The crown of the tooth is divided into surfaces which are named according to the direction in which they face. The anterior teeth , incisors and canines, have four surfaces and a ridge, while the posterior teeth, premolars
and molars, have five surfaces. The surfaces are named as follow:
- Facial or Labial / Buccal Surface: this is the outer surface of the anterior teeth, which is facing the face or the lip. The term “labial surface” is more commonly used.
The outer surface of the posterior teeth, premolars and molars, is termed the “buccal surface”.
- Palatal / Lingual Surface: it is the inner surface of all teeth, anterior and posterior. That of the maxillary tooth is termed “palatal
surface” because it face the palate, while the inner surface of the mandibular teeth is termed “lingual surface” because it faces the tongue.
- Mesial and Distal Surfaces: The surface of the tooth that faces the midline is termed “mesial
surface” while that faces away from the midline toward the back of the mouth is termed “distal surface”. The term “Proximal Surface” denotes any surface between two adjacent teeth, it could be mesial or distal. The area of the
mesial and/or the distal surface which touches its neighbor in the arch is termed the “Contact Area”.
- Incisal Edge or Ridge and Occlusal Surface: The cutting edge of the anterior teeth that is used for mastication is termed the “incisal
edge or ridge”, while in the posterior teeth it is termed the “occlusal surface”.
Division of Tooth Surfaces
For purpose of facilitating localization of various areas within a specific
surface of the tooth, the surface is divided into thirds in horizontal direction, as well as, in mesiodistal and bucco-lingual /palatal directions. (Fig. 7)
Fig. 7. Divisions of toot surfaces. Vertical divisions into thirds and buccal and proximal horizontal divisions.
Line Angles and Point
“Line angles” are imaginary lines formed by the junction of two surfaces. The line where surface A met surface B is called AB line angle. If A is the labial surface and B is the mesial surface of an anterior tooth,
accordingly the line that is formed by the meeting of them is termed “Mesio-Labial Line Angle”.
“Point Angle”, on the other hand, is the point where three surfaces met. The point where the labial and mesial surface join with
the incisal ridge of an anterior tooth is termed “Mesio-Labio-Incisal Point Angle”. (Fig. 8)
Fig. 8. Line angles (Left) and point angles (Right) .
Line Angles: Labioincisal, 2. Mesiolingual, 3. Mesiolabial, 4. Lingoinicsal, 5. Distolingual, 6. Distolabia, 7.
Mesio-occlusal, 8. Mesilingual, 9. Mesiobucca, 10. Bucco-occlusal, 11. Linguo-occlusal, 12. Distolingual, 13. Distobuccal, 14. Disto-occlusal
Point Angles: 1. Mesiolabioincisal, 2. mesiolinguoincisal, 3. distolabioincisal,
4. distolinguoincisal, 5. mesiolinguocclusal, 6. Mesiobuccoocclusal, 7. Destolinguooclusal, 8. distobuccoocclusal.
- Cusps: This are conical or pyramidal
projections on the crown portion of the tooth that makes up a major division of its occlusal surface. Cusps are found in premolars, molars and canines and each cusp represent a calcified developmental lobe. (Figs 9 and 10)
- Tubercle: It is
a small elevation on some portion of the crown which is produced by extra formation of enamel. It is mostly present in the second deciduous molar and the first permanent molars.
- Cingulum: It is a convex bulk of the cervical third of the lingual
surface of the anterior teeth that represent the lingual developmental lobe.
- Ridges: This are linear elevations on the surfaces of the crown and are named according to their location and/or shape. Several types can be identified as follows:
- Marginal Ridge: Is the linear elevation which is found on the mesial and distal boundaries of the lingual surface of the anterior teeth and the mesial and distal boundaries of the occlusal surface of the posterior teeth.
- Triangular Ridge: Is a linear ridge which descends from the tip of the cusp toward the central area of the occlusal surface of the posterior teeth.
- Transverse Ridge: Is the union of two triangular
ridges which transversly cross the occlusal surface of posterior teeth usually the lower first premolar.
- Oblique Ridge: It is the union between two triangular ridges of two cusps, not facing each others and crossing the occlusal
surface of the maxillary molars. It extends obliquely from mesiolingual cusp to distobuccal cusp.
Other ridges are named according to their site and present as elevations are “incisal, labial, lingual, buccal and cervical ridges”.
Fig. 9. Examples of major cusps of lower second premolar (upper), Lower
first molar (middle) and upper first molar (lower)
Fig. 10. Ridges of upper first permanent molar.
- Fossa: This is an irregular depression or concavity and is named according
to its shape and location.
- Central Fossa: present on the occlusal surface of molars and formed by the converging of ridges and cusps that terminate at the center as a depression.
- Lingual Fossa:
Is a depression formed on the lingual surface of the anterior teeth.
- Mesial or Distal triangular Fossa: Found on the occlusal surfaces of premolars and molars, mesial or distal to the triangular ridges.
It is a long depression or valley between ridges and cusps, the inclines of which meets at an angle. A Sulcus has a developmental groove at the junction of its inclines.
- Developmental Grooves: It is a groove or line in the bottom of the Sulcus
which denotes union of the primary parts or lobes of the crown of the tooth. (Figs 11 and 12)
- Supplemental Grooves: These are groves which branch from the developmental grooves, they do not indicate union between primary lobes.
These are small pinpoint depression located at the junction of developmental grooves and/or at their terminals.
Fig. 11. Developmental grooves of lower 5 (left), lower first molar (middle) and upper first molar (right).
Fig. 12. Fossae and pits of lower second premolar (left), lower first molar (middle) and upper first molar (right).
Fig. 13. General outline of some of the lobes.
a: Labial aspect of maxillary central incisor, mesial lobe (1), labial lobe (2) and distal lobe (3),
b and e: Mesial and occlusal aspect of maxillary first premolar,
mesial lobe (1), buccal lobe (2), distal lobe (3) and lingual lobe (4),
c: Occlusal aspect of mandibular first molar, mesiobuccal lobe (1), distobuccal lobe (2), distal lobe (3), mesiolingual lobe (4) and distolingual lobe (5),
Occlusal aspect of maxillary first molar, mesiobuccal lobe (1), distobuccal lobe (2), mesiolingual lobe (3), distolingual lobe (4) and fifth lobe (fifth cusp) (5)
- Inclined Planes:
These are slopes from cusp tips to the developmental grooves.
- Height of Contour: This is the point of maximum convexity of tooth surface.
- Developmental Lobe: Is the first site of calcium deposition. Each tooth begins it development from four
or more growth centers which are known as “Developmental Lobes”.
- The anterior teeth, the maxillary premolars and the mandibular first premolar develop from four developmental lobes, three labial and one lingual.
- The mandibular
second premolar may be two-cuspid, and show the same number and arrangement of the developmental lobes as the mandibular first premolar, or three-cuspid and, therefore, have five lobes three labial and two lingual.
- All molar teeth have two buccal
and two lingual lobes, except the mandibular first molar which may have a fifth buccal lobe.
- The lobes grow until they fuse, and the line of fusion is marked by a line that is termed the “Developmental Groove”, that can be seen on the
tooth after its eruption but soon disappear due to wear and attrition.
- The developmental lobes are represented by cusp, cingulum and mamelon.■