Fracture strength and fatigue resistance of dental resin-based composites

ObjectivesThe aim of this study was to evaluate in vitro the influence of fiber-reinforcement on the fracture strength and fatigue resistance of resin-based composites.MethodsOne hundred rectangular bar-shaped specimens (2 mm × 2 mm × 25 mm) made of resin-based composite were prepared in a stainless steel split-mould: (i) thirty specimens of particulate filler composite (PFC) (Filtek Z100, 3 M ESPE, St Paul, MN, USA), (ii) thirty specimens of fiber-reinforced composite (FRC) (Everstick C&B, Sticktech Ltd., Turku, Finland) and (iii) forty specimens of PFC and FRC combined in two longitudinal layers of equal thickness. Each specimen was trimmed into a cylindrical hourglass shape. The fracture strength (cantilever beam test, n = 10) and the fatigue resistance (rotating cantilever beam test; staircase method: 10⁴ cycles, 1.2 Hz, n = 20) were determined. Fracture strength, fatigue resistance and work-of-fracture were calculated. The fracture surfaces of failed specimens were analyzed with SEM. Data was analyzed by logistic regression, one-way ANOVA followed by Tukey's post hoc test and, a Student's t-test.ResultsANOVA revealed that fiber-reinforcement had significant effect (P < 0.001) on fracture strength, fatigue resistance, and work-of-fracture. Student's t-test showed significant differences (P < 0.001) in fatigue resistance compared to fracture strength.ConclusionsWithin the limitations of this study, the following conclusions can be drawn (i) the fatigue resistance of resin-based composites is lower than their fracture strength and (ii) FRC are more fatigue resistant than PFC or combinations of FRC and PFC.     

Influence of various irradiation processes on the mechanical properties and polymerisation kinetics of bulk-fill resin based composites

ObjectivesTo assess the effect of irradiation time and distance of the light tip on the micro-mechanical properties and polymerisation kinetics of two bulk-fill resin-based composites at simulated clinically relevant filling depth.MethodsMicro-mechanical properties (Vickers hardness (HV), depth of cure (DOC) and indentation modulus (E)) and polymerisation kinetics (real-time increase of degree of cure (DC)) of two bulk-fill resin-based composites (Tetric EvoCeram^® Bulk Fill, Ivoclar Vivadent and x-tra base, Voco) were assessed at varying depth (0.1–6 mm in 100 μm steps for E and HV and 0.1, 2, 4 and 6 mm for DC), irradiation time (10, 20 or 40 s, Elipar Freelight2) and distances from the light tip (0 and 7 mm). Curing unit's irradiance was monitored in 1 mm steps at distances up to 10 mm away from the light tip on a laboratory-grade spectrometer.ResultsMultivariate analysis (α = 0.05), Student's t-test and Pearson correlation analysis were considered. The influence of material on the measured mechanical properties was significant (η² = 0.080 for E and 0.256 for HV), while the parameters irradiation time, distance from the light tip and depth emphasise a stronger influence on Tetric EvoCeram^® Bulk Fill. The polymerisation kinetics could be described by an exponential sum function, distinguishing between the gel and the glass phase. The above mentioned parameters strongly influenced the start of polymerisation (gel phase), and were of less importance for the glass phase.ConclusionsBoth materials enable at least 4 mm thick increments to be cured in one step under clinically relevant curing conditions.Clinical significanceThe susceptibility to variation in irradiance was material dependent, thus properties measured under clinically simulated curing conditions might vary to a different extent from those measured under ideal curing conditions.     

Fracture toughness of cross-linked and non-cross-linked temporary crown and fixed partial denture materials

ObjectivesTemporary crowns and fixed partial dentures are exposed to considerable functional loading, which places severe demands on the biomaterials used for their fabrication (= temporary crown & bridge materials, t-c&b). As the longevity of biopolymers is influenced by the ability to withstand a crack propagation, the aim of this study was to investigate the fracture toughness of cross-linked and non-cross-linked t-c&bs.MethodsFour different t-c&bs (Luxatemp AM Plus, Protemp 3 Garant, Structur Premium, Trim) were used to fabricate bar shaped specimens (2 mm × 5 mm × 25 mm, ISO 13586). A notch (depth 2.47 mm) was placed in the center of the specimen using a diamond cutting disc and a sharp pre-crack was added using a razor blade. 60 specimens per material were subjected to different storage conditions (dry and water 37 °C: 30 min, 60 min, 4 h, 24 h, 168 h; thermocycling 5–55 °C: 168 h) prior to fracture (3-point bending setup). The fracture sites were inspected using SEM analysis. Data was subjected to parametric statistics (p = 0.05).ResultsThe K_IC values varied between 0.4 and 1.3 MPa m^0.5 depending on the material and storage time. Highest K_IC were observed for Protemp 3 Garant. Fracture toughness was significantly affected by thermocycling for all dimethacrylates (p < 0.05) except for Structur Premium. All dimethacrylates showed a linear-elastic fracture mechanism, whereas the monomethacrylate showed an elasto-plastic fracture mechanism.SignificanceDimethacrylates exhibit a low resistance against crack propagation immediately after curing. In contrast, monomethacrylates may compensate for crack propagation due to plastic deformation. However, K_IC is compromised with increasing storage time.     

Mathematical model for assessing true irradiance received by luting materials while curing through modern CAD/CAM resin composites

Statement of problemMeasurement of irradiance passing through a dental restoration for properly curing a dual- or light-polymerized luting composite is imprecise due to surface reflection.ObjectiveTo provide a mathematical correction of measured transmitted irradiance for predicting true transmitted light intensity through CAD/CAM restorations.MethodsA total of 432 specimens were fabricated. Seven modern CAD/CAM resin-based composites (RBCs) and one CAD/CAM glass-ceramic (control group) were sectioned and polished into specimens of 0.5–5 mm thickness (in 0.5 mm steps, n = 6). Irradiance of a violet-blue LED light curing unit (LCU) (power modes: Standard, High and Plasma) was measured after passing through each specimen with a spectrometer. Data was compared based on 95% confidence intervals and using univariate ANOVA followed by Tukey HSD (α = 0.05).ResultsThe measured transmitted irradiance passing through the specimens decreased exponentially. Significantly highest values of transmitted irradiance were measured for 0.5 mm thick specimens for all materials (p < 0.05). The decadic absorption coefficient for CAD/CAM-RBCs ranged from 0.292 mm⁻¹ to 0.387 mm⁻¹ while the control group (glass-ceramic) reached a significantly lower value of 0.283 mm⁻¹. The reflection ratio for all materials ranged from 12.6% to 18.5%.SignificanceA correction can be implemented to predict the true transmitted irradiance after passing through a dental restoration as function of initial irradiance, specimen thickness and material specific parameters. For a practitioner, this model may be applied depending on the specific treatment conditions, the individual LCU's radiant emittance and restoration thickness for the tested materials.     

Scratch test of thermoplastic denture base resins for non-metal clasp dentures

PurposeSeveral thermoplastic denture base resins have been introduced for the fabrication of non-metal clasp dentures. Although the surface of these materials is easily damaged, the surface roughness and characteristics of scratches created have not been evaluated. The purpose of this study was to evaluate the surface roughness of thermoplastic resins using a scratch test for the development of future materials.MethodsFour thermoplastic (polyamide: Valplast^®; VLP and Lucitone FRS^®; FRS, polyethylene terephthalate: EstheShot^®; ES, and polyester: EstheShot Bright^®; ESB) and two conventional acrylic (Heat-polymerizing: Urban^®; HC, and Pour type auto-polymerizing: Pro-Cast DSP^®; PR) denture base resins were examined. Eight specimens, approximately 10 mm × 10 mm × 30 mm in size, of each material were fabricated. Scratch test was performed by a scratch tester with a diamond indenter of 10-μm radius and cone angle 90°, applying a constant load of 500 mN, and 2-mm-long scratches were made. The scratch marks were studied under 3D laser measuring microscope and cross-section profiles at approximately 0.5 mm, 1.0 mm, and 1.5 mm from the starting point were extracted and measured with analysis software. Data from 24 cross-section profiles in each denture base material were analyzed.ResultsThe maximum depths of ES, ESB and FRS were greater than VLP, PR and HC, and the scratch widths of ES, ESB, FRS and VPL were greater than PR and HC.ConclusionsThe results showed that the surface of thermoplastic denture base resins was easily damaged compared to polymethyl methacrylate.     

Effect of self-setting α-tricalcium phosphate between segments for bone healing and hypoaesthesia in lower lip after sagittal split ramus osteotomy

AimsThe aim of this study was to evaluate hypoaesthesia of the lower lip and bone formation using self-setting α-tricalcium phosphate (Biopex^®) between the proximal and distal segments following sagittal split ramus osteotomy (SSRO) with bent absorbable plate fixation.Subjects and methodsThe subjects were 40 patients (80 sides) who underwent bilateral SSRO setback surgery. They were divided into a Biopex^® group (40 sides) and a control group (40 sides). The Biopex^® was inserted into the anterior part of the gap between the segments in the Biopex^® group. Trigeminal nerve hypoaesthesia in the region of the lower lip was assessed bilaterally using the trigeminal somatosensory-evoked potential (TSEP) method. Ramus square, ramus length, and ramus width, the square of the Biopex^® at the horizontal plane under the mandibular foramen were assessed preoperatively, immediately after surgery, and 1 year postoperatively by computed tomography (CT).ResultsThe mean measurable period and standard deviation were 9.3 ± 15.7 weeks in the control group, 5.3 ± 8.3 weeks in the Biopex^® group, and there was no significant difference. Ramus square after 1 year was significantly larger than that prior to surgery and new bone formation was found between the segments in both groups (P < 0.05). In the Biopex^® group, the square of the Biopex^® after 1 year was significantly smaller than that immediately after surgery (P < 0.05).ConclusionThis study suggested that inserting Biopex^® in the gap between the proximal and distal segments was useful for new bone formation and it did not prevent the recovery of lower lip hypoaesthesia after SSRO with bent absorbable plate fixation.     

Effect of implant abutment connection designs,and implant diameters on screw loosening before and after cyclic loading: In-vitro study

AimsThe purpose of this in-vitro study was to evaluate the screw loosening of two different forms of implant abutment connection designs, and two implant diameters by measuring removal torque value (RTV) before and after cyclic loading.Materials and methodsTwenty implant fixtures were divided equally into 2 groups (N = 10): group I fixture with conical hybrid connection (CH), and group II fixture with internal hex connection (IH). Each group was divided equally into two subgroups according to implant diameters: subgroup A (3.3 mm), and subgroup B (4.2 mm). Each fixture was vertically placed in the center of an acrylic resin block. The samples were fixed to the jig, and an implant abutment connected it with a 20 Ncm tightening torque. The samples were subjected to eccentric cyclic loading (at a distance of 5 mm) away from center of abutment at 100,000 cycles. A digital torque gauge was used to evaluate screw loosening by measuring RTVs in (Ncm) before and after cyclic loading. The removal torque loss ratio before and after cyclic loading and the removal torque loss ratio between before and after cyclic loading were calculated and analyzed using the SPSS statistical analysis.ResultsFor GI the initial removal torque loss ratio measurement was (14.45 ± 3.18) and decreased significantly after loading, it was (11.47 ± 3.64). For GII the initial removal torque loss ratio measurement was (20.47 ± 4.99) and increased significantly after loading, being (35.35 ± 4.26). There is no significant effect upon screw loosening for two implant diameters.ConclusionWithin the limitations of this study, the results suggested that conical hybrid connections showed a better screw stability than an internal hex connection. Therefore, the use of conical implants can be promoted as they have better screw stability compared to other systems.     

In vitro failure and fracture resistance of veneered and full-contour zirconia restorations

ObjectivesThis study evaluated the failure and fracture resistance of zirconia-based fixed partial dentures (FPDs) under the influence of different surface treatments and adjustment procedures.MethodsSeven groups (n = 8/group) of three-unit zirconia-based FPDs were fabricated in anatomic design (AD) or anatomically reduced design (ARD) and surfaces were prepared according to clinical relevance: #1: AD – sintered; #2: AD – sintered – glazed; #3: AD – sintered – sandblasted – glazed; #4: AD – sintered – polished – grinded (contact points adjusted); #5: AD – sintered – polished – grinded – repolished; #6: ARD – sintered – veneered; #7: control: analogous to #3 but without thermal cycling (TC) and mechanical loading (ML). FPDs were adhesively bonded to polymethylmethacrylate abutment teeth. TCML (TC: 6000 × 5°/55°; ML: 1.2 × 10⁶ × 50 N, 1.6 Hz) was conducted in a chewing simulator with steatite spheres as antagonists. Failures were monitored and fracture resistance was determined after ageing. Data were analysed statistically with Mann–Whitney U-test (Kolmogorov–Smirnov-test; α = 0.05). FPDs were subjected to scanning electron microscopy for fractographic failure analysis.ResultsNone of the FPDs failed during TCML, but showed wear at contact points. Median fracture force ranged between 1173.5 N (#4) and 1316.0 N (#3) without significant (p = 0.910) differences between the groups or in comparison to the control (p > 0.462).ConclusionsZirconia restorations showed high resistance to failures and fracture under different surface treatment variations. Full-contour polished or glazed zirconia FPDs might be an alternative to common veneered restorations.     

The effect of fatigue loading on the screw joint stability of zirconium abutment

PurposeThis study evaluated the effect of fatigue loading on the screw joint stability of a zirconium abutment connected to an external hexagon implant in vitro.Materials and methodsFifteen titanium and 15 zirconia abutments of 3 different heights (5, 8, and 11 mm) were connected to external titanium implants with titanium screws. A torque gauge was used to measure the reverse torque values before and after loading. An air cylindrical loading device was used to simulate mastication at a 45-degree angle to the longitudinal axis of the implant.ResultsThere were significant differences (P < 0.05) before and after the loading of titanium (5 mm) and zirconia (5, 8, and 11 mm) abutments.ConclusionZirconia abutments for external hexagon implants had durability rates similar to those of titanium abutments after repeating load on the reverse torque of the abutment screw, indicating that the zirconia abutment could be reliably used instead of the titanium abutment.     

Treatment outcome of the two-part semi-rigid oral appliance in obstructive sleep apnea

AimThe aim of this study was to assess the effectiveness of the two-part semi-rigid oral appliance, Silensor^® (Erkodent, Tuttlingen, Germany) which prevents the mandible from retracting during mouth opening.Materials and methodsTen patients with mild or moderate obstructive sleep apnea (2 males and 8 females; mean age = 62.5 ± 10.0 years) were recruited and lateral cephalometric radiographs were taken. The patients underwent polysomnography before and after 3 months of receiving treatment with the Silensor^®. The relationship between the improvement in the polysomnographic variables after the therapy and the cephalometric features was analyzed.ResultsA significant difference was observed in the apnea–hypopnea index after 3 months of Silensor^® therapy (1st (baseline), 17.1 ± 5.5; 2nd (therapy of Silensor^®), 11.0 ± 7.2, p = 0.011). Furthermore there was a significant positive correlation between the improvement in the degree of slow wave sleep (%) and the mandibular plane angle (R = 0.662, p = 0.037), as well as between the improvement in degree of slow wave sleep (%) and the lower face height (R = 0.845, p = 0.002). A significant negative correlation was observed between the improvement in degree of sleep efficiency (%) and the soft palate area (R = ?0.809, p = 0.005).ConclusionThese results suggested that keeping the nasopharyngeal airway space during mouth opening improves apnea–hypopnea index of some patients with mild or moderate obstructive sleep apnea and quality of sleep in obstructive sleep apnea patients with a long lower face height and a small soft palate.     

Effects of different abutment material and surgical insertion torque on the marginal adaptation of an internal conical interface: An in vitro study

PurposeTo evaluate the marginal adaptation at implant–abutment connection of an implant featuring a conical (45° taper) internal hexagonal abutment with a connection depth of 2.5 mm, comparing the performance of two identical abutments of different material (titanium grade-4 and Co-Cr-alloy).MethodsTwenty implants (3.75 mm × 15 mm) were connected to non-matching abutments (5.5 mm × 10 mm) of two different materials (titanium grade-4: n = 10; Co-Cr-alloy: n = 10). The specimens were separately embedded in epoxylite resin, inside copper cylinders, and submerged without covering the most coronal portion (5 mm) of the fixture. Five specimens per group were stressed simulating a surgical 100 Ncm insertion torque, while the others had no torque simulation. All specimens were subjected to a non-axial static load (100 N) in a universal testing machine, under an angle of 30° with respect to the implant axis. Once 100 N load was reached, low shrinkage self-curing resin was injected inside the cylinders, and load was maintained until complete resin polymerization. Specimens were cut and analyzed with optical and scanning-electron-microscope (SEM) to evaluate the marginal adaptation at the implant–abutment connection. Statistical analysis was performed using one-way ANOVA (p = 0.02).ResultsNone of the 20 samples failed. The implant–abutment connection was able to guarantee a good optical seal; SEM analysis confirmed the absence of microgaps.ConclusionsWithin the limits of this study (small sample size, limited time) the marginal adaptation of the implant–abutment connection was not affected by the abutment material nor by the application of surgical insertion torque.     

Limited reciprocity in curing efficiency of bulk-fill resin-composites

ObjectiveThe aim of the study was to identify experimental limits of the general reciprocity hypothesis that the same photo-cure outcomes will result from applying essentially constant energy densities, despite reciprocal variations in the irradiance and irradiation time-period, for a representative set of bulk fill (BF) and non-BF resin composites.MethodsSix BF and two non-BF resin-composites were selected. The unset pastes were inserted into white acetal molds (5 mm id) with (n = 6) depths (1, 2, 3, 4, 5 and 6 mm). Three light curing units (LCUs) of increasing radiant emittance capability: 1200, 2000 and 3200 mW/cm² were used. Composite specimen groups (n = 3, per depth, per LCU) were irradiated on the upper surface only. For each specific composite, the irradiation times for each LCU were reduced reciprocally, as the LCU irradiance increased, to deliver a constant energy density (J/cm²) to that composite. However, the required energy density for a given composite differed in accordance with each composite manufacturer recommendations. After storing for 24 h at 37 °C, light transmission measurements were made through each specimen and re-expressed as Apparent Absorbance (A′). Vickers hardness (H_V) measurements (n = 10) were made on both top and bottom surfaces, for each specimen, and H_V versus “depth” profile plots created. From the top-surface data, a Depth-of-Cure parameter could be derived. Data were statistically evaluated for differences between top and bottom H_V values and for other predefined variables of interest.ResultsIrradiation with the LCU of 1200 mW/cm² generally gave the highest H_V/depth for most materials tested compared to the other curing lights with higher power output, regardless of top and bottom measurements (p < 0.001). However, this difference was material-dependent. With one BF composite, 1200 and 2000 mW/cm² irradiance did not show a significant difference between top and bottom H_V. Composites with higher translucency showed reduced differences in top/bottom H_V than more opaque composites.SignificanceReciprocity was found to be limited with most materials examined, such that irradiance periods of 10 s, gave generally better H_V outcomes than by using LCUs of superior radiant emittance while reciprocally reducing irradiance time to maintain constant dose of energy density.     

Improvement on properties of experimental resin cements containing an iodonium salt cured under challenging polymerization conditions

ObjectiveThe use of resin cements in clinical practice entails photopolymerization through prosthetic devices, which precludes light penetration. The objective of this study was to modify experimental resin cements (ERCs) with diphenyliodonium hexafluorophosphate (DPI) in an attempt to improve chemical and mechanical properties of materials cured with reduced irradiance and final radiant exposure.MethodsA co-monomer base containing a 1:1 mass ratio of 2.2-bis[4-(2-hydroxy-3-methacryloxypropoxy)phenyl]propane (bis-GMA) and triethyleneglycol dimethacrylate (TEGDMA) was prepared, with 1 mol% of camphorquinone and 2 mol% of ethyl 4-(dimethylamino)benzoate as initiator system. The resin was divided into 4 fractions according to the DPI concentrations (0, 0.5, 1 and 2 mol%). The challenging polymerization condition was simulated performing the light activation (12, 23 and 46 s) through a ceramic block (3 mm thick). The irradiance was assessed with a calibrated spectrometer (1320 mW/cm²), resulting in three levels of radiant exposure (0.58, 1.1 and 2.2 J/cm²). The polymerization kinetics was evaluated in real-time using a spectrometer (Near-IR). Water sorption and solubility was analyzed and the cohesive strength of resins obtained through the microtensile test. Polymerization stress was assessed by Bioman method.ResultsResins containing DPI had higher degree of conversion and rate of polymerization than the control (without DPI). The use of DPI reduced water sorption and solubility, and led to higher cohesive strength compared to resins without the iodonium salt. However, the stress of polymerization was higher for experimental resins with DPI.SignificanceEven under remarkably reduced irradiance, cements containing a ternary initiating system with an iodonium salt can present an optimal degree of conversion and chemical/mechanical properties.     

Experimental composites containing quaternary ammonium methacrylates reduce demineralization at enamel-restoration margins after cariogenic challenge

ObjectiveThis study evaluated the influence of experimental composites containing quaternary ammonium monomers (QAM) at different concentrations and alkyl chains on demineralization at enamel-composite margins after cariogenic challenge.MethodsStandardized 4 × 4 mm cavities were cut into 35 bovine enamel blocks, which were randomly divided into seven groups (n = 5) and restored with the following experimental composites and commercial materials: (G12.5) – 5% dimethylaminododecyl methacrylate (DMADDM) with a 12-carbon alkyl chain (G12.10) – 10% DMADDM, (G16.5) – 5% dimethylaminohexadecyl methacrylate (DMAHDM) with a 16-carbon alkyl chain (G16.10) – 10% DMAHDM, (CG) – control group (without QAM), (GZ250) – commercial composite (Filtek Z250^®), and (GIC) – glass ionomer cement (Maxxion R^®). After restorative procedures, initial microhardness was measured and experimental composites were subjected to Streptococcus mutans biofilm formation for 48 h. After cariogenic challenge, the samples were washed and microhardness was reassessed. A 3D non-contact profilometer was used to determine surface roughness and enamel demineralization was assessed by micro-CT. Microhardness results were analyzed by the Kruskal–Wallis and Mann-Whitney tests and micro-CT results were analyzed by Tukey’s HSD test (95% confidence interval).ResultsNone of the materials could prevent mineral loss at the enamel-restoration margins. The addition of 10% DMAHDM yielded the lowest, albeit statistically significant, mineral loss (p < 0.05). 3D non-contact profilometry showed enamel surface roughness modification after biofilm exposure. The CG had the highest roughness values. Micro-CT analysis revealed mineral loss, except for GIC.SignificanceThe addition of 10% QAM with a 16-carbon chain in experimental composites reduced mineral loss at the enamel-restoration margins after cariogenic challenge.     

Sliding contact wear and subsurface damage of CAD/CAM materials against zirconia

ObjectiveMost previous work conducted on the wear behavior of dental materials has focused on wear rates and surface damage. There is, however, scarce information regarding the subsurface damage arising from sliding contact fatigue. The aim of this study was to elucidate the wear mechanisms and the subsurface damage generated during sliding contact fatigue in 5 contemporary CAD/CAM materials against a zirconia indenter.MethodsForty discs (Ø12 mm, 1.55 mm thick) were cut out of IPS e.max CAD (e.CAD), Suprinity PC (SUP), Enamic (ENA), Vitablocs Mark II (VMII) and Lava Ultimate (LU) blocks and mirror polished. After cementation onto a dentin-like composite, off-axis mouth-motion cycling was conducted with a spherical zirconia indenter (r = 3.18 mm) in water (200 N load, 2 Hz frequency) for 5 different cycling periods (10², 10³, 10⁴, 10⁵, 10⁶ cycles, n = 8). Analysis of the wear scars was conducted using light-microscopy, scanning-electron-microscopy and optical profilometry. Subsurface damage was assessed using sagittal and transverse sections of the samples.ResultsFatigue wear mechanisms predominated in glassy materials (e.CAD, SUP, VMII), accompanied by extensive subsurface damage, whereas abrasive wear mechanisms were responsible for the large wear craters in the resin composite (LU) with an absolute absence of subsurface fracture. A combination of both mechanisms was observed in the polymer-infiltrated reinforced-glass (ENA), displaying large wear craters and severe subsurface damage.SignificanceWell-controlled laboratory simulation can identify wear and subsurface damage susceptibility of various classes of restorative materials. Both wear and subsurface fracture are determining factors for the long-term success of restorations.     

Graded structures for damage resistant and aesthetic all-ceramic restorations

ObjectivesClinical studies revealed several performance deficiencies with alumina- and zirconia-based all-ceramic restorations: fracture; poor aesthetic properties of ceramic cores (particularly zirconia cores); and difficulty in achieving a strong ceramic–resin-based cement bond. We aim to address these issues by developing a functionally graded glass/zirconia/glass (G/Z/G) structure with improved damage resistance, aesthetics, and cementation properties.MethodsUsing a glass powder composition developed in our laboratory and a commercial fine zirconia powder, we have successfully fabricated functionally graded G/Z/G structures. The microstructures of G/Z/G were examined utilizing a scanning electron microscopy (SEM). The crystalline phases present in G/Z/G were identified by X-ray diffraction (XRD). Young's modulus and hardness of G/Z/G were derived from nanoindentations. Critical loads for cementation radial fracture in G/Z/G plates (20 mm × 20 mm, 1.5 or 0.4 mm thick) bonded to polycarbonate substrates were determined by loading with a spherical indenter. Parallel studies were conducted on homogeneous yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) controls.ResultsThe G/Z/G structure consists of an outer surface aesthetic glass layer, a graded glass–Y-TZP layer, and a dense Y-TZP interior. The Young's modulus and hardness increase from surface to interior following power-law relations. For G/Z/G plates of 1.5 and 0.4 mm thick, critical loads for cementation radial fracture were 1990 ± 107 N (mean ± S.D., n = 6) and 227 ± 20 N (mean ± S.D., n = 6), respectively, which were ～30 and 50% higher than those for their monolithic Y-TZP counterparts (1388 ± 90 N for 1.5 mm and 113 ± 10 N for 0.4 mm thick; mean ± S.D., n = 6 for each thickness). A 1-sample t-test revealed significant difference (p < 0.001) in critical loads for radial fracture of G/Z/G and homogeneous Y-TZP for both specimen thicknesses.SignificanceOur results indicate that functionally graded G/Z/G structures exhibit improved damage resistance, aesthetics, and potentially cementation properties compared to homogeneous Y-TZP.     

N-Acetylcysteine modulates the effects of composites on human gingival keratinocytes

ObjectiveThe aim of this study was to evaluate, if antioxidants, like N-Acetylcysteine, can modulate effects of composite eluates on human gingival keratinocytes.MethodsComposite samples of ceram.x® universal, Filtek? Supreme XTE, and Admira® Fusion were stored 72 h in cell culture medium to prepare eluates, according to ISO 10993-12:2012. Human gingival keratinocytes were exposed to these eluates with or without 3 mM N-Acetylcysteine. Following cell observation by iCELLigence®, exposure periods were determined at 1d and 4d. Cell morphological analysis combined with live/dead staining was performed. Tissue-specific biomarkers of terminal differentiation, Involucrin and Filaggrin, were analyzed by indirect immunofluorescence (IIF) and Western blot (WB). qPCR profiling was performed on genes encoding for: inflammation, apoptosis, turn-over of extracellular matrix, adhesion, proliferation and differentiation. For statistical analysis one-way Anova was used (p < 0.05).ResultsCells exposed to N-Acetylcysteine exhibited morphological changes but no cell death. After adding 3 mM N-Acetylcysteine to HGK cultures, increased fluorescence intensity and protein amounts of Involucrin and Filaggrin indicated enhanced differentiation (p < 0.05). Gene expression was modulated by: (i) composition of the composite eluates, (ii) NAC and (iii) exposure time. Filtek? Supreme XTE showed a significant increased gene expression in inflammatory genes (p < 0.05), which was amplified by the addition of NAC at 1d. Concerning exposure time, modulated gene expression showed eluate dependency, substantiated by Filtek? Supreme XTE modulation at day 1 and Admira® Fusion at day 4.SignificanceN-Acetylcysteine-emerging effects on gingival keratinocytes were threefold: (i) increase of differentiation, (ii) modulation of composite-related effects and (iii) in parts counteraction of eluate-induced effects.     

Maxillary sinus floor augmentation and dental implant placement using dentin matrix protein-1 gene-modified bone marrow stromal cells mixed with deproteinized boving bone: A comparative study in beagles

ObjectiveThe aim of the study was to evaluate the effects of the combined use of dentin matrix protein-1 (DMP1) gene-modified bone marrow stromal cells (BMSCs) and Bio-Oss^® for maxillary sinus floor augmentation (MSFA) implant placement in dogs.Materials and methodsBMSCs were derived from bone marrow of six beagles and cultured. The cells were transduced with a lentiviral vector overexpressing the DMP1 gene and enhanced green fluorescent protein (EGFP) gene (Lenti-DMP1/EGFP) in test group, and with a lentiviral vector encoding EGFP gene (Lenti-EGFP) in control group. Six dogs received sinus augmentations using the bilateral approach with a simultaneous implant placement at each site respectively. At the same concentration, 2 × 10⁷ cells/ml, one sinus was grafted using a mixture of autologous DMP1/EGFP gene-modified BMSCs and Bio-Oss^® (DMP1 group), and the contralateral sinus was grafted with autologous EGFP gene-modified bMSCs and Bio-Oss^® (EGFP group). After a 3 month healing period, bone regeneration and osseointegration were evaluated using histologic and histomorphometric methods.ResultsThe bone-implant contact (BIC) and the bone area fraction in the DMP1 group (BIC: 34.67% ± 8.23%, bone area fraction: 35.16% ± 3.32%) were significantly greater compared with the EGFP group (BIC: 26.06% ± 5.16%, bone area fraction: 20.74% ± 1.63%) (P < 0.05). No significant difference between the residual bone substitute material volume (BSMV) in the DMP1 group (35.86 ± 7.35) and the EGFP group (32.16 ± 9.16) was found in our study (P > 0.05).ConclusionBMSCs modified with the DMP1 gene can be used as an adjunct to Bio-Oss^® to enhance new bone formation and the osseointegration of dental implants in MSFA of dogs.     

Thermal shrinkage reveals the feasibility of pulse-delay photocuring technique

ObjectivesTo resolve the feasibility of the pulse-delay photocuring technique as a clinical strategy for reducing the detrimental polymerization stress induced in dental composites during the photocuring process.MethodsModel dental composites with high and low-filler contents were cured with the pulse-delay photocuring technique using different combinations of photocuring variables (irradiance, exposure time, and delay time). Irradiance used ranged from 0.1 W/cm² to 4 W/cm². The exposure time of the first pulse varied from 0.2 s to 27.2 s and the delay times ranged from 10 s to 120 s. The radiant exposure was varied from 4 J/cm² to 20 J/cm². A cantilever-beam based instrument (NIST Standards Reference Instrument 6005) was used to implement the photocuring technique for the measurement of the polymerization properties (the degree of monomer conversion, polymerization stress induced due to shrinkage, and temperature change due to the reaction exotherm and curing light absorbance) simultaneously in real-time. These properties were compared with those obtained using the conventional photocuring technique (i.e., using a constant irradiance for a fixed exposure time, a uniform exposure).ResultsThere exists a minimum radiant exposure, such that a reduction in the polymerization stress can be achieved without sacrificing the degree of monomer conversion by using the pulse-delay over the conventional photocuring technique. More specifically, stress reductions of up to 19% and 32% was observed with the pulse-delay when compared with the conventional photocuring technique at an irradiance of 0.5 W/cm² and 4 W/cm², respectively. The reduction occurred when the exposure time of the first pulse was greater than, but closer to, the gelation time (i.e., lower than the vitrification time) of the composite, regardless of the delay time used. Lower thermal shrinkage (contraction) during the post-curing time, rather than the stress relaxation during the delay time or lower degree of monomer conversion as claimed in the literature, is the cause of the reduction in the polymerization stress.SignificanceThe study clarifies a long-standing confusion and controversy on the applicability of the pulse-delay photocuring technique for reducing the polymerization stress and promotes its potential clinical success for dental restorative composites.