Impact and flexural strength, and fracture morphology of acrylic resins with impact modifiers.

OBJECTIVES: This study evaluated the impact and flexural strength and analyzed the fracture behavior of acrylic resins. METHODS: Eighteen rectangular specimens were fabricated of Lucitone 550, QC 20 (both unreinforced acrylic resins), Impact 1500 (extra strength impact), Impact 2000 (high impact) according to the manufacturers' instructions. The impact strength was evaluated in notched specimens (50x6x4mm) and flexural strength in unotched (64x10x3.3mm), using three-point bending test, as well as, stress at yield, Young modulus and displacement at yield. Fragments from mechanical tests were observed by SEM. Data from impact strength, stress at yield and displacement at yield were analyzed by 1-way ANOVA and Tukey test (alpha=0.05). Young modulus values were analyzed by One-way ANOVA and Dunnett T3 multiple comparisons test (alpha=0.05). RESULTS: Mean values of impact strength and stress at yield values were higher (P<.005) for Impact 2000 while Young modulus was higher (P<.05) for Lucitone 550; Impact 1500 and Impact 2000 showed significant values (P<.05) in the displacement at yield. Impact fractures of the all acrylic resins were brittle. Bending fractures of Lucitone 550 and Impact 2000 were brittle, QC 20 fractures were ductile and Impact 1500 showed brittle (75%) and ductile (25%) fractures. CONCLUSION: Within the limitations of this study, the Impact 2000 showed improved mechanical properties with high capacity of stress absorption and energy dissipation before the fracture.

Changes in the masticatory cycle after treatment of posterior crossbite in children aged 4 to 5 years.

INTRODUCTION: The aim of this study was to analyze the dynamics of the masticatory cycle after occlusal adjustment through coronoplasty on deciduous teeth (selective grinding) and composite resin occlusal guidance for posterior crossbite correction. METHODS: Nineteen children, aged 4 to 5 years, with functional posterior crossbites diagnosed at their clinical examinations, were selected. Analyses of masticatory cycles, including lateral excursive movements, were performed before and after treatment by using kinesiographic examinations in the 3 reference planes (frontal, horizontal, and sagittal). The examinations were done during bubble-gum chewing. The data were submitted to sign, Mann-Whitney, and paired t tests concerning the variables: left maximum lateral movement, right maximum lateral movement, total lateral movement (LM), vertical movement (VM), anteroposterior maximum movement, maximum lateral movement (MLM), and total maximum lateral movement (TMLM). RESULTS: The treatment produced increased TMLM, a significant increase of the MLM opposite the crossbite, no statistically significant difference of the crossbite side, and a significant decrease of LM. Left crossbite children showed significant decreases in the LM after treatment; right crossbite children showed significant increases in VM after treatment. CONCLUSIONS: Functional crossbite treatment with selective adjustment and occlusal guiding changed masticatory cycle patterns, producing larger horizontal and vertical planes. The cycles became more symmetric in the frontal plane, increasing the tendency of bilateral alternate mastication.

Effect of a denture cleanser on weight, surface roughness, and tensile bond strength of two resilient denture liners / Efeito do detergente na dentadura pelo peso, superfície rugosa, e força adesiva de tensão de dois revestimentos resilientes na dentadura

Purpose: This study evaluated the effects of a denture cleanser on weight change, roughness, and tensile bond strength on 2 denture resilient lining materials. Material and methods: Forty specimens of microwave acrylic resin (Onda-Cryl), 30 mm in diameter and 4 mm thick, were prepared to verify weight change and surface roughness. The specimens were divided into 4 groups consisting of 10 specimens each, relined with a resilient liner (Coe Soft or Dentusoft), and treated (Polident or tap water). To evaluate tensile bond strength, 12 specimens were used for each group. All specimens were stored in artificial saliva for 15 days, immersed once a day in Polident or tap water, and evaluated at 0 hours and after 24 hours and 7 and 15 days. Roughness was evaluated by use of a profilometer. Weight changes were recorded in milligrams and expressed as the percentage of weight difference between the periods of evaluation. Tensile bond strength was determined with a universal testing machine. The specimens were placed under tension until failure by a cross-speed of 5 mm/min with a 500 Kg load cell. The type of failure was determined by use of stereoscopic microscopy at original magnification ž8. The data were submitted to analysis of variance and compared by Tukey's test (alfa=.05). Results: Specimens immersed in Polident showed higher and significant (P<.05) weight changes (percent) than those immersed in water between 24 hours and 7 days (Coe Soft-Polident: 0.48 ± 0.09; Coe Soft-water: 0.28 ± 0.09; Dentusoft-Polident: 0.44 ± 0.16; Dentusoft-water: 0.22 ± 0.15) and between 7 and 15 days (Coe Soft-Polident: 0.08 ± 0.04; Coe Soft-water: - 0.17 ± 0.05; Dentusoft-Polident: 0.01 ± 0.04; Dentusoft-water: - 0.24 ± 0.03). There was significant difference (P<.05) in roughness (in micrometers), between treatments after 7 days (Coe Soft-water: 4.07 ± 0.22; Coe Soft-Polident: 3.36 ± 0.52; Dentusoft-water: 3.68 ± 0.72; Dentusoft-Polident: 3.26 ± 0.41) and 15 days (Coe Soft-water: 4.88 ± 0.29; Coe Soft-Polident: 3.53 ± 0.61; Dentusoft-water: 4.42 ± 1.12; Dentusoft-Polident: 3.68 ± 0.37). Tensile bond strength was highest (P<.05) after 15 days (Coe Soft-water: 5.19 ± 0.93; Coe Soft-Polident: 4.40 ± 0.38; Dentusoft-water: 4.42 ± 1.15; Dentusoft-Polident; 4.84 ± 1.14). Most failures were cohesive (Coe Soft: 76.04 percent and Dentusoft: 82.29 percent)...