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Class II Amalgam Preparation

#29 DO

Overview

Outline Form

extent of preparation on tooth surface 

*amalgam preparations are typically larger than comparable composite preparations due to material requirements

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Resistance Form

internal form of tooth preparation that allows restoration to resist forces of mastication

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Retention Form

 form, shape, and configuration of the tooth preparation that resists the displacement or removal of restoration from the preparation under lifting and tipping masticatory forces

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Convenience Form

form of tooth preparation  that allows for physical and visual accessibility to preparation 

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Armamentarium

Instruments and Materials

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Video Demonstration

 

Procedure

Optional: Draw the outline of the cavity preparation using the red-blue pencil

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Envision the cavity preparation outline and extent:  

  • Clinical note: The final preparation dimensions are determined by the extent of the carious lesion or existing, faulty restoration

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Obtain proper isolation of the tooth (e.g. rubber dam, isovac/isolite, etc.)

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Place a wedge in between adjacent teeth to separate the contact areas 

  • Optional: Place a matrix band or metallic fender wedge interproximally to protect the adjacent tooth from iatrogenic damage

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Orient the bur over the mesial pit, perpendicular to the occlusal surface 

 

Bur options: pear shaped carbide bur (330)

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Maintaining the 1-1.5mm bur depth, extend the occlusal outline proximally 

  • Always keep the bur perpendicular to the long axis of the tooth

  • Follow the anatomical grooves of the tooth​

  • Leave a small "enamel shell" interproximally to protect the adjacent tooth from iatrogenic damage

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Penetrate the mesial pit to a depth of 1-1.5mm for the initial punch-cut

  • Optional: The length of the bur may be used to approximate preparation depth 

  • Clinical note: The final preparation depth is determined by the extent of the carious lesion or existing, faulty restoration

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Use the probe to ensure appropriate dimensions of occlusal extension and evenness of the pulpal floor: 

  • BL width of occlusal outline: at least 1mm 

  • BL width of isthmus: 1-1.5mm  

  • ​BL dimension must accommodate the smallest condenser

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Extend the occlusal outline buccally on mesial pit to create the proximal dovetail

  • Follow the anatomy of the grooves

Bur options: pear shaped carbide burs (330, 245)

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Place the bur in the most mesial portion of the preparation outline, orient the bur 5º mesially, and brush upward to create occlusal divergence at the proximal wall of the dovetail

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Place the bur within occlusal preparation outline, adjacent to enamel shell and parallel to long axis of tooth

 

Bur options: pear shaped carbide burs (330, 245)

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To create the proximal box, extend the preparation gingivally until the gingival contact breaks 

  • The gingival contact is broken <0.5mm

  • When the gingival contact is broken, the tip of the explorer will pass through the broken contact 

 

Drop the box = creating the proximal box!

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Widen the proximal box to the full length of the contact area by moving bur in the buccal-lingual direction, maintaining the bur depth at the level of the broken gingival contact

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Break the enamel shell using a hand instrument 

  • Position the flat side of the instrument against the enamel shell and push the instrument outward toward the adjacent tooth

 

Hand instrument option: enamel hatchet

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Break the buccal and lingual contacts using rotary instruments (high speed with water) or hand instruments

 

Bur options: pear shaped carbide burs (330, 245)

Hand instrument options: enamel hatchet

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Shape proximal box using rotary instruments (high speed with water)

  • The exit angles are 90º degrees to the tooth surface, following the direction of the enamel rods

  • The buccal and lingual walls are occlusally convergent 

  • The axial wall is occlusally divergent 

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Optional: Remove the wedge to check that the buccal, lingual, and gingival contacts are broken < 0.5mm. Replace the wedge after checking

  • The tip of the explorer should pass through buccal, lingual, gingival contacts

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Create the reverse S curve by connecting and smoothing the transition between cavosurface outline and proximal box using rotary instruments (high speed with water)

Bur options: pear shaped carbide burs (330, 245)

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Clinical note: Extend the ideal preparation to remove remaining caries

  • Check for caries using the explorer or spoon excavator

  • Remove remaining caries using the largest round bur that fits into the preparation with rotary instruments (slow speed) or the spoon excavator

 

Bur options: round carbide burs (4, 6)

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Smooth the preparation with rotary instruments (high speed with water or slow speed) or hand instruments 

  • Remove unsupported enamel

  • Remove steps/ledges 

  • Bevel the axiopulpal line angle

 

Bur options: straight fissure or pear shaped carbide burs (556, 330) 

Hand instrument options:  enamel hatchet, enamel hoe, gingival margin trimmer 

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Clean and dry the preparation using the air-water syringe 

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​​Examine the preparation and evaluate for proper dimensions

  • Use the explorer to check for smoothness 

  • Use the probe to check for appropriate depth and width

  • Clinical note: The final dimensions of the preparation are determined by the extent of the carious lesion or existing, faulty restoration

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  • Smooth preparation

  • BL walls occlusally convergent  

  • Reverse S curve present 

  • Dovetails present

  • MD and axial walls occlusally divergent  

  • Pulpal floor: 1.5mm-2mm depth

  • BL width: at least 1mm

  • Proximal, buccal, and lingual contacts: broken by at least 0.5mm

  • Proximal box BL width: length of the contact area

  • Proximal box MD width: approximately 2mm

  • At least 90 degree exit angles

  • Gingival floor depth: at least 3mm with broken gingival contact

Checklist

 

*Hover for summary details

 

Common Mistakes

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Occlusal Convergence

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Proximal Extension

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Proximal Box Flare

References

Hilton, T. J., Ferracane, J. L., &amp; Broome, J. C. (2013). Summitt's fundamentals of operative dentistry a contemporary approach. Quintessence Publishing Co. Inc. 
Ritter, A. V., Boushell, L. W., Walter, R., &amp; Sturdevant, C. M. (2019). Sturdevant's art and Science of Operative Dentistry. Elsevier.