Biomechanics of tooth preparation/ orthodontics website

BIOMECHANICS OF TOOTH
PREPARATION
INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com

The design of a preparation for a cast
restoration & the execution of the design are
governed by five principles:-
1) Preservation of tooth structure
2) Retention & Resistance
3) Structural durability
4) Marginal integrity
5) Preservation of the periodontium

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 complete
coverage restorations.
2. Preparation of teeth with minimum practical
- convergence angle (taper) between axial walls.
-

3. Preparation of the occlusal surface so reduction
follows the anatomic planes to give uniform
thickness in the restoration.

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.

2. Retention & Resistance
For a restoration to accomplish its purpose, it must
stay in place on the tooth.
RETENTION:- prevents the removal of the
restoration along the path of insertion or long axis
of the tooth preparation.
RESISTANCE:- prevents dislodgment of the
restoration by forces directed in an apical or oblique
direction & prevents any movement of the
restoration under occlusal forces.

Essential elements of retention:-
2 opposing vertical surfaces in same preparation.
1) External surfaces:- eg. Buccal & lingual walls of
full veneer crown.
An extracoronal restoration is an example of
veneer, or sleeve retention.
Scan0001.jpg

2) Internal surfaces:- eg. Buccal & lingual walls of the
proximal box of a proximo-occlusal inlay.
An intracoronal restoration resists displacement by
wedge retention.

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.
The relationship of one wall of a preparation to the
long axis of that preparation is the INCLINATION
of that wall.

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.

Tooth preparation taper should be kept minimal
because of its adverse effects on retention.
ed taper ed retention
ed taper ed retention.
Greater the surface area of a preparation, greater
is its retention.

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 torquing
or twisting forces in a horizontal plane increases
the resistance of a restoration.

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.

SUBSTITUTION OF INTERNAL FEATURES:-
The basic unit of retention for a cemented
restoration is two opposing axial walls with a
minimal taper.
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.

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, since it is the primary
means of insuring that the preparation is neither
undercut nor over-tapered.

The path of insertion must be considered in two
dimensions:-
- Faciolingually
- Mesiodistally
The faciolingual orientation of the path can affect
the esthetics of metal-ceramic or partial veneer
crowns.

The mesiodistal inclination of the path must parallel
the contact areas of adjacent teeth.
So if path is inclined mesially or distally, the
restoration will be held up at the proximal contact
areas & be “locked out”.

3. STRUCTURAL
DURABILITY
A restoration must contain a bulk of material
that is adequate to withstand the forces of
occlusion.
The bulk must be confined to the space
created by the tooth preparation.

OCCLUSAL REDUCTION:-
An important feature for providing adequate bulk of
metal & strength to the restoration is occlusal
clearance.
Gold alloys – 1.5mm (FC) & 1mm(NFC)
Metal-ceramic crowns- 1.5-2mm(FC)&1-1.5mm(NFC)
All-ceramic crowns – 2mm of clearance on preparation

The occlusal reduction should reproduce the basic
inclined plane pattern of the occlusal surface
without overshortening the preparation rather than
being cut as one flat plane.

FUNCTIONAL CUSP BEVEL:-
It is an integral part of the occlusal reduction.
A wide bevel on the lingual inclines of the maxillary
lingual cusps & the buccal inclines of the
mandibular buccal cusps provides space for an
adequate bulk of metal in an area of heavy occusal
contact.

If a wide bevel is not placed on the functional cusp,
several problems may occur :
- If the crown is waxed & cast to normal contour it can
cause a thin area or perforation in the casting.
- To prevent this the crown may be waxed to optimal
thickness resulting in overcontouring & poor occlusion.

- If an attempt is made to obtain space for an
adequate bulk in a normally contoured casting
without a bevel, it will result in over inclination of the
buccal surface which will destroy excessive tooth
structure while lessening retention.

AXIAL REDUCTION:-
It plays an important role in securing space for an
adequate thickness of restorative material.
Inadequate axial reduction can cause thin walls & a
weak restoration subjected to distortion or
a bulbous, overcontoured restoration which will
strengthen the restoration but may have a
disastrous effect on periodontium.

Other features that will provide space for metal that
will improve the rigidity & durability of the
restoration are:
The offset, the occlusal shoulder, the isthmus,
the proximal groove & the box.

4. MARGINAL INTEGRITY
The restoration can survive in the biological
environment of the oral cavity only if the margins
are closely adapted to the cavosurface finish line of
the preparation.

MARGIN PLACEMENT:-
Whenever possible the margin of the preparation
should be supragingival.
Advantages of supragingival margins are:-
-   T                       1) They can be easily finished.
-   2) They are more easily kept clean
-   3) Easy to make impressions with less damage to
soft tissue.
R 4) Restorations –easily evaluated –at recall.

Subgingival margins of cemented restorations are a
major factor in periodontal disease.
A subgingival margin is justified if any of the following
pertains:-
1)Dental caries, cervical erosion extend subgingivally.
2)Proximal contact area extends to the gingival crest
3)Additional retention is needed
4)Margin of a metal-ceramic crown is to be hidden
behind the labiogingival crest.
5)Root sensitivity not controlled by more, conservative
means like the application of dentin bonding agents.
6)Modification of axial contour is indicated.

MARGIN ADAPTATION:-
The junction between a cemented restoration and
the tooth is always a potential site for recurrent
caries because of the dissolution of luting agent and
inherent roughness.
The more accurately the restoration is adapted to
the tooth, the lesser the chance of recurrent caries
or periodontal disease.

MARGIN GEOMETRY:-
For the evaluation of the margin the following
guidelines for margin design should be considered.
1)  Ease of preparation without overextension or
unsupported enamel.
    2)  Readily identifiable in impression & on die.
    3) A distinct boundary to which the wax pattern
can be finished.
    4) Provide sufficient bulk of material.
5) Conservation of tooth structure.

MARGIN DESIGNS:-

Feather Edge
Chisel Edge
Bevel
Chamfer
Shoulder
Sloped
Shoulder
Shoulder with
Advantages
Conservation of
tooth structure
Conservation
Removes unsupported enamel,
allows finishing of metal
Distinct margin, adequate bulk
easier to control.
Bulk of restorative Material
Bulk of material + advantage of
bevel
Bulk of material + Adv. Of bevel
Disadvantages
Does not provide
sufficient bulk
Location of margin
difficult to control
Extends preparation
into sulcus
Care is needed to
remove unsupported
lip of enamel
Less conservative
Less conservative
Less conservative
apically extends
Indications
Not recommended
Occasionally on tilted
tooth
Facial margin of
maxillary partial
coverage restoration
Cast metal
restoration
Facial margin of
metal ceramic
Facial margins of
metal ceramic
crowns
Facial margin of
posterior metal
ceramic crowns withwww.indiandentalacademy.com

5. Preservation of periodontium
The placement of finish lines has a direct bearing on
the ease of fabricating a restoration & on the ultimate
success of the restoration.
Margins that are smooth & fully exposed to cleansing
action give best results.
Finish lines should be placed in enamel when it is
possible to do so.
Subgingival restorations are described as a major
etiologic factor in periodontitis.
The deeper the restoration margin resides in the
gingival sulcus the greater the inflammatory response.

Thank you
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Biomechanics of tooth preparation/ orthodontics website

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