Chapter 24. Dentistry
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2. Tooth Growth
a. Chemical Action of Toothpaste
1. Structure of Teeth
⑴ Chemical Composition
① Teeth have a denser crystalline structure than bone and a lower water content.
⑵ Tooth = Crown + Root
① Crown: The portion visible above the gums.
② Root: The portion embedded within the gums.
⑶ Tooth Tissues = Enamel + Dentin + Pulp
① Enamel (also called porcelain-like layer): the outermost covering of the tooth
○ 96% of enamel is inorganic mineral crystals whose main component is calcium.
○ The hardest tissue in the human body—harder than bone.
○ Withstands approximately 60–70 kg of masticatory force.
○ Avascular tissue; non-vital.
② Dentin: a hard, pale-yellow tissue that makes up most of the tooth.
○ 70% of dentin is inorganic mineral crystals whose main component is calcium.
○ Pulp cavity: minute pores within dentin.
○ Function: cushions and attenuates mechanical shocks transmitted to enamel.
○ Odontoblasts: reside in the pulp; synthesize dentin to protect the pulp, with only their processes extending into dentin.
○ Avascular tissue; vital (because of odontoblasts).
○ Factors involved in dentin formation: OP-1 (osteogenic protein), BMP-2, BMP-4.
③ Dental pulp
○ A fully matured tooth can be maintained even without the pulp: this is why pulpectomy (root canal therapy) is possible.
○ Central region: fibroblasts + GAGs + collagen; contains blood vessels and nerves; supplies nutrients and senses stimuli.
○ Peripheral region: contains odontoblasts; forms dentin to repair dentin when damaged.
⑷ Periodontium: structures surrounding the tooth
① Cementum: tissue covering the tooth root
○ 50% of cementum is inorganic mineral crystals whose main component is calcium.
○ Anchors the collagen fibers of the ligament to the tooth.
○ Composed of calcium phosphate (hydroxyapatite), proteoglycans, and collagen.
② Alveolar bone (jawbone supporting the teeth)
○ Osteoblasts are present on its surface.
③ Gingiva (gums): pink mucosa located just below the crown
○ Protects the alveolar bone.
④ Periodontal ligament (PDL): a thin membrane connecting cementum and alveolar bone
○ Function: acts as a cushion; dental implants lack a PDL, so they do not absorb shock.
○ Composed of fibroblasts.
○ Traversed by blood vessels and nerves.
○ Tensile loading of the PDL → PDL cells differentiate into alveolar osteoblasts → alveolar bone formation.
○ Loss of PDL function leads to resorption of the alveolar bone.
2. Tooth Growth
⑴ Number of Teeth = 4 × (Incisors × 2 + Canines × 1 + Premolars × 2 + Molars × 3) = 32
⑵ Timing of Tooth Eruption
| Arch | Incisors | Canines | First Molars* | Second Molars* |
|---|---|---|---|---|
| Upper | 8–13 months | 16–22 months | 13–19 months | 25–33 months |
| Lower | 6–12 months | 17–23 months | 14–18 months | 23–31 months |
Table 1. Timing of Tooth Eruption: Primary Dentition (Deciduous Teeth). Primary teeth don’t have premolars; these columns correspond to primary (first/second) molars.
| Arch | Incisors | Canines | Premolars | First Molars | Second Molars | Third Molars (Wisdom) |
|---|---|---|---|---|---|---|
| Upper | 6.5–8.5 years | 10–12 years | 9.5–11.5 years | 6–7 years | 11.5–12.5 years | 17–21 years |
| Lower | 6–8 years | 9–11 years | 9.5–12 years | 6–7 years | 11–13 years | 17–21 years |
Table 2. Timing of Tooth Eruption: Permanent Dentition
⑶ Tooth Development from Birth to Adulthood
① At birth: Some primary and permanent tooth germs are already present, but none are visible.
② 6–8 months: The first teeth erupt, typically the two lower central incisors.
③ 2 years and half: All 20 primary (deciduous) teeth are present.
④ 6–8 years: Permanent teeth begin to erupt, starting with the first molars and the lower incisors.
⑤ Around 10 years: Following the incisors, the premolars and canines erupt.
⑥ Around 12 years: The second permanent molars erupt.
⑦ 17–21 years: The third molars (“wisdom teeth”) erupt; they’re so called because they typically appear at an age associated with growing maturity—or “the age when wisdom begins.”
3. Dental Diseases
⑴ Periodontitis (“loose teeth” disease): inflammation of the periodontal tissues
① Cause: dental plaque
○ Plaque: a biofilm (mass of bacteria) living in the mouth.
○ Bacteria in plaque survive by using sugars from food.
○ Plaque → invades the alveolar bone (periodontitis) → hardens into calculus (tartar).
② Periodontitis can occur even in otherwise healthy people: plaque must be removed by physical (mechanical) means, not immunologic ones.
⑵ Dental caries (tooth decay)
① About 85% of the general population is affected.
② Causes:
○ Carbohydrates + plaque → acid production.
○ Acid + tooth → caries (acid dissolves mineral).
○ Streptococci form dextran from carbohydrates; dextran is a polysaccharide essential for plaque formation.
③ Stages:
○ Stage 1 caries: confined to enamel; remove decay and restore with amalgam or gold.
○ Stage 2 caries: extends into dentin; remove decay and restore with amalgam or gold.
○ Stage 3 caries: reaches the pulp and nerve; root canal therapy (nerve therapy) and, if needed, extraction of teeth.
⑶ Oral cancer: a malignant tumor arising in the oral cavity and adjacent tissues
① Accounts for ~5% of all cancers; 7th–8th leading cause of cancer death.
② Cause: smoking, alcohol use, poor oral hygiene, syphilis, drugs, impaired immunity, and chronic irritation of the oral mucosa.
③ Treatment: surgery, radiation therapy, and chemotherapy.
① Apatite is a phosphate mineral with the formula Ca5(PO4)3X. Representative apatites include hydroxyapatite (X = OH-) and fluorapatite (X = F-). Their dissolution equilibria in water and solubility product constants (Ksp) at 25 °C are as follows:
② Ca5(PO4)3OH(s) ⇌ 5 Ca2+(aq) + 3 PO43-(aq) + OH-(aq), Ksp = 2.3 × 10-59
③ Ca5(PO4)3F(s) ⇌ 5 Ca2+(aq) + 3 PO43-(aq) + F-(aq), Ksp = 3.2 × 10-60
④ The outer layer of the tooth is composed of densely packed hydroxyapatite, which protects the tooth. Although hydroxyapatite is an insoluble compound, its solubility increases in acidic solution, leading to tooth damage. One method to prevent this is to replace the OH- ions in hydroxyapatite with F- ions, converting it to fluorapatite. For reference, because F- is a weaker base than OH- (since HF is a stronger acid), fluorapatite is less soluble than hydroxyapatite in acidic solutions.
Input: 2019.11.29 16:40