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PRINCIPLES OF TOOTH PREPARATION (LECTURE-1)

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Title: BIOMECHANICS OF TOOTH PREPARATION Author: ABC Last modified by: SMITA SINGH Created Date: 1/10/2005 6:32:58 PM Document presentation format – PowerPoint PPT presentation

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Title: PRINCIPLES OF TOOTH PREPARATION (LECTURE-1)


1
PRINCIPLES OF TOOTH PREPARATION(LECTURE-1)
2
  • 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

3
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.

4

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.
5
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

6
  • 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.

7
  • 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.

8
  • 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.
9
  • 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.


10
  • 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.

11
  • 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.

12
Leverage Action From The Oblique Forces
Rotation around the vertical axis
Width of the preparation
13
  • REFERENCES
  • Fundamentals of Fixed Prosthodontics, Herbert
    T. Shillingburg
  • Contemporary Fixed Prosthodontics ,Stephen
    F.Rosensteil
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