significant differences between the luting agents were observed for σbf and σ0. At z = -t2, VD had the highest σbf and σ0, whereas VV had the poorest results. No significant differences in m were observed across groups. A linear increase in flexural strength of the porcelain was associated with increased σf and Ef of the luting agents at position z = -t2. In conclusion, resin coating and use of luting agents with better physical properties generally improved the mechanical performance of porcelain.

Biomechanical behaviour of a fractured maxillary incisor restored with direct composite resin only or with different post systems.

AIM: To compare stress distribution between a fractured maxillary central incisor restored with direct composite resin only (CR) or associated with different post materials, using finite element analysis. METHODOLOGY: A three-dimensional model of a sound maxillary central incisor and supporting structures was constructed, using data from the dental literature. Changes were made in the crown region to create a tooth with a restored crown fracture. A composite resin restoration only and restorations associated with different tapered post systems (glass fibre, carbon fibre, titanium and zirconia ceramic) were also evaluated, resulting in six experimental models. A static chewing pressure of 2.16Nmm(-2) was applied to two areas of the palatal surface of the tooth. Stress distribution was analysed under a general condition and in the structures of the models separately. RESULTS: The maximum stresses were concentrated as follows: at the cemento-enamel junction in the model with a sound maxillary central incisor, restored with CR and with a composite resin restoration associated with fibre posts; in the enamel at the post-enamel interface on the palatal surface of the model with a titanium post; and in the post of the model with zirconia ceramic post. CONCLUSIONS: None of the restorations evaluated was able to recover the stress distribution of the sound tooth. The models restored with composite resin associated with a glass or carbon fibre post had similar stress distributions to that of the model restored with CR. The different post materials were shown to have a substantial influence on stress distribution, with less stress concentration when fibre posts were used.

Influence of post and resin cement on stress distribution of maxillary central incisors restored with direct resin composite.

The current study evaluated the influence of two endodontic post systems and the elastic modulus and film thickness of resin cement on stress distribution in a maxillary central incisor (MCI) restored with direct resin composite using finite element analysis (FEA). A three-dimensional model of an MCI with a coronary fracture and supporting structures was performed. A static chewing pressure of 2.16 N/mm2 was applied to two areas on the palatal surface of the composite restoration. Zirconia ceramic (ZC) and glass fiber (GF) posts were considered. The stress distribution was analyzed in the post, dentin and cement layer when ZC and GF posts were fixed to the root canals using resin cements of different elastic moduli (7.0 and 18.6 GPa) and different layer thicknesses (70 and 200 microm). The different post materials presented a significant influence on stress distribution with lesser stress concentration when using the GF post. The higher elastic modulus cement created higher stress levels within itself. The cement thicknesses did not present significant changes.