RETENTION AND RESISTANCE FORM in FPD.ppt

CONTENTS
 INTRODUCTION
 PRINCIPLES OF TOOTH PREPARATION
 DEFINITIONS
 FACTORS AFFECTING RETENTION
– DEGREE OF TAPER.
– TOTAL SURFACE AREA OF THE CEMENT FILM.
– AREA OF CEMENT UNDER SHEAR STRESS
– ROUGHNESS OF TOOTH SURFACE
– OTHER FACTORS AFFECTING RETENTION
 FACTORS AFFECTING RESISTANCE
– LEVERAGE AND RESISTANCE
– PREPARATION LENGTH AND RESISTANCE
– RESISTANCE AND TOOTH WIDTH
– TAPER AND RESISTANCE

 PATH OF INSERTION
 ROTATION AROUND A VERTICAL AXIS
 METHOD TO EVALUATE TAPER
 METHODS TO ANALYSE RESISTANCE FORM
 RETENTION AND RESISTANCE FEATURES IN
VARIOUS PREPARATIONS
– FULL VENEER CROWNS
– PARTIAL VENEER CROWNS
– PIN MODIFIED ¾ CROWNS
– INLAY PREPARATION
– ONLAY PREPARATION
 RETENTION AND RESISTANCE FOR
EXTENSIVELY DAMAGED TEETH
 RETENTION AND RESISTANCE FOR
ENDODONTICALLY TREATED TEETH
 SUMMARY & CONCLUSION
 REFERENCES

Introduction
 Teeth do not possess the regenerative
ability found in most other tissues….
 Restorative material must be used to
reestablish form and function.
 For a restoration to accomplish its
function……
 No cements that are compatible with living
structure can hold solely through
adhesion……
 The geometric configuration of the tooth
preparation …….

PRINCIPLES OF TOOTH
PREPARATION
 Definition: “Tooth preparation is defined
as the mechanical treatment of dental
disease or injury to hard tissues that
restores a tooth to original form”.

BIOLOGIC
Conservation of tooth
structure
Avoidance of
overcontouring
Supragingival margins
Harmonious occlusion
Protection against tooth
fracture
MECHANICAL
Retention form
Resistance form
Deformation
ESTHETIC
Minimum display of metal
Maximum thickness of
porcelain
Porcelain occlusal surfaces
Subgingival margins

 According to Glossary of Prosthodontic Terms
7/1999 – Resistance form: The features of a tooth
preparation that enhance the stability of restoration
and resist dislodgement along an axis other than
the path of placement.
 Retention form: The feature of a tooth preparation
that resists dislodgement of a crown in a vertical
direction or along the path of placement.
 In practice retention and resistance are closely
related and they are not always clearly
distinguishable.

 Four factors influence retention
during tooth preparation:
1. Degree of taper.
2. Total surface area of the cement film.
3. Area of cement under shear stress.
4. Roughness of the tooth surface.

 Degree of taper and
retention.
– The ability of a cement bond
to withstand a force
depends largely on the
direction of force.
– More nearly parallel the
opposing walls of the
preparation greater will be
retention.
– Jogensen said as retention
decreases taper increases.

 William A. Kent, Herbert T. Shillingburg,
Manville G. Duncamon, Quintessence Int. vol.
19, 1988. - Conducted a study to evaluate the
degree of taper of 418 dies of preparations, cut
over a time span of 12 years. The dies were
categorized by location and type of preparation.
The degree of taper of six opposing sets of
surfaces were measured and compared. They
concluded the mean of tapers of preparations
ranged from 8.6 – 26.6°. Internal features such as
grooves and boxes have a lower degree of taper
compared with external axial walls and
preparations in the anterior segment have a lower
degree of taper than in posterior segment.

 Jeffrey Nodlander, Dennis Weir, Warren Stoffer and
Shigaro Ochi, JPD, vol. 60, 1988. – Conducted a study to
measure the convergence angles of a full coverage
preparations performed in a clinical environment. Teeth
were prepared by the participants to attempt a 4-10°
convergence angle. The convergence angles of all
preparations were determined by projecting the faciolingual
and mesiodistal silhouttes of the dies with an overhead
projector. They concluded that, the ideal convergence
angle of 4-10° is seldom achieved. The convergence angle
for mandibular preparations were greater than maxillary.
Auxillary retention should be used in molar region because
these preparations were found to have larger convergence
angle.

 Ideal taper should be
within the range of 2-
6.5°.
 A taper of 16° have
been proposed as
being clinically
achievable while
affording adequate
retention.

Surface Area:
– Greater the area of cement film, greater
the retention.
– Total surface area of the preparation is
influenced by the size of the tooth.
– The extent of coverage by the
restoration.
– Features such as grooves and boxes.

 Area under Shear Stress:
– Most important feature for retention is that the
total surface area of cement which will
experience shearing rather than tensile
stress.
– To achieve this the preparation must have
opposing walls nearly parallel to each other.

To obtain the greatest area of cement
under shear stress, the direction in
which a restoration can be removed
must be limited to one path.

 Full veneer crown has excellent
retention. When compared to partial
veneer crown because reducing the
path of insertion to a narrow range.

 If axial wall left unveneered retention is
achieved by substituting a grooves, boxes
or pinholes for the missing wall.
 A grooved lingual wall must be distinct and
perpendicular to the axial wall.

 The length of the preparation is an
important factor in retention.
 Greater surface area with a larger diameter
will have greater retention

 Surface roughness:
– Adhesion of dental cements depends primarily
on projections of the cement into microscopic
irregularities.
– Jorgensen found retention of castings cemented
with ZnPO4 cement on test dies with a 10° taper
to be twice as great on preparations with 40µm
scratches than 10µm.
– Failure rarely occurs at the cement tooth
interface. So roughening hardly influences
retention.

 Mohammed F. Ayad, Stephen F. Rosenstiel and
Mirfat Salama. (JPD Vol.77, 1997) – Conducted a
study to determine the relationship between
surface characteristics of teeth prepared for
complete cast crowns and retention of respective
cemented restorations. Three luting cements: Zinc
phosphate, glass ionomer and adhesive resin
cements were used. And they concluded that cross
cut carbide bur improved retention of complete cast
crowns cemented with zinc phosphate cement
compared with tooth preparations completed with
finishing burs and the resin cements demonstrated
the greatest retention.

 Bernard G.N. Smith (JPD, Vol.23, 1970) –
conducted a study to test the hypothesis
that increasing the roughness of the dentin
surface increases the retention of a
cemented gold casting. Thirty six
preparations were made from extracted
teeth and were divided into 3 groups of
smooth, fine, and rough. Open top gold
crowns were cast for each tooth and
cemented with zinc phosphate. He
concluded that by altering the roughness of
a prepared dentin surface within a range of
5 to 120 microinches does not significantly
affect the retention of the completely vented
cast crown.

 Other factors affecting retention:
 Materials being cemented:
– Retention is affected by both the casting alloy
and the core material.
– More reactive the alloy is more adhesion.
– Base metal alloys are better retained than less
reactive high gold content metals.
 Type of luting agent:
– Studies show that adhesive resin cements are
more retentive than compared to conventional
ZnPO4 and GIC cements.

 Film thickness of the luting agent. Studies
showed that increased thickness of the cement film
will have some effect on a restoration.
 This may be important if a slightly oversized
casting is made using die spacers.
 Stanley G. Vermilyea, Michael J. Kuppler, Eugen
F. Hugger, JPD, 50, 1983. – Conducted a study to
determine the influence of die relief agent on the
retention of cast restorations using 3 cementing
mediums. They concluded that forces required to
dislodge unspaced castings were higher when
ZnPO4 as luting medium. Die relief agents resulted
in a 32% reduction in the dislodging forces.

 Steven M. Carter, Peter R. Wilson, IJP, vol. 9,
1996.- Conducted a study to see the effect on die
spacing on pre and post cementation crown
removal and crown elevation. Different layers of die
spacer was used. They observed that the force
required to remove a crowns before cementation
decreased with increased layers of die spacers.
Following cementation the mean crown elevation
decreased and the removal force is increased.
 Study was conducted in our department under the
able guidance of Sir to evaluate the effect of die
spacer thickness on crown retention. It was
concluded that the cast crowns fabricated with 4
layers of die spacer relief will provide maximum
retention and at least two layer of die relief is
essential to avoid crown to tooth contact or
frictional fit.

 Resistance:
– It prevents dislodgement of a restoration by forces
directed in an apical, oblique or horizontal direction.
 Leverage and Resistance:
– Leverage occurs when the line of action of a force
passes out side the supporting tooth structure.
– If the force passes within the margin of a crown no
tipping of the restoration when compared to the line of
action passing outside the margins of the restoration

 A force applied at an oblique
angle will produce a line of action
passing out side the supporting
tooth structure.
 If a line drawn from the center of
rotation perpendicular to the
cement film on the opposite wall
of the preparation the point where
the line intersects the cement film
is known as tangent point.

 If the tangent points of all the arcs of rotation
around a given axis are connected they form the
tangent line. The area above the tangent line is
resisting area.
 To have effective resistance the tangent line should
extend atleast halfway down the preparation.

Preparation length and resistance:
-Shortening of preparation
will reduce the resistance.
-Small restoration is less likely
to fail through tipping than
long restoration.
-In short crown lifting force is
small when compared to long
crown.

 Resistance and tooth width:
– A wider preparation has a greater retention but a
narrow tooth can have greater resistance to
tipping.
Because of smaller diameter
a tangent line falls low on the
wall opposite to axis of
rotation. Resulting in a large
resisting area.
Weak resistance can be
enhanced by placing
vertical grooves.

 Taper and Resistance:
– More tapered a preparation less is resistance.
No taper the resisting area cover half
the axial wall.
Ideal taper < ½ the axial wall.
Over tapered small resisting area
near the occlusal surface.

 William W. Dodge, Roger M. Weed,
Ramon J. Baez and Richard N. Buchanan
(Quintessence Int, Vol.3 1985) –
conducted a study to compare the effect of
varying the convergence angle on retention
and resistance in complete veneer crown
preparations. 15 stainless steel dies were
machined with 10°, 16° and 22° taper. They
concluded that resistance is more sensitive
to changes in a convergence angle than
retention form. There is no significant
difference in retention values between
preparations with 10° and 16°. 16° of
occlusal convergence was considered
adequate for retention and resistance.

 Taper of a preparation is directly
proportional to height : width ratio.
 Taper that permit an effective resisting area
for a preparation in which height equals
width is double than in a preparation where
height is only half width.

 Formula to calculate:
– Taper:
T = arc sin (2r/w),
- Height of tangency point.
r = (w sin T) / 2
- Preparation height
h = [w tan (90°- T/2)]/2.

Rotation around a vertical axis:
A partial veneer crown
which has no grooves
offer little resistance
to rotation.
Axial symmetry
of a full veneer
crown preparation
may allow
rotation of the
restoration.

 Path of insertion:
– A path must be selected that will allow the
margins of the retainers to fit against their
respective finish lines with minimal tooth
structure removal.
Parallel to the long
axis of the tooth
Parallel to the incisal
2/3rd of labial surface.
Perpendicular to the
occlusal plane

 Methods to analyze resistance form:

RETENTION AND RESISTANCE FEATURES
IN VARIOUS PREPARATIONS
Full veneer crowns

 Gerald D. Woolsey and Joseph A. Matich
(JADA, Vol.97, 1978)- conducted a study to
see the effects of parallel axial grooves on
resistance form of complete crown
preparations. Two experiments were
performed on four sets of dies to test
parallel proximal and parallel buccolingual
grooves. They found that dies with a
grooves oriented on the proximal surfaces
provided complete resistance to horizontal
dislodgement. Whereas dies with grooves
oriented on the buccal and lingual surfaces
provided only partial resistance to horizontal
displacement.

Partial veneer crowns – 3/4 crown

 Mitsuo Kishimoto, Herbert T.
Shillingburg, and Manville g. Duncanson
(JPD Vol.49, 1983) – Conducted a study to
examine the effect of preparation design as
well as a location and number of the
features on the retention and resistance of
¾ crowns. ¾ crowns designs with variations
in retention resistance features like proximal
grooves in the lingual half of proximal
surface, V-shaped and round proximal
grooves, proximal boxes were made. They
concluded that preparations with boxes or
four grooves are the most retentive of the ¾
crown designs. Lingual placement of
proximal grooves enhance retention and V-
shaped grooves offered least resistance.

 Eyal A. Galun, Charles J. Goodcare et al
(JPD Vol.56, 1986) – Conducted a study to
evaluate the contribution of pin hole
modification to the retention and resistance
form of partial veneer crowns by using two
different pin lengths, two techniques of
reproducing the pin in the wax and two
methods of cementation. They concluded
that the absence of boxes or grooves, pins
markedly enhance both retention and
resistance. In the presence of proximal,
boxes, pins contributed only to retention
and had no significant effect on resistance.
If pins are used, the cement should be
spiralled into the pin hole.

 Mitsuo Kishimoto, Herbert T.
Shillingburg, and Manville G. Duncanson
(JPD Vol.49, 1983) – Conducted as study
to evaluate the variables of the proximal
box, isthmus and flares in relation to the
retention and resistance of MOD onlays.
They concluded that boxes are more
effective than an isthmus in increasing
retention value. The combination of boxes
and isthmus showed the highest retention
and resistance. There was no difference in
resistance between boxes and isthmus.

 Retention and Resistance in extensively
damaged tooth preparations
– When it is necessary to compensate for
mutilated or missing cusps inadequate length
and in extreme cases even a missing clinical
crown the principle of substitution is used
– Boxes may be substituted where grooves are
utilized.
– Grooves may be used where axial walls have
been shortened.
– Pins may be employed where much of the
supragingival structure has been destroyed.

– Rules observed to avoid excessive tooth
destruction while creating retention.
– Central core must not be invaded.
– No retentive features should extend farther into
the tooth than 1.5mm from central fossa.
– Any mechanical retentive feature is added it is
kept in the safe area peripheral to the vital core.
– No wall of dentin should be reduced to a
thickness less than its height for retention.

 Box forms:
– Substitutes for grooves serves the dual purpose
of caries removal and retention form.
– More than 180° of the tooth circumference
should remain between the box and the groove.

 Grooves – multiple grooves effective as boxes for
resistance.
 Can be placed at angles of oversized box forms for
resistance when the walls of the box are at a
considerable distance.

 Pins: Increases retention can be placed
when insufficient axial wall length.
– Pin holes parallel to the path of insertion form
part of cast restoration.
– Non-parallel pins to retain amalgam or
composite resin core.

 Guidelines should be followed in drilling
pin holes.
– Place them in sound dentin.
– Do not undermine enamel.
– Avoid perforation into the periodontal
membrane.
– Do not encroach upon the pulp.
– Should be placed halfway between outer
surface and the pulp on ledges surrounded by
atleast 0.5mm dentin.
– Safest locations for the pinholes are the line
angles or corners of the teeth.

 Maxillary premolars have
concavities on the mesial and
distal aspects of the root.
 Centers of mesial, facial and
lingual surface of mandibular
molars overlie root concavities.
 The trifurcations of maxillary
molars are situated in the
centers of mesial, distal and
facial surfaces.

RETENTION AND RESISTANCE IN
ENDODONTICALLY TREATED TEETH
 Retention form
– Preparation geometry
– Post length
– Post diameter
– Post surface texture
– Luting agent

 Resistance form
– Stress distribution
– Rotational resistance

CONCLUSION
 All preparations require the incorporation of factors
to prevent the dislodgement of restoration by
functional stresses.
 More recent concepts tend to emphasize the
duplication of a preconceived design rather than
the assessment of biomechanical requirements of
individual retainers.
 The modification of an ideal design for preparation
entails judgment in the application of the basic
principles of retention and resistance.
 An analysis of these principles and factors should
enable the dentist to effectively apply them during
the design of any preparation.

References
 Rosensteil (2001) Contemporary Fixed Prosthodontics. 3rd
edition.
 Shillingburg (1981) Fundamentals of Fixed Prosthodontics.
2nd edition.
 Tylman (1989) Theory & Practice of Fixed Prosthodontics. 8th
edition.
 Shillingburg, Richard Jacobi, S.E. Brackett. (1987)
Fundamentals of Tooth Preparations for Cast metal and
Porcelain restorations.
 M.H. Parker, R.B. Gunderson, F.M. Gardner, M.J. Calverley :
Quantitative determination of taper adequate to provide
resistance form : Concept of limiting taper. JPD, 1988; 59:
281-288.
 S.M. Carter, Peter Wilson: The effect of die-spacer on crown
retention. IJP, 1996, 9: 21-29.
 S. Vermilyea, M.J. Buffler, E. Maget : The effect of die relief
agent on the retention of full coverage castings. 1983; 50:
207-210.

 W. Kent, H.T. Shillingburg, M.G. Duncansson: Taper of clinical
preparation for cast restoration. Quint. Int. 1988; 19: 339-345.
 P.J.B. Leempod, L.M. Leemens, P.A. Snvek, M.A. Vent Hoff:
The convergence angle of tooth preparations for complete
crowns. JPD, 1987; 58: 414-416.
 J. Nordlander, D. Weir, W. Stoffer, S. Ochi : The taper of
clinical preparations for fixed prosthodontics. JPD, 1988; 60:
148-151.
 G. Golun, Charles Goodacre, R. Dykima, B. Keeth Moore, L.
Sowinski; The contribution of a pinhole to the retention and
resistance form of veneer crowns. JPD, 1986; 56: 292-297.

 R.M. Weid, R.J. Baez : A method for determining adequate
resistance form of complete cast crown preparation. JPD,
1989; 52: 330-334.
 Anthony Tjan, Gary Miller : Biogeometric guide to groove
placement on three quarter crown preparation. JPD, 1979;
42: 405-410.
 R. Potts, H.T. Shillingburg, M. Duncanson, Retention and
resistance of preparation for cast restorations. JPD, 1980; 43:
303-308.
 Merle Parker, Ashton Trier, Thomas Striano: Evaluation of
resistance form for prepared tooth. JPD, 1991; 56: 730-733.
 Anthony Tjan, Ruben Sarkissian : Effect of preparation finish
on retention and fit of complete crown. JPD 1986; 56: 283-88.
 M. Harry Parker: Resistance form in tooth preparation. DCNA
2004, 387-396.

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