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At least 739 records · Page 41Linked to original sources
PubMed2006

Robotic magnetic navigation for atrial fibrillation ablation.

OBJECTIVES: We assessed feasibility of magnetic catheter guidance in patients with atrial fibrillation (AF) undergoing circumferential pulmonary vein ablation (CPVA). BACKGROUND: No data are available on feasibility of remote navigation for AF ablation. METHODS: Forty patients underwent CPVA for symptomatic AF using the NIOBE II remote magnetic system (Stereotaxis Inc., St. Louis, Missouri). Ablation was performed with a 4-mm tip, magnetic catheter (65 degrees C, maximum 50 W, 15 s). The catheter tip was guided by a uniform magnetic field (0.08-T), and a motor drive (Cardiodrive unit, Stereotaxis Inc.). Left atrium maps were created using an integrated CARTO RMT system (Stereotaxis Inc.). End point of ablation was voltage abatement >90% of bipolar electrogram amplitude. RESULTS: Remote ablation was successful in 38 of 40 patients without complications. The median mapping and ablation time was 152.5 min (range, 90 to 380 min) but was much longer in the first 12 patients (192.5 min vs. 148 min; p = 0.012). Median ablation time was 49.5 min (range, 17 to 154 min), but it was much shorter in the last 28 patients than in the first 12 patients (49 min vs. 70 min; p = 0.021). Patients receiving remote ablation had longer procedure times than control patients (p < 0.001) with similar mapping time but shorter ablation time on right-sided pulmonary veins. Many more mapping points regardless of their location were collected remotely (p < 0.001). CONCLUSIONS: Remote magnetic navigation for AF ablation is safe and feasible with a short learning curve. Although all procedures were performed by a highly experienced operator, remote AF ablation can be performed even by less experienced operators.

Adult↗
PubMed2004

Numerical investigation of the intravascular coronary stent flexibility.

Nowadays stent therapy is widely adopted to treat atherosclerotic vessel diseases. The high commercial value of these devices and the high prototypation costs require the use of finite element analyses, instead of classical trial and error technique, to design and verify new models. In this paper, we explore the advantages of the finite element method (FEM) in order to investigate new generation stent performance in terms of flexibility. Indeed, the ability of the stent to bend in order to accommodate curvatures and angles of vessels during delivery is one of the most significant prerequisites for optimal stent performance. Two different FEM models, resembling two new generation intravascular stents, were developed. The main model dimensions were obtained by means of a stereo microscope, analyzing one Cordis BX-Velocity and one Carbostent Sirius coronary stent. Bending tests under displacement control in the unexpanded and expanded configuration were carried out. The curvature index, defined as the ratio between the sum of rotation angles at the extremes and the length of the stent, yielded comparative information about the capability of the device to be delivered into tortuous vessels and to conform to their contours. Results, expressed in terms of the bending moment-curvature index, demonstrated a different response for the two models. In particular the Cordis model showed a higher flexibility. Lower flexibility in the expanded configurations for both models was detected. However this flexibility depends on how the contact takes place between the different parts of the struts.

Coronary Vessels↗
PubMed2004

Muscles that influence knee flexion velocity in double support: implications for stiff-knee gait.

Adequate knee flexion velocity at toe-off is important for achieving normal swing-phase knee flexion during gait. Consequently, insufficient knee flexion velocity at toe-off can contribute to stiff-knee gait, a movement abnormality in which swing-phase knee flexion is diminished. This work aims to identify the muscles that contribute to knee flexion velocity during double support in normal gait and the muscles that have the most potential to alter this velocity. This objective was achieved by perturbing the forces generated by individual muscles during double support in a forward dynamic simulation of normal gait and observing the effects of the perturbations on peak knee flexion velocity. Iliopsoas and gastrocnemius were identified as the muscles that contribute most to increasing knee flexion velocity during double support. Increased forces in vasti, rectus femoris, and soleus were found to decrease knee flexion velocity. Vasti, rectus femoris, gastrocnemius, and iliopsoas were all found to have large potentials to influence peak knee flexion velocity during double support. The results of this work indicate which muscles likely contribute to the diminished knee flexion velocity at toe-off observed in stiff-knee gait, and identify the treatment strategies that have the most potential to increase this velocity in persons with stiff-knee gait.

Gait↗
PubMed2004

Bending properties, porosity, and ash fraction of black bear (Ursus americanus) cortical bone are not compromised with aging despite annual periods of disuse.

In many species, including humans, disuse causes an imbalance in bone remodeling that leads to increased bone porosity as a result of increased bone resorption and decreased bone formation. However, black bears (Ursus americanus) may not develop disuse osteopenia, to the extent that other animals do, during long periods of disuse (i.e. hibernation) because they maintain osteoblastic bone formation during hibernation, even though bone resorption is increased during hibernation. Black bears may also have a mechanism to rapidly and completely recover the bone lost (by increased resorption during hibernation) during their remobilization period. Our findings suggest that cortical bone bending strength (211-328 MPa), bending modulus (16.0-29.5 MPa), fracture energy (0.0118-0.0205 J mm(-2)), porosity (2.3-7.1%), and ash fraction (0.638-0.672) are not compromised with age in black bears, despite annual periods of disuse. In fact, the ultimate strength (p=0.01), modulus (p=0.04), and ash fraction (p=0.03) of cortical bone were shown to significantly increase with age (2-14 yrs). Female bears give birth and nurse during hibernation; however, we found no significant (p>0.16) differences between male and female bone properties. Other animals require remobilization periods 2-3 times longer than the immobilization period to recover the bone lost during disuse. Our findings support the idea that black bears, which hibernate 5-7 months annually, have evolved a biological mechanism to mitigate the adverse effects of disuse on bone porosity and mechanical behavior.

Adaptation, Physiological↗
PubMed2005

Passive and active mechanical properties of the superficial and deep digital flexor muscles in the forelimbs of anesthetized Thoroughbred horses.

The superficial (SDF) and deep digital flexor (DDF) muscles are critical for equine forelimb locomotion. Knowledge of their mechanical properties will enhance our understanding of limb biomechanics. Muscle contractile properties derived from architectural-based algorithms may overestimate real forces and underestimate shortening capacity because of simplistic assumptions regarding muscle architecture. Therefore, passive and active (=total - passive) force-length properties of the SDF and DDF muscles were measured directly in vivo. Muscles from the right forelimbs of four Thoroughbred horses were evaluated during general anesthesia. Limbs were fixed to an external frame with the muscle attached to a linear actuator and load cell. Each muscle was stretched from an unloaded state to a range of prefixed lengths, then stimulated while held at that length. The total force did not exceed 4000 N, the limit for the clamping device. The SDF and DDF muscles produced 716+/-192 and 1577+/-203 N maximum active isometric force (F(max)), had ascending force-length ranges (R(asc)) of 5.1+/-0.2 and 9.1+/-0.4 cm, and had passive stiffnesses of 1186+/-104 and 1132+/-51 N/cm, respectively. The values measured for F(max) were much smaller than predicted based on conservative estimates of muscle specific tension and muscle physiological cross-sectional area. R(asc) were much larger than predicted based on muscle fiber length estimates. These data suggest that accurate prediction of the active mechanical behavior of architecturally complex muscles such as the equine DDF and SDF requires more sophisticated algorithms.

Algorithms↗
PubMed2005

A comparison of mechanical properties derived from multiple skeletal sites in mice.

Laboratory mice provide a versatile experimental model for studies of skeletal biomechanics. In order to determine the strength of the mouse skeleton, mechanical testing has been performed on a variety of bones using several procedures. Because of differences in testing methods, the data from previous studies are not comparable. The purpose of this study was to determine which long bone provides the values closest to the published material properties of bone, while also providing reliable and reproducible results. To do this, the femur, humerus, third metatarsal, radius, and tibia of both the low bone mass C57BL/6H (B6) and high bone mass C3H/HeJ (C3H) mice were mechanically tested under three-point bending. The biomechanical tests showed significant differences between the bones and between mouse strains for the five bones tested (p < 0.05). Computational models of the femur, metatarsal, and radius were developed to visualize the types of measurement error inherent in the three-point bending tests. The models demonstrated that measurement error arose from local deformation at the loading point, shear deformation and ring-type deformation of the cylindrical cross-section. Increasing the aspect ratio (bone length/width) improved the measurement of Young's modulus of the bone for both mouse strains (p < 0.01). Bones with the highest aspect ratio and largest cortical thickness to radius ratio were better for bending tests since less measurement error was observed in the computational models. Of the bones tested, the radius was preferred for mechanical testing because of its high aspect ratio, minimal measurement error, and low variability.

Animals↗
PubMed2005

Strain and elongation of the human gastrocnemius tendon and aponeurosis during maximal plantarflexion effort.

Regarding the strain and elongation distribution along the tendon and aponeurosis the literature is reporting different findings. Therefore, the purpose of this study was to examine in vivo the elongation and the strain of the human gastrocnemius medialis tendon and aponeurosis simultaneously at the same trial during maximal voluntary plantarflexion efforts. Twelve subjects participated in the study. The subjects performed isometric maximal voluntary contractions of their left leg on a Biodex-dynamometer. The kinematics of the leg were recorded using the Vicon 624 system with 8 cameras operating at 120 Hz. Two ultrasound probes were used to visualise the tendon (myotendinous junction region) and the distal aponeurosis of the gastrocnemius medialis respectively. The main findings were: (a) the absolute elongation of the gastrocnemius medialis tendon was different to that of the aponeurosis, (b) the strain of the gastrocnemius medialis tendon did not differ from the strain of the aponeurosis, (c) during the "isometric" plantarflexion the ankle angle exhibited significant changes, and (d) the non-rigidity of the dynamometer arm-foot system and the coactivity of the tibialis anterior both have a significant influence on the moment exerted at the ankle joint. Thus the strain of the human gastrocnemius medialis tendon and aponeurosis estimated in vivo using two-dimensional ultrasonography is uniform. To calculate the elongation of the whole tendon it is necessary to multiply the strain calculated for the examined part of the tendon by the total length of the tendon.

Achilles Tendon↗
PubMed2005

Estimation of the binding force of the collagen molecule-decorin core protein complex in collagen fibril.

Decorin belongs to the small leucine proteoglycans family and is considered to play an important role in extracellular matrix organization. Experimental studies suggest that decorin is required for the assembly of collagen fibrils, as well as for the development of proper tissue mechanical properties. In tendons, decorins tie adjoining collagen fibrils together and probably guarantee the mechanical coupling of fibrils. The decorin molecule consists of one core protein and one glycosaminoglycan chain covalently linked to a serine residue of the core protein. Several studies have indicated that each core protein binds to the surface of collagen fibrils every 67 nm, by interacting non-covalently to one collagen molecule of the fibril surface, while the decorin glycosaminoglycans extend from the core protein to connect to another decorin core protein laying on adjacent fibril surface. The present paper investigates the complex composed of one decorin core protein and one collagen molecule in order to obtain their binding force. For this purpose, molecular models of collagen molecules type I and decorin core protein were developed and their interaction energies were evaluated by means of the molecular mechanics approach. Results show that the complex is characterized by a maximum binding force of about 12.4 x 10(3) nN and a binding stiffness of 8.33 x 10(-8) N/nm; the attained binding force is greater than the glycosaminoglycan chain's ultimate strength, thus indicating that overloads are likely to damage the collagen fibre's mechanical integrity by disrupting the glycosaminoglycan chains rather than by causing decorin core protein detachment from the collagen fibril.

Biomechanical Phenomena↗
PubMed2005

Determination of trunk muscle forces for flexion and extension by using a validated finite element model of the lumbar spine and measured in vivo data.

Muscle forces stabilize the spine and have a great influence on spinal loads. But little is known about their magnitude. In a former in vitro experiment, a good agreement with intradiscal pressure and fixator loads measured in vivo could be achieved for standing and extension of the lumbar spine. However, for flexion the agreement between in vitro and in vivo measurements was insufficient. In order to improve the determination of trunk muscle forces, a three-dimensional nonlinear finite element model of the lumbar spine with an internal fixation device was created and the same loads were applied as in a previous in vitro experiment. An extensive adaptation process of the model was performed for flexion and extension angles up to 20 degrees and -15 degrees, respectively. With this validated computer model intra-abdominal pressure, preload in the fixators, and a combination of hip- and lumbar flexion angle were varied until a good agreement between analytical and in vivo results was reached for both, intradiscal pressure and bending moments in the fixators. Finally, the fixators were removed and the muscle forces for the intact lumbar spine calculated. A good agreement with the in vivo results could only be achieved at a combination of hip- and lumbar flexion. For the intact spine, forces of 170, 100 and 600 N are predicted in the m. erector spinae for standing, 5 degrees extension and 30 degrees flexion, respectively. The force in the m. rectus abdominus for these body positions is less than 25 N. For more than 10 degrees extension the m. erector spinae is unloaded. The finite element method together with in vivo data allows the estimation of trunk muscle forces for different upper body positions in the sagittal plane. In our patients, flexion of the upper body was most likely a combination of hip- and lumbar spine bending.

Adolescent↗
PubMed2004

The influence of alumina abrasion and cement lute on the strength of a porcelain laminate veneering material.

OBJECTIVES: The purpose of the current study was to examine whether alumina abrasion regime influenced the necessary surface roughness conducive to mechanical interlocking prior to composite resin bonding. The impact of alumina abrasion and cement lute on the bi-axial flexure strength and associated Weibull Moduli (m) on the cemented surfaces of disc specimens of porcelain laminate veneer (PLV) materials was therefore investigated. METHODS: Sets of 20 Vitadur-alpha dentine porcelain discs (15 mm diameter, 1.7 mm thickness) were condensed and prepared by abrading with 25, 50 or 110 microm alumina. Further specimen groups were stored wet or coated with a resin cement. Mean fracture strengths, standard deviations and associated m were determined using a ball-on-ring assembly. The surface roughness was assessed using profilometry and scanning electron microscopy. RESULTS: The one-way ANOVA revealed a significant reduction (P<0.05) between the means of the dry control and alumina abraded specimens. A significant increase in m was identified for 50 and 110 microm alumina abraded cemented specimens (6.1+/-1.4 and 6.0+/-1.3, respectively) compared with the 25 microm alumina abraded cemented and non-alumina abraded cemented controls (3.9+/-0.9 and 3.5+/-0.8, respectively). Increasing the alumina particle size utilised for the abrasion resulted in a decrease in the surface roughness (R(a) value) which is the arithmetic mean of the absolute departures of the roughness profile from the mean line. SIGNIFICANCE: Alumina abrasion acted to replace surface defects with a narrower distribution of defects. Composite resin polymerisation shrinkage may prevent surface flaws from being extended by healing the key surface defects and would appear to suggest that the use of a resin-based luting material may reduce the potential for crack propagation. Given the arrangement of ions across crack tips it is more likely that the resin composite cement enhanced strength by acting to heal key defects rather than imposing a compressive stress on the porcelain surface as suggested previously.

Aluminum Oxide↗
PubMed2004

Effect of sandblasting, grinding, polishing and glazing on the flexural strength of two pressable all-ceramic dental materials.

OBJECTIVES: During laboratory fabrication procedures and/or clinical adjustments, pressable materials: IPS Empress and Empress 2, may be ground, polished or sandblasted. These treatments may affect their strength by introducing microscopic flaws and defects. This study investigates the effect of these procedures on the mean flexural strength of these materials. METHODS: One hundred and forty disc specimens (14mmx1 mm) of IPS Empress and Empress 2 were prepared, and divided into seven groups of 20 specimens for each material. Groups were untreated, polished, polished and glazed, ground, ground and glazed, sandblasted, sandblasted and glazed. Surface roughness, mean biaxial flexural strength and Weibull modulus were appraised, and a scanning electron microscope was used to describe surface features. Statistical significance among groups of population was analysed using one-way Anova and Tukey's multiple comparison tests. RESULTS: Untreated and sandblasted groups showed significantly the highest roughness values, and polished the lowest for each material (p<0.05). Ground groups showed significantly lower roughness values than the sandblasted groups, and significantly higher roughness than the polished groups for each material (p<0.05). Polished groups for each material demonstrated significantly the highest mean flexural strength values (p<0.05). No significant difference in the mean strength values was found between untreated, sandblasted and ground groups for each material (p>0.05). Heat treatment had no effect on roughness or strength values of all treated groups of both materials. The Weibull modulus values for both materials varied with different treatments. They showed higher values for polished and untreated groups, and lower values for ground and sandblasted groups. CONCLUSIONS: Surface roughness may not be the only feature that determines strength. Other issues such as porosity, microstructural residual stresses, surface and bulk defects may also be pertinent.

Aluminum Silicates↗
PubMed2004

Determinants of in vitro gap formation of resin composites.

OBJECTIVES: To investigate whether polymerization shrinkage, flow, modulus, and bond strength influence marginal gap formation of resin composite restorations in vitro. METHODS: Eleven proprietary resin composites were studied. Shrinkage was measured by the 'bonded-disk method' (n=3). Flow was measured as the diameter of a constant volume of resin composite to which a load of 20 N had been applied for 60 s (n=3). Modulus was measured in 3-point bending (n=6). Bond strength mediated to human dentin by an adhesive system was measured in shear (n=6). Gap formation was measured in a light microscope along the margins of all-dentin, butt-joint cavities restored with the adhesive system and each resin composite and expressed as the widest gap in percent of the cavity diameter (n=6). RESULTS: Significant differences were found between the resin composites regarding all determined properties. A significant linear correlation was found between flow and gap formation (r=-0.68, P<0.025). Three-dimensional regression analysis showed a significant correlation between polymerization shrinkage (X(1)), flow (X(2)), and gap formation (r=0.79, P(1)<0.05, P(2)<0.005). The highest coefficient of correlation was found when the first part of the polymerization shrinkage (from 0 to 10 s) was disregarded (r=0.90, P(1)<0.005, P(2)<0.0005). CONCLUSIONS: With the adhesive system used, polymerization shrinkage and flow were found to be significant determinants of gap formation around resin composite restorations in vitro.

Analysis of Variance↗
PubMed2004

Polymerisation characteristics of resin composites polymerised with different curing units.

OBJECTIVES: The aim of this study was to compare the plasma arc light source Apollo 95E and the conventional halogen lamp Elipar Visio regarding a number of polymerisation characteristics of different resin composites. METHODS: Four different resin composites (Arabesk Top, Herculite XRV, Pertac II, Tetric) were irradiated using the Apollo 95E unit for one, two or three cycles of 3 s and using the Elipar Visio unit for 40 s. The investigated polymerisation characteristics were: flexural strength and modulus of elasticity, bond strength to dentine, depth of polymerisation, and quantity of remaining double bonds. The data were treated statistically by analysis of variance and by Scheffé test. RESULTS: The modulus of elasticity and the flexural strength resulting from curing with Apollo 95E for 1 x 3 s were equal to or less than those resulting from curing with Elipar Visio. The bond strength to dentine and the depth of polymerisation with Apollo 95E used for 1 x 3 s were equal to or less than that obtained with the conventional lamp, depending on the resin composite. Irradiation of Herculite XRV resulted in a higher quantity of remaining double bonds than did Elipar Visio. In general, two or three curing cycles of 3 s with Apollo 95E were necessary to produce mechanical properties not significantly worse than with 40 s of conventional curing. CONCLUSIONS: The efficiency of plasma arc curing with Apollo 95E strongly depends on the resin composite. For most resin composites tested, plasma arc curing for 3 s resulted in inferior mechanical properties as compared to conventional curing.

Composite Resins↗
PubMed2005

Influence of surface and heat treatments on the flexural strength of Y-TZP dental ceramic.

OBJECTIVE: The aim of the investigation was to assess the influence of sandblasting, grinding, grinding orientation, polishing and heat treatment on the flexural strength of a yittria stabilized tetragonal zirconia polycrystals ceramic (Y-TPZ). METHODS: The specimens (160 beams) were equally divided into four groups according to the surface treatment (sandblasted, polished, ground parallel to the tensile axis, ground perpendicular). Twenty specimens from each group underwent heat treatment under the firing conditions used to fire a layer of porcelain and glaze. After treatment, the three-point flexure test was used to calculate the flexural strength and X-ray diffraction analysis was used to estimate the relative amount of monoclinic phase. The reliability of strength was assessed through the Weibull distribution. Statistical analysis was conducted with multiple regression analysis, one-way ANOVA and Tukey's pairwise multiple comparisons. Treated and fractured surfaces were observed with SEM. RESULTS: The following values of strength and relative content of monoclinic phase of zirconia were measured for each group: sandblasted (1540MPa; 9.5%); ground parallel (1330MPa; 8.3%); ground perpendicular (1525MPa; 8.3%); ground parallel and heated (1225MPa; monoclinic content not detectable); ground perpendicular and heated (1185MPa; monoclinic content not detectable); polished and heated (1165MPa; monoclinic content not detectable); polished (1095MPa; 0.8%); sandblasted and heated (955MPa; 0.3%). CONCLUSIONS: The present study suggests that sandblasting and grinding may be recommended to increase the strength of dental Y-TZP, provided they are not followed by heat treatment. Fine polishing may remove the layer of compressive stresses and therefore, lower the mean flexural strength.

Analysis of Variance↗
PubMed2004

The in-vitro clinical failure of all-ceramic crowns and the connector area of fixed partial dentures: the influence of interfacial surface roughness.

OBJECTIVES: To assess the effect of interfacial surface roughness on the flexure strength and fracture mode and origin utilizing an in-vitro assessment of the clinical failure conditions expected for all-ceramic crowns and the connector area of fixed partial dentures (FPDs) using bilayered ceramic specimens tested in bi-axial flexure. METHODS: Sets of 20 bilayered composite discs, with core:dentine thickness ratio of 2:1 and interfacial surface roughnesses determined by alumina abrasion with different alumina particle sizes, were tested in bi-axial flexure with both the reinforcing core and veneering dentine loaded in tension. Mean flexure strengths, standard deviations and associated Weibull Moduli (m) were determined. Optical microscopy was employed for identification of the fracture mode and origin for the failure all-ceramic crowns and the connector area of FPDs. RESULTS: The interfacial surface roughness influenced the bi-axial flexure strength and reliability of the flexure strength data when both the reinforcing core and veneering dentine porcelain were tested in tension. The number of fracture fragments, frequency of occurrence of specimen delaminations, Hertzian cone formations and sub-critical radial cracking in the bilayered dental ceramic composite disc-shaped specimens was also dependent on the interfacial surface roughness and the surface loaded in tension. CONCLUSIONS: The fracture resistance, failure mode and failure origin in bilayered ceramics tested to represent the clinical failure mode of all-ceramic crowns and FPDs are dependent upon the interfacial surface roughness and the modulus of the material in tension.

Analysis of Variance↗
PubMed2006

The physical-mechanical performance of the new Ketac Molar Easymix compared to commercially available glass ionomer restoratives.

OBJECTIVES: To evaluate the time dependence of physical-mechanical performance of glass ionomer restoratives. METHODS: The physical-mechanical properties of Ketac Molar Easymix in comparison to four handmix glass ionomer restoratives were evaluated by testing the compressive and flexural strength, acid erosion and solubility. By testing the properties in the early and late setting phase it was determined how fast the level of material characteristics is achieved. IR spectroscopy was used for monitoring the acid-base reaction. RESULTS: The compressive strength after 1 and 24h setting time show that Ketac Molar Easymix and Fuji IX demonstrate a significantly higher performance. Ketac Molar Easymix exhibits by far the highest flexural strength 1h after start of setting. After 24h, it remains at this high level and only Vitro Molar and Fuji IX come close. Ketac Molar Easymix and Fuji IX show the lowest solubility in neutral media after 24h and 7d. It is remarkable that the solubilities of Ionofil Molar, Vitro Molar and Vidrion R stored in water for 24h are higher than those of Ketac Molar Easymix and Fuji IX obtained after 7d water immersion. In acidic media Ketac Molar Easymix is least prone to acid erosion. CONCLUSIONS: For clinical success of a filling material the early achievement of a high physical-mechanical performance is mandatory to minimize the risk of early damage to a restoration. With the high flexural strength reached after 1h combined with the lowest susceptibility for acid-attack and solubility in water, Ketac Molar Easymix, from 3M ESPE, provides the best performance of any materials tested.

Analysis of Variance↗
PubMed2005

The influence of surface modification techniques on the performance of a Y-TZP dental ceramic.

OBJECTIVES: The purpose of the current study was to examine the influence of the pre-cementation surface modification techniques, namely alumina abrasion and surface grinding, routinely employed by dental practitioners prior to cementation and placement of crown and bridge restorations on the performance of a Y-TZP dental ceramic. METHODS: Twelve sets of 30 Lava ceramic discs (13 mm diameter, 1.5mm thickness) supplied by the manufacturer were randomly selected. Six groups were abraded utilising 25, 50 and 110 microm alumina and stored dry or in a water bath at 37+/-1 degrees C for 24 h. Four groups were ground utilising a fine or a coarse grit diamond bur, specimens were ground dry or while using water as a coolant. The mean bi-axial flexure strengths, standard deviations and associated Weibull moduli (m) were determined. The surface roughness, hardness and phase composition were assessed utilising profilometry, the Vicker's hardness indentation and X-ray diffraction, respectively. RESULTS: No significant difference (P > 0.05) was identified in the bi-axial flexure strength of the 25, 50 and 110 microm alumina-abraded and the control specimens stored dry and wet for 24h. However, a significant increase in m was identified for the alumina-abraded specimens stored dry (10.7+/-1.9, 10.6+/-1.9 and 10.6+/-1.9) compared with the control (7.5+/-1.3) and the specimens stored in a water bath. In addition, the alumina abrasion regimes reduced the surface roughness compared with the controls. The coarse grinding regime significantly reduced both the bi-axial flexure strength and the associated m compared with the control whilst no significant difference was identified for the fine grinding regimes. The surface modification techniques initiated a phase transformation mechanism and resulted in the formation of a layer of compressive stresses on the surface of the disc-shaped specimens. CONCLUSIONS: The combination of the reduced surface roughness and the formation of a surface layer of compressive stress as a result of the alumina abrasion regimes investigated increased the reliability of the bi-axial flexure strength. The presence of water in the current study did not detrimentally influence the performance of the Y-TZP ceramic under investigation. Coarse grinding significantly reduced the bi-axial flexure strength and m due to the increased surface roughness. The Y-TZP specimens in the current investigation underwent a toughening mechanism as a result of a phase transformation mechanism which generated a transformation compressive stress that opposes the externally applied, crack-propagating tensile stress.

Air Abrasion, Dental↗
PubMed2006

Effect of relining, water storage and cyclic loading on the flexural strength of a denture base acrylic resin.

OBJECTIVES: This study investigated the effect of relining, water storage and cyclic loading on the ultimate flexural strength (FS(U)) and on the flexural strength at the proportional limit (FS(Pl)) of a denture base acrylic resin (Lucitone 550-L). METHODS: Rectangular bars of L were made (64 mm x 10 mm x 2 mm) and relined (1.3mm) with four relining resins (Kooliner-K, Ufi Gel Hard-UGH, Tokuso Rebase Fast-TR and New Truliner-NT). In addition, specimens relined with L and intact L specimens were made (64 mm x 10 mm x 3.3 mm). A three-point flexural test was applied on the specimens (n=10) after (1) polymerization; (2) water storage (30 days); (3) cyclic loading (10,000 cycles at 5 Hz) and (4) water storage (30 days)+cyclic loading. Data (MPa) were analyzed with three-way ANOVA and Tukey's HSD tests (alpha=0.05). To test for a possible correlation between FS(U) and FS(Pl), a linear regression coefficient "r" was calculated. RESULTS: After water storage, L-UGH and L-TR demonstrated an increased FS(U) (41.49-50.64 MPa and 49.95-57.36 MPa, respectively) (P<0.05). Only L-TR demonstrated an increased FS(Pl) (20.58-24.21 MPa) after water storage (P<0.05). L-L had the highest FS(U) (between 78.57 and 85.09 MPa) and FS(Pl) (between 31.30 and 34.17 MPa) (P<0.05). The cyclic loading decreased the FS(U) and FS(Pl) of all materials (P<0.05). Regression analysis showed a strong linear correlation between the two variables (r=0.941). CONCLUSIONS: Water storage improved the FS(U) of L-UGH and L-TR and the FS(Pl) of L-TR. L-L produced the highest FS(U) and FS(Pl). The FS(U) and FS(Pl) of all materials were detrimentally influenced by cyclic loading.

Acrylic Resins↗