Retention in fpd /certified fixed orthodontic courses by Indian dental academy

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Retention in fixed partial denture

Biomechanical principles of
tooth preparation
B OGICAL
IOL

M CH
E ANICAL

E HT
ST E IC


Biomechanical principles of
tooth preparation
 Preservation of tooth structure.
 Retention and resistance form.
 Structural durability.
 Marginal integrity.
 Preservation of periodontium.


Introduction
 Restoration should remain firmly attached

to tooth structure to meet functional
biological and esthetic requirements.


Definition (GPT)
 That quality inherent in the prosthesis

acting to resist the forces of dislodgement
along the path of placement


Factors influencing retention
 Magnitude of dislodging forces.
 Geometry of tooth preparation .
 Roughness of the fitting surface of the

restoration.
 Materials being cemented.
 Thickness of luting agent.


Magnitude of dislodging
forces.
 Depends on the stickiness of the food and

the surface area and texture of the
restoration.


Geometry of tooth preparation
 Taper
 Surface area
 Stress concentration.
 Type of preparation.


Geometry of tooth preparation
 Most fixed prosthesis depends on the

geometric form of the preparation rather
than on adhesion.
 As most cements are non adhesive( act by
increasing the frictional resistance between
tooth and restoration.
 Cylindrical tooth preparation.

Preparation is cylindrical if two horizontal
sections of prepared tooth surface is coincident

 Fig 7-27 and 7-28®


A crown depends
on external
retention to
resist removal

An inlay depends
on internal
retention to hold
it within
preparation

Taper
 Axial walls of the preparation

must taper slightly to permit the
restoration to seat.
 Inclination of the wall.
 Angle of convergence
 Angle of divergence.
 Two opposing surfaces with 3°

taper would give a taper of 6 °.
 Maximum retention is obtained if
a tooth preparation has parallel
walls .

 Parallel walls are impossible to create in the

mouth.
 Preparation walls are tapered
• To visualize preparation walls

• Prevent undercuts.
• Compensate for the inaccuracies in the
fabrication process
• More nearly complete seating of the restoration
during cementation.

Ward was one of the first to recommend taper
of 3 to 12 °respectively.

Relationship between the degree of axial wall
taper and magnitude of retention was first
demonstrated by Jorgensen in 1955


 To minimize stress in the cement interface

between the preparation and restoration a
taper of 2.5 ° to 6.5 ° is optimum.
 Slight increase in the stress as the taper

increased from 0-15 °

 At 20 ° stress concentration was found to

increase sharply.

Some of the tapers suggested by authors

 Ohm and silness

on vital teeth
19.2° MD and 23 FL

on non vital teeth
12.8 ° MD and 22.5 ° FL

 Mack 16.5 °
 Weed worked on dental students preparation

12.7 ° on typhodonts and 22.5 ° in clinical preparations.

 Kent and his associates

mean of 15.8 ° between mesial and distal walls
13.4 ° facial and lingual walls.
Overall mean of 14.3 °

 Recommended convergence angle

between the opposing walls is 6 degree
which has been shown to optimize the
retention for zinc phosphate cement.
 Guyer describes optimal taper to be in
range of 2-6.5 °.


OPTIMUM DEGREE OF TOOTH
PREPARATION TAPER
ARCH

M/D

F/L

Overall

Anterior tooth

10

10

10

Premolar

14

14

14

Molar

17

21

19

Maxillary

Isthmus

7

Box

7

Mandibular
Anterior tooth

10

10

10

Premolar

16

12

14

Molar

24

20

22

Isthmus
Box

12

12

Too small a taper will lead to unwanted

undercuts.
Too large a taper will not be retentive.


Length and surface area
 Occlusogingival length is an important

factor in retention and resistance. Longer
preparation will have more surface area
 Length must be great enough to interfere
with the arc of casting pivoting about a
point on the opposite side of restoration.


 Walls of shorter preparation should have

as shorter taper as possible.


 Greater the area of contact of the cement film

bound to the preparation and to the internal detail
of the casting the greater the retention of the
casting.
 Greater the surface area greater the retention.
 Total surface area is influenced by the
Size of the tooth
Extent of coverage of the restoration
Features such as grooves, boxes pin holes that
are placed in the restoration


Doubling the height
of the preparation
would nearly
double the area of
its axial walls

Doubling the diameter
of the preparation
would doubles the
area of its axial walls
and quadraples the
area of occlusal
surface.

 Basic unit of retention for a cemented

restoration is two opposing walls with
a minimal taper.
 It may not always be possible to use
opposing walls for retention.
 It may be desirable to leave a surface
uncovered for a partial veneer.
 Features like boxes grooves and
pinholes can be incorporated to
enhance the retention

 Kent etal reported difference between

degree of taper of full crown preparation
18.4 – 22.2 degree and boxes and grooves
in axial surface 7.3degreee
 Taper of these features nearly same as the
taper of the instruments used to cut them.


Freedom of displacement
 Retention is improved by geometrically

limiting the number of paths along which a
restoration can be removed from the tooth
preparation.
 Maximum retention can be achieved by
only one path ..


 Full veneer crowns with long parallel axial

walls and grooves will produce such
retention.
 Short over tapered preparation would be
without retention because the restoration
can be removed along an infinite number of
paths


Stress concentration
 Stresses not uniform throughout the cement.
 Concentrated around the junction of

occlusal and axial walls.
 Changes in the geometry of the tooth
preparation (rounding the internal line
angles) may reduce stress concentrations
and thus increase the retention.

Type of preparation
 Different types of preparation have different

retentive values.
 Different types of preparation include
Type of coverage of the restoration.
Inclusion of internal features.


Retention of different preparation designs

Roughness of the surfaces
being cemented.
 If the internal surface of the restorations

smooth failure occurs through the cement
restoration interface
 Worley and Smith in their study conclude
that the retention of the restoration can be
increased by roughening or grooving the
restoration.


 Casting must be prepared by the air abrading the







fitting surface by 50 micro m alumina.
It has shown to increase the retention by 64 %
Deliberately roughening of the preparation hardly
influences the retention and it is not recommended
as this adds to difficulty of impression making and
waxing.
Oilo and Jogerson found retention of castings on
cemented with zinc phosphate cement on test dies
with 10 degree taper to be twice as great on
preparations with 40 micro m scratches as on those
with 10 micro m scratches.
Smith found no significant difference in castings
cemented on 14 degree taper preparations whose
roughness varied by a factor of 24 from smoothest
to roughest.

Materials being cemented.
 More reactive the alloy more adhesion is with the luting

agent.
 Base metal alloys are better retained than less reactive high
gold content metals.
Effect of adhesion on different core materials.
 Chan kc etal in his study found that cement adhered better
to amalgam than to composite resin or cast gold
 De wals JP etal found that higher value of retention was
found with composite rather than amalgam cores. It may
be due to dimensional changes of the core materials
although the clinical implications are not clear.

Type of luting agent
 Decision regarding which agent to use

depends on other factors.


Film thickness of luting agent
 Conflicting evidence about the effect of the

increased thickness of the cement film on
retention of the restoration.
 It may be important if a slightly oversized
casting is made.


Loss of retention
 Loss of retention should be detected early,

otherwise extensive caries develops.

Detection of loss of retention
 Patients awareness to looseness or sensitivity to

temperature or sweets.
 Periodical examination which includes attempt to
seat prosthesis by lifting retainers up and down
while they are held between the hands and a
curved explorer placed under the connector.

 If the casting is loose the occlusal motion draws

the fluids to be drawn under the casting and when
the casting is reseated with the cervical force the
fluid is expressed generally producing bubbles as
the air and liquid are simultaneously displaced.
 When the retainer becomes loose the retainer
should be removed so that the abutment teeth can
be evaluated.
 If the restoration can be dislodged from the other
prepared teeth without damage and no caries
present it is possible to recement the restoration.

 If the prosthesis removal reveals lack of adequate

retention as evidenced by the preparation form the
teeth should be modified to improve their
retention and resistance.
 Some fixed partial dentures may become loose
even when maximal retentive preparations have
been developed . It may be caused by excessive
span length or heavy occlusal forces.


Summary
Greater retention
Taper

Parallel

Surface area

Large

Type of
preparation

Molar complete
crown

Surface
texture

Rough

Film
thickness

Effect uncertain

Luting agent

Adhesive resin

Lesser retention
6 degree

Excessive
Small

Premolar
complete crown
Partial crown

Intracoronal
restoration
Smooth

Glass ionomer
Zinc oxide
Polycarboxylate eugenol
Zinc phosphate


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