Torstein R. Meling

On mechanical properties of square and rectangular stainless steel wires tested in torsion.

Forty different sizes and types of square and rectangular stainless steel wires, supplied by five different manufacturers, were tested in torsion. The study simulated the situation occurring when torque is applied to an individual tooth. We used standard brackets with 0.018-inch slot heights, with an interbracket distance of 4 mm. The results show that variation in cross-sectional dimension and edge bevel leads to variable torsional play (third-order clearance). As an example 0.016 x 0.022-inch wires have a mean torsional play of as much as 18.5 degrees, with a range of 16.6 degrees to 20.4 degrees. We have shown that when 0.016 x 0.022-inch wires are used, one must apply from 24.6 degrees to 29.2 degrees of twist to get 20 Nmm of torsional moment. This variation is mostly due to a rather wide range in torsional play. As a result, the prediction by which a predetermined torsional moment can be delivered becomes uncertain. The results show that because the working range in torsion of stainless steel wires is somewhat limited, precise delivery of torsional moment, based on the condition present in the oral cavity, is difficult. Torsional stiffness varies considerable within the various dimensional groups, this being the result of variation in cross-sectional geometry and material properties.

On the variability of cross-sectional dimensions and torsional properties of rectangular nickel-titanium arch wires

Twenty-five rectangular superelastic or conventional work-hardened nickel-titanium alloy wires, commonly used in the 0.018-inch edgewise technique, supplied by seven different manufacturers, along with one braided nickel-titanium and two beta-titanium wires, were studied with respect to wire dimensions, edge bevel, and mechanical properties in longitudinal torsion at 37 degrees C. The wires were twisted 25 degrees and studied in deactivation, simulating application of torque to an individual tooth. Standard Siamese brackets. with stated slot heights of 0.018 inches and measured slot heights of 0.0187 inches, were used. Most wires were within +/-0.0005 inches of the stated dimensions, but had more edge bevel than previously reported for stainless steel and chrome-cobalt alloy wires. Variations in wire dimensions and edge bevel led to variable torsional (third-order) clearance. The torsional stiffness varied among manufacturers within the various wire sizes, this being the result of differences in actual cross-sectional geometry and material properties. None of the tested wires exhibited superelastic properties under the current conditions, and only one wire had a superelastic tendency.

On bracket slot height: A methodologic study

Effective bracket slot height is estimated by using a formula that describes the relationship between bracket slot height, wire dimensions, wire edge bevel, and torsional play (third-order clearance). With a torque measuring instrument, the torsional play was estimated for 10 different brackets (0.018-inch stated slot) of the same manufacturer and type. One arch wire with known dimensions and edge bevel was used for all the measurements. With known torsional play, wire dimensions and edge bevel, the bracket slot height could be calculated. This was performed five times for each bracket and the method error for estimation of torsional play for a single measurement was 0.04 degrees, corresponding to 0.36 mm in slot height. The brackets tested had a mean slot height of 0.476 +/- 0.003 mm, with a range of 0.470 to 0.481 mm (0.0187 +/- 0.0001 inches, range 0.0185 to 0.0189). The variation in bracket slot height was much greater than the method error. The method used to measure bracket slot height seems to have a high degree of accuracy and is easier to implement than conventional methods.

The effect of temperature on the elastic responses to longitudinal torsion of rectangular nickel titanium archwires

To investigate responses to longitudinal torsion and the effect of temperature on the torsional stiffness of superelastic nickel titanium archwires, eight batches of rectangular wires were tested at 18, 27, 37, and 40 degrees C ambient temperature. The wires were twisted 25 degrees and studied in deactivation. The resulting torque-twist diagrams show that only half the wires had discernible deactivation plateaus at body temperature. The plateaus were generally narrow (1 to 3 degrees) and started at 20 to 23 degrees of torsional twist. The clinical significance of these deactivation plateaus is debatable. Only one wire had a well-defined plateau that was fairly wide (6 degrees) and started at a lower level of twist (17 degrees). The wires without plateaus when deactivated from 25 degrees of twist were retested at body temperature. All exhibited deactivation plateaus subsequent to activating twists of 45 and 60 degrees, and the plateaus became more distinct as the degree of prior activation increased. This indicates that the stress imparted on the alloys by 25 degrees of activating twist is insufficient to induce martensitic transformation at body temperature. As prescriptions advocate bracket pretorque of less than 25 degrees for a maxillary central incisor, the clinical relevance of alloys requiring large activations before they demonstrate deactivation plateaus is questionable. Half the wires tested were markedly influenced by ambient temperature changes; the other half were relatively insensitive to temperature. Responsiveness to thermal stimuli seemed closely related to superelastic tendency.

An evaluation of the torsional moments developed in orthodontic applications. An in vitro study

The amount of play between brackets and wire and the magnitude of the torsional moments developed have been investigated for six different rectangular wires. The study simulated the situation occurring when torque is applied to an individual central incisor. The observed play between wire and bracket ranged from about 5 degrees between an 0.018 x 0.025 inch in an 0.018-inch bracket slot, to approximately 20 degrees for a 0.016 x 0.022 inch in an 0.018-inch bracket slot. The observed rate of change after the play had been eliminated, varied from 0.24 degrees/Nmm to 0.37 degrees/Nmm for the steel wires, whereas 0.017 x 0.025-inch nitinol had a rate of change of approximately 1.07 degrees/Nmm. The use of ligatures demonstrated that a torque effect could be observed even though the play had not been completely eliminated.

The effect of short-term temperature changes on the mechanical properties of rectangular nickel titanium archwires tested in torsion.

Due to their exceptional temperature sensitivity, superelastic nickel titanium wires may be affected by temperature changes associated with ingestion of cold or hot food. It has been assumed that the alterations in archwire stiffness associated with short-term cooling or heating are transient. This investigation studied the effect of these temperature changes on the torsional stiffness of nickel titanium alloys. Eight rectangular superelastic wires were activated to 20 degrees, in longitudinal torsion at body temperature and subjected to cold (10 degrees C) or hot (80 degrees C) water with the strain held constant. The torsional stiffness of some wires was strongly affected. The effect of hot water disappeared quickly, but the wires remained at a level of reduced torsional stiffness (up to 85% less than baseline) after short applications of cold water. The most thermodynamic archwires showed incremental reductions in torsional stiffness when cold water was repeatedly applied. Furthermore, the torsional stiffness remained low (up to 50% less than baseline) and showed no tendency to increase even after 2 hours of post-exposure restitution. It is conceivable that some wires may provide inadequate forces for tooth movement after ingestion of cold liquids.

The effect of friction on the bending stiffness of orthodontic beams: a theoretical and in vitro study.

The purpose of this study was to investigate the effect of friction on the bending stiffness of orthodontic beams. A theoretical and experimental model have been established where tensile and compressive forces are applied to an arch wire to simulate the effect of additional friction during activation and deactivation, respectively. The results show that tensile force increases wire stiffness, and that compressive force increases flexibility. Thus more force will be needed during activation and more force will be lost during deactivation. The amount of force lost increases nearly linearly with increasing friction. During activation, the percentage increase in force due to friction for a given deflection is about equal to the loss of force due to friction during deactivation. Friction affects thin flexible wires more than heavy wires. Careful ligation is recommended in the leveling phase to reduce the negative side effects of friction.

The effect of cross-sectional dimensional variations of square and rectangular chrome-cobalt archwires on torsion

The purpose of this investigation was to study the control of cross-sectional dimensions and edge bevel by various manufacturers in the production of chrome-cobalt archwires and this effect on transmitting torque through an .018 inc slot bracket system. Twenty-seven different square and rectangular chrome-cobalt wires commonly used in the edgewise technique supplied by five different manufacturers were studied with respect to dimensions, edge bevel, and mechanical properties in torsion. The mechanical study simulated application of torque to an individual tooth. Standard brackets with .018 inch slot heights were used. The results show that variation in cross-sectional dimension and edge bevel leads to variable torsional play (third-order clearance). As an example, .016 x .016 wires have a mean torsional play of as much as 26.8 degrees, with a range of 21.0 to 32.3 degrees. When using .016 x .016 wires, one must apply from 34.8 to 48.6 degrees of twist to get 20 N-mm of torsional moment. This variation is primarily due to the rather wide range in torsional play. As a result, the prediction by which a predetermined torsional moment can be delivered becomes uncertain. The results also show that because the working range in torsion of chrome-cobalt wires is somewhat limited due to high torsional stiffness, precise delivery of torsional moment based on the condition present in the oral cavity is difficult. The torsional stiffness varies between manufacturers within the various dimensional groups as a result of differences in cross-sectional geometry and material properties.

The effect of second-order couple on the application of torque

Complex combinations of linear forces, moments, and couples are developed by the arch wire during orthodontic treatment. For instance, application of torque to a canine during distal driving may create force interactions if the tooth tips distally toward the extraction site. This investigation studied the effect of second-order couples and bracket angulations on the application of torque to a single tooth. By using a test apparatus to simulate application of torque to a single tooth, 0.016 x 0.022 inch stainless steel wires were tested in longitudinal torsion simultaneous to fixed amounts of second-order couples or fixed degrees of second-order bracket angulation. Application of a second-order couple through a bracket to a longitudinally twisted arch wire produces a third-order couple, since the bracket slot walls exert forces on the wire, tending to detwist it. This third-order couple will usually be small as the distance between the two couple members is short. Nevertheless, it may have a restraining effect on the third-order wire-bracket interaction. The results show that application of second-order couples or bracket angulations lead to an increase in exerted torque for angles of twist below 22 degrees. Because of torsional play, a wire twisted 18 degrees in a 0.018-inch bracket slot did not exert any torque unless it was subjected to a second-order couple. Thus, in an in vivo situation where forces interact, the actual torsional play may be substantially less than predicted from theoretical models only regarding third-order mechanics. The restraining effect of second-order couples tapered when the torque created by longitudinal twisting became much larger than the torque exerted by the second-order couple. Second-order couples of biologically acceptable magnitudes had little effect on the level of torque after the third-order clearance had been eliminated.

Short-term temperature changes influence the force exerted by superelastic nickel-titanium archwires activated in orthodontic bending ☆ ☆☆ ★ ★★ ♢

BACKGROUNDnAlterations in mouth temperature may lead to changes in the force exerted by an activated superelastic wire. It has been assumed that variations in archwire stiffness associated with short-term cooling or heating are transient. This investigation studied the effect of short-term cooling or heating on the bending force exerted by nickel-titanium archwire.nnnMATERIAL AND METHODSnSix rectangular superelastic wires and one conventional nickel-titanium wire were tested in bending at 37 degrees C. The test specimens were deflected 0.5 mm, and the bending force was measured continually. The activated specimens were subjected to cold (10 degrees C) or hot (80 degrees C) water under constant deflection, simulating an inserted archwire that is subjected to cold or hot drinks or food during a meal.nnnRESULTSnThe conventional nickel-titanium wire was marginally affected by brief cooling or heating. In contrast, some of the superelastic wires were strongly affected by short-time application of cold or hot water. Whereas the effect of brief heating disappeared quickly, some wires continued to exert sub-baseline bending forces (up to 32% less) after short-time application of cold water and showed little or no tendencies toward increase even after 30 minutes of postexposure restitution (up to 43% less).nnnCONCLUSIONSnShort-term exposures to hot liquid increased the bending force exerted for a given deflection transiently. The effect of short-term exposures to cold liquid was not always transient; the bending force remained sub-baseline for a number of the thermosensitive wires tested for a prolonged time.