Thermal gradients and residual stresses in veneered Y-TZP frameworks

Objectives

The occurrence of “chipping” of all-ceramic restorations with Y-TZP frameworks has resulted in various designs and cooling procedures recommended for reducing such behavior. In this paper the temperature gradients during fast and slow cooling for conventional and anatomical designs are compared as well as an optical procedure to directly compare the influence of cooling rate on residual stress.

Methods

This investigation quantifies the temperature gradients between the inner and outer surfaces of crowns measured with thermocouples during two different cooling methods with uniform and anatomical frameworks. In the first method the crown was removed from the furnace after commencement of cooling whereas for the second method the crown was cooled to the glass transition temperature (600 °C) before removal. Direct observation of the residual stresses was made with an optical polarimeter and thin slices of veneered copings.

Results

This study observed that slow cooling decreases the temperature differences but still differences of up to 88 °C were observed. For the fast cooled crown, temperature differences of more than 100 °C for the uniform and 140 °C for the anatomical framework at temperatures above the glass transition temperature were recorded. Optical polarimeter observations indicated much lower stresses within the porcelain layer upon cooling by removing the crown below the glass transition temperature.

Conclusion

Slow cooling during the final veneering of dental restorations with zirconia frameworks reduces the temperature gradients and residual stresses within the porcelain layer, which represent one possible cause for chipping. An anatomical designed framework did not show the same reduction extent.     

饰瓷工艺对氧化锆陶瓷颜色和透光性的影响

目的 测定采用不同饰瓷工艺制作的氧化锆试件的色彩参数和透射比,探讨饰瓷工艺对氧化锆全瓷修复体颜色和透光性的影响.方法 制作15片厚度为(0.50±0.01)mm、直径为10 mm的氧化锆圆盘试件,表面烧结0.10 mm结合层瓷.按随机排列表法将试件分为3组(ZP、ZC和ZPC组),每组5片.ZP组采用直接热压铸技术压铸0.60 mm厚饰瓷层;ZC组采用涂层技术涂塑烧结0.60 mm厚饰瓷层;ZPC组先压铸0.30 mm厚饰瓷层,再涂塑烧结0.30 mm厚饰瓷层.用电脑比色仪和分光光度计测定饰瓷前后试件的色彩参数L*、a*、b*值和可见光积分透射比τ,计算色饱和度C*ab和色差ΔE,进行单因素方差分析(α=0.05)和Turkey's多重检验.测定A2色标准比色片的色彩参数,计算各组与比色片的色差.结果 ZP、ZPC、ZC 3组的a*值分别为-1.35±0.07、-0.64±0.06、-0.36±0.05,差异有统计学意义(P＜0.05);b*值分别为27.01±0.07、25.48±0.11、23.28±0.25,差异有统计学意义(P＜0.05);C*ab值分别为27.04±0.08、25.49±0.11、23.28±0.25,差异有统计学意义(P＜0.05).3组中ZP组的L*值和τ值最大,分别为87.53±0.48和1.64±0.03;ZPC组的L*值和τ值最小,分别为82.14±0.18和1.47±0.01(P＜0.05).ZC组与比色片的色差最小ΔE=1.04,ZP组与比色片的色差最大AE=4.86.结论 热压铸技术饰瓷的氧化锆陶瓷明度较高、透光性较好;热压铸后涂层技术饰瓷的氧化锆陶瓷明度较低、透光性较差;涂层技术饰瓷的氧化锆陶瓷与比色片的色差最小,能准确再现比色结果.     

Effects of core characters and veneering technique on biaxial flexural strength in porcelain fused to metal and porcelain veneered zirconia

PURPOSE

The purpose of this study was to assess the impact of the core materials, thickness and fabrication methods of veneering porcelain on prosthesis fracture in the porcelain fused to metal and the porcelain veneered zirconia.

MATERIALS AND METHODS

Forty nickel-chrome alloy cores and 40 zirconia cores were made. Half of each core group was 0.5 mm-in thickness and the other half was 1.0 mm-in thickness. Thus, there were four groups with 20 cores/group. Each group was divided into two subgroups with two different veneering methods (conventional powder/liquid layering technique and the heat-pressing technique). Tensile strength was measured using the biaxial flexural strength test based on the ISO standard 6872:2008 and Weibull analysis was conducted. Factors influencing fracture strength were analyzed through three-way ANOVA (α≤.05) and the influence of core thickness and veneering method in each core materials was assessed using two-way ANOVA (α≤.05).

RESULTS

The biaxial flexural strength test showed that the fabrication method of veneering porcelain has the largest impact on the fracture strength followed by the core thickness and the core material. In the metal groups, both the core thickness and the fabrication method of the veneering porcelain significantly influenced on the fracture strength, while only the fabrication method affected the fracture strength in the zirconia groups.

CONCLUSION

The fabrication method is more influential to the strength of a prosthesis compared to the core character determined by material and thickness of the core.     

Influence of firing time and framework thickness on veneered Y-TZP discs curvature

Objectives

The objective of the present work was to study the curvature of very thinly, veneered Y-TZP discs of different framework thicknesses submitted to different firing times.

Methods

Fifteen 20-mm-wide Y-TZP discs were produced in three different thicknesses: 0.75, 1, 1.5 mm. One disc from each group was left unveneered while the others were layered with a 0.1 mm veneering ceramic layer. All discs underwent five firing cycles for a total cumulative firing time of 30 min, 1, 2, 5 and 10 h at 900 °C. The curvature profile was measured using a profilometer after the veneering process and after each firing cycle respectively. A fitted curve was then used to estimate the, curvature radius. The coefficient of thermal expansion (CTE) measurements were taken on veneering, ceramic and Y-TZP beam samples that underwent the same firing schedule. Those data were used to calculate the curvature generated by CTE variations over firing time.

Results

All bilayered samples exhibited a curvature that increased over firing time inversely to framework thickness. However non-veneered samples did not exhibit any curvature modification.

Significance

The results of the present study reveal that even a very thin veneer layer (0.1 mm) can induce a significant curvature of Y-TZP discs. The dilatometric results showed that Tg and CTE, variations are not sufficient to explain this curvature. A chemical-induced zirconia volume, augmentation located at the framework sub-surface near the interface could explain the sample, curvature and its increase with firing time.     

饰面瓷厚度对氧化锆全瓷冠抗压缩破坏力的影响

目的：研究饰面瓷厚度对氧化锆全瓷冠抗压缩破坏力的影响。方法：常规 CAD-CAM制作氧化锆筒状基底冠25个,厚度均为0．5 mm,随机分为 A、B、C、D、E 5组,饰面瓷厚度依次为0．5、1．0、1．5、2．0、2．5 mm。万能材料试验机测试各组试件抗压缩破坏力值。采用 SPSS13．0统计软件对测试结果进行统计分析。用扫描电镜（SEM）观察试件断裂面微观结构。结果：A ～E 组试件抗压缩破坏力值依次为：（1279．96±42．85）N、（2235．44±50．14）N、（2216．38±48．97）N、（2169．22±60．40）N、（2028．70±47．37）N。A 组、E 组氧化锆全瓷修复体的抗压缩破坏力值均低于其他3组（P ＜0．01）;其余组间抗压缩破坏力值无统计学差异（P ＞0．05）。扫描电子显微镜观察发现,A 组、E 组氧化锆全瓷修复体基底瓷－饰面瓷结合界面处不平坦,较为疏松;饰瓷层出现气孔及微裂纹增多、增大。结论：饰面瓷厚度低于1．0 mm 及高于2．0 mm 时,修复体的微观结构可能会受到影响,从而氧化锆全瓷冠整体抗压缩破坏力的性能受影响,建议临床应用时严格控制饰面瓷厚度。     

Effect of autoclave induced low-temperature degradation on the adhesion energy between yttria-stabilized zirconia veneered with porcelain

Objective

To investigate the effect of autoclave induced low-temperature degradation on the adhesion energy between yttria-stabilized zirconia veneered with porcelain.

Methods

The strain energy release rate using a four-point bending stable fracture test was evaluated for two different porcelains [leucite containing (VM9) and glass (Zirox) porcelain] veneered to zirconia. Prior to veneering the zirconia had been subjected to 0 (control), 1, 5, 10 and 20 autoclave cycles. The specimens were manufactured to a total bi-layer dimension of 30 mm × 8 mm × 3 mm. Subsequent scanning electron microscopy/energy dispersive spectrometry, electron backscatter diffraction and X-ray diffraction analysis were performed to identify the phase transformation and fracture behavior.

Results

The strain energy release rate for debonding of the VM9 specimens were significantly higher (p < 0.05) compared to the Zirox specimens across all test groups. Increasing autoclave cycles lowered the strain energy release rate significantly (p < 0.05) from 18.67 J/m² (control) to the lowest of 12.79 J/m² (cycle 10) for only the VM9 specimens. SEM analyses showed predominant cohesive fracture within the porcelain for all cycle groups. XRD analysis of the substrate prior to veneering confirmed a tetragonal to monoclinic phase transformation with increasing the number of autoclave cycles between 5 and 20. The monoclinic phase reverted back to tetragonal phase after undergoing conventional porcelain firing cycles. EBSD data showed significant changes of the grain size distribution between the control and autoclaved specimen (cycle 20).

Significance

Increasing autoclave cycles only significantly decreased the adhesion of the VM9 layered specimens. In addition, a conventional porcelain firing schedule completely reverted the monoclinic phase back to tetragonal.     

Load-bearing capacity and failure types of anterior zirconia crowns veneered with overpressing and layering techniques

Objectives

The aim of this study was to test whether the load bearing capacity of anterior zirconia crowns veneered with overpressed or layered, is similar and to evaluate the failure types.

Methods

Standardized zirconia frameworks were fabricated and randomly divided into 8 groups (N = 120, n = 15 per test group). Four groups were veneered with one of the layered veneering porcelains: Zirox, GC Initial ZR, VITA VM9 or IPS e.max Ceram and the other four groups were veneered with overpressed veneering porcelains: PressX Zr, GC Initial LF, VITA PM9 or IPS e.max ZirPress. The crowns were cemented on their corresponding CoCr abutment and the specimens were loaded at an angle of 45° in a Universal Testing Machine to determine the fracture load. Data were analyzed using one-way and two-way ANOVA, followed by a post hoc Scheffé test, t-test and Weibull analysis (alpha = 0.05).

Results

Within three manufacturers of veneering porcelain, fracture load values were not statistically significant between overpressed and layered porcelain systems. Within one manufacturer of veneering porcelain, the overpressed crowns (IPS e.max ZirPress: 1519 ± 334 N) demonstrated significantly higher (p < 0.05) fracture load than that of the layered one (IPS e.max Ceram: 894 ± 160 N). Except with IPS e.max ZirPress, where exclusively only chipping of the veneering porcelain was observed, all other porcelain systems showed predominantly framework fractures together with fracture of the veneering porcelain.

Conclusion

Overpressed veneering porcelains for zirconia frameworks exhibited similar or better fracture load compared with layered ones.     

Influence of thermal gradients on stress state of veneered restorations

Objectives

To assess transient and residual stresses within the porcelain of veneered restorations (zirconia and metal) as a result of cooling rate and porcelain thickness.

Methods

Porcelain-on-zirconia (PZ) and porcelain-fused-to-metal (PFM) crowns were fabricated with 1 or 2 mm of porcelain. Thermocouples were attached both internally and externally to the crowns to record transient temperatures. For fast cooling, the furnace was opened after the holding time and switched off. Slow cooling was accomplished by opening the furnace at 50 °C below the glass transition temperature (T_g) of the material. An axially symmetric FEA model simulated thermal stresses. Time-dependent temperature equations from thermocouple readings were set as boundary conditions. Framework materials and the porcelain below T_g were considered to behave elastically. Visco-elastic behavior was assumed for porcelain above the T_g modeling properties as dependent on cooling rate.

Results

Differences in residual stress were found for fast and slow cooled PZ and PFM crowns. Significant transient stress waves were observed within the porcelain when fast cooling through T_g. They are believed to be related to non-uniform volumetric changes originated from thermal gradients. Results were confirmed by modeling and physical testing of crowns containing a defect.

Significance

Residual stresses do not distinguish PZ from PFM. High magnitude transient stresses observed within the porcelain during fast cooling may explain clinical fractures involving internal defects. Stress waves may also originate internal micro-cracking which could grow under function. Therefore, slow cooling, especially for all-ceramic crowns with thick porcelain, is important to prevent thermal gradients and high-magnitude transient stresses.     

Fracture resistance of porcelain veneered zirconia crowns with exposed lingual zirconia for anterior teeth after thermal cycling: An in vitro study

Statement of problem

In some clinical conditions minimally invasive complete crown tooth preparations are indicated. This is especially true when gross removal of tooth structure would weaken the remaining tooth or violate the vitality of the dental pulp.

Objective

The purpose of this study was to investigate the influence of (1) exposed lingual zirconia with veneered zirconia crowns, and (2) reduced lingual thickness of monolithic lithium disilicate crowns on the fracture resistance of the crowns after cyclic loading. Metal-ceramic crowns with exposed lingual metal served as controls.

Materials and methods

Twenty-four maxillary central incisor crowns were fabricated in identical shape on metal testing dies in 3 groups: metal-ceramic crowns (MC, n = 8), veneered zirconia crowns (VZ, n = 8), and monolithic lithium disilicate crowns (MO, n = 8). A conservative preparation design with 0.75 mm lingual clearance was used for each crown system. All crowns were cemented to their corresponding crown preparations with self-adhesive resin cement (Multilink Automix). The crowns were subjected to 1000 cycles of thermal cycling, then cyclic loading of 111 N by means of a stainless steel ball, and 50,000 cycles of loading were applied for the fatigue test. Fatigue loading was followed by a continuously increasing compressive load, at a crosshead speed of 1 mm/min until failure. The compressive load (N) required to cause failure was recorded. Means were calculated and analyzed with one-way ANOVA and the Tukey HSD test (α = .05).

Results

There was a significant difference between MO vs. MC (P = .0001), MO vs. VZ (P = .0001), and VZ vs. MC (P = .012).

Conclusions

There was a significant difference in the mean fracture resistance of MC, VZ, and MO crowns in this in vitro study. The MC group recorded the highest mean fracture strength.     

Effect of different veneering techniques on the fracture strength of metal and zirconia frameworks

PURPOSE

To determine whether the fracture strengths and failure types differed between metal and zirconia frameworks veneered with pressable or layering ceramics.

MATERIALS AND METHODS

A phantom molar tooth was prepared and duplicated in 40 cobalt-chromium abutments. Twenty metal (IPS d.SIGN 15, Ivoclar, Vivadent, Schaan, Liechtenstein) and 20 zirconia (IPS e.max ZirCAD, Ivoclar) frameworks were fabricated on the abutments. Each framework group was randomly divided into 2 subgroups according to the veneering material: pressable and layering ceramics (n=10). Forty molar crowns were fabricated, cemented onto the corresponding abutments and then thermocycled (5-55℃, 10,000 cycles). A load was applied in a universal testing machine until a fracture occurred on the crowns. In addition, failure types were examined using a stereomicroscope. Fracture load data were analyzed using one-way ANOVA and Tukey HSD post-hoc tests at a significance level of 0.05.

RESULTS

The highest strength value was seen in metal-pressable (MP) group, whereas zirconia-pressable (ZP) group exhibited the lowest one. Moreover, group MP showed significantly higher fracture loads than group ZP (P=.015) and zirconia-layering (ZL) (P=.038) group. No significant difference in fracture strength was detected between groups MP and ML, and groups ZP and ZL (P>.05). Predominant fracture types were cohesive for metal groups and adhesive for zirconia groups.

CONCLUSION

Fracture strength of a restoration with a metal or a zirconia framework was independent of the veneering techniques. However, the pressing technique over metal frameworks resisted significantly higher fracture loads than zirconia frameworks.     

贵金属烤瓷与氧化锆全瓷修复对种植体周围组织影响的临床比较

目的评价贵金属烤瓷、氧化锆全瓷单冠修复对种植体周围组织的影响。方法选择2008年6月—2010年12月间完成种植单冠修复的患者78例共120颗种植体,贵金属烤瓷修复与氧化锆全瓷修复各60颗。分别于修复后12个月复诊,检测种植体周围边缘骨吸收量(MBL)、牙周探诊深度(PD)、改良菌斑指数(mPLI)、改良龈沟出血指数(mSBI)、龈乳头指数等临床指标,以及调查患者对种植义齿的满意度。结果贵金属烤瓷与氧化锆全瓷修复在MBL、PD、mPLI、mSBI、龈乳头指数以及患者满意度方面的差异均无统计学意义(P>0.05)。结论贵金属烤瓷与氧化锆全瓷修复对种植体周围组织的影响无差异,均能取得较好的临床修复效果,长期效果有待于进一步观察。     

Effects of multiple firings on the mechanical properties and microstructure of veneering ceramics for zirconia frameworks

Objectives

The purpose of this study is to evaluate the effects of multiple firings on the mechanical properties and microstructure of veneering ceramics used with zirconia frameworks.

Methods

Five different veneering ceramics for zirconia frameworks were used: Vintage ZR (ZR), Cerabien ZR (CZR), Vita VM9 (VM9), Cercon ceram KISS (KISS), IPS e.max ceram (e.max), and one veneering ceramic used for PFM frameworks: Vintage MP (MP). Twenty specimens were fabricated of each veneering ceramic. Ten specimens were fired twice and another ten specimens were fired ten times. Three-point flexural strength following the ISO 6872 and Vickers hardness were measured, and fracture toughness (K_IC) was calculated. Density and porosity were determined. Specimens were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM).

Results

For all materials, density increased and porosity decreased after 10 firings. Significant differences in density and porosity were observed between 2 and 10 firings, with the exception of VM9 (P < 0.05). There were no significant differences in flexural strength between 2 and 10 firings except for MP. The Vickers hardness of ZR, VM9, KISS and MP increased significantly after 10 firings (P < 0.001). There were no significant differences in fracture toughness for ZR, CZR, VM9 and MP between 2 and 10 firings. However, e.max underwent a significant increase in fracture toughness (P = 0.000), and there was a significant decrease in the toughness of KISS after 10 firings (P = 0.007).

Conclusion

Multiple firings could be effective for improving the densification and the hardness of veneering ceramics for zirconia restorations.

Clinical significance

By 10 firings, the density and hardness of the veneering ceramics used with zirconia frameworks were raised, and porosity was reduced. However, no significant changes occurred in flexural strength, fracture toughness or microstructure.     

Influence of residual thermal stresses on the edge chipping resistance of PFM and veneered zirconia structures: Experimental and FEA study

Objective

Chipping fractures of the veneering porcelain are frequently reported for veneered all-ceramic crowns. In the present study, the edge chipping test is used to measure the toughness and the edge chipping resistance of veneered zirconia and porcelain-fused-to-metal (PFM). The aim is to describe an edge chipping method developed with the use of a universal testing machine and to verify the accuracy of this method to determine the influence of residual thermal stresses on the chipping fracture resistance of veneering porcelain. A finite element analysis (FEA) was used to study the residual stress profiles within the veneering porcelain.

牙科用氧化锆增韧纳米复相铝瓷粉体的制备与性能研究

目的 研制牙科用氧化锆增韧纳米复相铝瓷粉体并测试其理化性能，探讨影响铝瓷粉体性能的制备工艺。方法 表面诱导沉淀法制备纳米复相铝瓷粉体，记录分散剂加入量及pH值对氧化铝悬浮液沉降物体积的影响；观察超声时间对粉体团聚的影响；X射线衍射分析添加稳定剂前后粉体晶相的组成，扫描电子显微镜下观察其形貌。结果 pH=9、分散剂加入量wt=0．2％～0．3％时，悬浮液分散性好；超声处理后粉体粒径下降；稳定剂处理后复相铝瓷粉体含有单斜相和四方相氧化锆。在原始颗粒较大的氧化铝表面均匀分布着直径约80～100nm的氧化锆细颗粒。结论 控制制备工艺，采用表面诱导沉淀法可得到粒度细、分散好的氧化锆增韧纳米复相铝瓷粉体；经稳定剂处理后，复相铝瓷中的氧化锆成为部分稳定氧化锆。     

不同表面处理方法对氧化锆支架材料与Vitadur alpha瓷结合性能的影响

目的：研究不同表面处理方法对氧化锆支架材料与Vitadur alpha瓷结合情况的影响,选择适用于临床的表面处理方法。方法：选用Vitadur alpha瓷,通过粉浆涂塑法与成品氧化锆支架材料进行烧结,通过肉眼观察、扫描电镜、能谱分析、热冲击实验等方法,研究其烧结结合情况,测试界面的结合剪切强度,并对结果进行方差分析。结果：氧化锆陶瓷支架材料完全烧结后,与Vitadur alpha瓷能达到良好的烧结效果,能谱分析结果提示两者存在化学结合。不同表面处理方法对Vitadur alpha瓷与氧化锆陶瓷支架材料界面结合剪切强度有影响：其中水砂纸细磨组抗剪切强度（35．1662MPa）最高,机械喷砂组（27．8283MPa）优于末处理组（20．9677MPa）,车针粗磨组抗剪切强度（15．3025MPa）最低,各组结果间有显著性差异（P〈0．01）。结论：结合临床实际．机械喷砂处理后,Vitadur alpha瓷能够与氧化锆陶瓷支架材料达到良好的匹配。     

Influence of thickness and cooling rate on development of spontaneous cracks in porcelain/zirconia structures

Background: Clinical studies reporting the outcome of zirconia‐based restorations indicate that the strength of the zirconia frameworks is sufficient to withstand masticatory forces. However, a significant incidence of cohesive fracture of the veneering porcelain has been reported. The aim of this study was to investigate spontaneous crack development (chipping, rupture) in a range of porcelains veneered to a zirconia core as a result of thermal stresses induced by changes in thickness and cooling rate. The hypothesis tested was that crack incidence would increase with increased veneer thickness and faster cooling rates. Methods: Zirconia spheres (diameter 7.8 mm) were veneered with 1.5 gm (thickness ratio 1:2) and 2.5 gm (thickness ratio 1:1) of five nominally compatible commercially available porcelains. The manufacturers’ firing cycles and a rapid cooling firing cycle were followed. Results: Multiple regression analysis showed positive associations between the occurrence of cracks and the three covariates (materials, thickness and cooling rate). The incidence of cracks and rupture of the veneering porcelain increased with a faster cooling rate and increased thickness of the specimens in three porcelain–zirconia combinations. Conclusions: Crack incidence increased with increased porcelain veneer thickness and faster cooling rates in nominally compatible porcelain/zirconia systems in the geometrically configured specimens tested.     

Transient thermal stresses developed during speed sintering of 3 mol% yttria-stabilized tetragonal zirconia polycrystals

ObjectivesTo investigate transient thermal stresses that developed in 3Y-TZP green compacts during speed sintering.MethodsA total of 312 disc-shaped green compacts (Ø17.1 ×1, 1.5, 2, 2.5, 3 mm) were cold-isostatically pressed from 3Y-TZP powder (Zpex, Tosoh Corp.) for speed sintering studies as well as compositional analysis and biaxial flexural strength measurements (both at room temperature and following heating at 90 °C/min to 500 °C). Flexural strength was determined using the piston-on-3-ball method. Phase assemblies were analyzed using the X-ray diffraction method. Effects of heating/cooling rates on transient stresses were investigated by conducting definitive sintering studies to determine the threshold for fracture. Finite element analysis (FEA) was used to validate the experimental findings using measured thermomechanical properties.ResultsThe bulk and relative density of the green compact were 2.95 ± 0.03 g/cm³ and 48.52% ± 0.45%. The flexural strength was drastically decreased from 10.3 ± 0.4 MPa to 1.09 ± 0.07 MPa following heating at 90 °C/min to 500 °C. The monoclinic and tetragonal contents were 54.9% and 45.1%, respectively. The threshold for fracture was located at 500 °C during the first heating stage with a 90 °C/min heating rate in specimens of 2.5 mm thickness or greater. No fractures occurred in the second heating stage and cooling phase. The FEA estimated that the principal transient tensile stress was ∼1.14 MPa at 500 °C during the heating phase, which exceeded the corresponding flexural strength (1.09 ± 0.07 MPa).SignificanceAdvanced FEA methods are an accurate and efficient tool to analyze the history of transient stresses during sintering of ceramic dental restorations.     

Effect of slow-cooling protocol on biaxial flexural strengths of bilayered porcelain-ceria-stabilized zirconia/alumina nanocomposite (Ce-TZP/A) disks

Objective

The present study investigated the biaxial flexural strengths of bilayered ceria-stabilized zirconia/alumina nanocomposite (Ce-TZP/A) disks with various layering porcelains veneered using a slow-cooling protocol.

Biaxial flexural strength of the bilayered disk composed of ceria-stabilized zirconia/alumina nanocomposite (Ce-TZP/A) and veneering porcelain

Objective

Herein we investigated the flexural strengths of bilayered ceria-stabilized zirconia/alumina nanocomposite (Ce-TZP/A) disks using different veneering porcelains.

The influence of various core designs on stress distribution in the veneered zirconia crown: a finite element analysis study

PURPOSE

The purpose of this study was to evaluate various core designs on stress distribution within zirconia crowns.

MATERIALS AND METHODS

Three-dimensional finite element models, representing mandibular molars, comprising a prepared tooth, cement layer, zirconia core, and veneer porcelain were designed by computer software. The shoulder (1 mm in width) variations in core were incremental increases of 1 mm, 2 mm and 3 mm in proximal and lingual height, and buccal height respectively. To simulate masticatory force, loads of 280 N were applied from three directions (vertical, at a 45° angle, and horizontal). To simulate maximum bite force, a load of 700 N was applied vertically to the crowns. Maximum principal stress (MPS) was determined for each model, loading condition, and position.

RESULTS

In the maximum bite force simulation test, the MPSs on all crowns observed around the shoulder region and loading points. The compressive stresses were located in the shoulder region of the veneer-zirconia interface and at the occlusal region. In the test simulating masticatory force, the MPS was concentrated around the loading points, and the compressive stresses were located at the 3 mm height lingual shoulder region, when the load was applied horizontally. MPS increased in the shoulder region as the shoulder height increased.

CONCLUSION

This study suggested that reinforced shoulder play an essential role in the success of the zirconia restoration, and veneer fracture due to occlusal loading can be prevented by proper core design, such as shoulder.