Lobes

A. Introduction:

The previous section pointed out that animal teeth developed evolutionally from a one-lobed, conical crown, through the tritubercular. or three-lobed form, to the four or more lobed crowns found in the primates. Lobe was defined as a primary division of the tooth in the previous unit. Until recently, it was believed that each lobe developed from a separate calcification center, but this theory has fallen into disfavor in the last few years. Therefore, lobes will be considered only as anatomical divisions of a tooth, often separated by distinguishable primary grooves. The lobe pattern in the human dentition plays a part in the form and function of each individual tooth.

B. Permanent Anterior Teeth:

All anterior teeth are composed of four lobes. There are three labial lobes, named mesiolabial. middle labial, and distolabial lobes. (Occasionally the middle labial lobe is referred to simply as the labial lobe.) The remaining lobe encompasses the cingulum. and is termed the lingual lobe.

Evidence for the presence of the three labial lobes is sometimes found on the incisal edge of newly erupted incisors in the form of mamelons. which are the slightly rounded incisal terminations of the labial lobes. When the incisors are in functional occlusion, the mamelons are abraded away soon after eruption, but may still be visible, even in adults, when the incisor has not been in active occlusion.

Further evidence of separation of the labial lobes of all anterior teeth is found in the form of two shallow depressions in the incisal portion of the labial surface. These linear, vertical depressions are called mesiolabial and distolabial developmental depressions.

Maxillary Central Incisor Labial View ,,.„„,„«

Maxillary Central Incisor Labial View ,,.„„,„«

Imbrication Lines

Maxillary Central Incisor

1. Distolabial lobe

2. Middle Labial lobe

3. Mesiolabial lobe

4. Lingual lobe

Maxillary Central Incisor

1. Distolabial lobe

2. Middle Labial lobe

3. Mesiolabial lobe

4. Lingual lobe

Permanent Posterior Teeth:

1. Premolars - Most premolars also exhibit four lobes, three buccal and one lingual. They are named mesiobuccal. middle buccal, distobuccal. and lingual lobes. The one exception is the mandibular second premolar which, in the majority of cases, exhibits two lingual cusps. When this is the case, there are five lobes, three on the buccal and two lingually located. The three buccal lobes are named as in the four-lobed type, while the lingual lobes are termed mesiolingual and distolingual lobes.

Maxillary First Premolar

1. Distobuccal lobe

2. Middle Buccal lobe

3. Mesiobuccal lobe

4. Lingual lobe

Maxillary First Premolar

1. Distobuccal lobe

2. Middle Buccal lobe

3. Mesiobuccal lobe

4. Lingual lobe

Mesiobuccal

Developmental

Depression

Mesiobuccal

Developmental

Depression

Distobuccal

Developmental

Depression

Distobuccal

Developmental

Depression

"Central Groove Maxillary Premolar (4 Lobes)

Mesiobuccal Developmental Depression

Lingual Groove

Mesiobuccal Developmental Depression

Lingual Groove

Distobuccal Developmental Depression

Central Groove

Mandibular Second Premolar (5 Lobes)

Distobuccal Developmental Depression

Central Groove

Mandibular Second Premolar (5 Lobes)

As with the labial surface of the anterior teeth, the buccal surface of premolars normally displays lobe division in the form of mesiobuccal and distobuccal developmental depressions. When viewed from the occlusal aspect, the central groove serves as the separation between the buccal lobes and the lingual lobe(s). In the case of the mandibular second premolar with two lingual cusps, the lingual groove separates the two lingual lobes.

Permanent Molars:

a. Maxillary Molars - Normally, maxillary molars have four lobes, two buccal and two lingual, which are named in the same manner as the cusps that represent them (mesiobuccal, distobuccal. mesiolingual. and distolingual lobes). Unlike the anterior teeth and premolars, molars do not exhibit facial developmental depressions. Evidence of lobe separation can be found in the central groove, which divides buccal from lingual lobes. The two lingual lobes are separated by the distolingual groove, and the two buccal lobes are divided by the buccal groove.

Buccal Groove

Maxillary First Molar

1. Mesiobuccal lobe

2. Distobuccal lobe

3. Mesiolingual lobe

4. Distolingual lobe

Oistal Portion Central Groove

Maxillary First Molar

1. Mesiobuccal lobe

2. Distobuccal lobe

3. Mesiolingual lobe

4. Distolingual lobe

Buccal Groove

Oistal Portion Central Groove

Mesial Portion Central Groove

Maxillary First Molar (4 Lobes)

Mesial Portion Central Groove

Distolingual Groove

Maxillary First Molar (4 Lobes)

b. Mandibular first molars - These teeth normally have five cusps and five lobes. They are named for the cusps, exactly like the lobes of maxillary molars, with the addition of the fifth lobe, the distal lobe. Separational evidence is found in the central groove, as well as the lingual groove, buccal groove, and distobuccal groove.

Mandibular First Molar

Buccal Groove

Mandibular First Molar

¿_ LAsJoouccai ooe

4 Vesoi^uol lobe

5 Dtsorguo iobe

Mesial Portion Central Groove

Buccal Groove

Mesial Portion Central Groove

Distobuccal Groove

Distal Portion Central Groove

Mandibular First Molar (5 Lobes)

Distobuccal Groove

Distal Portion Central Groove

Lingual Groove

Mandibular First Molar (5 Lobes)

c. Other mandibular molars - The crowns of most other mandibular molar- . ~: fit four cusps and four lobes, with terminology the same as it is for maxillary molars. The developmental grooves indicative of lobe division include the central, buccal, and lingual grooves.

Buccal Groove

Mesial Portion Central Groove

Lingual Groove Mandibular Second Molar (4 Lobes)

Buccal Groove

Mesial Portion Central Groove

Distal Portion Central Groove

Lingual Groove Mandibular Second Molar (4 Lobes)

Distal Portion Central Groove

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