|TOOTH DEVELOPMENT FROM LOBES
Tooth crowns develop from lobes or primary growth centers (Fig. 1-62). Most normal teeth show evidence of having developed from three to five lobes. As a general rule, the facial portion of anterior teeth (incisors and canines) forms from three lobes, and the lingual cingulum area forms from one lobe. Evidence of three facial lobes can sometimes be seen as a labial ridge separated from the rest of the facial surface by two shallow depressions dividing the facial surface into three parts (seen clearly on a maxillary central incisor in Fig. 1-63A) or three mamelons on an incisal edge (Fig. 1-63B). To summarize, anterior teeth normally develop from four lobes: three facial lobes and one lingual lobe forming the cingulum.
As on anterior teeth, the facial portion of the facial cusp of a premolar forms from three lobes, often evident by a buccal ridge and a depression on either side dividing the facial surface into three parts. Each lingual cusp forms from one lobe. Therefore, a two-cusp-type premolar forms from four lobes: three facial and one lingual, the same as for an anterior tooth. However, a three-cusp-type premolar with two lingual cusps forms from five lobes: three facial and two lingual, one for each lingual cusp.
As a general rule, each molar cusp forms from one lobe. For example, maxillary or mandibular molars with five cusps form from five lobes, and those with four cusps form from four lobes. Some maxillary molars have as few as three cusps and form from three lobes. A small fifth cusp (of Carabelli) may also be present on some maxillary molars, and when it is large, it may have formed from a separate lobe.
Two types of tooth unusual occurrences (called anomalies), peg-shaped maxillary lateral incisors (seen later in Chapter 11) and some extra teeth (also called supernumerary teeth), form from less than three lobes. Guidelines for determining the number of lobes that form each tooth are presented in Table 1-5.
|Guidelines for Determining the Number of Lobes Forming Adult Tooth