Straightwire brackets allow for essentially four types of pre-adjusted tooth movement:

a. angulation (tip)
b. in – out (depth)
c. rotational control
d. inclination (torque)

Tip, in–out, and rotation control have become highly uniform in all current and popular appliance prescriptions. Torque, on the other hand, is available in a variety of ranges.

Unfortunately, most clinicians using straightwire brackets, assume they achieve the torque values outlined in their bracket prescription.

The actual torque (effective torque) is defined as the torque resulting from the combination of built-in pre-adjustments, slot size, and arch wire choice. A German study in 1993 arrived at a formula to calculate the effective torque. This study considered the necessary minimal edge rounding as well as the nominal sizes for orthodontic square and rectangular wires (F.Sernetz, Kieferorthop. Mitteilungen 7:13 –26). Other studies have shown that for every 0.001” slop between the arch wire and the bracket slot there is a loss of 4 degrees of effective torque.

From a clinical perspective this means that if you are using the standard Ricketts prescription of 22 degrees (maxillary central incisor) torque in an 0.022” bracket slot, your thickest working wire, which is 0.019” x 0.025” will only produce an effective torque of 12.4 degrees.

Variable arch wire shapes and bracket slots virtually eliminate the effectiveness of third order pre-adjustments. For example, common 0.019” x 0.025” archwires in 0.022” bracket slots can vary in deviation at any point between 7 and 36 degrees.

Inconsistent, unpredictable, inaccurate and unreproducible actual torque forces lead to numerous clinical problems including:

a. greater chair time
b. extended treatment time
c. unpredictable torque forces imparted to the tooth crowns
d. difficulty in replicating mechanics from patient to patient
e. potential for long term case instability
f. frustration for the doctor, staff and patient

A JCO article by Dr Siatkowski discussed the loss of anterior torque control due to variations in bracket slot and archwire dimensions. He stated that the once reliable intra-arch protraction mechanics have now become unreliable. He noticed with incisors tipping lingually instead of maintaining their antero-posterior positions, under force systems designed to protract the posterior teeth.

This undependability coincides with the introduction of an increasing number and variety of pre-adjusted bracket systems. Another study by Kusy and Whitley indicated that variations in bracket slot and arch wire dimensions may count for the failure of protraction mechanics, and for the loss of incisor axial inclination control during other forms of tooth movement. (Angle Orthod. 69:71-80, 1999)

SOLUTIONS TO THE PROBLEM:

Given the variations in bracket slot and wire sizes that currently exist it would be sensible for practitioners to perform their own measurements. A micrometer or digital caliper can be used to measure dimensions of archwires when they are purchased from the manufacturer.

Bracket slots may be measured relatively inexpensively by using spark plug feeler gauges. These can be found in most auto spare part stores as they are used for setting spark plug gaps. Incisor brackets with grossly oversized slots should be returned to their manufacturer as “defective goods”.

Alternatively, for en masse protraction mechanics, lingual root torquing auxiliaries can be added, over the main archwire, to ensure adequate AP incisor control. I favour the ART auxiliary for this purpose.

The third option is to ensure that the orthodontic supplier specifies the measured central incisor bracket/wire torque for each of its archwires and each bracket slot size. To this day I know of only one company, Ortho Specialties, which currently does this. This company introduced a 0.020” slot variable torque bracket system. Ortho Specialties hypothesized that if all actual torque ranges in each bracket system could be replicated on one system then, considering the relative standardization of the other planes of tooth movement, an all inclusive appliance is possible. The company has achieved this through their choice of a 0.020” x 0.030” slot and a selection of built-in torques that, when used in combination with specific commonly available wires, would replicate the actual torques of any technique and appliance.

The main advantage of the VTA (variable torque appliance) system is that actual torque information is supplied. This is because this system is not in itself a technique, but a way of replicating other known techniques. Reproducibility of mechanical sequences is also more predictable and easier to achieve.

CONCLUSION:

Orthodontic practitioners should be aware of the limitations of pre-adjusted edgewise brackets. It is important that the clinician realises that prescribed torque values are very rarely achieved due to the slop of the archwire in the bracket slot.

To overcome these problems a practitioner can a) use higher torque brackets (eg. Ricketts prescription), b) a variable torque appliance (VTA), c) place extra torque in the archwire or d) employ torquing auxiliaries (e.g. ART auxiliaries).