Cavity Preparation Class I Quiz

Class I Cavity Preparation for Amalgam

• Class I cavities are pit and fissure type cavities
• They involve occlusal surfaces of molars and premolars
• They also involve the occlusal two-thirds of buccal and lingual surfaces of molars
• Palatal pits in maxillary anterior teeth are also involved in Class I cavities
• Pits and fissures are susceptible to caries due to imperfect enamel lobe coalescence
• These areas are retentive for food, promoting caries production
• Class I cavities lesions start small and may remain unnoticed until substantial size
• The lesions spread conically in both enamel and dentin, enamel spreading towards the surface and dentin towards the pulp
• Class I cavities can be simple occlusal, compound (two surfaces), or complex (more than two). Specific types include occluso-buccal, occluso-lingual / occluso-palatal, and occluso-bucco-lingual.

Clinical Procedures

• Cavity preparation for amalgam
• Application of sealer (varnish)
• Matrixing (when necessary), vital in compound and complex cavities
• Amalgam manipulation involving: selection and proportioning of alloy and mercury, trituration, condensation, carving, and finishing and polishing.

Cavity Preparation Steps

• Outline form and initial depth
• Resistance and retention forms
• Convenience form
• Decay removal
• External cavity wall finishing
• Cavity toilet

Class I Simple Occlusal Cavity Outline Form

• The outline form of Class I cavities should be conservative, involving cleansable areas
• The outline is governed by the extent of caries in enamel and dentin, and the necessary extension to eliminate pits and fissures
• It varies according to the number of cusps and orientation of pits and fissures.

Steps of Operative Procedure

• Begin Class I occlusal preparation by entering the deepest or most carious pit with a No. 1/2 carbide bur at high speed with air-water spray.
• Extend the cavity with a suitable size fissure bur (or alternatively, No. 245 bur).
• Establish the proper depth of 1.5 mm (half the bur length). The initial pulpally depth is 0.2mm inside the DEJ or 1.5 mm from the central groove.
• A No. 245 bur has a 3mm head length and 0.8mm tip diameter. The sides converge toward the bur shank, which creates occlusal convergence of facial and lingual walls for adequate retention.
• Rounded internal line angles, created by the bur, make the tooth resistant to fracture from occlusal force.
• Necessary cavity extensions through pits, fissures, and developmental grooves are made with bur at a right angle to the tooth surface and a parallel long axis to the tooth crown.

Designing the Outline Form

• Include only faulty, defective occlusal pits, fissures, carious, and undermined enamel.
• Extend around cusps to maintain tooth structure and prevent pulpal horns from being approached too closely.
• Keep a minimum width in sweeping curves to prevent fracture.
• Cavity width is kept 1/4 - 1/3 intercuspal distance terminating on smooth surfaces to maintain strength and function of cusps but wide enough for condensation.

Setting Mesial and Distal Margins

• Positioned midway between proximal fossae and crest of proximal ridges.
• Placed parallel to these ridges.
• Mesial/distal walls are slanted or slightly diverging from the pulpal floor outward to avoid marginal ridge undermining.
• Preserves marginal ridge strength.
• Groove extensions kept narrow (mesio-distally) where possible to preserve cusp strength (but at least as wide as the narrowest condenser).
• Eliminate any weak enamel walls that are less than 0.5mm apart.

Extending Outline Form

• Include enamel undermined due to caries.
• Use enameloplasty on shallow fissures to conserve tooth structure.
• Establish an optimal conservative depth in pulpal wall.

Resistance and Retention Forms

• Resistance achieved by boxing the preparation.
• Relatively flat pulpal floor in sound tooth structure that resists forces and provides stable seat for the restoration.

Minimal Extension

• Minimal external wall extension.
• Strong ideal enamel margins.
• Adequate depth to ensure restoration thickness.
• Slight occlusal convergence of opposing external walls provides primary retention.

Convenience Form

• Establish depth and width for restoration.
• Although conserving tooth structure is crucial, convenience form may need extensive preparation to allow better access and visibility.

Removal of Carious Dentin

• In small cavities, caries removal is during cavity extension.
• In moderate/deep cavities, caries removal using large spoon excavators or round burs.
• Resistance/retention form may improve with flat floor peripheral to the excavated area.

Pulp Protection

• No liner/base in shallow cavities.
• In deeper cavities (0.5- 1.0mm remaining dentin), apply a thin layer of light-cured RMGI (resin-modified glass-ionomer) base.
• RMGI base insulates the pulp, bonds to dentin, releases fluoride, is strong, and resists condensation forces.

Enamel Wall Planning

• Finish enamel walls free of loose/undermined enamel.
• Trim walls to meet tooth surface at a right cavo-surface angle.
• Use sharp chisels, hatchets, fissure burs or stones for cleaning.
• Round all sharp corners in enamel, as they can contain short enamel rods.

Cavo-Surface Margins

• Well-defined and well-supported
• Easier for visualization and carving, with marginal integrity
• Important to achieve approximate 90-100 degree cavo surface angle for amalgam at the margin (approximately 80-90 degrees).
• A butt-joint marginal area allows strength for both.

Debridement and Inspection

• A sharp explorer to check cavity details/loosen any debris
• Flush cavity with water, then dry with air
• Facilitates amalgam adaptation, optimizes restoration physical properties by eliminating voids.

Buccal/Palatal Pits Preparation

• Shape varies from oval-round to oblong based on extent of caries
• Walls consistently joined and are slightly convergent towards cavosurface margin
• Axial wall follows buccal/lingual surface contour

Additional Considerations

• No. 245, 330 or 169 bur can be used for preparation, dependent on lesion size
• Retention grooves added as needed using No. 1/4 or 33 1/2 burs, per caries extent and wall angulation.

Compound Class I Cavities

• Occluso-buccal/occluso-lingual cavity extensions through fissures towards respective surfaces
• Dentin cutting at dentino-enamel junction using No.56 bur until the occlusal ridge is undermined and removed.
• If caries is still gingival to the level of the pulpal seat, a step is indicated.
• A #56 bur is used to cut dentin at the dentino-enamel junction with pressure & mesio-distal movement for gingival direction.

Axial Wall

• Follows external tooth surface contour
• 0.5mm depth inside the DEJ is ideal for retentive locks, otherwise 0.2mm is permissible

Resistance & Retention Form

• Round axio-pulpal line angle to prevent stress concentration, provide bulk, and maintain thickness
• Axial wall direction is parallel to corresponding exterior surface. This prevents pulp exposure and creates uniform thickness
• Retention grooves may be cut as needed in dentin, along axio-mesial or axio-distal line angles.

Buccal Fissure Extension

• Facial occlusal fissure is continuous with facial surface fissure.

• Extend through facial ridge onto facial surface. This procedure creates a tooth preparation appearance that has undergone extension through the ridge.

• Facial surface portion of extension is cut with bur's side, to create sharper line angles (to improve appearance) via bur from the facial aspect and, another sharp angle through occlusal direction use No. 169 bur.

• Further retention form ensured by preparing locks with No. 1/4 round bur and final tooth preparation.