Mechanical properties of 3D-printed and milled composite resins for definitive restorations: An in vitro comparison of initial strength and fatigue behavior

OBJECTIVE

To evaluate the flexural strength and fatigue behavior of a novel 3D-printed composite resin for definitive restorations.

MATERIALS AND METHODS

Fifty disc-shaped specimens were manufactured from each of a nanohybrid composite resin (NHC), polymer-infiltrated ceramic network (PICN), and 3D-printed composite resin (3D) with CAD-CAM technology. Biaxial flexural strength (σin ) (n = 30 per group) and biaxial flexural fatigue strength (σff ) (n = 20 per group) were measured using piston-on-three-balls method, employing a staircase approach of 105  cycles. Weibull statistics, relative-strength degradation calculations, and fractography were performed. The results were analyzed with 1-way ANOVA and Games-Howell post hoc test (α = 0.05).

RESULTS

Significant differences in σin and σff among the groups (p < 0.001) were detected. The NHC group provided the highest mean ± standard deviation σin and σff (237.3 ± 31.6 MPa and 141.3 ± 3.8 MPa), followed by the PICN (140.3 ± 12.9 MPa and 73.5 ± 9.9 MPa) and the 3D (83.6 ± 18.5 MPa and 37.4 ± 23.8 MPa) groups. The 3D group exhibited significantly lower Weibull modulus (m = 4.7) and up to 15% higher relative strength degradation with areas of nonhomogeneous microstructure as possible fracture origins.

CONCLUSIONS

The 3D-printed composite resin exhibited the lowest mechanical properties, where areas of nonhomogeneous microstructure developed during the mixing procedure served as potential fracture origins.

CLINICAL SIGNIFICANCE

The clinical indications of the investigated novel 3D-printed composite resin should be limited to long-term provisional restorations. A cautious procedure for mixing the components is crucial before the 3D-printing process, since nonhomogeneous areas developed during the mixing could act as fracture origins.

Effect of airborne particle abrasion and regeneration firing on the strength of 3D-printed 3Y and 5Y zirconia ceramics

OBJECTIVES

This study aimed to assess the effect of airborne particle abrasion (APA) and regeneration firing (RF) on the subsurface damage and strength distribution of 3D-printed 3Y-TZP and 5Y-PSZ zirconia parts for dental applications.

METHODS

Disc-shaped specimens were prepared using vat photopolymerization (VPP) technology from 3Y and 5Y zirconia ceramics, followed by thermal debinding and sintering. APA treatment with 50 µm Al2O3 particles and RF at 1000 °C for 15 min were applied. Microstructural analysis was conducted using FIB-SEM, and XRD analysis determined crystalline phase content. Biaxial flexural strength was measured using the ball on three balls method and analyzed with Weibull statistics. ANOVA and Tukey HSD test were employed to compare strength differences between groups.

RESULTS

APA treatment increased the flexural strength of the 3Y specimens but decreased it for the 5Y specimens. RF treatment reversed the effect, restoring the strength to as-sintered levels for both materials. APA-treated 3Y specimens exhibited characteristic strength values above 1400 MPa, attributed to phase-transformation toughening. As sintered 5Y specimens showed strength values above 600 MPa. APA treatment increased the Weibull modulus of the 5Y specimens, indicating a narrower defect size distribution.

SIGNIFICANCE

The study demonstrates that the impact of APA and RF treatments on the mechanical properties and reliability of VPP-fabricated 3Y-TZP and 5Y-PSZ ceramics is comparable to conventionally prepared zirconia. VPP technology for 3D printing provides a viable approach for future manufacturing of dental restorations with potential clinical applications.

Bonding Performance of Surface-Treated Zirconia Cantilevered Resin-Bonded Fixed Dental Prostheses: In Vitro Evaluation and Finite Element Analysis

Debonding of zirconia cantilevered resin-bonded fixed dental prostheses (RBFDPs) remains the main treatment complication, therefore, the present in vitro study aimed to evaluate the effect of different surface pretreatments on the bonding of zirconia RBFDPs. Eighty milled zirconia maxillary central incisors, with complementary zirconia cantilevered RBFDPs, were randomly subjected to four different surface pretreatments (n = 20): as-machined (AM); airborne-particle abraded (APA); coated with nanostructured alumina coating (NAC); incisor air-abraded and RBFDP coated (NAC_APA). After bonding, half of each group (n = 10) was stored in deionized water (150 days/37 °C), thermocycled (37,500 cycles, 5-55 °C), and cyclically loaded (50 N/1.2 × 10⁶). Load-bearing capacity (LBC) was determined using a quasi-static test. Additionally, finite element analysis (FEA) and fractography were performed. t-test and one-way ANOVA were used for statistical-analysis. Before aging, the NAC group provided superior LBC to other groups (p < 0.05). After aging, the AM specimens debonded spontaneously, while other groups exhibited comparable LBC (p ˃ 0.05). The FEA results correlated with the in vitro experiment and fractography, showing highly stressed areas in the bonding interface, cement layer, and in RBFDP's retainer wing and connector. The NAC RBFDPs exhibited comparable long-term bonding performance to APA and should be regarded as a zirconia pretreatment alternative to APA.

In vivo ageing of zirconia dental ceramics - Part II: Highly-translucent and rapid-sintered 3Y-TZP

OBJECTIVE

3Y-TZP ceramics with reduced alumina content have improved translucency and are used in monolithic dental restorations without porcelain-based veneers. The workflow can be further streamlined with rapid sintering. This study was designed to assess how these approaches affect ageing when the materials are exposed to the oral environment in vivo.

METHODS

43 discs were fabricated from 3Y-TZP powder with 0.05% Al₂O₃ and sintered with conventional or rapid regimens (1450 °C 2 h, 1530 °C 2 h, or 1530 °C 25 min). Their surfaces were polished or airborne-particle abraded with 50 μm Al₂O₃. The discs were incorporated in complete dentures of 16 volunteers and worn continuously for up to 48 months. Ageing changes on disc surfaces were monitored every 6 months by X-ray diffraction, scanning electron microscopy and atomic force microscopy. Data was statistically analysed with linear models.

RESULTS

The amount of monoclinic phase on polished surfaces increased linearly, reaching up to 40% after 48 months in vivo. The ageing process observed for rapid sintering was 1.6 times faster compared to conventional sintering. A nano-scale increase in roughness with microcracking was also detected on polished surfaces. Airborne-particle abraded surfaces did not exhibit clear signs of ageing during the course of the study.

SIGNIFICANCE

Highly-translucent 3Y-TZP ceramics are more susceptible to ageing than classic 3Y-TZP. After 4 years in vivo, the extent of degradation did not yet constitute grounds for clinical concern, but was more pronounced in materials prepared with rapid sintering.

In vivo aging of zirconia dental ceramics - Part I: Biomedical grade 3Y-TZP

OBJECTIVE

In vivo aging of biomedical grade 3Y-TZP ceramics in the oral environment was assessed and compared to artificially accelerated in vitro hydrothermal aging extrapolations at 37°C.

METHODS

88 discs were pressed and sintered (1450-1500°C) from two commercial 3Y-TZP compositions containing 0.25% Al₂O₃ to generate finer- and coarser-grained specimens. As-sintered (AS) and airborne-particle abraded (APA; 50μm Al₂O₃) surfaces were investigated. In vivo aging was performed by incorporating specimens in lingual flanges of complete dentures of 12 edentulous volunteers who wore them continuously for up to 24 months. For comparison, in vitro hydrothermal aging at 134°C was also performed and analysed by XRD and (FIB)-SEM. Data was statistically analysed with linear regression models.

RESULTS

Finer and coarser-grained specimens exhibited statistically insignificant differences in aging in vivo. The monoclinic fraction (X_m) on AS surfaces abruptly increased to ∼8% after 6 months. The aging process then proceeded with slower linear kinetics (∼0.24%/month). After 24 months, X_m reached ∼12%. The calculated maximum transformed layer was 0.385μm representing one layer of transformed grains. APA surfaces were highly aging resistant. The initial X_m of ∼4.0% linearly increased by 0.03%/month in vivo. In vitro aging exhibited an initial induction period, followed by linear aging kinetics. Coarser-grained AS surfaces aged significantly faster than fine-grained (2.41%/h compared to 2.16%/h). APA discs aged at a rate of 0.3%/h in vitro. Microcracking within a single grain and pull-out of grain clusters were observed on aged AS surfaces.

SIGNIFICANCE

Biomedical grade 3Y-TZP was susceptible to in vivo aging. After 2 years in vivo, the aging kinetics were almost 3-times faster than the generally accepted in vitro-in vivo extrapolation.

Anterior Esthetic Rehabilitation of an Alveolar Cleft Using Novel Minimally Invasive Prosthodontic Techniques: A Case Report

Missing lateral incisors are the most frequent dental disorder associated with cleft alveolus. When orthodontic closure of the edentulous space is not possible, more aggressive prosthodontic treatments are required. Contemporary resin-bonded fixed dental prostheses (RBFDPs) represent a promising, time-efficient alternative treatment with fewer biological complications. This clinical report proposes a modified approach to the esthetic rehabilitation of a patient with a complete unilateral cleft lip and palate on the left side and an incomplete cleft lip and alveolar cleft on the right side. Digital diagnostics, treatment planning, and clinical procedures involved in the fabrication of facially bonded RBFDPs are presented. This modified technique enables the concurrent replacement of lateral incisors and correction of the malformed central incisors as well as increasing the retention of the restorations.

Lactational exposure to dioxin-like polychlorinated biphenyl 169 and nondioxin-like polychlorinated biphenyl 155: Effects on rat femur growth, biomechanics and mineral composition

Exposure to polychlorinated biphenyls (PCBs), which are persistent lipophilic environmental pollutants, has a variety of adverse effects on wildlife and human health, including bone mineralization, growth and mechanical strength. The present study evaluated the effects of lactational exposure to nondioxin-like PCB-155 and dioxin-like PCB-169, individually and in combination, on pubertal rat femur development and its biomechanics. After offspring delivery, Wistar rat mothers were divided into four groups, i.e., PCB-169, PCB-155, PCB-155+169 and control, and were administered PCBs intraperitoneally. Data on bone geometry, biomechanics and mineral composition were obtained by analysis of femurs from 42-day-old offspring by microCT scanning, three-point bending test and inductively coupled plasma mass spectrometry. Decreased somatic mass and femur size, i.e., mass, periosteal circumference and cross sectional area, were observed in the PCB-169 and PCB-155 groups. Additionally, lactational exposure to planar PCB-169 resulted in harder and more brittle bones containing higher amounts of minerals. Combined exposure to structurally and functionally different PCBs demonstrated only mild alterations in bone width and mineralization. To conclude, our results demonstrated that alterations, observed on postnatal day 42, were primarily induced by PCB-169, while toxicity from both of the individual congeners may have been reduced in the combined group.

Ageing kinetics and strength of airborne-particle abraded 3Y-TZP ceramics

OBJECTIVE

The combined effects of alumina airborne-particle abrasion and prolonged in vitro ageing on the flexural strength of 3Y-TZP ceramic have been studied. The aim was to identify the different effects on the surface and subsurface regions that govern the performance of this popular bioceramic known for its susceptibility to low-temperature degradation (LTD).

METHODS

As-sintered or airborne-particle abraded 3Y-TZP discs were subjected to ageing at 134°C for up to 480h. Biaxial flexural strength was measured and the relative amount of monoclinic phase determined using X-ray diffraction. The transformed zone depth (TZD) was observed on cross-sections with scanning electron microscopy coupled with a focused ion beam. Segmented linear regression was used to analyze the flexural strength and TZD as functions of the ageing time.

RESULTS

A two-step linear ageing kinetics was detected in airborne-particle abraded specimens, reflecting the different microstructures through which the LTD proceeds into the bulk. A 10μm thick altered zone under the abraded surface was involved in both the surface strengthening and the increased ageing resistance. When the zone was annihilated by the LTD, the strength of the ceramic specimens and the speed of LTD returned to the values measured before abrasion. Even at prolonged ageing times, the strength of abraded groups was not lower than that of as-sintered groups.

SIGNIFICANCE

Both the ageing kinetics and the flexural strength were prominently affected by airborne-particle abrasion, which altered the subsurface microstructure and phase composition. Airborne-particle abrasion was not harmful to the 3Y-TZP ceramics' stability.

Alterations in geometry, biomechanics, and mineral composition of juvenile rat femur induced by nonplanar PCB-155 and/or planar PCB-169

Exposure to widespread lipophilic and bioaccumulative polychlorinated biphenyls (PCBs) induces diverse biochemical and toxicological responses in various organs, including the bone. The aim of this study was to evaluate the changes in growth rate, geometry, serum, and bone biochemical parameters and biomechanics of juvenile rat femur induced by lactational exposure to nonplanar PCB-155 and planar PCB-169 individually and in combination. Fifteen lactating Wistar rats were divided into four groups (PCB-169, PCB-155, PCB-155+169, and control), and PCBs were administered intraperitoneally at different time points after delivery. Femurs from 22-day-old offspring were analyzed by microCT, three-point bending test and inductively coupled plasma-mass spectrometry (ICP-MS) to obtain data on bone geometry, biomechanics and mineral composition. The serum levels of calcium, phosphate and alkaline phosphatase were also determined. Lactational exposure to planar PCB-169 resulted in shorter and thinner femurs, reduced endosteal and periosteal perimeters, smaller total cross-sectional and medullary areas, and lowered serum bone marker levels and calcium levels in the bone, while femur mechanical properties were not significantly altered. The changes observed in the combination exposure (PCB-155+169) group were similar to those observed in the PCB-169 group but were less pronounced. In summary, our results demonstrate that alterations in lactationally exposed offspring were primarily induced by planar PCB-169. The milder outcome in the combined group suggested that the PCB-169-mediated toxic effects on the bone might be reduced by a nonplanar PCB-155 congener. © 2016 Wiley Periodicals, Inc. Environ Toxicol 32: 1135-1146, 2017.

The influence of crown ferrule on fracture resistance of endodontically treated maxillary central incisors

Background: Prefabricated zirconia posts can contribute to increasing the fracture resistance of the endodontically treated teeth. Purpose. This in vitro study compared the fracture resistance of endodontically treated central maxillary incisors prepared with 2mm ferrule length to the ones without ferrule. Material and methods: Twenty-four caries-free maxillary central incisors were divided into 2 groups of 12. In group A circumferential external dentin shoulders were prepared for 2mm external dentin ferrule length. There was no ferrule preparation in Group B. Zirconia VALLPOST BO-S (Ø 1,6mm), Ljubljana, Slovenia were used with retention forms in the coronary part. Core build-up was made of pressed ceramics (IPS e.max Press, Ivoclar, Liechtenstein). Crowns were manufactured from the same ceramic material (IPS e.max Press, Ivoclar). After root canal treatment and post space preparation, all posts were cemented with an adhesive resin cement (Multilink Automix, Ivoclar). The specimens were embedded in acrylic resin blocks (ProBase Polymer/Monomer, Ivoclar) and loaded at an angle of 45o to the long axis in an Instron Testing Machine 4301 (Instron Corp., USA) at a crosshead speed of 1mm/min until fracture. Fracture patterns and loads were recorded. A significance level of p<0.05 was used for all comparisons. Two-way analysis of variance was used for statistical analysis. Failure patterns were analyzed with the optical microscope Stereo Discovery V.8 (Carl Zeiss, Germany) and compared using the chisquare nonparametric test. Results: The mean values (±SD) of fracture loads (N) for the Groups A and B were 664.63N (±49.14) and 519.36N (±71.65) recpectively. Significantly lower failure loads were recorded for the specimens in the group B. Failure patterns within the groups revealed non-catastrophic failure in 70% of the specimens for group A and 85% for group B. Conclusions: Within the limitations of this in vitro study, it can be concluded that zirconia VALLPOST BO-S (Ø 1,6mm) with press-ceramic cores and crowns, can be used for restoration of endodontically treated teeth. The teeth prepared with 2mm external dentin ferrule length were found to be more fracture resistant than teeth without ferrule.

Implant-prosthetic rehabilitation after radiation treatment in head and neck cancer patients: a case-series report of outcome

Background

Slovenia has a high burden of head and neck cancer. Patients are mostly treated with surgery followed by radiation therapy. Advanced surgical and prosthodontic techniques have expanded the rehabilitation options. The aim of the study was to review the outcome of implant-prosthetic treatment after radiation therapy.

Patients and methods

Twenty irradiated head and neck cancer patients who received a removable implant-supported denture at the University Medical Centre Ljubljana were included in the study. Kaplan-Meier survival analysis, Cox proportional hazard models and logistic regression were used to assess the implant survival and success rate.

Results

Twenty patients had 100 implants inserted. The estimated implant survival rate was 96% after 1 year and 87% after 5 years. Failures were mostly observed before loading (91.2%). Implants inserted in the transplanted bone were significantly more likely to fail. Out of 89 implants supporting the dentures, 79 implants (88.7%) were successful, meaning that they were functionally loaded and exhibited no pain, radiolucency or progressive bone loss. Prosthetic treatment was significantly less successful in older patients. The attachment system and the number of implants did not have a statistically significant influence on the success rate.

Conclusions

Implant-supported dentures have been shown to be a reliable treatment modality after head and neck cancer surgery and radiation therapy. Possible early failures should be communicated with the patients.

Fracture resistance and reliability of new zirconia posts

STATEMENT OF PROBLEM

The radicular portion of zirconia endodontic posts often need to be reshaped to achieve a definitive form and may be airborne-particle abraded to improve adhesion during luting. Therefore, the surface of the tetragonal zirconia ceramics may be transformed and damaged, influencing the mechanical properties of the material.

PURPOSE

This study compared the fracture resistance of prefabricated zirconia posts with a new retentive post-head after different surface treatments.

MATERIAL AND METHODS

Experimental zirconia posts of 2 different diameters, 1.3 mm and 1.5 mm, were produced from commercially available zirconia powder. A cylindro-conical outline form was used for the root portion of the system and a post-head with 3 retentive rings was designed. Sixty posts of each diameter were divided into 3 groups (n=20). Group 1 was ground with a coarse grit diamond bur; Group 2 was airborne-particle abraded with 110-microm fused alumina particles, and Group 3 was left as-received (controls). Posts were luted into the root-shaped artificial canals with the Clearfil adhesive system and Panavia 21 adhesive resin luting agent. The posts were loaded in a universal testing machine at an inclination of 45 degrees with the constant cross-head speed of 1 mm/min. The fracture load (N) necessary to cause post fracture was recorded, and the statistical significance of differences among groups was analyzed with 1-way ANOVA followed by the Fischer LSD test (alpha=.05). The variability was analyzed using Weibull statistics.

RESULTS

Load to fracture values of all zirconia posts depended primarily on post diameter. Mean fracture loads (SD) in Newtons were 518.4 (+/-101.3), 993.6 (+/-224.1), and 622.7 (+/-110.3) for Groups 1 through 3, respectively, for thicker posts, and 385.9 (+/-110.3), 627.0 (+/-115.1), and 451.2 (+/-81.4) for Groups 1 through 3, respectively, for thinner posts. Airborne-particle-abraded posts exhibited significantly higher resistance to fracture (P<.05) than those in the other 2 groups for diameters 1.3 mm and 1.5 mm. Grinding reduced Weibull modulus compared with controls, and the values were 4.1 and 6.5 for thicker and thinner posts, respectively.

CONCLUSION

Within the limitations of this study, the results suggest that grinding leads to a significant drop in load to fracture of zirconia posts, whereas airborne-particle abrasion increased the fracture load.

Study of water migration by micro MR imaging

Magnetic Resonance (MR) microscopy is a noninvasive tool that enables high resolution imaging of water concentration in non-magnetic materials. In this study, water migration in a dense porous material (dental cement) was monitored by 1D MR microscopy. The measured concentration profiles were later analyzed via three different migration models: an imbibition model, a diffusion model and a combined model. From the best fit to the experimental data, model parameters were obtained.

Effect of surface coating on water migration into resin-modified glass ionomer cements: a magnetic resonance micro-imaging study

Magnetic resonance micro-imaging was applied to study water diffusion into resin-modified glass ionomer cement restoration and to evaluate the effect of surface coating over restoration. Two cavities were prepared on the labial surface of extracted teeth and restored with resin-modified glass ionomer cement; one was protected with surface coating and the other was not. Immediately after restoration, the teeth were immersed in water. Progress of water diffusion into restorations was monitored by T(1) weighted spin-echo MRI at one-day intervals after the start of immersion. To quantify the water diffusion, a model was developed and compared with imaging data. Best fit yielded an effective water diffusion coefficient D = (2.3 +/- 0.4) 10(-12) m(2)/sec. Experimental results demonstrated that surface coating protects the dental cement against water intrusion from the surface of the restoration which faces the oral cavity. Such coating, however, does not prevent water penetration from the dentine side.

Strength and reliability of surface treated Y-TZP dental ceramics

This work was undertaken to evaluate the effects of dental grinding and sandblasting on the biaxial flexural strength and Weibull modulus of various Y-TZP ceramics containing 3 mol% yttria. In addition, the susceptibility of pristine and mechanically treated materials to low-temperature degradation under the conditions adopted for testing the chemical solubility of dental ceramics was investigated. The results revealed that surface grinding and sandblasting exhibit a counteracting effect on the strength of Y-TZP ceramics. Dental grinding lowered the mean strength and Weibull modulus, whereas sandblasting provided a powerful method for strengthening, but at the expense of somewhat lower reliability. The finest-grained material exhibited the highest strength after sintering, but it was less damage tolerant than tougher, coarse-grained materials. Upon extraction with the acetic acid solution and the ammonia solution, a significant amount of tetragonal zirconia had transformed to monoclinic, but extensive microcracking and attendant strength degradation had not yet occurred. Standard grade Y-TZP ceramics are more resistant in an alkaline than in an acidic environment, and there was a strong grain-size dependence of the diffusion-controlled transformation. Since a special Y-TZP grade containing a small amount of alumina exhibited the highest damage tolerance and superior stability in an acidic environment, this material shows considerable promise for dental applications.

The effect of surface grinding and sandblasting on flexural strength and reliability of Y-TZP zirconia ceramic

OBJECTIVES

This study was conducted to evaluate the effect of grinding and sandblasting on the microstructure, biaxial flexural strength and reliability of two yttria stabilized tetragonal zirconia (Y-TZP) ceramics.

METHODS

Two Y-TZP powders were used to produce fine grained and coarse grained microstructures. Sixty discs from each material were randomly divided into six groups of ten. For each group, a different surface treatment was applied: dry grinding, wet grinding, sandblasting, dry grinding + sandblasting, sandblasting + dry grinding and a control group. Biaxial flexural strength was determined and data were analyzed using one-way ANOVA, followed by Tukey's HSD test (p < 0.05). In addition, Weibull statistics was used to analyze the variability of flexural strength. The relative amount of transformed monoclinic zirconia, corresponding transformed zone depth (TZD) and the mean critical defect size Ccr were calculated.

RESULTS

There was no difference in mean strength between the as sintered fine and coarse grained Y-TZP. Significant differences (p < 0.05) were found between the control group and ground fine grained material for both wet and dry grinding. Sandblasting significantly increased the strength in fine and coarse grained materials. All surface treatment procedures reduced the Weibull modulus of Y-TZP. For both materials, the highest amount of the monoclinic phase and the largest TZD was found after sandblasting. Lower amounts of the monoclinic phase were obtained after both grinding procedures, where the highest mean critical defect size Ccr was also calculated.

SIGNIFICANCE

Our results indicate that sandblasting may provide a powerful technique for strengthening Y-TZP in clinical practice. In contrast, grinding may lead to substantial strength degradation and reduced reliability of prefabricated zirconia elements, therefore, sandblasting of ground surfaces is suggested.

Micro magnetic resonance imaging of water uptake by glass ionomer cements

OBJECTIVES

The purpose of the present study was: 1) to visualize the water penetration into glass ionomer cement samples prepared in two different setting modes as a function of time, and 2) to assess the potential use of micro magnetic resonance imaging by studying penetration processes.

METHODS

An encapsulated form of resin-modified glass ionomer cement (Fuji II LC, GC) was used in this study. The mixed cement was syringed into quartz tubes (4 mm ID x 10 mm long). Half of the samples were radially exposed to a light source for 120 s; the other half were allowed to set chemically in a photographic darkroom. One hour after the start of mixing, samples were extruded from the quartz tubes, immediately immersed in distilled water, and stored at 37 degrees C. Eight specimens were prepared with each setting mode and imaged at different times. Micro magnetic resonance imaging was performed on a Bruker Biospec System equipped with micro-imaging utilities. A spin echo technique was used. A small tube containing a mixture of normal and deuterated water was added as a standard to which the signals from the samples were normalized. The average signal, as calculated by the image processing software from each region, was divided by the signal from the standard sample to obtain the normalized intensity. The results were analyzed by a Student's t-test.

RESULTS

After 24 h of immersion, water diffused 1 mm into the chemical-cured material and approximately 0.5 mm in the light-cured samples. After 96 h, the water had reached the center of all chemical-cured samples but not of the light-cured samples. After 192 h, water had reached the center of the cylinders of both groups of samples.

SIGNIFICANCE

MRI microscopy is a good method for monitoring the water permeability of glass ionomer cements. The technique is nondestructive thus, the process can be followed on the same sample without destroying it. By using some special imaging techniques, refinement of the method will be possible.