[Comparative evaluation of elastic modulus of orthodontic elastomeric ligatures]

The application of elastomeric ligatures is one of the methods of archwires ligation to orthodontic brackets. Knowledge about modulus of elasticity of these ligatures is useful for selecting a specific brand of elastomeric ligature for a special condition. The purpose of this study was to compare elastic modulus of seven commercially available elastomeric ligatures in the initial condition and after 28 days immersion in a simulated oral environment. Changes in the modulus of elasticity were also surveyed for each brand of ligatures after 28 days. In this experimental laboratory-based study, elastomeric ligatures were obtained from different companies and their modulus of elasticity were measured in the initial condition and after 28 days immersion in a simulated oral environment using Zwick testing machine at 5mm/mm cross head speed. The data were analyzed using SPSS software and ANOVA, Duncan and student's t-test. Different brands of elastomeric ligatures showed significant differences in elastic modulus both in the as-received condition and after 28 days immersion in the simulated oral environment [p<0.05]. Ligatures from American Orthodontics and Ortho Technology showed the lowest and ligatures from Dentaurum revealed the highest elastic modulus in the as-received condition. Modulus of elasticity in the ligatures from American Orthodontics, Ortho Technology and ITO increased significantly while in ligatures from Dentaurum it was decreased significantly after 28 days immersion in the simulated oral environment [p<0.05]. Ligatures from American Orthodontics and Ortho Technology are suitable for immediate use after bonding and in sliding mechanics while ligatures from Dentaurum are not suitable to use immediately after bonding

Comparing the dental root biomechanical properties following two different methods of instrumentation and obturation

Clinically, vertical root fracture occurs commonly in endodonttcaliy treated teeth. Previous studies have shown little changes in root fracture strength following endodontic treatment. The purpose of this study was evaluation of changes in dental root biomechanical properties following two different methods of instrumention by Rotary [R] or Hand [H] files and two different methods of obturation by lateral [L] or vertical [V] technique. In this invitro study, one hundred extracted human mandibular premolars with straight root, closed apices and free of caries were selected. For the teeth to be indentical and simple to study, all teeth were decoronated 3mm above the CEJ. With making artificial PDL, all teeth were mounted in acrylic molds. Teeth were randomly distributed into four experimental groups according to the instrumentation and obturation techniques used each with a sample size of 25. Hand instrumentation was done using stainless steel [S.S] files and Step-Back technique. Rotary instrumentation was done using Ni-Ti files [Race] and crown-down technique. Loading was applied using a crosshead conical tip mounted in an Zwick testing machine. The crosshead tip was initially placed into the canal orifice of each tooth, advanced vertically until it contacted the gutta-percha automatically and at a constant rate [2mm/min]. Root fracture was noted with observation of a sudden deflection in the running graph. Load to fracture was recorded in Newton [N]. The energy to fracture, slope of elastic area and displacement was calculated using origin V.5.Q software through running graphs. The data were analyzed using ANOVA and Duncan test. Maximum mean load to fracture was observed in RL group [524 N]. Minimum mean load to fracture was observed in RV [Rotary and Vertical] group [319 N]. A significant difference in mean load to fracture was found between the [Rotary and Vertical] RV and [Hand and Lateral] HL, [Hand and Vertical] HV and [Rotary and Lateral] RL and [Rotary and Lateral] RL and [Rotary and Vertical] RV groups [P<0.05]. Both the energy and displacement were significantly correlated with load to fracture. The fracture strength of roots obturated through vertical compaction of gutta-percha was lower than that of lateral compaction regardless of method of instrumentation. Fracture strength of root was not affected by the method of instrumentation [hand or rotary]

Evaluation of tensile properties of several brands of elastomeric ligatures used in Iran

Elastomeric ligatures are used to secure archwires into orthodontic brackets. Unless the ligatures show adequate tensile properties, they may tear while inserting in the brackets or between patient's appointments. The latter may cause unwanted tooth movement and prolong orthodontic treatment. In this study the comparison of tensile properties [tensile strength, extension to tensile strength, and toughness] of seven commercially available molded clear elastomeric ligatures was performed. In this experimental laboratory-based study, eiastomeric ligatures were obtained from American Orthodontics, Ortho Technology, GAC, Techno Tak, All Star, Dentaurum and ITO companies and their tensile properties were measured using Zwick testing machine with 5mm/min cross head speed in the as-received condition and after 28 days immersion in a simulated oral environment. The data was analysed by SPSS software. All of the ligatures tested showed a significant decrease in tensile strength, extension to tensile strength and toughness excluding ITO's toughness after 28 days [P<0.001], Different brands of elastomeric ligatures showed significant differences in tensile properties both in the as-received condition and after 28 days immersion in the simulated oral environment [P<0.001]. Under the study conditions, ligatures from American Orthodontics and Techno Tak companies showed superior tensile properties and ligatures from Ortho Technology company exhibited weaker tensile features compared to other groups. There were significant differences in tensile properties of different brands of elastomeric ligatures in both the as-received condition and after 28 days immersion in the simulated oral environment. There was a significant reduction in strength properties of all types of elastomeric ligatures after 28 days immersion in a simulated oral environment. Thus, it is recommended that all ligatures should be replaced at each visit to reduce risk of tearing

[Evaluation of load-deflection rate changes of nickel-titanium wire after single clinical application in three different crowdings]

Desired mechanical properties of nickel-titanium wires have resulted in some orthodontists to recycle these wires and apply them repeatedly. The aim of this study was to evaluate load-deflection changes of nickel-titanium wires after single clinical application in three different types of crowding. 80 nickel-titanium wires [Global trade mark] with 0.014 inch diameter were randomly divided into four groups as: control, mild, moderate and severe crowding. Wires were used in patients for a period of 6 +/- 1 weeks Then, load-deflection experiment, using Zwick machine was done. Data were analyzed by Hottelling, Fisher's exact and Duncan tests. Hottelling analysis showed a significant difference between the groups. Fisher's exact test showed that there was a significant difference between forces at 0.5, 1.0, 1.5, 2, 2.5 mm deflection loading and unloading but there was no significant difference in 3 mm loading deflection. This study showed that although single application of nickel-titanium wire [at least for the wire used in this study] for one orthodontic visit causes significant decrease in force levels in different types of crowding, residual force after single clinical application is still enough for repeated clinical application

[Evaluation of load-deflection rate of stainless-steel orthodontic wires available in Iran's commercial market]

Different behavior of stainless-steel wires manufactured by different companies is observed in clinical situations. The purpose of this study was to compare the load-deflection rate of a specific size of stainless-steel wire from different companies which are available in Iran,s commercial market. This comparison allows classification of these wires and also the wire with best characteristics will be clarified. This experimental study was carried on cross-sectional base. Round 0.016 inch stainless-steel wires from 9 different companies were used in this investigation. 14 wires from each company were chosen on random base. 14 Wires from each company were divided in two groups of 7, group A and B with 3 and 1.5mm deflection respectively. The tests were done using Zwick testing machine model Z250. Loading and Unloading forces of wires in predetermined distances were measured. The total of 126 tests were measured. Upon these measurements, the load-deflection curve was drown. Using SPSS software the one-way analysis of variance [ANOVA] was done and each wire from a company was compared to other companies using Tukey analysis. The load deflection curves obtained were similar to typical curve of stainless-steel wires. The one-way analysis of variance [ANOVA] showed that there was no significant difference between the wires belonged to group A in three different deflection, i.e. 2.5, 2, and 1.5mm at a level of 5%, but he difference between the wires belonged to group B in 1.5 mm deflection was significant [P<0.001]. In this study it was found out that among the 9 different wires, tested, the RMO wites produced the minimum force levels and Dentaurum wires produced the maximum force levels in both groups. This means that RMO wires had better tissue compatibility. Moreover, it was found out that even the wires made by the same factory did not have a similar behavior. Among the tested wires, the Ortho technology wires had minimum and the GAC ones had maximum similarity to each other