The factors that dictate dental arch forms in the present fixed appliance therapy are the shape of the arch wire forms and the bracket system in which they are intended to be used. Nowadays, most people perceive a “broader smile” as desirable. To achieve a broader smile, it is necessary to expand the dental arches. However, this can create an unfavourable effect on the stability of the arch form. If there is a conflict between the patient’s desired aesthetic outcome and arch form stability, the orthodontist must be able to inform the patient about it so that both parties can make a decision about it.
Orthodontists utilise the cubic spline curve as a mathematical representation of the arch form. The cubic spline curve is composed of separate cubic polynomial segments connecting series of points called knots in such a manner that the curve is smooth through the knots. Through the spline curve method, the orthodontist is able to evaluate arch form contribution to stability.
For many years, the basis for standard arch form has been the catenary and trifocal ellipse curves. After quite some time however, most orthodontists have recommended using the original lower model of the patient as the arch form throughout the treatment. Though this method is all right, it is quite impractical for the reason that the model has to be brought out during every patient’s visit. Because of this, the standardization of the arch form with size options has been developed to solve this issue.
There has been a contemporary theory by Andreiko which states that the shape of the mandible should determine the arch form, instead of using the catenary or ellipse curve as the basis. In this theory, the teeth must be aligned and and contained within the limits of the mandibular bone. This new approach takes skeletal and dental anatomy more into consideration than mathematical representation.
In the past, orthodontists need to make bends in the arch wires in order to make adjustments in tooth position. Now, with the development of preadjusted brackets, the need to bend the arch wires has been reduced.
Perhaps the newest method of designing the arch form is through the use of digital scans of the patient’s teeth. Through this, the orthodontist can do mapping of the arch form and anatomy through the computer interface and can design the arch form specifically for the individual patient.