The force delivery of orthodontic buccal retractors of smaller diameter wires : an in vitro study

Abstract

Load deflection rate (LDR) refers to the amount of force produced per unit deflection. A buccal retractor on a removable orthodontic appliance is commonly used to retract an upper canine. The traditional unsupported 0.7mm retractor is difficult to adjust and unstable in the vertical direction. Zietsman and Botha (2000) suggested a supported retractor by inserting the distal leg of a 0.5mm retractor in a tube which is soldered onto a molar clasp or continues into the palatal acrylic for more support. The aim of this study was to assess the force delivery of removable orthodontic buccal retractors of smaller diameter wires. In addition, the relation of load deflection rate (LDR) to coil type, wire diameter and wire length was also examined. Two different coils, the standard and reverse coil types, were made using three different wire sizes namely the 0.020, 0.022 and 0.028 inch diameter wires. The 0.028 inch diameter wire was used as a control. Unitek stainless steel wires were used. The wires were mounted on a modified measuring scale device based on the one used by Bass and Stephens (1970). The wires were deflected by using 40g weights at 14, 16, 18 and 20mm, and the deflections were read off on the ruler using a magnifying glass to the nearest quarter of a millimetre. The readings were recorded in a customised form and then transferred to a MS-Excel spreadsheet for further analysis. Means, standard deviations and distributions were calculated for all the variables. The interactions between the diameter, wire length and coil type were assessed and compared statistically using the student’s paired t-test. P values less than or equal to 0, 05 were considered significant. The results of the study showed that there was no statistically significant difference between the load deflection rate (LDR) values of the 0.020 and 0.022 wires. The LDR of the 0.028 diameter wire showed a statistically significant difference when compared to that of the 0.020 and 0.022 wires (P<0, 01). The results also showed that increasing the length of the wire decreased the LDR. The LDR is an indication of the force required per unit deflection. A wire with a high LDR value will transmit a high force for a short span of time compared to a wire with a low LDR value which will transmit a low force over a longer time. The results of the study indicated that smaller diameter wires yielded low LDR values when compared to the 0.028 diameter wire. The clinical relevance of this study could be pointed out to the ability of the smaller diameter wires to deliver lower forces over longer periods which might lead to an increase in rate of tooth movement. The recommendations from this study is that clinicians need to change the mindset when using removable appliances, as the current trends in fixed orthodontics focus is on use of lighter continuous forces, by using wires with low deflection rate to increase the rate of tooth movement. This was a laboratory study; therefore clinical application should be interpreted with caution.