Principles of tooth preparation Flashcards
PRINCIPLES OF TOOTH PREPARATION
There are three main principles of tooth preparation:
1-Biological consideration
2-Mechanical consideration
3-Esthetic consideration
Biological considerations
two main aspects
1-Prevention of damage during tooth preparation:
This includes:
a-Adjacent teeth
B-Soft tissues
C-Pulp
2-Conservation of tooth structure
methods can be used to prevent damage to adjacent teeth
1-use metal matrix band around the adjacent tooth for protection
But this method is not the preferred method
2-the preferred method is the use of proximal enamel of the tooth being prepared
for protection of the adjacent proximal surface
factors can influence heat production these include:
1-coolants
2-cutting instruments
3-mode of cutting
Guide lines for tooth structure conservation
1-The best method to conserve the tooth structure is not to touch the tooth at all
(dental implants)
2-Use adhesive rather than conventional restoration
3-Use partial coverage rather than complete coverage restoration
4-preparation of the teeth with minimal axial taper
5-Reduction of occlusal surface should follow anatomic planes
7-selection of the most conservative margin design compatible with the type of
Restoration
-deep chamfer finish line (rounded shoulder) is the most conservative finish
than shoulder finish line.
Marginal integrity (finish line) is
Ending of tooth preparation should terminate at definite position and configuration
List requirement of finish lines
1-It should be easily prepared
2-easily identified in both impression and die
3-facilitates distinct boundary to which the wax pattern
4-should provide sufficient bulk of material
5-should conserve tooth structure.
Location of cervical margins
should be placed, whenever possible, supragingival
Advantages of supragingival margins
1-Easily prepared with out trauma to soft tissues
2-Facilitates their reproduction in impression
3-Easily finished
4-Easily cleaned by patient
5-Restoration can be easily evaluated at recall visit.
Situations justifying subgingival margins
1-Dental caries cervically
2-For esthetic reasons on the labial surfaces of anterior teeth.
3- insufficient crown length for retention
4- proximal contact area extend to gingival crest
5- when modification of axial contour is indicated.
Methods of checking margins of cast restorations include:
1-Direct vision (only for supra-gingival margins)
2-Sharp dental explorer
3-Dental floss
4-Bitewing radiographs to evaluate the proximal mesial and distal margins.
Types of finish line
A-Featheredge margins, it is more conservative,
B-Chisel (knife edge) , It used only in the lingual surface of lingually tilted mandibular teeth
C-Bevel finish line
Indicated for cast gold restoration like inlay-onlay and in full cast gold crowns
D-Chamfer finish line , Indicated for cast metal crown margins and lingual margin of metal-ceramic crown, -Instruments used are tapered diamond with a round tip
-The chamfer should never be prepared wider than half the tip of the diamond bur
E-Shoulder finish line , This type of finish line is usually recommended for the all-ceramic
crowns and the Facial margin of metal-ceramic crown.
F-Rounded shoulder , It is a modification form of shoulder finish line characterized by a rounded internal
Angle, it is indicated for the all ceramic crown.
G-Sloped shoulder , characterized by 120-degree
It is advantage reduces the possibility of leaving unsupported enamel
-it is indicated in facial margins of metal-ceramic crowns
H-Shoulder with bevel , this type is often recommended for the facial surface of posterior metal ceramic
restoration with supragingival margins
Disadvantage of Shoulder finish line
it is not Conservative.
-the 90-degree internal line angle would concentrate stress in the tooth and
Increase its liability to fracture.
-instruments used are tapered diamond with flat end
Mechanical considerations divided into:
1-Retention form
2-Resistance form
3-Structural durability.
Retention form:
the quality of preparation that prevents the removal of the
restoration along its path of insertion.
Checking the path of insertion is by ..
closing one eye and view the preparation with one eye from
distance 30cm
Several factors could affect the path of insertion
1-reduction of axial surface
2-proximal grooves
3-pinholes
4-proximal surface of adjacent teeth.
Factors affecting the retention form
1-Magnitude of dislodging forces
2-Geometry of prepared tooth
*Taper
*surface area
*Preparation features
3-Roughness of fitting surface of restoration
4-Materials being cemented
5-Type of luting cement
magnitude of dislodgment force is dependent on …
the stickness of food
For example chewing of a very sticky food
Taper is
maximum retention is obtained if the opposing walls of the
preparation are parallel
recommended convergence between opposing walls is 6 degree.
Undercut is
An undercut:
is termed as reverse taper.
surface area of the preparation is influenced by:
1-The length of the tooth
2-The size of the tooth
3-The extent of coverage
4-Preparation features.
Tipping path is
the path along which the restoration could be displaced under the displacing
Occlusal forces.
Factors affecting the resistance of restoration of displacement
1-Magnitude and direction of dislodging force
2-Geometry of the tooth preparation
-Taper
-Length
-Preparation features
3-Phsical properties of the luting cement
