Evaluation of the initial resistance to sliding between various esthetic bracket and archwire combinations: An experimental model using non-aligned brackets

Objective. Esthetic brackets and archwires are becoming increasingly popular in orthodontic practice due to the growing cosmetic demands of patients. To date, little research has been done to determine the frictional values of esthetic archwire and bracket combinations. Several variables influence the frictional resistance between brackets and archwires, such as the bracket and archwire material, the dimensions of the bracket slot and wire, bracket angulation, ligation method, inter-bracket distance, and moisture conditions. The goal of this in-vitro study was to evaluate the initial frictional resistance to sliding (IRS) generated by various combinations of ceramic brackets and orthodontic archwires by using an experimental model with three non-aligned brackets.;Methods. Two types of esthetic orthodontic brackets were tested: active self-ligating brackets and conventional twin ceramic brackets. The following .018" diameter orthodontic archwires were tested: stainless steel, nickel titanium, rhodium-coated nickel titanium (GAC Bioforce), Teflon RTM-coated nickel titanium (in both .018" and .020" diameters)(Forestadent Titanol), and unidirectional fiber-reinforced polymer (UFRP) composite (.018")(BioMers ClearWire). Ceramic brackets of four different designs were mounted in a three-bracket non-aligned pattern with the middle bracket placed 0.5 mm higher than the adjacent brackets to mimic the initial leveling and aligning phase of orthodontic tooth movement. Each of the twenty-four esthetic bracket-archwire combinations were tested ten times. Each archwire was pulled at a rate of 2.0 mm per minute for 30 seconds using a MTS Instron machine. The resultant static frictional force was measured in Newtons (N), and the data were analyzed utilizing a two-way ANOVA.;Results. Since archwire diameter, bracket angulation, inter-bracket distance and testing conditions were held constant, friction (IRS) was directly influenced by archwire and bracket material, surface characteristics, bracket slot composition, and method of ligation.;Conclusions. The Teflon-coated nickel titanium (NiTi) archwire (Titanol) demonstrated significantly lower frictional resistance to sliding than all the other archwires. No statistical difference in friction was noted between the unidirectional fiber-reinforced polymer (UFRP) composite archwire (ClearWire) and the rhodium-coated NiTi (Bioforce) archwire. Both Bioforce and ClearWire exhibited significantly less friction than uncoated nickel titanium and stainless steel archwires, and had significantly greater friction than .018" and .020" Titanol wires. When compared with conventional twin ceramic brackets with elastomeric ligation, ceramic self-ligating brackets demonstrated the lowest frictional resistance to sliding. A combination of the stainless steel archwire with both the QuicKlear self-ligating bracket and Transcend twin bracket with a ceramic slot created the greatest amount of friction. The ClearWire composite archwire had the lowest frictional values when used in combination with ceramic self-ligating brackets. When considering the composite archwire (ClearWire), its application in the clinical setting may be limited to treatment of minor malocclusions due to its tendency to craze or fracture with moderate deflection. For best results, deflection of this archwire should be minimal and ligation forces should be kept low.