Oral Anatomy and Physiology I - Incisors

Permanent Anterior Teeth Properties

Definition and Overview

• Permanent anterior teeth: These include the incisors and canines, which are visible when the patient smiles.

• Importantly, these teeth are aligned in a smooth curving arc starting from the distal of one canine to the distal of the opposite canine.

Composition

• All anterior teeth consist of four developmental lobes:

  • Three labial lobes: Mesiolabial, middle labial, and distolabial lobes.

  • One lingual lobe.

• Separation between the labial developmental lobes is outlined by two vertical labial developmental depressions:

  • Mesiolabial depression

  • Distolabial depression.

Succedaneous Nature

• Permanent anterior teeth are succedaneous teeth, meaning each one replaces its corresponding primary tooth.

• Often, permanent incisors appear to spread across the dental arch due to initial open spacing during eruption, particularly noted in the maxillary arch. However, with the eruption of permanent canines, these spaces usually close.

Crown Characteristics

• Anterior teeth have a long crown with a distinctive incisal surface known as the incisal ridge, which functions as the masticatory surface.

• The crown outline of anterior teeth appears triangular when viewed proximally, with the apex at the incisal ridge and the base at the cervical area.

• These teeth are typically wider mesiodistally than labiolingually.

• Height of contour for both labial and lingual surfaces is located in the cervical third.

• Contact areas for anterior teeth are usually smaller than for posterior teeth and are centered labiolingually on the proximal surface.

• The curvature of the CEJ (Cementoenamel Junction) on all anterior teeth is greater than that of the posterior teeth.

Lingual Surface Features

• All anterior teeth feature a cingulum, which is a raised rounded area located on the cervical third of the lingual surface. The prominence of this feature may vary;

• Additional features may include:

  • Ridges present on the lingual surface.

  • The lingual surface is bordered by marginal ridges on both the mesial and distal sides.

Complex Lingual Anatomy

• Some anterior teeth exhibit more intricate lingual surfaces that feature:

  • Fossae: Shallow and wide depressions.

  • Developmental pits: Located at the deepest part of each fossa.

  • Developmental grooves (primary grooves): Sharp, deep V-shaped linear depressions that delineate the junction of developmental lobes.

  • Supplemental grooves: Shallower and less regular linear depressions compared to developmental grooves.

Root Structure

• Anterior teeth typically possess a single root, with exceptions noted in certain canines.

• Root shapes may vary:

  • Triangular or oval (egg or ovoid-shaped) or elliptical in cervical cross-section, typically leading to a narrower lingual surface.

Maxillary vs. Mandibular Anterior Teeth

• Maxillary anterior teeth roots typically show great lingual inclination with slight distal inclination.

• Conversely, mandibular anterior teeth roots can vary, ranging from nearly vertical to strongly inclined lingually.

Dental Biofilm Considerations

• Gingival margins and grooves on the lingual surface of anterior teeth may facilitate retention of dental biofilm.

• Proper oral care and hygiene should be discussed with patients to mitigate any potential issues arising from biofilm retention.

Lingual Bar Fixed Retainer

• A lingual bar fixed retainer is a metal bar placed on the lingual surfaces of teeth, providing stability and acting as an indirect retainer post-orthodontic treatment.

• Potential challenges include difficulty with home care and instrumentation, although the design typically accommodates flossing.

Permanent Incisors: General Features

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

• Types:

  • Central incisors: Closest to the midline.

  • Lateral incisors: Second positioned from the midline. Each quadrant contains one of each type.

Crown Features

Eruption and Root Completion Ages

• Data from Nelson S. Wheeler’s Dental Anatomy, Physiology and Occlusions, 10th ed.

Additional Functions of Incisors

• The lingual surfaces of incisors are shovel-shaped, aiding in guiding food into the mouth.

• Incisors support the lips and facial muscles, maintaining the vertical dimension of the face.

• They play a critical role in speech articulation.

• Incisors assist in guiding jaw closure during occlusion.

Mamelons and Developmental Features

• Newly erupted incisors often display three mamelons: rounded enamel extensions situated on the incisal ridge.

• Mamelons typically undergo attrition, leading to less visible features over time due to occlusal contact.

Unique Considerations

• Only permanent teeth that display two incisal angles formed from the incisal ridge and each proximal surface.

• Distinguished by an almost straight incisal ridge, cingulum on the lingual surface, and height of contour located at the cervical third.

Dens in Dente

• Defined as a tooth within a tooth; most commonly found in the maxillary lateral incisors.

• May have a hereditary component.

Localized Enamel Hypoplasia

• Enamel hypoplasia affects the crowns of permanent incisors and molars in children with congenital syphilis.

• Infection can be transmitted from mother to fetus via the placenta, possibly causing Hutchinson incisors during tooth development.

• Hutchinson incisors are characterized by a screwdriver-shaped crown with a notched incisal ridge.

Talon Cusps

• A sharp, small extra cusp can occasionally develop as a projection from the cingulum of incisor teeth, potentially affecting occlusion.

Permanent Maxillary Incisors: General Features

• Maxillary incisors are the four most anterior teeth in the maxillary arch, larger in all dimensions than their mandibular counterparts.

• Central incisors are larger than lateral incisors but share similar forms. Both types are wider mesiodistally than labiolingually.

Prominent Features

• Lingual surface features, including marginal ridges and lingual fossa, are more pronounced on maxillary incisors than on mandibular ones.

• Roots are relatively short compared to other maxillary teeth, often lacking concavities and exhibiting bulbous crowns which can create deep concavities at the CEJ.

Lip Competence

• Lip competency influences maxillary incisor positioning; competent lips maintain proper tooth inclination, while incompetent lips may lead to maxillary incisors being positioned anteriorly to the lower lip.

Maxillary Central Incisor Specific Features

• Erupt between 7 and 8 years of age, with root completion by age 10.

• These incisors are notably prominent due to their size and anterior position, representing the largest of all incisors with the widest crown mesiodistally.

Root and Pulp Features

• Single, conical root that is smooth and slightly straight, approximately one and a half times the length of the crown, with a rounded apex.

• Root cross-section appearance is triangular at the labial aspect and narrows toward the lingual.

• Contains one pulp canal, considerably large, with a pulp chamber having three sharp elongations correlating with developmental lobes.

Labial View Features

• The crown narrows at the cervical third, expanding toward the incisal edge; the incisal ridge appears nearly straight.

• Labial developmental depressions may extend from the cervical to the incisal, showcasing the division of the surface into lobes.

• Crown features imbrication lines that run mesiodistally, with grooves formed by perikymata; mesial outline is slightly rounded, and the distant outline is round with a noticeable difference between MI and DI angles.

Lingual View Features

• The lingual surface is narrower compared to the labial surface, with a well-developed cingulum that is slightly off-center towards the distal.

• The wider, shallow lingual fossa is located below the cingulum and displays variability in depth and diameter.

Proximal View Features

• Molar curvature on the mesial surface is deep and pronounced, aiding in tooth identification relative to left/right distinction.

• Overall crown shape remains triangular from proximate view, with height of contour at cervical third.

Incisal View Features

• Triangular crown shape from an incisal viewpoint, with the labial outline being broadly rounded.

• Cingulum shows slight distal placement which aids in orientation and classification of the tooth.

Maxillary Central Incisor Trauma Considerations

• Maxillary central incisors are prone to traumatic fractures and displacement due to anterior placement and early eruption, leading to potential avulsion, involving complete tooth socket displacement.

• Fracture may lead to pulpal pathologies requiring endodontic therapy or tooth vitality loss.

Diastema

• An open contact between the maxillary central incisors represents a diastema, which may be considered unattractive by certain patients.

Mesiodens

• A supernumerary tooth of common occurrence is known as a mesiodens, typically located between the two maxillary central incisors.

• May affect spacing within the maxillary arch, creating additional aesthetic concerns.

Permanent Maxillary Lateral Incisors

• Erupt between 8 and 9 years of age, completing root development by age 11. They typically erupt post-maxillary central incisors.

Crown and Root Features

• The crown exhibits the greatest degree of form variation among permanent teeth, aside from third molars. Resembles a maxillary central incisor but smaller with a slightly rounder crown.

• The root is usually conical and smooth, occasionally curving slightly distally; it is one to one and a half times shorter than the root.

• The root shape in cervical cross-section is oval, possibly featuring a linguogingival groove that may continue onto the crown.

Pulp features

• Contains one pulp canal and chamber devoid of pulp horns, indicative of a simpler root structure.

Labial View Characteristics

• Features labial developmental depressions less frequently compared to central incisors. The crown appears smaller and less symmetrical, with rounder incisal angles aiding in right/left distinction.

Lingual View Characteristics

• The coul_alive of functions offers a narrower capability in comparison to the labial surface. The cingulum is prominent and centered, with a deeper lingual fossa than the central incisor.

• Marginal ridges on lateral incisors tend to be more developed, featuring one or more possible grooves or pits.

Proximal View Features

• The crown is triangular in shape from this angle, with higher CEJ curvature noted on the mesial side compared to distal, distinguishing right from left accurately.

Incisal View Features

• Incisal view reveals rounded crown outline either labially or lingually, contributing to classification clarity.