Investigation of the effects of arch size and implant angulation on the accuracy of digital impression using two intraoral scanners: An in vitro study.

OBJECTIVES: The aim of this in vitro study was to evaluate the effect of arch size and implant angulation on the accuracy of digital impression in two intraoral scanners of Trios (3shape) and CEREC (Omnicam). MATERIAL AND METHODS: Four acrylic models each including six implants at sites 11, 12, 15, 17, 23, and 27 were used, including large with parallel implants, large with angled implants, small with parallel implants, and small with angled implants. After tightening the scan bodies, distance measurements were done using a coordinate measuring machine. Then, each model was scanned 10 times using each scanner. Trueness and precision measurements were finally computed. RESULTS: The trueness values ranged from 20 to 260 µm in CEREC Omnicam, and from 40 to 1030 µm in Trios. The precision values ranged from 30 to 190 µm in CEREC Omnicam, while from 50 to 770 µm in Trios. The multivariate test analysis indicated that the measured distances via two scanners and different models show different behaviors. Pairwise interactions between these three variables were significant (p < .05). Pairwise interactions between these variables were also significant. (p < .0001). CONCLUSIONS: Arch width could affect the accuracy of digital impression; by rotating toward the second quadrant and end points of the scan, errors have increased. However, the angulation of the implants had no effect on the accuracy of digital impression. The CEREC Omnicam scanner showed higher accuracy (trueness and precision) compared to the Trios (3shape) one.

Color Stability of Different Denture Teeth Following Immersion in Staining Solutions.

Objectives: This study aimed to assess the color change (∆E00) of 7 brands of denture teeth (conventional acrylic and composite teeth) following immersion in staining solutions. Materials and Methods: Maxillary central incisor denture teeth made of 4 conventional acrylic resins (Vitapan, SR Vivodent PE, Beta Star and Crystal) and 3 composite resins (Finex, Emeral and Phonares II) were randomly divided into four groups (n=5). Denture teeth of different brands were immersed in tea, coffee, cola, and turmeric solutions. The solutions were incubated at 37°C. The baseline color of the teeth was measured using an intraoral spectrophotometer. The color of the teeth was measured after 24 h (∆E12), 1 week (∆E13), 2 weeks (∆E14), and 1 month (∆E15). ∆E00 was calculated and analyzed using one-way ANOVA. Pairwise comparisons were performed by the Tukey's post-hoc test (P<0.05). Results: The color stability of all teeth was significantly affected by the solutions (P<0.001). The type of tooth and coloring solution had significant interactions at all times (P<0.05). Turmeric caused the maximum color change in all teeth after 1 month. Repeated measures ANOVA showed that ∆E00 of all teeth was significantly affected by the duration of immersion in the solutions (P<0.001). Conclusion: within the limitations of this study, 1-month immersion of denture teeth in coffee, tea and cola solution altered the ∆E values; however, they were within the acceptable range, except for Beta Star. Turmeric solution caused unacceptable color change in all denture teeth even after 24 h of immersion.

Marginal adaptation of three-unit interim restorations fabricated by the CAD-CA systems and the direct method before and after thermocycling.

BACKGROUND: Limited information is available regarding the marginal adaptation of three-unit interim restorations fabricated by different techniques from different materials. Also, the efficacy of computer-aided design/computer-aided manufacturing (CAD-CAM) systems for their fabrication is still questionable. This study aimed to assess the marginal adaptation of three-unit interim restorations fabricated by the CAD-CAM systems and the direct method from different materials before and after thermocycling. MATERIAL AND METHODS: A sound mandibular second premolar and a second molar were prepared for a three-unit all-ceramic restoration. Metal dies were fabricated to simulate a case of mandibular first molar missing, and were mounted in an acrylic block. Seventy-two three-unit interim restorations were fabricated by different techniques from different materials in six groups (n=12). In the first four groups, restorations were fabricated from Tempron, Visalys Temp, Unifast III, and Acropars by the direct technique, while the Amann Girrbach and Arum CAD-CAM systems and Ceramill Temp blocks were used in the last two groups. Marginal gap in each group was measured under a stereomicroscope at ×100 magnification. The specimens then underwent 5000 thermal cycles (5-55°C), and the marginal gap was measured again afterwards. ANCOVA and Bonferroni test (for pairwise comparisons) were applied for statistical analyses (α=0.05). RESULTS: Amann Girrbach and Arum CAD-CAM systems were not significantly different regarding the marginal gap of restorations (P=0.999). Among the traditional interim materials, Acropars showed significantly higher marginal gap than others (P<0.001). No significant difference was noted between other traditional materials. CAD-CAM interim materials showed significantly smaller marginal gap than traditional materials (P<0.001). CONCLUSIONS: The CAD-CAM interim materials yielded superior marginal adaptation in three-unit interim restorations compared with traditional interim materials. The type of CAD-CAM system had no significant effect on the final marginal adaptation of restorations. Key words:Dental marginal adaptation, interim dental prosthesis, CAD-CAM.

Microtensile bond strength of CAD/CAM-fabricated polymer-ceramics to different adhesive resin cements.

OBJECTIVES: This study evaluated the microtensile bond strength (µTBS) of polymer-ceramic and indirect composite resin with 3 classes of resin cements. MATERIALS AND METHODS: Two computer-aided design/computer-aided manufacturing (CAD/CAM)-fabricated polymer-ceramics (Enamic [ENA; Vita] and Lava Ultimate [LAV; 3M ESPE]) and a laboratory indirect composite resin (Gradia [GRA; GC Corp.]) were equally divided into 6 groups (n = 18) with 3 classes of resin cements: Variolink N (VAR; Vivadent), RelyX U200 (RXU; 3M ESPE), and Panavia F2 (PAN; Kuraray). The µTBS values were compared between groups by 2-way analysis of variance and the post hoc Tamhane test (α = 0.05). RESULTS: Restorative materials and resin cements significantly influenced µTBS (p < 0.05). In the GRA group, the highest µTBS was found with RXU (27.40 ± 5.39 N) and the lowest with VAR (13.54 ± 6.04 N) (p < 0.05). Similar trends were observed in the ENA group. In the LAV group, the highest µTBS was observed with VAR (27.45 ± 5.84 N) and the lowest with PAN (10.67 ± 4.37 N) (p < 0.05). PAN had comparable results to those of ENA and GRA, whereas the µTBS values were significantly lower with LAV (p = 0.001). The highest bond strength of RXU was found with GRA (27.40 ± 5.39 N, p = 0.001). PAN showed the lowest µTBS with LAV (10.67 ± 4.37 N; p < 0.001). CONCLUSIONS: When applied according to the manufacturers' recommendations, the µTBS of polymer-ceramic CAD/CAM materials and indirect composites is influenced by the luting cements.

Effect of Selected Luting Agents on the Retention of CAD/CAM Zirconia Crowns Under Cyclic Environmental Pressure.

OBJECTIVES: The aim of this study was to evaluate the retention strength of zirconia crowns luted with two types of resin cement under environmental pressure changes. MATERIALS AND METHODS: Thirty zirconia crowns were fabricated by using computer-aided design/computer-aided manufacturing (CAD/CAM) system and were cemented by Panavia F2.0 (PAN), hand-mixed RelyX Unicem (UNH), or auto-mix RelyX Unicem Aplicap (UNA) cements on the corresponding extracted human molars. The samples were randomly divided into three groups according to the cement type. After 3000 thermal cycles, the cemented crowns were subjected to 24 pressure cycles (0 to 5 atmospheres). The retention force (N) of the specimens was measured in a universal testing machine. To normalize the retentive force, the recorded force was divided by the surface area of each tooth for measuring the retentive strength (MPa). The mean retention strengths (and forces) of the groups were compared by using one-way analysis of variance (ANOVA) and Tukey's honest significant difference (HSD) test (α=0.05). The failure modes were also examined by using a stereomicroscope. RESULTS: The retention values related to the evaluated resin cements were significantly different; the UNA group showed the highest retention strength (6.45±0.35 MPa) followed by the UNH (4.99±0.47 MPa) and PAN (4.45±0.39 MPa, P<0.001) groups. The adhesive failure mode was predominant in all the groups. CONCLUSIONS: The choice of resin cements and their mixing methods, which lead to differences in porosity, may affect the retention strength of zirconia crowns.

Fracture Resistance of Ceramic Laminate Veneers Bonded to Teeth with Class V Composite Fillings after Cyclic Loading.

PURPOSE: Porcelain laminate veneers (PLVs) are sometimes required to be used for teeth with composite fillings. This study examined the fracture strength of PLVs bonded to the teeth restored with different sizes of class V composite fillings. MATERIALS AND METHODS: Thirty-six maxillary central incisors were divided into three groups (n=12): intact teeth (control) and teeth with class V composite fillings of one-third or two-thirds of the crown height (small or large group, resp.). PLVs were made by using IPS e.max and bonded with a resin cement (RelyX Unicem). Fracture resistance (N) was measured after cyclic loading (1 × 106 cycles, 1.2 Hz). For statistical analyses, one-way ANOVA and Tukey test were used (α=0.05). RESULTS: There was a significant difference between the mean failure loads of the test groups (P=0.004), with the Tukey-HSD test showing lower failure loads in the large-composite group compared to the control (P=0.02) or small group (P=0.05). The control and small-composite groups achieved comparable results (P > 0.05). CONCLUSIONS: Failure loads of PLVs bonded to intact teeth and to teeth with small class V composite fillings were not significantly different. However, extensive composite fillings could compromise the bonding of PLVs.

Management of Limited Interocclusal Distance with the Aid of a Modified Surgical Guide: A Clinical Report.

In completely edentulous patients, limited interarch distance can compromise conventional prosthetic fabrication. Bone reduction through various surgical procedures has been recommended to restore an acceptable interarch distance. In such circumstances, a surgical guide built on a mounted cast can be used to minimize and control the amount of bone reduction performed. In the present report, an innovative method of fabrication of surgical guide has been described.

Practical guidelines for fabricating mandibular guide flange prostheses: A new technique for measuring the flange angulation.

BACKGROUND AND AIM: When a mandibulectomy causes discontinuity, the patient will need a rehabilitative prosthesis to achieve a proper occlusal relationship. TECHNIQUE: This article describes step-by-step guidelines for measuring the patient-specific mandibular guide flange angulation. In the presented technique, the flange angulation is determined by dividing the horizontal overlap of the maxillary posterior teeth plus the maxillary buccal clasp thickness by the vertical distance of the mandibular continuous clasp up to the maxillary buccal clasp. DISCUSSION: The mandibular guiding flange prosthesis must achieve an angulation that is appropriate for the particular circumstances of each patient to minimize the complications with mandibular deviation. CLINICAL RELEVANCE: The introduced method for measuring the patient-specific mandibular guide flange angulation can help prosthodontists to prepare the mandibular guiding flange prosthesis with higher accuracy and predictability.

Comparison of Flexural Strength of Resin Cements After Storing in Different Media and Bleaching Agents.

The aim of this study was to determine the effects of different storage media and bleaching treatments on the flexural strength of two resin cements (Panavia and BisCem). One hundred rectangular-shaped specimens were prepared with two resin cements and were stored in five media types (n = 10): distilled water (DW), lactic acid (LA), sodium hydroxide (NH), in-office bleaching (OB) and home bleaching (HB). There was significant interaction between the solutions and cements (p < 0.05). The lowest three-point flexural strength was found in sodium hydroxide for both cements (p < 0.05). Both cements exhibited significant increase in flexural strength following home and in-office bleaching (except Panavia in OB) (p < 0.05) compared with immersion in distilled water. Panavia recorded significantly higher flexural strength compared with BisCem in all media (p < 0.002), with the exception of in-office bleaching.

Effect of increased crown height on stress distribution in short dental implant components and their surrounding bone: A finite element analysis.

STATEMENT OF PROBLEM: Implants in posterior regions of the jaw require short dental implants with long crown heights, leading to increased crown-to-implant ratios and mechanical stress. This can lead to fracture and screw loosening. PURPOSE: The purpose of this study was to investigate the dynamic nature and behavior of prosthetic components and preimplant bone and evaluate the effect of increased crown height space (CHS) and crown-to-implant ratio on stress concentrations under external oblique forces. MATERIAL AND METHODS: The severely resorbed bone of a posterior mandible site was modeled with Mimics and Catia software. A second mandibular premolar tooth was modeled with CHS values of 8.8, 11.2, 13.6, and 16 mm. A Straumann implant (4.1×8 mm), a directly attached crown, and an abutment screw were modeled with geometric data and designed by using SolidWorks software. Abaqus software was used for the dynamic simulation of screw tightening and the application of an external load to the buccal cusp at a 75.8-degree angle with the occlusal plane. The distribution of screw load and member load at each step was compared, and the stress values were calculated within the dental implant complex and surrounding bone. RESULTS: During tightening, the magnitude and distribution of the preload and clamp load were uniform and equal at the cross section of all CHSs. Under an external load, the screw load decreased and member load increased. An increase in the CHS caused the corresponding distribution to become more nonuniform and increased the maximum compressive and tensile stresses in the preimplant bone. Additionally, the von Mises stress decreased at the abutment screw and increased at the abutment and fixture. CONCLUSIONS: Under nonaxial forces, increased CHS does not influence the decrease in screw load or increase in member load. However, it contributes to screw loosening and fatigue fracture by skewing the stress distribution to the transverse section of the implant.

Effect of the coefficient of friction and tightening speed on the preload induced at the dental implant complex with the finite element method.

STATEMENT OF PROBLEM: To prevent screw loosening, a clear understanding of the factors influencing secure preload is necessary. PURPOSE: The purpose of this study was to investigate the effect of coefficient of friction and tightening speed on screw tightening based on energy distribution method with exact geometric modeling and finite element analysis. MATERIAL AND METHODS: To simulate the proper boundary conditions of the screw tightening process, the supporting bone of an implant was considered. The exact geometry of the implant complex, including the Straumann dental implant, direct crown attachment, and abutment screw were modeled with Solidworks software. Abutment screw/implant and implant/bone interfaces were designed as spiral thread helixes. The screw-tightening process was simulated with Abaqus software, and to achieve the target torque, an angular displacement was applied to the abutment screw head at different coefficients of friction and tightening speeds. The values of torque, preload, energy distribution, elastic energy, and efficiency were obtained at the target torque of 35 Ncm. Additionally, the torque distribution ratio and preload simulated values were compared to theoretically predicted values. RESULTS: Upon reducing the coefficient of friction and enhancing the tightening speed, the angle of turn increased at the target torque. As the angle of turn increased, the elastic energy and preload also increased. Additionally, by increasing the coefficient of friction, the frictional dissipation energy increased but the efficiency decreased, whereas the increase in tightening speed insignificantly affected efficiency. CONCLUSION: The results of this study indicate that the coefficient of friction is the most influential factor on efficiency. Increasing the tightening speed lowered the response rate to the frictional resistance, thus diminishing the coefficient of friction and slightly increasing the preload. Increasing the tightening speed has the same result as reducing the coefficient of friction.

Dynamic nature of abutment screw retightening: finite element study of the effect of retightening on the settling effect.

STATEMENT OF PROBLEM: A fundamental problem in fully understanding the dynamic nature of screw loosening is lack of recognition of the entire process of screw tightening and retightening. PURPOSE: The purpose of this study was to explain the dynamic nature of abutment screw retightening by using finite element methods to investigate the effect of the coefficient of friction and retightening on the settling effect. MATERIAL AND METHODS: Precise computer models were designed of a Straumann dental implant, a directly attached crown, an abutment screw, and the bone surrounding the implant. All threaded interfaces were designed with a spiral thread helix with a specific coefficient of static and kinetic friction, and the surfaces were characterized as fine, regular, and rough. Abaqus software was used for dynamic simulation, which involved applying rotational displacement to the abutment screw and torque controlling during the steps of tightening, relaxation, retightening, and second relaxation and at different coefficients of friction. The obtained torque and preload values were compared to the predicted values. RESULTS: When surfaces changed from fine to rough, the remaining torque and preload decreased, and the settling effect increased. Upon retightening, the remaining torque and preload increased, and the settling effect also decreased. CONCLUSIONS: The reduction of the coefficient of friction contributes to increases in the preload and decreases in the settling effect. Retightening reduced the settling effect and had an insignificant effect on the preload. At high coefficients of friction, the retightening effect was intensified.

Effects of impression levels and trays on the accuracy of impressions taken from angulated implants.

BACKGROUND AND PURPOSE: It is crucial to keep the misfit of the abutment-fixture unit at the lowest possible rate. There are a few controversial studies on the accuracy of impression making of angulated implants, and much fewer (and controversial) studies on the abutment-level impression technique, which is a convenient and clinically favorable method. Besides, there are no studies on comparison of sectional vs. full-arch trays. We aimed to assess these. METHODS: A trapezoidal model with four angulated implants installed at 20° and 30° buccal tilts was fabricated. Forty impressions were taken from this model, with two groups of full-arch and sectional custom trays (n = 2 × 20), each divided into two subgroups of implant-level and abutment-level techniques (n = 2 × 2 × 10 in four subgroups). Absolute and non-absolute linear and angular impression errors were estimated by comparing the fabricated casts with the model, using a coordinate measuring machine. The effects of sectional/full-arch trays and abutment-level and fixture-level techniques on impression accuracies were analyzed using one- and two-way analyses of variance (ANOVA), Tukey, Mann-Whitney, and one-sample t-tests (α = 0.05, Mann-Whitney's α using the Bonferroni Bonferroni method). RESULTS: No significant differences between the absolute linear errors of the two trays (P = 0.100 [ANOVA]) and the two levels (P = 0.400 [ANOVA]) were observed. The assessment of absolute angular errors showed no significant differences (all P values ≥ 0.4 [ANOVA]). The difference between the linear errors in the full-arch vs. sectional trays was not significant in the fixture-level group (P = 0.290). However, in the abutment-level group, the linear error was significantly greater in the sectional tray compared to full-arch tray (P = 0.013, α = 0.025 [Mann-Whitney]). CONCLUSIONS: Using sectional trays might not be advantageous over full-arch trays. Sectional trays are not recommended for taking abutment-level impressions. The abutment-level impression technique is as accurate as the studied fixture-level technique. Increasing the angle of implants' divergence from 40° to 60° might not usually lead to a significant increase in the errors, particularly when using abutment-level impressions.

In vitro evaluation of the fracture resistance and microleakage of porcelain laminate veneers bonded to teeth with composite fillings after cyclic loading.

PURPOSE: There is insufficient data regarding the durability of porcelain laminate veneers bonded to existing composite fillings. The aim of the present study was to evaluate the fracture resistance and microleakage of porcelain laminate veneers bonded to teeth with existing composite fillings. MATERIALS AND METHODS: Thirty maxillary central incisors were divided into three groups (for each group, n=10): intact teeth (NP), teeth with class III composite fillings (C3) and teeth with class IV cavities (C4). Porcelain laminate veneers were made using IPS-Empress ceramic and bonded with Panavia F2 resin cement. The microleakage of all of the specimens was tested before and after cyclic loading (1 × 10(6) cycles, 1.2 Hz). The fracture resistance values (N) were measured using a universal testing machine, and the mode of failure was also examined. The statistical analyses were performed using one-way ANOVA and Tukey post hoc tests (α=.05). RESULTS: There was a significant difference in the mean microleakage of group C4 compared with group NT (P=.013). There was no significant difference in the fracture loads among the groups. CONCLUSION: The microleakage and failure loads of porcelain laminate veneers bonded to intact teeth and teeth with standard class III composite fillings were not significantly different.

Active tactile sensibility of single-tooth implants versus natural dentition: a split-mouth double-blind randomized clinical trial.

BACKGROUND: Unlike passive sensitivity of implants/teeth that is assessed more, only three controversial studies have compared active tactile sensibility (ATS) of implants and teeth. PURPOSE: We aimed to explore the difference between the ATS of teeth and single-tooth implants. METHODS: The ATS of single-tooth implants and contralateral teeth was measured in 25 patients after they bit on gold and placebo foils 0- to 70-µm thick, each for five times, in a random order blinded to patients and assessor, carried out at two sessions. Based on the experimental range of 0 µm (mock trials) to 70 µm, the sigmoid shape of psychometric curve was estimated to locate the 50% values as the ATS thresholds for each tooth or implant. ATS Data were analyzed using paired and unpaired t-tests and multiple linear regression (α = 0.05, ß ≤ 0.1). Also, equivalence testing approach was used to assess semi-objectively the clinical significance. RESULTS: Average ATS values for teeth and implants were 21.4 ± 6.55 µm and 30.0 ± 7.55 µm, respectively (p = .0001 [paired t-test]). None of the geometric characteristics of implants nor duration of implant in function were correlated with the ATS (p > .4 [regression]). Age was positively associated with the ATS of both implants and teeth (p ≤ .019 [regression]). Tooth ATS (but not implant ATS) was significantly higher in males compared with females (p = .050 [unpaired t-test]), which contributed to a generalizable tooth-implant difference higher than 8-µm clinical equivalence margin in females. The ATS was not significantly different between arches or between anterior/posterior regions (p > .6). CONCLUSION: There was a slight but statistically significant difference between implant and tooth tactile sensitivities.

Effect of Resin Cement Mixing Method on the Retention Strength of a CAD/CAM Zirconia Crowns.

Several treatments have been suggested to improve the retention of zirconia-based restorations luted with different cements. Resin cements are believed to improve crown retention under certain circumstances. The aim of the present study was to examine the effect of three cements with different mixing methods on the retention of CAD/CAM zirconia crowns. Thirty extracted human molars were randomly divided into three groups and prepared for all-ceramic crowns (6° taper, 4-mm height and a 1.2 mm rounded shoulder finish line). A zirconia crown (Tizian CAD/CAM) was fabricated for each tooth. The crowns were air-abraded using airborne particles, adjusted, and cemented to the corresponding tooth with one of the following cements: Panavia F2 (PAN group), RelyX Unicem (UNH group) or RelyX Unicem Aplicap (UNA group). After 3,000 rounds of thermal cycling, retention was measured using a specific retentive jig and a universal testing machine. The retention strength was measured by dividing the retention force by the surface area of each tooth. The means of the pull-out test results for each group were compared using analysis of variance and Tukey's HSD test (α = 0.05). The mode of failure was examined using a stereomicroscope. The mean retention value was 6.45 (0.34) MPa for the UNA group, 4.99 MPa (0.47) for the UNH group, and 4.45 (0.39) for the PAN group; the differences among the three test groups were significant. A mixed failure was observed in 83.3 % of specimens, while no cohesive failure occurred in the crowns. Within the limitations of the present study, of the three tested cements, Relyx Unicem Aplicap cement was associated with the highest retention force for Tizian zirconia crowns.

Microleakage of different post systems and a custom adapted fiber post.

OBJECTIVE: The effects of closely adapting a prefabricated fiber to the post space remain unknown. The purpose of this study was to quantify the microleakages of a custom adapted fiber-reinforced post, a prefabricated quartz fiber post and a cast post using nondestructive methods. MATERIALS AND METHODS: Sixty-five extracted human premolars were endodontically treated and randomly divided into three groups (n=15), which were restored using a cast post-and-core, a custom adapted fiber post (Refropost) with a microhybrid microfiller resin composite (Gradia), or a prefabricated quartz fiber post (DT light post) and two groups of control (n=10). All groups were cemented using a dual polymerizing resin cement (Panavia F2.0). A composite core (Z100) was used for the fiber posts. The microleakage was calculated for the experimental and control groups before and after thermal cycling and cycling loading using a radiotracer solution (thallium 201 chloride) and a gamma counter device. Data were subjected to statistical analysis of ANOVA and Tukey HSD at significant level of P< 0.05. RESULTS: Significantly lower microleakage values were found for the cast post-and-core (mean value =16.04 ×10(4)) and custom adapted fiber post groups (mean value=14.36×10(4)). Thermal cycling and cyclic loading had no significant effect on the microleakage value of any tested group. CONCLUSION: Post systems with improved adaptation showed similar microleakage to casting posts.

Verification of the mechanostat theory in mandible remodeling after tooth extraction: animal study and numerical modeling.

Bone tissue is capable of remodeling in response to environmental and physiological conditions. Mechanical loading is thought to be one of the most important environmental factors, affecting bone status. Mechanostat theory, which classifies bone behavior on the basis of the mechanical strain, has been vastly applied in cases of long bones. The present study surveyed the applicability of mechanostat theory in interpretation and anticipation of structural changes of the mandible due to tooth extraction, through experimental measurements and numerical modeling. The mandibular left first molar tooth of a 2-year-old male mongrel dog was extracted. Computed tomography (CT) images of the mandible were taken before, immediately after, and 1 year after extraction. Finite element models were constructed from the CT images and analyzed to evaluate the von Mises strain distribution within the mandible, specifically near the extraction site. Marked resorption around the extraction site and significant height reductions in the buccal and lingual ridges around the socket were observed one year after tooth removal. The deep socket made by extraction was filled and replaced by a smooth surface through bone remodeling in response to masticatory loading. Resultant strain analyses revealed a noticeable decline in strain level around the socket after tooth extraction, which activated bone remodeling according to mechanostat theory. At 1-year post-extraction, the strain magnitudes had increased significantly to near pre-extraction values. Alterations in the strain magnitudes along the mesio-distal axis were quantified for each stage and compared with the Mechanostat predefined regions. The FE results were surveyed in combination with the ridge height reductions, along the mesio-distal direction. Strain contours of the post-extraction stages were compatible with predictions by mechanostat theory. These findings validate the ability of mechanostat theory to describe mandible remodeling and predict post-extraction structural changes that occur near the extraction site. These results could be applied in dental treatment planning and implant design.

The effect of thermocycling on tensile bond strength of two soft liners.

OBJECTIVE: Failure of soft liners depends mostly on separation from the denture base resin; therefore measurement of the bond strength is very important. The purpose of this study was to compare the tensile bond strength of two soft liners (Acropars, Molloplast-B) to denture base resin before and after thermocycling. MATERIALS AND METHODS: Twenty specimens fromeach of the two different soft liners were processed according to the manufacturer's instructions between two polymethyl methacrylate (PMMA) sheets. Ten specimens in each group were maintained in 37°C water for 24 hours and 10 were thermocycled (5000 cycles) among baths of 5° and 55°C. The tensile bond strength was measured using a universal testing machine at a crosshead speed of 5 mm/min. Mode of failure was determined with SEM (magnification ×30). Two-way ANOVA was used to analyze the data. RESULTS: The mean and standard deviation of tensile bond strength of Acropars and Molloplast-B before thermocycling were 6.59±1.85 and1.51±0.22 MPa, respectively and 5.89±1.52 and1.37±0.18 MPa, respectively after thermocycling. There was no significant difference before and after thermocycling. Mode of failure in Acropars and Molloplast-B were adhesive and cohesive, respectivley. CONCLUSION: The bond strength of Acropars was significantly higher than Molloplast-B (P<0.05).