Class I Amalgam Restoration - Complete Guide (Sturdevant's)

Definition of Class I Cavity

PART 1: STEPS OF CAVITY PREPARATION

STEP 1: OUTLINE FORM AND INITIAL DEPTH

Principles:

1. All friable or weakened enamel must be removed
2. All faults (pits, fissures, grooves) must be included
3. Margins placed to allow good finishing of the restoration
4. Extension for prevention - extend margins to self-cleansable areas

Features of Outline Form:

1. Preserving cuspal strength - do not undermine cusp tips
2. Preserving marginal ridge strength - maintain minimum 1.6 mm from preparation margin to contact area for proximal ridges; avoid over-extension
3. Minimizing faciolingual extensions - keep preparation as narrow as practical
4. Using enameloplasty - shallow grooves/fissures may be eliminated by saucerizing with a flame-shaped diamond rather than being included in the cavity
5. Connecting two faults when they are close (< 0.5 mm apart)
6. Restricting depth into dentin to a maximum of 0.2 mm beyond the DEJ in the initial cut

Bur Selection:

• No. 245 carbide bur (pear-shaped) - most commonly used for Class I preparations in posterior teeth
• No. 330 bur (small pear-shaped) - for conservative/small preparations
• Initial entry at the most carious or faulted area

STEP 2: PRIMARY RESISTANCE FORM

Features:

1. Flat pulpal floor - resists forces directed along the long axis
2. Slight rounding of the internal line angles - reduces stress concentration
3. Adequate depth - minimum 1.5 mm (occlusal enamel thickness ~1 mm + 0.5 mm into dentin); ideal pulpal floor depth ~1.5-2 mm from DEJ
4. Preservation of cusps and marginal ridges
5. Isthmus width - no wider than one-third of the intercuspal distance (buccal to lingual cusp tip) to preserve cusp strength
6. Box shape with flat floor and walls flaring slightly

STEP 3: PRIMARY RETENTION FORM

Principles:

1. Convergence of external walls occlusally - opposing walls converge slightly toward the occlusal opening (walls taper ~2-5 degrees); this locks the amalgam in place
2. Dovetail extensions where needed (especially in two-surface preparations)
3. Retention grooves or locks placed in dentin at the axio-mesial and axio-distal line angles for extensive preparations
4. Minimum extension - no unnecessary widening; the dimensions must be functionally necessary

Cavosurface Angle:

• 90-100 degrees at the cavosurface margin
• Results in 80-90 degrees of amalgam at the margin
• Bevel is contraindicated for amalgam (unlike composite) because amalgam has low edge strength and chips if the marginal angle is < 80 degrees
• A butt-joint margin provides the strongest union between enamel and amalgam

STEP 4: CONVENIENCE FORM

• Widening or extending the outline form to allow direct vision or appropriate instrumentation
• Adequate access without excessive sacrifice of tooth structure

STEP 5: REMOVAL OF REMAINING CARIOUS DENTIN

• Done after outline and resistance/retention forms are established
• Use large round bur (No. 2 or No. 4) at slow speed OR a spoon excavator
• Remove softened, infected dentin
• If deep caries approaches the pulp: consider indirect pulp capping with calcium hydroxide before further excavation
• Stop when dentin is firm (does not indent with explorer) - "affected dentin" that is remineralizable may be left if pulp protection is applied

STEP 6: FINISHING OF WALLS AND MARGINS (Enamel Wall Finishing)

• Enamel walls and margins must rest on sound dentin (no unsupported enamel rods)
• Use gingival margin trimmer and enamel hatchets to refine margins
• Butt-joint margin (90 degrees) maintained throughout - NO bevel for amalgam
• Remove any unsupported enamel rods with hand instruments
• Refine cavosurface angle with flame-shaped carbide bur

STEP 7: CLEANSING OF THE CAVITY (Final Procedures)

• Rinse with air-water syringe
• Inspect for debris, caries remnants
• Disinfectants (e.g., chlorhexidine) may be used - not considered essential by Sturdevant but recommended by many
• Cotton pellet moistened with water for final cleaning
• Dry gently - avoid desiccation of dentin (do not over-dry)

PART 2: DIMENSIONS OF CLASS I CAVITY PREPARATION

PART 3: BASE AND LINER APPLICATION (Pulp Protection)

Decision Based on Remaining Dentin Thickness (RDT):

Shallow/Ideal Depth Preparations (RDT > 2 mm):

• No liner or base required
• Dentin provides adequate thermal and chemical protection
• Simply clean and restore

Moderately Deep Preparations (RDT ~0.5-1.5 mm):

• Apply Resin-Modified Glass Ionomer (RMGI) base
• Thickness: 0.5-0.75 mm (thin layer)
• Applied only over the deepest portion of the excavation - NOT over all dentin
• Peripheral dentin must remain exposed for mechanical support of the restoration

Why RMGI?

1. Insulates the pulp from thermal changes
2. Bonds chemically to dentin (glass ionomer component)
3. Releases fluoride (caries inhibition)
4. Strong enough to resist condensation forces of amalgam
5. Reduces microleakage
6. Light-activated (RMGI variety) - fast set

• Load RMGI on a periodontal probe or cement-placing instrument
• Place only at the deepest portion of the floor
• Light-cure per manufacturer instructions
• Do not cover the entire dentin floor

Very Deep Excavations (RDT < 0.5 mm, near pulp):

• Apply calcium hydroxide (Ca(OH)₂) liner first - directly over the deepest point
• Ca(OH)₂ stimulates secondary (reparative) dentin formation
• Then cover with RMGI base (0.5-0.75 mm)
• This two-layer approach: Ca(OH)₂ → RMGI → amalgam

Dentin Desensitizer:

• After any liner/base is placed, regardless of depth, apply a dentin desensitizer before restoring
• Reduces postoperative sensitivity

PART 4: AMALGAM RESTORATION TECHNIQUE

Materials Required:

• Pre-proportioned amalgam capsule (alloy + mercury)
• Amalgamator/triturator
• Amalgam carrier (gun type)
• Condensers (various sizes - small, medium, large)
• Burnishers (ball, beaver-tail, football-shaped)
• Carving instruments (Hollenback No. 3, discoid-cleoid)
• Dappen dish (for excess)

4A. TRITURATION (Mixing)

• Place amalgam capsule in the triturator
• Check triturator settings according to the specific alloy's manufacturer instructions
• Trituration time: typically 8-15 seconds (alloy-specific; most modern amalgams use high-speed trituration ~8-10 s)
• Properly triturated amalgam: smooth, slightly glossy, consolidates into one mass without crumbling, no excess mercury visible
• Undertrituration - crumbly mix, poor properties
• Overtrituration - sticky, sets too fast, hot on hand

4B. AMALGAM PLACEMENT AND CONDENSATION

1. Loading the Carrier:

• Empty triturated amalgam into dappen dish
• Load into the amalgam carrier (Tofflemire carrier)
• Use multiple increments (incremental technique)

2. Insertion:

• Dispense the first increment (small) into the preparation
• Condensation begins immediately (amalgam is plastic for only ~3-4 minutes from start of trituration)

3. Condensation Technique:

• Use large condensers in wide areas, smaller condensers in corners/narrow areas
• Condense each increment before adding the next
• Apply firm pressure in overlapping strokes
• Direction: condense from center toward walls (push amalgam against the walls)
• Each increment: ~1.0 mm depth
• Slightly overfill the preparation (excess amalgam above cavosurface margins)
• Purpose of condensation: expresses excess mercury (improving amalgam properties), adapts amalgam to walls/floor, eliminates voids

4C. PRE-CARVE BURNISHING

• After filling is complete (preparation slightly overfilled), burnish the surface before carving
• Use a ball burnisher or beaver-tail burnisher
• Burnish with firm strokes in all directions
• Purpose: adapts amalgam to margins, creates a surface for carving, compacts the surface layer, improves marginal integrity
• Sturdevant specifically recommends pre-carve burnishing as it significantly improves marginal adaptation

4D. CARVING

• Begin carving before the amalgam gets hard (within the working time, ~3-5 minutes from trituration start for conventional amalgam)
• Use the Hollenback No. 3 carver (most commonly used for occlusal surfaces) and discoid-cleoid carver
• Carve along the direction of the cavosurface margin (carver blade resting half on the amalgam, half on the tooth surface)
• Remove excess amalgam flush with the cavosurface margin
• Re-create:
  • Occlusal anatomy (fossae, grooves, triangular ridges)
  • Proper contour matching adjacent tooth surfaces
  • Correct marginal ridge height matching the adjacent tooth
  • Occlusal contacts - maintain proper contact with opposing teeth

Critical Points During Carving:

• Do NOT over-carve (leave amalgam deficient at margins)
• Carving instrument resting on both tooth and amalgam prevents over-excavation
• Check occlusal contacts with articulating paper
• Remove high spots by carving (do not adjust with the handpiece immediately after placement - freshly set amalgam is very weak and cuts easily)

4E. POST-CARVE BURNISHING

• After carving, perform post-carve (final) burnishing
• Use a ball or beaver-tail burnisher with firm pressure along the margins
• Purpose: smoothes amalgam surface, adapts margins further, produces a superior surface finish
• Research (Sturdevant) shows post-carve burnishing reduces microleakage and improves marginal adaptation

4F. FINAL CHECK

• Remove rubber dam/isolation
• Ask patient to close gently on articulating paper
• Mark any high occlusal contacts - adjust with a sharp carver or slow-speed instrument
• Check proximal contacts (if any) with dental floss
• Inspect all margins for voids, overhangs, or deficiencies

PART 5: FINISHING AND POLISHING

• Do not polish at the same appointment as placement (freshly set amalgam is weak)
• Polish at a subsequent appointment (24-48 hours minimum) after full hardening
• Steps:
  1. Use rubber cups/wheels and abrasive pastes (aluminum oxide, Shofu Porte-Polish system)
  2. Finish with fine abrasive stones for occlusal surfaces
  3. Progress from coarser to finer abrasives
  4. Final high shine with a dry, unimpregnated rubber cup
• Purpose of polishing:
  1. Reduces surface roughness (reduces plaque and corrosion)
  2. Improves marginal seal
  3. Reduces tarnish and corrosion over time
  4. Enhances appearance

PART 6: TYPES OF CLASS I AMALGAM PREPARATIONS

A. Conservative (Simple) Class I

• Involves only the occlusal surface (small pits/fissures)
• Single pit only (e.g., mesial pit, distal pit, central fossa)
• Outlined to include only the faulted area

B. Extensive Class I

• Involves multiple pits and connecting fissures
• May extend into buccal or lingual grooves
• Restorative technique requires more complex carving

C. Class I Occlusolingual (OL) Preparation

• Occurs when lingual pit of upper molars/incisors is involved
• Extension on the lingual surface requires:
  • Separate outline on lingual pit
  • Connected to the occlusal preparation if the ridge between them is carious
  • Do NOT routinely connect unless caries or severely undermined ridge

D. Class I Occlusofacial (OF) Preparation

• Extension involves the buccal groove of lower molars
• Keep extension no wider than necessary
• Buccal extension follows groove pattern

SUMMARY TABLE: KEY NUMBERS to Remember

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• Read the full chapter on Class I amalgam restorations
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PRIMARY STEPS OF TOOTH PREPARATION

(Initial Tooth Preparation Stage - Sturdevant's)

DEFINITION OF TOOTH PREPARATION (Sturdevant)

"Tooth preparation is the mechanical alteration of a defective, injured, or diseased tooth to receive a restorative material that re-establishes a healthy state for the tooth, including esthetic corrections where indicated, and normal form and function."

DEFINITION OF INITIAL TOOTH PREPARATION (Sturdevant)

"Initial tooth preparation is the extension and initial design of the external walls of the preparation at a specified, limited depth so as to provide access to the caries or defect, reach sound tooth structure (except for later removal of infected dentin on the pulpal or axial walls), resist fracture of the tooth or restorative material from masticatory forces principally directed with the long axis of the tooth, and retain the restorative material in the tooth."

STEP 1: OUTLINE FORM AND INITIAL DEPTH

Definition (Sturdevant):

"Outline form is placing the preparation margins in the positions they will occupy in the final preparation, while preparing an initial depth of 0.2-0.8 mm pulpally of the DEJ (dentinoenamel junction)."

Principles of Outline Form:

1. All friable and/or weakened enamel should be removed
2. All faults (pits, fissures, defects) should be included in the outline
3. All margins should be placed in a position to afford good finishing of the restoration margins
4. Margins should be in cleansable/accessible areas (extension for prevention - now more conservative)
5. All margins should be placed on sound tooth structure
6. Extension of the outline should be as conservative as possible
7. Extend the outline form to provide convenience form

Features of Outline Form:

1. Preserving cuspal strength
   • Do not undermine or weaken cusps
   • Isthmus width limited to no more than 1/3 of the intercuspal distance
2. Preserving marginal ridge strength
   • Maintain minimum 1.6 mm of tooth structure between the preparation margin and the proximal contact area
   • Avoid over-extension toward the marginal ridge
3. Minimizing faciolingual extension
   • Keep the preparation as narrow as clinically justified
   • Do not extend into non-carious areas
4. Using enameloplasty
   • Shallow, non-carious fissures can be saucerized with a flame-shaped diamond bur rather than being incorporated into the full cavity
   • Removes minimal enamel, reduces preparation size
5. Connecting two faults or preparations
   • When two faults are < 0.5 mm apart, connect them into one outline
   • When faults are farther apart, treat as separate preparations
6. Restricting initial depth
   • Pulpal floor: 0.2 mm pulpally beyond the DEJ (initial depth)
   • Maximum initial dentin extension: 0.2 mm into dentin at this stage
   • Prevents unnecessary pulp damage during the initial cut

STEP 2: PRIMARY RESISTANCE FORM

Definition (Sturdevant):

"Primary resistance form is that shape and placement of the preparation walls that best enables both the restoration and the tooth to withstand, without fracture, masticatory forces delivered principally in the long axis of the tooth."

Principles of Resistance Form:

1. Box shape with relatively flat floor
   • A flat pulpal floor resists occlusal forces by distributing them broadly
   • A rounded/saucer-shaped floor may allow rocking of the restoration, creating wedging forces that can shear tooth structure
2. Flat pulpal/axial floor
   • Relatively parallel to the cusp tips and the occlusal plane
   • Distributes masticatory loads evenly
3. Adequate thickness of restorative material
   • Minimum amalgam thickness: 1.5 mm (to resist fracture under occlusal loads)
   • Preparation depth must ensure this minimum bulk
4. No unsupported (undermined) enamel
   • All enamel walls must be supported by sound dentin beneath
   • Unsupported enamel fractures under masticatory load
5. Slightly rounded internal line angles
   • Sharp internal line angles concentrate stress and can initiate fractures in tooth and restoration
   • Rounded line angles distribute stress more evenly
6. Capping weak cusps
   • When a cusp is undermined or weakened, include it in the preparation so the restoration covers and protects it (cusp capping)
   • Prevents cusp fracture in service
7. Preserve cusps and marginal ridges wherever possible
   • Conservation of sound tooth structure improves overall tooth strength
8. Cavosurface angle of 90-100 degrees (butt-joint margin)
   • For amalgam: provides maximum bulk at the margin for both enamel and amalgam
   • Bevel is contraindicated for amalgam (amalgam chips if its marginal angle is < 80 degrees)
   • For composite: beveling is appropriate
9. Bonding of restorative material (modern principle)
   • Bonded restorations add resistance form even in less retentive preparations

Features of Resistance Form (Summary):

STEP 3: PRIMARY RETENTION FORM

Definition (Sturdevant):

"Primary retention form is that shape or form of the conventional preparation that resists displacement or removal of the restoration by tipping or lifting forces."

• Note: Although resistance form and retention form are separate entities, features that enhance one often enhance the other.

Principles of Retention Form:

1. Box form
   • The basic box shape provides inherent retention by limiting the paths of displacement
2. Convergence of walls toward the occlusal surface (small undercut)
   • Opposing external walls converge slightly occlusally (2-5 degrees taper)
   • This convergence mechanically locks the amalgam in the preparation
   • Prevents restoration from being lifted out
3. Retentive grooves
   • Placed in dentin at the axiomesial and axiodistal line angles
   • Cut with a small round bur or inverted cone bur
   • Especially important in deeper preparations and Class II preparations
   • Provide point-specific mechanical locking
4. Pins and slots
   • For extensive preparations where groove retention is insufficient
   • Threaded pins placed in dentin provide strong anchorage
   • Slots are shallow channels cut into the preparation floor or walls
5. Dovetail
   • Extension of the preparation into an adjacent area forming a dovetail shape
   • Prevents rotation/displacement of the restoration
   • Classic example: occlusal dovetail in a Class II preparation
6. Acid etching (for resin composite restorations)
   • Micromechanical retention from etched enamel/dentin surface
   • Not applicable for conventional amalgam

Features of Retention Form:

Key Distinction: Resistance vs. Retention Form

STEP 4: CONVENIENCE FORM

Definition (Sturdevant):

"Convenience form is that shape or form of the preparation that provides for adequate observation, accessibility, and ease of operation in preparing and restoring the tooth."

Principles of Convenience Form:

1. Adequate access and visibility to the preparation for the operator
2. Sufficient instrumentation access for burs, condensers, carvers
3. Extension of walls (distal, mesial, facial, or lingual) when needed to access deeper portions
4. Should not compromise resistance or retention form unnecessarily
5. Minimum sacrifice of sound tooth structure to gain access

Features / Examples of Convenience Form:

1. Widening the isthmus slightly when a narrow preparation prevents condensation of amalgam
2. Extending a wall mesially or distally to expose a carious area that cannot otherwise be seen or reached
3. Occlusal flaring of proximal box walls to allow matrix placement and condensation
4. Beveling of the gingival floor (gingival bevel) in proximal boxes to allow better access and seating of matrix
5. Using a dovetail on the occlusal surface of a Class II preparation - provides both retention AND convenience for accessing the proximal box
6. Ensuring the preparation opening is wide enough for amalgam carrier tip access

COMPLETE OVERVIEW TABLE