Thermally induced fracture for core-veneered dental ceramic structures

Crack Propagation and Fatigue Performance of Partial Posterior Indirect Restorations: An Extended Finite Element Method Study

Dental ceramics are susceptible to slow, progressive crack growth after cyclic loading. The purpose of this study was to investigate the progressive patterns of cracks in two different types of CAD/CAM ceramic materials used with three different partial posterior indirect restoration (PPIR) designs and to determine the materials' failure risk using a fatigue test. Standard initial cracks were formed in Standard Tessellation Language (STL) files prepared for three different PPIRs. The materials chosen were monolithic lithium disilicate (LS) and polymer-infiltrated ceramic networks (PICNs). The extended finite element method (XFEM) was applied, and the fatigue performance was examined by applying a 600 N axial load. The cracks propagated the most in onlay restorations, where the highest displacement was observed. In contrast, the most successful results were observed in overlay restorations. Overlay restorations also showed better fatigue performance. LS materials exhibited more successful results than PICN materials. LS materials, which can be used in PPIRs, yield better results compared to PICN materials. While inlay restorations demonstrated relatively successful results, overlay and onlay restorations can be specified as the most and the least successful PPIR types, respectively.

Mathematical tools for recovery of the load on the fissure according to the micro-CT results

In the present paper X-ray microtomographic research of a molar tooth was conducted. The study revealed regions with a reduced mineral density in the vicinity of the fissure tip. The basic assumption investigated is that corrosion induced enamel mineral density decrease is enhanced by high tensile stresses generated by mechanical load on the occlusal surface of the tooth during crushing of food. Magnitude and location of tensile stress concentration occurs at the fissure tip and may be determined by solving the problem of the stress-strain state of the tooth crown enamel with a wedge-shaped notch. The study of stresses in the vicinity of fissure tip make it possible to construct the boundaries of enhanced enamel virtual fracture. Comparison of the sizes and locations of areas with a reduced enamel mineral density with the sizes and locations of areas of virtual enamel fracture made it possible to establish their approximate congruence. This circumstance made it possible to recreate by mathematical means the nature and magnitude of the force load on the lateral surface of the fissure. Degree of influence of the main parameters of the fissure on the geometrical characteristics of the virtual fracture, such as its area and diameters, were determined.

On interaction between fatigue of reconstruction plate and time-dependent bone remodeling

BACKGROUND AND OBJECTIVE

The fibula free flap (FFF) has been extensively used to repair large segmental bone defects in the maxillofacial region. The reconstruction plate plays a key role in maintaining stability and load-sharing while the fibula unites with adjacent bone in the course of healing and remodeling. However, not all fibula flaps would fully unite, and fatigue of prosthetic devices has been recognized as one major concern for long-term load-bearing applications. This study aims to develop a numerical approach for predicting the fatigue life of the reconstruction plate by taking into account the effect of ongoing bone remodeling.

METHODS

The patient-specific mandible reconstruction with a prosthetic system is studied in this work. The 3D finite element model with heterogeneous material properties obtained from clinical computerized tomography (CT) data is developed for bone, and eXtended Finite Element Method (XFEM) is adopted for the fatigue analysis of the plate. During the remodeling process, the changing apparent density and Young's modulus of bone are simulated in a step-wise fashion on the basis of Wolff's law, which is correlated with the specific clinical follow-up. The maximum biting forces were considered as the driving force on the bone remodeling, which are measured clinically at different time points (4, 16 and 28 months) after reconstruction surgery.

RESULTS

Under various occlusal loadings, the interaction between fatigue crack growth and bone remodeling is investigated to gain new insights for the future design of prosthetic devices. The simulation results reveal that appropriate remodeling of grafted bone could extend the fatigue life of fixation plates in a positive way. On the other hand, the rising occlusal load associated with healing and remodeling could lead to fatigue fracture of fixation plate and potentially cause severe bone resorption.

CONCLUSION

This study proposes an effective approach for more realistically predicting fatigue life of prosthetic devices subject to a tissue remodeling condition in-silico. It is anticipated to provide a guideline for deriving an optimal design of patient-specific prosthetic devices to better ensure longevity.

Effects of composite resin core level and periodontal pocket depth on crack propagation in endodontically treated teeth: An extended finite element method study

STATEMENT OF PROBLEM

Preserving teeth with radicular cracks with or without a periodontal pocket is an alternative to extraction. However, an effective protocol for the restoration of radicular cracks is lacking.

PURPOSE

The purpose of this study was to examine the composite resin core level and periodontal pocket depth effects on stress distribution, maximum von Mises stress, and crack propagation in endodontically treated teeth by using the extended finite element (FE) method.

MATERIAL AND METHODS

Four 3-dimensional models of a cracked endodontically treated mandibular first molar were constructed: PP2C2 (periodontal pocket depth, 2 mm; composite resin core level, 2 mm below the canal orifice level); PP2C4 (periodontal pocket depth, 2 mm; composite resin core level, 2 mm below the crack level); PP4C2 (periodontal pocket depth, 4 mm; composite resin core level, 2 mm below the canal orifice level); and PP4C4 (periodontal pocket depth, 4 mm; composite resin core level, 2 mm below the crack level). The crack initiation was at the same level in all models. A static 700-N load was applied to the models in a vertical direction.

RESULTS

The highest stress in dentin was observed in PP2C2, whereas PP2C4 exhibited the lowest stress and least crack propagation. Stress was high in the dentin and supporting bone. No reduction in crack propagation was observed in the PP4 models, regardless of the composite resin core level.

CONCLUSIONS

The periodontal pocket depth (2 mm and 4 mm) and composite resin core level (2 mm below the crack level and 2 mm below the canal orifice level) affected stress concentration in dentin, resulting in different patterns of crack propagation in the FE models.

Influence of thermal and thermomechanical stimuli on a molar tooth treated with resin-based restorative dental composites

OBJECTIVES

In-vivo experimental techniques to understand the biomechanical behavior of a restored tooth, under varying oral conditions, is very limited because of the invasive nature of the study and complex tooth geometry structure. Therefore, 3D-Finite element analyses are used to understand the behavior of a restored tooth under varying oral conditions. In this study, the distribution of maximum principal stress (MaxPS) and the location of MaxPS on a restored tooth using six different commercially available dental resin composites under the influence of thermal and thermomechanical stimuli are performed.

METHODS

An intact tooth was scanned using µ-CT and segmented to obtain separate geometric models of the tooth, including enamel and dentine. Then, a class II mesial-occlusal-distal (MOD) cavity was constructed for the tooth model. The restored tooth model was further meshed and imported to the commercial Finite Element (FE) software ANSYS. Thermal hot and cold stimuli at 50 °C and 2 °C, respectively, were applied on the occlusal and lingual surface of the tooth model with the tooth's ambient temperature set at 37 °C. A uniform loading of 400 N was applied on the occlusal surface of the tooth to imitate the masticatory forces during the cyclic thermal stimuli.

RESULTS

The results of this study showed that the restorative materials with higher thermal conductivity showed a lower temperature gradient between the restoration and enamel, during the application of thermal stimuli, leading to a higher value of MaxPS on the restoration. Moreover, on applying thermal stimuli, the location of MaxPS at the restoration-enamel junction (REJ) changes based on the value of the coefficient of thermal expansion (CTE). The MaxPS distribution on the application of simultaneous thermal and mechanical stimuli was not only dependent on the elastic modulus of restorative materials but also their thermal properties such as the CTE and thermal conductivity. The weakest part of the restoration was at the REJ, as it experienced the peak stress level during the application of thermomechanical stimuli.

SIGNIFICANCE

The findings from this study suggest that restorative materials with lower values of elastic modulus, lower coefficient of thermal expansion and higher values of thermal conductivity result in lower stresses on the restoration. The outcomes from this study also suggest that the thermal and mechanical properties of a restorative material can have a considerable effect on the selection of restorative materials by dental clinicians over conventional restorative materials.

Mechanical failure of posterior teeth due to caries and occlusal wear- A modelling study

OBJECTIVES

The purpose of the present work is to explore the effect of occlusal wear and different types and degrees of caries on the mechanical performance and structural integrity of posterior teeth.

METHODS

Three-dimensional (3D) computational models with different combinations of caries parameters (caries location, caries size and caries induced pulp shrinkage) and occlusal wear factors (enamel thickness, marginal ridge height and cuspal slope) were developed and analyzed using the extended finite element method (XFEM) to identify the stress distribution, crack initiation load and ultimate fracture load values. The effect of a non-drilling conservative treatment using resin infiltration on the recovery of mechanical properties of carious molar teeth was also investigated.

RESULTS

Presence of fissural caries, worn proximal marginal ridge and decreased enamel thickness due to occlusal wear, imparted a significant negative effect on the crack initiation load value of the lower molar models. Accordingly, models with intact and strong proximal marginal ridge, generally exhibited higher crack initiation loading, regardless of caries size and location. Presence of fissure caries drastically decreased (55%-70%) the crack initiation load compared to sound teeth. The depth of the fissural lesion and the presence of proximal caries did not have a major effect on crack initiation load values. However, increasing the caries size resulted in lower final fracture load values in most of the cases. Accordingly, the groups with combined and connected large fissural and proximal lesions experienced the largest drop in the fracture load values compared to sound tooth models. The worst condition consisted of two connected large proximal and fissural caries with no proximal marginal ridge, in which the fracture load dramatically decreased to only 25% of that for sound teeth with intact marginal ridge. On the other hand, decreased cuspal slope due to occlusal wear and shrinkage of the pulp due to caries appeared to have a protective role and a direct relation with the fracture resistance of the tooth. Following the application of resin infiltration on the carious models, the crack initiation load and the fracture load could recover up to 75% and 90% of the values for the corresponding sound tooth models, respectively.

SIGNIFICANCE

Presence of fissural caries, if not treated (either with remineralization, resin infiltration or restoration), can be a major risk factor in the initiation of tooth fracture. When combined with decreased enamel thickness and loss of proximal marginal ridge due to mechanical or chemical wear, the weakening effect of the caries will be amplified specially in teeth with steep cuspal slopes. The application of a conservative treatment with resin infiltration can be an effective approach in prevention of further mechanical failure of demineralized enamel. The findings of this study emphasize the importance of early interventions in the management of caries for the prevention of future cuspal or tooth fracture especially in subjects with higher risk factors for tooth fracture such as caries, wear and bruxism.

A REVIEW OF MECHANICAL BEHAVIOR OF DENTAL CERAMIC RESTORATIONS

Dental ceramics are well known for restoring the function and aesthetic of lost or damaged teeth. Understanding these materials’ mechanical and aesthetic properties can make a suitable choice for those materials. The longevity of dental ceramics depends on several factors, including manufacturing method, clinical process, and the oral cavity’s aqueous environment. Failure mechanisms in restorative ceramics are complex and a combination of several factors. Different microstructures in the crystalline phase will involve the propagation of cracks and eventually the fatigue of ceramic materials. Large grains reduce mechanical performance compared to small grain sizes. Aesthetic materials used for veneering are weaker than the core materials and fail when even subjected to small loads. The soft bonding in the core–veneer interface and possible residual stresses created during the veneering method are drawbacks of these systems. Studies on the mechanical behavior of these materials have grown significantly in recent years and provide helpful information about static and fatigue experimentation and the failure behavior of various materials used in dental crowns.

Novel CAD-CAM zirconia coping design to enhance the aesthetics and strength for anterior PLZ crowns

BACKGROUND

The aesthetics and strength of anterior porcelain layered zirconia (PLZ) crowns are mainly affected by the zirconia (Zr) coping design. There is a need for a modified design to enhance aesthetics with strength.

OBJECTIVES

The purpose of this study was to compare the fracture resistance of anterior PLZ crowns having modified CAD-CAM Zr coping designs (in terms of thickness and marginal collar designs) with standard Zr copings.

METHODS

Fifty PLZ crowns were fabricated and divided into two groups: Gr 1: Standard Zr Coping (SZC) (control gr) with 0.5 mm thickness (Facial-F, Mesial-M, Distal-D, incisal-I, and Palatal-P) without a collar; Gr 2: Collar Zr Coping (CZC) (test gr) with 2.5 mm collar height on M, D, P and 0.2 mm F and variable facial wall thickness. Subgroups: Gr 2a: (CZC-0.5 mm) facial wall thickness 0.5 mm; Gr 2b: (CZC-0.4 mm) facial wall thickness 0.4 mm; Gr 2c: (CZC-0.3 mm) facial wall thickness 0.3 mm; Gr 2d: (CZC-0.2 mm) facial wall thickness 0.2 mm. The fracture load was determined and analysed using One-way ANOVA and Dunnet test.

RESULTS

The minimum fracture load was 927.36 ± 127.80 N observed for Gr 2c (CZC at 0.3 mm) while the maximum fracture load was 1373.61 ± 146.54 N observed for Gr 2a (CZC at 0.5 mm). A highly significant difference in mean fracture load among various Zr coping groups (p< 0.001) was determined.

CONCLUSIONS

Novel Zr coping design for anterior PLZ crowns can provide better aesthetics with strength. Reducing the thickness of Zr coping in the aesthetic zone to 0.2 mm and providing a modified collar design (2.5 mm collar height on M, D, P, and 0.2 mm F) would provide strength without jeopardizing aesthetics.

Mechanical Behavior and Fracture Loads of Screw-Retained and Cement-Retained Lithium Disilicate Implant-Supported Crowns

PURPOSE

To evaluate the fatigue survival, fracture loads and failure modes of monolithic lithium disilicate screw-retained crowns, attached to titanium insert, and cement-retained crowns.

MATERIALS AND METHODS

Internal tapered connection implants, embedded in acrylic resin at 30° inclination, were restored with lithium disilicate restorations, simulating a maxillary premolar (n = 20), with different designs: screw-retained titanium base abutment-crowns, and cement-retained crowns. The specimens were submitted to cyclic mechanical loading (1.2 × 10⁶ cycles with a load of 0-250 N at 2 Hz). Surviving specimens were subjected to single load to fracture in a universal testing machine and failure modes were determined with the aid of an optical microscope. Maximum load values were analyzed statistically using the t-test and differences in failure modes were analyzed using the chi-squared test (α = 0.05).

RESULTS

All specimens survived the cyclic mechanical loading. Fracture load was significantly higher for screw-retained crowns (821.69 ±196.71 N) than the cement-retained crowns (577.03 ± 137.75 N) (p = 0.005). Ceramic failure was the predominant mode, with no statistical difference between groups.

CONCLUSIONS

Screw-retained and cement-retained lithium disilicate crowns survived the cyclic mechanical loading. The use of titanium inserts to support a monolithic restoration enhances the fracture strength of the crown/abutment system.

A simple 3-point flexural method for measuring fracture toughness of the dental porcelain to zirconia bond and other brittle bimaterial interfaces

PURPOSE

Porcelain fused to zirconia prostheses are widely used, but porcelain chipping, fracture, spalling and delamination are common clinical problems. Conventional bond strength testing is inherently unsuited for studying interfacial failure by cracking in brittle materials. Instead, fracture toughness is a more meaningful parameter because it can assess the robustness of the interface when subjected to loading, but fracture mechanics approaches have only rarely been used. Our purpose was to develop a novel, simple, 3-point flexural methodology and mathematical analysis to measure the fracture toughness of the porcelain to zirconia interface.

METHODS

Equations were derived to estimate the fracture toughness of the bond by computing the interfacial energy release rate for a novel simple 3-point flexural test model. The test was validated using two different configurations of layered zirconia/porcelain beams (n = 10), approximating the dimensions of a fixed dental prosthesis, fabricated from a tetragonal polycrystalline zirconium dioxide partially stabilized with yttria and a feldspathic dental porcelain.

RESULTS

Cracking along the bimaterial interface was produced and measured as a discrete event. Fracture toughness means (standard deviations) computed from the measured energy release rate, for the porcelain to zirconia interface in two different specimen configurations were 7.9 (1.3) and 5.3 (1.6) J/m².

CONCLUSIONS

Equations were derived to measure interfacial fracture toughness of brittle materials using a novel simple 3-point flexural test method. The test was then validated; estimates for the fracture toughness for the porcelain to zirconia bond, overlapped with previously published data derived from more complex 4-point notched tests.

Effect of different implant configurations on biomechanical behavior of full-arch implant-supported mandibular monolithic zirconia fixed prostheses

Mechanical failure of zirconia-based full-arch implant-supported fixed dental prostheses (FAFDPs) remains a critical issue in prosthetic dentistry. The option of full-arch implant treatment and the biomechanical behaviour within a sophisticated screw-retained prosthetic structure have stimulated considerable interest in fundamental and clinical research. This study aimed to analyse the biomechanical responses of zirconia-based FAFDPs with different implant configurations (numbers and distributions), thereby predicting the possible failure sites and the optimum configuration from biomechanical aspect by using finite element method (FEM). Five 3D finite element (FE) models were constructed with patient-specific heterogeneous material properties of mandibular bone. The results were reported using volume-averaged von-Mises stresses (σ_VM^VA) to eliminate numerical singularities. It was found that wider placement of multi-unit copings was preferred as it reduces the cantilever effect on denture. Within the limited areas of implant insertion, the adoption of angled multi-unit abutments allowed the insertion of oblique implants in the bone and wider distribution of the multi-unit copings in the prosthesis, leading to lower stress concentration on both mandibular bone and prosthetic components. Increasing the number of supporting implants in a FAFDPs reduced loading on each implant, although it may not necessarily reduce the stress concentration in the most posterior locations significantly. Overall, the 6-implant configuration was a preferable configuration as it provided the most balanced mechanical performance in this patient-specific case.

A comparative evaluation of fracture load of monolithic and bilayered zirconia crowns with and without a cervical collar: an in vitro study

Introduction

The use of zirconia based all-ceramic restorations are preferred nowadays owing to superior biologic and esthetic properties. However, these restorations have also reported higher incidences of fracture and chipping. The clinical success may be enhanced by optimizing the core design, through the introduction of monolithic zirconia, or the layered crowns can be strengthened by adding the cervical collar to them.

Objective

This study was performed with the objective to compare and evaluate the fracture load of monolithic and bilayered zirconia crowns with and without a cervical collar.

Methods

A prospective observational study was carried out to compare 45 fabricated zirconia crowns of three different designs on a customized metal mould. The samples were oriented on the metal mould and subjected to confocal microscope for the evaluation of marginal integrity followed by cementing the crown on the metal mould and subjecting it to the universal testing machine for the analysis for the flexural strength. Data were analyzed using one way Anova and t- test for inter and intra groups. The significance level was set at P ≤ 0.05.

Results

The monolith zirconia crowns and layered zirconia crowns with cervical collar reported more flexural strength.

Conclusion

The amount of force required to fracture the zirconia crowns was higher in the case of monolith zirconia crown and layered zirconia crown with cervical collar as compared to the layered zirconia crown without cervical collar.

Modelling of stress distribution and fracture in dental occlusal fissures

The aim of this study was to investigate the fracture behaviour of fissural dental enamel under simulated occlusal load in relation to various interacting factors including fissure morphology, cuspal angle and the underlying material properties of enamel. Extended finite element method (XFEM) was adopted here to analyse the fracture load and crack length in tooth models with different cusp angles (ranging from 50° to 70° in 2.5° intervals), fissural morphologies (namely U shape, V shape, IK shape, I shape and Inverted-Y shape) and enamel material properties (constant versus graded). The analysis results showed that fissures with larger curved morphology, such as U shape and IK shape, exhibit higher resistance to fracture under simulated occlusal load irrespective of cusp angle and enamel properties. Increased cusp angle (i.e. lower cusp steepness), also significantly enhanced the fracture resistance of fissural enamel, particularly for the IK and Inverted-Y shape fissures. Overall, the outcomes of this study explain how the interplay of compositional and structural features of enamel in the fissural area contribute to the resistance of the human tooth against masticatory forces. These findings may provide significant indicators for clinicians and technicians in designing/fabricating extra-coronal dental restorations and correcting the cuspal inclinations and contacts during clinical occlusal adjustment.

Clinical performance of tooth- or implant-supported veneered zirconia single crowns: 42-month results

OBJECTIVES

The objective of this clinical study was to compare and assess the clinical performance of tooth-supported and implant-supported zirconia single crowns with sintered veneering caps.

METHODS

In this prospective study, 118 patients with a total of 220 single crowns placed on 106 teeth (69 vital teeth, 37 endodontically treated teeth) and 114 implants in molar and premolar regions were examined during a mean observation period of 42 months. The restorations were evaluated for technical failures such as veneering porcelain fractures (chipping), surface quality, marginal fit, and the interface quality of the coping and sintered veneering. The soft tissue status was assessed using the modified Silness and Löe's plaque and gingival index (mPI) and the modified Muhlemann sulcus bleeding index (mSBI). Tooth-supported crowns were checked for secondary caries and hypersensitivity during the follow-up period. Recalls were performed every 6 months.

RESULTS

The 3-year Kaplan-Meier success probability was 98.2% and 100% for implant- and tooth-supported crowns, respectively. A significant difference could be detected between the implant-supported and tooth-supported zirconia single crowns, in terms of their chipping rate (p = 0.039). Veneering material fractures were recorded on two implant-supported restorations (1.8%). No veneering fractures occurred on tooth-supported single crowns. The plaque and gingival index and sulcus bleeding index showed stable and healthy soft peri-implant and periodontal tissues. Neither loss of vitality nor secondary caries occurred on tooth-supported crowns.

CONCLUSIONS

Zirconia-based single crowns with a sintered veneering cap showed promising clinical results on both tooth and implant abutments; however, the dental implants were more prone to complications. In terms of clinical significance, high-strength ceramic with a sintered veneering cap can be recommended for prosthetic treatment of both tooth- and implant-supported single crowns in molar regions.

CLINICAL RELEVANCE

This study provides valuable information for further application of all-ceramic restorations.

Thermal cracks of implant-based zirconia four-unit restorations: A fractographic analysis of two restorations fractured during production

Two zirconia-based 4-unit restorations intended for the same patient fractured during the veneering process even though the prolonged cooling protocol recommended by the manufacturers was used. Fractographic analyses revealed that both restorations fractured as a result of thermal shock, but at different times during production. Further investigation is necessary to optimize the firing protocols for large zirconia-based restorations and avoid fracture due to thermal shock.

Viscoelastic finite element analysis of residual stresses in porcelain-veneered zirconia dental crowns

The main problem of porcelain-veneered zirconia (PVZ) dental restorations is chipping and delamination of veneering porcelain owing to the development of deleterious residual stresses during the cooling phase of veneer firing. The aim of this study is to elucidate the effects of cooling rate, thermal contraction coefficient and elastic modulus on residual stresses developed in PVZ dental crowns using viscoelastic finite element methods (VFEM). A three-dimensional VFEM model has been developed to predict residual stresses in PVZ structures using ABAQUS finite element software and user subroutines. First, the newly established model was validated with experimentally measured residual stress profiles using Vickers indentation on flat PVZ specimens. An excellent agreement between the model prediction and experimental data was found. Then, the model was used to predict residual stresses in more complex anatomically-correct crown systems. Two PVZ crown systems with different thermal contraction coefficients and porcelain moduli were studied: VM9/Y-TZP and LAVA/Y-TZP. A sequential dual-step finite element analysis was performed: heat transfer analysis and viscoelastic stress analysis. Controlled and bench convection cooling rates were simulated by applying different convective heat transfer coefficients 1.7E-5 W/mm² °C (controlled cooling) and 0.6E-4 W/mm² °C (bench cooling) on the crown surfaces exposed to the air. Rigorous viscoelastic finite element analysis revealed that controlled cooling results in lower maximum stresses in both veneer and core layers for the two PVZ systems relative to bench cooling. Better compatibility of thermal contraction coefficients between porcelain and zirconia and a lower porcelain modulus reduce residual stresses in both layers.

Bond strength enhancement of zirconia-porcelain interfaces via Nd:YAG laser surface structuring

OBJECTIVES

The aim of this study was to evaluate the effect of laser surface structuring on the bond strength of feldspar-based porcelain to zirconia, as compared to conventional sandblasting treatment.

MATERIALS AND METHODS

Thirty cylindrical zirconia substrates, previously sintered, were divided in three groups according to the type of surface conditioning: 1) sandblasting with 50 µm Al₂O₃; 2) laser structuring (Ø25 µm holes); and 3) laser structuring (Ø50 µm holes). Porcelain was injected onto the zirconia substrates. X-ray diffractometry (XRD) was used to evaluate the influence of the laser treatment on zirconia crystallographic phases. Shear bond strength test was performed. Micrographs using SEM were used to evaluate the zirconia surface after each surface treatment and to evaluate the fracture surface after the shear test.

RESULTS

The laser-structured groups presented the highest shear bond strength (65 ± 16 MPa and 65 ± 11 MPa, for the 25 µm and 50 µm holes, respectively). The sandblasting samples presented shear bond strength of 37 ± 16 MPa. XRD analysis showed that there was no phase transformation on the thermally affected surface due to laser action. Microcracks were created at some holes due to the high temperature gradient generated by laser.

SIGNIFICANCE

Laser structuring significantly increased (up to 75%) the shear bond strength of zirconia to veneering porcelain as compared to conventional sandblasting treatment. Therefore, laser structuring arises as a surface conditioning method for producing stronger and long lasting zirconia-porcelain interfaces.

ADM guidance-Ceramics: guidance to the use of fractography in failure analysis of brittle materials

OBJECTIVES

To provide background information and guidance as to how to use fractography accurately, a powerful tool for failure analysis of dental ceramic structures.

METHODS

An extended palette of qualitative and quantitative fractography is provided, both for in vivo and in vitro fracture surface analyses. As visual support, this guidance document will provide micrographs of typical critical ceramic processing flaws, differentiating between pre- versus post sintering cracks, grinding damage related failures and occlusal contact wear origins and of failures due to surface degradation.

RESULTS

The documentation emphasizes good labeling of crack features, precise indication of the direction of crack propagation (dcp), identification of the fracture origin, the use of fractographic photomontage of critical flaws or flaw labeling on strength data graphics. A compilation of recommendations for specific applications of fractography in Dentistry is also provided.

SIGNIFICANCE

This guidance document will contribute to a more accurate use of fractography and help researchers to better identify, describe and understand the causes of failure, for both clinical and laboratory-scale situations. If adequately performed at a large scale, fractography will assist in optimizing the methods of processing and designing of restorative materials and components. Clinical failures may be better understood and consequently reduced by sending out the correct message regarding the fracture origin in clinical trials.

THERMAL RESIDUAL STRESSES IN BILAYERED, TRILAYERED AND GRADED DENTAL CERAMICS

Layered ceramic systems are usually hit by residual thermal stresses created during cooling from high processing temperature. The purpose of this study was to determine the thermal residual stresses at different ceramic multi-layered systems and evaluate their influence on the bending stress distribution. Finite elements method was used to evaluate the residual stresses in zirconia-porcelain and alumina-porcelain multi-layered discs and to simulate the 'piston-on-ring' test. Temperature-dependent material properties were used. Three different multi-layered designs were simulated: a conventional bilayered design; a trilayered design, with an intermediate composite layer with constant composition; and a graded design, with an intermediate layer with gradation of properties. Parameters such as the interlayer thickness and composition profiles were varied in the study. Alumina-porcelain discs present smaller residual stress than the zirconia-porcelain discs, regardless of the type of design. The homogeneous interlayer can yield a reduction of ~40% in thermal stress relative to bilayered systems. Thinner interlayers favoured the formation of lower thermal stresses. The graded discs showed the lowest thermal stresses for a gradation profile given by power law function with p=2. The bending stresses were significantly affected by the thermal stresses in the discs. The risk of failure for all-ceramic dental restorative systems can be significantly reduced by using trilayered systems (homogenous or graded interlayer) with the proper design.

Influence of interlayer design on residual thermal stresses in trilayered and graded all-ceramic restorations

Residual thermal stresses are formed in dental restorations during cooling from high temperature processing. The aim of this study was to evaluate the influence of constructive design variables (composition and interlayer thickness) on residual stresses in alumina- and zirconia-graded restorations. Restorations' real-like cooling conditions were simulated using finite elements method and temperature-dependent material properties were used. Three different designs were evaluated: a bilayered restoration (sharp transition between materials); a trilayered restoration with a homogenous interlayer between core and veneer; and a trilayered restoration with a graded interlayer. The interlayer thickness and composition were varied. Zirconia restorations presented overall higher thermal stress values than alumina ones. Thermal stresses were significantly reduced by the presence of a homogeneous interlayer. The composition of the interlayer showed great influence on the thermal stresses, with the best results for homogeneous interlayers being observed for porcelain contents in the composite ranging between 30%-50% (vol.%), for both alumina and zirconia restorations. The interlayer's thickness showed a minor contribution in the thermal stress reduction. The graded interlayer showed an optimized reduction in restorations' thermal stresses. The use of graded interlayer, favoring enhanced thermal stress distributions and lower magnitude is expected to reduce the risk of catastrophic failure.

Fracture behaviors of ceramic tissue scaffolds for load bearing applications

Healing large bone defects, especially in weight-bearing locations, remains a challenge using available synthetic ceramic scaffolds. Manufactured as a scaffold using 3D printing technology, Sr-HT-Gahnite at high porosity (66%) had demonstrated significantly improved compressive strength (53 ± 9 MPa) and toughness. Nevertheless, the main concern of ceramic scaffolds in general remains to be their inherent brittleness and low fracture strength in load bearing applications. Therefore, it is crucial to establish a robust numerical framework for predicting fracture strengths of such scaffolds. Since crack initiation and propagation plays a critical role on the fracture strength of ceramic structures, we employed extended finite element method (XFEM) to predict fracture behaviors of Sr-HT-Gahnite scaffolds. The correlation between experimental and numerical results proved the superiority of XFEM for quantifying fracture strength of scaffolds over conventional FEM. In addition to computer aided design (CAD) based modeling analyses, XFEM was conducted on micro-computed tomography (μCT) based models for fabricated scaffolds, which took into account the geometric variations induced by the fabrication process. Fracture strengths and crack paths predicted by the μCT-based XFEM analyses correlated well with relevant experimental results. The study provided an effective means for the prediction of fracture strength of porous ceramic structures, thereby facilitating design optimization of scaffolds.

Topological design of all-ceramic dental bridges for enhancing fracture resistance

Layered all-ceramic systems have been increasingly adopted in major dental prostheses. However, ceramics are inherently brittle, and they often subject to premature failure under high occlusion forces especially in the posterior region. This study aimed to develop mechanically sound novel topological designs for all-ceramic dental bridges by minimizing the fracture incidence under given loading conditions. A bi-directional evolutionary structural optimization (BESO) technique is implemented within the extended finite element method (XFEM) framework. Extended finite element method allows modeling crack initiation and propagation inside all-ceramic restoration systems. Following this, BESO searches the optimum distribution of two different ceramic materials, namely porcelain and zirconia, for minimizing fracture incidence. A performance index, as per a ratio of peak tensile stress to material strength, is used as a design objective. In this study, the novel XFEM based BESO topology optimization significantly improved structural strength by minimizing performance index for suppressing fracture incidence in the structures. As expected, the fracture resistance and factor of safety of fixed partial dentures structure increased upon redistributing zirconia and porcelain in the optimal topological configuration. Dental CAD/CAM systems and the emerging 3D printing technology were commercially available to facilitate implementation of such a computational design, exhibiting considerable potential for clinical application in the future. Copyright © 2015 John Wiley & Sons, Ltd.

Fatigue Resistance of Y-TZP/Porcelain Crowns is Not Influenced by the Conditioning of the Intaglio Surface

OBJECTIVES

The objective of this study was to investigate the effects of treatments of the intaglio surface of Y-TZP frameworks and luting agents on the fatigue resistance of all-ceramic crowns.

METHODS

A research design was chosen that attempted to reduce the likelihood of Hertzian cracking and to increase the probability of fracture initiation at the intaglio surface of the framework. Ninety identical preparations were machined in a dentin-like epoxy composite. Each preparation was restored with a Y-TZP framework made by a CAD/CAM system and veneered using feldspathic ceramic. Prior to cementation the intaglio surface of the ceramic was treated using one of four treatments: 1) cleaning with isopropyl alcohol; 2) application of an overglaze; 3) sandblasting with 125 μm aluminum oxide powder; and 4) sandblasting with 30 μm silica powder (CJ). One of three luting cements were used: 1) zinc phosphate; 2) glass ionomer; and 3) adhesive resin cement (PN). All three cements were tested against frameworks that were alcohol cleaned. Only the PN cements were tested against frameworks that had been sandblasted or glazed. Altogether, six groups of 15 specimens each were tested. Fatigue resistance was evaluated using stepwise loads at 1.4 Hz until failure: 5000 cycles at maximum load of 200 N, followed by 10,000 cycles at maximum loads of 800, 1000, 1200, and 1400 N. The cement thickness and failure modes were analyzed using a stereomicroscope and scanning electron microscopy. The results were analyzed using the Kaplan-Meier and Mantel-Cox log rank tests (5%), a one-way analysis of variance, Tukey multiple comparison test, and Weibull nonparametric test.

RESULTS

The predominant failure mode was chipping of the veneer. The crowns cemented with the adhesive resin cement exhibited chipping failure at higher mean loads than did crowns cemented with cements that usually do not bond strongly with dentin. When the adhesive cement was used, glazing and sandblasting intaglio framework surface treatments exhibited lower mean loads at chipping than did crowns whose intaglio surface was only cleaned with alcohol. Weibull analysis indicated that all specimens had a high ratio of late-to-early failures.

CONCLUSIONS

The fatigue experiment produced a pattern of failures that is very similar to that observed in clinical trials of Y-TZP crowns that are veneered with feldspathic porcelain. Crowns cemented with an adhesive resin cement exhibited chipping at a significantly higher mean load than those cemented with luting cements that do not usually form strong bonds with dentin. When cemented with adhesive resin cement, glazing or sandblasting the intaglio surface of the framework significantly reduced the mean fatigue loads at which chipping of veneers occurred, as compared to crowns whose intaglio surface had only been cleaned with alcohol. For this cement glazing or sandblasting the intaglio surface of the crown is not recommended.

Influence of peak oral temperatures on veneer-core interface stress state

Objective: There is a growing interest for the use of Y-TZP zirconia as core material in veneered all-ceramic prostheses. The objective of this study was to evaluate the influence of CET on the stress distribution of a porcelain layered to zirconia core single crowns by finite elements analysis.

Material and methods: CET of eight different porcelains was considered during the analysis.

Results: Results of this study indicated that the mismatch in CET between the veneering porcelain and the Y-TZP zirconia core has to be minimum (0.5–1 μm/mK) so as to decrease the growing of residual stresses which could bring chipping.

Conclusions: The stress state due to temperature variation should be carefully taken into consideration while studying the effect of mechanical load on zirconia core crown by FEA. The interfacial stress state can be increased by temperature variation up to 20% with respect to the relative failure parameter (interface strength in this case). This means that stress due to mechanical load combined to temperature variation-induced stress can lead porcelain veneer–zirconia core interfaces to failure.

Numerical Simulation of Biomechanical Behaviours in Novel Dental Restorations

Besides the prevention strategies against early stage dental caries, restoration is a preferable way to prevent decayed tooth from further deterioration. This study aimed to compare the mechanical strengths of carious tooth, traditionally restored tooth, and novel conservatively restored teeth under occlusal loading. The two-dimensional (2D) finite element method (FEM) was applied to quantify and compare maximum tensile stresses thereby predicting the initiation of crack. Taking into consideration of peak tensile stresses, it was found that the conservative (minimal intervention) restorations exhibited better fracture resistance than traditional restoration.

Zirconia-Based Screw-Retained Prostheses Supported by Implants: A Retrospective Study on Technical Complications and Failures

BACKGROUND

Little information is yet available on zirconia-based prostheses supported by implants.

PURPOSE

To evaluate technical problems and failures of implant-supported zirconia-based prostheses with exclusive screw-retention.

MATERIAL AND METHODS

Consecutive patients received screw-retained zirconia-based prostheses supported by implants and were followed over a time period of 5 years. The implant placement and prosthetic rehabilitation were performed in one clinical setting, and all patients participated in the maintenance program. The treatment comprised single crowns (SCs) and fixed dental prostheses (FDPs) of three to 12 units. Screw-retention of the CAD/CAM-fabricated SCs and FDPs was performed with direct connection at the implant level. The primary outcome was the complete failure of zirconia-based prostheses; outcome measures were fracture of the framework or extensive chipping resulting in the need for refabrication. A life table analysis was performed, the cumulative survival rate (CSR) calculated, and a Kaplan-Meier curve drawn.

RESULTS

Two hundred and ninety-four implants supported 156 zirconia-based prostheses in 95 patients (52 men, 43 women, average age 59.1 ± 11.7 years). Sixty-five SCs and 91 FDPs were identified, comprising a total of 441 units. Fractures of the zirconia framework and extensive chipping resulted in refabrication of nine prostheses. Nearly all the prostheses (94.2%) remained in situ during the observation period. The 5-year CSR was 90.5%, and 41 prostheses (14 SCs, 27 FDPs) comprising 113 units survived for an observation time of more than 5 years. Six SCs exhibited screw loosening, and polishing of minor chipping was required for five prostheses.

CONCLUSIONS

This study shows that zirconia-based implant-supported fixed prostheses exhibit satisfactory treatment outcomes and that screw-retention directly at the implant level is feasible.

Clinical risk factors related to failures with zirconia-based restorations: an up to 9-year retrospective study

OBJECTIVES

The first objective of this study was to retrospectively evaluate zirconia-based restorations (ZBR). The second was to correlate failures with clinical parameters and to identify and to analyse chipping failures using fractographic analysis.

METHODS

147 ZBR (tooth- and implant-supported crowns and fixed partial dentures (FPDs)) were evaluated after a mean observation period of 41.5 ± 31.8 months. Accessorily, zirconia implant abutments (n=46) were also observed. The technical (USPHS criteria) and the biological outcomes of the ZBR were evaluated. Occlusal risk factors were examined: occlusal relationships, parafunctional habits, and the presence of occlusal nightguard. SEM fractographic analysis was performed using the intra-oral replica technique.

RESULTS

The survival rate of crowns and FPDs was 93.2%, the success rate was 81.63% and the 9-year Kaplan-Meier estimated success rate was 52.66%. The chipping rate was 15% and the framework fracture rate was 2.7%. Most fractographic analyses revealed that veneer fractures originated from occlusal surface roughness. Several parameters were shown to significantly influence veneer fracture: the absence of occlusal nightguard (p=0.0048), the presence of a ceramic restoration as an antagonist (p=0.013), the presence of parafunctional activity (p=0.018), and the presence of implants as support (p=0.026). The implant abutments success rate was 100%.

CONCLUSIONS

The results of the present study confirm that chipping is the first cause of ZBR failure. They also underline the importance of clinical parameters in regards to the explanation of this complex problem. This issue should be considered in future prospective clinical studies.

CLINICAL SIGNIFICANCE

Practitioners can reduce chipping failures by taking into account several risk parameters, such as the presence of a ceramic restoration as an antagonist, the presence of parafunctional activity and the presence of implants as support. The use of an occlusal nightguard can also decrease failure rate.