Orthodontic Bracket Manufacturing Tolerances and Dimensional Differences between Select Self-Ligating Brackets

Abstract

In all manufacturing processes there are tolerances; however, orthodontic bracket manufacturers seldom state the slot dimensional tolerances. This experiment develops a novel method of analyzing slot profile dimensions using photographs of the slot. Five points are selected along each wall, and lines are fitted to define a trapezoidal slot shape. This investigation measures slot height at the slot's top and bottom, angles between walls, slot taper, and the linearity of each wall. Slot dimensions for 30 upper right central incisor self-ligating stainless steel brackets from three manufacturers were evaluated. Speed brackets have a slot height 2% smaller than the nominal 0.559 mm size and have a slightly convergent taper. In-Ovation brackets have a divergent taper at an average angle of 1.47 degrees. In-Ovation is closest to the nominal value of slot height at the slot base and has the smallest manufacturing tolerances. Damon Q brackets are the most rectangular in shape, with nearly 90-degree corners between the slot bottom and walls. Damon slot height is on average 3% oversized.

Orthodontic bracket slot dimensions as measured from entire bracket series

“…Major et al 7 reported the Speed brackets to be slightly undersized at the base 0.556 mm (0.0219 inches) with ''pronounced rounding'' and ''large fillets'' where the base meets the wall and slight convergence. Such bases-to-wall issues were also present throughout this sample.…”

“…Major et al 7 measured 30 upper right central 0.022-inch stainless steel self-ligating brackets from three manufacturers. They reported slot heights from 2% smaller to 3% over the nominal size.…”

Dimensional accuracy of ceramic self-ligating brackets and estimates of theoretical torsional play

“…Regarding slot dimensions, Bhalla et al 6 reported that 0.022-inch InOvation-C (Dentsply, York, Penn), Clarity-SL (3M Unitek) brackets had slot tops that were wider than the given value by 11.4% and 9.5%, respectively, and that the bracket slots diverged. According to Major et al, 5 In-Ovation metal brackets (Dentsply), and Damon metal brackets (Ormco) were oversized, and InOvation bracket (Dentsply), slots were divergent, whereas Damon bracket slots (Ormco) were nearly parallel. Moreover, Cash et al, 4 in their study of conventional and self-ligating brackets, reported that all brackets were larger than stated by the manufacturer, with an error range of 5% to 24%.…”

“…5,12 As the slot-archwire difference is a major contributor to torsional play, the authors aimed to measure the actual slot sizes and wire dimensions and thus accurately quantify the theoretical torsional play in ceramic self-ligation brackets.…”

“…However, previous studies have reported dimensional inaccuracies in the brackets and archwires, often creating excessive torsional play. [4][5][6] As a result, torque is insufficiently controlled, especially in the incisors in extraction cases, and this compromises clinical outcome.…”