13.1 Caries Risk Assessment, Prevention, and Operative Dentistry

5.1 Caries Risk Assessment, Prevention, and Operative Dentistry

INBDE High-Yield Core Concept: Dental caries is a dynamic, multifactorial biofilm-mediated disease. Understanding the interplay between microflora, substrate, and host factors—alongside fluoride pharmacology, cavity preparation geometry, and moisture control—is vital for passing the INBDE.

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Caries Etiology & The Cariogenic Triad

The initiation and progression of dental caries require the simultaneous presence of four primary factors: a susceptible host (tooth structure and saliva), a cariogenic microflora, a fermentable carbohydrate substrate, and sufficient time.

               +-----------------------+
               |         HOST          |
               | (Tooth & Saliva)      |
               +-----------+-----------+
                           |
                           |
    +----------------------+----------------------+
    |                                             |
+---+-------------------+             +-----------+-----------+
|    MICROFLORA         |    TIME     |       SUBSTRATE       |
| (S. mutans, Lacto)    +-------------+ (Fermentable Carbs)   |
+-----------------------+             +-----------------------+

Primary Cariogenic Bacteria

• Streptococcus mutans: The principal initiator of enamel caries. S. mutans is acidogenic (produces lactic acid from carbohydrates) and aciduric (survives at low pH). It synthesizes extracellular glucans from sucrose via glucosyltransferase, facilitating firm adherence to the acquired pellicle.
• Lactobacillus species: Secondary invaders responsible for the progression of deep dentinal caries. They thrive in low-pH environment once cavity formation has occurred.
• Actinomyces viscosus / Actinomyces naeslundii: Strongly associated with root surface caries in patients with gingival recession.

Substrate Dynamics

Sucrose is the most cariogenic sugar because S. mutans utilizes it to synthesize insoluble extracellular glucans (sticky matrix) in addition to organic acid byproducts (primarily lactic acid).

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Caries Risk Assessment: CAMBRA Framework

CAMBRA (Caries Management by Risk Assessment) categorizes patients into Low, Moderate, High, or Extreme risk to tailor preventive and therapeutic interventions.

Key Indicators (WREC Mnemonic): White spots, Restorations in past 3 years, Enamel lesions, Cavities into dentin automatically place a patient in High or Extreme risk.

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Fluoride Pharmacology, Chemistry & Toxicology

Systemic vs. Topical Fluoride

• Systemic Fluoride: Ingested during tooth development (pre-eruptive). Incorporates directly into the developing hydroxyapatite crystal lattice to form fluorapatite.
• Topical Fluoride: Post-eruptive mechanism. Inhibits demineralization, enhances remineralization, and inhibits the bacterial enzyme enolase, blocking bacterial glycolysis.

Chemical Dynamics & Critical pH

Demineralization occurs when the oral pH drops below the critical pH of the specific mineral lattice:

$\text{Hydroxyapatite } Ca_{10}(PO_4)_6(OH)_2 \longrightarrow \text{Critical pH} = 5.5$

$\text{Fluorapatite } Ca_{10}(PO_4)_6F_2 \longrightarrow \text{Critical pH} = 4.5$

Because fluorapatite has a lower critical pH (4.5), fluoridated enamel resists acid dissolution significantly longer during cariogenic bacterial challenges.

pH Scale & Enamel Dissolution Thresholds:
|--- 7.0 (Neutral Saliva)
|
|--- 5.5 *** Hydroxyapatite Critical pH (Demineralization Begins) ***
|
|--- 4.5 *** Fluorapatite Critical pH (Fluoridated Enamel Demineralization Begins) ***
|
|--- 2.0 (Gastric Acid / Erosion)

Water Fluoridation Standards

The U.S. Public Health Service standard for community water fluoridation is 0.7 ppm (0.7 mg/L) $F^-$. This concentration provides optimal cariostatic benefit while minimizing dental fluorosis.

Fluoride Toxicology

• Certainly Toxic Dose (CTD): $5\text{ mg/kg}$ of elemental fluoride ($F^-$).
• Certainly Lethal Dose (CLD): $32\text{ to } 64\text{ mg/kg}$ of elemental fluoride ($F^-$).

Acute Fluoride Toxicity Emergency Protocol:
- Ingestion < 5 mg/kg: Give oral calcium (milk) to bind F-, monitor.
- Ingestion >= 5 mg/kg: Induce vomiting, give oral milk/calcium gluconate, admit to emergency room.
- Ingestion >= 15 mg/kg: Immediate emergency hospitalization, IV calcium gluconate, cardiac monitoring (due to severe hypocalcemia and hyperkalemia).

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GV Black Cavity Classification System

Dr. G.V. Black established the classic anatomical classification of cavity preparations:

Class I   : Pits & Fissures (Occlusal of posteriors, Lingual of maxillary incisors)
Class II  : Proximal of Posteriors (Premolars & Molars)
Class III : Proximal of Anteriors (NOT involving incisal angle)
Class IV  : Proximal of Anteriors (INVOLVING incisal angle)
Class V   : Cervical Third of Facial/Lingual (All teeth)
Class VI  : Incisal Edges or Occlusal Cusp Tips

• Class I: Faults in structural pits and fissures (occlusal surfaces of molars/premolars, occlusal two-thirds of facial/lingual surfaces of molars, lingual surfaces of maxillary incisors).
• Class II: Proximal surfaces of premolars and molars.
• Class III: Proximal surfaces of incisors and canines not involving the incisal angle.
• Class IV: Proximal surfaces of incisors and canines involving the incisal edge/angle.
• Class V: Gingival/cervical third of facial or lingual surfaces of any tooth.
• Class VI: Incisal edges of anterior teeth or occlusal cusp tips of posterior teeth.

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Operative Hand Instruments & Rotary Burs

Hand Instrument Formula

Hand cutting instruments are identified by a 3-number or 4-number formula stamped on the handle (e.g., 10-85-8-14):

$\text{Formula: [Blade Width (tenths of mm)] - [Primary Cutting Edge Angle (centigrades)] - [Blade Length (mm)] - [Blade Angle (centigrades)]}$

• 1st Number (10): Blade width in tenths of a millimeter ($10 = 1.0\text{ mm}$).
• 2nd Number (85 - present in 4-number formula): Primary cutting edge angle in centigrades relative to the long axis of the handle ($85 = 85% \text{ of } 360^\circ = 306^\circ$). Present when cutting edge is not perpendicular to blade (e.g., Gingival Margin Trimmer).
• 3rd Number (8): Blade length in millimeters ($8\text{ mm}$).
• 4th Number (14): Blade angle relative to long axis of handle in centigrades ($14 = 14% \text{ of } 360^\circ = 50.4^\circ$).

Instrument Classification:
- Excavators (Spoon): Removal of soft, necrotic dentin.
- Hatchets: Cutting edge parallel to handle axis; smooths internal cavity walls.
- Chisels (Enamel): Cutting edge perpendicular to handle axis; cleaves unsupported enamel.
- Gingival Margin Trimmers: Distal trimmer (2nd number >90); Mesial trimmer (2nd number <85).

Rotary Cutting Burs

• Carbide Burs: Made of tungsten carbide. Function by shearing/cutting blade action. Highly efficient for rapid bulk reduction, endodontic access, and removing old amalgam/composite restorations. Operate best at high speeds with light pressure.
• Diamond Burs: Function by abrasive grinding. Preferred for enamel reduction, beveling, and crown preparation wall tapering.

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Rubber Dam Isolation Technique & Clamp Selection

Rubber dam isolation provides moisture control, improves visibility, protects the airway from aspiration, and prevents microbial contamination of exposed pulp tissue.

Clinical Pearl: Always secure rubber dam clamps with dental floss tied to the outer bow to prevent accidental airway aspiration if the clamp fractures or slips.