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About tooth anatomy






The crown is the part of the tooth that is visible above the gumline. It is covered by a hard shell of enamel known as the anatomical crown. The enamel is hard enough to endure the strain of a lifetime of biting, chewing and grinding. It also protects the tooth from bacteria and changes in temperature when eating hot or cold foods. Nonetheless, it is brittle and may crack or chip. The enamel is translucent, and most of the color of the tooth actually is produced by the dentin below the enamel.

As people age, their gums often start to recede, which makes the crown appear larger as more of the root becomes exposed. Conversely, the crown may appear smaller as the enamel gradually wears away over time. The area of the tooth where the crown meets the root is called the neck (cemento-enamel junction).

The root is the part of the tooth that pushes into the upper (maxilla) or lower (mandible) jawbone, and the structure of the root differs from tooth to tooth. For example, canines and incisors have a single root, while molars may have up to four roots depending on the location in the mouth. Each root has a tiny opening at the bottom called the apical foramen. Blood vessels and nerves enter the root through this space.

A substance called cementum covers the tooth root and attaches to fibers that fasten the root to the jawbone. Similar to enamel, it is a hard protective layer of the tooth.

Just beneath the enamel (covering the crown) and the cementum (covering the root) is the dentin. This substance resembles bone and makes up most of a tooth’s structure and is responsible for the tooth’s color. It typically has a whitish to yellowish hue. As people grow older, the dentin usually darkens or discolors. Exposure to too much fluoride or to certain antibiotics (e.g., tetracycline) during pregnancy also can cause discoloration of teeth in children.

The pulp is located beneath the dentin at the core of the tooth. It is composed of blood vessels, nerves and connective tissue. The blood supply of the pulp provides the nutrients that keep the tooth alive. The pulp is made up of two parts. The pulp chamber is found in the crown of the tooth. The root canal is the part of the pulp located in the root of the tooth. This is where blood vessels and nerves enter the tooth and become part of the pulp chamber. The part of the root that points upward into the teeth is called the pulp horns.

Baby teeth, more accurately called primary teeth or deciduous teeth, are a person’s first set of teeth. They begin to form during the first trimester of pregnancy (first 12 weeks) and begin to emerge (erupt) through an infant's gums around 6 months of age. All twenty primary teeth are typically fully erupted by the time a child is 3 years old and are later replaced by 32 permanent teeth.

Twenty primary teeth – eight incisors (four front teeth on both the upper and lower jaw), four canines (located in the front of the jaw behind the incisors) and eight molars (larger teeth in the back of the mouth) – typically erupt by the time a child is 3 years old. Eruption of the lower teeth generally precedes eruption of the top teeth. Eruption times for girls are generally earlier than those for boys.

 

 

 

The following schedule for the top primary teeth was established by the American Dental Association (ADA):

· Central incisor: 8 months to 12 months

· Lateral incisor: 9 months to 13 months

· Canine (cuspid): 16 months to 22 months

· First molar: 13 months to 19 months

· Second molar: 25 months to 33 months

Eruption of the bottom primary teeth occurs as follows:

· Central incisor: 6 months to 10 months

· Lateral incisor: 10 months to 16 months

· Canine (cuspid): 17 months to 23 months

· First molar: 14 months to 18 months

· Second molar: 21 months to 31 months

The first primary teeth to emerge are usually the bottom two incisors, followed by the top four incisors and then the rest of the bottom incisors. After the incisors the rest of the primary teeth begin to fill the mouth, usually with two developing at a time. The roots of primary teeth are 50 percent formed when eruption first begins, and roots are completed by the time a child is between the ages of 18 months and 3 years.

Primary teeth have several roles. They help with biting and chewing, and are crucial to a child’s ability to speak. They may also make a child appear more attractive. Finally, primary teeth serve as a guide for permanent teeth. For this reason, primary teeth usually have more space between them to allow permanent teeth more room to erupt.

Primary teeth are smaller and whiter than permanent teeth. Some of the other ways in which primary teeth differ from permanent teeth include:

· Crowns are shorter in primary teeth

 

· Enamel depth is more consistent and thinner in primary teeth

 

· Pulp chambers of primary teeth are larger relative to the size of the tooth

 

· Roots are longer and more slender in primary teeth

Eventually, a child’s primary teeth begin to fall out. The middle teeth in front (incisors) typically fall out at age 6. Molars in the back are shed between the ages of 10 and 12. By age 13, children usually have their permanent teeth.

A total of 32 permanent teeth replace the original 20 primary teeth. Meanwhile, permanent first and second premolars (bicuspids) replace the primary first and second molars and permanent incisors and canines replace primary incisors and canines. The permanent molars which grow towards the back of the mouth do not replace baby teeth. This is why there are more teeth in permanent dentition.

 

 


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