Morphology of Permanent Incisors

Morphology of Permanent Incisors: Maxillary Centrals and Laterals

Learning Objectives

• List the differences between set, class, and arch traits of teeth.

• Describe the general and specific descriptions of anterior teeth.

• Define the function of anterior teeth: biting, cutting, speech articulation, jaw guidance, lip/face support, and vertical dimension maintenance.

• Compare the size of similar anterior teeth.

• Identify approximate eruption dates for anterior teeth (not provided in this transcript).

• Identify and illustrate the following terms and anatomical landmarks: marginal ridges, incisal edge, cingulum, pits, fossa, labial developmental depressions, imbrication lines, mamelons, facial and labial heights of contour.

• Identify and describe the anatomical similarities and differences of type traits for maxillary and mandibular central and lateral incisors from all views.

• Following specific instructions, color and/or annotate the anatomical structures of the maxillary and mandibular central and lateral incisors.

Permanent Anterior Teeth: General Features/Definitions

• Included Teeth: Permanent anterior teeth consist of the incisors and canines.

  • Maxillary arch: Canines (labial, lingual), Lateral incisors, Central incisors.

  • Mandibular arch: Canines (labial, lingual), Lateral incisors, Central incisors.

• Tooth Numbers:

  • Maxillary canines: #$6$, #$11$

  • Maxillary lateral incisors: #$7$, #$10$

  • Maxillary central incisors: #$8$, #$9$

  • Mandibular canines: #$22$, #$27$

  • Mandibular lateral incisors: #$23$, #$26$

  • Mandibular central incisors: #$24$, #$25$

• Lobes: All anterior teeth are formed from four developmental lobes:

  • $3$ labial lobes: mesiolabial, middlelabial, and distolabial.

  • $1$ lingual lobe.

• Developmental Depressions: Two vertical labial developmental depressions separate the labial developmental lobes (mesiolabial and distolabial depressions).

• Succedaneous Teeth: All permanent anterior teeth are succedaneous, meaning they replace primary teeth of the same type (e.g., a primary central incisor is replaced by a permanent central incisor).

• Incisal Ridge: The masticatory surface of an anterior tooth is called the incisal ridge, which is part of the incisal surface.

• Tooth Form:

  • Proximal View: Crown outline is triangular.

  • Labial/Lingual View: Crown outline is trapezoidal.

  • Function based on form:

    • Incisors' triangular proximal form aids in biting and cutting food.

    • Canines' tapered shape and prominent cusp function to pierce or tear food.

• Height of Contour: For all anterior teeth, the height of contour for both the labial and lingual surfaces of the crown is located in the cervical third.

• Contact Areas: Anterior tooth contact areas are typically:

  • Centered labiolingually on their proximal surfaces.

  • Smaller in area compared to posterior tooth contacts.

• Cementoenamel Junction (CEJ) Curvature: The CEJ curvature on each proximal surface of all anteriors is greater (more pronounced) than that of posteriors.

• Cingulum: A raised, rounded area on the cervical third of the lingual surface of all anterior teeth, varying in prominence.

• Marginal Ridge: The lingual surface of anteriors is bordered mesially and distally by a rounded, raised border called the marginal ridge.

• Fossa/Fossae: A shallow, wide depression found on the lingual surface of some anterior teeth.

• Developmental Pits: May be present in the deepest part of a fossa.

• Developmental Groove (Primary Groove): A sharp, deep, V-shaped linear depression found on the lingual surface of some anteriors.

• Supplemental Groove (Secondary Groove): A shallower, more irregular linear depression than a developmental groove, also found on the lingual surface.

• **Anatomical Landmark Identification (Referencing a diagram):

  1. Supplemental Groove

  2. Developmental Groove

  3. Developmental Pit

  4. Marginal Ridge

  5. Fossa

  6. Incisal Ridge

  7. Cingulum

• Roots:

  • Anteriors usually have one single root.

  • Maxillary anterior roots have a great lingual and slight distal inclination.

  • Mandibular anterior roots vary from nearly vertical to great lingual inclination.

Permanent Incisors: General Features

• Location: The eight most anterior teeth in the permanent dentition, with four in each dental arch.

• Types: Central incisors (closest to midline) and Lateral incisors (second from midline).

• Functions:

  • Biting and cutting food during mastication.

  • Involved in speech articulation.

  • Assisting in guiding the jaw closed.

  • Supporting the lips and face, maintaining vertical dimension.

• Mamelons:

  • Newly erupted incisors have three mamelons, which are rounded enamel extensions on the incisal ridge (visible from labial or lingual views).

  • Mamelons typically undergo slight attrition (wearing away due to tooth-to-tooth contact) shortly after eruption as teeth move into occlusion.

  • They are most noticeable immediately after eruption and become less detectable over time.

  • If mamelons remain long after eruption, it indicates these teeth are not in occlusion.

• Incisal Edges: After eruption and attrition, the incisal ridges can flatten and become incisal edges.

• Incisal Angles: Incisors are the only permanent teeth with two incisal angles, formed by the incisal ridge and each proximal surface.

• Surfaces: All incisors have $5$ surfaces: mesial, distal, facial (labial), lingual, and incisal.

• Lobes and Anatomy:

  • The facial surface is formed from $3$ lobes: mesial, middle, and distal.

  • Incisors have $2$ shallow vertical developmental depressions separating these $3$ labial lobes.

  • The $3$ labial lobes also contribute to the $3$ mamelons on the incisal edge.

  • The $4$th lobe forms the lingual bulge (the cingulum).

• Height of Contour: For all incisors, the height of contour for both labial and lingual surfaces is at the cervical third, consistent with all anteriors.

Permanent Maxillary Incisors: General Features

• Size: Each maxillary incisor crown is larger in all dimensions, especially mesiodistally (MD), compared with a mandibular incisor.

• Resemblance: Maxillary central and lateral incisors resemble each other more than they resemble the incisors of the mandibular arch.

• Relative Size: A maxillary central incisor is generally larger than a maxillary lateral incisor, but they share a similar overall form.

• Lingual Features: All lingual surface features (marginal ridges, lingual fossa, cingulum) are more prominent on maxillary incisors than on mandibular incisors.

• Incisal Ridge Position: From a proximal view, the incisal ridge is just labial to the mid-root axis line.

• Root Features: Each root is short compared to other maxillary teeth and usually lacks root concavities.

Clinical Considerations with Permanent Maxillary Incisors: Lingual Features

• Shovel-shaped form: Increased prominence of lingual marginal ridges and a deeper lingual fossa.

• Accentuated Cingulum: May have deepened grooves.

• Talon Cusp: A projection from the cingulum can be present.

• Attrition: Incisal edge may show severe attrition.

• Lingual Pit Caries Risk: Lingual pits are at increased risk for caries due to increased dental biofilm retention and weaker enamel walls.

• Linguogingival Groove: A vertically-placed groove originating in the lingual pit, extending cervically and slightly distally onto the cingulum; a concern for caries.

• Supragingival Deposits: Dental biofilm and stain can accumulate in prominent lingual surface concavities.

Permanent Maxillary Central Incisors (#8 and #9)

• Overall Features:

  • Most prominent teeth in the permanent dentition due to large size and anterior position.

  • Largest of all incisors.

  • Strong mesial contact with the other maxillary central incisor.

  • Widest crown mesiodistally of any permanent anterior tooth.

• Root Features:

  • $1$ conical root, smooth and straight with a rounded apex (cone-shaped).

  • Widest in the cervical $1/3$.

  • Only slightly longer than the crown, giving the smallest crown-to-root ratio of any permanent tooth.

  • Root tapers narrower toward the lingual.

  • No root depressions.

  • Wider and shorter root than the lateral incisor root.

  • Pulp cavity mirrors the shape of the tooth.

  • $1$ large root canal; $3$ pulp horns: mesial, distal, central.

  • Root is oval or egg-shaped in cross-section.

• Labial View Features:

  • Crown outline is trapezoidal.

  • Crown is narrowest at the cervical third, widening toward the incisal edge.

  • Incisal ridge is nearly straight.

  • Often has imbrication lines (slight ridges) running mesiodistally in the cervical third, with grooved perikymata between them.

  • Mesial outline is slightly rounded with a sharp mesioincisal (MI) angle (almost $90^{\circ}$).

  • Distal outline is rounder with a definite rounded distoincisal (DI) angle (greater than $90^{\circ}$).

  • The difference in MI and DI angle sharpness helps distinguish right from left.

  • Mesial contact with the other maxillary central is in the incisal third.

  • Distal contact with the maxillary lateral is at the junction of the incisal and middle third (more cervical than the mesial contact).

• Lingual View Features:

  • Lingual surface of the crown is narrower than the labial surface.

  • Cingulum is wide and well-developed, located slightly off-center toward the distal.

  • Lingual fossa is wide and shallow, immediately incisal to the cingulum.

  • Mesial marginal ridge is longer and straighter than the distal marginal ridge.

• Proximal View Features (Mesial and Distal):

  • Crown is triangular-shaped.

  • CEJ curvature on the mesial surface is deeper incisally than on the distal, aiding right/left distinction.

  • Has the greatest mesial CEJ curvature of any tooth in the permanent dentition.

  • Height of contour for both labial and lingual surfaces is greater than any other permanent tooth, located at the cervical third.

  • Lingual surface contour is