Reduction of the Hypocone of the Maxillary First Molar and Class III Malocclusion

This study was conducted to examine the incidence of evolutionary changes of the maxillary molars in orthodontic patients and to investigate the association of this event with craniofacial growth. Among 4,892 Japanese patients treated at the orthodontic clinic of Hiroshima University Dental Hospital, 59 patients presented with a defect of the distolingual cusp of the maxillary first molar. They consisted of 52 female patients and seven male patients with a mean age of 14.7 years. Eruption of the maxillary first and second molars was delayed and the third molar was congenitally missing in all of these patients. Thirty-seven patients exhibited Class III malocclusion (Class III evolution group). The Z scores of cephalometric measurements were compared between the Class III evolution group and the Class III control group. The latter group consisted of 37 Class III subjects with no evolutionary dental anomalies and no orthodontic history. The tooth developmental score (TDS) for each age at the initial record was compared with Nolla's standard tooth developmental score (STDS) for equivalent ages using paired t-tests. The Z scores for Ptm'-A and Ar-A were significantly smaller in the Class III evolutionary group than in the Class III control group (P < .05). The TDS for the maxillary first and second molars were significantly smaller than the STDS for these molars (P < .01). The TDS for the maxillary second molar was significantly larger than the STDS for the maxillary third molar (P < .01). This study revealed that an evolution change of the maxillary molar is more common (P < .05) in female patients than in male patients, is more frequent (P < .05) in subjects with Class III malocclusion, and is related to the anteroposterior undergrowth of the maxilla.