Flexural strength and Weibull characteristics of stereolithography additive manufactured versus milled zirconia

Statement of problemZirconia restorations can be processed by using stereolithography additive manufacturing (AM) technologies. However, whether additive manufactured zirconia could achieve flexural strength values comparable with those of milled zirconia is unclear.PurposeThe purpose of this in vitro study was to compare the flexural strength and Weibull characteristics of milled and additive manufactured zirconia.Material and methodsA total of 40 zirconia bars (25×4×1.2 mm) were obtained by using 2 manufacturing procedures, subtractive (CNC group) (IPS e.max ZirCAD; Ivoclar Vivadent AG) and additive manufacturing (AM group) (3DMix ZrO₂; 3DCeram) technologies and assigned to 2 subgroups according to accelerating artificial aging procedures (mastication simulation): nonaged and aged (n=10). Flexural strength was measured in all specimens by using 3-point bend tests according to ISO/CD 6872.2 with a universal testing machine (Instron Model 8501; Instron Corp). Two-parameter Weibull distribution values, including the Weibull modulus, scale (m), and shape (0) were calculated. Flexural strength values were analyzed by using 2-way ANOVA and Student t statistical tests (α=.05).ResultsThe manufacturing procedure (P<.001), the mastication simulating aging procedure (P<.001), and the interaction between them (P<.001) significantly affected flexural strength values. The CNC group exhibited statistically higher flexural strength values than those in the AM group when the specimens were tested before performing an aging procedure (P<.001) and after mastication simulation (P<.001). Moreover, mastication simulation produced a significant reduction in flexural strength for both the CNC group (P<.039) and the AM group (P<.001).ConclusionsThe manufacturing process reported a significant effect on the flexural strength of the zirconia material tested. Mastication simulation as a means of accelerating artificial aging resulted in the significantly decreased flexural strength values of milled and additively manufactured zirconia material, with the Weibull moduli being significantly higher for the milled groups versus the milled specimens.     

Marginal and internal fit of three-unit zirconia fixed dental prostheses: Effects of prosthesis design,cement space,and zirconia type

PurposeThe purpose of this study was to compare the marginal and internal fit of three-unit fixed dental prostheses (FDPs) fabricated using CAD/CAM with two designs, two cement space (CS), and two zirconia types.MethodsA master model with two zirconia abutments and a missing tooth was scanned with an intraoral scanner. FDPs were fabricated with two designs (Full contour: FC, Framework: FW), two zirconia types (multi-layer: L, single-layer: W), and two CS values (30 and 45 μm for L and 30 μm for W). There were six experimental groups. The fit of the FDPs was evaluated using the replica method. The space between an abutment and the FDPs in the marginal (MO), chamfer (CH), axial (AX), and occlusal (OC) areas was measured under an optical microscope and the data was statistically analyzed using three-way ANOVA and Bonferroni test (p < 0.05).ResultsFW-l-45 μm showed a significantly smaller space than those for the FC in MO (p = 0.011), CH (p = 0.001) and AXE (p = 0.003). FW-l-30 μm showed a significantly smaller space than that for the 45 µm in AXE (p = 0.000) and OC (p = 0.016). FW-W-30 µm showed a significantly smaller space than that for the L in MO (p = 0.000), CH (p = 0.000), AXE (p = 0.000), and OC (p = 0.002).ConclusionsThe design and CS of the FDPs affected the fit. FDPs with single-layer zirconia showed better fit than that obtained with multi-layer zirconia.     

Comparison of properties and cost efficiency of zirconia processed by DIW printing,casting and CAD/CAM-milling

ObjectivesThe aim of this study was to evaluate the mechanical properties and cost efficiency of direct ink writing (DIW) printing of two different zirconia inks compared to casting and subtractive manufacturing.MethodsZirconia disks were manufactured by DIW printing and the casting process and divided into six subgroups (n = 20) according to sintering temperatures (1350 °C, 1450 °C and 1550 °C) and two different ink compositions (Ink 1, Ink 2). A CAD/CAM-milled high strength zirconia (3Y-TZP) was added as reference group. The biaxial flexural strength (BFS) was measured using the piston-on-three-balls test. X-ray-diffraction (XRD) was used for microstructural analysis. The cost efficiency was compared for DIW printing and subtractive manufacturing by calculation of the manufacturing costs of one dental crown.ResultsUsing XRD, monoclinic and tetragonal phases were detected for Ink 1, for all other groups no monoclinic phase was detected. The CAD/CAM-milled ceramic showed a significantly higher BFS than all other groups. The BFS of Ink 2 was significantly higher than the BFS of Ink 1. At a sintering temperature of 1550 °C the mean BFS of the printed Ink 2 was 822 ± 174 MPa. The BFS of the cast materials did not show a significantly higher BFS than the corresponding printed group for any tested parameter-set. The manufacturing costs of DIW printed crowns are lower than the manufacturing costs of CAD/CAM-milled crowns.ConclusionDIW has a high potential to replace subtractive processes for dental applications, as it shows promising mechanical properties for appropriate ink compositions and facilitates a highly cost effective production.     

The effect of aging on the wear performance of monolithic zirconia

ObjectivesInvestigate the effect of aging on the wear behavior of glazed vs polished monolithic zirconia and to establish if glazing provides protection against low temperature degradation.Methods40 1-mm-diameter spheres made from four differently treated monolithic zirconia (VITA YZ® HT); polished, polished-aged, glazed and glazed-aged (n = 10), were tested in a wear testing machine (UFW200) against bovine enamel in artificial saliva as per the following settings (ISO20808:2016): ball-on-disc configuration, 5 N vertical load, 0.1 m/s sliding speed, 400 m sliding distance and 37 °C temperature. Vertical substance loss (mm) wear of zirconia and enamel specimens was measured. Data were statistically analyzed with Kruskal-Wallis one-way ANOVA test (α > 0.05).ResultsGlazed-aged zirconia specimens resulted in the greatest amount of enamel wear (0.823 mm ± 0.157) followed by glazed (0.729 mm ± 0.289), polished-aged (0.377 mm ± 0.201) then polished (0.247 mm ± 0.125). In the groups with the same surface finish, aging showed no statistical difference in wear (P > 0.008). Glazing resulted in a higher enamel wear compared with polishing that was statistically significant (P < 0.008) except when the polished specimens were aged and the glazed specimens were not aged.SignificanceAging increases abrasiveness of monolithic zirconia regardless of the type of surface finish. The effect of aging is “latent” and only revealed under mechanical loading during wear simulation which increases surface roughness and wear by adversely affecting zirconia’s mechanical properties, making it less capable to maintain its initial surface smoothness. The glaze layer may protect zirconia from LTD, however, it is susceptible to aging which further increases its abrasiveness.     

The effect of framework fabrication technique on the fit accuracy of full arch screw retained implant supported prostheses

PurposeThe purpose of this study was to compare the accuracy of fit of Co-Cr full arch screw-retained implant-supported fixed dental prosthesis fabricated among three different methods: conventional casting, milling, and additive manufacturing technology.Materials and methodsA master model of a completely edentulous mandible with five internal connection implants was utilized. Thirty full arch Co-Cr screw-retained implant-supported frameworks were fabricated by three different methods: conventional casting, milling, and additive manufacturing (AM) technology. The marginal fit was measured using a coordinate measuring machine in x-, y-, and z-axes, as well as the three-dimensional discrepancy. The casting group were measured twice: before the adaptation procedure and again after the adaptation procedure (sectioning and laser welding). For comparisons of marginal fit of frameworks between different groups one-way analysis of variance and Games Howell test was used. Paired t-test was used to compare cast frameworks before and after adaption.ResultsThere were statistically significant differences in marginal fit and width distortion between groups (P <.05). The mean of total distortion of each group was 94.6 µm (SD 50.5 µm) for casting group before adaptation, 92.44 µm (SD 49.6 µm) for casting group after adaptation, 71.4 µm (SD 37.2 µm) for additive manufacturing group, while for the milling group the total distortion was 50.1 µm (SD 27.5 µm).ConclusionFull arch screw-retained implant-supported frameworks fabricated with any of the three fabrication techniques using cobalt-chromium material exhibited acceptable marginal fit. Milling fabrication technique showed the most accurate marginal fit. Adaptation procedure for the cast group has significantly improved the marginal fit.     

Reliability and aging behavior of three different zirconia grades used for monolithic four-unit fixed dental prostheses

ObjectiveTo investigate the fracture resistance and phase composition of tooth supported four-unit fixed dental prostheses (FDPs) made from three different zirconia grades after loading and aging.MethodsSeventy-two FDPs were fabricated from 3Y-TZP, 4Y-PSZ and 5Y-PSZ. This resulted in 24 FDPs per grade, subdivided into three groups (n = 8): a control group (C), a hydrothermally aged (H₂O, 85 °C, 90 days) group (A) and a group subjected to loading (2.5M cycles, 98N) with simultaneous thermal cycling (H₂O, 5–55 °C) subsequent to treatment A (AL). Subsequently, FDPs were statically loaded to fracture. Phase composition was quantified by X-ray diffraction (XRD) and μ-Raman spectroscopy. Focused ion beam (FIB) - Scanning electron microscopy (SEM) was used for visualization in-depth.ResultsCompared to 3Y-C FDPs (1233 ± 165N), reduced fracture load was found for 5Y-C FDPs (889 ± 80 N; p < .001). This did not apply for 4Y-C samples (1065 ± 111N). Treatments (A, AL) did not negatively affect the fracture load for the three grades of zirconia (p > .645). Both A and AL increased monoclinic phase content for 3Y and 4Y FDPs, whereas FIB-SEM suggests no transformability of 5Y-PSZ. 5Y-AL FDPs showed cracks and fractures at the abutment walls and restoration margins after dynamic loading. Reduced fracture load of 5Y samples as compared to 3Y and 4Y was associated with deficient transformability in the fracture zone.SignificanceAging and loading did not negatively affect the fracture resistance of monolithic four-unit FDPs made from three grades of zirconia. Due to cracks after dynamic loading, 5Y-PSZ cannot be recommended for the clinical application of four-unit FDPs.     

Bone microarchitecture and turnover in the irradiated human mandible

ObjectivesThe aim of this study was to assess the microarchitecture and turnover in irradiated cancellous mandibular bone and the relation with radiation dose, to elucidate the effects of radiotherapy on the mandible.Patients and methodsMandibular cancellous bone biopsies were taken from irradiated patients and controls. Micro-CT scanning was performed to analyze microstructural bone parameters. Bone turnover was assessed by histomorphometry. Local radiation dose at the biopsy site (Dmax) was estimated from radiotherapy plans.ResultsTwenty-seven irradiated patients and 35 controls were included. Osteoid volume (Osteoid Volume/Bone Volume, OV/BV) [0.066/0.168 (median/interquartile range (IQR), OV/BV; %), P < 0.001], osteoid surface (Osteoid Surface/Bone Surface, OS/BS) [0.772/2.17 (median/IQR, OS/BS; %), P < 0.001] and osteoclasts number (Osteoclasts per millimetre bone surface, Ocl/mmBS; mm²) [0.026/0.123 (median/IQR, Ocl/mmBS; mm²), P < 0.001] were decreased; trabecular number (Tb.N) was lower [1.63/0.63 (median/IQR, Tb.N; 1/mm⁻¹), P = 0.012] and trabecular separation (Tb.Sp) [0.626/0.24 (median/IQR, Tb.Sp; μm), P = 0.038] was higher in irradiated mandibular bone. With higher Dmax, trabecular number increases (Spearman's correlation R = 0.470, P = 0.018) and trabecular separation decreases (Spearman's correlation R = −0.526, P = 0.007). Bone mineral density (BMD, milligrams hydroxyappetite per cubic centimetre, mgHA/cm³) [1016/99 (median/IQR, BMD; mgHA/cm³), P = 0.03] and trabecular separation [0.739/0.21 (median/IQR, Tb.Sp; μm), P = 0.005] are higher whereas connectivity density (Conn Dens) [3.94/6.71 (median/IQR, Conn Dens), P = 0.047] and trabecular number [1.48/0.44 (median/IQR, Tb.N; 1/mm⁻¹), P = 0.002] are lower in Dmax ≤50 Gy compared to controls.ConclusionsRadiotherapy dramatically impairs bone turnover in the mandible. Deterioration in microarchitecture only affects bone irradiated with a Dmax of <50 Gy. The 50 Gy value seems to be a critical threshold to where the effects of the radiation is more detrimental.     

Discrepancy at the implant abutment-prosthesis interface of complete-arch cobalt-chromium implant frameworks fabricated by additive and subtractive technologies before and after ceramic veneering

Statement of problemSelective laser melting additive manufacturing (AM) technologies can be used to fabricate complete-arch cobalt-chromium (Co-Cr) implant-supported prostheses. However, the discrepancy at the implant-prosthesis interface with these fabrication techniques and after ceramic veneering remains unclear.PurposeThe purpose of the present in vitro investigation was to measure the discrepancy at the implant abutment-prosthesis interface before and after the ceramic veneering of frameworks fabricated by using subtractive and selective laser melting AM technologies.Material and methodsA completely edentulous cast with 6 implant abutment replicas (Multi-unit Abutment RP Replicas; Nobel Biocare Services AG) was prepared. A total of 20 Co-Cr frameworks were fabricated using subtractive or computer numerical control milling (CNC group) and additive (AM group) technologies (n=10). A coordinate measurement machine was used to measure the linear and angular discrepancy at the implant abutment-prosthesis interface. Subsequently, a ceramic veneer was applied to each framework following the same standardized protocol. A bonding layer (Chromium-Cobalt Bonding; Bredent), 2 opaquer layers (Powder opaque and liquid UF; Creation CC), a layer of dentin ceramic (Dentine A3; Creation CC), a layer of enamel ceramic (Enamel S-59; Creation CC), and a glaze layer (Glaze paste and Liquid GL; Creation CC) were applied following the manufacturer’s firing protocol. Coordinate measurement machine assessment was repeated to measure the linear and angular discrepancies after ceramic veneering procedures. Data were analyzed by using the Wilcoxon signedrank and Mann-Whitney U tests (α=.05).ResultsNo statistically significant differences (P>.05) were demonstrated in assessing the discrepancies at the implant abutment-prosthesis interface between the groups except for the XZ angle of the CNC group (P<.05). Ceramic techniques produced significantly higher linear and angular discrepancies in both groups (P<.001) with a mean ±standard deviation increase in the 3-dimensional gap of 36.9 ±15.6 μm in the CNC group and 38.9 ±16.6 μm in the AM group. The AM group presented significantly higher discrepancy in the x-axis than the CNC group (P<.001).ConclusionsManufacturing procedures did not significantly influence the discrepancy at the implant abutment-prosthesis interface, which was significantly increased after ceramic veneering, except for the XZ angle of the CNC group. The differences between the discrepancies at the implant abutment-prosthesis interface before and after ceramic application revealed no significant discrepancies among the groups, except in the AM group that presented a significantly higher discrepancy on the x-axis compared with the CNC group.     

A three-dimensional study of hemimandibular hyperplasia,hemimandibular elongation,solitary condylar hyperplasia,simple mandibular asymmetry and condylar osteoma or osteochondroma

PurposeTo investigate the morphological features of hemimandibular hyperplasia (HH) in comparison to other condylar hyperplasia-associated asymmetries, including hemimandibular elongation (HE), solitary condylar hyperplasia (SCH), simple mandibular asymmetry (SMA) and condylar osteoma or osteochondroma (COS).Materials and methodsA total of 31 HH, 9 HE, 6 SCH, 10 SMA and 10 COS patients were included in this study. Clinical documentation, panoramic radiography and computed tomography data were retrospectively reviewed. The three-dimensional measurements were performed on multi-planar reformation images and volume rendering images. The accuracy of the subjective radiological signs was evaluated using sensitivity, specificity and receiver operating curve analysis. Discriminant analysis was performed to generate predictive formulas using quantitative data.ResultsThe condyles in HH were regularly or irregularly enlarged, with significantly enlarged anterior-posterior length [16.2/5.29 (mean/SD, mm) P < 0.001] and volume [5.3/2.9(mean/SD, cm³) P < 0.001] compared to the normal values. The condyles in HE and SMA were normally shaped, and the quantitative measurements were within the normal range. The ramus heights in the HH patients [55.7/5.4(mean/SD, mm)] were enlarged in comparison to the contralateral side (P < 0.001) and normal values (P < 0.001). The ramus heights in the HE [52.4/7.1 (mean/SD, mm), P < 0.001] and SMA [50.3/5.0(mean/SD, mm), P = 0.002] patients were enlarged in the contralateral side comparison but were within the normal range. The mandibular body heights in HH were enlarged in the premolar [16.6/1.3 (mean/SD, mm), P < 0.001] and molar [24.8/1.4 (mean/SD, mm), P < 0.001] regions. The inferior convexity of the lower mandibular border and inferiorly displaced mandibular canal produced high specificity, sensitivity and area under the curve for the diagnosis of HH. Discriminant analysis could predict the diagnoses with a cross-validation accuracy rate of 85.7%.ConclusionsHH is a distinct clinical entity characterized by enlargement of the condyle, ramus and mandibular body. The inferior convexity of the lower mandibular border and inferiorly displaced mandibular canal is accurate and specific for the diagnosis of HH. The condyles in HE are not hyperplastic. The term “condylar hyperplasia” alone cannot be used to refer to HH or HE.     

Three-dimensional trueness and margin quality of monolithic zirconia restorations fabricated by additive 3D gel deposition

PurposeThe aim of this in vitro study was to evaluate the three-dimensional trueness and margin quality of monolithic zirconia restorations fabricated by additive 3D gel deposition, compared with those by subtractive milling.MethodsTen single crowns and ten 4-unit FPDs of different occlusal geometries and margin thickness were fabricated by additive 3D gel deposition (additive group) and subtractive milling (subtractive group). An intraoral scanner was used to digitalize the restorations. 3D deviation analysis was applied and root mean square (RMS) was used to assess the trueness. Margin quality was characterized using optical stereomicroscopy and 3D laser scanning microscopy.ResultsFor single crowns with shallow fossae and grooves and normal margin, RMS value of additive group and subtractive group showed no significant difference in external surface, while additive group showed higher RMS value in intaglio surface. As for 4-unit FPDs with deep fossae and grooves and thin margin, RMS value of additive group in external surface was significantly lower than that of subtractive group and in intaglio surface there was no significant difference between two groups. With a 0.5 mm chamfer design, single crowns in additive group showed flawless margin with a smooth contour line, whereas minor flaws could be observed in 4-unit FPDs with thin margin. In subtractive group, restorations showed minor flaws or defects of various number and severity.ConclusionsMonolithic zirconia restorations fabricated by additive 3D gel deposition have comparable trueness and better margin quality than those fabricated by subtractive milling. Besides it is more capable of enabling complex geometry.     

Marginal and internal fit of three-unit fixed dental prostheses fabricated from translucent multicolored zirconia: Framework versus complete contour design

Statement of problemTranslucent multicolored zirconia materials enable more esthetic complete contour zirconia fixed dental prostheses (FDPs) than conventional zirconia, which exhibits low translucency and high opacity and is monochromatic. However, how the marginal and internal fit of translucent multicolored zirconia FDPs compare with those of traditional frameworks that require veneering is unclear.PurposeThe purpose of this in vitro study was to compare the marginal and internal fit of frameworks and complete contour 3-unit FDPs fabricated from translucent multicolored zirconia.Material and methodsFrameworks with a thickness of 0.5 mm and complete contour FDPs with a thickness of 0.8 to 1.5 mm were manufactured by using a workflow similar to one from a zirconia master model (mandibular left second premolar-mandibular left second molar). Two polyvinyl siloxane replicas were made for each specimen to measure the marginal and internal fit. Measurement locations were mesial, lingual, buccal, and distal for each abutment. In these locations, the marginal opening (MO), chamfer area (CA), axial wall (AW), and occlusal area (OC) were measured. The data were analyzed with 2-way ANOVA and the Bonferroni post hoc test (α=.05).ResultsFrameworks showed significantly better mean ±standard deviation fit values than complete contour 3-unit FDPs at measurement areas MO (frameworks: 112 ±22 μm, complete contour FDPs: 144 ±37 μm) (P=.013), CA (frameworks: 89 ±12 μm, complete contour FDPs: 110 ±22 μm) (P=.006), and OC (frameworks: 182 ±36 μm, complete contour FDPs: 244 ±64 μm) (P=.008). At the measurement area AW (frameworks: 47 ±7 μm, complete contour FDPs: 50 ±9 μm of each location, no significant difference was observed between frameworks and complete contour FDPs (P=.361).ConclusionsDesign differences in 3-unit FDPs fabricated from translucent multicolored zirconia influenced the marginal and internal fit. Frameworks had smaller marginal fit than complete contour FDPs for translucent multicolored zirconia.     

Influence of different combinations of CAD-CAM crown and customized abutment materials on the force absorption capacity in implant supported restorations – In vitro study

ObjectivesTo evaluate the force absorption capacity of implant supported restorations utilizing different CAD-CAM materials for the fabrication of crowns and customized abutments.Methods80 titanium inserts were scanned to design customized abutments and crowns. The specimens were divided into four groups (n = 20/material): (Z): zirconia, (P): PEEK, (V): VITA Enamic, and (E): IPS e.max. Each group was subdivided into two subgroups according to customized abutment material: (Z) zirconia, and (P) for PEEK. For the assessment of force absorption, all specimens were loaded in a universal testing machine, applied loads curves were collected from the machine’s software, and resulting loads curves were collected from forcemeter below the assembly. The slopes of all curves were analyzed using Two-way multivariate analysis of variance with pairwise comparisons using Tukey Post Hoc test (p < 0.05).ResultsThe curve progression of the applied and resulting forces varied among the investigated materials for each specimen. For zirconia abutments, ZZ showed the highest slope values of the applied and resulting force curves, followed by EZ, VZ, and PZ demonstrating statistically significant differences (P < .001). As for PEEK abutments, ZP and EP showed the least slope values, followed by PP then VP demonstrating statistically significant differences (P < .001). For Zirconia and e.max crowns, using PEEK abutments significantly increased slope loss. As for PEEK and Vita Enamic crowns changing abutment material did not significantly affect slope loss.SignificanceCombining rigid crown materials with less rigid abutments might enhance their force absorption capacity. However, with less rigid crown materials a stiff substructure might be mandatory to preserve their force absorption behavior.     

Addition of metal chlorides to a HOCl conditioner can enhance bond strength to smear layer deproteinized dentin

ObjectivesTo evaluate the effect of smear layer deproteinization using hypochlorous acid (HOCl) with/without metal chlorides (SrCl₂ and ZnCl₂) on the microtensile bond strength (µTBS) of two simplified adhesives to dentin.MethodsHuman dentin surfaces with a standardized smear layer were pretreated using a 105 ppm HOCl solution with/without SrCl₂ (0.05 M, 0.1 M, 0.2 M, 0.4 M) or ZnCl₂ (0.05 M, 0.1 M, 0.2 M) for 5 s, 15 s, or 30 s. After the deproteinizing solution was washed out with water for 5 s, 15 s, or 30 s, pretreated surfaces were bonded with one-step self-etch adhesive Bond Force II or universal adhesive Clearfil Universal Bond Quick, and µTBS was measured after 24 h. Additionally, the deproteinizing effects of HOCl solutions with/without the metal chlorides were compared by measuring changes in the amide:phosphate ratio using attenuated total reflection Fourier transform infrared spectroscopy. Statistical analysis was performed using multifactor ANOVA, Tukey’s post hoc tests and t-tests (p < 0.05).ResultsPretreatment with pure HOCl for 15 s and 30 s significantly decreased the amide:phosphate ratio (p < 0.05), indicating effective deproteinization, but the µTBS of both adhesives increased significantly only if HOCl was washed out for 30 s (p < 0.05). Increasing the concentrations of metal chlorides enabled shortening of the wash-out time down to 5 s while maintaining the improved µTBS (p < 0.05). The deproteinizing effect of HOCl was not significantly altered by the addition of metal chlorides (p > 0.05).SignificanceThe effectiveness of smear layer deproteinization using HOCl can be improved by the addition of metal chlorides, as their increasing concentration allowed to shorten the wash-out time from 30 s down to 5 s     

Effect of full arch two scanning techniques on the accuracy of overdenture conventional and CAD/CAM Co-Cr bars

PurposeThis work evaluates the internal and marginal adaptation of implant-assisted overdenture cobalt–chromium (Co–Cr) bars manufactured using conventional as well as CAD/CAM subtractive and selective laser melting (SLM) utilizing two scanning techniques. Methods: An edentulous study model containing four dental implants placed at teeth sites 36, 33, 43, and 46 was used. The study cast was scanned and compared to the virtual casts developed from two scanning techniques, straight and zigzag motion, using the in silico superimposition process. Then, conventional techniques were used to produce full-arch bars that were compared to the bars fabricated using the two scanning techniques and CAD/CAM subtractive and additive techniques. Results: The conventional impression and casting techniques had the smallest marginal gap among the groups (P-value < 0.05). The CAD/CAM subtractive milling techniques in groups II and III had significantly smaller marginal gaps than SLM technique used in groups IV and V (P-value < 0.05). The analysis of the internal gap within each group showed statistically significant differences between different implant sites in all groups (P-value < 0.001), except when using the conventional impression and casting techniques in group I (P-value = 0.20). Conclusion: The conventional impression and fabrication techniques were better than the digital impression and CAD/CAM subtractive and additive techniques for the fabrication of full-arch bars. However, both straight and zigzag scanning techniques and the CAD/CAM subtractive technique had marginal and internal gaps that were within clinically accepted ranges, and the SLM was found to be unsuitable for long-span framework fabrication with either scanning technique used.     

Polymers for conventional,subtractive, and additive manufacturing of occlusal devices differ in hardness and flexural properties but not in wear resistance

ObjectivesTo investigate the wear resistance of polymers for injection molding, subtractive and additive manufacturing of occlusal devices in comparison with enamel antagonist wear and material properties (i.e., hardness, flexural strength, and flexural modulus).MethodsInjection molding was compared with milling and the additive technologies stereolithography, low force stereolithography, and digital light processing. For each material, eight specimens were produced for wear measurements. Extracted human premolars served as indenters. All samples were subjected to two series of a 2-body wear test consisting of 200,000 circular loading cycles with an applied load of 1) 20 N and 2) 50 N in a thermocycling environment (5/55 °C, 30 s, 3860 cycles, H₂O). Wear resistance was characterized by means of maximum depth and volume of the resulting traces. In addition, enamel wear of the indenters and Vickers hardness, flexural strength, and flexural modulus of the polymers were determined. Wear was statistically analyzed with linear general models for repeated measures and material properties with one-way ANOVA with post-hoc Tukey-HSD tests.ResultsWear of the antagonists was not influenced by the material (P ≥ 0.343). Likewise, no differences in wear resistance were found between materials after cyclic loading with 20 N or 50 N (P ≥ 0.074). Material properties investigated revealed decreased values for the resins for the additive manufacturing with the exception of flexural strength of one material.SignificanceWithin the limitations of this in-vitro study, arylates for conventional, subtractive, and additive manufacturing of occlusal devices differ in material properties but not in wear resistance and antagonist wear.     

Different CAD/CAM-processing routes for zirconia restorations: influence on fitting accuracy

The aim of the present in vitro study was to evaluate the influence of different processing routes on the fitting accuracy of four-unit zirconia fixed dental prostheses (FDPs) fabricated by computer-aided design/computer-aided manufacturing (CAD/CAM). Three groups of zirconia frameworks with ten specimens each were fabricated. Frameworks of one group (CerconCAM) were produced by means of a laboratory CAM-only system. The other frameworks were made with different CAD/CAM systems; on the one hand by in-laboratory production (CerconCAD/CAM) and on the other hand by centralized production in a milling center (Compartis) after forwarding geometrical data. Frameworks were then veneered with the recommended ceramics, and marginal accuracy was determined using a replica technique. Horizontal marginal discrepancy, vertical marginal discrepancy, absolute marginal discrepancy, and marginal gap were evaluated. Statistical analyses were performed by one-way analysis of variance (ANOVA), with the level of significance chosen at 0.05. Mean horizontal discrepancies ranged between 22???m (CerconCAM) and 58???m (Compartis), vertical discrepancies ranged between 63???m (CerconCAD/CAM) and 162???m (CerconCAM), and absolute marginal discrepancies ranged between 94???m (CerconCAD/CAM) and 181???m (CerconCAM). The marginal gap varied between 72???m (CerconCAD/CAM) and 112???m (CerconCAM, Compartis). Statistical analysis revealed that, with all measurements, the marginal accuracy of the zirconia FDPs was significantly influenced by the processing route used (p?<?0.05). Within the limitations of this study, all restorations showed a clinically acceptable marginal accuracy; however, the results suggest that the CAD/CAM systems are more precise than the CAM-only system for the manufacture of four-unit FDPs.     

A retrospective clinical evaluation of extensive tooth-supported fixed dental prostheses after 10 years

Statement of problemThe survival and success of tooth-supported fixed dental prostheses (FDPs) in long-term studies vary greatly, depending on the patient and the size of the FDP. Influencing factors for FDP survival or success may include advanced patient age at the time of FDP treatment, treatment severity, and use of new and cheaper FDP materials. As the patient population ages, prosthodontists will treat tooth wear in a greater number of older adults; however, recent long-term studies on such treatments are lacking.PurposeThe purpose of this retrospective clinical study was to examine extensive, tooth-supported FDPs made at 2 specialist clinics in Sweden after 10 years and to compare the outcomes with those of previous studies.Material and methodsPatients rehabilitated by using FDPs of at least 5 units at 2 specialist clinics in Sweden between 2002 and 2006 were recalled after 10 years. Clinical examinations were supplemented by reviewing clinical records and existing radiographs. Statistical analysis was performed by using the Student t test, chi-squared test, Fisher exact test, and Kruskal-Wallis test (α=.05).ResultsA total of 152 patients were recalled for clinical examination. Of these, 78 patients attended and were examined. The mean age of the examined group was 70 years (range 36-94), lower than that of those not attending (80 years; range 46-100; P<.05). The mean number of units of the 78 examined FDPs was 7.3 (range 5-12) and 8.0 (range 5-14) for those not examined. FDP configurations in terms of number of units, abutments, pontics, and post-and-cores did not differ significantly between the 2 groups (P>.05). The survival proportion of the examined 78 FDPs (all units of the original FPD) was 74.4%. The success proportion (FDPs without complications) was 52.6%. The most frequent complications were caries (14.1%), endodontic complications (11.5%), loose retainers (7.7%), root fractures (5.1%), and framework fractures (3.8%). FDPs with post-and-cores (P<.05) and cantilevers (P=.054), especially when in combination (P<.05), showed more complications than FDPs without. Chipping fractures in porcelain were found in 38% of the FDPs (7.7% of the units), with more porcelain fractures on Co-Cr frameworks than on gold and titanium alloy frameworks (P<.05).ConclusionsThis long-term retrospective study indicated that the prognosis for complicated and extensive FDPs in aging patients does not worsen with increased clinical complexity. New materials, treatment complexity, and older patients did not seem to markedly influence prognosis.     

基于增材制造的口腔全瓷修复成型工艺研究进展与展望

全瓷修复相比金属和烤瓷修复具有明显优势,临床应用广泛。以等材制造和减材制造为代表,全瓷固定义齿的传统成型工艺发展较为成熟,但仍有许多不足之处。随着科技的快速进步,增材制造技术的出现正在改变口腔修复领域的传统制造模式,已有很多学者尝试将其用于加工全瓷固定义齿。本文将简要阐述全瓷修复成型工艺的技术现状和增材制造技术的原理及优势,着重论述基于口腔修复用陶瓷材料的增材制造技术分类及工艺探索的相关进展。     

Inlay‐retained cantilever fixed dental prostheses to substitute a single premolar: impact of zirconia framework design after dynamic loading

The purpose of this in‐vitro study was to evaluate the influence of the framework design on the durability of inlay‐retained cantilever fixed dental prostheses (IR‐FDPs), made from zirconia ceramic, after artificial ageing. Forty‐eight caries‐free human premolars were prepared as abutments for all‐ceramic cantilevered IR‐FDPs using six framework designs: occlusal–distal (OD) inlay, OD inlay with an oral retainer wing, OD inlay with two retainer wings, mesial–occlusal–distal (MOD) inlay, MOD inlay with an oral retainer ring, and veneer partial coping with a distal box (VB). Zirconia IR‐FDPs were fabricated via computer‐aided design/computer‐aided manufacturing (CAD/CAM) technology. The bonding surfaces were air‐abraded (50 μm alumina/0.1 MPa), and the frameworks were bonded with adhesive resin cement. Specimens were stored for 150 d in a 37°C water bath during which they were thermocycled between 5 and 55°C for 37,500 cycles; thereafter, they were exposed to 600,000 cycles of dynamic loading with a 5‐kg load in a chewing simulator. All surviving specimens were loaded onto the pontic and tested until failure using a universal testing machine. The mean failure load of the groups ranged from 260.8 to 746.7 N. Statistical analysis showed that both MOD groups exhibited significantly higher failure loads compared with the other groups (i.e. the three OD groups and the VB group) and that there was no significant difference in the failure load among the OD groups and the VB group. In conclusion, zirconia IR‐FDPs with a modified design exhibited promising failure modes.     

Evaluation of wear behavior of dental restorative materials against zirconia in vitro

ObjectivesTo evaluate two-body wear (2BW) and three-body wear (3BW) of different CAD/CAM and direct restorative materials against zirconia using a dual-axis chewing simulator and an ACTA wear machine.Methods3 CAD-CAM resin-based composite or polymer infiltrated ceramic network blocs, 1 lithium disilicate CAD-CAM ceramic (LS₂), 3 direct resin composites, amalgam and bovine enamel were tested. For 2BW, 8 flat specimens per material were produced, grinded, polished, stored wet (37 °C, 28d) and tested (49 N, 37 °C, 1,200,000 cycles) against zirconia. For 3BW, specimens (n = 10) were stored accordingly, and tested against a zirconia antagonist wheel (3Y-TZP, d = 20 mm, h = 6 mm; 200,000 cycles, F = 15 N, f = 1 Hz, 15% slip) in millet seed suspension. Wear resistance was analysed in a 3D optical non-contact profilometer, measuring vertical wear depth and volume loss for 2BW and mean wear depth and roughness (R_a) for 3BW. Vickers hardness (15 s, HV2) was measured. Statistical analysis was performed using non-parametric tests (Mann-Whitney-U test, p < 0.05).Results2BW and 3BW have a different impact on material surfaces. Similar wear resistance was observed for direct and indirect resin based materials with analogous filler configurations in both methods. Bovine enamel exhibited the best wear resistance in 2BW, but the least wear resistance in 3BW against zirconia. Regarding 2BW, a direct/indirect composite material pair of the same manufacturer showed the significantly highest mean volume losses (2.72/2.85 mm³), followed by LS₂ (1.41 mm³). LS₂ presented the best wear resistance in 3BW (mean wear depth 2.85 µm), combined with the highest mean Vickers hardness (598 MPa). No linear correlation was found between Vickers hardness and both wear testing procedures. The zirconia antagonists showed no recordable signs of wear.SignificanceDental restorative materials behave differently in 2BW and 3BW laboratory testing. Vickers hardness testing alone cannot hold for a correlation with wear behavior of materials. Micromorphological investigation of material composition can reveal insights in wear mechanisms related to variations in filler technologies.