The changes in maxillary arch form induced by two closely analogous orthodontic therapies were assessed from study models of Angle's Class II, division 1 patients. The traditional linear arch width and length dimensions proved difficult to interpret, however, as these parameters combined size and shape together. By contrast, when the dental arch datum point Cartesian coordinates were subjected to a simplified form of finite element analysis, this rigorous geometric technique disclosed that the dental arch size changes resulting from orthodontic treatment, were greatest in the sample where bilateral first premolar extraction was a component of the orthodontic treatment. Also the analysis showed that the dental arch shape changes were greatest in the non-extraction compared with extraction sample. This study therefore emphasized the complexity of dental arch changes resulting from orthodontic treatment, compared with extraction sample.