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

Objectives

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

Methods

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

Results

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

Conclusion

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

Thermal gradients and residual stresses in veneered Y-TZP frameworks

Objectives

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

Methods

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

Results

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

Conclusion

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

Slow crack growth and reliability of dental ceramics

Objective

To determine the slow crack growth (SCG) and Weibull parameters of five dental ceramics: a vitreous porcelain (V), a leucite-based porcelain (D), a leucite-based glass-ceramic (E1), a lithium disilicate glass-ceramic (E2) and a glass-infiltrated alumina composite (IC).

Methods

Eighty disks (Ø 12 mm × 1.1 mm thick) of each material were constructed according to manufacturers’ recommendations and polished. The stress corrosion susceptibility coefficient (n) was obtained by dynamic fatigue test, and specimens were tested in biaxial flexure at five stress rates immersed in artificial saliva at 37 °C. Weibull parameters were calculated for the 30 specimens tested at 1 MPa/s in artificial saliva at 37 °C. The 80 specimens were distributed as follows: 10 for each stress rate (10⁻², 10⁻¹, 10¹, 10² MPa/s), 10 for inert strength (10² MPa/s, silicon oil) and 30 for 10⁰ MPa/s. Fractographic analysis was also performed to investigate the fracture origin.

Results

E2 showed the lowest slow crack growth susceptibility coefficient (17.2), followed by D (20.4) and V (26.3). E1 and IC presented the highest n values (30.1 and 31.1, respectively). Porcelain V presented the lowest Weibull modulus (5.2). All other materials showed similar Weibull modulus values, ranging from 9.4 to 11.7. Fractographic analysis indicated that for porcelain D, glass-ceramics E1 and E2, and composite IC crack deflection was the main toughening mechanism.

Significance

This study provides a detailed microstructural and slow crack growth characterization of widely used dental ceramics. This is important from a clinical standpoint to assist the clinician in choosing the best ceramic material for each situation as well as predicting its clinical longevity. It also can be helpful in developing new materials for dental prostheses.     

Cyclic fatigue testing of denture teeth for bulk fracture

Objectives

Clinical experience with implant-supported dentures indicates that fracture and chipping of teeth are becoming an issue. Tooth fracture and chipping rates of approximately 2.5% per year are being experienced at one university. There has been no standardized test developed for bulk fracture or chipping of denture teeth. Such a test would aid in the development of improved teeth and in their evaluation.

Methods

Central incisor teeth were embedded in acrylic and loaded on incisal edges at 90° to their long axes. Teeth tested included ones commercially available and two sets made from improved materials designed to increase toughness. Cyclic loading was done at 5 Hz from 20 N to 135 N, 150 N, 175 N and 200 N. Data was analyzed using lifetime analysis software fit at each of the accelerated loads and then extrapolated to clinical use loads (Alta 7, Reliasoft Corp.) Clinical use loads were derived from extrapolation of probability of failure (Pf) data to 2.5% Pf.

Results

When carefully embedded, teeth could be reproducibly loaded to failure by bulk fracture involving a failure mode similar to that seen clinically. Clinical use loads were calculated to be in the range of 70 N. Results from accelerated loading could be fit to similar probability of failure distributions allowing extrapolation to clinical use loads.

Significance

This work was able to develop a clinically valid bulk fracture test for the fatigue failure of incisor denture teeth. It appears that teeth fabricated with improved materials will be expected to perform better clinically. Thus both the null hypotheses were rejected.     

Mechanical properties of denture base resin cross-linked with methacrylated dendrimer

Objectives

The aim of this study was to investigate the mechanical properties of denture base resin cross-linked with methacrylated dendrimer.

Methods

The test specimens (3 mm × 10 mm × 65 mm) were fabricated from autopolymerizing resin with the powder/liquid ratio of 10 g/7 ml. The monomer liquid of resin was applied with the mixture of methylmethacrylate and crosslinker dendrimer (DD1) or crosslinker ethyleneglycol dimethacrylate (EGDMA) with five different volume percentages (vol%). The dendrimer crosslinker in this study is a methacrylated molecule (MW = 3617 g/mol) with 12 methacrylate groups. Quantity of crosslinkers varied from 1.1 to 9.1 vol%. The specimens (n = 8/group) were polymerized in distilled water maintained at 55 °C under pressure of 0.4 MPa for 20 min. Test specimens were stored dry at room temperature before testing. The flexural strength (MPa) and flexural modulus (GPa) was measured with three-point bending test at a crosshead speed of 5 mm/min. Surface microhardness (MHN) of matrix area of polymer (n = 8/group) was measured with a load of 245.3 mN by 10 s. Data were analyzed with two-way ANOVA.

Results

ANOVA showed that the addition of DD1 had a significantly higher effect (p < 0.05) on flexural modulus and hardness of matrix area than EGDMA but on flexural strength (p > 0.05). The effect of quantity differences of crosslinker was statistically significant only on flexural strength (p < 0.05).

Significance

The results of this study suggest that dendrimer-crosslinked resin gives better stiffness than that of EGDMA.     

Eight-year outcome of posterior inlay-retained all-ceramic fixed dental prostheses

Aim

The main goal of this prospective clinical study was to evaluate the outcome of inlay-retained fixed dental prostheses (FDPs) made from heat-pressed lithium-disilicate glass-ceramic.

Methods

Forty-five FDPs were placed in 42 patients (21 women, mean age 36.1 years and 21 men, mean age 42.0 years). The FDPs replaced 4 premolars and 19 molars in the maxilla and 4 premolars and 18 molars in the mandible. Preparations were performed in accordance with general principles for ceramic inlay restorations. Five of the 45 FDPs were hybrid-retained restorations, i.e. one abutment tooth with an inlay retainer and one with a full crown retainer.All FDPs were pressed in one piece using lithium-disilicate ceramic (IPS e.max Press, Ivoclar Vivadent). The minimum dimensions for the proximal connector were 4 mm in height and 4 mm in width (16 mm²) with a minimum occlusal ceramic thickness of 1.5 mm. The surfaces of the inlay retainer were conditioned by etching with hydrofluoric acid 5% and silane application. Standard adhesive luting techniques were performed using a dentin adhesive (Syntac Classic, Ivoclar Vivadent) and a resin composite (Variolink II, Ivoclar Vivadent). Clinical follow-up examinations were performed annually.

Results

The mean observation periods were 70 months (minimum 4, maximum 123 months). Twenty-seven FDPs (60%) failed during the observation period and had to be replaced. The Kaplan–Meier survival rate for inlay-retained FDPs was 57% after 5 years and 38% after 8 years, while for hybrid-retained FDPs it was 100% after 5 and 60% after 8 years.

Conclusions

Inlay-retained FDPs made from lithium-disilicate ceramic present a high clinical failure rate and therefore cannot be recommended.     

Wear performance of substructure ceramics and veneering porcelains

Aim

The aim of this in vitro study was to compare the two-body wear resistance of substructure zirconia and veneering porcelain versus steatite and human enamel antagonists, respectively.

Materials and methods

Two-body wear tests were performed in a chewing simulator with steatite and enamel antagonists (enamel cusps). A pin-on-block design with a vertical load of 50 N for 1.2 × 10⁵ cycles; (f = 1.6 Hz; lateral movement: 1 mm, mouth opening: 2 mm) was used for the wear test. For quantification of the wear resistance, wear tests were performed with standardized steatite spheres. Human enamel was used as a reference. Five zirconia ceramics and four veneering porcelains were investigated. One zirconia ceramic was tested with superficial glaze, which was applied after polishing or sandblasting, respectively. Surface roughness R_a (SP6, Perthen-Feinprüf, G) and wear depth were determined using a 3D-Profilometer (Laserscan 3D, Willytec, G). SEM (Quanta FEG 400, FEI, USA) pictures were used for evaluating wear performance of both, ceramics and antagonists.

Results

No wear was found for zirconia substructures. Veneering porcelain provided wear traces between 186.1 ± 33.2 μm and 232.9 ± 66.9 μm (steatite antagonist) and 90.6 ± 3.5 μm and 123.9 ± 50.7 μm (enamel). Wear of the steatite antagonists varied between 0.812 ± 0.256 mm² and 1.360 ± 0.321 mm² for zirconia and 1.708 ± 0.275 mm² and 2.568 ± 0.827 mm² for porcelain. Enamel generally showed wear, cracks or even fractures at the ridge, regardless whether opposed by zirconia or porcelain/glaze. Enamel was polished, when opposed to zirconia, or plowed, provoked and grinded, when opposed to porcelain/glaze.

Conclusion

The results of the wear test with steatite or enamel antagonists indicated no measurable wear on zirconia surfaces. Porcelain showed higher wear than zirconia, but comparable or lower wear than an enamel reference. Antagonistic wear against zirconia was found to be lower than wear against porcelain.     

Porous polyethylene implant reconstruction of the orbit after resection of spheno-orbital meningiomas: A novel technique

Background

Spheno-orbital meningiomas (SOM) are complex tumors involving the middle cranial fossa and orbit. Following resection of these tumors, reconstruction of the orbit can be challenging. Inadequate reconstruction may lead to cosmetic deformities and functional complications.

Objective

The development of a technique for orbital reconstruction which is technically straightforward, cost-effective, with an excellent functional and cosmetic outcome.

Technique

Twelve patients with SOM each underwent a modified orbital-zygomatic craniotomy with osteotomies based on individual tumor location. After tumor resection, the lateral orbit and orbital roof, where necessary, were reconstructed using a 1 mm porous polyethylene sheet, customized to reconstruct the bony anatomy of the lateral and superior orbit and secured with 1.5 × 4 mm titanium screws into the orbital roof and rim.

Results

All patients demonstrated stabilization or improvement of vision and satisfactory cosmetic outcomes. One complication (surgical site infection) was noted. Follow-up imaging was obtained at regular intervals. The resection cavity was easily visualized without interference from the implant in each case. One patient developed tumor recurrence requiring re-operation 20 months after the initial procedure.

Conclusions

Use of a 1 mm porous polyethylene sheet for reconstruction of the lateral orbit offers an excellent functional and cosmetic outcome, does not interfere with postoperative imaging, is technically straightforward, and is cost-effective.     

Failure modes of Y-TZP crowns at different cusp inclines

Objectives

To compare the reliability of the disto-facial (DF) and mesio-lingual (ML) cusps of an anatomically correct zirconia (Y-TZP) crown system. The research hypotheses tested were: (1) fatigue reliability and failure mode are similar for the ML and DF cusps; (2) failure mode of one cusp does not affect the failure of the other.

Methods

The average dimensions of a mandibular first molar crown were imported into CAD software; a tooth preparation was modelled by 1.5 mm marginal high reduction of proximal walls and occlusal surface by 2.0 mm. The CAD-based tooth preparation was milled and used as a die to fabricate crowns (n = 14) with porcelain veneer on a 0.5 mm Y-TZP core. Crowns were cemented on composite reproductions of the tooth preparation. The crowns were step-stress mouth motion fatigued with sliding (0.7 mm) a tungsten-carbide indenter of 6.25 mm diameter down on the inclines of either the DF or ML cusps. Use level probability Weibull curve with use stress of 200 N and the reliability for completion of a mission of 50,000 cycles at 200 N load were calculated.

Results

Reliability for a 200 N at 50,000 cycles mission was not different between tested cusps. SEM imaging showed large cohesive failures within the veneer for the ML and smaller for the DF. Fractures originated from the contact area regardless of the cusp loaded.

Conclusion

No significant difference on fatigue reliability was observed between the DF compared to the ML cusp. Fracture of one cusp did not affect the other.     

The fit of cobalt-chromium three-unit fixed dental prostheses fabricated with four different techniques: A comparative in vitro study

Objectives

This study sought to evaluate and compare the marginal and internal fit in vitro of three-unit FDPs in Co-Cr made using four fabrication techniques, and to conclude in which area the largest misfit is present.

Methods

An epoxy resin master model was produced. The impression was first made with silicone, and master and working models were then produced. A total of 32 three-unit Co-Cr FDPs were fabricated with four different production techniques: conventional lost-wax method (LW), milled wax with lost-wax method (MW), milled Co-Cr (MC), and direct laser metal sintering (DLMS). Each of the four groups consisted of eight FDPs (test groups). The FDPs were cemented on their cast and standardised-sectioned. The cement film thickness of the marginal and internal gaps was measured in a stereomicroscope, digital photos were taken at 12× magnification and then analyzed using measurement software. Statistical analyses were performed with one-way ANOVA and Tukey's test.

Results

Best fit based on the means (SDs) in μm for all measurement points was in the DLMS group 84 (60) followed by MW 117 (89), LW 133 (89) and MC 166 (135). Significant differences were present between MC and DLMS (p < 0.05). The regression analyses presented differences within the parameters: production technique, tooth size, position and measurement point (p < 0.05).

Significance

Best fit was found in the DLMS group followed by MW, LW and MC. In all four groups, best fit in both abutments was along the axial walls and in the deepest part of the chamfer preparation. The greatest misfit was present occlusally in all specimens.     

Non-invasive quantification of resin-dentin interfacial gaps using optical coherence tomography: Validation against confocal microscopy

Objectives

Regardless of the cause, gap formation at the tooth-restoration interface may result in treatment failure; non-destructive assessment and monitoring of these defects are important. The aim of this in vitro study is to assess the tooth-restoration interface using a non-invasive technique; swept source optical coherence tomography (SS-OCT) and to confirm the findings with confocal laser scanning microscope (CLSM).

Methods

Cylindrical class-I cavities (3 mm in diameter and 1.5 mm in depth) were prepared in the occlusal surface of human premolars. Each cavity was restored using an all-in-one adhesive system (Clearfil Tri-S Bond) and one of the three types of composites placed in bulk; Majesty Posterior, AP-X and Majesty LV (all by Kuraray Medical, Japan). Ten serial cross-sectional images of the whole restored cavity were obtained by SS-OCT at 1319 nm center wave length, to which locations the specimens were later trimmed, polished and observed under CLSM. An image analysis software was used to detect significant peaks in the signal intensity at the resin-dentin interface of the cavity floor. The presence and dimensions of gaps at the interface were also confirmed by CLSM.

Results

Increased SS-OCT signal intensity along the interface corresponded well to the interfacial gaps detected by CLSM. The actual gap size detected ranged from 26 μm to 1.9 mm in length, and the universal composite APX showed lowest interfacial gaps.

Conclusion

SS-OCT imaging technology can be used to non-invasively detect and quantify micrometer gaps at the bottom of composite restorations, and potentially become a monitoring tool for composite restorations both in the laboratory research, and in the clinics.     

Selective masking for thin indirect restorations: can the use of opaque resin affect the dentine bond strength of immediately sealed preparations?

Purpose

Evaluate the dentine bond strength using different methods of application of opaque resins to mask dentine discoloration.

Materials and methods

Dentine was exposed on 21 extracted molars which were assigned to 3 groups: G1: immediate dentine sealing (IDS). G2: IDS + opaque. G3: IDS with adhesive resin mixed with opaque. After 1 week, teeth were restored and beams were fabricated for microtensile testing. Optical microscopy was used to analyse the failure mode.

Results

Bond strength mean values were statistically different: G1 55.20 MPa > G2 45.79 MPa > G3 18.96 MPa. Failure modes were mostly adhesive for G1 and G3. G2 presented mostly mixed failures.

Conclusion

The use of opaque resin to mask discoloured dentine results in a decrease of bond strength.     

Transaminases in the acquired pellicle

Objective

Transaminases (AST, aspartate amino transferase; ALT, alanine amino transferase) are relevant enzymes in physiology and pathology of the human organism. The aim of the present in situ study was to demonstrate the presence of these enzymes in the enamel pellicle.

Methods

Bovine enamel slabs were fixed on buccal sites of individual upper jaw splints and worn for 3, 30 and 120 min by 5 subjects to allow pellicle formation. The in situ pellicles were tested for AST and ALT. Enzyme activities were measured photometrically via determination of the products pyruvate and oxalacetate using lactate-dehydrogenase and malate-dehydrogenase, respectively.

Results

Enzymatic AST- as well as ALT-activities are present in the acquired pellicle within 3 min. The enzyme activities exposed at the pellicles’ surfaces increased slightly with the pellicle formation time (ANOVA, AST: n.s., ALT: p = 0.021). However, the two enzymes show considerable intraindividual and interindividual variability. The mean AST-activity of the pellicle amounted to 1.07 ± 0.81 mU/cm² (ALT 1.18 ± 0.52 mU/cm²). The ALT-activity of the centrifuged saliva was 26.62 ± 11.09 mU/ml (AST 35.98 ± 29.35 mU/ml).

Conclusions

AST as well as ALT are present in the in situ pellicle layer and may contribute to the intrinsic maturation of pellicle proteins.     

The effect of porcelain thickness and surface liner application on the fracture behavior of a ceramic system

Objectives

To determine the effect of porcelain thickness and the application of a surface liner (SL) on the flexural strength (σ) of a ceramic system.

Methods

One hundred twenty bar-shaped specimens of yttria-stabilized zirconia-based ceramic were fabricated and randomly divided into two groups according to the application of SL: A - applied; B - not applied. The specimens were further divided according to the porcelain veneer thickness (n = 20): 0-0.5 mm; 1-1.0 mm; 2-2.0 mm. All specimens were tested in 3-point bending with the porcelain under tension. The maximum load was recorded at first sign of fracture, the σ was calculated and the mode of failure was determined. Data was statistically analyzed using one-way ANOVA, Tukey (α = 0.05) and Weibull distribution.

Results

The SL application had no effect (p = 0.723) while the porcelain thickness had a significant effect (p = 0.000) on the mean σ values. No significant differences in mean σ values were found with same porcelain thickness. A 0.5-mm thick porcelain layer (groups A0 and B0) showed greater mean σ values than other groups. The Weibull modulus (m) ranged from 6 (groups A1 and B2) to 9 (groups A0, A2 and B0). The predominant mode of failure for all groups was fracture initiation at the porcelain surface propagating to the ceramic interface.

Significance

The porcelain thickness influenced the mean σ values, but the SL had no significant effect on the flexural strength and the mode of failure of the ceramic system examined.     

Pilot study of unidirectional E-glass fibre-reinforced composite resin splints: up to 4.5-year clinical follow-up

Objectives

This prospective clinical pilot study evaluated the performance of fibre-reinforced-composite resin (FRC) splints on mandibular anterior teeth.

Methods

Between June-2003 and January-2008, 19 patients (7 females, 12 males, 45-72 years old) from a group of consecutive patients who completed periodontal therapy received E-glass FRC splints (everStick Perio, StickTech) in combination with two types of flowable and restorative resin-composites (Filtek Flow, Filtek Supreme, n = 11; Tetric Flow, Tetric-Ceram, n = 8). Only patients with vital teeth, presenting mobility of grade 3, having at least one canine with no mobility on both sides of the dental arch were assigned for a splint therapy. The patients were recalled for periodical follow-up controls first at 6 months and thereafter annually. The evaluation protocol involved technical failures [chipping, debonding or fracture (tooth/restoration)] and biological failures (caries)]. Periodontal pocket depth (PPD) and clinical attachment level (CAL) were measured 6 months after splinting and annually. Six sites were measured for each natural tooth at the mesiobuccal, buccal, distobuccal, distolingual, lingual and mesiolingual sites.

Results

All splints were applied from canine to canine in the mandible. In total, 5 recalls were performed and no drop-out was experienced. One partial debonding of the FRC splint with Tetric Flow/Tetric-Ceram combination was observed after 40 months. No caries was found around any of the splints and no teeth had to be extracted until the final follow up. The splinted teeth were found to be vital in the vitality tests. Overall survival rate was 94.8% (Kaplan-Meier). The survival rate was not significantly affected by the composite type (Filtek-Flow/Filtek Supreme: 100%, Tetric Flow/Tetric Ceram: 96% (p = 0.92) [Kaplan-Meier, Log Rank (Mantel-Cox) (CI = 95%)]. Hazard ratio for Tetric Flow/Tetric Ceram group was 0.05 (95% CI) and for Filtek Flow/Filtek Supreme group 0.00 (95% CI). Whilst overall PPD measurements of the dentition ranged between 6 and 12 mm, the CAL measurements ranged between 4.9 and 10 mm at baseline. The mean PPD for the splinted teeth decreased from 8.9 ± 1.8 mm to 5.2 ± 1.2 mm, and CAL decreased from 7.2 ± 1.6 mm to 4.6 ± 1 mm at the end point.

Conclusion

Direct tooth splinting with E-glass FRC material performed successfully up to 4.5 years. Periodontal status of the splinted teeth showed decreased PPD and CAL.     

Effect of surface pre-treatments on the zirconia ceramic-resin cement microtensile bond strength

Objective

To evaluate the influence of different surface treatments on the microtensile bond strength of resin cement to zirconia ceramic.

Materials and methods

Twelve cylinder-shaped (∅ 12 × 5.25 mm high) blocks of a commercial zirconium-oxide ceramic (Cercon^® Zirconia, DENTSPLY) were randomly divided into 4 groups (n = 3), based on the surface treatment to be performed: (1) airborne particle abrasion with 125 μm Al₂O₃ particles (S); (2) selective infiltration etching (SIE); (3) experimental hot etching solution applied for 30 min (ST) and (4) no treatment (C). Paradigm MZ100 blocks (3M ESPE) were cut into twelve cylinders of 4 mm in thickness. Composite cylinders were bonded to conditioned ceramics using a resin cement (Calibra^®, DENTSPLY), in combination with the proprietary adhesive system. After 24 h bonded specimens were cut into microtensile sticks and loaded in tension until failure. Bond strength data were analyzed with Kruskall-Wallis and Dunn's Multiple Range test for multiple comparisons (p < 0.05). Failure mode distribution was recorded and the interfacial morphology of debonded specimens was analyzed using a scanning electron microscope (SEM).

Results

Bond strength values achieved after SIE and ST treatment were significantly higher than after S treatment and without any treatment (p < 0.05). Premature failures were mostly recorded in the S group.

Significance

Conditioning the high-strength ceramic surface with SIE and ST treatments yielded higher bond strengths of the resin cement than when zirconia ceramic was treated with airborne particle abrasion or left untreated.     

Fracture strength and fracture patterns of root filled teeth restored with direct resin restorations

Objective

To compare fracture characteristics of root-filled teeth with variable cavity design and resin composite restoration.

Methods

80 extracted intact maxillary premolars were divided randomly into eight groups; (1) intact teeth; (2) unrestored MOD cavity; (3) unrestored MOD cavity plus endodontic access through the occlusal floor; (4) unrestored MOD plus endodontic access with axial walls removed; (5) MOD restored with resin composite; (6) MOD plus endodontic access, resin composite; (7) MOD plus extensive endodontic access, resin composite; (8) MOD plus extensive endodontic access, GIC core and resin composite. A ramped oblique load was applied to the buccal cusp in a servohydraulic testing machine. Fracture load and fracture patterns were recorded. Fracture loads were compared statistically using 1-way ANOVA, with Dunnett test for multiple comparisons.

Results

Unrestored teeth became progressively weaker with more extensive preparations. Endodontic access confined within the occlusal floor did not significantly affect strength compared to an MOD cavity. Loss of axial walls weakened teeth considerably [292 + 80 N vs 747 + 130 N for intact teeth]. Restoration increased the strength of prepared teeth particularly in teeth without axial walls. Teeth with a GIC core were not significantly weaker than intact teeth [560 + 167 N]. Failures were mostly adhesive at the buccal interface, with the fracture propagating from the buccal line angle of the occlusal floor (MOD and MOD plus access groups) or of the proximal box (axial wall removed).

Conclusions

Direct restorations increased fracture resistance of root filled teeth with extensive endodontic access. Both restored and unrestored teeth showed similar fracture patterns.     

Long-term microtensile bond strength of surface modified zirconia

Objective

To compare long-term microtensile bond strength of zirconia, surface-modified via a novel treatment, to current surface conditioning methods for zirconia, when resin bonded to dental composite.

Methods

Two ProCAD (porcelain) and 10 sintered ZirCAD (ZrO₂) blocks (18 mm × 14 mm × 12 mm) were obtained from manufacturers. Twelve Herculite XRV composite blocks were fabricated (18 mm × 14 mm × 12 mm). Bonding surface of blocks was polished through 1200-grit SiC and air-abraded (50 μm alumina, 0.28 MPa, 20 s). Blocks were then separated into six groups: (1) porcelain (control), HF-etched/silane-treated, (2) ZrO₂, tribochemical-coated/silane-treated, (3) ZrO₂, primer-treated, (4) ZrO₂, modified via novel 3.2 nm silica layer/silane-treated, (5) ZrO₂, modified via novel 5.8 nm silica layer/silane-treated, and (6) ZrO₂, modified via novel 30.4 nm silica layer/silane-treated. Blocks were bonded to composite using Clearfil Esthetic cement. Blocks were stored in distilled water (37 °C, 24 h), then cut into microtensile bars (n = 8/gp), then bond strengths were measured using a universal testing machine at 0, 1, 3, and 6 months. All groups were statistically analyzed (ANOVA, Tukey's, p < 0.05).

Results

At 6 months (aging), all silica seed layer specimens displayed microtensile bond strength similar to CoJet specimens but less than that of silane-modified dental porcelain.

Conclusion

The deposition of a silica layer on zirconia resulted in similar or superior long-term resin bond strength when compared to traditional silanation and bonding techniques for zirconia but lower than that for silane-treated dental porcelain.     

Fracture resistance of three porcelain-layered CAD/CAM zirconia frame designs

Objectives

Chipping is the most frequent clinical failure of zirconia crowns. Causes of chipping have not been completely understood and different possible reasons have been considered. The study was aimed at evaluating the fracture resistance of 3 different CAD/CAM zirconia frame designs veneered with porcelain.

Methods

Thirty extracted sound premolars were divided into 3 groups (n = 10). Chamfer preparations were performed, impressions were taken. Three zirconia frame designs (Aadva, GC) were realized: reproduction of the abutment contour (flat design, FD); wax-up as for porcelain-fused-to-metal crowns (PFM); anatomically guided, designed to keep constant the thickness of the overlying porcelain veneering (AG). Porcelain veneering was made with pressure layering technique (Initial Zr, GC). Crowns were cemented utilizing a self-adhesive resin cement (G-Cem, GC). After a 24-h water storage at 37 °C, using a universal testing machine (1 mm crosshead speed), crowned teeth were loaded in the central fossa in a direction parallel to the longitudinal axis of the tooth. Load at fracture was recorded in Newtons (N). Digital photographs of the specimens were taken in order to assess failure patterns. Between-group differences in fracture strength were statistically analyzed (One-Way Analysis of Variance, Tukey test, p < 0.05).

Result

Load at fractures differed significantly among the groups (p = 0.004). AG exhibited significantly higher fracture resistance 1721.6 (488.1) N than PFM 1004.6 (321.3) N and FD 1179.5 (536.2) N, that were comparable. Repairable failures occurred in 80% of AG, 70% of PFM, and 50% of FD specimens.

Significance

Anatomically guided zirconia frames resisted significantly higher loads than flat and PFM-like frame designs.     

Photoinitiator type and applicability of exposure reciprocity law in filled and unfilled photoactive resins

Objectives

To test the influence of photoinitiator type and filler particle inclusion on the validity of exposure reciprocity law.

Materials and methods

50/50 wt% Bis-GMA/TEGDMA resins were prepared with equimolar concentrations of camphorquinone/DMAEMA (0.20/0.80 mass%) (CQ) or Lucirin-TPO (0.42 mass%), and were used either unfilled or filled to 75 mass%. Specimens were cured with a halogen Swiss Master Light (EMS, Switzerland) using four different curing protocols: 400 mW/cm² for 45 s as reference protocol (18 J/cm²), 1500 mW/cm² for 12 s (18 J/cm²), 3000 mW/cm² for 6 s (18 J/cm²) and 3 s (9 J/cm²). Degree of conversion (DC) was measured in real time for 70 s by FT-NIRS and temperature rise using a thermocouple. Depth of cure was determined with a penetrometer technique.

Results

With respect to DC and depth of cure, exposure reciprocity law did not hold for any tested material, except for the depth of cure of filled CQ-based materials. At similar radiant exposure, DC was significantly higher (p < 0.05) for all unfilled and filled TPO-based materials compared with CQ-based materials. As exposure time was reduced and irradiance increased, TPO-based materials exhibited higher DC whilst an opposite trend was observed for CQ-based materials (p < 0.05). For similar curing regimes, depth of cure of CQ-based materials remained significantly greater than that of TPO-based materials. Adding fillers generally reduced DC, except at higher irradiance for CQ-based materials where a positive effect was observed (p < 0.05).

Significance

The validity of exposure reciprocity law was dependent on several factors, among which photoinitiator type and filler content were important. Lucirin-TPO is a highly reactive and efficient photoinitiator, which may allow the potential for a reduction in curing time of TPO-based photoactive materials in thin sections.