ORIGINAL ARTICLE Mandibular arch form: The relationship between dental and basal anatomy Valerie Ronay, a R. Matthew Miner, b Leslie A. Will, c and Kazuhito Arai d Vienna, Austria, Boston, Mass, and Tokyo, Japan Introduction: We investigated mandibular dental arch form at the levels of both the clinically relevant application points of the orthodontic bracket and the underlying anatomic structure of the apical base. The correlation of both forms was evaluated and examined to determine whether the basal arch could be used to derive a standardized clinical arch form. Methods: Thirty-five mandibular dental casts (skeletal and dental Class I) were laser scanned, and a 3-dimensional virtual model was created. Two reference points (FA, the most prominent part of the central lobe on each crown’s facial surface, and WALA, a point at the height of the mucogingival junction) were selected for each tooth from the right to the left first molars. The FA and WALA arch forms were compared, and the distances between corresponding points and intercanine and intermolar widths were analyzed. Results: Both arch forms were highly individual and the tooth values scattered. Nevertheless, a highly significant relationship between the FA and WALA curves was found, especially in the canine (0.75) and molar (0.87) areas. Conclusions: Both FA and WALA point-derived arch forms were individual and therefore could not be defined by a generalized shape. WALA points proved to be a useful representation of the apical base and helpful in the predetermination of an individualized dental arch form. (Am J Orthod Dentofacial Orthop 2008;134:430-8) T he size and shape of the dental arches have considerable implications for orthodontic diagno- sis and treatment planning. These factors have an effect on space available, stability of the dentition, and dental esthetics. Furthermore, the definition of arch form would improve the understanding of malocclusion and assist clinicians in producing orthodontic results that are consistent with the natural laws of biologic variation. Although most arch form studies have looked at similar patient samples—subjects with orthodontically untreated ideal occlusions—few come even close to agreement about the natural shape of the dental arch. It is commonly believed that the dental arch form is initially shaped by the configuration of its supporting bone. 1 Nevertheless, 2 opposing theories about modifying the dental arch form have coexisted for 100 years. 2,3 The bone-growing theory is that the supporting bone grows in response to normal stimulation, such as mastication, if the teeth are aligned in the ideal position. Angle 4 reported stable orthodontic treatment results of his expanded crowding patients and first advocated the bone-growing theory. In the latter part of the 19th century a basic biologic principle was introduced called Wolff’s law in which the bone structure changes in response to external force. According to this theory, tooth size is controlled by heredity, but size and shape of the supporting bones depend largely on environmen- tal stimuli including eruption of the teeth, pressure from tongue and cheek, and mastication. For example, a small mandible can result from the lack of healthy jaw function and indicates degeneration. 5 This approach resulted in fewer extractions and is often called the nonextraction theory. According to the “apical base” theory, the size and shape of the supporting bone are largely under genetic control, and there is a limit to expansion of a dental arch. In 1925, Lundström 6 proposed a new term—apical base—to describe the limits of expansion of the dental arch and wrote extensively on this topic. He stated that the apical base (1) is not changed after loss of teeth, (2) is not influenced by orthodontic tooth movement or masticatory function, and (3) limits the size of dental arch. If the teeth are orthodontically moved beyond this limit, labial or buccal tipping of the teeth, 6 periodontal problems, 7 or an unstable treatment result 8 could be expected. 2 One of Angle’s students, Tweed, 9 also observed unstable results after nonextraction treatment with Angle’s mechanics a Student, Clinic of Dentistry, Vienna University, Vienna, Austria. b Assistant clinical professor, Department of Developmental Biology, Harvard School of Dental Medicine, Boston, Mass. c Professor and graduate program director, Department of Orthodontics, Tufts University School of Dental Medicine, Boston, Mass. d Assistant professor, Department of Orthodontics, Nippon Dental University, Tokyo, Japan; visiting professor, Department of Developmental Biology, Harvard School of Dental Medicine, Boston, Mass. Reprint requests to: R. Matthew Miner, One Lyons St, Dedham, MA 02026; e-mail, r_miner@hsdm.harvard.edu. Submitted, April 2006; revised and accepted, October 2006. 0889-5406/$34.00 Copyright © 2008 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2006.10.040 430