lecture 8 dental anatomy

Maxillary Central Incisors (Teeth $11$ and $21$)

• Eruption and Development:

  • Eruption occurs between ages $7$ and $8$.

  • Root completion is finalized by age $10$.

• Specific Overall Features:

  • These are the most prominent teeth in the permanent dentition due to their large size and anterior position in the arch.

  • Smile Design: In aesthetic consideration, central incisors should be dominant. Each tooth moving posteriorly should appear smaller from a perspective view.

  • They are the largest of all incisors and typically share a mesial contact area.

  • The crown is the widest Mesiodistally ($MD$) of any permanent anterior tooth.

  • Root Structure: Features a smooth, single cone-shaped root with a rounded apex.

  • Pulp Cavity: Contains a single large root canal that mimics the overall shape of the root.

• Labial View Features:

  • Imbrication Lines: Slight ridges or lines that run $MD$ in the cervical third of the crown. These are surface manifestations of the striae of Retzius.

  • Perikymata: The specific spaces located between the imbrication lines.

  • Angle Identification: The Mesioincisal ($MI$) angle is sharper than the Distoincisal ($DI$) angle, which assists in distinguishing right from left.

  • Contact Area: The mesial contact is located in the incisal third.

• Lingual View Features:

  • CEJ Curvature: Usually exhibits more curvature toward the distal.

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

  • Lingual Fossa: Wide but shallow, located immediately incisal to the cingulum. Depth and diameter vary by individual.

  • Marginal Ridges: The mesial marginal ridge is longer than the distal marginal ridge.

  • Lingual Groove: A horizontally placed groove may separate the cingulum from the lingual fossa, sometimes giving the cingulum a scalloped appearance. (This is more common in maxillary laterals).

  • Lingual Pit: May be present at the incisal border of the cingulum within the lingual groove.

• Proximal View Features:

  • CEJ Depth: The curvature on the mesial surface is deep incisally. It has the greatest depth of curvature of any tooth surface in the entire permanent dentition, making it a primary feature for distinguishing right from left.

  • Height of Contour: Located at the cervical third for both labial and lingual surfaces; the value is greater on this tooth than any other in the permanent dentition.

  • Incisal Edge Position: Located slightly labial to the long axis of the tooth. The incisal outline slopes toward the lingual from its most labial and longest part.

  • Distal vs. Mesial: The distal view is similar to the mesial, but the CEJ curvature is notably less deep.

• Incisal View Features:

  • Crown Shape: Triangular overall, with a broadly rounded labial outline.

  • This view clearly shows the slight distal placement of the cingulum.

Maxillary Lateral Incisors (Teeth $12$ and $22$)

• Eruption and Development:

  • Eruption occurs between ages $8$ and $9$.

  • Root completion is finalized by age $11$.

• Specific Overall Features:

  • Variation: These teeth have the greatest degree of variation in form of any permanent tooth except for the third molars.

  • They generally resemble maxillary central incisors but are smaller and have a rounder crown.

  • Root Structure: A long conical root that is relatively straight but may curve slightly to the distal. The root is actually longer than that of the central incisor.

  • Pulp Cavity: Consists of a simple pulp canal and pulp chamber.

• Labial View Features:

  • The crown is less symmetrical and smaller than the central incisor.

  • Both the $MI$ and $DI$ angles are rounder than those found on the central incisor.

• Lingual View Features:

  • The lingual surface is narrower than the labial surface.

  • Cingulum: Prominent and centered, though narrower than the central incisor's cingulum.

  • Fossa and Ridges: The lingual fossa is deeper than that of the central. Marginal ridges are pronounced, with a long mesial marginal ridge and a shorter distal marginal ridge.

  • Grooves and Pits: A horizontal lingual groove separating the cingulum from the fossa is more common and better developed than on centrals. A lingual pit is also more common, located along the lingual groove.

• Proximal View Features:

  • CEJ Curvature: More curved on the mesial surface than the distal, helping distinguish right from left.

  • Incisal Edge: Usually positioned labial to the long axis of the tooth.

General Features of Mandibular Incisors

• Size and Symmetry: These are the smallest teeth in the permanent dentition and the most symmetrical.

• Uniformity: There is more uniformity in form among mandibular incisors than any other tooth category.

• Arch Comparisons: Mandibular lateral and central incisors resemble each other much more closely than maxillary central and lateral incisors do.

• Size Hierarchy: Generally, the mandibular lateral incisor is slightly larger than the mandibular central incisor (the opposite of the maxillary arch).

• Attrition: The incisal edges typically show wear due to attrition.

• Root Features:

  • Proximal Root Concavities: Present on both types; if deep, they can give the appearance of being double-rooted.

  • Root Shape: Elliptical (elongated oval) in cervical cross-section. The root is extremely narrow on labial/lingual surfaces and wide on proximal surfaces. The root is longer than the crown.

Mandibular Central Incisors (Teeth $31$ and $41$)

• Eruption and Development:

  • Eruption occurs between ages $6$ and $7$.

  • Root completion is finalized by age $9$.

• Specific Overall Features:

  • Antagonists: This tooth and the maxillary third molar are the only teeth in the dentition to have only one antagonist; all other teeth have two.

  • Mamelons: May be present on the incisal edges upon eruption.

  • Smallest Teeth: These are the smallest and simplest teeth in the permanent dentition.

  • Root: Simple root that is wider Labiolingually ($LL$) than Mesiodistally ($MD$). Features pronounced proximal root concavities and a shallow longitudinal depression along the mid-portion.

  • Pulp: One pulp canal and three pulp horns.

• Labial and Lingual Features:

  • Symmetry: The crown is bilaterally symmetrical with a fan shape.

  • Angles: Both $MI$ and $DI$ angles are sharp, though the $MI$ angle is slightly sharper.

  • Lingual Surface: Smooth with a small, centered cingulum. The lingual fossa, mesial marginal ridge, and distal marginal ridge are all barely noticeable.

• Proximal and Incisal Features:

  • CEJ: Curvature is higher incisally on the mesial than the distal.

  • Incisal View: Nearly symmetrical crown outline.

Mandibular Lateral Incisors (Teeth $32$ and $42$)

• Eruption and Development:

  • Eruption occurs between ages $7$ and $8$.

  • Root completion is finalized by age $10$.

• Specific Overall Features:

  • Slightly larger than the mandibular central incisor but resembles its form.

  • Root: Single, usually straight, and slightly longer/wider than the central's root. Pronounced proximal concavities exist, especially on the distal surface.

  • Pulp: One pulp canal and three pulp horns.

• Labial and Lingual View Features:

  • Asymmetry: The crown appears tilted or twisted distally on the root. The distal outline is rounder and shorter, while the mesial outline is flatter and longer.

  • Contact Areas: Mesial contact is in the incisal third; distal contact with the canine is also in the incisal third but more cervical than the mesial.

  • Lingual View: Lacks bilateral symmetry; appears twisted distally.

• Proximal and Incisal View Features:

  • Mesial View: More of the lingual surface is visible from this aspect due to the distal tilt of the incisal edge.

  • Incisal Edge: Not straight $MD$; the edge curves toward the lingual in its distal portion.

  • Cingulum: Appears displaced toward the distal. The mesial marginal ridge is longer than the distal marginal ridge.

  • Rare Variation: May have an accessory or bifurcated root (labial and lingual branches).

Permanent Canines (General Features)

• Stability: These are the longest (incisocervically) and most stable teeth in the dentition.

• Position: Located at the corners of the dental arches.

• Anchorage: Well-anchored in the bone by the canine eminence (maxilla).

• Function: Specifically designed for holding and tearing food.

• Force Dissipation: The V-shape proximal outline allows for the dissipation of occlusal forces.

Permanent Maxillary Canines

• Eruption and Development:

  • Eruption occurs between ages $11$ and $12$.

  • Root completion is finalized between ages $13$ and $15$.

  • Usually erupt after mandibular canines, maxillary incisors, and maxillary premolars.

• Labial Features:

  • Transition Form: The mesial half resembles an incisor, while the distal half is more premolar-like.

  • Cusp: Single round cusp. Upon eruption, the mesial cusp slope is shorter than the distal cusp slope.

  • Contact Areas: Mesial contact (with lateral incisor) is at the junction of the incisal and middle thirds. Distal contact (with first premolar) is more cervical, located at the middle third.

• Lingual Features:

  • Prominent mesial and distal marginal ridges.

  • Lingual Ridge: A vertical, centrally placed ridge runs from the cingulum to the cusp tip.

  • Fossae: Separates two shallow but visible fossae: the Mesiolingual ($ML$) fossa and the Distolingual ($DL$) fossa.

  • Tubercle: A small tubercle may be present on the cingulum.

• Proximal and Incisal Features:

  • Triangular Profile: Ideal for holding and tearing.

  • Root Grooves: A longitudinal groove is seen from the CEJ to the midpoint of the root.

  • Distal View: The distal marginal ridge is heavier/more developed than the mesial. The distal root surface may show a more pronounced developmental depression.

  • Incisal View: The $LL$ width is large, making it a very strong tooth. The outline is asymmetrical; the mesial part has greater $LL$ bulk, while the distal part appears thinner and "stretched."

Permanent Mandibular Canines

• Eruption and Development:

  • Eruption occurs between ages $9$ and $10$.

  • Root completion is finalized between ages $12$ and $14$.

  • Usually erupt before maxillary canines.

• Comparison to Maxillary Canines:

  • The crown can be as long as or longer than the maxillary canine.

  • The lingual surface is smoother with a less developed cingulum and two marginal ridges.

• Labial and Lingual View Features:

  • Outlines: Straight mesial outline and convex distal outline.

  • Cusp Apex: Closer to the mesial than the distal.

  • Contacts: Mesial contact is near the mesioincisal edge; distal contact is near the junction of the incisal and middle thirds.

• Proximal and Root Features:

  • Cusp Tip: Lingually inclined and wedge-shaped from the mesial view.

  • Root Apex: More pointed than the maxillary canine.

  • Variations: Bifurcated roots (buccal and lingual) are a common variation. Developmental depressions may be extremely pronounced, creating a double-rooted appearance in the apical third.

Clinical Considerations and Pathologies

• Maxillary Central Incisors:

  • Trauma: Highest risk for fracture or displacement due to anterior position and early eruption.

  • Avulsion: Complete displacement of the tooth from the socket; common in children before full root completion.

  • Diastema: An open contact. Treatment is controversial; may involve reducing a tight maxillary labial frenum or orthodontics.

  • Winged Incisors: Actually a case of tooth rotation (usually bilateral to the mesial), not a developmental disturbance.

• Maxillary Lateral Incisors:

  • Microdontia: Leads to a "peg lateral" (smaller crown shape).

  • Partial Anodontia (Hypodontia): Often congenitally missing (unilaterally or bilaterally).

• Mandibular Incisors:

  • Calculus Buildup: Supragingival deposits (biofilm, calculus, stain) collect in the lingual concavity because of the proximity to the submandibular and sublingual salivary gland ducts.

  • Crowding: Instrumentation is difficult due to inadequate arch size and physiological mesial drift.

  • Instrumentation Risk: Prolonged hand instrumentation can narrow the already thin labial and lingual root surfaces, potentially jeopardizing the crown during mastication via unsupported enamel.

• Canines:

  • Ectopic Eruption: Eruption in an abnormal or odd placement.

  • Impacted Canines: Often fail to erupt because their arch space is closed by earlier-erupting teeth. They may erupt labially or lingually to the arch. (Impacted canines occur in approximately $1$ in $5000$ cases, or $0.22\%$).

  • Dilaceration: Distorted angulations or curvatures along the root length.

  • Dentigerous Cyst: A developmental cyst forming within the dental tissue of an impacted crown; it typically attaches at the CEJ.

  • Abrasion and Attrition: Wear from toothbrushes or tooth-to-tooth contact. Cusp reshaping is sometimes performed for aesthetics or function after wear.