Čedomir Oblak

The effect of surface grinding and sandblasting on flexural strength and reliability of Y-TZP zirconia ceramic

OBJECTIVES This study was conducted to evaluate the effect of grinding and sandblasting on the microstructure, biaxial flexural strength and reliability of two yttria stabilized tetragonal zirconia (Y-TZP) ceramics. METHODS Two Y-TZP powders were used to produce fine grained and coarse grained microstructures. Sixty discs from each material were randomly divided into six groups of ten. For each group, a different surface treatment was applied: dry grinding, wet grinding, sandblasting, dry grinding + sandblasting, sandblasting + dry grinding and a control group. Biaxial flexural strength was determined and data were analyzed using one-way ANOVA, followed by Tukeys HSD test (p < 0.05). In addition, Weibull statistics was used to analyze the variability of flexural strength. The relative amount of transformed monoclinic zirconia, corresponding transformed zone depth (TZD) and the mean critical defect size Ccr were calculated. RESULTS There was no difference in mean strength between the as sintered fine and coarse grained Y-TZP. Significant differences (p < 0.05) were found between the control group and ground fine grained material for both wet and dry grinding. Sandblasting significantly increased the strength in fine and coarse grained materials. All surface treatment procedures reduced the Weibull modulus of Y-TZP. For both materials, the highest amount of the monoclinic phase and the largest TZD was found after sandblasting. Lower amounts of the monoclinic phase were obtained after both grinding procedures, where the highest mean critical defect size Ccr was also calculated. SIGNIFICANCE Our results indicate that sandblasting may provide a powerful technique for strengthening Y-TZP in clinical practice. In contrast, grinding may lead to substantial strength degradation and reduced reliability of prefabricated zirconia elements, therefore, sandblasting of ground surfaces is suggested.

Strength and reliability of surface treated Y-TZP dental ceramics

This work was undertaken to evaluate the effects of dental grinding and sandblasting on the biaxial flexural strength and Weibull modulus of various Y-TZP ceramics containing 3 mol% yttria. In addition, the susceptibility of pristine and mechanically treated materials to low-temperature degradation under the conditions adopted for testing the chemical solubility of dental ceramics was investigated. The results revealed that surface grinding and sandblasting exhibit a counteracting effect on the strength of Y-TZP ceramics. Dental grinding lowered the mean strength and Weibull modulus, whereas sandblasting provided a powerful method for strengthening, but at the expense of somewhat lower reliability. The finest-grained material exhibited the highest strength after sintering, but it was less damage tolerant than tougher, coarse-grained materials. Upon extraction with the acetic acid solution and the ammonia solution, a significant amount of tetragonal zirconia had transformed to monoclinic, but extensive microcracking and attendant strength degradation had not yet occurred. Standard grade Y-TZP ceramics are more resistant in an alkaline than in an acidic environment, and there was a strong grain-size dependence of the diffusion-controlled transformation. Since a special Y-TZP grade containing a small amount of alumina exhibited the highest damage tolerance and superior stability in an acidic environment, this material shows considerable promise for dental applications.

Survival-rate analysis of surface treated dental zirconia (Y-TZP) ceramics.

The role of surface preparation, hydrothermal ageing exposure and subsequent cyclic fatigue testing on the biaxial strength of a dental Y-TZP material are investigated. The initial strength and survival rate of a dental Y-TZP ceramic material to fatigue testing was found to be highly dependent upon surface preparation more so than exposure to various hydrothermal exposure conditions. The results suggest that the monoclinic phase generated by either surface damage (especially sandblasting) and to a lesser extent hydrothermal exposure does appear to mitigate strength and fatigue degradation. The results are discussed in terms of the size of defects generated following various surface treatments and the role of cyclic fatigue induced crack growth. A critical ratio is established between the monotonic strength and fatigue stress survival. From the specimens that failed and exhibited reduced strength after cycling a plot of averaged crack growth rate versus max cyclic stress intensity factor was established which closely matched existing results for Y-TZP ceramics.

Mechanical Fatigue and Hydrothermal Stability of Y-TZP Dental Ceramics

This study was designed to evaluate the effects of dental grinding and sandblasting on the ageing and fatigue behavior of pressure less-sintered biomedical grade Y-TZP ceramic. It was found that upon dental grinding and sandblasting, the surface of the material was heavily damaged in part plastically deformed, but the amount of transformed monoclinic zirconia was low. The partitioned tetragonal zirconia grains and pre-existing monoclinic zirconia in the ground and sandblasted surfaces hindered the propagation of the diffusion-controlled transformation during subsequent ageing. Dental grinding at a high rotation speed lowered the mean strength under static loading and the survival rate under cyclic loading. Sandblasting, in contrast, resulted in surface strengthening and substantially higher survival rate under cyclic loading. For all tested groups, about 10-15 % lower survival strength values were obtained when tested in artificial saliva, compared to dry specimens, implying that stress-assisted corrosion plays an important role in the fatigue behavior of dental zirconia.

Ageing and Fatigue Behaviour of Surface Treated Y-TZP Dental Ceramics

The effects of dental grinding and sandblasting on ageing and fatigue behaviour of pressure less sintered biomedical grade Y-TZP ceramics were investigated. Disc-shaped specimens were sintered to high density for subsequent surface treatment and testing. Accelerated aging experiments by autoclaving in artificial saliva were performed under isothermal conditions at 1340C. The amount of monoclinic zirconia in the ground and sandblasted specimens was < 5% and about 15 %, respectively and the corresponding strength values were 920 MPa and 1290 MPa. After autoclaving for 24 hours, the amount of transformed monoclinic zirconia in as-sintered material was increased to 15 %, resulting in about 10% strength reduction. In contrast, no strength reduction was observed with the ground and sandblasted specimens subjected to prolonged ageing. At any ageing condition, the highest survival rate during mechanical fatigue testing was observed with the sandblasted samples and the lowest with the ground samples. The strength of the surviving specimens tested in air corresponded well to the mean flexural strength of the particular group before fatigue testing, whereas lower survival strength values and a larger variability in strength were obtained with specimens that were subjected to prolonged accelerated ageing prior to mechanical fatigue testing in artificial saliva.

Nanoparticle concentrations and composition in a dental office and dental laboratory: A pilot study on the influence of working procedures

ABSTRACT During material treatment in dentistry particles of different size are released in the air. To examine the degree of particle exposure, air scanning to dental employees was performed by the Scanning Mobility Particle Sizer. The size, shape and chemical composition of particles collected with a low-pressure impactor were determined by scanning electronic microscopy and X-ray dispersive analysis. The average concentrations of nanoparticles during working periods in a clean dental laboratory (45,000–56,000 particles/cm3), in an unclean dental laboratory (28,000–74,000 particles/cm3), and in a dental office (21,000–50,000 particles/cm3), were significantly higher compared to average concentrations during nonworking periods in the clean dental laboratory (11,000–24,000 particles/cm3), unclean laboratory (14,000–40,000 particles/cm3), and dental office (13,000–26,000 particles/cm3). Peak concentration of nanoparticles in work-intensive periods were found significantly higher (up to 773,000 particles/cm3), compared to the non-working periods (147,000 particles/cm3) and work-less intensive periods (365,000 particles/cm3). The highest mass concentration value ranged from 0.055–0.166 mg/m3. X-ray dispersive analysis confirmed the presence of carbon, potassium, oxygen, iron, aluminum, zinc, silicon, and phosphorus as integral elements of dental restorative materials in form of nanoparticle clusters, all smaller than 100 nm. We concluded that dental employees are exposed to nanoparticles in their working environment and are therefore potentially at risk for certain respiratory and systematic diseases.

Zirconia ceramic fixed partial dentures after cyclic fatigue tests and clinical evaluation: a systematic review

ABSTRACT Zirconia fixed prosthetic dentures are extensively used for replacing missing teeth. The primary objective of this systematic review was to gather and present the results of all in vitro studies and clinical trials conducted on zirconia fixed prosthetic dentures. This review concentrated exclusively on bilayered zirconia and monolithic fixed prosthetic dentures. As such this paper can act as a guideline for more comparable future experimental work on zirconia ceramics. Future studies must use a more systematic approach such as the uniform use of abutment material, material for simulating periodontal support, data about fracture strength before and after fatigue, number of cycles, information about position and size of the indenter. The new digital techniques with long-term follow-up are desirable in further clinical studies.

Fracture resistance of endodontically treated maxillary incisors restored with zirconia posts: effect of the internal plateau preparation

ABSTRACT We report on a new approach to increase the fracture resistance of endodontically treated teeth. We propose a preparation of horizontal internal plateau (IP) in dentine and the use of retentive zirconia posts to achieve a more favourable load transmission. The aim of our work was to investigate the effect of the IP depth and the post diameter on the fracture resistance and the failure mode of maxillary central incisors. Seventy-two teeth were, divided into six groups of 12. IP 4 mm in diameter and 0, 1 or 2 mm in depth was prepared and zirconia posts of two diameters luted. Specimens were loaded until failure and fractures were classified as reparable or not. Two-way ANOVA, Tukey’s HSD test and Poisson regression were used for statistical analysis (α = 0.05). Significantly improved fracture resistance and predominantly favourable failure modes were found when 2-mm deep IP is prepared.

Properties and clinical application of zirconia bioceramics in medicine.

Background : A group of inorganic non-metal biomaterials, that are commonly used in clinical medicine to replace or repair tissues, can be classified as a bioceramics. This group includes bioactive glasses, glass-ceramics, hydroxy-apatite and some other calcium phosphates. In addition, some bio-inert engineering ceramics materials have become increasingly utilised, aluminum oxide, zirconium oxide and their composites being the most popular. With the developement of yttria stabilized tetragonal zirconium oxide ceramics (Y-TZP) medical community received a high strength biomaterial that is currently a material of choice for the manufacturing of medical devices. Y-TZP ceramics is becoming also increasingly used in dental medicine, where frameworks are manufactured by the use of computer-assisted technology. Conclusions : The article describes the basic properties of zirconia oxide ceramics important for the use in clinical medicine; high strength and fracture toughness, biocompatibility and negligible radiation. The ageing issue of this particular material, which is attributable to the thermo-dynamical instability of tetragonal zirconium oxide in hydrothermal conditions, is also discussed. When exposed to an aqueous environment over long periods of time, the surface of the Y-TZP ceramic will start transforming spontaneously into the monoclinic structure. The mechanism leading to the t-m transformation is temperature-dependent and is accompanied by extensive micro-cracking, which ultimately leads to strength degradation. The degradation might influence the clinical success rate of medical devices and therefore Y-TZP femoral heads are no longer made of pure zirconium oxide. Composites of zirconium and aluminium oxides are used instead, that are currently the strongest ceramic materials used in clinical medicine. In this work the clinical application of zirconia oxide ceramics in dental medicine is also presented. Conventional porcelain fused to metal technique is successfully replaced with Y-TZP ceramics in some clinical situations that are described in detail. It is important that computer design of the zirconia frameworks shortens and simplifies laboratory procedures and contributes to a precise final product.

Interdisciplinary treatment of ectodermal dysplasiawith oligodontia: review and case report

Background: Ectodermal dysplasia (ED) is a group of hereditary diseases with disorders in the development of two or more ectodermal structures, such as hair, teeth, nails and sweat glands. The symptoms of craniofacial anomalies express themselves with a larger number of unerupted teeth – oligodontia. Facial characteristics appear because of disorders in the growth of facial bones due to congenitally missing teeth related to underdeveloped alveolar ridge. Treatment planning in these patients requires interdisciplinary approach, and should include various possibilities of substitution of congenitally absent teeth, orthodontic treatment, orthognatic surgery, skeletal augmentation, surgical insertion of dental implants, prosthetic rehabilitation and genetic counseling. Case presentation: In a patient with ectodermal dysplasia, clinical examination and analysis of panoramic x-ray revealed 17 congenitally absent permanent teeth with extensive atrophy of the alveolar ridge, hypotrichosis, narrow face, prominently expressed chin with speech and mastication difficulties. Rehabilitation planning included an extensive interdisciplinary approach with orthodontically guided three-dimensional correction of the upper and lower jaw relationship that was followed by a surgical open lift of the sinus floor on the right side and insertion of four dental implants. In the mandibular arch, the missing teeth were replaced with a metal-porcelain bridge on the supporting teeth and with two bridges supported on the dental implants in the maxillary arch. After 12 years of interdisciplinary treatment the patient was successfully rehabilitated. Conclusion: Treatment of patients with ectodermal dysplasia and oligodontia is individual and must be targeted mainly at prevention, for it is necessary to conserve the patient’s own teeth and at the same time replace (the) missing tissues and teeth, and correct the facial structures.