PRINCIPLES OF TOOTH PREPARATION (LECTURE-1)

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PRINCIPLES OF TOOTH PREPARATION(LECTURE-1)
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• CONTENTS-
• Tooth preparation is governed by the principles
  of tooth preparation-
• 1) Preservation of tooth structure
• 2) Retention Resistance
• 3) Structural durability
• 4) Marginal integrity
• 5) Preservation of the periodontium
  (According to Shilinburg)
• Definition The process of removal of diseased
  and/or healthy enamel and dentin and cementum to
  shape a tooth to receive a restoration

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1. Preservation of Tooth Structure

• While replacing a lost tooth structure, a
  restoration must preserve the remaining tooth
  structure as much as possible.
• Tooth structure is conserved by using the
  following guidelines-
• 1. Use of partial coverage rather than complet
  coverage restorations.
• 2. Preparation of teeth with minimum practical
  convergence angle
• (taper) between axial walls.
• 3.  Preparation of the occlusal labial surface
  so reduction follows the anatomic
• planes to give uniform thickness in the
  restoration.

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4. Preparation of the axial surfaces so that the
tooth structure is removed evenly if
necessary, teeth should be orthodontically
repositioned. 5.  Selection of a
conservative margin compatible with the other
principles of tooth preparation. 6.
 Avoidance of unnecessary apical extension of the
preparation.
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2. Retention Resistance RETENTION-
Resistance to removal of restoration in the path
of insertion.RESISTANCE-Prevention of
dislodgement of a restoration from apical,
oblique and horizontal forces.

• Factors for Retention-
• Degree of taper
• Freedom of displacement
• Length of preparation
• Substitution of internal features

• Factors for Resistance-
• Degree of taper
• Freedom of displacement
• Length width of preparation
• Leverage action from the oblique forces.
• Rotation around the vertical axis

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• TAPER-
• The axial walls of the preparation must taper
  slightly to permit the restoration to seat.i.e. 2
  opposing external walls must gradually converge
  ANGLE OF CONVERGENCE.
• 2 opposing internal surfaces of the tooth
  structure must diverge occlusally ANGLE OF
  DIVERGENCE.

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• The more nearly parallel the opposing walls of a
  preparation, the greater should be the retention.
• Most retentive preparation should be one with
  parallel walls, but the parallel walls are
  impossible to create in the mouth without
  producing preparation undercuts.
• An UNDERCUT - is defined as a divergence between
  opposing axial walls, or wall segments, in a
  cervical- occlusal direction.

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• FREEDOM OF DISPLACEMENT-
• Retention is improved by geometrically limiting
  the numbers of paths along which a restoration
  can be removed from the tooth preparation.
• Maximum retention is achieved when there is only
  one path.

Limiting the freedom of displacement from
torqueing or twisting forces in a horizontal
plane increases the retention of a restoration.
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• LENGTH-
• Occlusogingival length is an important factor in
  both retention resistance.
• Longer preparations will have more surface area
  therefore will be more retentive .
• The length must be great enough to interfere with
  the arc of casting pivoting about a point on the
  margin on the opposite side of the restoration.
  
  

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• SUBSTITUTION OF INTERNAL FEATURES-
• It may not be possible always to use opposing
  walls for retention.
• Therefore, internal features such as the groove,
  the box form, the pin hole can be substituted
  for an axial wall or for each other.

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• PATH OF INSERTION-
• It is an imaginary line along which the
  restoration will be placed onto or removed from
  the preparation.
• It is of special importance when preparing teeth
  to be fixed partial denture abutments, since the
  paths of all the abutment preparations must
  parallel each other.
• Surveying visually- it is the primary means of
  insuring that the preparation is neither undercut
  nor over-tapered.

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Leverage Action From The Oblique Forces
Rotation around the vertical axis
Width of the preparation
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• REFERENCES
• Fundamentals of Fixed Prosthodontics, Herbert
  T. Shillingburg
• Contemporary Fixed Prosthodontics ,Stephen
  F.Rosensteil