Recent advances in dental zirconia: 15 years of material and processing evolution

OBJECTIVES

The objective was to discuss the research on zirconia published in the past 15 years to help the dental materials community understand the key properties of the types of zirconia and their clinical applications.

METHODS

A literature search was performed in May/2023 using Web of Science Core Collection with the term "dental zirconia". The search returned 5102 articles, which were categorized into 31 groups according to the research topic.

RESULTS

The current approach to improving the translucency of zirconia is to decrease the alumina content while increasing the yttria content. The resulting materials (4Y-, 5Y-, and above 5 mol% PSZs) may contain more than 50% of cubic phase, with a decrease in mechanical properties. The market trend for zirconia is the production of CAD/CAM disks containing more fracture resistant 3Y-TZP at the bottom layers and more translucent 5Y-PSZ at the top. Although flaws located between layers in multilayered blocks might represent a problem, newer generations of zirconia layered blocks appear to have solved this problem with novel powder compaction technology. Significant advancements in zirconia processing technologies have been made, but there is still plenty of room for improvement, especially in the fields of high-speed sintering and additive manufacturing.

SIGNIFICANCE

The wide range of zirconia materials currently available in the market may cause confusion in materials selection. It is therefore imperative for dental clinicians and laboratory technicians to get the needed knowledge on zirconia material science, to follow manufacturers' instructions, and to optimize the design of the prosthetic restoration with a good understanding where to reinforce the structure with a tough and strong zirconia.

Strength-limiting damage and defects of dental CAD/CAM full-contour zirconia ceramics

OBJECTIVE

This study aimed to compare the four-point flexural strength of CAM-milled and sintered (as-sintered, AS) specimens with those of high-polished (HP) specimens using chairside polishing systems to simulate clinical surface conditions.

METHODS

Seven full-contour zirconia CAM/CAM blanks with various yttria contents (3, 4, 5 mol%) including three high-translucent groups (5Y) were selected to prepare flexural specimens. The bend bar specimens (2.0 × 4.0 × 25.0 mm3) were fabricated by using STL file and dental CAM machine with the respective zirconia blanks (98 mm ϕ and 10-14 mm in height). Twelve bar specimens were machined from one zirconia puck and a total of 24 specimens were prepared from each group. The pre-sintered bar specimens were sintered by using a dental zirconia furnace at 1530-1550 °C for 2 h according to the instructions. All sintered specimens were divided into two groups: as-sintered (AS) group and high-polished (HP) groups (n = 12). HP groups were subjected to polishing one surface of specimens using a three-step polishing system and finally finished with diamond polishing. After cleaning and drying, the flexural strength of all specimens was determined by a fully articulating four-point flexure fixture consisting of a 1/4-point test configuration with an inner/outer span of 10/20 mm. Statistical differences between AS and HP groups were conducted with Weibull analysis. The fractured surfaces of zirconia specimens were observed using a field emission SEM and EDS to detect failure origins.

RESULTS

The mean AS flexural strength values were significantly lower than those of HP counterparts. However, Weibull moduli expressing the reliability of HP groups were generally decreased although not significantly in comparison to their AS. The fracture of the AS specimens mostly originated from extrinsic CAM-milling defects, while the HP specimens were fractured from intrinsic subsurface or volume defects including pores, large grain clusters, inclusions, and corner-located critical flaws. Two high-translucent (5Y) zirconia groups were not affected in their strength and reliability after polishing, whereas one 5Y zirconia significantly increased its strength but significantly lowered its reliability.

SIGNIFICANCE

The extrinsic and intrinsic strength-limiting defects should be considered in evaluating the flexural strength and reliability of dental CAD/CAM zirconia ceramics for full-contour restorations. For the materials tested in this study, more optimized processing of blanks and milling protocols of pre-sintered zirconia blanks should be developed including post-sintering surface finishing to reduce the flaw population regulating strength and reliability which will affect the survivability of dental zirconia prostheses.

An In Vitro Evaluation of the Effects of Air-Polishing Powders on Sound and Demineralised Enamel

AIM

To evaluate the effects of two air-polishing powders, during orthodontic treatment, on the surface roughness of sound and demineralised enamel.

MATERIALS AND METHODS

Forty-two caries-free human molars were collected, and the enamel surfaces were flattened and polished. Teeth were assigned to two groups (n = 21 each), a sound- and a demineralised-enamel group (subjected to pH-cycling over 2 weeks to create artificially induced white spot-like lesions). Within each group, teeth were further assigned to three groups (n = 7 each), air polished with either sodium bicarbonate, erythritol, or a negative control (water). Each sample was treated for 5 and 150 s. The average surface roughness (Ra) for each sample was measured using white-light-sensor profilometry.

RESULTS

On sound enamel, the Ra was roughly 0.17 ± 0.07 μm. After 150 s of air polishing, the Ra increased with erythritol (by 0.28 μm), and even more so with bicarbonate treatment (by 0.68 μm) (p < 0.01). On demineralised enamel, the Ra was roughly 0.79 ± 0.56 μm. The Ra increased after 5 s of air-polishing treatment similarly with erythritol and bicarbonate powders (by 1.03 μm and 1.04 μm, respectively) (p = 0.025), and even more after 150 s (by 2.48 μm and 2.49 μm, respectively) (p < 0.001).

CONCLUSIONS

On white spot lesions, one should be aware that enamel surface roughness will increase with both erythritol and bicarbonate air-polishing powders, especially with longer exposure times.

Pre-Treat Xenogenic Collagenous Blocks of Bone Substitutes with Saline Facilitate Their Manipulation and Guarantee High Bone Regeneration Rates, Qualitatively and Quantitatively

Deproteinized bovine bone mineral particles embedded in collagen (DBBM-C) are widely used for bone regenerations with excellent, albeit sometimes variable clinical outcomes. Clinicians usually prepare DBBM-C by mixing with blood. Replacing blood by saline represents an alternative. We investigated if saline treatment could improve DBBM-C i. handling in vitro and ii. biological performances in a rabbit calvarial model. In vitro, DBBM-C blocks soaked in saline or blood were submitted to compression tests. In vivo, four poly ether ether ketone (PEEK)cylinders were placed on 16 rabbit skulls, filled with DBBM-C soaked in blood or saline for 2-4-8-12 weeks before histomorphometry. DBBM-C blocks were fully hydrated after 30 s in saline when 120 s in blood could not hydrate blocks core. Stiffness gradually decreased 2.5-fold after blood soaking whereas a six-fold decrease was measured after 30 s in saline. In vivo, saline treatment allowed 50% more bone regeneration during the first month when compared to blood soaking. This difference was then no longer visible. New bone morphology and maturity were equivalent in both conditions. DBBM-C saline-soaking facilitated its handling and accelerated bone regeneration of highly qualitative tissues when compared to blood treatment. Saline pretreatment thus may increase the clinical predictability of bone augmentation procedures.

Fractal analysis at varying locations of clinically failed zirconia dental implants

OBJECTIVES

Previous studies have shown that the fracture toughness of ceramics can be determined from the fractal dimensions (D) of their fracture surfaces and that the surface should be leveled to obtain an accurate D measurement. This study was to determine the effects of leveling operations and distance from the failure origin on the D values.

METHODS

Twelve clinically failed zirconia implants from four different manufacturers: Axis Biodental (n=7), Z-Systems (n=3), Straumann (n=1), and Swiss Dental Solutions (n=1) were obtained from one of the authors and thoroughly cleaned. Epoxy replicas were made of three locations along the crack path in the center region of each fracture surface (near origin (O), hackle (H), and near compression curl (CC)) using a light body polyvinyl siloxane impression material. Surfaces were scanned in ScanAsyst mode with a scan size of 5μm×5μm and a scan rate of 0.592Hz using the atomic force microscope. The surface scans were then leveled using 1st order flattening operation in the AFM analysis software. The height data before and after the operation were imported into a custom MathCAD script, and FRACTALS software was used to determine the D value by Minkowski Cover algorithm, which was shown previously to be the algorithm with the highest precision. A Wilcoxon signed-rank test, two-way repeated-measures ANOVA, and one-way repeated-measures ANOVA were performed as detailed below.

RESULTS

The data were not normally distributed (S-W p≤0.05), so a non-parametric repeated measures test (Wilcoxon signed-rank test) was selected. The median D values before and after leveling were 2.161 and 2.174, respectively. There was a significant difference before and after leveling (p<0.001). The two-way repeated-measures ANOVA showed no significant difference among the D values for different implant brands (p=0.66) and scanning locations on the fracture surface (p=0.83). After eliminating the implant brand as a factor, the data passed normality and equal variance tests (S-W p=0.88, BF p=0.15). The mean D values and standard deviations from the three locations (O, H, CC) were 2.183±0.031, 2.179±0.024, and 2.175±0.018, respectively. One-way repeated measures ANOVA showed no significant effect of scanning location (p=0.74).

SIGNIFICANCE

The leveling operation successfully removed the tilt without decreasing surface tortuosity, as it increased the D values significantly. The fractal dimension was the same at the three locations on the fracture surfaces. This means that hackle and compression curl regions can be used to determine fracture toughness when the failure origin has been lost.

Calvarial Model of Bone Augmentation in Rabbit for Assessment of Bone Growth and Neovascularization in Bone Substitution Materials

The basic principle of the rabbit calvarial model is to grow new bone tissue vertically on top of the cortical part of the skull. This model allows assessment of bone substitution materials for oral and craniofacial bone regeneration in terms of bone growth and neovascularization support. Once animals are anesthetized and ventilated (endotracheal intubation), four cylinders made of polyether ether ketone (PEEK) are screwed onto the skull, on both sides of the median and coronal sutures. Five intramedullary holes are drilled within the bone area delimited by each cylinder, allowing influx of bone marrow cells. The material samples are placed into the cylinders which are then closed. Finally, the surgical site is sutured, and animals are awaken. Bone growth may be assessed on live animals by using microtomography. Once animals are euthanized, bone growth and neovascularization may be evaluated by using microtomography, immune-histology and immunofluorescence. As the evaluation of a material requires maximum standardization and calibration, the calvarial model appears ideal. Access is very easy, calibration and standardization are facilitated by the use of defined cylinders and four samples may be assessed simultaneously. Furthermore, live tomography may be used and ultimately a large decrease in animals to be euthanized may be anticipated.

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.

ADM guidance-Ceramics: Fracture toughness testing and method selection

OBJECTIVES

The objective is within the scope of the Academy of Dental Materials Guidance Project, which is to provide dental materials researchers with a critical analysis of fracture toughness (FT) tests such that the assessment of the FT of dental ceramics is conducted in a reliable, repeatable and reproducible way.

METHODS

Fracture mechanics theory and FT methodologies were critically reviewed to introduce basic fracture principles and determine the main advantages and disadvantages of existing FT methods from the standpoint of the dental researcher.

RESULTS

The recommended methods for FT determination of dental ceramics were the Single Edge "V" Notch Beam (SEVNB), Single Edge Precracked Beam (SEPB), Chevron Notch Beam (CNB), and Surface Crack in Flexure (SCF). SEVNB's main advantage is the ease of producing the notch via a cutting disk, SEPB allows for production of an atomically sharp crack generated by a specific precracking device, CNB is technically difficult, but based on solid fracture mechanics solutions, and SCF involves fracture from a clinically sized precrack. The IF test should be avoided due to heavy criticism that has arisen in the engineering field regarding the empirical nature of the calculations used for FT determination.

SIGNIFICANCE

Dental researchers interested in FT measurement of dental ceramics should start with a broad review of fracture mechanics theory to understand the underlying principles involved in fast fracture of ceramics. The choice of FT methodology should be based on the pros and cons of each test, as described in this literature review.

Hydrothermal degradation of a 3Y-TZP translucent dental ceramic: A comparison of numerical predictions with experimental data after 2 years of aging

OBJECTIVES

The purpose of the study was to assess the hydrothermal resistance of a translucent zirconia with two clinical relevant surface textures by means of accelerated tests (LTD) and to compare predicted monoclinic fractions with experimental values measured after two years aging at 37°C.

METHODS

Polished (P) and ground (G) specimens were subjected to hydrothermal degradation by exposure to water steam at different temperatures and pressures. The t-m phase transformation was quantified by grazing incidence X-ray diffraction (GIXDR). The elastic modulus and hardness before- and after LTD were determined by nanoindentation.

RESULTS

G specimens presented a better resistance to hydrothermal degradation than P samples. Activation energies of 89 and 98kJ/mol and b coefficients of 2.0×10(-5) and 1.8×10(-6) were calculated for P and G samples respectively. The coefficients were subsequently used to predict transformed monoclinic fractions at 37°C. A good correlation was found between the predicted values and the experimental data obtained after aging at 37°C during 2 years. Hydrothermal degradation led to a significant decrease of the elastic moduli and hardness in both groups.

SIGNIFICANCE

The dependency of the t-m phase transformation rate on temperature must be determined to accurately predict the hydrothermal behavior of the zirconia ceramics at oral temperatures. The current prevailing assumption, that 5h aging at 134°C corresponds to 15-20 years at 37°C, will underestimate the transformed fraction of the translucent ceramic at 37°C. In this case, the mechanical surface treatment influences the ceramic's transformability. While mild grinding could potentially retard the hydrothermal transformation, polishing after occlusal adjustment is recommended to prevent wear of the antagonist teeth and maintain structural strength.

Modulation of osteoblast behavior on TiNxOy coatings by altering the N/O stoichiometry while maintaining a high thrombogenic potential

INTRODUCTION

Titanium nitride oxide (TiNxOy) coatings are known to stimulate osteoblast proliferation and osseointegration when compared to microrough titanium implants. The objectives of the present study were to determine whether the beneficial effects of TiNxOy coatings observed with implant osseointegration are dependent on N/O stoichiometry, with the final goal of optimizing these benefits. MMS: TiNxOy coatings with various N/O compositions were deposited on microrough titanium plates (Ti-SLA, 11 × 11 mm). Human primary osteoblast (hOBs) proliferation and gene expression were analyzed for a time course of three weeks, with or without additional stimulation by 1.25 (OH)2 vitamin D3 100 nM. Platelet adhesion/activation and thrombin generation were also assessed.

RESULTS

hOBs proliferation gradually increased with the amount of oxygen contained in the coatings. The effect was observed from day 7 to reach a maximum at day 10, with a 1.8 fold increase for the best coating as compared to Ti-SLA. SEM views indicated that cells adhered, spread and elongated faster on oxygen-rich TiNxOy films, while the differentiation process as well as the thombogenic potential was not affected.

CONCLUSIONS

The effect of TiNxOy coatings on osteoblast is dependent on their chemical composition; it increases with the amount of oxygen. TiNxOy coatings may act as a catalyst for cell-adhesion and proliferation early after seeding. In contrast, thrombogenicity of Ti-SLA surface is not affected by TiNxOy application.

Fracture Rates and Lifetime Estimations of CAD/CAM All-ceramic Restorations

The gathering of clinical data on fractures of dental restorations through prospective clinical trials is a labor- and time-consuming enterprise. Here, we propose an unconventional approach for collecting large datasets, from which clinical information on indirect restorations can be retrospectively analyzed. The authors accessed the database of an industry-scale machining center in Germany and obtained information on 34,911 computer-aided design (CAD)/computer-aided manufacturing (CAM) all-ceramic posterior restorations. The fractures of bridges, crowns, onlays, and inlays fabricated from different all-ceramic systems over a period of 3.5 y were reported by dentists and entered in the database. Survival analyses and estimations of future life revealed differences in performance among ZrO2-based restorations and lithium disilicate and leucite-reinforced glass-ceramics.

Thermally induced fracture for core-veneered dental ceramic structures

Effective and reliable clinical uses of dental ceramics necessitate an insightful analysis of the fracture behaviour under critical conditions. To better understand failure characteristics of porcelain veneered to zirconia core ceramic structures, thermally induced cracking during the cooling phase of fabrication is studied here by using the extended finite element method (XFEM). In this study, a transient thermal analysis of cooling is conducted first to determine the temperature distributions. The time-dependent temperature field is then imported to the XFEM model for viscoelastic thermomechanical analysis, which predicts thermally induced damage and cracking at different time steps. Temperature-dependent material properties are used in both transient thermal and thermomechanical analyses. Three typical ceramic structures are considered in this paper, namely bi-layered spheres, squat cylinders and dental crowns with thickness ratios of either 1:2 or 1:1. The XFEM fracture patterns exhibit good agreement with clinical observation and the in vitro experimental results obtained from scanning electron microscopy characterization. The study reveals that fast cooling can lead to thermal fracture of these different bi-layered ceramic structures, and cooling rate (in terms of heat transfer coefficient) plays a critical role in crack initiation and propagation. By exploring different cooling rates, the heat transfer coefficient thresholds of fracture are determined for different structures, which are of clear clinical implication.

TiNOx coatings on roughened titanium and CoCr alloy accelerate early osseointegration of dental implants in minipigs

INTRODUCTION

Titanium nitride oxide (TiNOx) coatings are known for their biocompatibility, hardness and high resistance to corrosion and wear. Further, they can be applied by plasma vapor deposition onto a wide variety of metallic, mineral, or organic substrates. In cell cultures, TiNOx coatings applied onto SLA (sandblasted, large grit, acid etched)-roughened titanium surfaces increased human primary osteoblast proliferation by 1.5 times in the first 2 weeks after seeding, while maintaining a high degree of cell differentiation. Therefore, the objectives of the present study were (i) to determine whether these findings would translate into the enhanced osseointegration of TiNOx-coated implants in vivo and (ii) to compare the osseointegration of Ti-SLA (titanium-SLA) and CoCr-SLA (cobalt-chromium-SLA) implants coated with TiNOx.

METHODS

Forty-eight cylinders made of Ti-SLA, Ti-SLA-TiNOx (TiNOx-coated Ti-SLA) and CoCr-SLA-TiNOx (TiNOx-coated CoCr-SLA) were implanted into the lower jawbone of 8 minipigs. The bone-to-implant contact was determined after 1 week, 2 weeks, 1 month and 3 months.

RESULTS

Osseointegration proceeded normally on all 3 surfaces, with equal activity after the first week of healing. After 2 weeks, bone-to-implant contact was 1.8 times higher on TiNOx coatings, either deposited on Ti or on CoCr. These differences fell off after 1 and 3 months of healing.

CONCLUSIONS

When compared to standard SLA titanium, TiNOx coatings enhance implant osseointegration during the first month of healing. Furthermore, this stimulating effect is independent of the substrate, leading to similar results whether the coating is applied onto SLA-Ti or onto SLA-CoCr.

Resistance of three implant-abutment interfaces to fatigue testing

The design and retentive properties of implant-abutment connectors affect the mechanical resistance of implants. A number of studies have been carried out to compare the efficacy of connecting mechanisms between abutment and fixture.

Low temperature degradation of a Y-TZP dental ceramic

Bars of Y-TZP ceramic for dental restorations were subjected to hydrothermal degradation via in vitro exposure to water steam at 140°C for 7 days. X-ray diffraction, atomic force microscopy and scanning electron microscopy techniques were applied to observe and quantify the tetragonal-monoclinic (t-m) phase transformation associated with the process. Nanoindentation was used to assess the ceramic's mechanical properties before and after hydrothermal degradation. Texture associated with martensitic t-m transformation was observed at the grain surface. The t-m transformation followed nucleation-and-growth kinetics, with predominance of the nucleation process. The transformation occurred within a layer of 6 μm below the surface. Mechanical properties deteriorated with hydrothermal degradation, resulting in a 30% reduction of Young's modulus and hardness. A strong correlation was found between the increasing monoclinic fraction and the decline in mechanical response. It was thus concluded that the emergence of the monoclinic phase and the associated microcracking were the most likely causes for the degradation of mechanical properties.

Fatigue behavior in water of Y-TZP zirconia ceramics after abrasion with 30 μm silica-coated alumina particles

OBJECTIVE

The use of a 30 μm alumina-silica coated particle sand (CoJet™ Sand, 3M Espe), has shown to enhance the adhesion of resin cements to Y-TZP. The question is whether or not sandblasting 30 μm particles does negatively affect the fatigue limit (S-N curves) and the cumulative survival of Y-TZP ceramics.

METHOD

Four zirconia materials tested were: Zeno (ZW) (Wieland), Everest ZS (KV) (KaVo), Lava white (LV) and Lava colored (LVB) (3M Espe). Fatigue testing (S-N) was performed on 66bar of 3 mm × 5 mm × 40 mm with beveled edges for each zirconia material provided by the manufacturers. One half of the specimens were CoJet sandblasted in the middle of the tensile side on a surface of 5 mm × 6 mm. Cyclic fatigue (N=30/group) (sinusoidal loading/unloading at 10 Hz between 10% and 100% load) was performed in 3-point-bending in a water tank. Stress levels were lowered from the initial static value (average of N=3) until surviving 1 million cycles. Fatigue limits were determined from trend lines. Kaplan-Meier survival analysis was performed to determine the failure stress at the median percentile survival level for 1 million of cycles before and after sandblasting. The statistical analyses used the log-rank test. Characterization of the critical flaw was performed by SEM for the majority of the failed specimens.

RESULTS

The fatigue limits "as received" (ctr) were: LV=720 MPa, LVB=600 MPa, KV=560 MPa, ZW=470 MPa. The fatigue limits "after CoJet sandblasting" were: LV=840 MPa, LVB=788 MPa, KV=645 MPa, ZW=540 MPa. The increase in fatigue limit after sandblasting was 15% for Zeno (ZW) and Everest (KV), 17% for Lava (LV) and 31% for Lava colored (LVB). The KM median survival stresses in MPa were: ZW(ctr)=549 (543-555), ZW(s)=587 (545-629), KV(ctr)=593 (579-607), KV(s)=676 (655-697), LVB(ctr)=635 (578-692), LVB(s)=809 (787-831), LV(ctr)=743 (729-757), LV(s)=908 (840-976). Log-rank tests were significantly different (p<0.001) for all sandblasted groups vs. the "as received" except for Zeno (Wieland) (p=0.295). Failures started from both intrinsic and machined flaws.

SIGNIFICANCE

30 μm particle sandblasting did significantly improve the fatigue behavior of three out of four Y-TZP ceramic materials and can therefore be recommended for adhesive cementation procedures. This study was supported in part by grants from the Swiss Society for Reconstructive Dentistry (SSRD) and 3M Espe.

Direct comparison of the bond strength results of the different test methods: a critical literature review

OBJECTIVE

The goal of this paper is to undertake a literature search collecting all dentin bond strength data obtained for six adhesives with four tests (shear, microshear, tensile and microtensile) and to critically analyze the results with respect to average bond strength, coefficient of variation, mode of failure and product ranking.

METHOD

A PubMed search was carried out for the years between 1998 and 2009 identifying publications on bond strength measurements of resin composite to dentin using four tests: shear, tensile, microshear and microtensile. The six adhesive resins were selected covering three step systems (OptiBond FL, Scotch Bond Multi-Purpose Plus), two-step (Prime & Bond NT, Single Bond, Clearfil SE Bond) and one step (Adper Prompt L Pop).

RESULTS

Pooling results from 147 references showed an ongoing high scatter in the bond strength data regardless which adhesive and which bond test was used. Coefficients of variation remained high (20-50%) even with the microbond test. The reported modes of failure for all tests still included high number of cohesive failures. The ranking seemed to be dependant on the test used.

SIGNIFICANCE

The scatter in dentin bond strength data remains regardless which test is used confirming Finite Element Analysis predicting non-uniform stress distributions due to a number of geometrical, loading, material properties and specimens preparation variables. This reopens the question whether, an interfacial fracture mechanics approach to analyze the dentin-adhesive bond is not more appropriate for obtaining better agreement among dentin bond related papers.

Optimization of Plasma Treatment, Manipulative Variables and Coating Composition for the Controlled Filling and Coating of a Microstructured Reservoir Stent

The object of the study was to fill and coat the microcavities of a drug eluting stent using a batch dipping process. 316L coronary stents, which were coated with a 0.25 μm layer of TiNOx were used as substrates. The stents’ surface was dimpled with 0.21 μl microcavities separated by distances of 17–28 μm depending on location. The experiment consisted of (1) optimizing the procedures to fill the microcavities with a solution of therapeutic agent and (2) covering the filled microcavities with a protective “lid” that shielded the solution during stent insertion in the arteries and then controlled its release into the surrounding tissue. The filling solution was a water-propanol mix containing 20% L-arginine. The coating solution was comprised of poly-ethylene-glycol (PEG-8000) and dexamethasone. The filling quality was investigated after altering the following variables: plasma surface activation (type of gas, pressure, power, and duration), water-propanol percentage ratio of the filling solution, lifting speed from the bath, and effect of ultrasonic vibration (monofrequency versus multifrequency). The surface coating was evaluated by altering the PEG-8000-dexamethasone percentage ratio and recording the effects on coating thickness and structure, on elution rate, and on wear resistance. The optimized process is presented in detail.

Using implant connectors to support clinical abrasion probes: A methodological study

OBJECTIVES

To validate a procedure aimed at assessing the clinical wear rate of full crown veneering materials. To confirm its clinical applicability and alignment capacity in the ancillary profiling apparatus. To quantify the wear rate of a full crown veneering composite (Sinfony, 3M-ESPE) as compared to that of a ceramic (negative control) and an unfilled resin (positive control).

METHODS

The technique consisted in utilizing implant supported crowns which were either screw-fastened to endosseous implants via ITI-Straumann Octa connectors or to an Octa analog on an x-y measuring table. After initial contour profiling, 14 crowns were placed in the mouths of seven patients. Six crowns were veneered with composite, four with ceramic and four with an unfilled resin. The crowns were removed at 3, 6, 9, 12, 15, 18 and 24 months and digitalized contour profiles were obtained. Contour profiling consisted in securing the restorations to the x-y table and assessing the z-coordinate using an LVDT stylus. At the end of the experimental period, loss of material was computed as the differences between the initial- and subsequent measurements.

RESULTS

The total error of the method was estimated at +/-10 microm. The composite's annual wear rate was 75 microm/year, the ceramic wore at ca. 6 microm/year and the unfilled resin wore at 160 microm/year. Wear was unevenly distributed and concentrated on occlusal contact areas.

SIGNIFICANCE

(1) A technique utilizing ITI-Straumann octagonal implant connectors is suitable for clinical use. (2) The veneering composite wears at a rate that is superior to ADA guidelines for composites intended for direct fillings.

Resistance of internal-connection implant connectors under rotational fatigue loading

PURPOSE

To aid in developing mechanically optimized implant-abutment connectors, the fatigue resistance of 5 connector configurations of the Replace Select system (Easy abutment, Easy abutment without antirotational mechanism, Multi-unit abutment, Esthetic Alumina abutment, Esthetic Zirconia abutment) was investigated. Other purposes of the study were to determine whether the connector's antirotational mechanism participates in fatigue resistance and to compare the results with previous data on Straumann connectors.

MATERIALS AND METHODS

The repetitive, alternating, and multivectorial intraoral force pattern was reproduced by subjecting the test specimens to the rotating cantilever beam test. To this end, the samples were spun around their long axis while clamped into a revolving collet on one end and loaded normal to their long axis on the other end. The aim was to determine the load level at which 50% of the specimens survived and 50% fractured before 106 cycles. Means were determined using the staircase procedure. They were fitted with 95% confidence intervals for intergroup comparisons.

RESULTS

In the chosen testing configuration, 2 statistical groups emerged. The Easy abutments with and without antirotational mechanism were statistically similar, with mean failure loads in the 70 to 72 N range. Both ceramic and the Multi-unit abutments belonged to the second group, with mean failure loads in the 53 to 58 N range.

CONCLUSIONS

(1) The fatigue resistance of ceramic and the Multi-unit abutments was approximately 20% less than that of the Easy Abutments. (2) The antirotational mechanism did not participate in mechanical resistance. (3) The fatigue strength of the Easy abutment connectors was approximately 20% greater than the equivalent abutments in the Straumann system.

Rotational fatigue-resistance of seven post types anchored on natural teeth

PURPOSE

To develop a laboratory model aimed at duplicating the failure process of post and core restorations. The load pattern applied was to be repetitive (fatigue) and multivectorial. To determine and compare the resistance under fatigue loading of seven endodontic post/natural root combinations: stainless steel-, titanium-, ceramic-, composite-fiber/epoxy-, two glass-fiber/epoxy- and glass-fiber/acrylic posts.

MATERIALS AND METHODS

The repetitive, alternating and multivectorial intraoral force pattern was reproduced by subjecting the specimens to the rotating cantilever beam test. To this end, the samples were designed as rotation-symmetric structures comprising a root, a post, periodontal ligament- and bone analogs and a restoration analog. The following posts were tested: Unimetric-Ti, Unimetric-SS, Biopost, Composipost, Easypost, DT Lightpost, Everstickpost. The samples were spun around their long axes while being clamped into a revolving collet on one end and loaded normal to their long axis on the other end. The aim was to determine the load level at which 50% of the specimens survived- and 50% fractured before 10E6 cycles. The 50% means were determined using the staircase procedure.

RESULTS

In increasing order of magnitude, the resistances to fatigue loading were as follows: Biopost, Unimetric-Ti, Unimetric-SS, Composipost, Easypost, Everstickpost, DT Lightpost.

SIGNIFICANCE

The fatigue resistance of the two fibrous posts with the highest fatigue resistance was twice that of any of the ceramic or metal posts.

Fractographic ceramic failure analysis using the replica technique

OBJECTIVES

To demonstrate the effectiveness of in vivo replicas of fractured ceramic surfaces for descriptive fractography as applied to the analysis of clinical failures.

METHODS

The fracture surface topography of partially failed veneering ceramic of a Procera Alumina molar and an In-Ceram Zirconia premolar were examined utilizing gold-coated epoxy poured replicas viewed using scanning electron microscopy. The replicas were inspected for fractographic features such as hackle, wake hackle, twist hackle, compression curl and arrest lines for determination of the direction of crack propagation and location of the origin.

RESULTS

For both veneering ceramics, replicas provided an excellent reproduction of the fractured surfaces. Fine details including all characteristic fracture features produced by the interaction of the advancing crack with the material's microstructure could be recognized. The observed features are indicators of the local direction of crack propagation and were used to trace the crack's progression back to its initial starting zone (the origin). Drawbacks of replicas such as artifacts (air bubbles) or imperfections resulting from inadequate epoxy pouring were noted but not critical for the overall analysis of the fractured surfaces.

SIGNIFICANCE

The replica technique proved to be easy to use and allowed an excellent reproduction of failed ceramic surfaces. It should be applied before attempting to remove any failed part remaining in situ as the fracture surface may be damaged during this procedure. These two case studies are intended as an introduction for the clinical researcher in using qualitative (descriptive) fractography as a tool for understanding fracture processes in brittle restorative materials and, secondarily, to draw conclusions as to possible design inadequacies in failed restorations.

Oscillatory shear loading of bovine periodontal ligament--a methodological study

This study examined the stress response of bovine periodontal ligament (PDL) under sinusoidal straining. The principle of the test consisted in subjecting transverse tooth, PDL and bone sections of known geometries to controlled oscillatory force application. The samples were secured to the actuator by support plates fabricated using a laser sintering technique to fit their contours to the tooth and the alveolar bone. The actuator was attached to the root slices located in the specimen's center. Hence the machine was able to push or pull the root relative to its surrounding alveolar bone. After determining an optimal distraction amplitude, the samples were cyclically loaded first in ramps and then in sinusoidal oscillations at frequencies ranging from 0.2 to 5 Hz. In the present study the following observations were made: (1) Imaging and the laser sintering technique can be used successfully to fabricate custom-made support plates for cross-sectional root-PDL-bone sections using a laser sintering technique, (2) the load-response curves were symmetric in the apical and the coronal directions, (3) both the stress response versus phase angle and the stress response versus. strain curves tended to "straighten" with increasing frequency, and (4) the phase lag between applied strain and resulting stress was small and did not differ in the intrusive and the extrusive directions. As no mechanical or time-dependent anisotropy was demonstrable in the intrusive and extrusive directions, such results may considerably simplify the development of constitutive laws for the PDL.

Failure analysis of ceramic clinical cases using qualitative fractography

PURPOSE

To educate dental academic staff and clinicians on the application of descriptive (qualitative) fractography for analyses of clinical and laboratory failures of brittle materials such as glass and ceramic.

MATERIALS AND METHODS

The fracture surface topography of failed glass, glass fiber-reinforced composite, and ceramic restorations (Procera, Cerestore, In-Ceram, porcelain-fused-to-metal) was examined utilizing a scanning electron microscope. Replicas and original failed parts were scrutinized for classic fractographic features such as hackle, wake hackle, twist hackle, arrest lines, and mirrors.

RESULTS

Failed surfaces of the veneering porcelain of ceramic and porcelain-fused-to-metal crowns exhibited hackle, wake hackle, twist hackle, arrest lines, and compression curl, which were produced by the interaction of the advancing crack with the microstructure of the material. Fracture surfaces of glass and glass fiber-reinforced composite showed additional features, such as velocity hackle and mirrors. The observed features were good indicators of the local direction of crack propagation and were used to trace the crack back to an initial starting area (the origin).

CONCLUSION

Examples of failure analysis in this study are intended to guide the researcher in using qualitative (descriptive) fractography as a tool for understanding the failure process in brittle restorative materials and also for assessing possible design inadequacies.

Fractographic analyses of three ceramic whole crown restoration failures

OBJECTIVES

Develop fractographic techniques to perform failure analyses of three ceramic whole-crown clinical failures.

METHODS

Three clinical whole-crown failures of different ceramic material systems were studied. The systems included appropriate veneers applied to core materials of cold isostatically pressed alumina, injection molded alumina/alumina-magnesia spinel, and hot pressed lithium disilicate. The surface topography was examined for classical fractographic features, utilizing both optical and scanning electron microscopes independently as well as group examination by four fractographers.

RESULTS

All three restorations displayed many common fractographic features, the most important of which was wake hackle. Wake hackle occurs when an approaching crack front sweeps by a small bubble or discontinuity in a brittle material, leaving a trail. The resulting trails can be used to trace back crack progression to an initial starting area. Wake hackle was found to be common in the veneer portions of the crowns, and the strategy of creating a wake hackle map enabled crack paths to be traced from a core area outward in all three restorations. The fracture origin areas were located where hoop stresses are high. Another helpful fracture feature was compression curl, an indication of flexural stress, most visible under low incident optical lighting.

SIGNIFICANCE

This study indicates that fractographic analyses can be productively performed on ceramic whole-crown failures. Optical examination under low incident lighting and wake hackle mapping were developed as promising techniques. Hoop stress was implicated as a concern in the three studied specimens, and should be considered when analyzing other whole-crown failures.

Resistance of ITI implant connectors to multivectorial fatigue load application

PURPOSE

In an effort to provide patients with mechanically optimized implant connectors, this study was designed to assess the relative fatigue resistance of five types of connectors for the ITI implant system: (1) standard (screw-on) abutments torqued to 35, 70, and 140 Ncm; (2) metal Octa connectors torqued to 35 Ncm; (3) ceramic Octa connectors torqued to 15 and 35 Ncm; (4) cemented cast-on abutments; and (5) an experimental screw-retained composite core.

MATERIALS AND METHODS

To duplicate the alternating and multivectorial intraoral loading patterns, the specimens were subjected to the rotating cantilever beam test. The implants, their connectors, and abutments were spun around their longitudinal axes while a perpendicular force was applied to the external end. The objective was to determine the force level at which 50% of the specimens would survive 106 load cycles. The mean force levels at 50% failure and their 95% confidence intervals were determined using staircase analysis.

RESULTS

The fatigue resistances of the standard (screw-on) abutment, metal Octa connector, and ceramic Octa connector torqued to 35 Ncm were within a few percent of one another. The fatigue resistance of the cemented cast-on abutment was approximately half that of the screwed connectors, and the experimental screw-retained composite core's resistance was about 30%. Increasing the preload in the standard abutments and ceramic Octa connectors increased their fatigue resistance.

CONCLUSION

Preloaded screwed components were mechanically superior to cemented cast-on abutments and screw-retained composite buildups. For the screw-on connectors, augmenting preload (ie, torque) augmented the resistance to fatigue loading.

Reversible and Irreversible Peri-implant Lesions: Report and Etiopathogenic Analysis of 7 Cases

The purpose of this report is to review the aberrations in the integration process of cylindrical endosseous implants, though such aberrations are seldom observed. These issues are treated according to the following scheme: (1) infectious lesions, which consist of peri-implantitis, intraosseous infectious foci, and septic voids; (2) transitory lesions; (3) lesions related to occlusal overload; and (4) healing defects. In this report, we illustrate these categories with patient histories and discuss the clinical findings and etiopathogenies.

Stimulating effect of implant loading on surrounding bone

A number of algorithms have been proposed to model the adaptive behavior of bone under load. However, the predictions of several models have neither been compared nor have they been systematically related to in vivo data. To this end, the stress states of loaded implant-bone interfaces were analyzed before and after osseointegration using finite element (FE) techniques. In a preliminary step, an FE mesh of a cylindrical implant encased in a cancellous core surrounded by a cortical layer was constructed, and the stresses and strains that developed at the interface were determined. The implant was loaded with 100 N vertical and 30 N lateral loads. Using this structure, the peak compressive and tensile stresses were determined. Then bone remodeling predictions were assessed using three different models: von Mises equivalent strain, strain energy density and effective stress. Finally, a systematic search of the literature was conducted to relate the numerical predictions to existing in vivo data. The FE simulations led to the following conclusions: (1) calculated compressive stresses were lower than the ultimate compressive stresses of cortical and cancellous bone. (2) Calculated tensile stresses were generally superior to experimental data on the tensile strength of the bone-implant interface. (3) With one exception, the predictions of all models were homogeneously grouped on the stimulus scales. (4) The predictions of the models as to bone gain or loss were not consistent and at times contradictory. It is hypothesized that this effect is linked to a lazy zone that is too narrow. With respect to the application of the numerical models to in vivo data, peak strains and strain energy densities were consistent with in vivo data. No in vivo data were found that supported effective stress as a stimulus.

Monotonic flexure and fatigue strength of composites for provisional and definitive restorations

STATEMENT OF PROBLEM

Ordinarily, the mechanical strength of composites is characterized by their flexural strength. Information as to the material's fatigue strength is seldom provided.

PURPOSE

The purpose of this study was to compare the flexural strength and the resistance to fatigue loading of composites and an acrylic resin for provisional and definitive restorations.

MATERIAL AND METHODS

Artglass, Colombus, and Targis (composites) and Jet, Protemp II, Protemp Garant, and Provipont DC (provisional restorations) were subjected to mechanical tests. Fatigue tests (MPa) (n = 30 specimens/group) were conducted with the rotating-bending cantilever design. Monotonic flexural strength (MPa) (n = 10) was determined in 3-point bending tests. Fatigue resistance was analyzed via the staircase procedure, and flexural strength was examined by use of the 2-parameter Weibull distribution (confidence intervals at 95%).

RESULTS

The mean fatigue resistances (S(50)) in MPa +/- SD were: Targis, 62.1 +/- 7.0; Artglass, 58.5 +/- 3.7; Colombus, 54.6 +/- 6.2; Provipont DC, 29.5 +/- 3.2; Protemp II, 23.1 +/- 5.3; Jet, 22.8 +/- 8.3; Protemp Garant, 19.6 +/- 4.6. The flexure strengths (Weibull's S(0)) in MPa and their shape parameters (m) were: Colombus, 145.2 (13.1); Targis, 110.3 (7.8); Artglass, 5.9 (5.4); Jet, 150.9 (17.3); Provipont DC, 97.3 (23.8); Protemp II, 57.9 (6.4); Protemp Garant, 54.2 (12.8). The S(50) of Targis was significantly higher than that of Colombus but not different from Artglass. In flexion, the S(0) of Colombus was significantly higher than that of Artglass and Targis. The S(50) ranged between 40% and 60% of the S(0) for the composites and between 15% and 30% for the provisional restorative materials.

CONCLUSIONS

Correlations between monotonic flexure strength and resistance to fatigue loading were weak. Because fatigue tests are considered more pertinent than monotonic tests as to their predictive value, it is concluded that flexure strength data alone may not provide relevant information for long-term clinical performance. The material's resistance to fatigue loading should also be determined.

Fatigue resistance of ITI implant-abutment connectors -- a comparison of the standard cone with a novel internally keyed design

The Straumann Company has recently supplemented its standard 'morse-taper' configuration with an octagonal internal key. During the restorative phase of implant treatment, this additional feature was designed to ensure positional duplicability between the laboratory and the clinical environments. It was, however, unclear whether this keying mechanism would decrease the mechanical strength of the connection between the implant and the abutment. This applies to keyed male and female parts but also to combinations of the new and the standard designs. Specially constructed specimens analogs representing all three combinations were fitted with a T-shaped bar, preangled to 15 degrees and subjected to vertical force applications provided by a servohydrolic fatigue tester. The loading frequency was 2 Hz and the maximum cycle number was 106. The data were evaluated using the staircase technique. The specimens were also modeled and analyzed numerically using finite element procedures. The samples' failure locations were recorded and the displacement vs. cycle number plots were patterned in four groups. The fatigue tests and staircase analysis showed no difference in mechanical resistance between the standard and the internally keyed connectors. The finite element models revealed a stress concentration located at the apical edges of the octagonal connector. However, it appeared that this phenomenon was based on computational rather than mechanical grounds. The locations of the failure sites were distributed randomly across the structures, thereby indicating the absence of a locus of minor resistance. The patterns of the displacement vs. cycle number could not be attributed to specific combinations between the standard and the internally keyed designs. It was concluded that both connectors are equal in their mechanical resistance to bending and torquing forces.

Mechanical characterization of bovine periodontal ligament

This study is part of a research program that aims to develop a constitutive three-dimensional model of the periodontal ligament (PDL) through the identification of pertinent material parameters. As part of this program, bovine PDL was utilized to establish stress-strain responses under tensile and compressive loading conditions. Fresh bovine molars were secured, frozen and prepared to appropriate dimensional specifications. Bar-shaped specimens that comprised portions of dentine, PDL and bone were produced. Push-pull tests were conducted using a specifically constructed loading machine. Full range monotonic stress-strain diagrams were generated. The effect of a rate increase on cyclic S-E diagrams was also determined. The influence of specimen thickness was expressed in terms of modulus of elasticity, strength, uniaxial maximizer strain, and strain energy density. The overall load-response was heavily hysteretic in compression. On the tensile side, after a steep rise, the curve tended to flatten out asymptotically. Variations in rate that spanned four orders of magnitude had no effect on reciprocal load responses. The E-modulus was in the 4-8 MPa range, the strength of the PDL was 1-2 MPa, the maximizer strain was at 45-60% and the strain energy density ranged between 0.3 and 0.4 MPa.

Weibull parameters of composite resin bond strengths to porcelain and noble alloy using the Rocatec system

OBJECTIVE

The aim of the study was to determine the Weibull distribution parameters S(0) and m of the tensile bond strength of a composite resin sandwiched between a noble metal alloy and a feldspathic porcelain using the Rocatec system.

METHODS

Specimens were prepared either as Au-Pt-Pd alloy cylinders alone or as alloy cylinders and a feldspathic porcelain. Surface treatments included 110 microm Al(2)O(3) sandblasting (Rocatec Pre), tribochemical silica coating (Rocatec Plus) and silane application (ESPE-Sil) prior to composite bonding. The tensile bond strengths of ceramic-resin-ceramic and metal-resin-metal specimens were determined before and after thermocycling at 5-60 degrees C for 1000 cycles. Statistical analyses were conducted using two parameter Weibull distributions determined by the characteristic strength (S(0)) at the 63% probability of failure, the Weibull modulus (m) and the 95% confidence intervals.

RESULTS

With respect to the control group (no Rocatec) (7.5MPa), the Rocatec system significantly augmented the characteristic tensile bond strength S(0) of metal (17-18MPa) and porcelain (20-28MPa) groups. Thermocycling had no effect on the tensile bond strength of the metal samples and increased (not significant) the bond of the porcelain group. The Weibull modulus m for all groups ranged from 2.8 to 3.7.

SIGNIFICANCE

This study confirmed the efficacy of the Rocatec system for bonding composite resin to metal or ceramic substrates. However, the variability of the strength of the bond is shown in the low Weibull modulus values obtained. This reflect the difficulties in controlling flaw formation and warrants research for future improvement.

In vivo wear of three types of veneering materials using implant-supported restorations: a method evaluation

In a previous study, we determined the precision and the positional duplicability of a system that used implant connectors to transfer restorations between the oral cavity and the measuring device. The objectives of the present study were to determine whether this testing procedure was suitable for clinical use, and to apply the procedure to the longitudinal assessment of the wear of three types of veneering materials. Ten patients received a total of 12 restorations. The restorations were made either of ceramic (positive control), poly(methylmethacrylate) resin (negative control) or of composite resin. The restorations were profiled at baseline and at 3, 6, 9, 12, 18 and 24 months. For profiling, the restorations were secured to the x-y table of the measuring device using the octagonal connector of the ITI implant system. Numerical analyzes were performed using a commercial array-oriented software package. The ceramic and the composite wore at roughly 12-14 microm yr(-1) (height) 10-12 microm3 microm2 yr(-1) (volume). The resin wore at c. 50 micorm yr(-1) (height) and c. 45 microm3 microm2 yr(-1) (volume). The error of the procedure was estimated at +/- 13%. It was concluded that the procedure was applicable for clinical studies, and that the composite did not differ from the ceramic as to its wear rate.

Mechanical and structural characteristics of commercially pure grade 2 Ti welds and solder joints

This study aimed at determining whether data previously gathered for a laser welds and IR brazings using a Au-Pd alloy were applicable to titanium joints. As to its resistance under fatigue loading, Au-Pd alloy had shown a poor response to pre-ceramic laser welding and post-ceramic brazing. The present study was designed to assess the mechanical resistance, the microstructure and the elemental diffusion of laser welded, electric arch welded and brazed joints using commercially pure titanium as substrate metal. Mechanical resistance was determined by determining the joints' ultimate tensile strength and their resistance to fatigue loading. Elemental diffusion to and from the joints was assessed using microprobe tracings. Optical micrographs of the joints were also obtained and evaluated. Under monotonic tensile stress, three groups emerged: (1) the GTAW and the native (i.e. as received) substrate, (2) the annealed substrate and the laser welds and (3) the brazed joints. Under fatigue stress, the order was: first the native and annealed substrate, second the brazings and laser welds, third the GTAW joints. No Au-filler brazing withstood the applied fatigue loading. The micrographs showed various patterns, an absence of HAZ cracking and several occurrences of Widmanstätten structures. Elemental diffusion to and from the Ti substrate was substantial in the Ti filler brazings and virtually nil in the Au-based brazings. Under fatigue stress application, the titanium-based brazings as well as the laser- and electric arc welds performed equally well if not better than a previously tested AuPd alloy. There was a definite increase in grain size with increased heat application. However, no feature of the microstructures observed or the elemental analysis could be correlated with the specimen's resistance to fatigue stress application.

Effect of water exposure on the fracture toughness and flexure strength of a dental glass

OBJECTIVES

The low fusing dental glass (Duceram LFC) has been advertised as presenting a superior chemical resistance and augmented strength after 16h exposure to water or 4% acetic acid. The purpose of this study was to evaluate the effect of prolonged exposure to water on two mechanical properties (fracture toughness and flexure strength) of LFC.

METHODS

Disks and bars were mirror polished and annealed prior to aging in: (1) air (control), (2) water for 24h at 80 degrees C and (3) water for 8 weeks at 80 degrees C. Fracture toughness (K(Ic)) was determined by indentation fracture (IF) and indentation strength (IS) using a 19.6N Vickers indentation load. Flexure strength values were obtained from three-point bending at 0.1mm/min. Statistical analysis was performed using the Weibull distribution, Tukey and Bartlett tests (P<0.05).

RESULTS

Both techniques (IS and IF) showed a significant improvement in the K of Duceram LFC after 8 weeks in water (0.88 and 1.14MPa m(0.5)) as opposed to the 24-h values both in water and air (0.77-0.78MPa m(0.5)). However, for flexure strength the Weibull characteristic (S(0)) and the m parameter did not change significantly with water storage (S(0)=90-100MPa, Weibull m =7-8).

SIGNIFICANCE

The increase in toughness of Duceram LFC after aging in water is an interesting and favorable observation for a restorative material exposed to the oral environment. Nevertheless, in comparison with other contemporary ceramics, the toughness of this LFC remains in the range of soda-lime-glass or classic feldspar porcelains.

Incidence of fractures and lifetime predictions of all-ceramic crown systems using censored data

PURPOSE

To determine Kaplan-Meier survival estimates and Weibull lifetime predictions for four all-ceramic crown systems from long-term data (> 5 yrs).

MATERIALS AND METHODS

Single unit crowns of Cerestore (n = 30), Dicor (n = 30), Hi-Ceram (n = 22) and In-Ceram (n = 68) were placed in 95 patients treated in a university clinic. They were cemented using glass-ionomer (GI) for Cerestore, zinc phosphate (ZP) for Dicor, and 75% ZP (n = 51), 20% GI (n = 13) and 5% resin-based cement (n = 4) for In-Ceram crowns. The follow-up times were 8 yrs for Cerestore, 7 yrs for Dicor, 6 yrs for Hi-Ceram and 5 yrs for In-Ceram. The statistical analyses were based on censored data sets. A progressively censored Weibull distribution allowing for lifetime predictions beyond the actual observation time was used as well as the Kaplan-Meier Survival Product Limit Estimate (PLE), which provides survival estimates up to the maximum time of follow-up.

RESULTS

Cerestore, Dicor and Hi-Ceram demonstrated molar fractures in the first 2 yrs, whereas for In-Ceram these occurred during the third and fourth year of the study. The Kaplan-Meier Survival PLE was 69% for Cerestore at 8 yrs, 86% for Dicor at 7 yrs, 81% for Hi-Ceram at 6 yrs, 92% for In-Ceram at 5 yrs. The predicted Weibull characteristic time T0 (time at which 63% of the restorations would have failed) was 23 yrs for Cerestore, 34 yrs for Dicor, 31 yrs for Hi-Ceram, and 16 yrs for In-Ceram. However, when using data sets arbitrarily limited to the three first years of follow-up, T0 decreased significantly for Cerestore, Dicor and Hi-Ceram due to the number of early fractures. Such Weibull lifetime data illustrate the risk of predicting long-term (> 5 yrs) survival using short-term (< or = 3 yrs) data on ceramic restorations, which exhibit fracturing in the initial years.

Microstructures of brazings and welds using grade 2 commercially pure titanium

PURPOSE

Microstructural analyses of commercially pure titanium (CpTi) are scarce. The present report presents the micrographs, fractographs, elemental characteristics, and hardness profiles of brazed joints and weldments using machined rods of CpTi.

MATERIALS AND METHODS

CpTi rods were joined using four techniques: laser welding, electric-arc welding, electron-beam welding, and gold- and Ti-filler brazing. The specimens were then subjected to tensile and fatigue loading. After sectioning and patterning, optical micrographs of intact joints were obtained. Fractured surfaces were investigated using scanning electron microscopy (SEM). The joint's composition was determined by SEM-energy dispersive x-ray analysis. Hardness was determined at specific locations using a microindenter.

RESULTS

While laser welding left the parent metal's equiaxed structure fairly intact, electric-arc welding, electron-beam welding, and brazing created a heat-affected zone in the vicinity of the joint. The extent and characteristics of the heat-affected zone depended on the amount of heat transferred to the specimens. In this respect, brazing essentially increased grain size and altered their shape. Electron-beam welding augmented this phenomenon, yielding grains that encompassed the full diameter of the joint. Electric-arc welding disrupted the granular pattern and generated highly lamellar/acicular structures.

CONCLUSION

Hardness was not a good indicator of mechanical resistance, nor was the joint's structural continuity with the parent substrate. Still, acicular microstructures were characterized by a peculiar behavior in that such joints were highly resistant to tensile stresses while their fatigue strength ranged among the lowest of the joints tested.

Fracture toughness of aged dental composites in combined mode I and mode II loading

Resin-based laboratory dental composites for prosthetic restorations have been developed in the past years as a cost-effective alternative to conventional porcelain-fused-to-metal or full ceramic restorations. The fracture toughness at different stress states (K(Ic), K(IIc), and mixed-modes K(I), K(II) ) was assessed for three laboratory dental composite resins used for prosthetic restorations that were aged up to 12 months in a food simulating fluid (10% ethanol) at 37 degrees C. The materials were mainly di- methacrylate based resins reinforced with submicron glass filler particles. The Brazilian disk test was used on precracked chevron-notched specimens, and different stress states were obtained by angulating the precracked chevron notch relative to the diametral compressive loading direction. The stress intensity factors were calculated using Atkinson et al.'s relation. For all three materials, mode I fracture toughness values ranged between 0.48-0.64 MPa. m(0.5) and mode II values ranged between 0.93-1.2 MPa. m(0.5). Overall, aging time and storage media had little effect on toughness. Considering the inherently low toughness of these restorative materials, their use should be limited to low stress masticatory areas.

A new method to quantify wear using implant supported restorations

OBJECTIVES

To design a novel technique to assess the wear of prosthodontic veneering materials. Further to determine whether accurate transfer between the oral cavity and the measuring device is achievable and assess the reproducibility of the coordinates generated by the measuring system.

METHODS

The system is based on the repositioning capacity of an octagonal connector of the ITI implant system. The same type of connector was screwed onto the clinical implants that supported the experimental restorations and secured to the x-y table of the measuring device. The measuring setup also comprised a z-axis LVDT displacement gauge that allowed the entire surface of the restorations to be profiled and digitized. The system was under the control of a PC equipped with custom-made software that set the position of the stepping motors, lifted and lowered the z-axis probe, and registered and wrote the x-, y- and z-axis coordinates. Final numerical adjustments and analyses were performed using a commercial array-oriented software package. Validation procedures were performed using a specially designed calibration surface.

RESULTS

On repeated profile tracings, the measurement error was less than 2 microns. When the calibration surface was removed between measurements as during clinical trials, the measurement error increased to ca. 5 microns.

SIGNIFICANCE

The measurement error of the testing procedure including transfer to and from the mouth is +/- 5 microns.

Fracture toughness (KIc) of a dental porcelain determined by fractographic analysis

OBJECTIVES

Fractographic analysis of indentation cracks is performed following flexure testing as part of the ASTM (1999) standard for fracture toughness, KIc, determination in advanced ceramics. This method depends on the conduciveness of the material towards fractographic interpretation. The purpose of this study was to evaluate the use of fractography in fracture toughness methods with a feldspathic dental porcelain, in which KIc was measured fractographically as well as numerically using two controlled-flaw beam bending techniques.

METHODS

The following methods for KIc determination were applied to a dental porcelain containing a leucite volume fraction of 15-20%: (1) surface crack in flexure (SCF) (dependent upon fractographic analysis); and (2) indentation strength (IS) at indentation loads of 9.8 and 19.6 N (applying both standard numeric calculations and fractographic analysis). The testing environments were (1) ambient air (IS and SCF) and (2) flowing dry nitrogen (SCF).

RESULTS

No significant differences were found between numeric and fractographic KIc values for the IS technique at both indentation loads (9.8 and 19.6 N) in ambient air, although KIc values were sensitive to indentation load. Due to the presence of residual stresses, stable crack extension was observed fractographically in all IS specimens, as evidenced by differences between initial (ainitial) and critical (acritical) crack dimensions. For the SCF method, there was a significant difference in toughness between specimens tested in air versus dry nitrogen, however no fractographic evidence for chemically assisted slow crack growth (SCG) was observed.

SIGNIFICANCE

The SCF method as described by the ASTM standard was applicable to the feldspathic porcelain and produced very comparable results with the numeric toughness calculations of the IS procedure. However, fractographic analysis of the surface crack was somewhat difficult for this glassy ceramic compared with polycrystalline ceramics. Knowledge about stable crack extension or slow crack growth and its fractographic appearance is essential when estimating the toughness from examination of flaw dimensions on fractured surfaces since large calculation errors may occur if these effects are not taken into account.