The Effects of Adhesive Type and Thickness on Stress Distribution in Molars Restored with All‐Ceramic Crowns

Purpose: This study investigated the effects of luting cement type and thickness on the stress distribution within all‐ceramic crowns using finite element analysis. Materials and Methods: An all‐ceramic crown restoration of the mandibular right first molar was prepared according to standard dental processes and scanned using micro‐computed tomography. Eight 3D FE models were then developed that accounted for two adhesive systems, each with cement thickness of 60 μm, 90 μm, 120 μm, and 150 μm. The models were subjected to four loading conditions, and stresses in the veneer and core layers were evaluated. Results: The stress distribution and maximum stresses in the veneer, core, and cement are presented in corresponding loading conditions. The cement with higher elastic modulus resulted in lower tensile stresses in the veneer and core layers, and the shear strength of the cement was critical to the intactness of the all‐ceramic crown. Conclusion: The cement thickness acts as a cushion between the crown and dentin substrate. Although there is an optimal thickness (approximately 90 μm) that can reduce the stress level in ceramic crowns, cement thickness is not very important to stresses in the core or veneer in most cases when compared to the influence of loading conditions or cement moduli.     

下颌第一磨牙全瓷冠疲劳寿命预测的三维有限元分析

目的:对EmpressⅠ、EmpressⅡ、In-Ceram Alumina以及In-Ceram Zirconia4种不同材料的下颌第一磨牙全瓷冠进行疲劳寿命预测。方法:模拟1、5、10年间,相当于最大咬合力的循环疲劳载荷,加载于全瓷冠模型。通过有限元疲劳分析软件MSC.Fatigue预测各种材料全瓷冠的失效概率。结果:基于本有限元疲劳分析,EmpressⅠ、EmpressⅡ、In-Ceram Alumina以及In-Ceram Zirconia这4种均可应用于后牙全瓷冠。其中采用EmpressⅠ制作的全瓷冠较其他3种材料具有较高的失效概率(3.15%~7.486%)。结论:有限元疲劳分析有助于全瓷修复材料临床应用前的耐疲劳强度的判断。     

Residual Thermal Stress Simulation in Three‐Dimensional Molar Crown Systems: A Finite Element Analysis

Purpose: To simulate coefficient of thermal expansion (CTE)‐generated stress fields in monolithic metal and ceramic crowns, and CTE mismatch stresses between metal, alumina, or zirconia cores and veneer layered crowns when cooled from high temperature processing. Materials and Methods: A 3D computer‐aided design model of a mandibular first molar crown was generated. Tooth preparation comprised reduction of proximal walls by 1.5 mm and of occlusal surfaces by 2.0 mm. Crown systems were monolithic (all‐porcelain, alumina, metal, or zirconia) or subdivided into a core (metallic, zirconia, or alumina) and a porcelain veneer layer. The model was thermally loaded from 900°C to 25°C. A finite element mesh of three nodes per edge and a first/last node interval ratio of 1 was used, resulting in approximately 60,000 elements for both solids. Regions and values of maximum principal stress at the core and veneer layers were determined through 3D graphs and software output. Results: The metal‐porcelain and zirconia‐porcelain systems showed compressive fields within the veneer cusp bulk, whereas alumina‐porcelain presented tensile fields. At the core/veneer interface, compressive fields were observed for the metal‐porcelain system, slightly tensile for the zirconia‐porcelain, and higher tensile stress magnitudes for the alumina‐porcelain. Increasingly compressive stresses were observed for the metal, alumina, zirconia, and all‐porcelain monolithic systems. Conclusions: Variations in residual thermal stress levels were observed between bilayered and single‐material systems due to the interaction between crown configuration and material properties.     

Masking Ability of Zirconia with and without Veneering Porcelain

Purpose : The present study compared changes in CIE L*a*b* color coordinates of substrates of different colors when covered with zirconium oxide discs (Procera) and with such discs if veneered with two shades of porcelain. Material and Methods : Forty background substrates were fabricated and divided into four groups depending on the color of the substrates: white, black, gray, and tooth‐colored (Vita shade A3). The initial color of the substrates was measured using a colorimeter. The color of the substrates covered with plain zirconium oxide discs and with zirconium oxide discs veneered with porcelains of two shades (Vita shade A1 and B4) was measured. The color difference between the substrates, the substrates covered with plain discs, and the substrates covered with veneered discs was calculated, and the data were statistically analyzed with one‐way ANOVA and multiple paired t‐test. Results : For each group of substrates, the resulting colors were significantly different when the substrates were covered by either plain zirconium oxide discs or zirconium oxide discs veneered with Vita shade A1 or B4 porcelain. Conclusion : While zirconium oxide coping material alone has a degree of masking ability, the resulting color of a restoration can be further modified with the veneering porcelain.     

Fatigue analysis of computer‐aided design/computer‐aided manufacturing resin‐based composite vs. lithium disilicate glass‐ceramic

Resin‐based composite molar crowns made by computer‐aided design/computer‐aided manufacturing (CAD/CAM) systems have been proposed as an inexpensive alternative to metal‐ceramic or all‐ceramic crowns. However, there is a lack of scientific information regarding fatigue resistance. This study aimed to analyze the fatigue behavior of CAD/CAM resin‐based composite compared with lithium disilicate glass‐ceramic. One‐hundred and sixty bar‐shaped specimens were fabricated using resin‐based composite blocks [Lava Ultimate (LU); 3M/ESPE] and lithium disilicate glass‐ceramic [IPS e.max press (EMP); Ivoclar/Vivadent]. The specimens were divided into four groups: no treatment (NT); thermal cycling (TC); mechanical cycling (MC); and thermal cycling followed by mechanical cycling (TCMC). Thermal cycling was performed by alternate immersion in water baths of 5°C and 55°C for 5 × 10⁴ cycles. Mechanical cycling was performed in a three‐point bending test, with a maximum load of 40 N, for 1.2 × 10⁶ cycles. In addition, LU and EMP molar crowns were fabricated and subjected to fatigue treatments followed by load‐to‐failure testing. The flexural strength of LU was not severely reduced by the fatigue treatments. The fatigue treatments did not significantly affect the fracture resistance of LU molar crowns. The results demonstrate the potential of clinical application of CAD/CAM‐generated resin‐based composite molar crowns in terms of fatigue resistance.     

A practice‐based clinical evaluation of the survival and success of metal‐ceramic and zirconia molar crowns: 5‐year results

This practice‐based study evaluates the survival and success of conventionally luted metal‐ceramic and zirconia molar crowns fabricated by using a prolonged cooling period for the veneering porcelain. Fifty‐three patients were treated from 07/2008 to 07/2009 with either metal‐ceramic crowns (MCC) or zirconia crowns (ZC). Forty‐five patients (26 female) with 91 restorations (obser‐vational period: 64·0 ± 4·8 months) participated in a clinical follow‐up examination and were included in the study. Estimated cumulative survival (ECSv), success (ECSc) and veneering ceramic success (ECVCSc) were calculated (Kaplan–Meier) and analysed by the crown fabrication technique and the position of the restoration (Cox regression model) (P < 0·05). Five complete failures (MCC: 2, ZC: 3) were recorded (5‐year ECSv: MCC: 97·6%, (95% confidence interval (95%‐CI): [93%; 100%]/ZC: 94·0%, (95%‐CI): [87%; 100%]). Of the MCCs (n = 41), 85·0%, [95%‐CI: (77%; 96%)] remained event‐free, whereas the ECSc for the ZCs (n = 50) was 74·3% (95%‐CI): [61%; 87%]. No significant differences in ECSv (P = 0·51), ECSc (P = 0·43) and ECVCSc (P = 0·36) were detected between the two fabrication techniques. Restorations placed on terminal abutments (n = 44) demonstrated a significantly lower ECVCSc (P = 0·035), (5‐year VCF‐rate: 14·8%) than crowns placed on tooth‐neighboured abutments (n = 47), (5‐year VCF‐rate: 4·3%). In the present study, zirconia molar crowns demonstrated a 5‐year ECSv, ECSc and ECVCSc comparable to MCCs. Irrespective of the fabrication technique, crowns on terminal abutments bear a significantly increased risk for VCFs. Clinical investigations with an increased number of restorations are needed.     

2种材质桩核修复磨牙残冠的抗疲劳强度

目的比较2种桩核材料用于磨牙残冠桩核冠修复的抗疲劳强度。方法收集制备20个下颌第一磨牙残冠,分别用纤维桩和铸造插销式分体桩进行桩核冠修复,测定其抗疲劳强度,并对两种修复方式的疲劳强度和折裂模式进行比较。结果铸造插销式分体桩核冠用于磨牙残冠的修复可获得良好的抗疲劳强度,与纤维桩核冠修复相比较,差异具有统计学意义(P﹤0.05)。结论磨牙残冠选用铸造插销式分体桩核冠修复可以获得更好的力学性能,对于合并有夜磨牙、紧咬牙等习惯的患者,应该采取降低牙尖斜度等方法来减小垂直及侧向牙合力。     

基于FEA的种植体个性化选择及力学性能评价

目的:使用有限元方法进行个性化种植体分析,为医师选择适合的种植体形态提供参考依据。方法:通过自行开发的口腔虚拟种植系统重建患者颌骨模型,并选择相应种植体进行虚拟种植以模拟各颌牙缺损状态下种植体植入的位置及方向,利用FEA分析咀嚼运动中不同种植体产生的应力分布,同时进行力学性能评价。结果:实现了口腔种植体的个性化选择方案。结论:所提出的口腔种植力学性能评价方法,能够从应力角度为医师选择种植体形态提供参考依据。     

A Comparative Study of the Effect of Four Consecutive Firing Cycles on the Marginal Fit of All: Ceramic Crown System and Metal Ceramic Crown System

The marginal fit of new all ceramic crown system. To know the marginal adaptability of new all ceramic systems. Finesse all ceramic system and traditional metal ceramic system total 15 samples of all ceramic (test group) and 15 samples of metal ceramic crown system (control group) were fabricated and tested for marginal distortion at four firing cycles using image analyzer and special software (Leco Version La 32) in which instead of measuring at points an area was measured that gives a computed mean measured thickness of marginal distortion. Value obtained were evaluated for significance using two tailed, unpaired, student t test and Tukeys-Kramer multiple comparison test. Finesse all ceramic crown system showed continued clinically acceptable marginal distortion through all firing cycles (12.84 μm). Greatest distortion of metal ceramic system occurred during degassing cycle(16.90 μm). In respect of marginal fit all ceramic (finesse) crowns is better choice when esthetics is more concern.     

牙科玻璃渗透氧化铝陶瓷弯曲强度Weibull分析和循环载荷下亚临界裂纹扩展研究

目的：研究牙科玻璃渗透氧化铝陶瓷（IA,In-Ceram Alumina）弯曲强度（σ）Weibull分析意义和循环载荷下亚临界裂纹扩展（SCG）情况。方法：分别采用三点弯曲强度测量方法（3p）和循环疲劳（F）测试方法测得15个条形IA样本的弯曲强度以及15个条形IA样本的循环疲劳断裂循环次数,数据采用Weibull方法分析;亚临界裂纹扩展（SCG）行为通过裂纹扩展速度和最大应力强度因子（KImax）的函数关系来描述。结果：IA的Weibull模数为7.9,特征强度（σ63.21）为438.3MPa。IA在循环载荷下发生亚临界裂纹扩展后导致材料断裂,在应力强度因子K1为临界应力因子KIC的55%时发生裂纹传播。结论：牙科玻璃渗透氧化铝陶瓷循环载荷下存在亚临界裂纹扩展;采用弯曲强度测试法并结合使用Weibull分析方法能加深对口腔陶瓷修复失败机制的理解。