Testing of Orthodontic Springs Using Geometric Variables

Abstract

A common method of space closure utilizes loops formed in orthodontic arch wires. The loops must provide adequate force characteristics to maintain controlled tooth movement. Generally, a moment-to-force (M/F) ratio of 10 mm is considered effective. The 1/2 Baldwin Spring is an effective loop, but it is difficult to fabricate and potentially irritating in the oral cavity. T- and L-loops are commonly used in orthodontics. They are conceivably less effective for controlled tooth movements but are simple to fabricate and relatively more comfortable in the oral cavity. This study demonstrates how altering the vertical and horizontal dimensions of the T- and L-loops affects their force characteristics. The vertical dimensions were 6 mm and 7 mm for the T- and L-loop. The horizontal dimensions were 6 mm, 7 mm, and 8 mm for the T-loop and 4 mm, 5 mm, and 6 mm for the L-loop. The experimental groups were named according to loop shape, vertical dimension, and horizontal dimension: T66, T67, T68, T76, T77, T78; L64, L65, L66, L74, L75, L76. Specimens were fabricated from 0.016 in x 0.022 in stainless steel and tested at 1 mm, 2 mm, and 3 mm horizontal activation distances (dx). Then they were preactivated by 30° gable bends, heat-treated (30°-htx), and tested again. Experimental data collection included the moment about the z axis (Mr) and the force along the x axis (Fx). TheMr/ Fx ratio was calculated. The T-loop with 30°-htx provided the highest M/F ratios at 1 mm dx and ranged from 5.15 mm (T68) to 6.67 mm (T67) (p<0.05). The T77 with 30°-htx exhibited the most consistent M/F ratios at 1 mm, 2 mm, and 3 mm dx: 6.09 mm, 6.01 mm, 6.04 mm consecutively (p~.6309). The L-loop provided scattered data ranging from -0.50 mm (L65) to 4.15 mm (L75). In general, 30°-htx increased the M/F ratio in all groups. T6* loops provided higher M/F ratios than TT* loops at small dx. T*8 loops provided consistently low M/F ratios ranging from 4.92 mm (T68) to 5.18 mm (T78). Altering the vertical and horizontal dimensions of the Tand L-loop significantly affects their force characteristics (p<0.05). None of the geometrically altered T- and L-loop designs provided ideal M/F ratios. These data suggest that the best T-loop designs for space closure are T66,T67, and T77.