The Tooth: Functions and Terms

Objectives

• To identify the different functions of the teeth

• To identify the different tissues that compose the teeth

• To differentiate between clinical and anatomic eruption

• To define single, bifurcated, and trifurcated roots

• To recognize how the functions of teeth determine their shape and size

• To understand the individual functions and therefore the individual differences that exist among incisors, canines, premolars, and molars

• To name and identify the location of the various tooth surfaces

• To name and identify the line angles of the teeth

• To name and identify the point angles of the teeth

• To define the terminology used in naming the landmarks of the teeth

Functions of Teeth

Teeth serve several critical functions within the body:

• Protection of the Oral Cavity: They form a hard physical barrier, shielding oral structures from external damage.

• Acquiring and Chewing Food: Essential for the initial processing of food. They aid the digestive system by breaking down food into smaller, more manageable pieces.

• Defense and Attack: Teeth can act as formidable weapons. Mammals in the order Carnivora, such as lions and tigers, showcase this with their well-developed canines used for defense and hunting prey.

• Communication: Necessary for proper speech, phonetics, and even whistling.

• Sexual Attraction: In many cultures, the appearance of teeth can be a significant positive sexual attraction.

In dental anatomy, teeth are studied both individually and collectively, focusing on their functions, how they are anchored, and their relationships to one another.

Crown and Root

Each tooth is comprised of two main portions:

• Crown: The part of the tooth typically visible in the mouth and covered with enamel.

• Root: The part of the tooth embedded within the jawbone and covered with cementum.

These two portions meet at the cementoenamel junction (CEJ). The visible line demarcating this junction is called the cervical line (Fig. 2-1).

Tooth Eruption

The crown portion of the tooth erupts through the bone and gum tissue. Once erupted, it is generally no longer covered by gum tissue, except perhaps the cervical third of the crown in healthy young adults, which may be partially covered by gingiva (gum tissue) (Fig. 2-2). Eruption continues until the entire crown is exposed. Eruption of a tooth refers to its movement through surrounding tissues, making the clinical crown appear gradually longer.

Clinical vs. Anatomic Crown

There's a critical distinction between the anatomic and clinical crown:

• Anatomic Crown: This refers to the entire portion of the tooth that is covered by enamel, regardless of whether it has erupted or is visible in the mouth (Fig. 2-3).

• Clinical Crown: This is only the part of the tooth that is visible and exposed above the gingiva (gum line). Any unerupted area is not considered part of the clinical crown.

If the entire anatomic crown does not erupt, the visible part is the clinical crown, and the unerupted portion is considered part of the clinical root (Fig. 2-3).

Root Configurations

Teeth can have different root configurations:

• Single Root: A single, undivided root (Fig. 2-1).

• Multiple Roots: Roots that divide into segments:

  • Bifurcation: Division of the root portion into two segments (Figs. 2-3, 2-4).

  • Trifurcation: Division of the root portion into three segments (Fig. 2-5).
    Each root terminates in a single apex or terminal end.

Tooth Anchorage

Teeth are firmly anchored in the bony process of the jaw, ensuring their position relative to other teeth in the dental arch. The portion of the jaw supporting the teeth is called the alveolar process. The bony socket in which each tooth fits is known as the alveolus (Fig. 2-6).

• Maxillary Teeth: Teeth in the upper jaw, anchored in the maxillary bone.

• Mandibular Teeth: Teeth in the lower jaw, anchored in the mandible bone.

Tooth Tissues

There are four primary tissues that compose a tooth (Fig. 2-7):

1. Enamel

2. Dentin

3. Cementum

4. Dental Pulp

Enamel, dentin, and cementum are hard tissues, while the dental pulp is a soft tissue.

Enamel

Enamel forms the outer surface of the anatomic crown.

• Thickness: It is thickest over the tip of the crown and gradually thins out, ending at the cervical line.

• Coloration: Enamel's color varies based on its thickness and mineralization:

  • Thicker enamel appears whiter.

  • Thinner enamel shows variations from grayish-white at crown cusps' edges, to white in the middle, to yellow-white at the cervical line (where its translucency allows the yellow tint of underlying dentin to show through).

  • Higher mineralization increases translucency.

  • Tooth color is determined by a combination of enamel's mineralization and thickness, as well as skin pigmentation. Older individuals and those with darker skin may exhibit brownish or grayish tones, sometimes in place of yellow. Individuals with red or auburn hair may have slight reddish or brown-red coloration, often concentrated at the gingival area but dispersed throughout the tooth.

• Hardness and Composition: Enamel is the most densely mineralized and hardest tissue in the human body.

  • Chemical Composition: $96\%$ inorganic material and $4\%$ organic matter and water.

  • This dense mineralization provides exceptional resistance to wear, allowing it to grind, crush, and break down tough substances like nuts, seeds, ice cubes, and even particles of bone, grit, sand, and leather. Our ancestors, with diets of hard, uncooked, unprocessed foods, often wore the enamel off their teeth, with their life expectancy sometimes linked to their dental function.

• Durability and Protection: Modern humans rarely experience such severe enamel wear. Enamel is not only highly resistant to wear but also durable and resistant to bacteria, mild acids, and tooth decay.

• Self-Cleaning Ability: The densely packed, smooth surface of enamel gives the crown a self-cleaning property, making it difficult for food particles, bacteria, sticky carbohydrates, and other debris to adhere. This, combined with its hardness and wear resistance, makes enamel a nearly perfect outer covering for the crown.

Dentin

Dentin constitutes the main portion or body of the tooth, forming the greatest bulk of both the crown and root. It is enveloped by enamel on the crown and cementum on the root.

• Characteristics: Dentin is a hard, dense, calcified tissue. It is softer than enamel but harder than cementum or bone. It has a yellow color and is elastic.

• Composition: Chemical composition is $70\%$ inorganic material and $30\%$ organic matter and water.

• Self-Repairing Capacity: Unlike enamel, dentin can add to itself:

  • Secondary Dentin: Formed throughout the pulp chamber after tooth eruption, it can eventually fill the pulp chamber completely.

  • Reparative Dentin: Laid down specifically in response to caries (decay) or trauma.

Cementum

Cementum is a bone-like substance that covers the tooth root. It is important to note that the root is not always covered by a perfect layer; small patches of dentin may be exposed due to voids in the cementum.

• Main Function: To provide a medium for the attachment of the tooth to the surrounding alveolar bone.

• Density: It is less dense and hard than enamel or dentin, but it is more dense than bone and shares a physiological resemblance to it.

• Composition: Chemical composition is $45\%$ to $50\%$ inorganic and $50\%$ to $55\%$ organic components, making it less durable than dentin or enamel.

• Thickness: Cementum is relatively thin at the cervical line but gradually increases slightly in thickness towards the apex of the root.

• Dentinocemental Junction: The point where cementum and dentin unite.

• Types of Cementum:

  • Acellular Cementum: Covers the entire anatomic root.

  • Cellular Cementum: Confined to the apical third of the root. It has the unique ability to reproduce itself, which helps compensate for the attrition (wear) that occurs on the crown of the tooth. Its name derives from the fact that the cells forming it, called cementoblasts, become entrapped within the newly formed cementum.

• Anchorage and Nutrition: Cementum provides a self-adjusting mechanism of anchorage that protects and supports the tooth, yet it operates independently of the tooth’s primary nourishment system. Its nutrition is derived externally from blood vessels that come directly from the bone.

Dental Pulp

Dental pulp is the tooth's nourishing, sensory, and dentin-reparative system.

• Composition: It consists of blood vessels, lymph vessels, connective tissue, nerve tissue, and specialized dentin-formation cells called odontoblasts.

• Location: The pulp is centrally housed within the tooth, surrounded by dentin.

• Odontoblasts: These cells line the walls of the pulp cavity and have a primary function of laying down primary and secondary dentin. They form secondary dentin in response to trauma from chemical, mechanical, or bacterial causes.

• Vascular and Neural Functions:

  • Blood Vessels: Bring in the necessary nourishment to activate and support secondary dentin formation. They also supply white blood cells crucial for fighting bacterial invasions within the pulp.

  • Lymph Tissue: Filters fluids within the tooth.

  • Nerve Tissue: Responds to pain stimuli; however, it does not differentiate the specific cause of pain.

• Anatomic Divisions: The pulp is anatomically divided into two areas:

  • Pulp Chamber: Housed within the coronal portion of the tooth.

  • Pulp Canals (Root Canals): Located within the roots of the tooth.

  • Pulp Cavity: The collective term for the pulp chamber and pulp canals. The pulp cavity extends the entire length of the tooth's interior, from the tip of the pulp chamber (known as pulp horns) down to the apex of the root canal (Fig. 2-7).

Types of Teeth and Their Functions

The functions of teeth are diverse and depend on their individual shape, size, and location within the jaws. The three fundamental food processing functions of teeth are cutting, holding/grasping, and grinding.

Incisors

• Design: Specifically designed for cutting. The term "incisor" means "that which makes an incision, or cut."

• Biting Edge: The biting edge is called an incisal edge (Fig. 2-8).

• Lingual Surface: The tongue-facing (lingual) surface is often shovel-shaped, which aids in guiding food into the mouth (Fig. 2-9).

Canines

• Design: Primarily designed for holding or grasping food.

• Importance: Their significance is evident in animals like dogs (Canis genus, named after these teeth), which use canines as weapons to pierce, hold, and tear flesh.

• Human Function: In humans, canines also protect the jaw joint during side (lateral) jaw movements. Their considerable length and thickness provide lateral stress-bearing support during side-to-side jaw excursions.

• Dimensions: Canines are the longest teeth in the human dentition, and the maxillary canines have the longest roots among all teeth (Fig. 2-10). This makes them some of the best-anchored and most stable teeth.

• Root Shape: Canine roots have a triangular shape in cross-section (Fig. 2-11). This unique shape enables the canine to maintain its position at the corner of the mouth, resisting anterior, posterior, lateral, and rotational forces of displacement within its bony socket.

Premolars

• Number: There are four maxillary and four mandibular premolars.

• Characteristics: They represent a functional