Introduction: We previously reported that, on average, maxillary molars erupt with buccal crown torque and upright with age, mandibular molars erupt with lingual crown torque and upright with age, and these molar crown torque changes are accompanied by concurrent increases in maxillary and mandibular intermolar widths. Our purpose in this study was to determine the transverse skeletal changes that accompany these molar movements during growth.
Methods: Thirteen maxillary and mandibular transverse measurements were made by using casts and posteroanterior radiographs from 36 Class I untreated subjects from the Iowa Facial Growth Study at approximately ages 7.6, 10.3, 12.9, 16.5, and 26.4 years. Intermolar arch widths, maxillary basal bone widths (bijugale), and mandibular basal bone widths (bigonion, biantegonion) were recorded for each subject. Also recorded were the maxillary and mandibular cross-arch alveolar process widths measured at the molar gingival crest and the midalveolar level.
Results: The results indicate a pattern of width changes in the maxilla, the maxillary alveolar process, the maxillary first molars, the mandibular first molars, and the mandibular alveolar process that occurs as a gradient in the vertical dimension. The greatest width change occurs superiorly (jugale point). The smallest width change occurs inferiorly (midalveolar point of the mandible). On average, the basal bone of the maxilla increased in width by 5.37 mm; maxillary cross-arch midalveolar process width increased by 3.20 mm (left buccal surface to right buccal surface) and 4.94 mm (left palatal surface to right palatal surface); maxillary intermolar width increased by 3.08 mm; mandibular intermolar width increased by 2.05 mm; and mandibular cross-arch crest level alveolar width increased by 1.60 mm (left buccal surface to right buccal surface) and 1.02 mm (left lingual surface to right lingual surface). The basal bone of the mandible increased in width by 14.54 mm.
Conclusions: We concluded that transverse molar movements during growth mirror transverse maxillary basal bone width increases, maxillary cross-arch alveolar process width increases, and mandibular cross-arch alveolar process width increases. Furthermore, there is a pattern of width changes in the maxilla, the maxillary alveolar process, the maxillary first molars, the mandibular first molars, and the mandibular alveolar process that occurs as a gradient in the vertical dimension. On the other hand, transverse mandibular basal bone changes, measured as bigonion and biantegonion, do not fit this pattern.