Comparison of Three Different Types of Two-Implant-Supported Magnetic Attachments on the Stress Distribution in Edentulous Mandible.

Two-implant-retained mandibular overdentures with magnetic attachments can provide an effective treatment modality for edentulous patients. In this study, a three-dimensional finite element analysis was used to compare the biomechanical characteristics of three different types of magnetic attachments in two-implant-retained mandibular overdentures. Flat-type, dome-type, and cushion-type of the magnetic attachments were designed to retain the overdenture. Four types of load were applied to the overdenture in each model: 100 N vertical and oblique loads on the right first molar and a 100 N vertical load on the right canine and the lower incisors. The biomechanical behaviors of peri-implant bone, abutment, and mucosa were recorded. In vertical incisors, vertical right canine, and oblique molar loading condition, the flat-type group exhibited the highest levels of maximum equivalent strain/stress in the peri-implant bone. The total deformation of mucosa and the maximum equivalent strain/stress in the oblique molar loading condition are about two times as the vertical molar loading condition. These results suggested that both cushion-type and dome-type of the magnetic attachments are better choices in two-implant-retained mandibular overdentures, and oblique loading is more harmful than vertical loading.

New mini dental implant attachments versus O-ring attachment after cyclic aging: Analysis of retention strength and gap space.

Overdenture as a treatment modality for both partially and fully edentulous patients is costeffective and less expensive. The purpose of the present study was to examine the newly fabricated attachments by comparing them with conventional O-ring attachment in vitro in terms of retention force and cyclic aging resistance. A total of 150 samples were prepared and divided into five groups according to the materials used (O-ring attachment, Deflex M10 XR, Deflex Classic SR, Deflex Acrilato FD, and flexible acrylic resin). The retention force of different attachments was measured by a mini dental implant after three subsequent aging (0, 63, and 126) cycles in the circumstances similar to the oral environment. The gap space between the head of the implant and the inner surface of the attachments was detected. Two-way analysis of variance (ANOVA) analysis with multiple comparisons test was applied for statistical analysis. The results showed that Deflex M10 XR had the highest retention force and the lowest gap space after cyclic aging; in addition, by comparing the relative force reduction, the lowest values were obtained in the O-ring attachment and the highest values in the flexible acrylic resin attachment. The retention force measured after cyclic aging for the Deflex M10 XR attachment was greatly improved when compared with the O-ring attachment and other types of attachment materials; in addition, the Deflex M10 XR attachment exhibited the minimum gap space between the inner surface and the mini dental implant head. In conclusion, Deflex M10 XR has the ability to withstand weathering conditions and retains its durable and retentive properties after aging when compared with other attachments.

Surface topography and bond strengths of feldspathic porcelain prepared using various sandblasting pressures.

OBJECTIVE: The purpose of this study was to determine the bond strength of composite resin to feldspathic porcelain and its surface topography after sandblasting at different pressures. METHODS: In this in vitro study, 68 porcelain disks were fabricated and randomly divided into four groups of 17. The porcelain surface in group 1 was etched with hydrofluoric acid. Groups 2, 3, and 4 were sandblasted at 2, 3 and 4 bars pressure, respectively. Surface topography of seven samples in each of the four groups was examined by a scanning electron microscope (SEM). The remaining 40 samples received the same silane agent, bonding agent, and composite resin and they were then subjected to 5000 thermal cycles and evaluated for shear bond strength. Data were analyzed using one-way anova. The mode of failure was determined using stereomicroscope and SEM. RESULTS: The highest shear bond strength was seen in group 4. however, statistically significant differences were not seen between the groups (P = 0.780). The most common mode of failure was cohesive in porcelain. The SEM showed different patterns of hydrofluoric acid etching and sandblasting. CONCLUSION: Increasing the sandblasting pressure increased the surface roughness of feldspathic porcelain but no difference in bond strength occurred.

Three-Dimensional Finite Element Analysis of Anterior Single Implant-Supported Prostheses with Different Bone Anchorages.

The aim of this study was to evaluate the stress distribution of monocortical and bicortical implant placement of external hexagon connection in the anterior region of the maxilla by 3D finite element analysis (FEA). 3D models were simulated to represent a bone block of anterior region of the maxilla containing an implant (4.0 × 10.0 mm) and an implant-supported cemented metalloceramic crown of the central incisor. Different techniques were tested (monocortical, bicortical, and bicortical associated with nasal floor elevation). FEA was performed in FEMAP/NeiNastran software using loads of 178 N at 0°, 30°, and 60° in relation to implant long axis. The von Mises, maximum principal stress, and displacement maps were plotted for evaluation. Similar stress patterns were observed for all models. Oblique loads increased the stress concentration on fixation screws and in the cervical area of the implants and bone around them. Bicortical technique showed less movement tendency in the implant and its components. Cortical bone of apical region showed increase of stress concentration for bicortical techniques. Within the limitations of this study, oblique loading increased the stress concentrations for all techniques. Moreover, bicortical techniques showed the best biomechanical behavior compared with monocortical technique in the anterior maxillary area.

Step-stress analysis for predicting dental ceramic reliability.

OBJECTIVE: To test the hypothesis that step-stress analysis is effective to predict the reliability of an alumina-based dental ceramic (VITA In-Ceram AL blocks) subjected to a mechanical aging test. METHODS: Bar-shaped ceramic specimens were fabricated, polished to 1µm finish and divided into 3 groups (n=10): (1) step-stress accelerating test; (2) flexural strength-control; (3) flexural strength-mechanical aging. Specimens from group 1 were tested in an electromagnetic actuator (MTS Evolution) using a three-point flexure fixture (frequency: 2Hz; R=0.1) in 37°C water bath. Each specimen was subjected to an individual stress profile, and the number of cycles to failure was recorded. A cumulative damage model with an inverse power law lifetime-stress relation and Weibull lifetime distribution were used to fit the fatigue data. The data were used to predict the stress level and number of cycles for mechanical aging (group 3). Groups 2 and 3 were tested for three-point flexural strength (σ) in a universal testing machine with 1.0MPa/s stress rate, in 37°C water. Data were statistically analyzed using Mann-Whitney Rank Sum test. RESULTS: Step-stress data analysis showed that the profile most likely to weaken the specimens without causing fracture during aging (95% CI: 0-14% failures) was: 80MPa stress amplitude and 10(5) cycles. The median σ values (MPa) for groups 2 (493±54) and 3 (423±103) were statistically different (p=0.009). SIGNIFICANCE: The aging profile determined by step-stress analysis was effective to reduce alumina ceramic strength as predicted by the reliability estimate, confirming the study hypothesis.

The long-term effect of calcium hydroxide application on dentin fracture strength of endodontically treated teeth.

OBJECTIVE: The aim of this in vitro study was to evaluate the long-term effect of calcium hydroxide (CH) on the microtensile fracture strength (MTFS) of endodontically treated human teeth. MATERIALS AND METHODS: A total of 105 caries-free extracted human mandibular incisors were used. The teeth were divided into seven experimental groups of 15 teeth each. The root canals of all the teeth were rotary instrumented. The teeth in the control group were immediately obturated and tested for MTFS. The rest of the teeth were vertically compacted with CH and a sterile saline solution mixture and sealed with temporary filling. The teeth were stored in a moist environment for 30, 90, 180, 270, 360, and 540 days. On scheduled dates, the root canals were obturated and then the teeth were tested for MTFS with an Instron testing machine. The Kruscal-Wallis test and a post hoc Dunn's multiple comparison test was utilized. The statistical significance level was established at P < 0.05. RESULTS: The introduction of CH into the root canals seems to decrease the MTFS of teeth statistically significantly through the 180th, 270th, 360th, and 540th days, respectively, compared with the control group. The results clearly indicated that there are statistically significant differences between group 2 and group 7 (P < 0.001) and between group 3 and group 7 (P < 0.05). There were no statistically significant differences between the rest of the groups (P > 0.05). CONCLUSIONS: The results indicate that long-term CH treatments can significantly reduce the strength of the teeth, causing an increase in fracture risk.

Correlation of wear in vivo and six laboratory wear methods.

OBJECTIVE: We examined the correlation between clinical wear rates of restorative materials and enamel (TRAC Research Foundation, Provo, USA) and the results of six laboratory test methods (ACTA, Alabama (generalized, localized), Ivoclar (vertical, volumetric), Munich, OHSU (abrasion, attrition), Zurich). METHODS: Individual clinical wear data were available from clinical trials that were conducted by TRAC Research Foundation (formerly CRA) together with general practitioners. For each of the n=28 materials (21 composite resins for intra-coronal restorations [20 direct and 1 indirect], 5 resin materials for crowns, 1 amalgam, enamel) a minimum of 30 restorations had been placed in posterior teeth, mainly molars. The recall intervals were up to 5 years with the majority of materials (n=27) being monitored, however, only for up to 2 years. For the laboratory data, the databases MEDLINE and IADR abstracts were searched for wear data on materials which were also clinically tested by TRAC Research Foundation. Only those data for which the same test parameters (e.g. number of cycles, loading force, type of antagonist) had been published were included in the study. A different quantity of data was available for each laboratory method: Ivoclar (n=22), Zurich (n=20), Alabama (n=17), OHSU and ACTA (n=12), Munich (n=7). The clinical results were summed up in an index and a linear mixed model was fitted to the log wear measurements including the following factors: material, time (0.5, 1, 2 and 3 years), tooth (premolar/molar) and gender (male/female) as fixed effects, and patient as random effect. Relative ranks were created for each material and method; the same was performed with the clinical results. RESULTS: The mean age of the subjects was 40 (±12) years. The materials had been mostly applied in molars (81%) and 95% of the intracoronal restorations were Class II restorations. The mean number of individual wear data per material was 25 (range 14-42). The mean coefficient of variation of clinical wear data was 53%. The only significant correlation was reached by OHSU (abrasion) with a Spearman r of 0.86 (p=0.001). Zurich, ACTA, Alabama generalized wear and Ivoclar (volume) had correlation coefficients between 0.3 and 0.4. For Zurich, Alabama generalized wear and Munich, the correlation coefficient improved if only composites for direct use were taken into consideration. The combination of different laboratory methods did not significantly improve the correlation. SIGNIFICANCE: The clinical wear of composite resins is mainly dependent on differences between patients and less on the differences between materials. Laboratory methods to test conventional resins for wear are therefore less important, especially since most of them do not reflect the clinical wear.

The fracture load and failure types of veneered anterior zirconia crowns: an analysis of normal and Weibull distribution of complete and censored data.

OBJECTIVES: The aim of this study was to compare the fracture load of veneered anterior zirconia crowns using normal and Weibull distribution of complete and censored data. METHODS: Standardized zirconia frameworks for maxillary canines were milled using a CAD/CAM system and randomly divided into 3 groups (N=90, n=30 per group). They were veneered with three veneering ceramics, namely GC Initial ZR, Vita VM9, IPS e.max Ceram using layering technique. The crowns were cemented with glass ionomer cement on metal abutments. The specimens were then loaded to fracture (1 mm/min) in a Universal Testing Machine. The data were analyzed using classical method (normal data distribution (µ, σ); Levene test and one-way ANOVA) and according to the Weibull statistics (s, m). In addition, fracture load results were analyzed depending on complete and censored failure types (only chipping vs. total fracture together with chipping). RESULTS: When computed with complete data, significantly higher mean fracture loads (N) were observed for GC Initial ZR (µ=978, σ=157; s=1043, m=7.2) and VITA VM9 (µ=1074, σ=179; s=1139; m=7.8) than that of IPS e.max Ceram (µ=798, σ=174; s=859, m=5.8) (p<0.05) by classical and Weibull statistics, respectively. When the data were censored for only total fracture, IPS e.max Ceram presented the lowest fracture load for chipping with both classical distribution (µ=790, σ=160) and Weibull statistics (s=836, m=6.5). When total fracture with chipping (classical distribution) was considered as failure, IPS e.max Ceram did not show significant fracture load for total fracture (µ=1054, σ=110) compared to other groups (GC Initial ZR: µ=1039, σ=152, VITA VM9: µ=1170, σ=166). According to Weibull distributed data, VITA VM9 showed significantly higher fracture load (s=1228, m=9.4) than those of other groups. SIGNIFICANCE: Both classical distribution and Weibull statistics for complete data yielded similar outcomes. Censored data analysis of all ceramic systems based on failure types is essential and brings additional information regarding the susceptibility to chipping or total fracture.

Frictional resistance between orthodontic brackets and archwire: an in vitro study.

AIM: The purpose of this investigation was to determine the kinetic frictional resistance offered by stainless steel and Titanium bracket used in combination with rectangular stainless steel wire during in vitro translatory displacement of brackets. MATERIALS AND METHODS: In this study. Brackets: (All brackets used had a torque of - 7° and an angulation of 0°): (1) Dynalock (Unitek) 0.018'' slot, 3.3 mm bracket width, (2) Mini Uni-Twin (Unitek) 0.018'' slot, 1.6 mm bracket width, (3) Ultra-Minitrim (Dentaurum) 0.022'' slot 3.3 mm bracket width, (4) Titanium (Dentaurum) 0.022'' slot, 3.3 mm bracket width. WIRES: (1) 0.016 x 0.022'' stainless steel (Dentaurum), (2) 0.017 x 0.025''stainless steel (Unitek), (3) 0.018 x 0.025'' stainless steel (Dentaurum), elastomeric modules (Ortho Organisers), 0. 009'' stainless steel ligature wires, hooks made of 0.021 x 0.025'' stainless steel wires, super glue to bond the hooks to the base of the bracket, acetone to condition the bracket and wires before testing and artificial saliva. Brackets were moved along the wire by means of an Instron universal testing machine (1101) and forces were measured by a load cell. All values were recorded in Newtons and then converted into gms (1N-102 gm). 200 gm was then subtracted from these values to find out the frictional force for each archwire/bracket combination. For each archwire/ bracket combination three readings were taken under wet and dry condition and also with stainless steel ligature and elastomeric modules separately. RESULTS: The results showed that narrow brackets generated more friction than wider brackets. Frictional force was directly proportional to wire dimension. Titanium brackets generated more friction than stainless steel brackets. Archwire and bracket ligated with elastomeric module generated more friction than when ligated with stainless steel ligature wire. Frictional forces in the wet condition were greater than in the dry condition for all archwire to bracket combinations. CONCLUSION: Frictional force was seen to be inversely proportional to bracket width, frictional force was inversely proportional to bracket width, and in the wet condition were greater than in the dry condition for all archwire to bracket combinations. CLINICAL SIGNIFICANCE: This study of friction is its role in lessening the force actually received by a tooth from an active component such as a spring, loop or elastic. Hence greater applied force is needed to move a tooth with a bracket archwire combination demonstrating high magnitudes of friction compared with one with a low frictional value.

Spotlight on bond strength testing--unraveling the complexities.

The variability in methods and outcomes of bond strength testing reports is well documented in the dental literature. Many studies lack important information, which impairs the ability to reproduce them as well as to compare them to other studies in the literature. In order to critically discuss the important issues around bond strength testing methods, and to move closer to at least standardizing the reporting of such studies, the Academy of Dental Materials held a conference in 2009 entitled Adhesion in Dentistry-Analyzing Bond Strength Testing Methods, Variables, and Outcomes. Short synopses of the presentations are presented in this article. This article also provides a list of the variables that should be reported in bond strength studies, regardless of testing methods, to be used by authors conducting future studies, as well as journal reviewers and editors. The goal is to provide guidance and a rationale for what should be included in a study so that reporting might be more standardized and to enhance the possibility that more meaningful comparisons and conclusions may be drawn across studies. The table lists detailed descriptions covering all aspects of testing procedures, including variables related to tooth substrate, restorative material, specimen preparation, pre-testing conditions, testing methods, data reporting and analysis.

Numeric comparison of the static mechanical behavior between ProFile GT and ProFile GT series X rotary nickel-titanium files.

INTRODUCTION: This article aims to evaluate how different superelastic nickel-titanium (NiTi) alloys determine the static mechanical performance of endodontic files during bending and torsion. METHODS: Two NiTi rotary instruments with similar geometries and equal cross sections, ProFile GT (GT) and a GT Series X (GTX), were selected. The latter file is made from M-Wire, a NiTi alloy that, according to its manufacturer, has been thermomechanically processed to have larger flexibility at body temperature. The mechanical response was studied for a series of static bending and torsional loads by using finite element (FE) models. The materials were characterized according to previously published stress-strain curves. RESULTS: For the same load and boundary conditions, the GTX material significantly increased the instrument's performance. For instance, the deflection for a 1N force at the tip of the file was found to be 28.5% larger for the GTX file, whereas the maximum stress decreased 13.2%. CONCLUSIONS: Although not fully reflective of the instrument's behavior in a dynamic rotation intracanal system, the static results showed that the GTX file is more flexible and capable of stress relief at the most critical sections than the GT file, suggesting that it has a lower risk of fracture inside the root canals during its clinical use.

[Distribution features of surface stress on the bilateral condyles of the unilateral mandible model for 3 kinds of surgeries of temporomandibular joint ankylosis].

OBJECTIVE: To determine the distribution features of surface stress on the bilateral condyles of the normal mandible and the unilateral mandible model for 3 kinds of surgeries of temporomandibular joint ankylosis under normal occlusal strength. METHODS: The normal mandible 3-dimensional finite element model and the 3-dimensional finite element mandible model for 3 kinds of surgeries of unilateral temporomandibular joint ankylosis were built. The jaw triangle area and the condyle transverse ridge was fixed and bound at mid-point. And 100 N of load was applied to the first molars of the bilateral mandible to obtain a von Mises stress diagram of the condylar surface and the unit von Mises stress of the condylar surface. Statistical analysis was carried out. RESULTS: The transmission of von Mises stress at the operated side of fractured articular arthroplasty of the condylar was interrupted. The von Mises stress at the operated side of the condylar in the group of point-surface contact joint reconstruction was bigger than that of the normal group (P<0.05) whereas there was no significant difference between the group of surface-surface contact joint reconstruction group and the normal group (P>0.05). The von Mises stress of the condylar at the non-operated side in the group of fractured articular arthroplasty was bigger than that of the normal group (P<0.05), whereas there was no significant difference between the group of point-surface contact or surface-surface contact group and the normal group (P>0.05). CONCLUSION: The surface-surface articular reconstruction is beneficial to the recovery of the masticatory function without increasing the burden for contralateral temporomandibular joint and is helpful to the joint. Thus this surgical method is recommended.

Effects of stress rate and calculation method on subcritical crack growth parameters deduced from constant stress-rate flexural testing.

OBJECTIVES: To more efficiently determine the subcritical crack growth (SCG) parameters of dental ceramics, the effects of stressing rate and choice of statistical regression model on estimates of SCG parameters were assessed. METHODS: Two dental ceramic materials, a veneering material having a single critical flaw population (S) and a framework material having partially concurrent flaw populations (PC), were analyzed using constant stress-rate testing, or "dynamic fatigue", with a variety of testing protocols. For each material, 150 rectangular beam specimens were prepared and tested in four-point flexure according to ISO6872 and ASTM1368. A full-factorial study was conducted on the following factors: material, stress rate assumed vs. calculated, number of stress rates, and statistical regression method. RESULTS: The proportion of specimens for which the statistical models over-estimated reliability was not significantly different based on regression method for Material S (P = 0.96, power = 94%) and was significantly different based on regression method for Material PC (P < 0.001). The standard method resulted in SCG parameters, n and l nB, of 35.9 and -11.1 MPa(2)s for Material S and 12.4 and 9.61 MPa(2)s for Material PC, respectively. SIGNIFICANCE: The method of calculation that uses only the median strength value at each stress rate provided the most robust SCG parameter estimates. Using only two stress rates resulted in fatigue parameters comparable to those estimated using four stress rates having the same range. The stress rate of each specimen can be assumed to be the target stress rate with negligible difference in SCG parameter estimates.

Evaluation of multiple implant-bone parameters on stress characteristics in the mandible under traumatic loading conditions.

PURPOSE: The inter-relationships between various implant and bone parameters were evaluated for their influence on the von Mises stress distribution within the mandible using the finite element procedure. The maximum compressive stresses in cancellous and cortical bone were compared to the published stress-strain data to determine bone fracturing status when the maximum (traumatic) loading is applied. MATERIALS AND METHODS: Parameters considered herein include the implant diameter and length. Also considered are Young's modulus of cancellous bone and that of cortical bone, along with its thickness. The implant-bone system was modeled using two-dimensional plane strain elements, 50% osseointegration between implant and cancellous bone was assumed, and linear relationships were assumed between the stress value and Young's modulus of both cancellous and cortical bone at any specific point within the mandible. RESULTS: Implant length was more influential than implant diameter within cancellous bone, whereas implant diameter was more influential in cortical bone. A ranking of all the parameters indicated that the applied masticatory force had a more significant influence on the stress difference, in both cancellous and cortical bone, than all other parameters. Young's modulus of cortical bone and implant length were least influential in cancellous and cortical bone, respectively. Under traumatic loading, cancellous bone fractured for all parameter combinations. When all parameters were set to their average values, the cortical bone did not fracture under traumatic loading. However, it fractured if all the parameters were all set to the minimum values. CONCLUSION: Quantitative evaluation and ranking of the major implant and bone parameters will help provide practical guidelines that are useful for the design and testing of dental implants. The study may also be of interest to dental professionals in evaluating possible implant placement options under various clinical scenarios.

Morphologic classification and stress analysis of the mandibular bone in the premolar region for implant placement.

PURPOSE: The objective of this study was to assess the influence of mandibular morphology on the stress and strain distributions in the bone supporting an implant. MATERIALS AND METHODS: Subjects were selected from patients with missing mandibular premolar teeth. Computed tomographic scans of the premolar region were classified into nine groups based on the overall bone morphology and the relative position of the alveolar crest. Finite element models of the bone on the basis of this classification, with inserted implants and superstructures, were constructed. A load of 200 N applied 30 degrees buccal from the vertical axis was directed toward the occlusal surface. RESULTS: One hundred two partially (n = 98) and completely (n = 4) edentulous subjects were recruited. Their ages ranged from 40 to 79 years (mean age, 60.5 years; 41 men and 61 women). Approximately 56% of the patients showed bone widening in the caudal direction. The maximum compressive stresses were shown in the lingual cervical regions. An influence of overall bone morphology on stress distribution was not observed; however, the alveolar crest contour clearly affected the maximum stress and strain values. Compressive microstrain above 4,000 was observed only when the alveolar crest was lingually located relative to the body of the mandible. These high strain levels were concentrated at the lingual cervical margin, with volumes ranging from 0.04 to 0.07 mm3. CONCLUSION: The contour of the alveolar crest potentially plays a critical role in maintaining the bone levels surrounding implants.

Quantification of three-dimensional orthodontic force systems of T-loop archwires.

OBJECTIVE: To demonstrate the three-dimensional (3D) orthodontic force systems of three commercial closing T-loop archwires using a new method and to quantify the force systems of the T-loop archwires. MATERIALS AND METHODS: An orthodontic force tester (OFT) and a custom-made dentoform were developed to measure force systems. The system simulated the clinical environment for an orthodontic patient requiring space closure, which included measurement of three force components along, and three moment components about, three clinically defined axes on two target teeth. The archwires were attached to the dentoform and were activated following a standard clinical procedure. The resulting force system was measured using the OFT. RESULTS: The force systems of the T-loops on the teeth were 3D. Activation in one direction resulted in force and moment components in other directions (side effects). The six force and moment components as well as the moment-to-force ratios in the clinically defined coordinate system were quantified. CONCLUSIONS: The commercial archwires do not provide force systems for pure translation. Quantification of the force system is critical for the selection and design of optimal orthodontic appliances.

Measuring bond strength between fiber post and root dentin: a comparison of different tests.

PURPOSE: the aim of this study was to evaluate the ability of bond strength tests to accurately measure the bond strength of fiber posts luted into root canals. MATERIALS AND METHODS: the test methods studied were hourglass microtensile (HM), push-out (PS), modified pushout (MP), and pull-out (PL). The evaluated parameters were: bond strength values, reliability (using Weibull analysis), failure mode (using confocal microscopy), and stress distribution (using finite element analysis). Forty human intact single-rooted and endodontically treated teeth were divided into four groups. Each group was assigned one of the test methods. The samples in the HM and PS groups were 1.0 ± 0.1 mm thick; the HM samples were hourglass shaped and the PS samples were disk shaped. For the PL and MP groups, each 1-mm dentin slice was luted with a fiber post piece. Three-dimensional models of each group were made and stress was analyzed based on Von Mises criteria. RESULTS: PL provided the highest values of bond strength, followed by MP, both of which also had greater amounts of adhesive failures. PS showed the highest frequency of cohesive failures. MP showed a more homogeneous stress distribution and a higher Weibull modulus. CONCLUSION: the specimen design directly influences the biomechanical behavior of bond strength tests.

Comparison of an analytical expression of resin composite curing stresses with in vitro observations of marginal cracking.

PURPOSE: To observe the marginal failure at the enamel-resin composite interface upon curing, and utilize a recently developed analytical model for curing stresses to relate to the extent of interfacial failure. METHODS: Occlusal cavity preparations were restored with Heliomolar, Filtek Z100 or UltraSeal XT Plus resin composites. Teeth were restored with either bulk or incremental placement. The control group was not restored. Teeth were placed in an acrylic ring and embedded in cold mounting epoxy resin and the crown sectioned so that the tooth/restoration interface and cavosurface margin could be visualized with an optical microscope. A recently developed simplified analytical approach was utilized to evaluate the composite-enamel interface tensile stresses for these materials theoretically and thereby the fracture susceptibility of the resin-enamel interface during polymerization. RESULTS: White lines, enamel cracks and interfacial failure of the bond were evident for all three materials evaluated (P < 0.01). Gaps at the enamel-composite interface measured 1-10 microm and were more evident for Ultra Seal XT Plus and Z100 than Heliomolar. Conversely, cracking of the enamel adjacent to the interface was more evident for Heliomolar. Statistical analysis showed that enamel cracking and interfacial failure was significant for all groups (P < 0.01). An inverse relationship was noted between enamel cracking and interfacial failure (P < 0.05). The predictions for the extent of cracking from the mathematical modeling match the experimental data and prior studies.

Slumping during sculpturing of composite materials.

OBJECTIVES: This study investigated the slumping characteristics of four composite materials during sculpturing prior to their polymerization. METHODS: Four different composite materials were used to measure shape deformation due to slumping. Silicon impressions of the occlusal plane of three different molars were used as a mould for the composite samples. The surface of the samples was digitized with a laser scanner (400 slices, lateral resolution: 25 microm). Scans were made after 1-4 min. The 3D data sets were numerically superimposed with matching software and differences were calculated relative to the baseline measurement. RESULTS: The amount of surface deformation increases with increasing observation time. The average coefficient of variation was 0.2. The largest mean amount of slumping was observed for ELS with tooth mould 1 (150.0 microm), and for Clearfil Majesty with tooth mould 2 (98.3 microm) and mould 3 (42.8 microm). Miris 2 Dentin and Synergy D6 Enamel were rather similar and seem to exhibit little deformation. The slump flow of ELS and Clearfil Majesty was up to 400% higher than the formers. The deformation could be sorted in the following order "mould 3"<"mould 2"<"mould 1" for all materials and all observation time. There was a significant influence (p<0.05) of the three factors, time, mould and composite type (ANOVA). SIGNIFICANCE: This specific method provides a reproducible approach for the assessment of the handling characteristics of composite materials. The results can identify slumping differences and assist in collecting information about the feasibility of a material for certain indications.

Slumping resistance and viscoelasticity prior to setting of dental composites.

OBJECTIVES: The aim of this study was to develop a method for measuring the slumping resistance of resin composites and to evaluate the efficacy using rheological methodology. METHODS: Two commercial hybrid composites (Z100 and Z250) and a nanofill composite (Z350) were used to make disc-shaped specimens of 2mm thickness. Three kinds of aluminum molds with triangular, circular, and square shaped cutting surfaces were pressed onto the resin discs to make standardized imprints. The imprints were light-cured either immediately (non-slumped) or after waiting for 2min at 25 degrees C (slumped). White stone replicas were made and then scanned for topography using a laser 3-D profilometer. Slumping resistance index (SRI) was defined as the ratio of the groove depth of the slumped specimen to that of the non-slumped specimen. The pre-cure viscoelasticity of each composite was evaluated by an oscillatory shear test and normal stress was measured by a squeeze test using a rheometer. Correlation analysis was performed to investigate the relationship between the viscoelastic properties and SRI. RESULTS: SRI varied between the three materials (Z100