Influence of substructure design and occlusal reduction on the stress distribution in metal ceramic complete crowns: 3D finite element analysis.

PURPOSE: Occlusal reduction is considered a fundamental step for providing adequate and uniform space for the ceramic prosthesis; however, a flat occlusal surface is usually found. The prosthesis design influences the resistance to deformation and the stress state within the ceramic. This finite element (FE) study analyzes the influence of changing the substructure design on the stress distribution of a metal-ceramic crown in a premolar tooth with three types of occlusal reduction. MATERIALS AND METHODS: Each part of three-dimensional metal ceramic complete crown models was designed according to the space provided by different levels of occlusal reduction and the same external morphology of the tooth. Three models were designed: (1) correct occlusal reduction with a uniform thickness of the substructure (0.3 mm) and the veneering porcelain (1.5 mm); (2) flat occlusal reduction with different thicknesses of veneering porcelain to produce a uniform substructure; and (3) a flat occlusal reduction with different thicknesses of substructure for a uniform thickness of veneering porcelain. RESULTS: Stress distributions were very similar in the three models. The highest tensile stresses were concentrated immediately below the midline fissure in both the veneering porcelain and the metal alloy substructure. Although models with flat occlusal reduction had lower stress values, this preparation results from a reduction that removes a greater amount of sound tissue, which may increase the probability of dental pulp injury. CONCLUSIONS: Occlusal reduction must be anatomic; however, when a flat occlusal reduction already exists, the substructure must reproduce the correct anatomic form to allow a uniform thickness of the veneering porcelain.

Randomized clinical trial of implant-supported ceramic-ceramic and metal-ceramic fixed dental prostheses: preliminary results.

PURPOSE: The aim of this study was to determine the survival rates over time of implant-supported ceramic-ceramic and metal-ceramic prostheses as a function of core-veneer thickness ratio, gingival connector embrasure design, and connector height. MATERIALS AND METHODS: An IRB-approved, randomized, controlled clinical trial was conducted as a single-blind pilot study involving 55 patients missing three teeth in either one or two posterior areas. These patients (34 women; 21 men; age range 52-75 years) were recruited for the study to receive a three-unit implant-supported fixed dental prosthesis (FDP). Two implants were placed for each of the 72 FDPs in the study. The implants (Osseospeed, Astra Tech), which were made of titanium, were grit blasted. A gold-shaded, custom-milled titanium abutment (Atlantis, Astra Tech), was secured to each implant body. Each of the 72 FDPs in 55 patients were randomly assigned based on one of the following options: (1) A. MATERIAL: ceramic-ceramic (Yttria-stabilized zirconia core, pressable fluorapatite glass-ceramic, IPS e.max ZirCAD, and ZirPress, Ivoclar Vivadent) B. metal-ceramic (palladium-based noble alloy, Capricorn, Ivoclar Vivadent, with press-on leucite-reinforced glass-ceramic veneer, IPS InLine POM, Ivoclar Vivadent); (2) occlusal veneer thickness (0.5, 1.0, and 1.5 mm); (3) curvature of gingival embrasure (0.25, 0.5, and 0.75 mm diameter); and (4) connector height (3, 4, and 5 mm). FDPs were fabricated and cemented with dual-cure resin cement (RelyX, Universal Cement, 3M ESPE). Patients were recalled at 6 months, 1 year, and 2 years. FDPs were examined for cracks, fracture, and general surface quality. RESULTS: Recall exams of 72 prostheses revealed 10 chipping fractures. No fractures occurred within the connector or embrasure areas. Two-sided Fisher's exact tests showed no significant correlation between fractures and type of material system (p = 0.51), veneer thickness (p = 0.75), radius of curvature of gingival embrasure (p = 0.68), and connector height (p = 0.91). CONCLUSIONS: Although there were no significant associations between connector height, curvature of gingival embrasure, core/veneer thickness ratio, and material system and the survival probability of implant-supported FDPs with zirconia as a core material, the small number of fractures precludes a definitive conclusion on the dominant controlling factor.

Atomic force microscopy observation of the enamel roughness and depth profile after phosphoric acid etching.

The aim was to compare the enamel surface roughness (ESR) and absolute depth profile (ADP) (mean peak-to-valley height) by atomic force microscopy (AFM) before and after using four different phosphoric acids. A total of 160 enamel samples from 40 upper premolars were prepared. The inclusion criterion was that the teeth have healthy enamel. Exclusion criteria included any of the following conditions: facial restorations, caries lesions, enamel hypoplasia and dental fluorosis. Evaluations of the ESR and ADP were carried out by AFM. The Mann-Whitney U-test was used to compare continuous variables and the Wilcoxon test was used to analyze the differences between before and after etching. There were statistically significant differences (P

Thermal compatibility of dental ceramic systems using cylindrical and spherical geometries.

OBJECTIVE: To test the hypothesis that bilayer ceramic cylinders and spheres can provide valid confirmation of thermal incompatibility stresses predicted by finite element analyses. METHODS: A commercial core ceramic and an experimental core ceramic were used to fabricate open-ended cylinders and core ceramic spheres. The core cylinders and spheres were veneered with one of four commercial dental ceramics representing four thermally compatible groups and four thermally incompatible groups. Axisymmetric thermal and viscoelastic elements in the ANSYS finite element program were used to calculate temperatures and stresses for each geometry and ceramic combination. This process required a transient heat transfer analysis for each combination to determine input temperatures for the structural model. RESULTS: After fabrication, each specimen was examined visually using fiberoptic transillumination for evidence of cracking. There were 100% failures of the thermally incompatible cylinders while none of the thermally compatible combinations failed. Among the spheres, 100% of the thermally incompatible systems failed, 16% of one of the thermally compatible systems failed, and none of the remaining compatible combinations failed. The calculated stress values were in general agreement with the experimental observations, i.e., low residual stresses for the specimens that did not fail and high residual stresses for the specimens that did fail. SIGNIFICANCE: Simple screening geometries can be used to identify highly incompatible ceramic combinations, but they do not identify marginally incompatible systems.

Clinical performance and wear characteristics of veneered lithia-disilicate-based ceramic crowns.

OBJECTIVES: The objectives of this study were to characterize the clinical performance and wear characteristics of lithia-disilicate-based ceramic crowns. METHODS: Thirty posterior crowns were made using the heat-pressing technique and lithia-disilicate-based core ceramic. Subjects were recalled annually. The quality of crowns and adjacent gingival tissues were examined using nine criteria for acceptability. All crowns were examined and ranked from 4 (Excellent) to 1 (Unacceptable) for each criterion. Impressions were made for replica models at each appointment. Wear characteristics of dental ceramic and enamel were obtained by comparing the surface of the original model with the follow-up model using a laser scanner. RESULTS: Twenty-nine subjects returned for the 1-year recall examination. The maximum clenching force for the 30 subjects ranged from 125 to 815 N. All clinical criteria were ranked good to excellent at the 1-year recall exam and no fractures were observed. The mean occlusal wear volumes for the ceramic crowns after 1 year were 0.19 (0.065)mm3 for premolar sites and 0.34 (0.08)mm3 for molar sites. The mean occlusal wear volumes of opposing enamel after 1 year were 0.21 (0.06)mm3 for premolar teeth and 0.50 (0.22)mm3 for molar teeth. The mean occlusal wear volume of ceramic molar crowns was significantly lower than the volume of enamel wear of the opposing teeth (p

The effects of viscoelastic parameters on residual stress development in a zirconia/glass bilayer dental ceramic.

OBJECTIVES: The aim of this study was to test the hypothesis that the residual stresses in a zirconia-based bilayer dental composite system can be tailored through heat treatment above and below the glass transition temperature of glass veneers. METHODS: Ceramic bilayer disc specimens were prepared from a zirconia core and a glass veneer. Each bilayer ceramic group was heat treated 40 degrees C below, 20 degrees C and 40 degrees C above and at the glass transition temperature of the glass veneer, and cooled using a fast or a slow cooling rate. Specimens were tested for flexure strength using a biaxial bending fixture. Residual stresses were calculated using a fracture mechanics approach. RESULTS: Heat treatments produced significant differences (p < or =0.05) between the mean flexural strengths of the heat treatment groups when the specimens were cooled using a fast cooling rate. However, there was not a significant difference (p >0.05) between the mean flexural strengths of the heat treatment groups when a slow cooling rate was used. Fractures initiated from the veneer surfaces of the specimens. SIGNIFICANCE: Heat treatment above and below the glass transition temperature of the veneer layer, and the cooling rate have a significant effect on the flexural strength of the bilayer ceramic laminates. The existence of residual compressive stress is the most likely reason for the observed strength increases. Residual stresses can be modified using the elastic-viscoelastic relaxation behavior of a glass veneer.

Viscoelastic finite element analysis of an all-ceramic fixed partial denture.

In recent years metal-free ceramic systems have become increasingly popular in dental practice because of their superior aesthetics, chemical durability and biocompatibility. Recently, manufacturers have proposed new dental ceramic systems that are advertised as being suitable for posterior fixed partial dentures (FPDs). Reports indicate that some of these systems have exhibited poor clinical performance. The objective of this study was to use the viscoelastic option of the ANSYS finite element program to calculate residual stresses in an all-ceramic FPD for four ceramic-ceramic combinations. A three-dimensional finite element model of the FPD was constructed from digitized scanning data and calculations were performed for four systems: (1) IPS Empress 2, a glass-veneering material, and Empress 2 core ceramic; (2) IPS Eris a low fusing fluorapatite-containing glass-veneering ceramic, and Empress 2 core ceramic; (3) IPS Empress 2 veneer and an experimental lithium-disilicate-based core ceramic; and (4) IPS Eris and an experimental lithium-disilicate-based core ceramic. The maximum residual tensile stresses in the veneer layer for these combinations are as follows: (1) 77 MPa, (2) 108 MPa, (3) 79 MPa, and (4) 100 MPa. These stresses are relatively high compared to the flexural strengths of these materials. In all cases, the maximum residual tensile stresses in the core frameworks were well below the flexural strengths of these materials. We conclude that the high residual tensile stresses in all-ceramic FPDs with a layering ceramic may place these systems in jeopardy of failure under occlusal loading in the oral cavity.

Residual stresses in bilayer dental ceramics.

It is clinically observed that lithia-disilicate-based all-ceramic fixed partial dentures (FPD) can fail because of the fragmentation of the veneering material. The hypothesis of this study is that the global residual stresses within the surface of those veneered FPDs may be responsible for partial fragmentation of the veneering ceramic. Bilayer and monolithic ceramic composites were prepared using a lithia disilicate based (Li2OSiO2) glass-ceramic core and a glass veneer. A four-step fracture mechanics approach was used to analyze residual stress in bilayered all-ceramic FPDs. We found a statistically significant increase in the mean flexural strengths of bilayer specimens compared with monolithic glass specimens (p < or = 0.05). There was a statistically significant difference between the mean longitudinal and transverse indentation-induced crack sizes in bilayer specimens (p < or = 0.05), which indicates the existence of residual stress. Global residual stresses in the veneer layer, calculated using a fracture mechanics equation, were determined to be responsible for the increased strength and observed chipping, i.e., spallation in bilayer ceramic composites.

Mammalian cell delivery via aerosol deposition.

The objective of this study was to test the hypothesis that bovine dermal fibroblasts can survive aerosol delivery via an airbrush with mean cell survival rates greater than 50%. This technology has great implications for burn and other wound therapies, for delivery of genetically altered cells in gene therapies, and for tissue engineering with tissue scaffolds. Bovine dermal fibroblasts were suspended at a concentration of 200,000 cells/mL in Hank's Balanced Salt Solution, and delivered into six-well tissue culture plates using a Badger 100G airbrush. Cells were delivered through three nozzle diameters (312, 484, and 746 microm) at five different air pressures (41, 55, 69, 96, and 124 kPa). Nine repetitions were performed for each treatment group, and cell viability was measured using trypan blue exclusion assay. Mean cell viability ranged from 37 to 94%, and depended on the combination of nozzle diameter and delivery pressure (p < 0.0001). Linear regression analysis was used to develop a stochastic model of cell delivery viability as a function of nozzle diameter and delivery air pressure. This study demonstrates the feasibility of using an airbrush to deliver viable cells in an aerosol to a substrate.

Present and future approaches for the control of caries.

This article summarizes current and potential future approaches for the management of caries. Current surveys suggest that traditional "drill, fill, and bill" dentistry is still widely practiced in the United States in spite of considerable evidence that supports a minimally invasive treatment approach. Because there is a wide variability in treatment decisions on when and how to prevent new lesions, on how to arrest the progression of existing lesions, and on when and how to place initial and replacement restorations, the findings from some studies differ significantly from the results of other studies. While fluoride treatments are known to prevent a percentage of new lesions, they do not have the ability to prevent all caries lesions. Modern management of caries entails treating patients according to risk and monitoring early lesions in tooth surfaces that are not cavitated. Although we know that the dmfs score for children is a powerful predictor of caries increment in permanent teeth of these children a few years later, this score is rarely used in private practice as a measure of risk or as a measure of treatment success. Although these modern methods for caries management offer great promise for controlling the disease, they may take decades to apply in a standardized way so that the variability in treatment is reduced. However, during the next two decades, an alternative approach to caries prevention such as replacement therapy and a caries vaccine may become available as a more consistent method of controlling this disease.

Creep functions of dental ceramics measured in a beam-bending viscometer.

OBJECTIVE: To characterize the high temperature viscoelastic properties of several dental ceramics by the determination of creep functions based on mid-span deflections measured in a beam-bending viscometer (BBV). METHODS: Six groups of beam specimens (58 x 5.5 x 2.5 mm) were made from the following materials: (1) IPS Empress2 body--a glass veneer ceramic (E2V); (2) an experimental glass veneer (EXV); (3) Vita VMK 68 feldspathic body porcelain--a low-expansion body porcelain (VB); (4) Will-Ceram feldspathic body porcelain--a high-expansion body porcelain (WCB); (5) Vita feldspathic opaque porcelain--a medium-expansion opaque porcelain (VO); and (6) Will-Ceram feldspathic opaque porcelain--a high-expansion opaque porcelain (WCO). Midpoint deflections for each specimen were measured in a BBV under isothermal conditions at furnace temperatures ranging from 450 to 675 degrees C. Non-linear regression and linear regression analyses were used to determine creep functions and shear viscosities, respectively, for each material at each temperature. RESULTS: The shear viscosities of each group of dental ceramics exhibited bilinear Arrhenius behavior with the slope ratios (x) ranging from 0.19 for WCB to 0.71 for EXV. At the higher temperature ranges, activation energies ranged from 363 kJ/mol for VO to 386 kJ/mol for E2V. SIGNIFICANCE: The viscoelastic properties of dental ceramics at high temperatures are important factors in understanding how residual stresses develop in all-ceramic and metal-ceramic dental restorations.

Shear stress relaxation of dental ceramics determined from creep behavior.

OBJECTIVE: To test the hypothesis that shear stress relaxation functions of dental ceramics can be determined from creep functions measured in a beam-bending viscometer. METHODS: Stress relaxation behavior was determined from creep data for the following materials: (1) a veneering ceramic-IPS Empress2 body ceramic (E2V); (2) an experimental veneering ceramic (EXV); (3) a low expansion body porcelain-Vita VMK 68 feldspathic body porcelain (VB); (4) a high expansion body porcelain-Will Ceram feldspathic body porcelain (WCB); (5) a medium expansion opaque porcelain-Vita feldspathic opaque porcelain (VO); and (6) a high expansion opaque porcelain-Will Ceram feldspathic opaque porcelain (WCO). Laplace transform techniques were used to relate shear stress relaxation functions to creep functions for an eight-parameter, discrete viscoelastic model. Nonlinear regression analysis was performed to fit a four-term exponential relaxation function for each material at each temperature. The relaxation functions were utilized in the ANSYS finite element program to simulate creep behavior in three-point bending for each material at each temperature. RESULTS: Shear stress relaxation times at 575 degrees C ranged from 0.03 s for EXV to 195 s for WCO. SIGNIFICANCE: Knowledge of the shear relaxation functions for dental ceramics at high temperatures is required input for the viscoelastic element in the ANSYS finite element program, which can used to determine transient and residual stresses in dental prostheses during fabrication.

Work of adhesion of resin on treated lithia disilicate-based ceramic.

OBJECTIVE: This study is to test the hypothesis that chemical etching and silane coating of a ceramic surface will influence the work of adhesion (WA) of adhesive resin to dental ceramic. METHODS: A hot-pressed lithia disilicate-based ceramic was used as a model material to investigate the influence of probing media and surface treatments on WA using a dynamic contact angle analyzer. Eighty ceramic specimens were randomly divided into eight experimental groups and treated as follows: (1 and 3) as polished; (2 and 4) etched with 9.5% hydrofluoric acid (HF) for 1 min; (5) etched with 4% acidulated phosphate fluoride (APF) for 2 min; (6) silane coated; (7) etched with HF for 1 min and silane coated; (8) etched with APF for 2 min and silane coated. Advancing and receding contact angles (theta(a) and theta(r)) were measured using high purity water (gamma = 72.6 mN/m) for groups 1 and 2, and a liquid resin (gamma = 39.7) for groups 3-8 as probing liquids. RESULTS: The liquid resin medium yielded a lower WA than water. Silanization produced a significantly lower WA (p < 0.001) than non-silanated surfaces. Etching alone consistently yielded a greater WA for all surface treatments (p < 0.001). SIGNIFICANCE: The silanated ceramic surface exhibited a lower surface energy and did not enhance bonding to the liquid resin by work of adhesion.

Role of investment interaction layer on strength and toughness of ceramic laminates.

OBJECTIVES: The aim of this study was to test the hypothesis that the interaction of a core ceramic with investment material can significantly reduce the flexural strength and the fracture toughness of core/veneer ceramic laminates. METHODS: Ceramic composites were prepared from experimental core and experimental veneer and Empress 2 core and Empress 2 veneer ceramics. Four divesting techniques were used for each bilayer ceramic group. Core surfaces were etched with 1% HF solution for 15 or 30 min and grit blasted with 100 microm Al2O3 particles for 15 or 30 s. The effect of treatment on strength was analyzed statistically by means of two-way ANOVA. A linear regression graph was made for each group to analyze the relationship between flexural strength and the dimensions of critical cracks. RESULTS: The four surface divesting treatments produced no significant differences (p > 0.05) between the mean flexural strengths and the mean fracture toughnesses. However, groups with different core/veneer combinations showed statistically significant differences (p < or = 0.05) between the mean flexural strengths and between the mean fracture toughnesses. The Empress 2 core/experimental veneer combination exhibited the greatest fracture toughness values. The Empress 2 core/Empress 2 veneer combination exhibited the lowest mean fracture toughness and lowest mean flexural strength. SIGNIFICANCE: The investment interaction layer does not have a significant effect on the flexural strength and fracture toughness of the bilayer ceramic laminates for interfaces that are coherent and well bonded. However, the core/veneer combination of materials does affect the strength of bilayer ceramic laminates. The existence of global residual stress is the most likely reason for the observed strength increases.

Failure analysis of resin composite bonded to ceramic.

OBJECTIVE: To use fractographic principles to classify the mode of failure of resin composite bonded to ceramic specimens after microtensile testing. METHODS: A leucite-based ceramic (IPS Empress)-E1) and a lithia disilicate-based ceramic (IPS Empress2)-E2) were selected for the study. Fifteen blocks of E1 and E2 were polished through 1 microm alumina abrasive. The following ceramic surface treatments were applied to three blocks of each ceramic: (1) 9.5% hydrofluoric acid (HF) for 2 min; (2) 4% acidulated phosphate fluoride (APF) for 2 min; (3) Silane coating (S); (4) HF+S; (5) APF+S. An adhesive resin and a resin composite were applied to all treated surfaces and light cured. Twenty bar specimens for each group were prepared from the composite-ceramic blocks and stored in 37 degrees C distilled water for 30 days before loading to failure under tension in an Instron testing machine. Fracture surfaces were examined using scanning electron microscopy and X-ray dot mapping. Statistical analysis was performed using one-way ANOVA, Duncan's multiple range test, and Weibull analyses. RESULTS: Similar surface treatments were associated with significantly different bond strengths and modes of failures for E1 and E2. All fractures occurred within the adhesion zone. The microstructural difference between etched E1 and E2 ceramics was a major controlling factor on adhesion. SIGNIFICANCE: The quality of the bond should not be assessed based on bond strength data alone. Mode of failure and fractographic analyses should provide important information leading to predictions of clinical performance limits.

Fracture behavior of lithia disilicate- and leucite-based ceramics.

OBJECTIVE: This study was designed to characterize the fracture behavior of ceramics and test the hypothesis that variation in strength is associated with a variation in fracture toughness. METHODS: The following four groups of 20 bar specimens (25 x 4 x 1.2 mm) were fabricated (ISO standard 6872): E1, a hot-pressed leucite-based core ceramic (IPS Empress); E2, a hot-pressed lithia-based core ceramic (IPS Empress 2); ES, a hot-pressed lithia-based core ceramic (Experimental); and GV, a glass veneer (IPS Empress2 body). Specimens were subjected to four-point flexure loading in 37 degrees C distilled water. Fractographic analysis was performed to determine the fracture origin (c) for calculation of fracture toughness (KIC). Weibull analysis of flexure strength (sigma) data was also performed. RESULTS: Differences in mean sigma and KIC were statistically significant for E1 and GV (p<0.05). These differences are associated with processing effects and composition. SIGNIFICANCE: The higher mean sigma and KIC values of E2 and ES core ceramics suggest potentially improved structural performance compared with E1 although the Weibull moduli of E1 and E2 are the same.

Weibull analysis and flexural strength of hot-pressed core and veneered ceramic structures.

OBJECTIVE: To test the hypothesis that the Weibull moduli of single- and multilayer ceramics are controlled primarily by the structural reliability of the core ceramic.Methods. Seven groups of 20 bar specimens (25 x 4 x 1.2 mm) were made from the following materials: (1) IPS Empress--a hot-pressed (HP) leucite-based core ceramic; (2) IPS Empress2--a HP lithia-based core ceramic; (3 and 7) Evision--a HP lithia-based core ceramic (ES); (4) IPS Empress2 body--a glass veneer; (5) ES (1.1 mm thick) plus a glaze layer (0.1 mm); and (6) ES (0.8 mm thick) plus veneer (0.3 mm) and glaze (0.1 mm). Each specimen was subjected to four-point flexure loading at a cross-head speed of 0.5 mm/min while immersed in distilled water at 37 degrees C, except for Group 7 that was tested in a dry environment. Failure loads were recorded and the fracture surfaces were examined using SEM. ANOVA and Duncan's multiple range test were used for statistical analysis. RESULTS: No significant differences were found between the mean flexural strength values of Groups 2, 3, 5, and 6 or between Groups 1 and 4 (p>0.05). However, significant differences were found for dry (Group 7) and wet (Groups 1-6) conditions. Glazing had no significant effect on the flexural strength or Weibull modulus. The strength and Weibull modulus of the ES ceramic were similar to those of Groups 5 and 6. SIGNIFICANCE: The structural reliability of veneered core ceramic is controlled primarily by that of the core ceramic.

Dental caries: risk assessment and treatment solutions for an elderly population.

Caries remains one of the top three most common infectious diseases in the world today. Although caries prevalence decreased markedly in children and in adults up to age 40 between 1975 and 2000, the overall risk for caries in older age groups (45 to 64, 65 to 84, and > 85 years of age) has not decreased appreciably. In fact, the risk for caries in individuals 70 years of age and older has increased. The increase in restorative work needed between 1990 and 2030 will be highest in adults over the age of 44 years. Root caries prevalence and the number of restored teeth will be greatest in the elderly population. Approximately 30% of individuals over the age of 65 will have no permanent teeth. It is also apparent that additional caries risk factors are associated with a significant proportion of the older population, including reduced saliva flow, inadequate oral hygiene, frequent sugar intake, Asian ethnicity, and the presence of partial dentures. The principles of modern caries management focus on risk assessment, risk reduction, monitoring noncavitated carious lesions, and the assignment of specific treatment options according to risk. Because a relatively high proportion of elderly patients will remain at high risk for caries, therapeutic regimens for managing caries as an infectious disease must focus on the use of antibacterial treatment; high-fluoride dentifrices; supplementary low-dose, high-frequency fluoride rinses; patient education; and shorter recall intervals.

Fracture analysis of randomized implant-supported fixed dental prostheses.

OBJECTIVE: Fractures of posterior fixed dental all-ceramic prostheses can be caused by one or more factors including prosthesis design, flaw distribution, direction and magnitude of occlusal loading, nature of supporting infrastructure (tooth root/implant), and presence of adjacent teeth. This clinical study of implant-supported, all-ceramic fixed dental prostheses, determined the effects of (1) presence of a tooth distal to the most distal retainer; (2) prosthesis loading either along the non-load bearing or load bearing areas; (3) presence of excursive contacts or maximum intercuspation contacts in the prosthesis; and (4) magnitude of bite force on the occurrence of veneer ceramic fracture. METHODS: 89 implant-supported FDPs were randomized as either a three-unit posterior metal-ceramic (Au-Pd-Ag alloy and InLine POM, Ivoclar, Vivadent) FDP or a ceramic-ceramic (ZirCAD and ZirPress, Ivoclar, Vivadent) FDP. Two implants (Osseospeed, Dentsply) and custom abutments (Atlantis, Dentsply) supported these FDPs, which were cemented with resin cement (RelyX Universal Cement). Baseline photographs were made with markings of teeth from maximum intercuspation (MI) and excursive function. Patients were recalled at 6 months and 1-3 years. Fractures were observed, their locations recorded, and images compared with baseline photographs of occlusal contacts. CONCLUSION: No significant relationship existed between the occurrence of fracture and: (1) the magnitude of bite force; (2) a tooth distal to the most distal retainer; and (3) contacts in load-bearing or non-load-bearing areas. However, there was a significantly higher likelihood of fracture in areas with MI contacts only. CLINICAL SIGNIFICANCE: Because of the absence of a periodontal ligament, this clinical study demonstrates that there is a need to evaluate occlusion differently with implant-supported prostheses than with natural tooth supported prostheses. Implant supported prostheses should have minimal occlusion and lighter contacts than those supported by natural dentition. CLINICAL TRIALSGOV NO: K23 D2007-46.

Cytotoxicity of alloying elements and experimental titanium alloys by WST-1 and agar overlay tests.

OBJECTIVE: This study was performed to evaluate the biocompatibility of nine types of pure metals using 36 experimental prosthetic titanium-based alloys containing 5, 10, 15, and 20wt% of each substituted metal. METHODS: The cell viabilities for pure metals on Ti alloys that contain these elements were compared with that of commercially pure (CP) Ti using the WST-1 test and agar overlay test. RESULTS: The ranking of pure metal cytotoxicity from most potent to least potent was: Co>Cu>In>Ag>Cr>Sn>Au>Pd>Pt>CP Ti. The cell viability ratios for pure Co, Cu, In, and Ag were 13.9±4.6%, 21.7±10.4%, 24.1±5.7%, and 24.8±6.0%, respectively, which were significantly lower than that for the control group (p<0.05). Pure Pd and Pt demonstrated good biocompatibility with cell viabilities of 93.8±9.6% and 97.2±7.1%, respectively. The Ti-5Pd alloy exhibited the highest cell viability (128.4±21.4%), which was greater than that of CP Ti. By alloying pure Co or Cu with Ti, the cell viabilities for the Ti-xCo and Ti-xCu alloys increased significantly up to 10wt% of the alloying element followed by a gradual decrease with a further increase in the concentration of the alloying element. Based on the agar overlay test, pure Ag, Co, Cr, Cu, and In were ranked as 'moderately cytotoxic', whereas all Ti alloys were ranked as 'noncytotoxic'. SIGNIFICANCE: The cytotoxicity of pure Ag, Co, Cr, Cu, and In suggests a need for attention in alloy design. The cytotoxicity of alloying elements became more biocompatible when they were alloyed with titanium. However, the cytotoxicity of titanium alloys was observed when the concentration of the alloying element exceeded its respective allowable limit. The results obtained in this study can serve as a guide for the development of new Ti-based alloy systems.