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At least 1,279 records · Page 71Linked to original sources
PubMed1998

Rigidity and retention of root canal posts.

OBJECTIVE: To test the rigidity and the retention into roots of parallel root canal posts, one a spiral vented titanium post and the other a spiral serrated, hollow, stainless steel post. A serrated, stainless steel post was used as the control. MATERIALS AND METHODS: A three-point bending test was used to test rigidity. To test retention, ten posts of each type were cemented into the roots of extracted teeth with a resin cement and the tensile loads required to remove them were compared using Student's t and Mann-Whitney U tests. RESULTS: The serrated stainless steel posts were significantly more rigid than either of the other types. The titanium posts and the stainless steel hollow posts were not significantly different in rigidity. The serrated, stainless steel posts were significantly better retained than either of the other types. The titanium posts showed greater retention than the hollow posts. CONCLUSIONS: Within the limits of the study the stainless steel, serrated posts were superior to the two newer types in terms of rigidity and retention into roots.

Cementation↗
PubMed2004

Modulation of kinesin binding by the C-termini of tubulin.

The flexible tubulin C-terminal tails (CTTs) have recently been implicated in the walking mechanism of dynein and kinesin. To address their role in the case of conventional kinesin, we examined the structure of kinesin-microtubule (MT) complexes before and after CTT cleavage by subtilisin. Our results show that the CTTs directly modulate the motor-tubulin interface and the binding properties of motors. CTT cleavage increases motor binding stability, and kinesin appears to adopt a binding conformation close to the nucleotide-free configuration under most nucleotide conditions. Moreover, C-terminal cleavage results in trapping a transient motor-ADP-MT intermediate. Using SH3-tagged dimeric and monomeric constructs, we could also show that the position of the kinesin neck is not affected by the C-terminal segments of tubulin. Overall, our study reveals that the tubulin C-termini define the stability of the MT-kinesin complex in a nucleotide-dependent manner, and highlights the involvement of tubulin in the regulation of weak and strong kinesin binding states.

Adenosine Diphosphate↗
PubMed2006

Protonation-mediated structural flexibility in the F conjugation regulatory protein, TraM.

TraM is essential for F plasmid-mediated bacterial conjugation, where it binds to the plasmid DNA near the origin of transfer, and recognizes a component of the transmembrane DNA transfer complex, TraD. Here we report the 1.40 A crystal structure of the TraM core tetramer (TraM58-127). TraM58-127 is a compact eight-helical bundle, in which the N-terminal helices from each protomer interact to form a central, parallel four-stranded coiled-coil, whereas each C-terminal helix packs in an antiparallel arrangement around the outside of the structure. Four protonated glutamic acid residues (Glu88) are packed in a hydrogen-bonded arrangement within the central four-helix bundle. Mutational and biophysical analyses indicate that this protonated state is in equilibrium with a deprotonated tetrameric form characterized by a lower helical content at physiological pH and temperature. Comparison of TraM to its Glu88 mutants predicted to stabilize the helical structure suggests that the protonated state is the active form for binding TraD in conjugation.

Amino Acid Sequence↗
PubMed2004

Preferred dimerization of tetra-tolyl- and tetra-tosylurea derivatives of flexible and rigidified calix[4]arenes.

The dimerization of tetratolyl- and tetratosyl-urea derivatives 1 and 2, derived from a tetrapentoxy calix[4]arene in the cone conformation and of the corresponding tetra-urea derivatives 3 and 4, in which the cone conformation is rigidified by the two crown-3 tethers, have been studied. All six possible equimolar mixtures were examined by 1H NMR using CDCl3 and CD2Cl2 as solvents. While no heterodimers are found for the combinations 1/3 and 2/4 in either solvent, all remaining combinations lead to the (exclusive) formation of heterodimers in CD2Cl2. In CDCl3 heterodimers are only observed for the combinations of 3 with 2 or 4. These results are discussed in terms of entropic and enthalpic contributions and compared with MD-simulations in a box of chloroform solvent molecules.

Calixarenes↗
PubMed2006

Kalata B8, a novel antiviral circular protein, exhibits conformational flexibility in the cystine knot motif.

The cyclotides are a family of circular proteins with a range of biological activities and potential pharmaceutical and agricultural applications. The biosynthetic mechanism of cyclization is unknown and the discovery of novel sequences may assist in achieving this goal. In the present study, we have isolated a new cyclotide from Oldenlandia affinis, kalata B8, which appears to be a hybrid of the two major subfamilies (Möbius and bracelet) of currently known cyclotides. We have determined the three-dimensional structure of kalata B8 and observed broadening of resonances directly involved in the cystine knot motif, suggesting flexibility in this region despite it being the core structural element of the cyclotides. The cystine knot motif is widespread throughout Nature and inherently stable, making this apparent flexibility a surprising result. Furthermore, there appears to be isomerization of the peptide backbone at an Asp-Gly sequence in the region involved in the cyclization process. Interestingly, such isomerization has been previously characterized in related cyclic knottins from Momordica cochinchinensis that have no sequence similarity to kalata B8 apart from the six conserved cysteine residues and may result from a common mechanism of cyclization. Kalata B8 also provides insight into the structure-activity relationships of cyclotides as it displays anti-HIV activity but lacks haemolytic activity. The 'uncoupling' of these two activities has not previously been observed for the cyclotides and may be related to the unusual hydrophilic nature of the peptide.

Amino Acid Motifs↗
PubMed1997

A review of contemporary archwires: their properties and characteristics.

The materials used by orthodontists have changed rapidly in recent years and will continue to do so in the future. As esthetic composite archwires are introduced, metallic archwires will likely be replaced for most orthodontic applications in the same way that metals have been replaced by composites in the aerospace industry. Archwires are reviewed in the order of their development, with emphasis on specific properties and characteristics, such as strength, stiffness, range, formability, and weldability. Because an ideal material has not yet been found, archwires should be selected within the context of their intended use during treatment.

Chemical Phenomena↗
PubMed1997

Stiffness-deflection behavior of selected orthodontic wires.

Treatment of horizontal and vertical tooth discrepancies requires wires of low stiffness to produce forces as the teeth are leveled and aligned. In this investigation, the stiffness characteristics of several solid and multistrand nickel-titanium and stainless steel orthodontic wires were determined at selected clinically relevant deflections. Twenty specimens of 24 different wires were tested in both three-point and three-bracket bending modes. The unloading force deflection plot of each wire was described by a polynomial regression from which wire stiffnesses were obtained by mathematical differentiation. Graphs of the functional relationship between stiffness and deflection are presented. The results of this investigation show that wire stiffness can be altered not only by changing the size, but also by varying the number of strands and the alloy composition. An equally important finding was the dependence of stiffness on deflection for most of the wires measured. Comparisons were also made between the stiffness values obtained in three-point bending and the three-bracket bending systems.

Dental Alloys↗
PubMed1997

Variable modulus orthodontics advanced through an auxiliary archwire attachment.

Reducing the load deflection rates of orthodontic springs is important, for it provides relative constancy of the moment-to-force ratio applied to the teeth with concomitant, forecastable dental movement. Increasing patient comfort and reducing the number of office visits while lowering potential tissue damage are additional features of lower load deflection rate springs. A simple auxiliary attachment, which can be crimped into position on an archwire or onto segments of an archwire, is described. This attachment permits the clinician to incorporate a relatively high rate stiff wire to enhance the anchorage of the reactive teeth in one area of the dental arch, while allowing the use of lesser stiffness (lower load deflection rate spring) to engage teeth targeted for movement. The auxiliary allows the clinician various stiffness through the use of wire of one modulus (stainless steel, for example) in one area of the arch, and wire of a differing modulus (NiTi, for example) in another area of the same arch. The advantages and disadvantages of choosing wires of differing moduli are reviewed. Alternative methods of transforming the spring rate through changes in wire cross-section or length are also reviewed. Practical clinical applications of the auxiliary attachment are shown.

Dental Alloys↗
PubMed2000

The influence of orthodontic adhesive properties on the quality of orthodontic attachment.

The objective of this study was to determine the resilience, glass transition temperature (Tg), ultimate flexural strength (UFS), and penetration coefficient of 3 composite adhesives (Concise, Transbond, and Right On) and a glass ionomer cement (Fuji Ortho LC). For 25 minutes after initial set, the composite materials were significantly more resilient than Fuji Ortho LC (P = .000). Resilience values for all materials increased for up to 90 minutes after initial set, reflecting a continuation of their setting reaction. Right On remained the most resilient material at 90 minutes after initial set (P < or = .01). At 120 minutes, there was no significant difference in the resilience of Concise, Right On, and Fuji Ortho LC. However, Transbond was significantly less resilient than these materials (P < or = .01). The composite materials recorded significantly higher glass transition values (89-123 degrees C) than Fuji Ortho LC (54 degrees C). The composite materials were also significantly stronger (61-68 MPa) than Fuji Ortho LC (35 MPa) in flexion. The penetration coefficients of Concise and Transbond were significantly higher (P < or = .001) than Right On and Fuji Ortho LC. However, the relative penetration coefficients of the materials studied did not appear to influence the degree of bracket base penetration achieved by the cements. In conclusion, Fuji Ortho LC offered a reduced energy-absorbing capacity immediately after bracket placement and reduced cohesive and mechanical adhesive strength. In addition, the glass transition temperature of Fuji Ortho LC was low; this may result in compromised attachment at temperatures above 60 degrees C. Significant differences in the penetration coefficient of the materials studied have been recorded. However, the penetration of the 4 cements into a range of bracket base morphologies was uniformly good. Therefore, the differences in penetration coefficient recorded should not influence the quality of the bracket cement interface provided.

Acrylic Resins↗
PubMed2001

An evaluation of the quality of orthodontic attachment offered by single- and double-mesh bracket bases using the finite element method of stress analysis.

The objective of this study was to evaluate the influence of bracket base mesh geometry on the stresses generated in the bracket-cement-tooth continuum by a shear/peel load case. A validated three-dimensional finite element model of the bracket-cement-tooth system was constructed consisting of 15,324 nodes and 2971 finite elements. Cement geometric and physical properties were held constant and bracket base geometry was varied, representing a variety of single-mesh configurations and 1 double-mesh design. For the single-mesh designs, increasing wire diameter (100-400 microm) resulted in a decrease in enamel and cement stresses. Increases in wire mesh spacing (200-750 microm) increased the major principal stress recorded in the enamel and adhesive at all wire diameters. Within the bracket, the major principal stress increased significantly at wire spacing above 400-500 microm. However, within the impregnated wire mesh (IWM), the major principal stress decreased as wire space increased. When the double-mesh bracket base was considered, the combined mesh layers resulted in a decrease in the stresses recorded in the most superficial (coarse) mesh layer and an increase in the stresses recorded in the deepest (fine mesh) layer when compared with the single-layer designs in isolation. Modification of single-mesh spacing and wire diameter influences the magnitude and distribution of stresses within the bracket-cement-tooth continuum. The use of a double-mesh design results in a reduction in the stresses recorded in the most superficial mesh. Mesh design influenced stress distribution in this study, primarily by determining the flexibility of the bracket base.

Bicuspid↗
PubMed2002

Elastic flexural properties of multistranded stainless steel versus conventional nickel titanium archwires.

Based on a recent investigation that modeled the elastic properties (ie, strength, stiffness and range) of multistranded wires made from linearly elastic materials, three-stranded (triple) and six-stranded coaxial (coax) stainless steel (SS) wires were compared to single-stranded (single) SS and conventional nickel titanium (NiTi) leveling wires. To measure Young's modulus of elasticity (E), flexural tests were performed with an Instron mechanical testing machine in a three-point bending arrangement having a span length of 8.9 mm or 12.5 mm. A strong correlation between wire stiffness and the area moment of inertia demonstrated that strand interaction was negligible at low activations and that E = 199 GPa was constant even for the heavily drawn coax strands. Using the Instron with an extensometer, the 0.1% yield strengths (sigma(YS)) of the single SS wires and the straight inner strands within the coax wires were tested. The ratio of the sigma(YS) to the ultimate tensile strength averaged 0.81 for the single wires, 0.88 for the coax wires, and was subsequently assigned 0.85 for the triple wires. The average sigma(YS) values were 1.88, 1.83, and 1.78 GPa for the single, triple, and coax SS wires, respectively. For each NiTi wire, both the sigma(YS) and the elastic limit (sigma(EL)) via cyclic loading were measured. The conventional NiTi wires displayed nonlinear elasticity, as the average sigma(EL) values (1.10 GPa) were 50% higher than the average sigma(YS) values (0.73 GPa). Compared to the elastic properties of the conventional NiTi wires, the triple and coax SS wires generally matched the stiffness, but had only one-third to one-half of the strength and range. Since the properties of strength and range are both proportional to sigma(YS), fabrication using alloys with enhanced sigma(YS) values would allow multistranded SS archwires to compete better against conventional NiTi products.

Dental Alloys↗
PubMed2004

Stiffness in bending of a superelastic Ni-Ti orthodontic wire as a function of cross-sectional dimension.

Superelasticity is a property used in orthodontics to initiate tooth movement in the first stage of orthodontic treatment. It is the aim of all clinicians to accomplish biological tooth movement, which implies the use of low, continuous force and requires archwire with low stiffness. In this study, 15 nickel-titanium archwires with three different cross-sectional dimensions were tested in three-point bending to determine the nature of forces in a loading and unloading cycle. The evolution of stiffness in bending as a function of wire size is discussed. The applied forces or stiffness dependence on cross-sectional size differs from the linear-elastic prediction because of the superelasticity property. We discuss the origin of the nonconventional profile of curves and the nature of reversible large deformation of these alloys. Martensitic transformation is at the origin of nonlinear elasticity. The stiffness decreases with increasing deflection, and this phenomenon is emphasized in the unloading process. The value of stiffness appears to vary with wire size but depends on the ratio of volume of martensitic transformation. During martensitic transformation, the rigidity (elastic modulus) of the alloy is nonconstant. These results and their understanding should allow a different approach of biomechanical considerations, ie, a large-size square wire does not produce necessarily high forces.

Biomechanical Phenomena↗
PubMed2004

Mechanical properties and surface characteristics of three archwire alloys.

Recent developments in material science have presented newer archwire materials as well as improvements in the properties of existing ones. Proper selection and understanding of the biomechanical requirement of each case requires proper characterization studies on archwire alloys. The present study characterizes and compares three orthodontic archwire alloys, stainless steel, beta titanium alloy (TMA), and a newly introduced titanium alloy (TiMolium), for the parameters (1) ultimate tensile strength (UTS), 0.02% offset yield strength (YS), and modulus of elasticity (E); (2) load deflection characteristics; (3) frictional properties; (4) surface characteristics and (5) elemental analysis for TiMolium. Seven specimens of each archwire alloy were used for evaluating each parameter. An instron universal testing machine was used for tensile testing, three-point bend testing, and evaluation of frictional characteristics. Scanning electron microscope was used for surface evaluation and X-ray fluorescence for elemental analysis of TiMolium wire specimens. Stainless steel was the strongest archwire alloy with high UTS, E, 0.02% offset YS, and less friction at the archwire-bracket interface. TMA wires exhibited better load deflection characteristics with less stiffness than the other two wires. The surface of TMA appeared rough and exhibited very high values for friction at the archwire-bracket interface. TiMolium appeared to be an alpha-beta titanium alloy composed of titanium, aluminum, and vanadium and intermediate in nature for all the parameters evaluated.

Alloys↗
PubMed2005

Effect of time on the flexural strength of glass ionomer and composite orthodontic adhesives.

The purpose of this study was to compare the effects of time on the flexural strength of a resin-reinforced glass ionomer and a composite adhesive system, specifically at three time frames corresponding to the three stages of polymerization of Fuji Ortho LC. Ten rectangular specimens of each material were prepared in a metal mold (25 x 2 x 2 mm) and then stored at 37 degrees C and 100% humidity in an incubator. Six test groups were created, in which each specimen was fractured using a 3-point-bending test at a crosshead speed of 0.5 mm/min. The test results indicated that there were significant differences among the groups (P = .0001). The flexural strengths were significantly higher in the two groups (III and VI) that were fractured after seven days. This was true for both the Fuji Ortho LC (x = 77 +/- 6.1 MPa) and the Concise (x = 103.9 +/- 4.2 MPa). The flexural strength of the resin-modified glass ionomer adhesive was significantly lower than that for the composite whatever the time of fracture, 10 minutes, one hour, or seven days. The analysis of the strength-deformation curve of the group of Fuji Ortho LC, which was fractured within 10 minutes after setting (group I), showed viscoplastic behavior, whereas that of all the others groups showed elastic behavior. According to this study, clinicians must consider the mechanics of Fuji Ortho LC setting and, when this material is used, wait for at least one hour to ligate initial or repaired arch wires.

Acrylic Resins↗
PubMed2003

Retention and postinsertion maintenance of bar-clip, ball and magnet attachments in mandibular implant overdenture treatment: an in vivo comparison after 3 months of function.

It could be hypothesised that attachments, which provide more retention against vertical and horizontal dislodgement, will be associated with more favourable parameters of oral function. This in vivo study is designed to provide data regarding initial retention force, loss of retention force after 3 months of function and postinsertion maintenance and complications associated with the use of magnet, bar-clip and ball attachments in mandibular overdenture treatment. Eighteen edentulous subjects received two permucosal implants in the inter-foramina region of the mandible, a new denture and three successive suprastructure modalities (magnet-, bar-clip and ball attachments). The retention force of the attachments at baseline and after 3 months was measured in a standardised way. The amount and type of postinsertion maintenance that was related to the attachment were evaluated. No differences in retention force at baseline and after 3 months of loading were observed for all three attachment types. The mean retention forces of magnet attachments, bar-clip attachments and ball attachments were 8.1, 31.3 and 29.7 N respectively. Functional maintenance complications related to the attachments were predominantly observed in 11/36 magnet attachments. Functional problems in the ball attachment group were relatively rare, easily manageable and seen in 4/36 attachments. The bar-clip attachments exhibited no maintenance problems at all.

Adult↗
PubMed1998

Fracture properties of aged and post-processed dental composites.

The purpose of this study was to determine the flexure strength (sigma f), Young's modulus (E), and fracture toughness (KIC) of five dental composites after aging in water and air. The composites were, by weight, 75% or 79% glass filler and 25% or 21% resin composed of 60% Bis-GMA and 40% TEG-DMA. The filler was either strontium glass (75Sr or 79Sr) or a combination, by weight, of 90% strontium glass and 10% colloidal silica (75Sr10 or 79Sr10). The specimens, 2 x 4 x 70 mm bars, were aged in either air or distilled water at 37 degrees C and were tested in their respective aging media at a loading rate of 1.22 mm/s. The sigma f and E were tested in four-point loading and the KIC in three-point loading. The addition of the fillers to the unfilled resin resulted in a statistically significant increase in the flexure strength, flexure modulus, and fracture toughness. Aging in air had minimal effect on these properties. However, testing and aging in water led to a significant decrease in the mechanical properties in the first 6 months, but had limited effect from 6 to 12 months.

Aluminum Oxide↗
PubMed1998

The role of cuspal flexure in the development of abfraction lesions: a finite element study.

A tooth flexure mechanism has been proposed over the past 15 yr to explain non-carious cervical tooth loss. It is well established that cavity preparation weakens a tooth, resulting in more cuspal movement under occlusal load. The aim of this study was to investigate the effect that an occlusal restoration would have on the stress profile in the cervical region of a lower second premolar using two-dimensional finite element stress analysis. Under an eccentric 100 N occlusal load, a premolar with an occlusal amalgam restoration showed peak tensile and shear stresses in the buccal cervical region that were in excess of the known failure stress for enamel. Increases in the cavity depth of the occlusal amalgam restoration were found to increase cervical stress more than increases in cavity width. It was concluded that the weakening effect of cavity preparation may contribute to the development of non-carious cervical tooth loss.

Bicuspid↗