Machining variability impacts on the strength of a ‘chair-side’ CAD–CAM ceramic

Impact of CAD/CAM burs lifetime on surface properties of dental zirconia: Implications for biocompatibility of custom abutments

OBJECTIVE

To evaluate the effect of the deterioration of computer aided design/computer aided manufacturing (CAD/CAM) burs during zirconia milling, on surface roughness, contact angle, and fibroblast viability.

MATERIALS AND METHODS

Ceramic blocks were milled and 75 ceramic disks (8 × 1.5 mm) made and allocated into three groups (n = 25): G1-brand new 2L and 1L burs, G2-2L bur at the end of lifetime and brand new 1L bur and G3-both burs at the end of their lifetimes. Roughness (Ra, Rq, and Rz) was evaluated using a 3D optical profilometer, the contact angle by the sessile drop method and the cell viability of the mouse NIH/3T3 fibroblast, using the Alamar Blue assay at intervals of 24, 48, and 72 h (ISO 10993-5). Data were analyzed by one-way ANOVA and Kruskal-Wallis tests (p ≤ 0.05).

RESULTS

Roughness increased as the burs deteriorated and G3 (0.27 ± 0.04) presented a higher value for Ra (p < 0.001). The highest contact angle was observed in G3 (86.2 ± 2.66) when compared with G1 (63.7 ± 12.49) and G2 (75.3 ± 6.36) (p < 0.001). Alamar Blue indicated an increase in cell proliferation, with no significant differences among the groups at 24 and 72 h (p > 0.05).

CONCLUSIONS

The deterioration of the burs increased the surface roughness and decreased the wettability, but did not interfere in cell viability and proliferation.

CLINICAL SIGNIFICANCE

The use of custom zirconia abutments represents an effective strategy for single crowns restorations. Our findings suggest that these abutments can be efficiently milled using CAD/CAM burs within their recommended lifetime.

Effect of machining damage on the surface roughness and flexural strength of CAD-CAM materials

STATEMENT OF PROBLEM

Computer-aided design and computer-aided manufacturing (CAD-CAM) materials are available for different types of restorations. However, the longevity of the material is affected by chipping, milling damage, flexural strength, and surface roughness, and a standard edge chipping test or standardized measurements are unavailable for monitoring edge chipping of rotary instrument-milled materials.

PURPOSE

The purpose of this in vitro study was to analyze the surface roughness and edge chipping of different CAD-CAM diamond rotary instrument-milled dental material bars, correlate the effect of machining damage with material strength, and compare the flexural strength of rotary instrument-milled and sectioned CAD-CAM blocks.

MATERIAL AND METHODS

Five dental CAD-CAM materials were tested: lithium disilicate glass-ceramic (IPS e.max CAD), leucite-reinforced glass-ceramic (IPS Empress CAD); feldspathic porcelain (Vitablocs Mark II); feldspar ceramic-polymer infiltrated (Enamic), and composite resin (Lava Ultimate). Rectangular bars were designed and milled for each material (n=10). The surface roughness of the bars was measured using a profilometer. All edges of 3 selected bars were analyzed with scanning electron microscopy (SEM) for the chip length, depth, and area. The 3-point bend test was used to test the flexural strength of rotary instrument-milled and saw-cut bars with the same dimensions. Analysis of variance and the Tukey honestly significant difference post hoc test were used to determine the difference among the groups (α=.05).

RESULTS

IPS e.max CAD had the highest surface roughness and Lava Ultimate the lowest. Lava Ultimate had the smallest chipping factor and IPS Empress CAD the largest. The surface location significantly affected the chipping depth, area, and length (P<.05). A strong correlation was found between the decrease in flexural strength and the chipping length on the central tensile side of the rotary instrument-milled materials (R2=.62, P=.01), as well as the chipping depth (R2=.44, P=.01).

CONCLUSIONS

Edge chipping was significantly associated with the material type, milling surface, and edge location and strongly correlated with a decrease in flexural strength.

Influence of CAD-CAM milling strategies on the outcome of indirect restorations: A scoping review

STATEMENT OF PROBLEM

The influence of computer-aided manufacturing (CAM) parameters and settings on the outcomes of milled indirect restorations is poorly understood.

PURPOSE

The purpose of this scoping review was to summarize the current CAM systems, parameters, and setting changes, and their effects on different outcomes of milled indirect restorations and aspects related to their manufacture.

MATERIAL AND METHODS

The protocol of this review is available online (https://osf.io/x28ps/). Studies that used at least 2 different parameters (CAM units, number of axes, digital spacers, or protocols with different rotatory instruments, grit-sizes, milling speed, or others) for milling indirect restorations were included. A structured search up to July 2023 was performed by 2 independent reviewers for articles written in English in LILACS, MEDLINE via PubMed, EMBASE, Web of Science, and Scopus.

RESULTS

Of 1546 studies identified, 22 were included in the review. Discrepancies were found between the planned and actual measured cement space, with a decreasing linear relationship impacting restoration adaptation at different points. The CEREC MC XL milling machine was the most used system in the included studies, with variations in bur types, milling modes, and number of burs uses affecting internal fit and surface trueness. The results demonstrated the better adaptation of restorations made with 5-axis over 3-axis milling machines. Lithium disilicate and zirconia were the most commonly used materials, and crowns and inlays were popular designs. Marginal and internal adaptation were the primary outcomes assessed using the various techniques.

CONCLUSIONS

The study presented a comprehensive exploration of CAM systems and parameters, and their influence on indirect restorations. The planned cement space was not properly reproduced by the milling. Bur characteristics can affect restoration fit and trueness. The 5-axis units seem to result in better-adapted restorations compared with 3- and 4-axis units.

In-lab simulation of CAD/CAM grinding and intaglio surface treatments of 4YSZ monolithic restorations: Effect on its load-bearing capacity under fatigue

OBJECTIVES

To evaluate the effect of in-lab simulation of CAD/CAM grinding and intaglio surface treatments on the surface characteristics (topography and roughness) and fatigue behavior of adhesively luted 4YSZ simplified restorations.

METHODS

Ceramic discs (Ø = 10 mm, thickness = 1 mm) were randomly allocated into 6 groups considering: "In-lab simulation of CAD/CAM grinding" (ground or polished) and "intaglio surface treatments": Ctrl (without surface treatment), AlOx (aluminum oxide air abrasion) or GLZ (glaze spray application). The surface roughness of all samples was measured, the treated discs received a ceramic primer, were luted with resin cement onto a dentin analogue material (woven glass-reinforced epoxy resin) and tested under a cyclic fatigue test (step-stress approach, n = 15; 1.4 Hz, 10,000 cycles/step, step-size of 100N starting at 200N until failure). A complementary analysis was performed to corroborate the findings in the fatigue test that the glaze fill defects increase the mechanical properties of the ceramic. To do so, bars (n= 10; 1.0 × 1.0 × 12 mm; considering the groups: N-ID: non-indented; ID: indented; ID-GLZ: indented plus glaze spray application) were indented in a vickers hardness tester to produce a crack pattern, treated with glaze or not, and then submitted to flexural strength tests (FS). Fractographic and topographic analysis were performed.

RESULTS

In-lab simulation of CAD/CAM grinding decreased the fatigue failure load of the 4YSZ ceramic when comparing polished and ground groups, regardless of surface treatment. GLZ induced better fatigue performance compared to the air abrasion, regardless of the grinding condition (ground or polished surface). The results of the flexural strength test corroborated the findings in the fatigue test, as the ID-GLZ group presented superior FS than the ID group, however both had inferior FS than N-ID. There is an inverse association between roughness and fatigue failure load, as the higher the surface roughness, the lower the fatigue failure load. Failures in the fatigue and flexural strength tests started from the face subjected to tensile stresses.

CONCLUSION

In-lab simulation of CAD/CAM grinding had a detrimental effect on the fatigue behavior of 4YSZ and glaze spray induced better 4YSZ performance compared to the air abrasion. The intaglio surface treatments differently influenced the 4YSZ fatigue performance, however, only glaze spray can reverse the damage caused by the grinding.

Simulation of CAD/CAM milling on lithium disilicate: Mechanical and topographic analyses of surface grinding different protocols

The aim of this study was to evaluate the topography and the fatigue performance of lithium disilicate glass-ceramic after surface grinding through different laboratory protocols used to simulate the Computer-aided design/Computer-aided manufacturing (CAD/CAM) milling. Ceramic discs (IPS e.max CAD, Ø = 13.5 mm × 1.2 mm of thickness) were produced through different methodologies: milling in CAD/CAM system (CAD/CAM group); produced in-lab with a polished surface (POL group); or produced through in-lab methods and randomly distributed into five groups according to different grinding protocols to simulate the CAD/CAM milling [grinding with a CAD/CAM bur coupled to a mandrel (CAD/CAM Bur group); fine diamond bur using oscillatory movements (DBO group); fine diamond bur in x and y axes of the disc (DBXY group); #60-grit silicon carbide sandpaper (SiC group); and #60-grit wood sandpaper (WS group)]. The specimens were fatigue tested (n = 15) according to the step-stress method (initial load: 60 N; step-size: 20 N; 10,000 cycles/step; 20 Hz frequency). A roughness analysis was performed on all specimens, while fractal dimension (FD) and fractography were performed on representative samples. The Kaplan-Meier analysis showed that the POL (293.3 N) group presented better fatigue performance (higher load and number of cycles for failure) (p < 0.05) than the other groups (CAD/CAM = 222.7 N; CAD/CAM Bur = 181.3 N; DBO = 184.0 N; DBXY = 192.0 N; SiC = 182.6 N; WS = 182.6 N). For roughness, only the SiC (Ra = 1.616; Rz = 10.465) and WS (Ra = 1.673; Rz = 10.655) groups produced statistically similar Ra (μm) and Rz (μm) values to the CAD/CAM (Ra = 1.628; Rz = 9.571) group (p > 0.05). The surface created by CAD/CAM milling and POL group exhibited more complexity (FD) higher values than the experimental groups. For the ceramic surface topography images, the CAD/CAM milling visibly produced a uniform surface compared to the other groups; however, the POL group was the smoothest. The DBO, DBXY, SiC, and WS groups resulted in similar characteristics of surface topography. Therefore, although the SiC and WS groups showed similar roughness to the control group (CAD/CAM), no in-lab simulation method was fully capable to mimic the mechanical performance of the CAD/CAM-milled lithium disilicate glass-ceramic.

Effect of multiple firings on surface roughness and flexural strength of CAD-CAM ceramics

STATEMENT OF PROBLEM

Knowledge on the effect of multiple firings on surface roughness and the flexural strength of different types of monolithic computer-aided design and computer-aided manufacturing (CAD-CAM) ceramics is limited.

PURPOSE

The purpose of this in vitro study was to evaluate the effect of multiple firings on the surface roughness and flexural strength of 4 different CAD-CAM ceramics after thermocycling.

MATERIAL AND METHODS

Four different CAD-CAM ceramics (Lava All Zirconia 3-mol yttria tetragonal zirconia polycrystal [3-YTZP] [Z], VITA SUPRINITY [S], IPS e.max CAD [EX], IPS Empress CAD [E]) (n=33) were wet-sectioned to form rectangular 18×4×1.2-mm specimens. After glaze application, the specimens were divided into 3 subgroups according to the number of firings (1, 2, and 4) (n=11). The specimens were thermocycled (10 000 cycles), and surface roughness and flexural strength values were measured (n=10). One additional specimen from each group was analyzed by using scanning electron microscopy (SEM). Data were analyzed by 2-way analysis of variance (ANOVA) and the Tukey honestly significance difference (HSD) test (α=.05).

RESULTS

According to the 2-way ANOVA, the material, number of firings, and the interaction between the material and number of firings affected the surface roughness (P<.001). For flexural strength, material (P<.001) and number of firings (P<.039) were found significant. Multiple firings (2 or 4 firings) affected the surface roughness of E (P<.001). Regardless of the number of firings, the Z material had the highest flexural strength (P<.001). Four firings affected the flexural strength values only for the Z material (P≤.005).

CONCLUSIONS

CAD-CAM ceramic type affected the surface roughness and flexural strength values. The surface roughness of E was lower when fired 2 or 4 times than when fired once. The flexural strength of Z was lower when fired 4 times than when fired once.

Effect of repeated millings on the surface integrity of diamond burs and roughness of different CAD/CAM materials

OBJECTIVES

This study evaluated the surface integrity of two types of diamond burs and their effect on the roughness of different CAD/CAM materials.

MATERIALS AND METHODS

CAD/CAM materials, Vita Mark II (VM), IPS e.max CAD (EM), Celtra Duo (CD), G-Ceram (GC), and Lava Ultimate (LU) blocks were milled with CEREC MCXL. Five pairs of pointed cylinder burs (CB) and step burs (SB) were used repeatedly. The wear rate of burs was analyzed before and after milling with a scanning electron microscope (SEM) and weighing the burs. The milling time of the blocks at different milling stages was recorded from 2 (M2) to 8 times (M8) of use. The surface roughness (Ra) of blocks was measured with a profilometer. Data from the differences in surface roughness were analyzed using univariate analyses of variance and Kruskal-Wallis tests (alpha = 0.05).

RESULTS

For M2, the highest roughness (2.12 ± 0.1 μm) was observed with the VM block. EM blocks presented the highest roughness values (1.82 ± 0.3 μm, 1.85 ± 0.1 μm, and 1.86 ± 0.04 μm) at M4, M6, and M8, respectively. While the highest Ra values were observed for VM (2.12 to 1.43 µm), LU provided the lowest mean Ra (1.62 to 1.33 µm) among the tested materials. After the repeated use of burs up to 8 times, the surface roughness of all the tested materials decreased. Milling of LU resulted in chip deposits on the CB surface in SEM images. The duration of cutting was higher for CD and EM materials than those of other materials, but the milling duration for VM, GC, and LU blocks was similar. SB burs exhibited more wear than CB after repeated milling.

CONCLUSIONS

A repeated number of millings more than 4 times decreased the surface integrity of the milling burs, increased the surface roughness of CAD/CAM blocks except for LU, and increased the milling duration. Increased material hardness was more detrimental on bur surface integrity.

CLINICAL RELEVANCE

During milling CAD/CAM blocks, clinicians should note that the surface integrity of milling burs decreases after 4 times of use as a function of material hardness where step burs wear quicker than pointed cylinder burs.

Effect of CAD/CAM Bur Deterioration on the Surface Roughness of Ceramic Crowns

PURPOSE

To evaluate the effect of repetitive milling on the surface roughness of ceramic crowns.

MATERIALS AND METHODS

Twenty feldspathic ceramic crowns (VITABLOCS Mark II; Vita Zahnfabrik, Bad Säckingen, Germany) milled with hard milling protocol, and 20 zirconia crowns (5Y-TZP, inCoris TZI; Dentsply Sirona, York, PA) milled with soft milling protocol, using a 5-axis milling machine (inLab MC X5; Dentsply Sirona) were investigated. Atomic force microscopy (SPM SmartSPM™-1000) was used for surface roughness measurements (Ra, RMS). Pearson correlation between bur deterioration percentage as provided by the software of the milling machine and surface roughness was performed with an (r) range between 1 and -1. Trend analysis between the 2-half and among the 4-quarter bur lifetime was performed with linear contrast analysis and partial eta squared (η) for estimate of effect size. Analysis of mean variance between the zirconia and feldspathic ceramic groups was performed with a t-test, with α = 0.05.

RESULTS

Correlation analysis showed small correlation for Ra, RMS (r = -0.02, -0.16 for the feldspathic ceramic crowns and r = 0.044, 0.034 for the zirconia crowns), and no significant differences for the 2-half and 4-quarter groups (p > 0.05) for both crown types. Estimate of effect size was rated as medium for the 2-half groups (Ra = 0.420, RMS = 0.395) and large for the 4-quarter groups (RA = 0.22, F = 1.48, RMS = 0.19, F = 1.26) for the feldspathic ceramic crowns, and small effect size for the 2-half groups (Ra = 0.18, RMS = 0.20) and the 4-quarter groups (Ra = 0.035, RMS = 0.037) for the zirconia crowns. There was no significant difference (p = 0.178, F = 0.016. p = 0.088, F = 0.002) between the feldspathic ceramic and zirconia crowns with medium and large effect size (0.45, 0.57) for RA and RMS.

CONCLUSIONS

Milling zirconia and feldspathic ceramic crowns had small correlation between bur deterioration and surface roughness when the burs were used within their recommended lifetime. However, hard milled feldspathic ceramic crowns had higher surface roughness than the soft milled zirconia crowns. Bur deterioration had a larger effect on the surface roughness of the feldspathic ceramic crowns than the zirconia crowns.

In-lab simulation of CAD/CAM milling of lithium disilicate glass-ceramic specimens: Effect on the fatigue behavior of the bonded ceramic

The aim of this study was to evaluate the effect of in-lab simulation procedures performed on a lithium disilicate ceramic luted to a dentin-analogue material regarding the fatigue performance and topographic changes. Lithium disilicate ceramic (IPS e.max CAD) discs (Ø = 13.5 mm and 1.5 mm of thickness) were produced in different ways: milled in a CAD/CAM system (CAD/CAM - control group); mirror-polished (POL group); produced in-lab and ground with #60 silicon carbide paper (SiC group); with #60 wood sandpaper (WS group); with a fine diamond bur (DB group); or with a CAD/CAM bur adapted in a handpiece with a custom mandrel (MANDREL group). The ceramic discs were adhesively luted (Multilink N) onto dentin analogue discs (Ø = 12 mm and 2 mm of thickness) and fatigue testing (n = 19 discs) was performed by step-stress methodology (initial load of 200 N; step-size of 50 N; 10,000 cycles per step; 20 Hz). Surface roughness and contact angle analysis were also performed. According to Kaplan-Meier and post-hoc Mantel-Cox (log-rank), distinct fabrication methods affected the fatigue performance of bonded glass-ceramic discs (p< 0.001). The CAD/CAM group presented the lowest fatigue failure loads (1250 N) and number of cycles for failure (185,000), while the POL groups obtained the highest results (1752 N; 284,444 cycles). The in-lab groups had intermediate values (1355 - 1526 N; 206,052 - 238,684 cycles). Polished specimens presented the lowest roughness values (Ra = 0.041 μm), while the SiC (1.604 μm), WS (1.701 μm), and MANDREL (1.867 μm) groups showed statistically similar roughness values to the CAD/CAM group (1.738 μm). Despite differences before etching, the contact angle was similar among the milled and simulated groups after etching, except for the polished group. Even with some topographic similarities, the tested in-lab simulation methods were not able to mimic the milled specimens in terms of fatigue findings, leading to distinct magnitude of overestimations of the results.

Evaluating the effect of repeated use of milling burs on surface roughness and adaptation of digitally fabricated ceramic veneers

Objectives

This study aimed to evaluate how repeated use of milling diamond burs with different coarseness affects surface roughness, and marginal and internal adaptation of CAD/CAM veneers.

Methods

Forty leucite-reinforced glass-ceramic veneers were milled in 2 groups based on the milling mode (with fine or extra-fine bur sets). In each group, every 10 veneers were milled with a new bur set. All veneers were cemented to bovine teeth and then polished. Labial surface roughness was measured before cementation, and after polishing. Marginal and internal discrepancies were measured using a field emission scanning electron microscope. Three-way and two-way mixed repeated measures ANOVA were applied to assess changes in surface roughness values of veneers and discrepancy values, respectively. The Bonferroni correction was applied for multiple comparisons.

Results

Repeated use of a milling diamond bur set had a significant effect on surface roughness of the veneers (P < .001). Mean surface roughness of the fine milling mode was significantly higher in comparison to that of extra-fine mode before (P = .002) and after (P = .01) polishing. After polishing a significant decrease in surface roughness occurred in fine (P = .02), but not in extra-fine milling mode (P = .99). Repeated use of milling burs significantly affected marginal and internal adaptation between some repeated uses.

Conclusions

Marginal and internal adaptation were significantly affected by repeated use of milling diamond burs up to 10 times between some repeated uses. However, no specific pattern could be established.

Clinical significance

Repeated use of milling burs could affect surface roughness, surface microcracks, critical defects, and adaptation of CAD/CAM restorations. Therefore, it plays a major role in clinical success of the restorations.

Extended glaze firings for porcelain-veneered zirconia: Effects on the mechanical and optical behavior

OBJECTIVE

This study evaluated the effect of dwell time (conventional or extended) and cooling protocol (fast or slow) of self-glaze firings on the mechanical (flexural strength and crack propagation) and optical (color and translucency) properties of a porcelain-veneered zirconia system.

METHODS

Bilayer disc-shaped samples were prepared (Vita VM9 + In-Ceram YZ) and divided according to the final thermal treatment: glaze firing followed by slow cooling (furnace opening at 200 °C) (G-S) or fast cooling (furnace opening at 600 °C) (G-F, manufacturer-recommended protocol), extended glaze firing (15 min of dwell time) followed by slow cooling (EG-S) or fast cooling (EG-F), or no thermal treatment (CTRL). Porcelain roughness (Ra and Rz) was measured before and after glaze firings. Color (ΔE₀₀) and translucency (TP₀₀) alteration were also evaluated. Flexural strength was measured with the piston-on-three-ball test and crack propagation analysis was performed after Vickers indentations. Complementary analyzes of crystalline phase and scanning electron microscopy were carried out.

RESULTS

Significant effect of dwell time was observed, with extended glaze leading to higher flexural strength and shorter crack lengths. Cracks of EG groups were observed to end in clusters of crystals. Color and translucency changed below perceptibility thresholds. All treatments led to a smoother surface and EG groups reached the lowest Rz values. An extra SiO₂ peak was revealed in control and EG groups. No effect of cooling protocol was found.

SIGNIFICANCE

Extended glaze firing was able to improve the resistance to crack initiation and propagation of porcelain-veneered zirconia without clinically perceptible changes in optical properties.

Effect of milling, fitting adjustments, and hydrofluoric acid etching on the strength and roughness of CAD-CAM glass-ceramics: A systematic review and meta-analysis

STATEMENT OF PROBLEM

Whether procedures performed before the cementation of computer-aided design and computer-aided manufacturing (CAD-CAM) glass-ceramic restorations, including milling, fitting adjustment, and hydrofluoric acid etching introduce defects on the ceramic surface that affect the mechanical and surface properties is unclear.

PURPOSE

A systematic review and meta-analysis were conducted to assess the effect of milling, fitting adjustments, and hydrofluoric acid etching (HF) on the flexural strength and roughness (Ra) of CAD-CAM glass-ceramics.

MATERIAL AND METHODS

Literature searches were performed up to June 2020 in the PubMed/MEDLINE, Web of Science, and Scopus databases, with no publication year or language limits. The focused question was "Do milling, fitting adjustments, and hydrofluoric acid etching affect the flexural strength and roughness of CAD-CAM glass-ceramics?" For the meta-analysis, flexural strength and Ra data on milling, fitting adjustment, and HF etching versus control (polishing) were analyzed globally. A subgroup analysis assessed the effect of etching parameters (HF concentration and time) on the flexural strength and roughness of CAD-CAM glass-ceramics with different microstructures. Comparisons were performed with random-effect models at 5% significance.

RESULTS

Fourteen studies from 2764 potentially relevant records were included in the qualitative syntheses, and 12 in the meta-analysis. Milling and fitting adjustments increased roughness and reduced the flexural strength of CAD-CAM glass-ceramics. The effect of HF etching was dependent on the glass-ceramic microstructure, HF concentration, and etching time. For feldspathic- and leucite-reinforced ceramics, HF 5% applied for between 30 and 120 seconds increased roughness without affecting flexural strength. For lithium disilicate glass-ceramics, HF concentrations greater than 4.9% used for 20 seconds or more reduced the strength without affecting the surface roughness.

CONCLUSIONS

The flexural strength of CAD-CAM glass-ceramic is reduced by grinding procedures such as milling and fitting adjustment. Ceramic microstructure, HF concentration, and etching time determined the effect of hydrofluoric acid etching on the flexural strength and surface roughness of glass-ceramic materials.

Strength-limiting damage and its mitigation in CAD-CAM zirconia-reinforced lithium-silicate ceramics machined in a fully crystallized state

OBJECTIVES

The objective was to explore how clinically relevant machining process and heat treatment influence damage accumulation and strength degradation in lithium silicate-based glass ceramics machined in the fully crystallized state.

METHODS

A commercial zirconia-reinforced lithium silicate (ZLS) glass ceramic with a fully developed microstructure (Celtra® Duo) was studied. Disk-shaped specimens (nominal 10 mm diameter and 1 mm thickness) were fabricated either using a CAD-CAM process, creating a clinically relevant dental restoration surface, or were sectioned from water-jet cut cylindrical blocks with their critical surfaces consistently polished. Bi-axial flexure strength (BFS) was determined in a ball-on-ring configuration, and fractographic analysis was performed on failed specimens. XRD, AFM and SEM measurements were conducted before and after heat treatment. For each sample group, BFS was correlated with surface roughness. A two-way ANOVA and post-hoc Tukey tests were used to determine differences in BFS between machining and heat treatment groups (ɑ = 0.05).

RESULTS

A two-way ANOVA demonstrated that BFS was influenced by fabrication route (p < 0.01) with CAD-CAM specimens exhibiting significantly lower mean BFS. A factorial interaction was observed between heat treatment and machining route (p < 0.01), where a significant strengthening effect of post-manufacture heat treatment was noted for CAD-CAM specimens but not sectioned and polished samples. CAD-CAM specimens exhibited sub-surface lateral cracks alongside radial cracks near fracture origin which were not observed for polished specimens. BFS did not correlate with surface roughness for polished specimens, and no change in microstructure was detectable by XRD following heat treatment.

SIGNIFICANCE

The mechanical properties of the ZLS ceramic material studied were highly sensitive to the initial surface defect integral associated with manufacturing route and order of operations. CAD-CAM manufacturing procedures result in significant strength-limiting damage which is likely to influence restoration performance; however, this can be partially mitigated by post-machining heat treatment.

Low-fusing porcelain glaze application does not damage the fatigue strength of Y-TZP

This study evaluated and compared the effects of two glaze application methods (brush and spray) on the fatigue strength and surface characteristics (topography and roughness) of a translucent yttrium stabilized partially tetragonal zirconia polycrystal ceramic (Y-TZP) prior to and after grinding. Disc-shaped specimens of translucent Y-TZP (Vita YZ-HT; Vita-Zahnfabrik) were processed (ISO 6872-2015) and randomly allocated into 6 groups, according to the surface treatments performed on the tensile surface: Ctrl - as-sintered (no treatment); Gr - grinding with a diamond bur (181 μm-grit; #3101G); Br - glaze obtained from a powder-liquid mix and applied by brush (Vita Akzent; Vita Zahnfabrik); Sp - glaze application via spray (Vita Akzent Plus; Vita Zahnfabrik); Gr + Br and Gr + Sp - association of grinding + respective glaze method. Analyses of surface roughness (Ra and Rz), fatigue strength (staircase method), surface topography and fractography were carried out. The as-sintered condition had the smoothest surface, while grinding led to the rougher and more heterogeneous topography. Both glaze application methods showed a potential for topography evenness (smoothening effect), while the glaze spray method led to thinner layers of material, showing a limitation in reducing the roughness compared to the brush method. No deleterious effect on fatigue strength of the Y-TZP could be observed, as the glaze-spray application on the as-sintered surface showed the highest values. Fractography depicted two distinct fracture origin regions: from defects in the surface/sub-surface region for the Ctrl and Gr groups; and at the zirconia-glaze layer interface for Br, Sp, Gr + Br and Gr + Sp. The clinical relevance of this work is that the tested glaze application methods did not damage the fatigue strength of the tested Y-TZP.

Newer vs. older CAD/CAM burs: Influence of bur experience on the fatigue behavior of adhesively cemented simplified lithium-disilicate glass-ceramic restorations

This study evaluated the effect of the CAD/CAM burs experience (newer vs older as consequence of the milling sequence) on fatigue failure load (FFL), number of cycles for failure (CFF), and survival rates of lithium disilicate glass-ceramic simplified restorations adhesively cemented to a dentin analogue substrate. Three sets of CAD/CAM burs were used to mill disc-shaped ceramic specimens (1 bur set - 18 milled discs with 10 mm diameter and 1.5 mm thickness), considering the bur experience as a result of the milling sequence to compose the study groups: G1-6 - discs obtained from the 1st to 6th milling of each bur set; G7-12 - specimens from the 7th to 12th milling; G13-18 - discs from the 13th to 18th. Discs of dentin analogue (G10, 10 mm diameter and 2.0 mm thickness) were made to serve as substrate (base material) and randomly assigned into pairs with the respective ceramic discs. Then, the ceramic discs were adhesively cemented onto the dentin analogue substrate, composing a three-layer specimen that mimics a monolithic restoration of a posterior tooth. Specimens were tested under stepwise fatigue approach: frequency = 20 Hz, 5000 cycles at maximum load of 400 N to accommodate the testing assembly, followed by incremental steps of 200 N with initial load ranging from 10 to 1000 N, to a maximum of 20,000 cycles/each step, until the occurrence of failure (radial crack). FFL and CFF were recorded at the end of the testing and subjected to statistical analysis. Supplementary roughness analysis of the milled surface was performed (n = 18) using a contact profilometer. Residual stress after milling and acid etching were accessed via X-ray Diffractometry analysis. FFL and CFF were not affected by increase on bur experience (no statistical differences among groups), despite that, it affected both Ra and Rz parameters (G1-6 had the smoothest surface). The residual stress concentration was negligible (milling did not induce residual stress concentration). It is concluded that the fatigue behavior of adhesively cemented lithium-disilicate glass-ceramic restorations was not influenced by CAD/CAM bur experience (newer vs older as consequence of the milling sequence), and so the residual stress concentration induced by milling was negligible.

Evaluation of Wear Resistance of Dental Chairside CAD/CAM Glass Ceramics Reinforced by Different Crystalline Phases

The mechanical properties of crystalline phase of glass ceramics are critical. This study aimed to evaluate wear resistance of different crystalline-reinforced dental chairside computer-aided design/computer-aided manufacturing (CAD/CAM) glass ceramics. Materials of feldspar (Vita Mark II, VM), leucite (IPS Empress CAD, EC), lithium disilicate (IPS e.max CAD, EX), lithium disilicate enriched with zirconia (Vita Suprinity, VS), and enamel were embedded, grounded, and polished, respectively. Samples were indented with a Vickers hardness tester to test the fracture resistance (KIC). Two-body wear tests were performed in a reciprocal ball-on-flat configuration under artificial saliva. The parameters of load force (50 N), reciprocating amplitude (500 μm), frequency (2 Hz), and the test cycle (10,000 cycles) were selected. Specimen microstructure, indentation morphology, and wear scars were observed by scanning electron microscope (SEM), optical microscopy, and three-dimensional profile microscopy. EX, VS, and EC demonstrated significantly higher KIC values than the enamel, while ceramic materials showed smaller wear depth results. Cracks, massive delamination, and shallow plow were seen on the enamel worn scar. Long deep plow, delamination, and brittle cracks are more common for VM and EC, and short shallow plow and smooth subsurface are the characteristics of EX and VS. Greater fracture toughness values indicated higher wear resistances of the materials for the test glass ceramics. The CAD/CAM glass ceramics performed greater wear resistance than enamel. Feldspar- and leucite-reinforced glass ceramics illustrated better wear resistance similar to enamel than lithium disilicate glass ceramics, providing amicable matching with the opposite teeth.

The effect of extended glaze firing on the flexural fatigue strength of hard-machined ceramics

STATEMENT OF PROBLEM

It is unclear whether an extended glaze firing could improve the long-term mechanical performance of densely sintered CAD-CAM ceramics.

PURPOSE

The purpose of this in vitro study was to analyze the effect of an extended glaze firing on the flexural fatigue strength (FFS) of densely sintered milled (hard-machined) leucite-based (LEU) and lithium disilicate-based (DIS) ceramics.

MATERIAL AND METHODS

Disks were machined from ceramic blocks and divided into 6 groups (n=20) according to the material, LEU or DIS, and to the applied glaze firing: manufacturer-recommended glaze (G group), extended glaze (EG group), and control/no firing (C group). The surface roughness of the disks was measured before and after firing by using a contact profilometer, and data were compared by paired sample tests. Specimens were submitted to fatigue by using the staircase test design in water (piston-on-3 balls; 500 000 cycles, 20 Hz, and sinusoidal loading). Mean (±SD) FFS values were then calculated and analyzed by using 1-way analysis of variance and post hoc Tukey test (α=.05).

RESULTS

Surface roughness did not change after the firing (P>.05). The highest FFS value in both ceramics was obtained after EG firing (LEU-EG=80.52 ±6.3 MPa; DIS-EG=147.25 ±10.5 MPa), which was statistically superior to G firing (LEU-G=73 ±6.8 MPa, P=.003; DIS-G=134.34 ±15.6 MPa; P=.023) and C group (LEU-C=61.94 ±6.3 MPa; P<.001; DIS-C=134.13 ±17.3 MPa; P=.023).

CONCLUSIONS

EG firing optimized the biaxial flexural fatigue strength of hard-machined leucite and lithium disilicate ceramics compared with conventional glaze firing.

Effect of Finishing and Polishing on Roughness and Gloss of Lithium Disilicate and Lithium Silicate Zirconia Reinforced Glass Ceramic for CAD/CAM Systems

Objective:

To assess the efficacy of dedicated finishing/polishing systems on roughness and gloss of VITA Suprinity and IPS e.max CAD.

Method:

A total of 24 blocks of Suprinity and 24 of e.max were cut into a wedge shape using an InLab MC-XL milling unit. After crystallization, the 24 Suprinity wedges were divided into four subgroups: group A.1: Suprinity Polishing Set Clinical used for 30 seconds and group A.2: for 60 seconds; group A.3: VITA Akzent Plus Paste; and group A.4: spray. The 24 e.max wedges (group B) were divided into four subgroups according to the finishing procedure: group B.1: Optrafine Ceramic Polishing System for 30 seconds and group B.2: for 60 seconds; group B.3: IPS e.max CAD Crystall/Glaze paste; and group B.4: spray. After finishing/polishing, gloss was assessed with a glossmeter and roughness evaluated with a profilometer. Results were analyzed by applying a two-way analysis of variance for gloss and another for roughness (α=0.05). One specimen per each subgroup was observed with a scanning electron microscope.

Results:

For roughness, materials and surface were significant factors (p&lt;0.001). Suprinity exhibited significantly lower roughness than e.max. Also the Material-Surface Treatment interaction was statistically significant (p=0.026). For gloss, both material and surface treatment were significant factors (p&lt;0.001). VITA Suprinity showed significantly higher gloss than e.max. Also the Material-Surface Treatment interaction was statistically significant (p&lt;0.001).

Conclusions:

Manual finishing/polishing for 60 seconds and glazing paste are the most effective procedures in lowering the roughness of CAD/CAM silica-based glass ceramics. Manual finishing/polishing for 60 seconds allows milled silica-based glass ceramics to yield a higher gloss. VITA Suprinity displayed higher polishability than IPS e.max CAD.

Ceramic molar crown reproducibility by digital workflow manufacturing: An in vitro study

PURPOSE

This in vitro study aimed to analyze and compare the reproducibility of zirconia and lithium disilicate crowns manufactured by digital workflow.

MATERIALS AND METHODS

A typodont model with a prepped upper first molar was set in a phantom head, and a digital impression was obtained with a video intraoral scanner (CEREC Omnicam; Sirona GmbH), from which a single crown was designed and manufactured with CAD/CAM into a zirconia crown and lithium disilicate crown (n=12). Reproducibility of each crown was quantitatively retrieved by superimposing the digitized data of the crown in 3D inspection software, and differences were graphically mapped in color. Areas with large differences were analyzed with digital microscopy. Mean quadratic deviations (RMS) quantitatively obtained from each ceramic group were statistically analyzed with Student's t-test (α=.05).

RESULTS

The RMS value of lithium disilicate crown was 29.2 (4.1) µm and 17.6 (5.5) µm on the outer and inner surfaces, respectively, whereas these values were 18.6 (2.0) µm and 20.6 (5.1) µm for the zirconia crown. Reproducibility of zirconia and lithium disilicate crowns had a statistically significant difference only on the outer surface (P<.001). The outer surface of lithium disilicate crown showed over-contouring on the buccal surface and under-contouring on the inner occlusal surface. The outer surface of zirconia crown showed both over- and under-contouring on the buccal surface, and the inner surface showed under-contouring in the marginal areas.

CONCLUSION

Restoration manufacturing by digital workflow will enhance the reproducibility of zirconia single crowns more than that of lithium disilicate single crowns.

CAD/CAM machining Vs pre-sintering in-lab fabrication techniques of Y-TZP ceramic specimens: Effects on their mechanical fatigue behavior

This study evaluated the effects of different pre-sintering fabrication processing techniques of Y-TZP ceramic (CAD/CAM Vs. in-lab), considering surface characteristics and mechanical performance outcomes. Pre-sintered discs of Y-TZP ceramic (IPS e.max ZirCAD, Ivoclar Vivadent) were produced using different pre-sintering fabrication processing techniques: Machined- milling with a CAD/CAM system; Polished- fabrication using a cutting device followed by polishing (600 and 1200 SiC papers); Xfine- fabrication using a cutting machine followed by grinding with extra-fine diamond bur (grit size 30 μm); Fine- fabrication using a cutting machine followed by grinding with fine diamond bur (grit size 46 μm); SiC- fabrication using a cutting machine followed by grinding with 220 SiC paper. Afterwards, the discs were sintered and submitted to roughness (n=35), surface topography (n=2), phase transformation (n=2), biaxial flexural strength (n=20), and biaxial flexural fatigue strength (fatigue limit) (n=15) analyses. No monoclinic-phase content was observed in all processing techniques. It can be observed that obtaining a surface with similar characteristics to CAD/CAM milling is essential for the observation of similar mechanical performance. On this sense, grinding with fine diamond bur before sintering (Fine group) was the best mimic protocol in comparison to the CAD/CAM milling.

Microtensile bond strength and micromorphologic analysis of surface-treated resin nanoceramics

PURPOSE

The aim of this study was to evaluate the influence of different surface treatment methods on the microtensile bond strength of resin cement to resin nanoceramic (RNC).

MATERIALS AND METHODS

RNC onlays (Lava Ultimate) (n=30) were treated using air abrasion with and without a universal adhesive, or HF etching followed by a universal adhesive with and without a silane coupling agent, or tribological silica coating with and without a universal adhesive, and divided into 6 groups. Onlays were luted with resin cement to dentin surfaces. A microtensile bond strength test was performed and evaluated by one-way ANOVA and Tukey HSD test (α=.05). A nanoscratch test, field emission scanning electron microscopy, and energy dispersive X-ray spectroscopy were used for micromorphologic analysis (α=.05). The roughness and elemental proportion were evaluated by Kruskal–Wallis test and Mann–Whitney U test.

RESULTS

Tribological silica coating showed the highest roughness, followed by air abrasion and HF etching. After HF etching, the RNC surface presented a decrease in oxygen, silicon, and zirconium ratio with increasing carbon ratio. Air abrasion with universal adhesive showed the highest bond strength followed by tribological silica coating with universal adhesive. HF etching with universal adhesive showed the lowest bond strength.

CONCLUSION

An improved understanding of the effect of surface treatment of RNC could enhance the durability of resin bonding when used for indirect restorations. When using RNC for restoration, effective and systemic surface roughening methods and an appropriate adhesive are required.

Strength Determination of Brittle Materials as Curved Monolithic Structures

The dental literature is replete with “crunch the crown” monotonic load-to-failure studies of all-ceramic materials despite fracture behavior being dominated by the indenter contact surface. Load-to-failure data provide no information on stress patterns, and comparisons among studies are impossible owing to variable testing protocols. We investigated the influence of nonplanar geometries on the maximum principal stress of curved discs tested in biaxial flexure in the absence of analytical solutions. Radii of curvature analogous to elements of complex dental geometries and a finite element analysis method were integrated with experimental testing as a surrogate solution to calculate the maximum principal stress at failure. We employed soda-lime glass discs, a planar control (group P, n = 20), with curvature applied to the remaining discs by slump forming to different radii of curvature (30, 20, 15, and 10 mm; groups R30-R10). The mean deflection (group P) and radii of curvature obtained on slumping (groups R30-R10) were determined by profilometry before and after annealing and surface treatment protocols. Finite element analysis used the biaxial flexure load-to-failure data to determine the maximum principal stress at failure. Mean maximum principal stresses and load to failure were analyzed with one-way analyses of variance and post hoc Tukey tests (α = 0.05). The measured radii of curvature differed significantly among groups, and the radii of curvature were not influenced by annealing. Significant increases in the mean load to failure were observed as the radius of curvature was reduced. The maximum principal stress did not demonstrate sensitivity to radius of curvature. The findings highlight the sensitivity of failure load to specimen shape. The data also support the synergistic use of bespoke computational analysis with conventional mechanical testing and highlight a solution to complications with complex specimen geometries.