Effect of cements on fracture resistance of monolithic zirconia crowns.

Objectives The present study investigated the effect of cements on fracture resistance of monolithic zirconia crowns in relation to their compressive strength. Materials and methods Four different cements were tested: zinc phosphate cement (ZPC), glass-ionomer cement (GIC), self-adhesive resin-based cement (SRC) and resin-based cement (RC). RC was used in both dual cure mode (RC-D) and chemical cure mode (RC-C). First, the compressive strength of each cement was tested according to a standard (ISO 9917-1:2004). Second, load-to-failure test was performed to analyze the crown fracture resistance. CAD/CAM-produced monolithic zirconia crowns with a minimal thickness of 0.5 mm were prepared and cemented to dies with each cement. The crown-die samples were loaded until fracture. Results The compressive strength of SRC, RC-D and RC-C was significantly higher than those of ZPC and GIC (p < 0.05). However, there was no significant difference in the fracture load of the crown between the groups. Conclusion The values achieved in the load-to-failure test suggest that monolithic zirconia crowns with a minimal thickness of 0.5 mm may have good resistance against fracture regardless of types of cements.

Cutting efficiency of diamond burs operated with electric high-speed dental handpiece on zirconia.

Zirconia-based dental restorations are becoming used more commonly. However, limited attention has been given to the difficulties experienced, concerning cutting, in removing the restorations when needed. The aim of the present study was to compare the cutting efficiency of diamond burs, operated using an electric high-speed dental handpiece, on zirconia (Zir) with those on lithium disilicate glass-ceramic (LD) and leucite glass-ceramic (L). In addition, evaluation of the cutting efficiency of diamond burs on Zir of different thicknesses was performed. Specimens of Zir were prepared with thicknesses of 0.5, 1.0, 2.0, and 4.0 mm, and specimens of LD and L were prepared with a thickness of 1.0 mm. Cutting tests were performed using diamond burs with super coarse (SC) and coarse (C) grains. The handpiece was operated at 150,000 rpm with a cutting force of 0.9 N. The results demonstrated that cutting of Zir took about 1.5- and 7-fold longer than cutting of LD and L, respectively. The SC grains showed significantly higher cutting efficiency on Zir than the C grains. However, when the thickness of Zir increased, the cutting depth was significantly decreased. As it is suggested that cutting of zirconia is time consuming, this should be taken into consideration in advance when working with zirconia restorations.

Fracture resistance of monolithic zirconia molar crowns with reduced thickness.

OBJECTIVES: The purpose of the present study was to analyze the relationship between fracture load of monolithic zirconia crowns and axial/occlusal thickness and to evaluate the fracture resistance of monolithic zirconia crowns with reduced thickness in comparison with that of monolithic lithium disilicate crowns with regular thickness. MATERIALS AND METHODS: Monolithic zirconia crowns (Lava Plus Zirconia, 3M/ESPE) with specified axial/occlusal thicknesses and lithium disilicate crowns (IPS e.max press, Ivoclar/Vivadent) with regular thickness were fabricated using a dental CAD/CAM system and a press technique, respectively. The crowns cemented onto dies were loaded until fracture. Based on measurements of the crown thickness made by micro-CT and the fracture load, multiple regression analysis was performed. RESULTS: It was revealed that the occlusal thickness significantly affected the fracture load (p < 0.01), but the axial thickness did not (p = 0.2828). Although the reduction of the occlusal thickness decreased the fracture resistance of the monolithic zirconia crowns, the fracture load of the zirconia crowns with the occlusal thickness of 0.5 mm (5558 ± 522 N) was significantly higher than that of lithium disilicate crowns with an occlusal thickness of 1.5 mm (3147 ± 409 N). CONCLUSION: Within the limitations of the present study, it is suggested that monolithic zirconia crown with chamfer width of 0.5 mm and occlusal thickness of 0.5 mm can be used in the molar region in terms of fracture resistance.

Influence of grain size and veneer firing process on the flexural strength of zirconia ceramics.

The aim of the present investigation was to study the influence of grain size on the strength of yttria-stabilized zirconia upon exposure to an additional heat treatment, mimicking the veneering process. The green bodies of zirconia discs prepared by cold isostatic pressing were sintered at 1,425, 1,500, or 1,575°C to customize the grain size. The disc-shaped specimens were highly polished and then processed with or without additional heat treatments (five times at 850-930°C). Ten specimens from each treatment group were subjected to a biaxial flexural strength test according to the ISO 6872 standard. The grain size of the specimens was analyzed by the mean linear intercept technique using scanning electron microscopy. It was demonstrated that the mechanical strength of zirconia ceramics was not reduced by the veneer firing processes when polished specimens were used, but the strength was found to decrease with increasing grain size. The raw materials and the sintering process used should be selected appropriately to avoid grain coarsening because it was found that the strength was influenced by large grains.

Zirconia as a dental implant abutment material: a systematic review.

PURPOSE: The focus of this systematic review was to assess the published data concerning zirconia dental implant abutments from various aspects. MATERIALS AND METHODS: To identify suitable literature, an electronic search was performed using PubMed. The keywords "zirconia," "zirconium," "ceramic," "dental abutments," "dental implants," "plaque," and "bacteria" were included. Titles and abstracts were screened, and literature that fulfilled the inclusion criteria was selected for a full-text reading. Articles were divided into four groups: (1) studies on the mechanical properties of zirconia abutments, (2) studies on the peri-implant soft tissues around zirconia abutments, (3) studies on plaque accumulation on zirconia, and (4) clinical studies on the survival of zirconia abutments. RESULTS: The initial literature search resulted in 380 articles. For groups 1 to 4, 11, 4, 7, and 3 articles satisfied the inclusion and exclusion criteria, respectively. Only 1 randomized clinical study was identified. Review of the selected articles showed that zirconia abutments were reliable in the anterior region from both biologic and mechanical points of view. Furthermore, zirconia abutments may represent a material surface less attractive for early plaque retention compared to titanium. Three clinical follow-up studies indicated that zirconia abutments could function without fracture and peri-implant lesions. CONCLUSIONS: Based on the reviewed literature, zirconia has the potential to be used as a dental abutment material, although some issues have to be studied further.

Topography, microhardness, and precision of fit on ready-made zirconia abutment before/after sintering process.

BACKGROUND: Sintering porcelain on a ceramic abutment may change the microstructure and result in aging processes that influence the mechanical properties, internal strain, and the three-dimensional form of the abutment, thus causing a possible misfit between the abutment and the fixture. PURPOSE: The aim was to investigate topography, microhardness, and precision of fit on yttrium-stabilized zirconia (Y-TZP) abutments before/after the sintering process. MATERIALS AND METHODS: Ten Y-TZP abutment samples were ground to a shape used in the clinical situation and divided at random into two groups: before/after sintering. After the surface roughness was measured on all abutments, the abutments were connected to fixture replicas, embedded in resin, and cut in the longitudinal axis. Both sides of the cut samples were measured with respect to microhardness and minimum distance between fixture and abutment surface. t-Test, one-way analysis of variance, and Bonferroni multiple comparisons were used to investigate statistical significant differences. RESULTS: The surface roughness (S(a) and S(dr)) after sintering was significantly higher than before sintering. The total average values of microhardness after sintering were statistically lower than before sintering with a difference of 2%. The total distance between abutment/fixture before/after sintering demonstrated no statistically significant difference. Contact between abutment/fixture was most common at the top area of the fixture. CONCLUSION: A slight decrease of microhardness and contamination of porcelain particles immediately below the veneered part were found on the Y-TZP abutment after sintering. The sintering process did not affect the precision of fit.

The influence of a chemo-mechanical caries removal solution on the topography of dental ceramic materials.

The purpose of this study was to investigate the chemical influence of chemo-mechanical caries removal (CMCR) on surface topography of dental ceramic materials. Thirty samples of three different types of ceramics (Vita Omega, Vita Alpha, and Procera AllTitan) were manufactured and used. With an optical profilometer, designed with a confocal setting of its optics, (CLSP). and an atomic force microscope (AFM) the surface topography of the specimens was investigated at two different resolutions. The specimens were exposed to a solution (64 mm NaOCl, 27 mM amino acids, pH = 11) supposed to be more aggressive than Carisolv (34 mm NaOCl, 53mM amino acids, pH = 11), a CMCR system commercially available. After exposure for 5, 10, and 20 min, respectively, areas measured were relocated and the surface topography was reinvestigated. Parameters describing the amplitude, spatial, and hybrid topographical variation were calculated. In general, after 20 min CMCR solution exposure, the AFM topographical parameters were reduced for Vita Omega, and increased for Vita Alpha", whereas the CLSP topographical parameters of Procera AllTitan were slightly reduced. Thus, the results of this study showed minor influence on surface topography after exposure to CMCR solution for 20 min on the dental ceramics Vita Omega, Vita Alpha, and Procera AllTitan.

Ion leaching from dental ceramics during static in vitro corrosion testing.

Dental ceramics are often called inert materials. It can be hypothesized, however, that differences in the composition, microstructure, and environmental conditions will affect the degree of corrosion degradation in an aqueous environment. The aims of the study were, therefore, to study the ion dissolution from glass-phase ceramics, with or without crystalline inclusions, and from all-crystalline ceramics and to compare the effects of different corrosion media. Ceramic specimens were produced from glass-phase and oxide ceramics and given an equivalent surface smoothness, after which they were subjected to in vitro corrosion (Milli-Q water at 37 +/- 2 degrees C for 18 h and 4% acetic acid solution at 80 +/- 2 degrees C for 18 h, respectively). The temperature of the corrosion solution was slowly increased until it reached 80 +/- 2 degrees C to reduce the risk of microcrack formation at the surface. The analyses of ion leakage were performed with inductively coupled plasma optical emission spectroscopy. A large number of inorganic elements leached out from the various dental ceramics. The major leaching elements were sodium and potassium; in the acid-corrosion experiments, there were also magnesium, silicon, and aluminum and, on a lower scale, yttrium, calcium, and chromium. The various glass-phase ceramics displayed significant differences in ion leakage and significantly higher leakage values than all-crystalline alumina and zirconia ceramics. No significant difference in dissolution was found between high and low-sintering glass-phase ceramics or between glass-phase ceramics with high volume fractions of crystallites in the glass phase in comparison with those with lower crystalline content. It can be concluded, therefore, that none of the dental ceramics studied are chemically inert in an aqueous environment.