John Q. Whitley

Friction between different wire-bracketconfigurations and materials

Friction opposes tooth motion whenever sliding mechanics is employed. Understanding what friction is and how to manage it is of paramount importance to the successful practitioner. In this article, the coefficients of friction are summarized between different arch wire-bracket couples as a function of material, geometric, and external parameters. From this vantage point, friction can then be evaluated within the context of other factors that affect sliding-binding and notching.

Zirconia brackets: An evaluation of morphology and coefficients of friction

The frictional characteristics of two types of zirconia (Harmony, Hudson Ltd., Sheffield, U.K., and Toray, Yamaura Corp., Tokyo, Japan) brackets were compared with those of polycrystalline alumina (Transcend 2000, Unitek Corp., Monrovia, Calif.) brackets in both dry and wet states. To compare the couples, four arch wire alloys were studied: stainless steel, cobalt-chromium, nickel titanium, and beta-titanium. Under dry conditions, the highest frictional coefficients were seen with the Harmony/beta-titanium couple (uk = 0.64); the lowest values were seen with both Transcend 2000/stainless steel (uk = 0.13) and Toray/cobalt-chromium couples (uk = 0.13). Beta-titanium arch wires produced the highest coefficients of friction against each type of ceramic bracket, except against Toray arch wires in the wet state. The presence of human saliva produced only slight changes in the frictional behavior of zirconia brackets. We conclude that currently available zirconia brackets offer no significant improvement over alumina brackets with regard to their frictional characteristics.

Surface roughness of orthodontic archwires via laser spectroscopy.

Using specular reflectance, the surface roughness of six representative orthodontic archwire products was determined. Among the four alloy groups which are commonly used in orthodontics, stainless steel appears the smoothest, followed by cobalt-chrome, beta titanium, and nickel-titanium. A clearer understanding of the parameters which contribute to sliding mechanics will be possible when these results are combined with future experiments on the coefficient of friction.

Assessment of second-order clearances between orthodontic archwires and bracket slots via the critical contact angle for binding.

Twenty-six archwires and 24 brackets were selected from among the hundreds of products available that nominally have from 18 to 22 mil bracket slots and 14, 16, 17, 18, 19, and/or 21 mil archwire sizes. After the archwires and brackets were dimensioned, a minimization-maximization algorithm was applied to the measurements in order to establish the likely boundaries of the critical contact angle for binding (thetac) as defined by the presence and absence of second-order clearance. From among the myriad archwire-bracket permutations possible, 64 combinations were identified--20 using the bracket slot as the controlling dimension and 44 using the bracket width. Using a previously derived mathematical expression that relates the dimensions of each archwire-bracket couple to its calculated thetac, the corresponding sets of indices were plotted. The results show that the maximum value of the calculated thetac can never exceed about 5 degrees , or else sliding mechanics will always be hampered. Other outcomes were validated experimentally using 5 of the 64 archwire-bracket couples by measuring the resistance to sliding (RS) at 15 different contact angles (theta) ranging from theta=0 degrees to theta=12 degrees and by subsequently determining a measured thetac. These values agreed with the calculated thetac values. When the practitioner knows the thetac, treatment time might be reduced because the teeth do not need to be over-aligned prior to employing sliding mechanics (i.e., by not making theta<<thetac) and, further, because the contact angle beyond which the binding phenomenon retards or halts tooth movement does not need to be exceeded (i.e., by not making theta>thetac) These results underscore the importance of exact wire and bracket dimensions on packaging; otherwise, sliding mechanics can be compromised by miscalculating thetac.

Resistance to sliding of orthodontic appliances in the dry and wet states: influence of archwire alloy, interbracket distance, and bracket engagement.

Having established dimensional and mechanical characteristics, the resistances to sliding (RS) were measured in vitro for various archwires against stainless steel brackets. Using stainless steel ligatures, a constant normal force (300g) was maintained while second-order angulation (straight theta) was varied from -12 degrees to +12 degrees. Using miniature bearings to simulate contiguous teeth, five experiments each were run in the dry or wet states with human saliva at 34 degrees C as a function of four archwire alloys, five interbracket distances, and two bracket engagements. Outcomes were objectively analyzed to establish when theta=0, and the relative contact angles ( theta(r)) were replotted. Critical contact angles (theta(c)) that were determined via experimentation were in good agreement with theory. Slopes and y-intercepts were tabulated from linear regression equations of RS against theta plots in both the passive (theta < or = theta (c)) and active ( theta > or = theta(c)) configurations, for which theta = theta(c) identified the boundary between classical friction and binding phenomena. Stiffer archwires and shorter interbracket distances exacerbated binding, whereas, once corrected for differing bracket engagement, RS was independent of slot dimension. Unlike earlier results in the passive configuration, in the active configuration couples comprised of titanium alloys (NiTi and (beta-Ti) had higher RS values in the wet versus the dry state. For those archwire alloys evaluated, two empirical expressions were adduced that comprise the binding component, the yield strength or elastic limit, and the beam length, which implicitly represent the stiffness, flexibility, and interbracket distance.

EVALUATION OF TITANIUM BRACKETS FOR ORTHODONTIC TREATMENT : PART I. THE PASSIVE CONFIGURATION

The static and kinetic frictional coefficients of commercially pure titanium brackets were evaluated in the passive configuration in the dry and wet states against stainless steel, nickel-titanium, and beta-titanium arch wires. For comparison, stainless steel brackets were evaluated under identical conditions. Titanium brackets were grayer in color and rougher in texture than the stainless steel brackets. Bracket slots were up to 0.002 inch greater than the nominally stated values. Remarkably, the static and kinetic frictional coefficients of the couples formed by titanium and stainless steel brackets were comparable. When evaluated against stainless steel and nickel-titanium arch wires in the dry state at 34 degrees C, the static coefficient averaged.12 and.20, respectively, independent of bracket alloy. When evaluated against stainless steel and nickel-titanium wires in the wet state at 34 degrees C using human saliva, the static coefficient averaged.15 and.20, respectively, independent of bracket alloy. Only the beta-titanium arch wires increased by about 15%, when tested in either the dry or the wet state against titanium versus stainless steel brackets. Noteworthy, too, was the decrease of both coefficients in the beta-titanium wire couples from their previously reported values. Analyses of electron spectroscopy for chemical analysis spectra and depth profiles show that these new brackets are titanium only in the bulk. Indeed the immediate surfaces are composed of, at least, 80 atomic percent (at.%) carbon and oxygen; whereas, the titanium that is present (>11 at.%) is mostly in the form of titanium dioxide. The presence of this quite thin passivating layer, which resides on top of an oxygen-hardened titanium substrate, reduces the galling and fretting that would normally be expected in such materials. Pending the outcome of future angulation tests, these frictional measurements show that titanium brackets are not only comparable to stainless steel brackets but also are more biocompatible with nickel having been eliminated from their constitution.

Surface roughness of stainless steel and electroformed nickel standards using a HeNe laser

Abstract Using a helium neon laser a relationship was established between specular reflection and surface roughness for fifteen stainless steel mill finish plates and twenty-two electroformed nickel surfaces. Results showed that, at high angles of incidence, the specular reflectance was independent of the alloy type or finish process. Moreover, as the roughness increased, the specular reflectance decreased for both alloys. Using the Hensler and Tanner expression the rms roughness was determined directly for those samples having roughness less than 0.4 μm. The present investigation substantiates specular reflectance as an on-line optical technique to measure the surface roughness of relatively smooth test surfaces.

Frictional Resistances of Metal-lined Ceramic Brackets Versus Conventional Stainless Steel Brackets and Development of 3-D Friction Maps

The frictional resistances of 2 metal-lined ceramic brackets (Luxi and Clarity) were compared with 2 conventional stainless steel brackets (Mini-Taurus and Mini-Twin) in vitro. In method 1, we varied the second-order angulation from 0 degrees to 12 degrees while maintaining the normal or ligature force constant at 0.3 kg; in method 2, we varied the ligature force from 0.1 kg to 0.9 kg while maintaining the angulation at theta = 0 degrees or theta = 11 degrees. The hardware simulated a 3-bracket system in which the interbracket distances were always 18 mm. All couples were evaluated at 34 degrees C using the same size stainless steel archwire (19 x 26 mil) and ligature wire (10 mil). In the passive region, the static and kinetic frictional forces and coefficients of friction were key parameters; in the active region, the static and kinetic binding forces and coefficients of binding were critical parameters. From outcomes of methods 1 and 2, the 4 aforementioned parameters, and a knowledge of the critical contact angle for binding, 3-dimensional friction maps were constructed in the dry and wet states from which the frictional resistances could be determined at any ligature force or second-order angulation. Those 3-dimensional maps show that metal-lined ceramic brackets can function comparably to conventional stainless steel brackets and that 18-kt gold inserts appear superior to stainless steel inserts. As the morphologies of metal inserts are improved, these metal-lined ceramic brackets will provide not only good esthetics among ceramic brackets but also minimal friction among conventionally ligated brackets.

Design of ionophore-free H+-selective solvent polymeric membranes for further biomedical applications

Abstract Ionophore-free H + -selective solvent polymeric membrane electrodes based on aminated poly(vinyl chloride) (PVC-NH 2 ) are studied. Among the large number of amino-PVC derivatives, the electrodes fabricated from piperazine-modified PVC show the best analytical performances. The new pH electrodes exhibit a Nernstian response between pH 4 and pH 12, and their selectivity coefficients toward biologically interesting cations match the corresponding values of H + electrodes based on TDDA or ETH 5294. The new membrane material offers additional prospects for ion-selective electrode fabrication with technologies used in microelectronic device development.

Syntheses and kinetics of piperazine-modified poly(vinyl chloride)s for use as fixed-site proton carrier membranes

Abstract Analytical-grade piperazine was dissolved in methanol and reacted with commercial-grade poly(vinyl chloride) powders over a range of temperatures from 25 to 64°C and times from 0.17 to 49 days. The products were aminated polymers that also contained conjugated double bonds. Chemical analysis of the resulting powders showed that up to 1.6 wt% N was possible in this solid-liquid heterogeneous reaction, although insolubility of the aminated polymer in tetrahydrofuran was always realized by 0.8 wt% N content. In those polymer formulations that were soluble, membranes were cast from solution using o -nitrophenyloctyl ether and potassium tetrakis( p -chlorophenyl)borate as the plasticizer and negatively charged sites, respectively. After conditioning, many of these fixed-site proton carrier membranes showed Nernstian pH sensitivity in potentiometric cells, independent of synthesis parameters. The nitrogen content of the aminated PVC and the pH sensitivity of the membrane correlated below a limiting nitrogen content, where Nernstian behaviour was generally observed over a range from 0.4 to 0.8 wt% N content. A three-dimensional plot over the soluble region of the polymer predicted the overall dependence of the reaction parameters, from which an activation energy (17 kcal mol −1 ) was determined for the substitution process. A time-temperature correspondence was demonstrated that allowed the construction of a master curve of amination in which the shift factor was of the same form as either the Williams-Landel-Ferry equation or a rearranged Doolittle equation. This format was extended to three dimensions, where the pH sensitivity is shown as a function of both the weight percentage N content and the shift factor.