PRINCIPLE OF CAVITY PREPRATION 2 l Preclinical Operative MCQs

 

PRINCIPLE OF CAVITY PREPRATION 2 l Operative MCQs (multiple choice question) for dental students

Cavity Preparation for Amalgam Restoration Class 1

 

Definition:

•  Pit and fissure cavities that involves:

1. Occlusal surface of molars and premolars.
2. Occlusal 2/3 of buccal and lingual surface of molars.
3. Palatal pits in maxillary anterior teeth.

 

1st: Outline form :

• There are 3 important guidelines for extension for prevention):

1.    Carious tooth structure should be eliminated
2.    Margins should be placed on sound tooth structure (less caries susceptibility). Any enamel that has been undermined  by the removal of carious dentin should be.
3.    all continuous non-coalesced pits and fissures must be eliminated.

 

External:

• Sweeping curves.
• In upper premolar, buccal wall (convex curved), lingual (Semi- straight).
• Buccal pit cavity in lower molars - triangular.
• Buccal pit cavity in upper molars- round.
• Lingual pit in upper anterior - oval

 

Internal:

• lingual and buccal wall should be parallel to the respective tooth surface or converge occlusally).
• Mesial and distal walls should be diverging from pulpal floor to prevent undermining the marginal ridges.
• Pulpal floor should be perpendicular to long axis of the tooth except in lower first premolar, and parallel to occlusal  plane in order to:

1.      To bring force perpendicular to floor.
2.      Susceptibility to pulp exposure.

• Preparation should be widened only to obtain enamel margins supported by sound dentin.
• No sharp angle (although that, sound tooth structure should not be removed so easily for that).
• Dentine ledge is an option to prevent rotation.

 

2nd: Resistance form:

• Cavosurface angle should be 90° (To have sufficient bulk to prevent fracture of amalgam).

• Notes: If:

1. Angel >( 90° -110°) amalgam fracture or restoration fracture.
2. Angel <( 90°-110°) enamel fracture or tooth fracture.
3. Both will form Ditch: v-shaped space in the margin of restoration due to fracture of the undermined enamel of amalgam margin.

• Pulpal floor parallel to occlusal plane.

• Dentin walls at right angel to pulpal floor.

• Line angels and point angels should be slightly rounded to prevent stress concentration of forces at sharp area to  prevent fracture of tooth.

• Mesial and distal wall are diverge occlusally following enamel rods.

• facio-lingual width is one third the distance between the tips of cusps, unless they are unable to withstand the forces  it should be included.

• Depth should bulky as 1.5 mm or 0.5 mm apical to DEJ to resist fracture during function.

• If there is direct occlusal contact between the opposing tooth and the weakened marginal ridge, the marginal ridge should be removed and restored with amalgam.

• Extending around the cusps to conserve tooth structure and prevent the internal line angles from approaching the  pulp horns too closely.

• Keeping the facial and lingual margin extension minimal as possible between the central groove the cusp tips.

• Minimally extending into the marginal ridges (only enough to include the defect) without removing dentinal  support.

• Eliminating a weak wall of enamel by joining outlines that come close together (i.e., less than 0.5 mm apart).

 

3rd: Retention form:

• If the cavity provides effective retention; no special retentive features are required.
• Pulpal floor shouldn’t be rounded to prevent movement of restoration and leads to tooth structure fracture.
• Convergence occlusally from buccal and lingual walls in order to:

1. Parallel to enamel rods.
2. 90 degree of cavosurface angel to prevent undermining of enamel rods.
3.  Parallel to corresponding surface.

• Prevent undermining of marginal ridges.

4th: planning of enamel walls :

•  Enamel walls finished from any short or undermined enamel.
•  Enamel walls meet tooth surface at right cavosurface angel.
•  All sharp corners in enamel must be rounded.

 

Buccal, lingual, and palatal cavities:

• A triangle with its base forming gingival wall and its side forming the mesial and distal walls.
• The gingival wall is placed at or slightly occlusal to the height of contour of the teeth.
• All walls extended just to eliminate defective enamel and dentin.
• Enamel walls are placed in the direction of enamel rods perpendicular to axial wall.
• In buccal cavities - triangular in shape.
• In palatal cavities- oval or rounded in shape.

 

Buccal, lingual, and palatal extension:

• Mesial and distal walls of buccal extension should be parallel to each other, parallel to long axis, perpendicular to  gingival wall, perpendicular to buccal surface. Gingival wall parallel to pulpal floor parallel to occlusal plane.

• Gingival wall and pulpal floor perpendicular to long axis.