<P> When the teeth are biting down, the maxillary central incisors occlude with the mandibular central and lateral incisors . The contact point of the mandibular teeth is in the lingual fossa of the maxillary central incisor about 4 mm gingivally from the incisal edge . In this position, the maxillary incisors cover nearly half of the mandibular incisors' crowns . When the maxillary and mandibular incisors do not contact even when the mouth is fully closed, an anterior open bite occurs . This misalignment of teeth may result from some habits, such as thumb - sucking . On the other hand, when the contact of the mandibular incisors to the maxillary incisors is near or completely on the gingiva, a deep bite occurs . </P> <P> Sinodonty, a genetic variation occurring in Native Americans and some East Asian populations, is possibly a trait retained from an indigenous East Asian archaic human ancestor Homo Erectus Pekinensis . Among its features are shovel - shaped incisors that derive their name from the deeper - than - normal lingual fossa and prominent marginal ridges of the teeth . When seen from lingual view, the tooth is said to resemble a shovel and are rotated slightly inward . It is also common to see signs of attrition, which is wear over time from other tooth contact . The lingual of maxillary incisors and the facial of mandibular incisors are the most common places for attrition to occur . </P> <P> When space exists between the contacts of the maxillary central incisors, the condition is referred to as a diastema or "gap tooth ." One frequent cause of the space is the presence of a large labial frenum from the upper lip extending near the teeth . Treatment depends upon the cause and extent of the gap . Periodontal surgery may be required to reduce the frenum . A small space may be corrected with a filling, veneer, or crown . Larger spaces may require orthodontics . </P> <P> The maxillary incisors, both the central and lateral, are the most likely teeth to have a talon cusp, which is an extra cusp on the lingual surface . Talon cusps range from less than 1% to 6% of the population, and 33% of cases occur on the permanent maxillary central incisor . Deciduous teeth are unlikely to have talon cusps . Also, the permanent maxillary incisors are the most likely teeth to have a dilaceration, which is a sharp curve on a tooth . </P>

When has the permanent maxillary lateral incisor root completed its development approximately