Mathematical and descriptive classification of variations in dental arch shape in an Australian aborigine population

Analysis of change in arch form with premolar expansion

The arch forms of 38 cases (53 nonextraction and 23 extraction arches) in which expansion, while maintaining arch form, was the objective of the practitioner, were analyzed before treatment, after treatment, and an average of 6 to 8 years after retention. The cubic spline was used to fit a curve representing arch form. By superimposing the spline curves, changes in arch form were analyzed with the variables rebound change (RC), rebound index (RI), rebound number (RN), and stability number (SN). Traditional linear intraarch dimensions were also analyzed. Analysis of variance was used to determine differences between the maxillary and mandibular arches and between the extraction and nonextraction cases. Pearson correlation coefficients between spline variables and arch width variables were also computed. There was significantly more expansion in the maxillary arch than the mandibular arch during treatment, irrespective of extraction or nonextraction strategies. In the nonextraction cases, a greater amount of net expansion was achieved for all dimensions for the maxillary arch as compared with the mandibular arch. Overall, a relatively high stability in arch form was found. The findings suggest that stability may not be related to the amount of change produced during treatment. Significant expansion can be gained throughout the premolar regions and may be expected to be stable. The order of greatest net arch width gained was for the second premolars followed by first premolars, molars, and then the canines. The intercanine widths for both arches decreased toward pretreatment values, but were more stable in the maxillary arch in nonextraction cases. The cubic spline permits measurement of change in arch form both during treatment and retention periods.

Dental arch shape and tooth wear variability

The rapid rate of tooth wear frequently reported among certain contemporary aboriginal populations has often been attributed to dietary form and abrasives. Several investigators have reported a close correlation between food bulk and the wear planes formed over the dental arches, i.e., steep oblique wear vs. flat horizontal planes. In this investigation we demonstrate that arch shape is an additional and a significant factor influencing the distribution of wear facets and exposed dentin over occlusal surfaces. We examined 64 dental stone casts of Aboriginals from Yuendumu, Central Australia, born between 1900 and 1940. These casts offer a record of the variety of tooth wear and arch forms and their interrelationships. This group of individuals, some subsisting on abrasive and some on soft diets, have dentition which exhibit various wear rates and wear patterns probably due to the diversity of arch shape, size, and occlusal relationships. Hypsiloid or U-shaped maxillas had a more buccally directed wear in contrast to the parabolic or hyperbolic forms, which exhibit a heavier lingual loading. Varying occlusal conditions also contribute to differing wear patterns over the arches. Individuals with alternate intercuspation, for example, have a more horizontally directed wear. These and other conditions of shape, size, and occlusion emphasize the importance of morphological factors in the production of tooth wear rates and patterns in addition to dietary abrasives.