Influence of interbracket distances on the resistance to sliding of orthodontic appliances.

INTRODUCTION: The influence of interbracket distance (IBD) on the resistance to sliding (RS) was evaluated. METHODS: Commercially pure titanium brackets (CP-Ti; 0.018- and 0.022-in slots, width = 0.14-in) were tested against 0.016 x 0.022-in rectangular stainless steel (SS), nickel titanium (Ni-Ti), and beta-titanium (beta-Ti) archwires in the dry and wet (human saliva) states. With a custom testing apparatus that simulated a 3-bracket system, the RS was measured at a normal force of 300 cN and at second-order angles (theta) ranging from -9 degrees to +9 degrees. Twenty-three pairs of IBDs (written as IBD1_IBD2) were varied to simulate clinically relevant biomechanical scenarios with IBD ranging from 16 to 7 mm. RESULTS: In the dry state, the kinetic frictional coefficients (micro(k)) were equal to 0.12, 0.23, and 0.24 for the SS, Ni-Ti, and beta-Ti archwires against the CP-Ti brackets, respectively. The presence of saliva slightly increased micro(k). The RS was inversely proportional to the total IBD (IBD(T) = IBD1 + IBD2) regardless of archwire alloy or bracket slot. Elastic binding (BI = RS - frictional force in the passive region) did not depend on the order of the IBDs in the IBD1_IBD2 pair. CONCLUSIONS: For a specific archwire-bracket couple, the BI of an IBD1_IBD2 pair is equal to any other pair with an equal IBD(T). Although no significant difference was found between the coefficients of binding (micro(BI)) for SS and beta-Ti archwires, the micro(BI)'s of Ni-Ti archwires were lower and significantly different. The micro(BI) was linearly related (P<.01) to IBD(T) and total archwire beam length (L(T)).

Resistance to sliding of titanium brackets tested against stainless steel and beta-titanium archwires with second-order angulation in the dry and wet states.

INTRODUCTION: With the increased awareness of Ni allergies, alternative alloys for orthodontic products must be identified. The properties of these new products must be determined. METHODS: Rectangular (0.017 x 0.025 in) stainless steel (SS) and beta-titanium (beta-Ti) archwires were tested against commercially pure titanium brackets (CP-Ti, 0.018-in slot) in the dry state and with whole human saliva. Resistance to sliding (RS) was measured as a function of 5 normal forces (N, 200 to 950 cN), 32 angles (theta, -12 degrees to +12 degrees), and 1 interbracket distance (IBD, 18 mm). RESULTS: With clearance between the archwire and the bracket (passive region, theta < or =theta(c)), the frictional coefficients (mu) of the SS archwire and the CP-Ti bracket couples were 0.12 and 0.13 for the dry and wet tests, respectively; for the beta-Ti archwire and the CP-Ti bracket couples, the mu values were 0.29 and 0.28 for the dry and wet tests, respectively. For an theta without clearance (active region, theta > or =theta(c)), RS increased as a function of theta and N. To examine the rates of binding (mu(BI)) in this active region, the value of classical friction (mean of the passive region data) was subtracted from RS to yield BI, and the value of theta(c) was subtracted from each theta to yield relative contact angles (theta(r)). Because of the unique relationship between the frictional and mechanical properties of these SS and beta-Ti archwires tested against the CP-Ti brackets at a large IBD, the mu(BI) values for these archwire-bracket couples were nominally equivalent (24 to 30 cN per degree). Clinical outcomes would be unaffected by this 6 cN per degree (approximately 0.2 oz-force per degree) difference. When all kinetic data in the elastic region (theta(r) < or =5 degrees ) were combined, mu(BI) equaled 28 cN per degree. Above this region (theta(r) > or =5 degrees ), the data for the SS archwire and CP-Ti bracket couples were less scattered than those for the beta-Ti archwire and the CP-Ti bracket couples. This demarcation from linearity was designated as theta(z) and indicated the end of the elastic region and the beginning of the plastic region, above which sliding can eventually cease. This region (theta > or =theta(z)), the binding region (theta(c) < or =theta < or =theta(z)), and the classical friction region (theta < or =theta(c)) were described in a model. CONCLUSIONS: This model explains, in part, the equivalent values of mu(BI) for SS and beta-Ti archwires tested against CP-Ti brackets.

Thermal and mechanical characteristics of stainless steel, titanium-molybdenum, and nickel-titanium archwires.

INTRODUCTION: In recent years, nickel-titanium (Ni-Ti) archwires have been developed that undergo thermal transitions. Before the practitioner can fully utilize these products, the effect of those transitions within the clinical application must be understood. METHODS: The transitional temperatures and mechanical stiffnesses of 3 archwire alloys--stainless steel, beta-titanium, and Ni-Ti--were investigated were for 7 products. Among the nickel-titanium alloys, 2 were thought to represent classic Ni-Ti products and 3 copper (Cu)-Ni-Ti products. By using 2 techniques, differential scanning calorimetry to measure heat flow and dynamic mechanical analysis to measure storage modulus, transition temperatures were evaluated from -30 degrees C to +80 degrees C. RESULTS: With regard to the first technique, no transitions were observed for the stainless steel alloy, the beta-titanium alloy, and 1 of the 2 classic Ni-Ti products. For the other classic Ni-Ti product, however, a martensitic-austenitic transition was suggested on heating, and a reverse transformation was suggested on cooling. As expected, the Cu-Ni-Ti 27, 35, and 40 products manifested austenitic finish temperatures of 29.3 degrees C, 31.4 degrees C, and 37.3 degrees C, respectively, as the enthalpy increased from 2.47 to 3.18 calories per gram. With regard to the second technique, the storage modulus at a low frequency of 0.1 Hz paralleled static mechanical tests for the stainless steel alloy (183 gigapascal [GPa]), the beta-titanium alloy (64 GPa), and the Nitinol Classic (3M Unitek, Monrovia, Calif) product that represented a stable martensitic phase (41 GPa). The remaining 4 Ni-Ti products generally varied from 20 to 35 GPa when the low-temperature or martensitic phase was present and from 60 to 70 GPa after the high-temperature or austenitic phase had formed. CONCLUSIONS: From the clinical viewpoint, the Orthonol (Rocky Mountain Orthodontics, Denver, Colo), Cu-Ni-Ti 27, Cu-Ni-Ti 35, and Cu-Ni-Ti 40 (SDS/Ormco, Glendora, Calif) products increased at least twofold in stiffness as temperature increased, best emulating the stiffness of Nitinol Classic below the transformational temperature and the stiffness of TMA (SDS/Ormco, Glendora, Calif) above the transformational temperature. Of the 3 Cu-Ni-Ti products, the least differences were found between Cu-Ni-Ti 27 and Cu-Ni-Ti 35, thereby questioning the justification for 3 similar products.

Degradation of plastic polyoxymethylene brackets and the subsequent release of toxic formaldehyde.

PURPOSE: Heat, acids, alkalis, oxygen, abrasion, enzymes, and radiation are all viable mechanisms for the chemical breakdown of polyoxymethylene (POM), a plastic material used in some esthetic orthodontic brackets. The aim of this study was to establish the thermal characteristics of POM brackets and the chemical by-products in the as-received bracket, during thermal analyses of the bracket, and after abrasion of the slot and base of the bracket. MATERIAL: Plastic brackets and control rods made of POM were evaluated ex vivo by thermal and chemical analyses. RESULTS: POM brackets produce toxic formaldehyde gas whether heated or mechanically abraded. Patients who wear these brackets are being exposed to, at the very least, a potential irritant. Thermal analyses showed that the melting temperature of a POM bracket was approximately 178 degrees C, comparable to that for a commercial rod of POM. Both POM products started to degrade at approximately 250 degrees C, and, by 420 degrees C, both products had completely decomposed into their fundamental molecular structure, formaldehyde. A colorimetric assay with a fuchsin-aldehyde reagent (Schiff's reagent) showed that aldehydes were present in the effluent from thermal heating, from mechanical abrasion of the bracket's slot or base, and even from the as-received bracket. The only difference between the 6 to 8-week assays and the 12 to 14-week assays was the intensification of the color, which occurred because of an increase in aldehyde formation and the concomitant increase in double-bond formation. These observations are consistent with a report in the tribologic (ie, friction and wear) literature, which documented the degradation of POM when it rubs against steel. CONCLUSIONS: Because formaldehyde's inherent uses as a disinfectant base and embalming fluid preclude its beneficial presence in the human body, further use of POM for orthodontic brackets, crowns for children, and other prosthetic appliances is contraindicated because even radiography will promote its degradation.

Comparisons of surface roughnesses and sliding resistances of 6 titanium-based or TMA-type archwires.

Six titanium-based or TMA-type archwires (Beta III, Resolve, CNA, TMA, low-friction ion-implanted TMA or TMAL, and TiMolium) were studied as functions of composition, morphology, surface roughness, and sliding mechanics by using a scanning electron microscope, an x-ray energy dispersive wavelength analyser, a laser specular reflectometer, and a frictional testing machine. In the last instrument, all wires were coupled with 0.022-in stainless steel brackets in which normal forces were applied by 0.010-in stainless steel ligatures. With regard to composition, 5 wires were true beta-titanium alloys having nominal compositions of 80% titanium, 10% molybdenum, 6% zirconium, and 4% tin, and 1 was an alpha-beta alloy having a nominal composition of 90% titanium, 6% aluminum, 3% vanadium, and 1% other. Morphologies varied from surfaces with striations, scale, or layers of drawn material that suggested surface steps or fissures. Specular reflectance and optical roughness measurements divided the archwires into 2 groups of 3: Beta III, Resolve, and CNA had an overall mean value of 0.148 microm; and TMA, TMAL, and TiMolium had a mean overall value of 0.195 microm. These roughness measurements and their accompanying details of the compositional analyses suggested that there could be as few as 2 vendors manufacturing the 5 beta-titanium products. For 6 different values of angulation that embraced the passive and active regions of sliding, the coefficients of friction varied rather narrowly from 0.17 to 0.27 and were independent of surface roughnesses. Although these contemporary products are better than their predecessors of over a decade ago, other issues might be more important than surface finishes or frictional resistances because all products appear fairly comparable.