Fundamentals of Tooth Preparation

Comprehensive practice flashcards covering the fundamentals of tooth preparation, including the 9 steps, outline form, primary resistance and retention forms, caries excavation, pulp protection materials, and cavosurface margin finishing.

What is tooth preparation?

The process of shaping a tooth to receive a restorative material (such as amalgam, composite, or cast restoration) while preserving maximum healthy tooth structure and providing adequate strength, retention, and access.

What are the four steps comprising the initial stage of tooth preparation?

1. Step 1: Outline Form & Initial depth
2. Step 2: Primary Resistance Form
3. Step 3: Primary Retention Form
4. Step 4: Convenience Form

What are the five steps comprising the final stage of tooth preparation?

1. Step 5: Removal of any infected dentin or old restorative material (or both), if indicated
2. Step 6: Pulp protection, if indicated
3. Step 7: Secondary resistance and retention form
4. Step 8: Procedures for finishing external walls
5. Step 9: Final procedures (cleaning, inspecting, desensitizing)

In cavity preparation nomenclature, how are external walls differentiated from internal walls?

External walls extend directly to the external tooth surface (distal, facial, lingual, and gingival walls), whereas internal walls do not contact the external tooth surface (pulpal and axial walls).

What is the definition of outline form in tooth preparation?

Placing the preparation margins in the exact positions they will occupy in the final preparation (except for finishing enamel walls and margins), establishing the shape and perimeter on the surface of the tooth.

In conservative cavity preparation, what are the target measurements for isthmus width and dovetail relative to intercuspal distance (ICD)?

The isthmus width should be $\frac{1}{4}$ the ICD, and the dovetail should be $\frac{1}{3}$ the ICD.

Why is an S-curve (reverse curve) placed on the buccal proximal wall of a Class II amalgam preparation?

Because the lingual embrasure is wider than the buccal embrasure, extending straight bucco-proximally would cause excessive cutting of the buccal cusp. The S-curve allows the wall to meet the external tooth surface at a $90^\circ$ cavosurface angle without undermining enamel or creating a flare.

What is the required embrasure clearance for the buccal, lingual, and gingival walls of the proximal box in a Class II amalgam preparation?

The walls must extend into the embrasures and $0.5\,mm$ out of contact with the adjacent tooth surface to ensure margins are placed in cleansable areas accessible for inspection and finishing.

What is the recommended mesio-distal width of the gingival (cervical) floor for molars versus premolars?

For molars, the width is $1.0\text{--}1.5\,mm$. For premolars, the width is $0.75\text{--}1.0\,mm$.

Under what condition may facial and lingual proximal margins remain in contact with the adjacent tooth in Class II and III composite preparations?

They may remain in contact unless caries, a defect, or an old restoration extends beyond the contact area, which simplifies matrix placement, composite insertion, and contouring.

What are the recommended initial depths for pit-and-fissure preparations versus smooth-surface axial walls?

For pit-and-fissure preparations: $0.2\text{--}0.5\,mm$ pulpally of the DEJ (or $1.5\text{--}2.0\,mm$ from the cavosurface margin). For smooth surfaces: $0.2\text{--}0.8\,mm$ for the axial wall.

What structural criteria define a strong enamel margin during outline form preparation?

All enamel walls must consist of full-length enamel rods resting on sound dentin, or full-length rods supported on the preparation side by shortened rods that are also on sound dentin.

When should two adjacent defects or preparations be connected into a single outline form?

When they are separated by less than $0.5\,mm$ of sound tooth structure, in order to avoid leaving a weak, unsupported enamel wall between them.

Under what specific clinical conditions may an oblique or transverse ridge be crossed?

1. A carious groove crosses the ridge
2. The ridge is undermined by caries
3. Less than $0.5\,mm$ of sound enamel remains between the mesial and distal areas
4. Additional retention is gained by extending into the adjacent fossa

What is the definition of primary resistance form?

The shape and placement of preparation walls that best enable both the remaining tooth structure and the restoration to resist fracture caused by masticatory forces.

Why is a flat pulpal floor (A) preferred over a rounded pulpal floor (B) in resistance form?

A flat floor helps the tooth resist occlusal forces perpendicular to the long axis and prevents restoration movement, whereas a rounded floor allows non-bonded restorations to rock, creating a wedging force that shears tooth structure.

How is the pulpal floor oriented in a mandibular first premolar, and why is it an exception to the flat horizontal floor rule?

The pulpal floor is prepared parallel to a line connecting the buccal and lingual cusp tips (inclined lingually) rather than perpendicular to the long axis, specifically to conserve dentin over the large, prominent buccal pulp horn.

What is the minimum mesio-distal thickness of intact tooth structure required at the marginal ridge for molars and premolars?

Molars require $2.0\,mm$ and premolars require $1.6\,mm$ ($1.6\text{--}2.0\,mm$ range) to provide a uniform bulk of tooth structure that resists fracture under masticatory stress.

Why are internal line angles slightly rounded (coved) in tooth preparation?

Sharp internal line angles concentrate stress, which increases the likelihood of tooth fracture; rounding line angles (such as beveling the axio-pulpal line angle in Class II preps) dissipates stress concentrations.

What are the clinical rules for cusp capping based on cavity extension from the primary groove toward the cusp tip?

1. Extension $\le \frac{1}{2}$ of the distance: no cusp capping
2. Extension from $\frac{1}{2}$ to $\frac{2}{3}$: consider cusp capping
3. Extension $\ge \frac{2}{3}$: recommend cusp capping

What are the minimal occlusal thickness requirements for amalgam, cast metal/composite, and porcelain/ceramics?

• Amalgam: $1.5\,mm$
• Cast metal & composite: $1\text{--}2\,mm$
• Porcelain / ceramics: $2\,mm$

What is primary retention form, and how is it mechanically achieved in Class I and Class II amalgam preparations?

It is the shape of the preparation that resists displacement or removal of the restoration from tipping or lifting forces. For amalgam, opposing buccal and lingual walls are made slightly converging pulpo-occlusally to mechanically lock the material in place.

What is convenience form in tooth preparation?

The shape or form of the preparation that provides adequate observation, accessibility, and ease of operation during cavity preparation and restoration insertion.

What degree of wall divergence is required for cast restorations to fulfill convenience form?

Cavity walls must diverge $2^\circ\text{--}5^\circ$ pulpo-occlusally to provide an unimpeded path of insertion and withdrawal for the casting.

What is the proper procedural sequence when excavating deep dentin caries?

Excavation begins at the periphery of the cavity using a spoon excavator with a scooping motion, and the deepest portion overlying the pulpal floor is excavated last under strict isolation.

When carious dentin extends beyond the ideal pulpal or axial wall depth in a specific spot, how should it be managed?

Only the localized carious area is punched out or scooped out; the entire pulpal floor or entire axial wall must never be extended deeper.

What is the clinical distinction between infected dentin and affected dentin?

Infected dentin is necrotic, teeming with bacteria, irreversibly denatured, and must be eliminated. Affected dentin contains no bacteria, has intact or reversibly denatured collagen, is remineralizable, and may be preserved near the pulp to prevent exposure.

What chemical solution is used as a caries detector, and what does it stain?

1% acid red in propylene glycol, which stains irreversibly denatured collagen.

How much rotary cutting time without water coolant can produce burn lesions in the pulp?

$11\,\text{seconds}$ of rotary cutting without water coolant generates enough frictional heat to induce pulpal burn lesions.

What are the clinical properties, mechanisms, and limitations of calcium hydroxide liners?

Calcium hydroxide has a basic pH, is bacteriostatic, and stimulates tertiary (reparative) dentin formation upon direct contact with pulp tissue. However, it possesses low compressive strength and high solubility in oral fluids, so it must only be placed thinly over the deepest areas and never on preparation walls.

Why is zinc oxide eugenol (ZOE) contraindicated beneath composite restorations?

Eugenol inhibits the free-radical polymerization of composite resin.

What are the remaining dentin thickness (RDT) rules for placing liners and bases beneath metallic restorations?

• If excavation depth does not exceed $1\text{--}2\,mm$ from the initial pulpal/axial wall: no liner is indicated.
• If RDT is $< 0.5\,mm$: apply a thin layer of liner followed by a stronger base.
• Total bulk between the pulp and metallic restoration must equal at least $2.0\,mm$ ($\text{RDT} + \text{liner} + \text{base}$).

What mechanical features provide secondary ($2^\circ$) resistance and retention in compound or complex preparations?

1. Retention coves and grooves (vertical grooves on buccoaxial/linguoaxial line angles; horizontal grooves; incisal coves)
2. Preparation extensions (into facial or lingual grooves)
3. Skirts
4. Beveled enamel margins
5. Pins, slots, steps, and amalgam pins

What are the orientations of enamel rods in the gingival third of smooth surfaces, at grooves, and at cusps/ridges in permanent teeth?

1. Gingival third of smooth surfaces: incline apically
2. Center of grooves: converge
3. Height of cusps and ridges: diverge

Why is a $15^\circ\text{--}20^\circ$ bevel placed exclusively on the enamel of the gingival margin in a Class II amalgam preparation?

Because enamel rods in the gingival third of permanent teeth incline apically; the $15^\circ\text{--}20^\circ$ bevel removes unsupported enamel rods that would otherwise fracture under functional loading.

Why are cavosurface margins prepared as a $90^\circ$ butt joint for amalgam restorations but beveled at $45^\circ$ for Class III, IV, and V composites?

Amalgam has low edge strength and will fracture if beveled, requiring a $90^\circ$ butt-joint margin. Class III, IV, and V composites require a $45^\circ$ bevel to expose the ends of enamel rods for superior acid-etch bonding, increase surface area, and provide an esthetic marginal blend.

What are the protocol guidelines for final cleaning, inspecting, and desensitizing (Step 9)?

The cavity is cleaned of debris and moisture using water from a 3-way syringe and lightly dried with clean air without dehydrating the dentin; the application of a cavity disinfectant is unnecessary.

What is the most likely cause of an isthmus fracture during masticatory function in a newly placed Class II amalgam restoration?

Inadequate isthmus depth, which deprives the brittle amalgam of its required minimal bulk ($1.5\,mm$) to resist fracture.