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At least 1,099 records · Page 61Linked to original sources
PubMed1998

Effect of composite basing on the resistance to bulk fracture of industrial porcelain inlays.

OBJECTIVES: Bases are used in restorative dentistry for several reasons (i.e. isolation, elimination of undercuts, etc). Glass ionomers are the standard materials used as bases for porcelain inlays, despite the disadvantages of their mechanical properties. An alternative basing material is composite: a generous layer of posterior composite is cured and shaped in the cavity before an impression is taken. The composite basing technique has several clinical advantages. The aim of this study was to investigate the effect of the thickness of a composite base on the bulk fracture resistance of industrial porcelain, and to describe the procedure. METHODS: Fifteen porcelain (P) and 15 composite (C) bars, 1-, 2-, and 3-mm thick were joined to form 15 C/P bars, all 4-mm thick. Three groups were created: C 1 mm/P 3 mm (group 1), C 2 mm/P 2 mm (group 2), and C 3 mm/P 1 mm (group 3). The pairs were joined using Twinlook cement, subjected to a three-point bending test and loaded to fracture. The beam theory was used to support and explain the results. RESULTS: The fracture load means were: group 1, 197.7 +/- 18.7 N; group 2, 234.3 +/- 63.3 N, group 3, 336.3 +/- 31.3 N. Group 3 was significantly stronger than group 1 (P = 0.01) and group 2 (P = 0.03). Groups 1 and 2 were not statistically different. CONCLUSION: Composite basing is a tissue conserving method which may significantly increase the resistance to bulk fracture of adhesive porcelain inlays.

Acid Etching, Dental↗
PubMed1998

Stress-strain and thermal expansion characteristics of a phosphate-bonded investment mould material for dental super plastic forming.

OBJECTIVES: The stress-strain and thermal expansion characteristics of a phosphate-bonded investment material were measured to determine its suitability as a die material for superplastic forming of dental appliances. Titanium alloy denture bases and implant superstructures have been fabricated successfully using this investment material, but a greater understanding of its properties was sought to optimize the forming technique before exploitation could proceed. The effect of a boron nitride powder on the properties of phosphate-bonded investment was also investigated. METHODS: Three-point bend test specimens were cast in the phosphate-bonded investment material. Four groups of test samples were produced at specified water-to-powder ratios (w/p ratios). For one group 5 wt.% boron nitride powder was added to the investment powder before mixing with water. For stress-strain measurements specimens were heated to 920 degrees C and held for 30 min prior to the application of load. For thermal expansion measurements specimen length was measured for the entire heating and cooling cycle. RESULTS: Curves of stress versus strain and thermal expansion were recorded for all four test groups. For increasing initial w/p ratio failure stress decreased with lowest failure stresses for specimens containing boron nitride. Strain to failure also decreased with increasing initial w/p ratio, except for the group containing boron nitride which showed relatively large elongations to fracture. Specimen surfaces with higher initial w/p ratios were observed to have larger surface porosity. Thermal expansion curves for three groups were very similar, but differed considerably from the group containing boron nitride. CONCLUSIONS: The lowest initial w/p ratio should be used for highest hot strength. Hot deformation of phosphate-bonded investment dies for superplastic forming is an issue that requires further investigation because of potential die deformation and the resulting loss of fit between the superplastic superstructure and implant components. Boron nitride could be used in phosphate-bonded investment to decrease thermal expansion, but the consequences of reduced hot strength and increased strain to failure may make the modified material unsuitable as a die for superplastic forming.

Alloys↗
PubMed1998

The effect of cuspal flexure on a buccal Class V restoration: a finite element study.

OBJECTIVES: Failure of Class V restorations is a clinical occurrence which is often blamed on inadequate moisture control. However, the effects of occlusal forces and cuspal movement may also have an effect. The aim of this study was to examine the effects that cuspal movement had on the shear forces around a buccal Class V restoration in a lower first premolar. METHOD: A lower first premolar restored with a buccal Class V composite restoration and a Class I composite or amalgam restoration were modelled using the finite element method. The width of the occlusal restoration was varied from 2.1 to 3.7 mm and the depth varied from 1.7 to 3.7 mm. RESULTS: Compared to the unrestored premolar, the presence of an occlusal preparation restored with composite increased the interfacial forces around the buccal Class V restoration by 1-67%. Similarly, the presence of an amalgam occlusal restoration increased the interfacial forces by 9-228%. CONCLUSIONS: It was found that the presence of an occlusal restoration increased cuspal movement, which in turn increased the shear forces around the buccal Class V cavity. This effect was more pronounced with increases in cavity depth compared to cavity width, and when amalgam was the occlusal restorative material.

Bicuspid↗
PubMed1999

Flexural and thermal cycling of resins for veneering removable overlay dentures.

OBJECTIVES: This investigation compared the effect of flexural and thermal cycling upon resins, bonded via adhesive primers to cobalt-chromium alloy to identify appropriate materials for veneering overlay dentures which may flex in function. METHODS: The resins investigated were an acrylic resin, a crown and bridge resin, a urethane dimethacrylate and a hybrid composite resin systems. The first three were bonded via an adhesive primer/opaque system based upon 4-methacryloxyethyl trimellitate anhydride (4-META) and the last via one based on 10-methacryloxyldecyl dihydrogen phosphate (MDP). Cast alloy beams 30 x 4 x 0.8 mm with 2 mm resin veneers were stored in H2O for three days or three months, and subjected to 1000 thermo-cycles between 4 and 60 degrees C, and/or 5000 flexural cycles to a displacement of 0.1 mm. Three-point bend testing was carried out in an Instron 4505 UTM. The increase in the flexural moduli of the cast beams, due to the addition of the resin spines, together with the displacements and loads at yield were recorded. RESULTS: The adhesive resin systems varied in their abilities to withstand conditioning stresses, this appeared to reflect the rigidity of the resin component as well as the performance of the adhesive bond. CONCLUSIONS: While the composite/MDP system maintained the highest bond strength throughout the conditioning, its low displacement at yield indicates that it may be more suitable for rigid areas of a removable partial denture. Acrylic resin, with its high load and displacement at yield, make it the material of choice for veneering the more flexible saddle areas of partial dentures.

Acrylic Resins↗
PubMed1999

Comparison of the bond compatibility of titanium and an NiCr alloy to dental porcelain.

OBJECTIVES: The aim of this study was to investigate the bond compatibility between porcelain and titanium, by using three-point bending, oxide adherence and thermal expansion tests, and to compare the results with those of a conventional NiCr alloy-porcelain system. METHODS: The three-point bending test was used and the results were evaluated according to DIN draft 13927. Fractured surfaces of the metal and porcelain were examined macroscopically. The oxide adherence test was applied to titanium and NiCr alloy with appropriate oxidation degrees. After an oxide film was formed on the specimens, tensile strength test was applied. Oxide adherence strength values were set and fractured surfaces were examined macroscopically. In the thermal expansion test, thermal expansion curves and thermal expansion coefficients of titanium, NiCr alloy and tested porcelains were determined. Differences in thermal expansion values (delta alpha) in all metal-porcelain pairs were calculated to allow inferences to be made about residual stress levels. RESULTS: The bending strengths of all groups were found to be within the acceptable standard levels. At the end of the oxide adherence test, the results indicated that the adherence of the formed oxides to the metals were at a desired level. As a result of the thermal expansion test, the titanium-porcelain and NiCr-porcelain systems showed compressive thermal stress. However, the titanium-porcelain pair exhibited large positive delta alpha values. This results is found to be above the proposed thermal compatibility. CONCLUSIONS: It was concluded that the bond compatibility between titanium and porcelain was comparable with the NiCr-porcelain system.

Chromium Alloys↗
PubMed1999

The biaxial flexural strength of two pressable ceramic systems.

The biaxial flexural strength of Optimal pressable ceramics and IPS Empress pressable ceramics were tested, compared and the micro-structures and compositions explored. The materials evaluated were Optimal shaded (Opcs) and unshaded (Opcus) ceramics and IPS Empress shaded (Ems) and unshaded (Emus) ceramics. Twenty-one disc specimens per material were prepared, heat-treated and tested. The piston on three-ball test ASTM F394-78 (1991) was used to test the specimens in a universal testing machine at a crosshead speed of 0.15 mm/min. Specimens were viewed in a scanning electron microscope and X-ray diffraction used to determine the phases present. Mean strengths (MPa +/- SD) were: Ems 120.1 +/- 20.5, Emus 135.8 +/- 16.0, Opcs 139.1 +/- 14.3 and Opcus 138.0 +/- 11.5. There was no statistically significant difference between Optimal shaded, Optimal unshaded and Empress unshaded strength values (p > 0.05). Empress shaded strength values were significantly lower than the other materials tested (p < 0.05). Weibull analysis provided m values: Ems 6.1, Emus 10.2, Opcs 12.8 and Opcus 13.9 and 1% and 5% probabilities of failure. Secondary electron imaging revealed a dense dispersal of leucite crystals in the glassy matrix of the Optimal ceramics of an average size 5.5 +/- 9.7 micron 2 for the Optimal shaded ceramic and 6.6 +/- 13.3 micron 2 for the Optimal unshaded ceramic. Leucite crystal agglomerates were evident for the Empress shaded material and a uniform distribution of fine leucite crystals (1.9 +/- 1.8 micron 2) for the Empress unshaded ceramic. Crystal and matrix microcracking were present in most of the material microstructures, together with porosity and tabular alumina platelets in the Optimal ceramics. X-ray diffraction revealed the presence of tetragonal leucite and small amounts of cubic leucite. Optimal ceramics and Empress unshaded ceramic provided higher strength and Weibull m values compared with Empress shaded ceramic. Inherent material defects were characterised and cubic leucite was identified.

Aluminum Silicates↗
PubMed1999

An in vitro investigation of a poly(vinyl phosphonic acid) based cement with four conventional glass-ionomer cements. Part 1: Flexural strength and fluoride release.

OBJECTIVE: To investigate the flexural strength and fluoride release of four conventional glass-ionomer cements: Ketac-Molar (KM), HiFi (HF), Vivaglass Fil (VF), Ketac-Fil (KF) and a newly developed glass polyphosphonate cement, Diamond Carve (DC). METHOD: Disc specimens (10 mm diameter, 1 mm thick) were prepared and mould stored at 37 degrees C. After one hour, the specimens were removed from their mould and immersed in 20 ml of deionised water until required for testing. Biaxial flexural strength was determined at 1 hour and at 1, 7, 30 and 90 days after the start of mixing. Measurements of fluoride release from the specimens were carried out at 2 hours and at 1, 3, 7, 14, 30, 60 and 90 days after the start of mixing using a fluoride ion selective electrode. The results were analysed using ANOVA and student 't' tests. RESULTS: All the materials displayed different flexural strength patterns. KM and DC became stronger whilst KF and VF plateaued in strength with time. HF peaked in strength and then became weaker. At 90 days, the mean flexural strengths in decreasing order was as follows: KM > or = VF > or = DC > or = HF > KF. An initial fast rate of fluoride release followed by a slower but steady release of fluoride was observed in each of the materials. The mean cumulative fluoride release in decreasing order was as follows: VF > KF > or = HF > DC > KM. VF released significantly higher level and KM significantly lower level of fluoride than the other materials. CONCLUSIONS: The acid used to form the cement could not be used to predict changes in cement strength behaviour with respect to time. DC increased in strength with time and its flexural strength at 90 days was comparable to that of HF and VF. The cumulative and rate of fluoride release varied for the materials. DC had a low fluoride release consistent with a fast setting material with good early resistance to water.

Analysis of Variance↗
PubMed1998

The effect of casting conditions on the biaxial flexural strength of glass-ceramic materials.

OBJECTIVE: To assess the effect of mould and glass casting temperatures on the biaxial flexural strength (BFS) of two different types of castable glass-ceramic, using existing laboratory equipment and techniques. METHODS: Two castable glass-ceramic materials were evaluated. One glass (LG3) is based on SiO2-Al2O3-P2O5-CaO-CaF2, and is similar in composition to glasses used in the manufacture of glass-ionomer cements. The other glass (SG3) is based on SiO2-K2O-Na2O-CaO-CaF2, and is a canasite-based material. Both materials were used to produce discs of 12 mm diameter and 2 mm thickness using the same lost-wax casting process as used for metal castings. Mould temperatures of between 500 degrees C and 1000 degrees C and glass casting temperatures of between 1100 degrees C and 1450 degrees C were evaluated. The cast discs were cerammed and the biaxial flexural strength determined with a Lloyd 2000 R tester. RESULTS: A significant difference was found for the BFS in the range of mould temperatures evaluated, with the optimum investment mould temperature being 590 degrees C for LG3 and 610 degrees C for SG3 (p = 0.0002 and p = 0.019, respectively). No significant differences were seen between any of the glass casting temperatures evaluated. SIGNIFICANCE: The mould temperature for castable glass-ceramic materials produced using the lost-wax casting process can have a significant effect on BFS. The optimum mould temperature may differ slightly depending on the type of material being used. The glass casting temperature of these materials does not appear to have a significant effect on BFS.

Analysis of Variance↗
PubMed2002

Hydrogen and deuterium in myoglobin as seen by a neutron structure determination at 1.5 A resolution.

From the first days of protein neutron structure determination sperm whale myoglobin was an object under investigation [Nature 224 (1969) 143, J. Mol. Biol. 220 (1991) 381]. Nevertheless myoglobin is still of interest [Proc. Natl. Acad. Sci. USA 97 (2000) 3872]. The feasibility of the monochromatic neutron diffractometer BIX-3 at the JRR-3M reactor at the JAERI [J. Phys. Chem. Solids 60 (1999) 1623], to collect high-resolution diffraction data in a relatively short time stimulated us to repeat the structural determination of myoglobin. The structure of metmyoglobin has been determined up to a resolution of 1.5 A. The hydrogen atoms were replaced in part, by deuterium soaking the crystals for more than 10 years in D(2)O. A refinement of all atoms has been performed including the refinement of individual mean square displacements and occupancies of the exchangeable protons in backbone hydrogen bonds. A method is described to show clear negative scattering densities of the H atoms. Water molecules within the protein and on the molecule surface are shown. The exchangeability of H atoms is correlated with structural distribution and flexibility.

Animals↗
PubMed2000

Liners for waste containment constructed with class F and C fly ashes.

Hydraulic conductivity of a Class F fly ash containing residual organic carbon was evaluated in this study using laboratory and field tests. Compacted specimens of the Class F fly ash mixed with various materials (sand, Class C fly ash, and bottom ash) were prepared in the laboratory at various water contents and different compactive efforts. Hydraulic conductivity of the compacted specimens was measured using flexible-wall permeameters. A test pad was constructed to determine whether a low hydraulic conductivity liner could be constructed with Class F fly ash mixtures. Sealed double-ring infiltrometers and two-stage borehole permeameters were used to measure the field hydraulic conductivity of the test pad. Specimens were also removed from the test pad for hydraulic conductivity testing in the laboratory. Results of the study showed that mixtures of Class F and Class C fly ashes along with coarse aggregate can be compacted to hydraulic conductivities needed for landfill liners provided compaction is wet of optimum water content. The field tests showed that constructing a fly ash liner with hydraulic conductivities similar to those found in the laboratory is challenging, and requires careful attention to factors that result in cracks and permeable interlift regions that result in high field hydraulic conductivity. Leachate collected from the base of the test pad also showed that metal leaching must be considered when designing a liner with fly ash.

Carbon↗
PubMed1997

The effects of exercise on ligamentous stiffness in the wrist.

The purpose of this study was to determine if exercise alters wrist joint laxity, as measured by the mechanical behavior of the scaphoid bone. The load-displacement behavior of the scaphoid was studied in the palmar-dorsal direction in both wrists of 7 healthy volunteers (n = 14) before and after 2 exercise protocols (grip and push-up). When compared to the rested values, both exercise protocols significantly increased the displacement at 40 N by 47% (grip) and by 34% (push-up). Accordingly, the stiffness decreased significantly by 36% (grip) and by 32% (push-up). Partial recovery was documented after 1 hour of rest and there were no differences between any of the groups after 24 hours of rest. The increase in laxity documented during these exercise protocols reduces the ligament loads at comparable wrist positions and may thereby reduce the likelihood of traumatic ligamentous injury during participation in strenuous activity or sports.

Adult↗
PubMed1997

Percutaneous pinning of distal radius fractures: a biomechanical study.

Mechanical testing of extra-articular distal radius fractures stabilized with percutaneous pins was performed. Twelve fresh-frozen unembalmed radii stripped of all soft tissues were used. An osteotomy with dorsal comminution was made 2.5 cm proximal to the radial styloid. Three pin sizes and four pin configurations were used to fix the fracture, and the constructs were mechanically tested. Results show that at least a .062-inch pin was required for any significant changes in rigidity of the four pin configurations tested to be discerned. Cross-pinning with two radial styloid pins and placement of a pin from the ulnar corner of the radius as the most rigid construct in both torsion and cantilever bending.

Aged↗
PubMed1998

Scapholunate ligament reconstruction using a bone-retinaculum-bone autograft: a biomechanic and histologic study.

A new surgical technique has been recently described that involves reconstruction of the dorsal aspect of the scapholunate ligament (DSLL) with a bone-retinaculum-bone (BRB) autograft preparation from Lister's tubercle. In this study, the mechanic and histologic properties of the 2 tissues were compared. The BRB and DSLL specimens were harvested from 6 fresh-frozen human cadaveric forearms. The specimens were measured and then tested in tension with an MTS 810 servohydraulic materials testing machine at a rate of 10 mm/min. The BRB autograft was significantly weaker than the DSLL. However, because the mean cross-sectional area of the DSLL was more than 3 times as large as that of the BRB autograft, the failure stress (failure force/cross-sectional area) of the BRB autograft was not significantly different from that of the DSLL. Histologically, the DSLL and BRB autograft were also similar. These findings suggest that the BRB autograft may be appropriate graft material for scapholunate ligament reconstruction, but that structural parity with DSLL will ultimately depend on remodeling and hypertrophy during healing. This also highlights the importance of using a large BRB autograft to approximate the strength of the DSLL as much as possible, and that the BRB autograft must be protected postoperatively as it heals and remodels.

Aged↗
PubMed1998

Reconstruction of the scapholunate ligament in a cadaver model using a bone-ligament-bone autograft from the foot.

This study is an investigation of a new procedure in which the scapholunate interosseous ligament (SLIL) is reconstructed using a bone-ligament-bone autograft from the foot. After investigation, the dorsal medial portion of the navicular-first cuneiform ligament (NFCL) was chosen for testing as a potential donor since it is similar in length and thickness to the SLIL and it is easily harvested with minimal potential donor site morbidity. Eight SLILs and NFCLs were harvested from fresh-frozen cadavers. Biomechanical extensometry testing was performed using an Instron 1000 machine. A 5-mm-wide central portion of the NFCL was tested since this width was compatible with the technical aspects of reconstructing the SLIL. Both ligaments were tested for elastic properties, including stiffness, load to failure, and deformation to failure. Mean length of the NFCL was 7.6 mm (range, 5.5-8.5 mm). Stiffness of the NFCL was 10.6 x 10(5) Nm (range, 8.0-13.0 Nm) compared with 14.4 x 10(5) Nm for the SLIL (range, 10.0-19.5 Nm). Peak load to failure for the NFCL was 1,980 N (range, 1,530-2,940 N) compared with 2,940 N for the SLIL (range, 1,780-4,050 N). Total elongation to failure for the NFCL was 2.50 mm (range, 1.7-3.2 mm) compared with 3.2 mm for the SLIL (range, 2.1-5.2 mm). Thus, the biomechanical characteristics of the NFCL were found to be very similar to those of the SLIL. Having established the biomechanical similarities of the 2 ligaments, we are currently using the NFCL to reconstruct the sectioned SLIL in a fresh-frozen cadaver model. Early results suggest that this procedure is feasible for restoration of normal kinematics of the wrist.

Aged↗
PubMed2001

Age and ovariectomy impair both the normalization of mechanical properties and the accretion of mineral by the fracture callus in rats.

The impact of age and ovariectomy on the healing of femoral fractures was studied in three groups of female rats at 8, 32 and 50 weeks of age at fracture. In the two older groups, the rats had been subjected to ovariectomy or sham surgery at random at 26 weeks of age. At fracture, all rats received unilateral intramedullary pinning of one femur and a middiaphyseal fracture. Rigidity and breaking load of the femora were evaluated at varying times up to 24 weeks after fracture induction by three-point bending to failure. Bone mineral density (BMD) was measured by dual-energy X-ray absorptiometry. In the youngest group, 8-week-old female rats regained normal femoral rigidity and breaking load by 4 weeks after fracture. They exceeded normal contralateral values by 8 weeks after fracture. In the middle group, at 32 weeks of age, fractures were induced, and the femora were harvested at 6 and 12 weeks after fracture. At 6 weeks after fracture there was partial restoration of rigidity and breaking load. At 12 weeks after fracture, only the sham-operated rats had regained normal biomechanical values in their fractured femora, while the fractured femora of the ovariectomized rats remained significantly lower in both rigidity and breaking load. In contrast, for the oldest group of rats, 50 weeks old at fracture, neither sham-operated nor ovariectomized rats regained normal rigidity or breaking load in their fractured femora within the 24 weeks in which they were studied. In all fractured bones, there was a significant increase in BMD over the contralateral intact femora due to the increased bone tissue and bone mineral in the fracture callus. Ovariectomy significantly reduced the BMD of the intact femora and also reduced the gain in BMD by the fractured femora. In conclusion, age and ovariectomy significantly impair the process of fracture healing in female rats as judged by measurements of rigidity and breaking load in three-point bending and by accretion of mineral into the fracture callus.

Absorptiometry, Photon↗
PubMed2002

Changes in human knee ligament stiffness secondary to osteoarthritis.

Stiffness of the medial (MCL) and lateral (LCL) collateral ligaments was compared between a group of 10 patients undergoing total knee arthroplasty for varus degenerative osteoarthritis (OAP), a group of 10 osteoarthritic cadaveric knees (OAC), and a group of 10 non-osteoarthritic cadaveric knees (NOA). A load-elongation curve was obtained for the medial and lateral compartments of each knee using an instrumented Moreland spreader. A strain gage (SG) was attached to the spreader handle and strain was calibrated to load applied against the spread distance. In extension, medial compartment stiffness of the OAP, OAC, and NOA groups was 60.7+/-16, 52.8+/-9.3 and 21.4+/-5.0 N/mm, respectively. Lateral compartment stiffness in extension was 29.2+/-9.2, 33.3+/-10.3 and 19.5+/-5.3 N/mm, for OAP, OAC, and NOA, respectively. Differences in stiffness between the OAP and OAC groups were not statistically significant (p > 0.05). However, the osteoarthritic groups (OAP and OAC) demonstrated a statistically significantly (p < 0.05) increase in ligament stiffness when compared to the NOA group. Following knee arthroplasty, stiffer medial structures in extension may lead to flexion contracture and accelerated polyethylene wear. Adequate bone resection, in conjunction with soft tissue release may alleviate the threefold increase in stiffness observed in the medial compartment secondary to OA without jeopardizing joint stability.

Aged↗
PubMed1997

Early rupture and degeneration of cryopreserved arterial allografts.

Arterial allografts are used in vascular surgery to solve a major problem: vascular reconstruction in the infected area. To palliate the unavailability and to reduce the risk of viral disease transmission, vascular allografts are currently cryopreserved and stored in tissue banks. In our recent clinical experience, we observed several cases of rupture and degeneration of cryopreserved arterial allografts. All indications are that current cryopreservation protocols are probably the cause for these degenerations.

Aorta↗
PubMed1994

[A comparative study of the "Levame" type extension splint and the "Low Profile" type extension splint: impact on therapeutic indications].

The most widely used dynamic extension systems, both to straighten the digital flexion retractions as well as to ensure directed mobilization after surgical repair of extensor tendons, are the so-called "Levame" and "Low Profile" splints. This study analysed the application of forces for these two types of extension splints, the directional constraints which they impose and freedom of joint movement which they allow in order to more clearly define the type of motorization to be used as a function of the most frequent indications.

Equipment Design↗