Lower lip position and incisor overjet in a 12-year-old Nigerian population.

Relationship between variation of overjet and different postural positions of the lower lip in 1,016, 12-year old Nigerians was investigated using a modified Jackson classification (1962). The range and mean overjet values tended to decrease as the extent of incisal coverage increased with statistically significant differences in lip positions. Overjet greater than 6 mm was consistently associated with 'trapped' lower lip. The results of this study also indicate that lower lip may provide inadequate lip coverage to the upper incisors when the overjet is greater than 7 mm.