Failure probability, stress distribution and fracture analysis of experimental screw for micro conical abutment

Abstract The aim of this study was to evaluate the failure probability of two types of abutment screws after compressive load and to analyze the stress distribution with finite element method. Sixty (60) single-tooth implant restorations were assembled on titanium implants (e-fix, A.S. Technology - Titanium Fix). The groups were divided into Conventional screw (Screw neck 1.5 ø mm) and Experimental screw (Screw neck constricted with 1.2 ø mm). Specimens were subjected to single load to failure with compressive test according ISO 14801. The fractured specimens were subjected to stereomicroscopy for measurement of remaining screws inside the implant and characterization of fracture origin. Representative specimens were analyzed by scanning electronic microscopy. For finite element method (FEM), an identical 3D model of the two in vitro test groups were used with similar conditions (30º, 100 N load). The stress in the abutment screw was analyzed by von-Mises criteria. The results of strength means were 4132.5 ± 76 MPa and 4528.2 ± 127.2 for conventional and experimental groups, respectively. During microscopy, the mean (mm) of the remaining screw piece inside the implants were 0.97 ± 0.23 and 1.32 ± 0.12 for conventional and experimental groups, respectively. In FEM, the conventional group showed stress concentered in an unfavorable region (peak of 39.23 MPa), while the experimental group showed more stress areas but less concentration than the conventional group (36.6 MPa). In using the tested experimental geometry, the abutment screw can have its strength improved, and the origin of failure can be more favorable to clinical resolution.

Resumo O objetivo deste estudo foi avaliar a probabilidade de falha de dois tipos de parafusos para pilar protético após a compressão e analisar a distribuição da tensão com o método dos elementos finitos. Sessenta (60) restaurações unitárias foram montadas em implantes de titânio (e-fix, A.S. Technology - Titanium Fix). Os grupos foram divididos em parafusos convencionais (parafuso de pescoço 1,5 ø mm) e parafuso experimental (parafuso de pescoço estreitado com 1,2 ø mm). As amostras foram sujeitas ao teste de compressão de acordo com ISO 14801. Os espécimes fraturados foram submetidos a estereomicroscopia para a mensuração dos parafusos restantes dentro do implante e caracterização da origem da fratura. Os espécimes representativos foram analisados ​​por microscopia eletrônica de varredura. Para o método de elementos finitos (FEM), utilizou-se um modelo 3D idêntico dos dois grupos de teste in vitro com condições semelhantes (30º, 100 N). A tensão no parafuso do pilar foi analisada pelo critério de von-Mises. Os resultados de resistência a compressão foram 4132,5 ± 76 MPa e 4528,2 ± 127,2 para grupos convencionais e experimentais, respectivamente. Durante a microscopia, a média do remanescente do parafuso restante dentro dos implantes foi de 0,97 ± 0,23 e 1,32 ± 0,12 mm para os grupos convencionais e experimentais, respectivamente. Em FEM, o grupo convencional mostrou tensão concentrada em uma região desfavorável (pico de 39,23 MPa), enquanto o grupo experimental apresentou mais áreas de tensão, porém menor concentração do que o grupo convencional (36,6 MPa). Ao usar a geometria experimental testada, o parafuso do pilar pode ter sua resistência melhorada e a origem da falha pode ser mais favorável à resolução clínica.

Effect of hydrofluoric acid concentration and etching time on resin-bond strength to different glass ceramics

Abstract: The objective of this study was to evaluate the effect of the hydrofluoridric acid (HF) concentration and time of acid conditioning on bond strength of three glass ceramics to a resin cement. Thus, fifty blocks (10 mm x 5 mm x 2 mm) of each ceramic (LDCAD: IPS e.max CAD; LCAD: IPS Empress CAD and LDHP: IPS e.max Press) were made and embedded in acrylic resin. The surfaces were polished with sandpaper (#600, 800, 1000, and 1200 grits) and blocks were randomly divided into 15 groups (n = 10) according to the following factors: Concentration of HF (10% and 5%), conditioning time (20 s and 60 s) and ceramic (LDCAD, LDHP, and L). After conditioning, silane (Prosil / FGM) was applied and after 2 min, cylinders (Ø = 2 mm; h = 2 mm) of dual resin cement (AllCem / FGM) were made in the center of each block using a Teflon strip as matrix and light cured for 40 s (1,200 mW/cm2). Then, the samples were thermocycled (10,000 cycles, 5/55°C, 30s) and submitted to the shear bond test (50 KgF, 0.5 mm/min). The data (MPa) were analyzed with 3-way ANOVA and Tukey's test (5%). Failure analysis was performed using a stereomicroscope (20x) and a scanning electron microscope (SEM). ANOVA revealed that the "concentration" factor (p = 0.01) and the interaction "acid concentration X ceramic" (p = 0.009) had a significant effect, however, the "ceramic" (p = 0.897) and "conditioning time" (p = 0.260) factors did not influence the results. The LDHP10%60s (10.98 MPa)aA* group presented significantly higher bond strength than LDHP10%20s (6.57 MPa)bA, LCAD5%20s (6,90 ±3,5)aB and LDHP5%60s (5.66 ± 2,9MPa)aA* groups (Tukey). Failure analysis revealed that 100% of specimens had mixed failure. In conclusion, etching with 5% HF for 20 seconds is recommended for lithium disilicate and leucite-reinforced CAD/CAM ceramics. However, for pressed lithium disilicate ceramic, 10% HF for 60 s showed significantly higher bond strength to resin cement.

Influence of ceramic material, thickness of restoration and cement layer on stress distribution of occlusal veneers

Abstract The aim of this study was to evaluate stress distribution in an occlusal veneer according to the restorative material, restoration thickness, and cement layer thickness. A tridimensional model of a human maxillary first molar with an occlusal veneer preparation was constructed using a modeling software of finite element analysis. The model was replicated 9 times to evaluate the factors: restoration thickness (0.6, 1.2, and 1.8 mm) and cement layer thickness (100, 200, and 300 μm). Then, each model received different restorative materials (High Translucency Zirconia - [YZHT], Lithium Disilicate - [LD], Zirconia Reinforced Lithium Silicate - [ZLS], Feldspathic - [F], and Hybrid Ceramic - [HC]), totaling forty-five groups. An axial load (600 N) was applied on the occlusal face for static structural analysis. Solids were considered isotropic, homogeneous, and linearly elastic. Contacts were considered perfectly bonded. Fixation occurred in the dental root and a mechanical static structural analysis was performed. Descriptive statistical analysis and one-way ANOVA (α =10%) were performed for tensile stress peak values in the restoration and cement layer. The difference between groups was compared using the Tukey's test with 10% significance to match the percentage of the mesh convergence test. According to the results, the cement layer thickness did not influence stress distribution in the restoration (p ≥ 0.10). The thicker the restoration, the higher the tensile stress concentration in the restoration. The graphs showed higher stress concentration in the YZHT, followed by LD, F, ZLS, and HC. Also, the restorative material influenced stress concentration on the cement layer, which decreased according to the sequence HC>YZHT>ZLS>LD>F. HC stood out for causing the least stress concentration in the restoration. Cement layer thickness did not interfere in the mechanical performance of the restorations.