essay 9 - class II cavity preparation for amalgam stages of preparation. Different types of cavities and retentions in small and large carious lesions. Secondary retentive features pins and slots (locks, caves)

what are class II cavities?

• proximal surfaces of posterior teeth, usually involving

— 1 surface - M/D (if adjacent tooth is missing)

— 2 surfaces - MO/DO

— 3 surfaces - MOD

— 4 surfaces - MODB/MODL

describe amalgam

• direct restorative material

• metal like material composed of silver, tin, copper, mercury

• mixture is pressed into specifically prepared undercut tooth form and contoured to restore tooth’s form and function once the material has set

stages of class II tooth preparation

1. Initial tooth preparation

• Occlusal step:

— access proximal caries through occlusal surface

— enter at central fissure with bur parallel to long axis

• maintain depth: 1.5mm to 2mm

• outline form similar to class I

• proximal box:

— Create a proximal ditch and extend gingivally to the desired level of gingival wall

— Extend facial/lingual/gingival margins to:

- include all caries

- create 90 degree cavosurface angles

- clear adjacent tooth by -0.5mm

— axial and pulpal walls must be in sound dentin

— Proximal ditch is wider gingivally than occlusally → slight occlusal convergence.

— Remove isolated weak proximal enamel using a spoon excavator.

— Preparation must resist fracture under forces along the long axis and retain the restorative material

— Retention locks must be placed 0.2 mm inside the DEJ and oriented parallel to the DEJ

2. Final tooth preparation

• Remove remaining infected dentin/enamel on axial and pulpal walls using a slow‑speed round bur.

• secondary resistance and retention forms round axiopulpal line angles with a gingival margin trimmer for stress distribution. Proximal retention locks may further improve fracture resistance.

• Finish external walls - no occlusal bevels, ensure all margins meet the tooth surface at a 90° cavosurface angle.

what are small class II carious lesion?

• lesion is confined to enamel or superficial dentin

• marginal ridge may be intact or slightly undermined

• occlusal fissures may or may not be involves

• Types of preparations:

1. Conservative MOD or DO cavity

— limited proximal box

— minimal occlusal extension (if any)

2. slot preparation (box-only) when occlusal fissures are not affected

— no occlusal access step

— used when occlusal surface is intact

• retention features:

— retention grooves (also called proximal locks when placed in class II proximal boxes) placed at axiofacial and axiolingual line angles

— slight convergence of buccal and lingual walls

— flat gingival and axial walls to resist displacement

• key goals:

— preserve as much tooth structure as possible while ensuring sufficient mechanical retention

types of preparations for small class II carious lesions

• Types of preparations:

1. Conservative MOD or DO cavity

— limited proximal box

— minimal occlusal extension (if any)

2. slot preparation (box-only) when occlusal fissures are not affected

— no occlusal access step

— used when occlusal surface is intact

what is the goal of class II carious lesion prep

— preserve as much tooth structure as possible while ensuring sufficient mechanical retention

small class II carious lesion retention features during prep

• retention features:

— proximal locks placed at axiofacial and axiolingual line angles

— slight convergence of buccal and lingual walls

— flat gingival and axial walls to resist displacement

describe large class II carious lesions

• caries extend deep into dentin, possibly undermining cusps or crossing marginal ridges

• multiple surfaces may be involved (MOD)

• occlusal surface often compromised

• Types of preparations:

1. traditional MO/DO with extended occlusal step

2. MOD cavity = involves both proximal surfaces and the central groove

3. Cusp capping = for severely weakened cusps or marginal ridges

• Retention features:

— retention locks in gingivo-axial line angles

— secondary retentive features:

- dovetail locks in the occlusal portion

- pins (rarely used today)

— boxes with internal convergence

— flat pulpal and gingival floors for stress resistance

• Key goal:

• ensure structural durability and long term retention of restoration by compensating for significant tooth loss

types of preparations of large class II carious lesions

• Types of preparations:

1. traditional MO/DO with extended occlusal step

2. MOD cavity = involves both proximal surfaces and the central groove

3. Cusp capping = for severely weakened cusps or marginal ridges

retention features during preparation of large carious lesions

• Retention features:

— retention grooves in gingivo-axial line angles

— secondary retentive features:

- dovetail locks in the occlusal portion

- pins (rarely used today)

— boxes with internal convergence

— flat pulpal and gingival floors for stress resistance

key goal of preparation of large class II carious lesion

• Key goal:

• ensure structural durability and long term retention of restoration by compensating for significant tooth loss

state the secondary retentive features

1. Slots - horizontal groove placed entirely in dentin, usually on the gingival floor of a proximal box.

• Commonly used when: there’s limited occluso gingival height, marginal ridge is weak or missing, restoration is large or lacks enough vertical wall convergence

2. Retention locks/grooves - narrow vertical grooves placed at gingivo axial line angles (0.2mm inside DEJ) to prevent lateral dislodgement, or dovetail

3. Bevelling or rounding the axiopulpal line angle - reduces internal stress concentration and increases bulk of amalgam, decreases risk of fracture

4. Pins - used in complex amalgam restorations for additional mechanical retention in severely broken down teeth

5. Coves - small concavities placed in facial or lingual walls of a cavity. Improve mechanical retention, especially in flat or shallow preparations

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slots

1. Slots - horizontal groove placed entirely in dentin, usually on the gingival floor of a proximal box.

• Commonly used when: there’s limited occluso gingival height, marginal ridge is weak or missing, restoration is large or lacks enough vertical wall convergence

retention locks/grooves

2. Retention locks/grooves - narrow vertical grooves placed at gingivo axial line angles (0.2mm inside DEJ) to prevent lateral dislodgement, or dovetail

bevelling or rounding the axiopulpal line angle

3. Bevelling or rounding the axiopulpal line angle - reduces internal stress concentration and increases bulk of amalgam, decreases risk of fracture

pins

4. Pins - used in complex amalgam restorations for additional mechanical retention in severely broken down teeth

coves

5. Coves - small concavities placed in facial or lingual walls of a cavity. Improve mechanical retention, especially in flat or shallow preparations