The identification of a suitable arch form for treating each malocclusion is key for achieving a stable, functional, and esthetic occlusion. Clinically, it would be appropriate to have several preformed arch forms to choose from for individual patients after identification of patient’s pretreatment arch form [14].
Several factors have been claimed to affect arch size and form such as ethnic background, type of malocclusion, variability in eruptive paths of the teeth, growth of the supporting bones and the movement of the teeth after emergence due to habits and unbalanced muscular pressures [11–12, 20].
It was intended in this study to identify the forms of maxillary and mandibular arches in a Jordanian population. Several studies have been conducted on different populations [10, 13, 21–23]. Most of the conducted studies were describing the mandibular arch form because the mandible is considered as the reference element of diagnosis and treatment in orthodontics [24]. According to several authors, the stability of the form and dimension of the mandibular dental arch is a factor of stability of the results [25]. Only few studies dealt with maxillary arch [22, 26].
Sixth polynomial function was used in this study to determine the arch form from the digitized points of tooth positions on the dental arch. It has been reported that the sixth-degree polynomial equation was the function that best described dental arch configuration. Polynomial functions with lower degrees compromised the descriptions of some important dental arch regions, such as anterior curvature of the mandibular arch and posterior tooth alignment [27].
Five arch forms were identified in each of the maxilla and the mandible in this study. Some previous studies reported three arch forms for their studied populations, others reported 5 different arch forms and some others reported eight arch forms [10, 24, 27–28].
The method used to determine the arch form in this study ensured high accuracy and objectivity since allocation of the curves into their corresponding arch form and size was constructed by a computer software. Other studies allocated them manually, by visual observation or by simple calculations [10, 22, 24].
The results of this study showed that there are at least five arch forms describing dental arches for untreated young Jordanian adults with normal occlusion. However, arch form 1 (Catenary) was the most common form representing almost half of the studied sample with a slightly higher frequency in the maxilla than in the mandible.
Telles [28] reported an elliptical mandibular arch form for the majority of their sample representing almost two thirds of the subjects.
The second most common arch form was form 2, a form between elliptical and U-shaped arch, with a relatively large intercanine distance. About one quarter of the curves fell under this category. Ricketts [29] reported that one third of his sample exhibited this arch form while Triviño et al. [10] reported only 9 per cent of the studied mandibular arches belonged to this category.
Form 3 was not commonly observed in previous studies. This form has a morphology of projecting central incisors with a widening in the posterior region. It was described as "tudor" curve by architects. It was found in about 10% of our sample in both the maxilla and the mandible. Triviño et al. [10] reported a higher percentage (18%) of this form in their sample.
Form 4 was observed in a small number of our sample (around 7%). This form has a pointed anterior region. It was described in other studies with different frequencies. Raberin et al.25 found this form in 19.4% of his sample while Triviño et al. [10] found it in only 2% of his studied sample.
In form 5, the incisors are arranged in a straight line with the initiation of the curvature at the distal region of the lateral incisors, it was described as a quadrangular. This arch form occurred in a low frequency generally with a lower frequency in the maxilla than in the mandible. Triviño et al. [10] reported a similar figure in the mandible. On the other hand, Triviño and Vilella [30] reported a higher percentage of this arch form which was the predominant form in that study.
The differences in arch forms between our study and other studies could be attributed to different ethnic backgrounds, sample characteristics and study methodology.
Size 2 (medium) was the predominant size in the maxilla and the mandible, in almost all the forms, a finding that has been reported by other authors [29–30].
Comparing arch forms between females and males, there were differences in arch forms 1 and 3 in the maxilla and arch form 1 in the mandible contrary to the findings of other studies who reported similar forms but different sizes in both genders [24, 30].
Differences in the results could be explained by different ethnic backgrounds and reference points used for measurements.
The transverse measurements carried out in this study were intercanine and intermolar widths. The mean of these parameters were close to the values found in other studies conducted on samples with similar criteria to our sample (Staley et al., 1985). Other studies reported smaller intercanine widths but larger intermolar widths [32–33].
Out of the 5 arch forms that were found for the Jordanian population, two constituted the majority of all arch forms. The rest were less frequent. It is recommended that clinicians keep the most common arch forms in their armamentarium. However, if a patient presented with one of the less common arch forms, it is of paramount importance for the clinician to respect the original arch form of this patient. Therefore, with less common arch forms, clinicians should make adjustments to the archwires according to the patient’s arch form to reduce the chances of relapse especially in the intercanine width; since changes in intercanine width is associated with high risk of relapse [25].