To evaluate the vertical distance between the maxillary first molars (MFMs) and the maxillary sinus floor (MSF) and its interrelationship with sex, age, and vertical facial pattern in skeletal ClassⅡ patients to provide a reference for clinical orthodontic treatment.

Sixty teenagers and sixty adults with skeletal Class Ⅱ malocclusion who met the inclusion criteria were selected to evaluate the vertical relationship between the MFMs and the MSF on cone-beam CT (CBCT) images. The vertical distance between the roots of the MFMs and the MSF was measured. Statistical analysis was used to assess differences between patients by sex, age, and vertical facial pattern.

The contact percent of the roots of MFMs and MSF was 85% and 56% in skeletal Class Ⅱ teenagers and adults, respectively. The contact percent and penetration percent of the roots with MSF were higher in teenagers than in adults（P<0.05）. The penetration percent of the high-angle (HA) and the normal-angle(NA) groups was 34.1% and 36.6% respectively, which was significantly higher than that in the low-angle (LA) group(20.8%)（P<0.05）. The difference between the distance of the bilateral MFMs and the MSF was not significant in skeletal Class Ⅱ patients (P>0.05); No significant difference was found between different sexes of skeletal Class Ⅱ patients when comparing the distance of the MFMs and the MSF (P>0.05). The MFMs of skeletal Class Ⅱ teenagers were closer to the MSF than those of adults (P<0.05). In the adult group, the distance was not significantly different in different vertical facial patterns (P>0.05). In the teenager group, the MFMs were more closely related to the MSF in the NA and HA groups than in the LA group. Among them, the difference between the mesiobuccal roots and distalbuccal roots was significantly different (P<0.05). There was no significant difference between the groups of the palatal roots (P>0.05).

The MFMs were closer to the MSF in skeletal Class Ⅱ teenagers than in adults. The distance between the MFMs and MSF was associated with the vertical facial pattern in skeletal Class Ⅱ teenagers, while it was not associated with the vertical facial pattern in adult patients.