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Comparison of commercially available arch wires with normal dental arch in a group of Iranian population.

Statement of the Problem The stability of orthodontic treatment depends on preserving the patient’s pretreatment arch form and arch size during and after treatment. Purpose This investigation was aimed to study the size and shape of Iranian mandibular dental arch and evaluate the correlation of their average dental arch with commercially available preformed rectangular nickel-titanium arch wires. Materials and Method In this study, 148 subjects were selected among students of Shiraz University of Medical Sciences. The inclusion criteria were having Angle class I in molar and canine relationships, and normal growth pattern. Intercanine and intermolar widths were measured after scanning their mandibular dental casts. Three main arch form templates; square, ovoid and tapered (Orthoform TM; 3M, Unitek, CA, USA) and 12 commercially available preformed mandibular nickel-titanium arch wires were scanned. Intercanine and intermolar widths of arch wires were compared with dental arch widths of the study samples. Arch width, arch form and the most appropriate arch wire were determined for each cast. Student’s t-test was used to compare arch widths and arch depths of male and female subjects. Coefficient of variance was used to determine the variability of indices in the study samples. Results Most preformed arch wires were wider than the average width of the normal Iranian dental arch. The most frequent arch form in Iranian population was tapered. Inter molar width was the only statistically significant variable between males and females. Conclusion Variation in available preformed arch wires does not entirely cover the range of diversity of the normal dental arch of our population. Narrow arch wires with a tapered shape are better consistent with the Iranian lower arch.


Introduction
The most important part of orthodontic treatment is aligning the teeth on the patient's dental arches. Each patient has a special arch form and arch size. Stability of orthodontic treatment depends on preserving the patient's pretreatment arch form and arch size during and at the end of treatment. [1][2][3] Arch width and shape are important characteristics of the dental arch. Although different classifications of arch form have been suggested, three main arch forms (ovoid, tapered, square) are commonly used by the clinicians. [4] Arch perimeter, arch width and arch depth are used for arch size measurements. Inter-canine and inter-molar widths are accurate indices for showing muscle equilibrium. [5] Longitudinal studies have shown high probability of relapse after increasing arch width especially in the mandibular canine region. [6] When the Edgewise technique was first introduced in the 1920s, bending the arch wires in order to match the dental arch was an important part of orthodontic treatment and dental casts were used in order to form arch wires. [7] Today, dental casts are replaced by 3-D digital models to produce prefabricated arch wires. [1,8] Since the introduction of nickel-titanium wires, preformed types of these wires have been widely used, particularly in the initial phases of orthodontic treatment. [9] Invention of self-ligating and straight wire systems has further increased using rectangular nickeltitanium wires. [10] It is possible to change the form of preformed nickel-titanium wires with cold forming or by using a heat source. But these techniques are not recommended because of inducing significant changes in force level of the wire. [9] Some orthodontists do not care about the size of preformed nickel-titanium wires, since they believe the original arch size and arch shape will return back after using stainless steel arch wires with appropriate size and shape. This method is not recommended because it causes round tripping movement of the teeth during treatment and increases the subsequent side effects. [11] Dental arches vary in different races and populations. [2,[4][5] Therefore arch wires should be selected according to the related population's arch size and arch shape. In a study of American patients by Braun et al., thirty three preformed nickel-titanium wires were compared with normal dental arches. They reported that the intercanine and intermolar widths of upper and lower preformed arch wires were larger than the average dental arch widths in almost their entire sample. [11] Similar results were achieved by another study conducted in India. The average intermolar width exceeded the average dental arch width by 2.893 mm in the maxillary arches and 1.861 mm in the mandibular arches. The average intermolar-intercanine width ratios for natural arches (2.11 for mandibular and 1.75 for maxillary) were greater than the ratios for the wire-bracket assemblies (1.78 for mandibular and 1.75 for maxillary). [5] According to a study conducted by Tulin Taner et al. in Turkey, maxillary and mandibular arch widths increased during orthodontic treatment. Arch form in both maxilla and mandible of Turkish samples in that study was tapered before the treatment. Maxillary arch forms changed in 81% of samples during the treatment due to using arch wires incompatible with the patients' arch form. [3] Contrary to the studies mentioned before, Souichiro Oda et al. revealed that preformed nickeltitanium wires were significantly narrower than Japanese dental arch in canine and molar regions. The preformed arch wires were approximately 1 to 3mm narrower at the canine level and 2 to 5 mm narrower at the first molar level. [7] Because most of the available arch wires in Iran are designed according to normal dental arches of European and American population, this study was undertaken to compare the commercially available preformed nickel-titanium arch wires with the Iranian Angle class I normal occlusion dental arches and introduce the highest correlated arch wires with Iranian dental arch size and shape.

Materials and Method
Our study samples were 148 orthodontically untreated students, including 67 male and 81 female subjects.
They were selected through convenient sampling among the students of Shiraz University of Medical Sciences.
The inclusion criteria were having skeletal and dental class I, normal vertical growth pattern, normal overjet were also scanned.
Cast analysis was performed in order to select the most appropriate arch wire as well as to determine the arch shape for each subject. The indices, including intermolar width, intercanine width, canine depth, and molar depth were measured during these two stages to achieve higher accuracy ( Figure 1).

Results
The most frequent arch form in the study population was tapered (45%). Square and ovoid arch forms were respectively the next common arch forms ( Table 1). The most common arch wires for each arch shape are presented in Table 1. The same order of arch form frequency was seen in male and female groups.  (Table 3).

Discussion
Ethnicity is an important factor that influences the shape and dimension of dental arches.  Studies conducted by Uysal et al. [14][15] [5] whereas in other studies conducted in Japan the average width of preformed arch wires was found to be narrower than the mean of Japanese dental arches. [9,12] In spite of the availability of various brands of arch wires in Iran, only a few of them can be used safely to avoid post treatment instability. These facts suggest that manual arch wire adjustments may be necessary for prevention of side effects of stainless steel arch wires with inappropriate width. An orthodontist must be able to form suitable arch wires for each patient. Arch shape and arch widths in patients with class III, class II, long face and short face tendency are different from the normal population. Thus, further studies to compare preformed arch wires with these patients are required. [14][15] Also extraction cases with severe crowding may need special preformed nickel-titanium arch wires due to their smaller arches.
In the near future when digital models will eventually replace dental casts, arch wire selection with software may become one of the steps in designing treatment for each patient and using custom made arch wires may become frequent in patients with dentofacial deformity and cleft lip and palate patients.

Conclusion
Most of the preformed arch wires were wider in both intercanine and intermolar width than the average widths of our population dental arch. The variation in available preformed arch wires does not entirely cover the range of diversity of the normal dental arch. Narrow arch wires with tapered shape are better consistent with the Iranian lower arch.