Young Cheul Heo

Digital image correlation analysis of the load transfer by implant-supported restorations

This study compared splinted and non-splinted implant-supported prosthesis with and without a distal proximal contact using a digital image correlation method. An epoxy resin model was made with acrylic resin replicas of a mandibular first premolar and second molar and with threaded implants replacing the second premolar and first molar. Splinted and non-splinted metal-ceramic screw-retained crowns were fabricated and loaded with and without the presence of the second molar. A single-camera measuring system was used to record the in-plane deformation on the model surface at a frequency of 1.0Hz under a load from 0 to 250N. The images were then analyzed with specialist software to determine the direct (horizontal) and shear strains along the model. Not splinting the crowns resulted in higher stress transfer to the supporting implants when the second molar replica was absent. The presence of a second molar and an effective interproximal contact contributed to lower stress transfer to the supporting structures even for non-splinted restorations. Shear strains were higher in the region between the molars when the second molar was absent, regardless of splinting. The opposite was found for the region between the implants, which had higher shear strain values when the second molar was present. When an effective distal contact is absent, non-splinted implant-supported restorations introduce higher direct strains to the supporting structures under loading. Shear strains appear to be dependent also on the region within the model, with different regions showing different trends in strain changes in the absence of an effective distal contact.

Amelogenesis imperfecta, rough hypoplastic type, dental follicular hamartomas and gingival hyperplasia: report of a case from Central America and review of the literature

We report on a black male patient from Central America with amelogenesis imperfecta, rough hypoplastic type, dental follicular hamartomas, and gingival hyperplasia. Although previous reports have described this association of amelogenesis imperfecta with hyperplastic follicular hamartomas or central odontogenic fibroma-like lesions in blacks from South Africa, we have noticed non-black patients in the literature with similar findings.

A digital image correlation analysis on the influence of crown material in implant-supported prostheses on bone strain distribution

PURPOSE A digital image correlation (DIC) method for full-field surface strain measurement was used to analyze the effect of two veneering materials for implant supported crowns on the strain distribution within the surrounding bone. METHODS An epoxy resin model of a bone block was made by housing acrylic resin replicas of a mandibular first premolar and second molar together with threaded implants replacing the second premolar and first molar. Porcelain-veneered (G1 and G3) and resin-veneered (G2 and G4) screw-retained splinted crowns were fabricated and loaded with (G1 and G2) and without (G3 and G4) the presence of the second molar replica. A 2-dimensional DIC measuring system was used to record surface deformation of the bone block model at a frequency of 1.0 Hz during application of a 250-N load. RESULTS Maximum compressive strains (ɛ(XX), %) were found for the following regions: between molars, G1 (-0.21), G2 (-0.18), G3 (-0.26), and G4 (-0.25); between implants, G1 (-0.19), G2 (-0.13), G3 (-0.19), and G4 (-0.14). The magnitude of strains in the simulated bone block with the resin-veneered crowns was lower than that with porcelain-veneered crowns, irrespective of the presence or absence of the second molar. CONCLUSIONS The softer resin veneer helped to spread the load more evenly amongst the supporting teeth and implants, thus reducing the strains in the simulant bone block. Conversely, using the harder porcelain veneer resulted in the load being concentrated within one or two teeth or implants, thus leading to higher strain values in the bone block.

A study of polymerization shrinkage kinetics using digital image correlation

OBJECTIVE To investigate the polymerization shrinkage kinetics of dental resin composites by measuring in real time the full-field shrinkage strain using a novel technique based on digital image correlation (DIC). METHODS Polymerization shrinkage in resin composite specimens (Filtek LS and Z100) was measured as a function of time and position. The main experimental setup included a CCD camera and an external shutter inversely synchronized to that of the camera. The specimens (2 mm × 4 mm × 5 mm) were irradiated for 40s at 1200 mW/cm(2), while alternating image acquisition and obstruction of the curing light occurred at 15 fps. The acquired images were processed using proprietary software to obtain the full-field strain maps as a function of time. RESULTS Z100 showed a higher final shrinkage value and rate of development than LS. The final volumetric shrinkage for Z100 and LS were 1.99% and 1.19%, respectively. The shrinkage behavior followed an established shrinkage strain kinetics model. The corresponding characteristic time and reaction order exponent for LS and Z100 were calculated to be approximately 23s and 0.84, and 14s and 0.7, respectively, at a distance of 1.0mm from the irradiated surface, the position where maximum shrinkage strain occurred. Thermal expansion from the exothermic reaction could have affected the accuracy of these parameters. SIGNIFICANCE The new DIC method using an inversely synchronized shutter provided realtime, full-field results that could aid in assessing the shrinkage strain kinetics of dental resin composites as a function of specimen depth. It could also help determine the optimal curing modes for dental resin composites.

Acoustic properties of interfacial debonding and their relationship with shrinkage stress in Class-I restorations

OBJECTIVES (1) To investigate the properties, and their correlations, of the acoustic emission (AE) from interfacial debonding of Class-I composite restorations during curing. (2) To establish the relationship between the theoretical shrinkage stress and the level of interfacial debonding in such restorations as determined by AE measurement. METHODS An AE sensor was attached onto the surface of human molars with a Class-I composite restoration of 4mm (length)×3mm (width)×2mm (depth) to monitor their debonding from the tooth tissues during curing. Background signals were analyzed before curing to determine the threshold amplitude for noise filtering. Three groups (n=3) of composites with different levels of shrinkage were tested: (1) Z100™, (2) Filtek™ Z250, and (3) Filtek™ LS. All restorations were cured with an LED blue light operated at 1200mW/cm(2) for 40s. AE signals were recorded continuously from the start of curing for 10min, and their frequency, amplitude and duration were analyzed. Finally, the cumulative number of AE events was compared with the theoretical maximum shrinkage stress that could be generated by the composites. RESULTS The amplitude of the background signals was below 30dB, which was chosen as the threshold for noise filtering. The amplitude of all debonding events ranged from 30 to 50dB, and their duration was below 100μs. The peak frequency had two main bands: 100-200kHz and 700-800kHz. The duration time increased with increasing amplitude, but no correlation was found between the peak frequency and the other two parameters. The cumulative number of AE events was 30.67±2.31, 14.00±7.81 and 5.67±3.06 for Z100, Z250 and LS, respectively, which corresponded well with the theoretical maximum shrinkage stress they could produce, i.e. 42.5, 97.5 and 182.5MPa. R(2)=0.9955 for the linear regression. The theoretical shrinkage stress below which no AE events were detected was about 14.3MPa. CONCLUSIONS For the materials considered, the amount of interfacial debonding produced in a Class-I restoration during curing increased linearly with the theoretical maximum shrinkage stress of the composite. The theoretical stress below which no AE events were detected was similar to composite-dentin bond strength reported in the literature.

Determining the temporal development of dentin-composite bond strength during curing.

OBJECTIVES As composite restorations cure a competition develops between bond formation and shrinkage stress at the composite-dentin interface. Thus, understanding the temporal development of tooth-composite bond strength should enable better assessment of tooth-composite debonding. METHODS In this study, bond strengths of composite-dentin specimens obtained from tensile test at different curing times were used to determine the bond formation rate. By varying the composite thickness and output from the curing light, their effects on the rate of bond formation for two different materials (a conventional and a bulk-fill composite) were also investigated. The proportions of cohesive and adhesive failure were determined by analysis of electron micrographs of the fractured surfaces. RESULTS The development of dentin-composite bond strength (S) with time (t) can be described by the equation: S=Smax(1-exp(-αt)), where Smax is the final bond strength (∼12MPa for both composites) and α the rate of bond formation. Using bulk-fill and thinner specimens gave faster bond formation. In fact, the higher the irradiance at the interface, the higher the rate of bond formation. However, α had a maximum value of ∼0.6s(-1) and the rule of reciprocity did not hold. A minimum dose of ∼2J/cm(2) was required to achieve adequate bond strength. The predominant failure mode changed from cohesive in the composite and adhesive to interfacial at the adhesive-dentin interface, indicating the latter to be the weakest link in the cured dentin-composite assemblies considered. SIGNIFICANCE When combined with the temporal development of shrinkage stress, the current results will help determine the likelihood of tooth-composite debonding.

Accelerated fatigue testing of dentin-composite bond with continuously increasing load

OBJECTIVES The aim of this study was to evaluate an accelerated fatigue test method that used a continuously increasing load for testing the dentin-composite bond strength. METHODS Dentin-composite disks (ϕ5mm×2mm) made from bovine incisor roots were subjected to cyclic diametral compression with a continuously increasingly load amplitude. Two different load profiles, linear and nonlinear with respect to the number of cycles, were considered. The data were then analyzed by using a probabilistic failure model based on the Weakest-Link Theory and the classical stress-life function, before being transformed to simulate clinical data of direct restorations. RESULTS All the experimental data could be well fitted with a 2-parameter Weibull function. However, a calibration was required for the effective stress amplitude to account for the difference between static and cyclic loading. Good agreement was then obtained between theory and experiments for both load profiles. The in vitro model also successfully simulated the clinical data. SIGNIFICANCE The method presented will allow tooth-composite interfacial fatigue parameters to be determined more efficiently. With suitable calibration, the in vitro model can also be used to assess composite systems in a more clinically relevant manner.

Fracture Toughness of Nuclear Graphite NBG-18

Objective: Graphite components in a Very High Temperature Reactor (VHTR) may fracture under the actions of external and internal (irradiation-induced) stresses. Measurement of the fracture properties of graphite is therefore essential in the design and structural integrity assessment of VHTR cores. This study aimed to evaluate the fracture toughness and its associated statistical characteristics of nuclear graphite NBG-18, for which there had been very little data. The effect of specimen size on its fracture toughness was also studied.Materials and methods: Three-point-bending tests were conducted with a MTS machine (858 Mini Bionix II, MTS, US) on single-edge-notched beams (SENB) of NBG-18 to measure its fracture toughness. Three different specimen sizes were considered: (I) 200mm (Support Span S) × 50mm (Width W) × 25mm (Thickness T), (II) 100mm (S) × 20mm (W) × 10mm (T), and (III) 40mm (S) × 10mm (W) × 5mm (T). A notch was machined into each specimen to give a crack length-to-width (a/w) ratio of 0.4 using a 0.3mm-thick diamond blade. The acoustic emission (AE) technique was applied to monitor the damage evolution process during loading.Results: The mean and standard deviation (in brackets) of the critical stress intensity factor KIC (MPa·m1/2) for each group were: (I) 1.69 (0.04), (II) 1.36 (0.05), and (III) 1.33 (0.06). Specimen size was found to significantly influence the fracture toughness results: the smaller the specimen, the lower the mean fracture toughness and the larger the variation.Conclusions: The fracture toughness of NBG-18 appears to reduce with the specimen size. The lower fracture toughness of the smaller specimens could be attributed to a finite process zone ahead of the crack tip.Copyright

Relationships between tissue properties and operational parameters of a dental handpiece during simulated cavity preparation.

A preliminary study was conducted on the development of an intelligent dental handpiece with functionality to detect subtle changes in mechanical properties of tooth tissue during milling. Such equipment would be able to adopt changes in cutting parameters and make real-time measurements to avoid tooth tissue damage caused by overexertion and overextension of the cutting tool. A modified dental handpiece, instrumented with strain gauges, microphone, displacement sensor, and air pressure sensor, was mounted to a linear movement table and used to mill three to four cavities in >50 bovine teeth. Extracted sound frequency and density were analyzed along with force, air pressure, and displacement for correlations and trends. Experimental results showed a high correlation (coefficient close to 0.7) between the feed force, the rotational frequency, and the averaged gray scale. These results could form the basis of a feedback control system to improve the safety of dental cutting procedures. This article is written in memory of Dr Hongyan Sun, who passed away in 2011 at a young age of 37.