Fatigue loading and R-curve behavior of a dental glass-ceramic with multiple flaw distributions

Objectives

To determine the effects of surface finish and mechanical loading on the rising toughness curve (R-curve) behavior of a fluorapatite glass-ceramic (IPS e.max ZirPress) and to determine a statistical model for fitting fatigue lifetime data with multiple flaw distributions.

Materials and methods

Rectangular beam specimens were fabricated by pressing. Two groups of specimens (n = 30) with polished (15 μm) or air abraded surface were tested under rapid monotonic loading in oil. Additional polished specimens were subjected to cyclic loading at 2 Hz (n = 44) and 10 Hz (n = 36). All fatigue tests were performed using a fully articulated four-point flexure fixture in 37 °C water. Fractography was used to determine the critical flaw size and estimate fracture toughness. To prove the presence of R-curve behavior, non-linear regression was used. Forward stepwise regression was performed to determine the effects on fracture toughness of different variables, such as initial flaw type, critical flaw size, critical flaw eccentricity, cycling frequency, peak load, and number of cycles. Fatigue lifetime data were fit to an exclusive flaw model.

Results

There was an increase in fracture toughness values with increasing critical flaw size for both loading methods (rapid monotonic loading and fatigue). The values for the fracture toughness ranged from 0.75 to 1.1 MPa m^1/2 reaching a plateau at different critical flaw sizes based on loading method.

Significance

Cyclic loading had a significant effect on the R-curve behavior. The fatigue lifetime distribution was dependent on the flaw distribution, and it fit well to an exclusive flaw model.     

牙陶瓷材料及修复体循环疲劳的损伤模式

牙科全瓷系统具有良好功能性和美学性质，在临床上逐渐得到了应用；但全瓷修复体在反复咀嚼功能状态下常常出现疲劳失效．陶瓷材料的循环疲劳性能表征则成为研究热点之一。本文重点对牙科陶瓷循环疲劳损伤模式（包括脆性模式、准塑性模式和放射状裂纹）的研究进展进行综述。     

Evaluation of machinability and flexural strength of a novel dental machinable glass-ceramic

Objectives

To evaluate the machinability and flexural strength of a novel dental machinable glass-ceramic (named PMC), and to compare the machinability property with that of Vita Mark II and human enamel.

Methods

The raw batch materials were selected and mixed. Four groups of novel glass-ceramics were formed at different nucleation temperatures, and were assigned to Group 1, Group 2, Group 3 and Group 4. The machinability of the four groups of novel glass-ceramics, Vita Mark II ceramic and freshly extracted human premolars were compared by means of drilling depth measurement. A three-point bending test was used to measure the flexural strength of the novel glass-ceramics. The crystalline phases of the group with the best machinability were identified by X-ray diffraction.

Results

In terms of the drilling depth, Group 2 of the novel glass-ceramics proves to have the largest drilling depth. There was no statistical difference among Group 1, Group 4 and the natural teeth. The drilling depth of Vita MK II was statistically less than that of Group 1, Group 4 and the natural teeth. Group 3 had the least drilling depth. In respect of the flexural strength, Group 2 exhibited the maximum flexural strength; Group 1 was statistically weaker than Group 2; there was no statistical difference between Group 3 and Group 4, and they were the weakest materials. XRD of Group 2 ceramic showed that a new type of dental machinable glass-ceramic containing calcium-mica had been developed by the present study and was named PMC.

Conclusions

PMC is promising for application as a dental machinable ceramic due to its good machinability and relatively high strength.     

热处理对新型牙科陶瓷可切削性及强度的影响

目的 分析不同晶化温度和晶化时间对新研制的一种牙科云母玻璃陶瓷切削性能及强度的影响,探讨其延性切削行为的可能机理及其与强度间的关系.方法 用钻孔实验的方法,以VITA MARKⅡ为对照组,测定七组不同晶化工艺处理的新型牙科云母玻璃陶瓷单位时间的钻孔深度,进行车床切削实验定性分析,同时测量三点抗弯强度.结果 该新材料在680℃晶化60min,抗弯强度可达到155MPa,可切削性能优于对照组,一定热处理条件下可在车床高速车削时形成连续带状切屑.结论 该新材料在国际上首次实现了牙科陶瓷材料的延性切削,强度已能满足临床要求.     

Application of analytical stress solutions to bi-axially loaded dental ceramic–dental cement bilayers

OBJECTIVE: Increased consideration of dental ceramics and dental cements as a singular structural unit for in-vitro mechanical testing has resulted in the reporting of a wide range of analytical methods to calculate the failure stresses. Therefore a comparison of observations between studies is complicated by the use of dissimilar stress solutions despite the employment of a similar testing methodology and specimen geometry. MATERIALS AND METHODS: Three analytical solutions to calculate failure stresses in bi-axially loaded dental ceramic-dental cement bilayers were appraised for a commonly utilized testing geometry (ball-on-ring). Clinically relevant datasets were generated from the bi-axial flexure testing of uncoated and dental cement coated aluminous core ceramic exposed to differing ceramic surface preparations. A Weibull statistical approach was utilized in order to provide insight into the impact of the analytical method on both the scale (sigma(0)) and distribution (m) of the failure stress data. RESULTS: Calculation of the bi-axial flexure stress utilizing Timoshenko's analysis resulted in an increase in sigma(0) for the uncoated (6%) and dental cement coated (11-12%) aluminous core ceramic, when compared with the bilayered solutions reported by Rosenstiel and Hsueh. However, the shape of the failure distributions illustrated by the consistency of m and associated 95% confidence intervals was not influenced by the analytical stress solution employed. SIGNIFICANCE: The choice of the analytical method chosen to calculate failure stresses in bi-axially loaded dental ceramic-dental cement bilayers will impact on the magnitude of the reported strength. Comparison between the failure stresses of uncoated and cement coated dental ceramics is more accurately represented by bilayer solutions, which account for the mismatch between the elastic constants of dissimilar materials. However, within the context of dental cement coated dental ceramics of clinically relevant dimensions, the choice of solution is unlikely to impact on the interpretation of the observations previously reported in the dental literature.     

Influence of veneer and cyclic loading on failure behavior of lithium disilicate glass-ceramic molar crowns

Objective

This in vitro study was designed to investigate the influence of the veneer and cyclic loading on the failure behavior of lithium disilicate glass-ceramic (LDG) crowns on maxillary first molar.

Methods

Sixty-four LDG crowns were divided into 4 groups (n = 16). Thirty-two monolithic crowns were fabricated from IPS e.max Press (M), and the remaining bilayered crowns using cut-back technique and conventional manual layering technique from IPS e.max Press/Ceram (B). Monolithic or bilayered crowns were subjected to single-load-to-fracture (SLF) testing using a universal testing machine, before (M1 and B1) and after exposure to sliding-contact fatigue (SCF) testing (M2 and B2), consisting of 1,200,000 mechanical cycles (F_max = 98 N). Data were statistically analyzed using two-by-two factorial design ANOVA. Fractographic analysis was performed to determine the fracture modes of the failed specimens.

Results

The mean fracture load values (N ± S.D.) for M1, B1, M2 and B2 were 2686 ± 628 N, 1443 ± 327 N, 2133 ± 578 N and 1464 ± 419 N, respectively. Significant differences were found between the failure loads of all groups (P < 0.001), except between groups B1 and B2. Bulk fracture initiating from the occlusal surface is the primary failure mode of monolithic and veneered LDG crowns. Cracking that initiated from core–veneer interfacial defects and ultimately resulted in bulk fracture is another major failure origin of veneered all-ceramic crowns.

Significance

Veneer application resulted in significantly lower fracture load values compared to monolithic LDG crowns. Cyclic loading is an accelerating factor contributing to fracture for monolithic LDG crowns but not for bilayered ones.     

IPS-Empress 2 玻璃陶瓷结构及性能的研究

目的 研究新型IPS-Empress 2牙科高强度陶瓷的显微结构和机械性能。方法采用原子力显微镜、扫描电子显微镜和X射线衍射仪，分析IPS-Empress 2的显微结构和晶相，用三点弯曲实验和压痕法测试其弯曲强度和断裂韧性。结果IPS-Empress 2玻璃陶瓷主要由二硅酸锂晶体和磷酸锂晶体组成，二者形成相互交错的三维网络式结构；这种玻璃陶瓷在热压铸前后晶体相保持不变，其三点弯曲强度和断裂韧性分别为300MPa和3．1MPam^1/2。结论IPS-Empress 2玻璃陶瓷的高强度和韧性与高含量的二硅酸锂晶体、相互锁结的网络结构和裂纹偏转有关。     

Slow crack growth and reliability of dental ceramics

Objective

To determine the slow crack growth (SCG) and Weibull parameters of five dental ceramics: a vitreous porcelain (V), a leucite-based porcelain (D), a leucite-based glass-ceramic (E1), a lithium disilicate glass-ceramic (E2) and a glass-infiltrated alumina composite (IC).

Methods

Eighty disks (Ø 12 mm × 1.1 mm thick) of each material were constructed according to manufacturers’ recommendations and polished. The stress corrosion susceptibility coefficient (n) was obtained by dynamic fatigue test, and specimens were tested in biaxial flexure at five stress rates immersed in artificial saliva at 37 °C. Weibull parameters were calculated for the 30 specimens tested at 1 MPa/s in artificial saliva at 37 °C. The 80 specimens were distributed as follows: 10 for each stress rate (10⁻², 10⁻¹, 10¹, 10² MPa/s), 10 for inert strength (10² MPa/s, silicon oil) and 30 for 10⁰ MPa/s. Fractographic analysis was also performed to investigate the fracture origin.

Results

E2 showed the lowest slow crack growth susceptibility coefficient (17.2), followed by D (20.4) and V (26.3). E1 and IC presented the highest n values (30.1 and 31.1, respectively). Porcelain V presented the lowest Weibull modulus (5.2). All other materials showed similar Weibull modulus values, ranging from 9.4 to 11.7. Fractographic analysis indicated that for porcelain D, glass-ceramics E1 and E2, and composite IC crack deflection was the main toughening mechanism.

Significance

This study provides a detailed microstructural and slow crack growth characterization of widely used dental ceramics. This is important from a clinical standpoint to assist the clinician in choosing the best ceramic material for each situation as well as predicting its clinical longevity. It also can be helpful in developing new materials for dental prostheses.     

Evaluation of key parameters in a potentiostatic corrosion test for dental amalgam

OBJECTIVES: Participation in a Round Robin study of potentiostatic corrosion test guidelines for dental amalgam was undertaken for the purpose of developing an accurate set of draft guidelines. METHODS: Dispersalloy, sybraloy, aristalloy, phasealloy, and tytin were used as the amalgam specimens. They were prepared following the guidelines, then coupled to a copper wire, cemented into glass tubes, and polished to a 600-grit finish. A corrosion cell was prepared using a carbon counter-electrode, a standard calomel electrode (SCE) as the reference electrode, and amalgam as the working electrode. A 37 degrees C solution of 10 g/l NaCl with a minimum volume of 300 ml was used. Within 5 min of polishing, the open circuit potential (OCP) was recorded for 10 min. Next, the specimen was polarized to 0 mV versus SCE, and the currents were recorded for a 24-h time period. Corrosion results were analyzed statistically with one-way ANOVA (p < 0.05) and the multiple comparisons Student-Newman-Keuls post-hoc test. RESULTS: Problems that occurred with evaporation, beaker size, carbon electrode length, SCE cap removal, glass tube fracture, polishing technique, and fresh electrolyte are easily avoidable with further explanation or reminder notes. Observations made concerning starting time, initial OCP recording, millivoltage, and solution temperature were determined to be necessary for the accuracy of test results. Analysis of results should include clarification of units, and graph interpretations. Finally, the number of specimens per amalgam should be increased from one to three so that statistical analysis can be performed. Using three specimens per amalgam, the method revealed corrosion susceptibility as measured by the improved test: aristalloy > sybraloy > (dispersalloy, phasealloy, tytin). SIGNIFICANCE: Having run the initially proposed guidelines, a number of clarifying changes were made so that the corrosion susceptibilities of five dental amalgams could be clearly differentiated.     

Subcritical crack growth behavior and life data analysis of two types of dental Y-TZP ceramics

Objectives

The aim of this study was to evaluate the mechanical properties and the subcritical crack growth behavior of a presintered dental Y-TZP (Kavo Everest ZS) and a hot isostatic pressed Y-TZP (Kavo Everest ZH) and to perform life data analysis.

Methods

For each material 150 bending bars were produced. The initial fracture strength was determined in a four-point bending test. The subcritical crack growth parameters n and A were determined in a dynamic fatigue method at four decreasing loading rates from 110 MPa/s to 0.11 MPa/s in distilled water at 38 °C. For each loading rate Weibull statistics were performed and the Weibull moduli m and characteristic strengths σ₀ were calculated. Using these data, strength-fracture probability-life time (SPT) predictions were derived for 1 day, 1 year, 5 years and 10 years, based on a static crack growth mechanism.

Results

The “hipped” Y-TZP ceramic exhibited a higher initial strength (σ_c = 1618.18), characteristic strength (σ₀ = 837.15) and fracture toughness (K_IC = 4.52 MPa/m^1/2) than the pre-sintered ceramic (σ_c = 1431, σ₀ = 745.46 and K_IC = 3.17 MPa/m^1/2, respectively). Fatigue parameters, n and A, were 28.5 and 7.97 × 10⁻²⁴ for Everest ZH and 30.15 and 5.47 × 10⁻²⁰ for Everest ZS. The predicted fracture stress at 5% failure probability for a lifetime of 10 years was 259.34 MPa for Everest ZH and 263.2 MPa for Everest ZS.

Conclusions

Although the “hipped” Y-TZP showed favorable initial mechanical properties, no significant difference could be found in the susceptibility of both ceramics to subcritical crack growth and their long-term strength.     

周期性单轴压力对大鼠髁突软骨细胞结缔组织生长因子表达的影响

目的 探讨在髁突软骨细胞受到周期性单轴压力后,结缔组织生长因子(connective tissuegrowth factor, CTGF)表达的影响,为正畸治疗中髁突软骨受力后的改建提供生物学依据。方法 选取1周龄SD大鼠,提取并培养髁突软骨细胞,免疫组化鉴定。利用四点弯曲细胞力学加载仪对第3代细胞进行力值为2000u strain、0.5Hz的体外周期性单轴压力加载,分别在加力0min、30min、60min和120min后继续培养24h,应用蛋白印迹法检测在不同加力时间CTGF蛋白表达的变化。应用SPSS18.0软件对数据进行统计学分析。结果 CTGF的相对蛋白量在加力0min、30min、60min和120min后,分别为0、1.59、2.34和3.16,随着加载时间的增加,表达呈逐渐上升趋势。且组间差异具有统计学意义(P<0.05) 结论 周期性单轴压力可刺激大鼠髁突软骨细胞CTGF的表达。     

The corrosion of dental amalgam in artificial salivas: an electrochemical impedance study

An electrochemical impedance study of the corrosion of Tytin dental amalgam was carried out in electrolytes similar to artificial saliva with or without lactic acid, and in standard inorganic artificial saliva with higher ionic concentration. Spectra were recorded in the presence and absence of dissolved oxygen to show the importance of the formation of oxide film and the adsorption of the organic component, as well as the exposed surface microstructure. The data, supported by open circuit potential and polarisation curve experiments, obtained in the presence and absence of dissolved oxygen are interpreted in the light of possible corrosion mechanisms.     

Internal stress in children and parental attitude to dental treatment with passive restraint

PurposeThe purpose of this study was to examine the internal stress in uncooperative children treated under passive restraint and the changes of it in later treatments under cooperative condition with no need of restraint. We also conducted questionnaire among parents to survey their opinion towards the treatment with restraining device.MethodsThe subjects for electrocardiogram (ECG) recording were the child patients who were referred to the university clinic by the primary care dentists. We recorded ECG of the patients during dental treatment and assessed the autonomic nervous activity as a physiological response to stress. The recordings from uncooperative children were continued at every visit until they became cooperative and no longer restrained. A questionnaire survey was conducted among parents who have experienced the use of the restraining device to their children.ResultsThough the sympathetic nerve activity, which elevates with internal stress, and the heart rate were relatively high under restraint in uncooperative children, both diminished in the later session when children became cooperative enough to remove restraint. The questionnaire survey demonstrated that most parents whose children underwent treatment with restraining device showed positive attitudes toward the use of it.ConclusionsThe results of this study suggest that the experience of dental treatment with passive restraint would not necessarily be a traumatic event for children in a short term and parents would accept the necessity through the experience of the treatment in case urgent treatments are required.     

齿科铸造合金应力腐蚀破裂的研究进展

齿科铸造合金广泛应用于口腔修复,修复体在口腔环境中行使功能时,会发生电化学腐蚀,其中应力腐蚀破裂是危害性最大的一种腐蚀形式。本文就齿科铸造合金应力腐蚀破裂的研究进展作一综述。     

Analysis of subcritical crack growth in dental ceramics using fracture mechanics and fractography.

OBJECTIVES: The aim of this study was to test the hypothesis that the flexural strengths and critical flaw sizes of dental ceramic specimens will be affected by the testing environment and stressing rate even though their fracture toughness values will remain the same. METHODS: Ceramic specimens were prepared from an aluminous porcelain (Vitadur Alpha; VITA Zahnfabrik, Bad S?ckingen, Germany) and an alumina-zirconia-glass composite (In-Ceram Zirconia; VITA Zahnfabrik). Three hundred uniaxial flexure specimens (150 of each material) were fabricated to dimensions of 25 mmx4 mmx1.2 mm according to the ISO 6872 standard. Each group of 30 specimens was fractured in water using one of four different target stressing rates ranging on a logarithmic scale from 0.1 to 100 MPa/s for Vitadur Alpha and from 0.01 to 10 MPa/s for In-Ceram Zirconia. The fifth group was tested in inert environment (oil) with a target stressing rate of 100 MPa/s for Vitadur Alpha and 1000 MPa/s for In-Ceram Zirconia. The effects of stressing rate and environment on flexural strength, critical flaw size, and fracture toughness were analyzed statistically by Kruskal-Wallis one-way ANOVA on ranks followed by post hoc comparisons using Dunn's test (alpha=0.05). In addition, 20 Vitadur Alpha specimens were fabricated with controlled flaws to simplify fractography. Half of these specimens were fracture tested in water and half in oil at a target stressing rate of 100 MPa/s, and the results were compared using Mann-Whitney rank sum tests (alpha=0.05). A logarithmic regression model was used to determine the fatigue parameters for each material. RESULTS: For each ceramic composition, specimens tested in oil had significantly higher strength (P0.05). Specimens tested at faster stressing rates had significantly higher strength (P0.05). Regarding critical flaw size, stressing rate had a significant effect for In-Ceram Zirconia specimens (P0.05). Fatigue parameters, n and lnB, were 38.4 and -12.7 for Vitadur Alpha and were 13.1 and 10.4 for In-Ceram Zirconia. SIGNIFICANCE: Moisture assisted subcritical crack growth had a more deleterious effect on In-Ceram Zirconia core ceramic than on Vitadur Alpha porcelain. Fracture surface analysis identified fracture surface features that can potentially mislead investigators into misidentifying the critical flaw.     

Effect of chronic stress on capsaicin-induced dental nociception in a model of pulpitis in rats

ObjectiveChronic stress can alter nociceptive sensitivity. However, the effect of stress exposure on dental nociception has been less addressed. Therefore, the present study investigated the effects of chronic exposures to some social and psychological stresses on pulpal nociceptive responses.DesignThe stress groups were constructed as follows: forced swimming (n = 6), restraint (n = 6), and mild (n = 10) and severe (n = 15) crowding stresses. Rats were subjected to stress for 1 h per day for a week. At the end of the stress session, pulp irritation was induced by intradental application of capsaicin (100 μg). There were another capsaicin or capsaicin plus stress training groups that received articaine 5 min before the administration of capsaicin. Nociceptive responses were recorded for 40 min. The time (in s) of continuous shaking of the lower jaw and excessive grooming and rubbing of the mouth near the procedure site was measured as nociceptive behaviors. Data was analyzed using one-way analysis of variance (ANOVA) followed by post hoc Tukey’s test.ResultsSignificant nociceptive responses were evoked by the administration of capsaicin. Exposures to forced swimming (p < 0.01), restraint (p < 0.001), and both mild and severe crowding stresses (p < 0.05) exaggerated capsaicin-induced nociceptive reaction. There was, however, no significant difference in nociceptive reaction time between the different stress groups. Articaine buccal infiltration attenuated nociceptive time in capsaicin and capsaicin plus stress training groups (p < 0.001).ConclusionsThe current data support the association between chronic stress exposures and nociceptive behavior following intradental capsaicin administration.     

Effect of loading frequency on cyclic fatigue lifetime of a standard-diameter implant with an internal abutment connection

Objective

To investigate the effect of loading frequency on the fatigue lifetime of one standard-diameter titanium dental implant system.

Effects of stress rate and calculation method on subcritical crack growth parameters deduced from constant stress-rate flexural testing

Objectives

To more efficiently determine the subcritical crack growth (SCG) parameters of dental ceramics, the effects of stressing rate and choice of statistical regression model on estimates of SCG parameters were assessed.

Methods

Two dental ceramic materials, a veneering material having a single critical flaw population (S) and a framework material having partially concurrent flaw populations (PC), were analyzed using constant stress-rate testing, or “dynamic fatigue”, with a variety of testing protocols. For each material, 150 rectangular beam specimens were prepared and tested in four-point flexure according to ISO6872 and ASTM1368. A full-factorial study was conducted on the following factors: material, stress rate assumed vs. calculated, number of stress rates, and statistical regression method.

Results

The proportion of specimens for which the statistical models over-estimated reliability was not significantly different based on regression method for Material S (P = 0.96, power = 94%) and was significantly different based on regression method for Material PC (P < 0.001). The standard method resulted in SCG parameters, n and ln B, of 35.9 and −11.1 MPa² s for Material S and 12.4 and 9.61 MPa² s for Material PC, respectively.

Significance

The method of calculation that uses only the median strength value at each stress rate provided the most robust SCG parameter estimates. Using only two stress rates resulted in fatigue parameters comparable to those estimated using four stress rates having the same range. The stress rate of each specimen can be assumed to be the target stress rate with negligible difference in SCG parameter estimates.     

牙科陶瓷循环和静态疲劳的对比实验

目的 比较牙科陶瓷循环和静态加载后的强度下降规律以及结构损伤差异。方法应用赫兹触压实验方法对可切削陶瓷标准试件（VitamarkⅡ）进行循环和静态加载，测试试件的残余三点挠曲强度，观测试件表面及表面下损伤，数据进行单因素方差分析。结果试件在两种加载模式下，首次加载后残余强度均降低，同时出现外锥型损伤。循环加载至1000次时，内锥型损伤出现并伴随强度进一步降低，差异有统计学意义（P〈0．05）。不同加载时间下的静态加载试件强度下降的差异无统计学意义，同时不伴随内锥状裂纹的产生。结论静态加载和循环加载都能降低陶瓷试件的强度，但循环加载使损伤更严重。     

Fracture toughness and cyclic fatigue resistance of resin composites with different filler size distributions

Objectives

To verify the influence of filler size distributions on fracture toughness (K_Ic), initial fracture strength (IFS) and cyclic fatigue resistance (CFR) of experimental resin composites.

Methods

Four composites were prepared with same inorganic content (78 wt%), in which 67 wt% was constituted by glass particles with d₅₀ of 0.5; 0.9; 1.2; 1.9 μm K_Ic of the composites was determined by the single-edge notched beam (SENB) method. To evaluate the IFS and the CFR a biaxial bending test configuration was used. The CFR was determined under cyclic loading for 10⁵ cycles using the ‘staircase’ approach. The fracture surfaces of IFS and CFR specimens were analyzed under scanning electron microscope (SEM).

Results

There was a positive linear correlation between d₅₀ vs. K_Ic and statistical difference was found only between C0.5 (1.24 ± 0.10 MPa m^0.5) and C1.9 (1.41 ± 0.17 MPa m^0.5). There were no statistical differences among IFS means, which ranged from 155.4 ± 18.8 MPa (C0.9) to 170.7 ± 23.1 MPa (C1.2). C0.5 (93.0 ± 18.6^a MPa) showed the highest and C0.9 the lowest CFR (82.5 ± 8.0^c MPa). There was no correlation between CFR with d₅₀ values or with K_Ic means. SEM images showed the morphology with brittle fracture patterns for the surfaces of IFS specimens and a more smooth fracture surface for CFR specimens.

Significance

Resin composites showed different failure mechanisms for quasi-static and fatigue loading. For K_Ic and IFS, composites with larger filler size distributions showed better results due to crack deflection; while under cyclic loading, viscous behavior was predominant and composites with smaller particles showed higher fatigue resistance.