A mechanical study on new ceramic crowns and bridges for clinical use

By the recent development of new ceramics, i.e. castable glass ceramics and high strength porcelain, the clinical use of all-ceramic bridges as well as all-ceramic crowns have been expected. The purpose of this study was to evaluate the mechanical properties of new ceramics and to analyze the stress distributions in new ceramic crowns and bridges. The Young's modulus, flexural strength and diametral tensile strength of four types of new ceramics (DICOR, BIORAM-C, OPTEC, and HI-CERAM) were measured, and the fracture loads of new ceramic anterior crowns and bridges on the metal abutments were evaluated. Three dimensional finite element analyses of new ceramic anterior crowns and bridges were also carried out to investigate the effects of various mechanical factors; locations of loading point, types of ceramics, thickness of crowns, luting materials, core materials, and designs of fixed joints. In each experiment, the loading forces were applied at 45 degree to the tooth axis. The results were summarized as follow; 1) DICOR showed the highest flexural strength. HI-CERAM hard core porcelain showed the highest Young's modulus and tensile strength. 2) HI-CERAM crowns showed the highest fracture load among the new ceramic crowns. DICOR bridges were significantly stronger than BIORAM-C bridges. The stress analyses of the experimental cases indicated that the fractures of crowns and bridges occurred by the concentration of tensile stresses. 3) By the load at the incisal edge, the highest tensile stresses were caused in the crown. In the crown with 0.5 mm thickness at axial wall, high tensile stresses were observed at more wide regions of palatal side than in the crown with 0.75 mm or 1 mm thickness. However, in the case with an enamel layer remained on the surface of the abutment tooth, the stresses were reduced in spite of the crown thickness. When the abutment tooth was restored with a metal post and core, the stresses of the crown decreased in comparison with the natural abutment tooth. 4) In case of bridges, high tensile stresses concentrated at the fixed joints under any loading point. The stresses tended to rise slightly according to the increase of the Young's modulus of bridges. The aluminous core material which had high Young's modulus was effective for the reduction of the stresses at the surface of the bridge. Those tensile stresses were reduced remarkably by increasing the thickness of the fixed joints toward the labial and vertical side.(ABSTRACT TRUNCATED AT 400 WORDS)     

Strength comparison of anterior teeth restored with ceramic endocrowns vs custom-made post and cores

Purpose

The aim of the study was to compare strength of the anterior teeth restored with ceramic endocrowns versus custom-made post and core.

Tensile stress in glass-ceramic crowns: effect of flaws and cement voids.

The objective of this study was to analyze the relative effect of loading site, occlusal thickness, ceramic flaws, elastic modulus of the cement, and voids in the cement layer on tensile stress that develops in molar glass-ceramic crowns under applied loads. Finite-element stress analyses were performed. Resin cement with a thickness of 0.05 mm was used. A central conical flaw (0.05 mm [diameter] x 0.05 mm) and a circular grooved flaw located under the cusp tips were included in all flaw cases. A void space confined within the occlusal region of the cement layer was also included in selected cases. For a ceramic thickness of 0.5 mm and a vertical distributed load applied at a distance of 1.3 mm from the vertical axis, the maximum tensile stresses were 100 MPa for a crown with flaws and a void, 87 MPa for a crown with no flaws and a void, and 75 MPa for a crown with flaws and no void. For a 1.5-mm-thick crown with flaws and a void, the tensile stress decreased to 22 MPa. When the load of 600 N was concentrated at the central point of the occlusal region, the peak tensile stress in a crown with flaws and no void was increased to 325 MPa. For the conditions analyzed in this study a large void in a flawed occlusal region of a thin molar crown (0.5 mm) is proposed as a mechanism of crown failure.     

Stress distribution in metal-ceramic crowns with a facial porcelain margin

The use of metal-ceramic restorations with porcelain butt-joint facial margins has increased in the past several years. Although these crowns exhibit improved esthetics compared with metal-ceramic crowns which display a metal gingival collar or metal knife-edge margin, the effectiveness of this design in resisting intra-oral forces is not known. The objective of this study was to analyze the stress distribution induced by simulated intra-oral loads on crowns with variable coping configurations. The copings, with a thickness of either 0.1 or 0.3 mm, were modeled with a facial termination of metal at three locations: at the gingival floor, 0.9 mm above the gingival floor, and 4.2 mm above the gingival floor. The coping and crown dimensions were based on a prepared maxillary central incisor with a facial shoulder and a lingual chamfer. Both Ni-Cr and Au-Pd alloy copings were employed in the crown models. Finite element stress and analyses were performed on crowns which were subjected to several loading conditions. A cement film thickness of 0.030 mm was assumed. For all cases, the stresses which developed in porcelain and cement near the facial and lingual margins due to a vertical load of 200 N were predominantly compressive in nature. For the crowns with Ni-Cr copings, the tensile stress in porcelain ranged from 11.0 MPa (for crowns with a facial metal thickness of 0.3 mm) to 12.5 MPa (for a metal thickness of 0.1 mm). The corresponding stresses for crowns with Au-Pd alloy copings were 8.3 MPa and 8.6 MPa, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

对瓷贴面粘结层的有限元应力分析

目的 :对可能影响瓷贴面粘结层应力分布的诸多因素进行分析 ,为临床应用提供理论参考。方法 :用MSC/NASTRAN有限元大型结构分析软件系统建立 6种不同牙体缺损及瓷贴面修复体实验模型 ,并应用二维有限元应力分析法分析 6种瓷贴面修复模型在不同功能负荷条件下粘结层的应力分布规律。结果 :瓷贴面粘结层应力主要集中于切端粘结点及颈部粘结点。在同一条件下切端加载时粘结层应力值最高 ,当载荷从牙切端移向牙颈部时 ,应力值逐渐降低。尤其当加载位于修复体边缘时粘结层压应力值明显升高。常规修复设计模型粘结层各项应力值随加载角度水平加大也有增加 ,剪切应力值可高达 5 1.3 8MPa。当加载条件相同时 ,各相同切端减少组粘结层VonMises应力值变化不大 ,但当切端减少大于 4mm时粘结层拉应力有明显增高。结论 :应重点注意粘结层边缘的处置 ,尤其当修复体边缘位于加载部位时 ,更应注意调整咬合 ,尽量使牙齿在功能活动时与瓷贴面边缘少有接触。在瓷贴面临床设计时应避免选择切端对刃接触 ;在制作及粘结后还应调改好咬合 ,尽量引导患牙沿牙长轴方向受力。在需切端重建的瓷贴面修复设计中无论包绕型设计或对接边缘设计对粘结层应力均无明显影响。但当切端牙体缺损大于 4mm时 ,应慎重挑选粘结剂     

上中切牙全瓷冠切端瓷厚度与应力关系的三维有限元分析

目的　为全瓷冠的临床设计提供依据。方法　采用三维有限元法分析不同切端瓷厚度 (切龈向厚度 )的上中切牙全瓷冠受载的应力状况。结果　全瓷冠唇舌侧颈缘有张应力、压应力集中 ;全瓷冠表面应力 >冠内应力 >制备体应力 ;切端瓷厚度为 1.5mm及 2 .0mm全瓷冠的应力分布相似 ,应力值也非常接近。结论　建议在临床制作前牙全瓷冠时 ,预备切端瓷厚度在 1.5mm以保障全瓷冠的抗折能力 ,而磨除过多的切端牙体组织并不能提高全瓷冠的强度     

Design optimization and evolution of bonded ceramics for the anterior dentition: a finite-element analysis.

OBJECTIVE: Finite-element method was used to explore the stress distribution of incisors restored with porcelain veneers. The design of the incisal palatal finish line was analyzed as a function of incisal overlap and initial tooth substance loss (coronal fractures). METHOD AND MATERIALS: The treatment of intact and fractured incisors was investigated using 8 different designs of porcelain veneer. The palatal finish line varied from butt margins to extended chamfers. The stress distribution was assessed in a 2-dimensional finite-element model, reproducing a buccolingual cross section of an incisor. A palatal 50-N horizontal force was applied to the incisal edge to simulate an extreme functional load. The palatal surface tangential stresses were calculated. RESULTS: Considerable differences were detected in the stresses at the level of the incisal-palatal restoration margin. The margins of restorations with limited incisal overlap (butt margin or minichamfer) showed low tensile stresses or even compressive stresses. Restorations with a long chamfer extending into the palatal concavity were subjected to the highest tensile stresses. In the presence of moderate crown fractures (incisal one third) or severe wear, butt margins limited the palatal extension of ceramic, thus reducing the amount of stress at the restoration interface. In the presence of severe crown fractures (incisal two thirds), the margins (either butt or chamfered) were subjected to low tensile forces when located in the smooth convex area of the cingulum. CONCLUSION: Because of the geometry and natural elastic modulus of mineralized tooth structures, a concentration of tensile stresses is formed at the palatal concavity of teeth restored with porcelain veneers. Long chamfers extending into the palatal concavity are unfavorable because thin extensions of ceramic are generated in an area of maximum tensile stresses. Minichamfers or butt margins are generally recommended, especially in the presence of moderate crown fractures or severe wear.     

A comparison of the depths produced using three different tooth preparation techniques

STATEMENT OF PROBLEM: Inadequate preparation of teeth for metal ceramic crowns can significantly influence the ultimate form and longevity of the definitive restoration, as well as adversely affect the supporting tissues. PURPOSE: The purpose of the study was to determine the effect that 3 different tooth preparation techniques had on an operator's ability to appropriately and consistently prepare teeth for metal ceramic crowns. MATERIAL AND METHODS: Thirty typodont maxillary central incisor teeth were mounted individually and randomly allocated to 3 equal groups (A, B, and C). One operator was asked to prepare each tooth for a metal ceramic crown. A freehand approach was used to prepare the teeth in Group A, which acted as a control. Groups B and C were prepared with the assistance of a silicone index and a suitable depth gauge bur, respectively. A silicone index of the unprepared tooth, into which contrasting silicone was injected to occupy the space created by tooth preparation, was sectioned in the midline. Images of the sectioned index were captured with an optical microscope (resolution +/- 0.02 mm), which was attached to a personal computer. A calibrated image analysis program was used to measure the depth of preparation (in millimeters) at 5 points (labial cervical, mid-labial, incisal, mid-palatal, and palatal cervical) on 2 occasions. These results were pooled and averaged to give a mean labial and palatal preparation depth (in millimeters) and incisal edge reduction. The data were analyzed by use of a 1-way analysis of variance and Scheffe's post hoc statistical test (P<.05). RESULTS: The mean depths of labial and palatal preparation for Groups A, B, and C were 1.28 and 0.47 mm; 1.46 and 0.56 mm; and 1.45 and 0.63 mm, respectively. The difference between the groups' labial preparation depth was not significant (P=.06), but the difference for palatal preparation depth was significant (P=.01). The mean incisal reduction was 3.00 mm for Group A, 2.74 mm for Group B, and 2.13 mm for Group C (P=.00). CONCLUSION: Within the limitations of this study, it was concluded that preparation of teeth for metal ceramic crowns without the use of devices to help gauge reduction depth can result in insufficient labial reduction and incisal overreduction.     

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.     

Influence of resin cement polymerization shrinkage on stresses in porcelain crowns

Objective

The aim of this study was to analyze the influence of polymerization shrinkage of the cement layer on stresses within feldspathic ceramic crowns, using experimentally validated FEA models for (1) increasing occlusal cement thickness; and, (2) bonded versus non-bonded ceramic-cement interfaces.

Methods

2-D axial symmetric models simulated stylized feldspathic crowns (1.5 mm occlusal thickness) cemented with resin-cement layers of 50–500 μm on dentin preparations, being loaded (500 N) or not. Ceramic–cement interface was either bonded or not. Cement was bonded to the dentin in all models. Maximum axial shrinkage of 0%, 1%, 2%, 3%, 4% and 4.65% were simulated. The first principal stresses developing in the cementation surface at the center and at the occluso-axial line-angle of the crown were registered.

Results

Polymerization shrinkage of the cement increased tensile stresses in the ceramic, especially in loaded non-bonded crowns for thicker cement layers. Stresses in loaded non-bonded crowns increased as much as 87% when cement shrinkage increased from 0% to 4.65% (100–187 MPa), for a 500 μm-thick cement. Increasing polymerization shrinkage strain raised the tensile stresses, especially at the internal occlusal-axial line-angle, for bonded crowns.

Significance

Changes in the polymerization shrinkage strain (from 0% to 4.65%) have little effect on the tensile stresses generated at the cementation surface of the ceramic crowns, when the occlusal cement thickness is thin (approx. 50 μm for bonded crowns). However, as the cement becomes thicker stresses within the ceramic become significant.     

The influence of design parameters on the FEA-determined stress distribution in CAD-CAM produced all-ceramic dental crowns.

INTRODUCTION: Knowledge of factors, which influence stress and its distribution is of key importance to the successful production of durable all-ceramic restorations. The objective of this study was to evaluate, by finite element analysis (FEA), the influence of the shape of the preparation and the cement layer on the stress distribution in CAD-CAM produced all-ceramic crowns and in their cement layer. MATERIAL AND METHODS: The CAD models of multi-layer all-ceramic crowns for posterior tooth 46 of three patients produced with CAD-CAM-technology were translated into a three-dimensional FEA program. The stress distribution due to the combined influences of bite forces, residual stresses caused by the difference in expansion coefficient of the two ceramic layers, and the influence of shrinkage of the cement was investigated. RESULTS: The tensile stresses in the crown for the chamfer knife-edge preparation might put the integrity of the currently available ceramic materials at risk, while a non-uniform cement layer might result in stresses exceeding the bond strength. It was concluded that for long lasting restorations in the posterior region it is advisable to make a chamfer with collar preparation, the cement layer as uniform, and the difference in thermal expansion for the two ceramics as small as possible. SIGNIFICANCE: This study indicates that for full ceramic crowns in the posterior region, specific design rules should be followed, and that FEA utilizing CAD-CAM data can be a successful tool to develop design guidelines for all-ceramic restorations.     

Influence of metal thickness on stress distribution in metal-ceramic crowns

The objective of this study was to calculate the stress distribution induced in anterior metal-ceramic crowns fabricated with either gold-alloy or nickel-alloy copings of reduced thickness using plane stress analyses. Two-dimensional finite element models of three crown designs were subjected to a simulated biting force of 200 N which was distributed over porcelain near the lingual metal-ceramic junction. Based on plane stress analyses, the maximum tensile and compressive stresses in porcelain for the three cases were 29.5 MPa and 123.1 MPa, respectively. The highest tensile strains in porcelain for veneered Ni-Cr and Au-Pd copings with conventional dimensions were 0.016% and 0.014%, respectively. The maximum stresses and strains in porcelain for the crowns with a conventional coping thickness (0.3 mm) and a reduced coping thickness (0.1 mm) were not significantly different. All values were below the critical failure values of porcelain.     

Retention of zirconium oxide ceramic crowns with three types of cement

STATEMENT OF PROBLEM: Information about the retentive strength of luting agents for zirconium oxide-based crowns is limited. It is unknown if this type of high-strength ceramic restoration requires adhesive cementation to enhance retention. PURPOSE: The purpose of this in vitro study was to determine the ability of selected luting agents to retain a representative zirconium oxide ceramic crown under clinically simulated conditions. MATERIAL AND METHODS: Recently extracted human molars were prepared with a flat occlusal surface, 20-degree taper, and approximately 4-mm axial length. The axial and occlusal surface areas were determined, and specimens were distributed equally by total surface area into 3 cementation groups (n=12). Zirconium oxide ceramic copings (Procera AllZirkon) with an occlusal bar to facilitate removal were fabricated using computer-aided design/computer-assisted manufacturing (CAD/CAM) technology. All copings were airborne-particle abraded with 50-mum Al(2)O(3) and then cleaned in an ultrasonic bath with isopropyl alcohol. Provisional cement was removed from the prepared teeth, followed by a pumice prophy. After trial insertion, the copings were cleaned with phosphoric acid, rinsed, dried, and dehydrated with isopropyl alcohol. They were then cemented with a seating force of 10 kg per tooth, using either a composite resin cement with adhesive agent (Panavia F 2.0 and ED Primer A & B [PAN]), a resin-modified glass ionomer cement (Rely X Luting [RXL]), or a self-adhesive modified composite resin (Rely X Unicem [RXU]). The cemented copings were thermal cycled at 5 degrees C and 55 degrees C for 5000 cycles with a 15 second dwell time, and then removed along the path of insertion using a universal testing machine at 0.5 mm/min. The removal force was recorded, and the stress of dislodgement was calculated using the surface area of each preparation. A 1-way analysis of variance was used to analyze the data (alpha=.05). The nature of failure was also recorded. RESULTS: Mean dislodgement stresses were 5.1, 6.1, and 5.0 MPa for PAN, RXL, and RXU, respectively. The 1-way analysis of variance revealed no differences in mean crown removal stress among the 3 cementation groups. The predominant mode of failure was cement remaining principally on the zirconium oxide copings in 46% of the specimens, followed by cement found on the tooth in 25.7% of the specimens. CONCLUSIONS: Within the limitations of this study, the 3 luting agents, with mean removal stresses ranging from 5.0 to 6.1 MPa were not significantly different. The use of a composite resin cement with a bonding agent did not yield higher coping retention compared to the other 2 cements tested.     

氧化铝渗透陶瓷底瓷全瓷冠2年临床观察

目的 探讨氧化铝渗透陶瓷作为全瓷修复体底瓷的修复效果,为临床推广应用提供参考。方法 61例门诊患者,修复体159件,其中上颌98件,下颌61件。前牙全冠132个,后牙全冠27个。通过12～24个月的观察,采用美国加利福尼亚牙科学会的质量评价体系对氧化铝渗透陶瓷作为底瓷所做的前、后牙全冠修复进行临床效果评价。结果 对61例患者159颗全冠的临床观察结果 表明,氧化铝渗透陶瓷作为底瓷所制作的全瓷修复体具有良好的的边缘密合性,色泽稳定,强度高,其失败率为1.25%,效果满意。结论 氧化铝渗透陶瓷作为底瓷的全瓷冠修复是一种效果良好的全瓷修复方式。     

Colour measurements of all ceramic crown systems

The objectives of this study were: (i) to determine variability among colour parameters of five different ceramic crown systems; and (ii) to measure the effect of using coloured luting agents on restoration colour. The crown systems studied were Cerestore, Dicor, Hi-Ceram, Renaissance, and Vitadur-N. Five crowns for each system were made according to manufacturer's instructions with the same nominal shade (Vita Lumin Vacuum A2) to fit an Ivorine central incisor tooth. Restoration thickness was adjusted to within +/- 0.1 mm (+/- 0.05 mm in the mid-facial area where colour measurements were to be made) with the aid of a dial calliper prior to glazing or, in the case of Dicor, surface staining. Where a core was part of the system this was fabricated to the minimum recommended thickness. The crowns were cemented using luting agents of five different colours in a randomly chosen sequence. The colour of each restoration/cement combination was measured three times using a small-area colorimeter (Minolta CR-121). The variance of each colour parameter (L*, a*, b*) was statistically compared for each crown system using an analysis of variance procedure, as was the effect of the cement. Observed differences were related to visual perception by using the colour difference formula. There were statistically significant differences among the variances of the crown systems and the cements, with significant interactions between crown systems and direction of colour and between cement and direction of colour. Restorations made with different ceramic crown systems had noticeably different colour despite having the same nominal shade. Changing the shade of the luting agent had a perceivable effect on Dicor crowns and, to a lesser extent, on Vitadur-N crowns but not on the other systems due, presumably, to the opacity of their core materials.     

Finite Element Stress Analysis of Diastema Closure with Ceramic Laminate Veneers

Purpose: The purpose of this study was to use finite element stress analysis to examine the relative importance of variables such as porcelain laminate veneer (PLV) extensions, loading angle, and loading level for the case of feldspathic ceramic veneering of teeth to manage diastema. Materials and Methods: A 3D maxillary central incisor including its internal anatomy and morphology was constructed with ANSYS software for different extensions of PLV. Internal boundaries defining the dentinoenamel junction, the pulp‐dentinal junction, the interface between the enamel‐luting cement, and the porcelain‐luting cement were well defined. The von Mises stresses distribution and stress intensity were analyzed on the free extension of PLV for varying extensions, various angulations (0°, 30°, and 60°) on the incisal edge, and for different loading levels (50, 150, and 250 N). Results: The numerical values of stress were recorded. A significant difference in stress was observed. Increased stresses occurred with increased extensions, angulations, and loading levels. At 0° angulation, compressive stresses were visualized in finite element analysis for various magnitudes of force. Higher stress values of 182 MPa and 211 MPa were obtained for the 2.5‐mm extension in the mesial surface and in both proximal surfaces for 0° angulation at 250 N magnitude of force. The stress occurring at 30° and 60° angulations was the combination of compressive and tensile stress. Higher values of 261 MPa and 232 MPa were observed when forces were applied on the mesial extension of the PLV and on both the proximal surfaces for 2.5 mm at 30°, 250 N magnitude of force. A maximum stress value of 507 MPa was observed when PLV were increased in mesial width by 2.5 mm for 60° angulation at 250 N magnitude of force. Conclusion: The extensions of PLV in diastema closure have more of an esthetic than functional consideration, but critical factors such as angulations and the loading level acting on the free extension of PLV are important.     

(牙合)面厚度与肩台宽度连续变化的下颌第一磨牙全瓷冠三维有限元分析

目的 探讨(牙合)面厚度和肩台宽度同时连续变化情况下下颌第一磨牙全瓷冠的优化设计,以期为全瓷冠的临床设计提供理论基础.方法 运用逆向工程及计算机辅助设计技术建立(牙合)面厚度和肩台宽度连续变化的下颌第一磨牙三维有限元模型,设定(牙合)面厚度变化范围为1～3 mm,肩台宽度变化范围为0.4～1.2 mm,并在全瓷冠颊尖垂直向(牙合)面加载225 N的力,观察验面厚度和肩台宽度同时变化对全瓷冠、粘接剂、基牙、牙周膜和牙槽骨等效应力峰值的影响,同时进行灵敏度分析.结果 随着(牙合)面厚度和肩台宽度的增加,除牙周膜和牙槽骨的等效应力峰值无明显变化外,全瓷冠、粘接剂和基牙的等效应力峰值均下降.当(牙合)面厚度≥1.87 mm、肩台宽度≥0.66 mm时,全瓷冠、粘接剂和基牙的应力响应曲线斜率均位于-1～1之间,即在此区间内等效应力峰值变化相对较小.灵敏度分析显示,肩台宽度对全瓷冠和基牙等效应力峰值的影响较大,(牙合)面厚度对粘接剂等效应力峰值的影响较大.结论 在本项研究所设定的参数范围内,下颌第一磨牙全瓷冠的(牙合)面厚度应不小于1.87 mm,肩台宽度应不小于0.66 mm.     

(牙合)面厚度与肩台宽度连续变化的下颌第一磨牙全瓷冠三维有限元分析

目的 探讨(牙合)面厚度和肩台宽度同时连续变化情况下下颌第一磨牙全瓷冠的优化设计,以期为全瓷冠的临床设计提供理论基础.方法 运用逆向工程及计算机辅助设计技术建立(牙合)面厚度和肩台宽度连续变化的下颌第一磨牙三维有限元模型,设定(牙合)面厚度变化范围为1～3 mm,肩台宽度变化范围为0.4～1.2 mm,并在全瓷冠颊尖垂直向(牙合)面加载225 N的力,观察验面厚度和肩台宽度同时变化对全瓷冠、粘接剂、基牙、牙周膜和牙槽骨等效应力峰值的影响,同时进行灵敏度分析.结果 随着(牙合)面厚度和肩台宽度的增加,除牙周膜和牙槽骨的等效应力峰值无明显变化外,全瓷冠、粘接剂和基牙的等效应力峰值均下降.当(牙合)面厚度≥1.87 mm、肩台宽度≥0.66 mm时,全瓷冠、粘接剂和基牙的应力响应曲线斜率均位于-1～1之间,即在此区间内等效应力峰值变化相对较小.灵敏度分析显示,肩台宽度对全瓷冠和基牙等效应力峰值的影响较大,(牙合)面厚度对粘接剂等效应力峰值的影响较大.结论 在本项研究所设定的参数范围内,下颌第一磨牙全瓷冠的(牙合)面厚度应不小于1.87 mm,肩台宽度应不小于0.66 mm.

Abstract：

Objective To investigate the effect of different occlusal thickness and shoulder finish line depth on stress distribution of all-ceramic crowns and to select optimal occlusal thickness and shoulder finish line depth using continuous variation of parameters. Methods This analysis was performed using mandibular first molar finite element modl. The range of occlusal thickness was set from 1 mm to 3 mm, and that of shoulder finish line depth was from 0. 4 mm to 1.2 mm. Load of 225 N was applied perpendicularly to the occlusal surface of the tooth at all buccal cusps to simulate functional occlusal force. The maximum equivalent stresses in crown, cement layer, abutment, periodontal ligament, alveolar bone were calculated, and the sensitivities of stresses to the variables were also evaluated. Results The maximum equivalent stresses in crown, cement layer and abutment decreased as occlusal thickness and shoulder finish line depth were increased, while no obvious change were found in maximum equivalent stresses in periodontal ligament and alveolar bone. When occlusal thickness exceeded 1.87 mm and shoulder finish line depth exceeded 0. 66 mm,the tangent slope rate of the maximum equivalent stress response curves ranged from - 1 to 1. Data indicated that occlusal thickness played a more important role in reducing maximum equivalent stress in cement layer than finish line depth did, and shoulder finish line depth was a more effective parameter in reducing maximum equivalent stress in crown and abutment than occlusal thickness was. Conclusions Occlusal thickness exceeding 1.87 mm and shoulder finish line depth exceeding 0. 66 mm are optimal design for ceramic crown on mandibular first molar from biomechanical point of view.     

Numerical investigation of macro- and micro-mechanics of a ceramic veneer bonded with various cement thicknesses using the typical and submodeling finite element approaches

Objectives

This study investigates the influence of cement thickness on the macro- and micro-mechanical responses in a ceramic veneer adjacent to an incisal overlapped incisor.

Methods

Seven finite element (FE) ceramic veneer macro-models with different cement thicknesses (10–180 μm) were generated. A 10 N load was applied with an angulation of 60° to the longitudinal tooth axis. Seven FE micro-models corresponding to the macro-models were constructed at an enamel–adhesive interface where the stress concentration was found. Based on an interfacial scanning electron microscope (SEM) micrograph, morphology of resin tags in the micro-models was generated. The micro-model boundary conditions were determined from the macro-model results. The principal stress on each node in the macro- and micro-models was calculated to investigate interfacial mechanics. A tensile test was performed to obtain an ultimate cement tensile strength to determine the material failure parameters.

Results

The highest stress concentration within the cement was found at the resin tag base of the enamel–adhesive interface in lingual side. Maximum stress values from 10.6 to 14.7 MPa for the micro-models were higher (44–48%) than that from 7.2 to 10.0 MPa for the macro-models when the cement layers increased. Based on the ultimate tensile strength (11.8 MPa), bonding failure could found when the micro-models with the cement layers presented more than about 50 μm. This seems to correspond with data from previous studies.

Conclusions

Higher stresses develop in the adhesive as the cement thickness increases. Cement thicknesses less than 50 μm might reduce the adhesive bonding failure.     

Optimization of resilience and stress distribution in porcelain veneers for the treatment of crown-fractured incisors

The present study was conducted to define, when restoring extensive loss of dentin, the configuration of the restoration that will best reproduce the biomechanical properties of the intact original tooth in terms of resilience and stress distribution. The treatment of 1/3-crown fractures and 2/3-crown fractures was investigated using different designs of facial porcelain veneers with and without underlying composite buildup. The stress distribution and tooth compliance were assessed in a numeric model reproducing a 2-dimensional buccolingual cross section of an incisor. A 50-N facial force was applied to simulate an incisal impact situation. The facial surface tangential stresses were calculated, and the maximum displacement (horizontal direction) at the most incisal node of the enamel surface was also recorded and used to calculate the tooth compliance (i.e., displacement/load or resilience) for each test condition. Tensile stresses were generated on the facial surface of the porcelain laminates with a similar pattern for all test conditions, the cervical part of the crown being the most quiescent area. Substantial differences appeared in the incisal half of the crown, the lowest stresses being observed for extensively fractured teeth restored without composite buildup (facial peaks at approximately 33 MPa). Fractured teeth restored with minimal veneers and a "dentin-like" composite buildup showed stress patterns similar to the intact tooth (facial peaks at approximately 50 MPa). The natural tooth gave the highest tooth compliance or flexibility. All restorative designs featured increased tooth stiffness. However, the original tooth compliance was almost restored when composite was used to replace the missing dentin, with the porcelain acting only as a facial and incisal enamel substitute. When restoring crown-fractured incisors, tooth compliance and stress distribution can be modulated by the combination of composite and ceramics. Optimized configurations can be reached to reproduce the original biomechanical behavior of the intact tooth. The use of ceramic alone generates low stress concentrations, but also less compliant restored teeth.