A quite common question in post-graduate exams. Includes all the retention and resistance forms from Class I amalgam cavity to post and core.

1. RETENTION AND RESISTANCE FORMS IN CAVITY DESIGNS
Dr. Sheetal Kotni
Conservative Dentistry and Endodontics

2. Contents

3. Retention &
Resistance forms
in cavity designs
Conservativ
e Dentistry
Fundamentals in
tooth preparations
Initial tooth
preparation
Final tooth
preparation
Direct
restorations
Class I, II, III, IV,
V, VI cavity
Tooth
coloured
restoration
Composite,
GIC
Non tooth
coloured
restoration
Amalgam
Conventional
Pin-retained
DFG
Indirect
restorations
Class II cast
metal inlay,
onlay
Class II ceramic
inlay, onlay
Endodontics
Post & Post
space
preparation

4. Fundamentals of tooth preparation
01

5. DEFINITION
Primary resistance form may be defined as the shape and placement of the preparation walls that best enable the remaining
tooth structure and the restoration to withstand, without fracture, masticatory forces delivered principally in the long axis of the
tooth.
PRINCIPLES RATIONALE
Box shape with a relatively
horizontal floor
Helps the tooth resist occlusal loading by virtue of being at
right angles to the forces of mastication that are directed in
the long axis of the tooth
Restricting the extension of the external walls To allow strong cusp and ridge areas to remain with sufficient
dentin support
Slight rounding of internal and external line
angles
To reduce stress concentrations in tooth structure and
increased resistance to fracture
Reducing and covering (capping) weak cusps and
enveloping or including enough of a weakened
tooth within the restoration in extensive tooth
preparations
To prevent or resist fracture of the tooth by forces
in the long axis and obliquely (laterally) directed forces (most
resistance to oblique or lateral forces is attained later in the
final tooth preparation stage)
Providing enough thickness of restorative material To prevent its fracture under load
Bonding the material to the tooth structure, when appropriate.
Initial Tooth
Preparation stage
Primary Resistance
form

6. The end of the cutting instrument
prepares a relatively horizontal pulpal
wall of uniform depth into the tooth,
following the original occlusal surface
contours and the DEJ.

7. Cusp reduction
strongly recommended
when the outline form has
extended 2/3rds the distance
from a primary groove to a
cusp tip.
Exception-Bonded
restoration
In pulpless teeth, special consideration is applied in obtaining resistance form . The weakened cusps
may need to be reduced, enveloped, and covered with restorative material to prevent the cracking
or splitting of the remaining tooth structure.

8. Occlusal
contact
assessment
• Greater the occlusal forces and contacts, greater chances of fracture
• Further posterior the tooth, the greater is the effective masticatory force because the tooth is closer to the
temporomandibular joint
The amount
of remaining
tooth
structure
• Sometimes unsupported, but not friable, enamel may be left. This is usually for esthetic reasons in anterior teeth,
especially on the facial surfaces of maxillary teeth where stresses are minimal and a bonded restoration typically is used.
• Large teeth due to sufficient bulk may not be emphasized for resistance form.
Type of
restorative
material
• The minimal occlusal thickness for amalgam for appropriate resistance to fracture is 1.5 mm; cast metal, 1 to 2
mm (depending on the region); and ceramics, 2 mm. The thickness requirement is greater for posterior teeth than
for anterior teeth for composite due to increased wear potential.
Bonding
• Bonding may increase the strength of the remaining unprepared tooth
Factors affecting the need to develop resistance form in a preparation

9. Primary
Retention form
DEFINITION
Primary retention form is the shape or form of the conventional preparation that prevents displacement or
removal of the restoration by tipping or lifting forces for nonbonded restorations.
In many respects,
retention form and
resistance form are
accomplished at the
same time e.g., pins
placed in a manner so
that one portion of a
tooth supports another
portion of the tooth

10. Amalgam
• Macromechanical retention
• Occlusal convergence- it cannot
be dislodged without some type
of fracture occurring
• Occlusal convergence should
not be excessive - result in
unsupported enamel rods at the
cavosurface margin
• Studies show that bonded
amalgams do not result in long-
term reinforcement of teeth or
improved resistance to fracture
Composite
• Micromechanical retention
• Bevels or flares
Cast metal
• Parallel vertical walls
• Angle of divergence (2-5
degrees per wall) from the line
of draw
• The degree of divergence
needed primarily depends on
the length of the prepared
walls: Greater the ht., more the
divergence.
• Use of a luting agent
PRINCIPLES OF RETENTION FORM IN INITIAL TOOTH PREPARATION STAGE

11. Add your title
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（1）Mechanical preparation features
（2）Treatments of the preparation walls with etching, priming, and
adhesive materials
SECONDARY RETENTION AND RESISTANCE FORMS
After any remaining enamel pit or fissure, infected dentin, or old restorative material (if indicated) has been removed,
and pulpal protection has been provided by appropriate liners and bases, additional resistance and retention features
may be deemed necessary for the preparation

12. Vertically oriented retention grooves
Horizontally oriented retention grooves in most Class
III and Class V preparations for amalgam and in some
root-surface tooth preparations for composite.
Retention coves for the incisal retention of Class III
amalgams.
Retention grooves in Class II preparations for
extensive tooth preparations for amalgam
involving wide faciolingual proximal boxes,
cusp capping, or both
Retention Grooves and Coves
MECHANICAL
FEATURES

13. Preparation
Extensions
Extending the preparation for
molars onto the facial or lingual
surface to include a facial or
lingual groove.
Results in additional
vertical, almost-parallel
walls for retention in cast
metals
Enhances resistance for
the remaining tooth
owing to envelopment.
Skirts
Cast gold restorations
that extend the
preparation around line
angles of the tooth
Provide
additional, opposed
vertical walls for added
retention
Increase resistance form
by enveloping the tooth,
resisting fracture of the
remaining tooth from
occlusal forces.
Beveled
Enamel
Margins
May improve retention
form slightly when
opposing bevels are
present
Used primarily to afford
a better junctional
relationship between the
metal and the tooth
Composite - bevels or flares
increase the surface area of
etchable enamel and the
effectiveness of the bond
by etching more enamel
rod ends

14. • High need for retention form for
amalgam restorations
• Pins or slots> Steps, amalgam pins
Pins, Slots,
Steps, and
Amalgam Pins
• Enamel walls are etched for bonded
restorations that use ceramic,
composite, and amalgam
Enamel Wall
Etching
• DBA
• Retention of indirect restorations may
be enhanced by the luting agent used
Dentin
Treatment

15. Resistance and retention forms
in cavity designs for
composite
02

16. Class III Composite & GIC Class IV Composite & GIC Class V Composite & GIC
• The extension of the preparation-
extension of the fault or defect.
• Not necessary to reduce sound tooth
structure to provide “bulk for strength”
or to provide conventional retention and
resistance forms.
• If a large amount of tooth structure is
missing and the restoration is in a high
stress area, groove retention form
indicated even when the preparation
periphery is entirely in enamel.
• Also, wider enamel bevels - greater
surface area for etching-stronger bond
between the composite and the tooth
and better esthetics
Beveling of incisal margins(not required
in case of GIC)
Gingival retention grooves when
involving root
Roughening of internal walls
Due to adequate bonding, mostly no
retention form necessary
If the fracture is confined to enamel,
adequate retention - simply beveling the
sharp cavosurface margins in the
fractured area- flame-shaped diamond
instrument
+ retention form-increasing the surface area
with a wider enamel bevel, adding retentive
features in the preparation internal walls
Bevel- flame-shaped or round diamond
instrument, 450 angle to the external
surface, width- 0.5 to 2.0 mm
+ retention with grooves, undercuts,
dovetails.
Gingival retention groove with 1/4 round
bur, 0.2mm into DEJ, 0.25mm depth(1/2
of bur). This groove should extend the
length of the gingival floor and slightly up
the facioaxial and linguoaxial line angles
Smaller round burs define the internal
angles enhancing proper placement of
the retention grooves.

17. Class III Composite & GIC

18. Gingival retention groove.
Class IV Composite & GIC

19. Class V Composite

20. Class VI Composite

21. Add your title
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• R & R---------
• More flared
cavosurface forms
w/o flat pulpal or
axial walls
• No overextension to prevent
weakening and fracture
• Unprepared occlusal margin-
No bevel- may result in thin
composite in areas of heavy
occlusal contact.
Re t e n t i o n - B o n d i n g
Small to Moderate Class I Moderate to Large Class I
Class I Direct Composite Restoration

22. Class II Direct Composite Restoration
• Mode of retention-Micromechanical bonding
• Not necessary to use mechanical retention features
Extensive Class II Direct Composite Restorations
Composites can also be used for
• Selected cases for extensive restorations - Cusp reduction and capping, extentions, 2o retention features,
more resistance forms
• Foundation for indirect restorations (crowns and onlays) in cases where there is insufficient natural tooth
structure to provide adequate retention and resistance form for the crown.
 Primary retention-Micromechanical bonding
 When a full-coverage preparation is anticipated, secondary retention features may be incorporated,
however, because of
(1) the decreased amount of tooth structure available for bonding,
(2) the increased concern for retaining the composite in the tooth. These features include grooves, coves,
locks, or slots.
In case of RC Treated tooth
Pulp chamber can be opened, and extensions can be made several millimeters into each treated canal.

23. • Cusps to be capped as early to provide
more access and visibility for the
preparation.
• The depth cut- instrumentparallel to
the cuspal incline (from cusp tip to
central groove), 1.5 to 2 mm deep. For
a large cusp, multiple depth cuts can
be made.
• Then, the instrument is used to join
the depth cuts and extend to the
remainder of the cuspal form.
• The reduced cusp has a relatively flat
surface that may rise and fall with the
normal mesial and distal inclines of
the cusp.
• Enough clearance with the opposing
tooth to result in 1.5 to 2 mm of
composite to restore form and
function.
• The cusp reduction should be blended
in with the rest of the occlusal step
portion of the preparation.

24. • Must be placed entirely in dentin, not
undermining and weakening any adjacent
enamel.
• At times, bevels may be placed on
available enamel margins to enhance
retention form, even on occlusal areas.
• Retention form for foundations must be
placed far enough inside the DEJ (at least
1 mm) to remain after the crown
preparation is done subsequently.
Otherwise, the potential retentiveness
may be lost for the foundation
2O Retention forms

25. INDIRECT TOOTH COLOURED RESTORATIONS-CERAMIC INLAYS, ONLAYS
• All margins should have a 90-degree butt-joint cavosurface
angle to ensure marginal strength of the restoration.
• All line and point angles, internal and external, should be
rounded to avoid stress concentrations in the restoration and
tooth, reducing the potential for fractures.
• Excessive divergence must be avoided since resistance and
retention form are required to help preserve the adhesive
interface.
• Cusp reduction and capping if required. In case of cusp
capping, reduction is by 2 mm, 90-degree cavosurface angle.
When capping cusps, especially centric holding cusps, it might
be necessary to prepare a shoulder to move the facial or
lingual cavosurface margin away from any possible contact
with the opposing tooth, either in maximum intercuspal
position or during functional movements.
• Isthmus at least 2 mm wide to decrease the possibility of
fracture of the restoration.
• Ideally, there should be no undercuts that would prevent the
insertion or removal of the restoration. Small undercuts, if
present, can be blocked out using a resin-modified glass
ionomer (RMGI) liner.
• The pulpal floor should be smooth and relatively flat.

27. CAD/CAM Techniques
• Clinical procedures for CAD/CAM
systems differ somewhat from the
procedures previously described.
• Tooth preparations for CAD/CAM inlays
must reflect the capabilities of the CAD
software and hardware and the CAM
milling devices that fabricate the
restorations.
• E.g. The CEREC system automatically
“blocks out” any undercuts during the
optical impression

28. Resistance and retention forms
in cavity designs for amalgam
03

29. Class I
Amalgam
cavity
designs
Conservative
Class I
Extensive
Class I
Occlusolingual
Class I cavity
Occlusofacial
Class I cavity

30. Conservative Class I cavity for Amalgam
The resistance principles are as follows:
• Extending around the cusps to conserve tooth
structure and prevent the internal line angles from
approaching the pulp horns too closely
• Keeping the facial and lingual margin extensions as
minimal as possible between the central groove and
the cusp tip
• Extending the outline to include fissures, placing the
margins on relatively smooth, sound tooth structure
• Minimally extending into the marginal ridges (only
enough to include the defect) without removing
dentinal support
• Eliminating a weak wall of enamel by joining two
outlines that come close together (i.e., <0.5 mm apart)
• Extending the outline form to include enamel
undermined by caries
• Using enameloplasty on the terminal ends of shallow
fissures to conserve tooth structure
• Establishing an optimal, conservative depth of the
pulpal wall
• No. 245 bur-sides slightly convergent toward the
shank- occlusal convergence of the facial and lingual
preparation walls- retention form .
• The slightly rounded corners of the end of the No.
245 bur produce slightly rounded internal line
angles-fracture resistance to the tooth.
• The No. 330 bur is a smaller version of the No. 245
bur.

31. • Distal extension into the distal
marginal ridge to include a fissure
or caries occasionally requires a
slight tilting of the bur distally (≤10
degrees)-Slight occlusal divergence
to the distal wall to prevent
undermining the marginal ridge of
its dentin support.
• Because the facial and lingual
prepared walls converge, this slight
divergence does not present any
retention form concerns.
• Narrow occlusal isthmus is less
prone to fracture.

32. The primary resistance form is provided by
the following:
• Sufficient area of relatively flat pulpal
floor in sound tooth structure to resist
forces directed in the long axis of the
tooth and to provide a strong, stable seat
for the restoration
• Minimal extension of external walls,
which reduces weakening of the tooth
• Strong, ideal enamel margins
• Sufficient depth (i.e., 1.5 mm) that
results in adequate thickness of the
restoration, providing resistance to
fracture and wear
The primary retention form is provided by
the following:
• The parallelism or slight occlusal
convergence of two or more opposing,
external walls

33. Operative Dentistry: Modern Theory and Practice- Marzouk

35. If enameloplasty is unsuccessful in eliminating a mesial (or
distal) fissure that extends to the crest of a marginal ridge
or beyond, three alternatives exist:
1. Make no further change in the outline form
2. Extend through the marginal ridge when margins would
be lingual to the contact
3. Include the fissure in a conservative Class II tooth
preparation

37. Extensive Class I cavity for Amalgam
Primary resistance form-
Extending the outline of the tooth preparation to include only
undermined and defective tooth structure while preparing
strong enamel walls and allowing strong cuspal areas to
remain.
Primary retention-
by the occlusal convergence of the enamel walls
Secondary retention form-
undercut areas that are occasionally left in dentin (and that
are not covered by a liner) after removal of infected dentin.
Usually, no secondary resistance or retention
form features are necessary for extensive Class I amalgam
preparations.

38. Class I Occlusolingual cavity for Amalgam
• Occlusal portion- Occlusal convergence
• Lingual portion- An axial depth of 0.5 mm
inside the DEJ is indicated if retentive
grooves are required; an axial depth of
0.2 mm inside the DEJ is permissible if
retentive grooves are not required.
• The No. 245 bur results in the mesial and
distal walls converging lingually because
of the shape of the bur.
• The occlusal and lingual convergences
usually provide a sufficient preparation
retention form; no retention grooves are
needed.
• Rounded axiopulpal line angle
Primary retention features

39. Secondary Retention Features
• Additional retention in the
lingual extension-if the
extension is wide
mesiodistally or if it was
prepared without a lingual
convergence.
• No. 1/4 or No. 169 bur
• The depth of the grooves at
the gingival floor is one-half
the diameter of the No. 1/4
bur.
• The cutting direction for
each groove is the bisector
of the respective line angle.

40. Class I Occlusofacial cavity
for Amalgam
Operative Dentistry:
Modern Theory and
Practice- Marzouk

41. • Specifically for RC treated
teeth
• Confined to occlusal and
buccal/lingual surfaces
• Pulp chamber is flat except
the extension
• Bulky walls of pulp chamber
should accomodate ledges
on both walls. Eliminate
excess loading on subpulpal
floor

42. Class II Amalgam Restorations Involving Only One Proximal Surface
The primary resistance form is provided by
(1) the pulpal and gingival walls being relatively level and
perpendicular to forces directed with the long axis of the
tooth;
(2) restricting the extension of the walls to allow strong cusps
and ridge areas to remain with sufficient dentin support, at the
same time establishing the peripheral seat;
(3) restricting the occlusal outline form (where possible) to
areas receiving minimal occlusal contact
(4) the reverse curve optimizing the strength of the amalgam
and tooth structure at the junction of the occlusal step and
proximal box;
(5) slightly rounding the internal line angles to reduce stress
concentration in the tooth structure (automatically created by
bur design except for the axiopulpal line angle)
(6) providing enough thickness of the restorative material to
prevent its fracture from the forces of mastication.
The primary retention form is provided by the
occlusal convergence of the facial and lingual walls and by the
dovetail design of the occlusal step, if present.

44. Secondary Resistance and Retention Forms
Secondary resistance form-Restricting extensions of external walls, using the
gingival margin trimmer or a bur to round the axiopulpal line angle
The use of retention grooves in proximal boxes is controversial. It has been
reported that proximal retention grooves in the axiofacial and axiolingual line
angles may increase the fracture resistance and significantly strengthen the
isthmus, superior to the axiogingival grooves.
Other investigators- retention grooves located occlusal to the axiopulpal line
angle provide more resistance.
Reports- With high-copper amalgams, proximal retention grooves are
unnecessary in preparations that include dovetails.
The use of retention grooves is recommended in tooth preparations with
extensive proximal boxes.

46. In box-only preparations-
• Slight convergence of facial
and lingual walls.
• Retention grooves. The
proximal retention grooves
should have a 0.5-mm
depth at the gingival point
angle, tapering to a depth
of 0.3 mm at the occlusal
surface
In slot preparation in cases of root caries-
• A No. 1/4 bur-retention grooves in the
occlusoaxial and gingivoaxial line angles, 0.2 mm
inside the DEJ or 0.3 to 0.5 mm inside the
cemental cavosurface margin
• The depth of these grooves is one-half the
diameter of the bur head (i.e., 0.25 mm), and the
bur is directed to bisect the angle formed by the
junction of the occlusal (or gingival) and axial
walls.
• Ideally, the direction of the occlusal groove is
slightly more occlusal than axial, and the direction
of the gingival groove would be slightly more

47. Class II Amalgam Restorations Involving Both Proximal Surfaces
• 90-degree cavosurface
amalgam margin
• Cusp reduction and capping
• Large proximal box
preparations also need
secondary retention features
(i.e., retention grooves, pins,
slots) for an adequate etention
form.

48. Operative Dentistry: Modern Theory and Practice- Marzouk

49. • Flat ledges
• Squaring up Surrounding walls
• Each flat table or ledge should be opposed by similar
flat component to resist displacement
Operative Dentistry: Modern Theory and Practice- Marzouk

50. Class III Amalgam Restorations
The resistance form is provided by
(1) cavosurface and amalgam margins of 90 degrees,
(2) enamel walls supported by sound dentin,
(3) sufficient bulk of amalgam
(minimal 1-mm thickness), and
(4) no sharp preparation internal angles.
The box-like preparation form provides primary
retention form.
Secondary retention form is provided by a gingival
groove, an incisal cove, and sometimes a lingual
dovetail.

52. Class V Amalgam Restorations
Reference to previous seminar on “Class V Cavity designs”

53. Sturdevant's Art and Science of Operative
Dentistry, 6th ed.

54. • The depth of the grooves - 0.25 mm (half of bur diameter).
• Adequate retention grooves - the only retention form to the preparation.
• In a large Class V amalgam preparation, extending the retention groove circumferentially around all the
internal line angles of the tooth preparation may enhance the retention form.
Sturdevant's Art and Science of Operative Dentistry, 6th ed.

55. Resistance and retention forms
in cavity designs for Complex
Amalgam Restorations
04

56. • Fracture resistance- Amalgam restorations with cusp coverage >>> amalgam
restorations without cusp coverage in weakened teeth.
• Resistance and retention forms increased by the use of pins and slots.
• Resistance form is more difficult to develop than for a cusp-capping onlay (skirting
axial line angles of the tooth) or a full crown.
• Not as effective in protection as an extracoronal restoration
Pin-Retained Amalgam Restorations
Pros
• Amalgam with pins have greater retention than using boxes only or bonding systems.
Cons
• Craze lines or fractures, internal stresses in dentin.
• Increases the risk of penetrating into the pulp or perforating the external tooth
surface.
• Decreases the tensile and horizontal strength of pin-retained amalgam restorations

57. SLOTS PINS
Defination For a complex restoration, a slot is a horizontal
retention groove in dentin
A pin-retained restoration is defined as any
restoration requiring the placement of one or
more pins in dentin to provide adequate
resistance and retention forms.
Use Used in conjunction with pin retention or as an
alternative to it
Whenever adequate resistance and retention
forms cannot be established with slots, locks,
or undercuts only
Indication In short clinical crowns and in cusps that have
been reduced 2 to 3 mm for amalgam.
Used more frequently in preparations with
few or no vertical walls
Sacrifice of tooth structure More Less
Chances of fracture Less likely to create microfractures in dentin,
however, and to perforate the tooth or
penetrate into the pulp
More likely
Inflammatory response No Medium sized self-threading pins may elicit
an inflammatory response if placed within
0.5 mm of the pulp
Retention potential Similar

59. INITIAL TOOTH PREP FINAL TOOTH PREP
Cusp reduction in case of a weekened cusp. Diminishes
retention form by reducing the height of walls.
Coves-horizontal plane
locks-vertical plane.
When possible, opposing vertical walls should be formed
to converge occlusally, to enhance the primary retention
form.
Also, a facial or lingual groove can be extended arbitrarily
to increase the retention form.
These locks and coves should be prepared before
preparing the pinholes and inserting the pins
The pulpal and gingival walls should be relatively flat and
perpendicular to the long axis of the tooth.
When additional retention is indicated- pins. Slots
prepared along the gingival floor, axial
to the DEJ instead of, or in addition to, pinholes

61. • The threads engage dentin as the pin is inserted, thus retaining it.
• The elasticity (resiliency) of dentin permits insertion of a threaded pin
into a hole of smaller diameter
• Self-threading pins are the most retentive of the three types of pins ,
being three to six times more retentive than cemented pins-Vertical,
horizontal stresses, craze lines

62. FACTORS AFFECTING RETENTION OF THE PIN IN DENTIN AND AMALGAM
Type/Retentiv
eness
Self threading>Friction lock>Cemented pins
Surface
Characteristics
• The number and depth of the elevations (serrations or threads)
• The shape of the self-threading pin gives it the greatest retention value.
Orientation,
Number, and
Diameter
• Placing pins in a non-parallel manner increases their retention.
• Bending is not advisable-the bends may interfere with adequate condensation of amalgam around the pin
and decrease amalgam retention, may weaken the pin and risk fracturing dentin.
• Pins should be bent only to provide for an adequate amount of amalgam (approximately 1 mm) between the
pin and the external surface of the finished restoration (on the tip of the pin and on its lateral surface).
• Only the specific bending tool should be used
• In general, increasing the number of pins increases their retention in dentin and amalgam. As the number
of pins increases,
(1) the crazing of dentin and the potential for fracture increase,
(2) the amount of available dentin between the pins decreases, and
(3) the strength of the amalgam restoration decreases.
Also, as the diameter of the pin increases, retention in dentin and amalgam generally increases.
• As the number, depth, and diameter of pins increase, the danger of perforating into the pulp or the external
tooth surface increases.
Extensn into
dentin, Amalgam
• Self-threading pin extension into dentin and amalgam should be approx. 1.5 to 2 mm to preserve the
strength of dentin and amalgam. Extension greater than this is unnecessary and contraindicated

65. Tooth prep for amalgam foundations
Pin Retention Slot Retention Pulp chamber retention
• Pinholes must be located farther from
the external surface of the tooth
(farther internally from the DEJ)
• More bending of the pins to allow for
adequate axial reduction of the
foundation w/o exposing the pins
during tooth preparation.
Placed slightly more axial (farther
inside the DEJ) than indicated for
conventional amalgam preparations
The number of remaining vertical
walls determines the indication for
slots.
(1) dimension of the pulp chamber is
adequate
(2) dentin thickness in the region of the pulp
chamber is adequate to provide rigidity and
strength to the tooth. Extension into the
root canal space 2 to 4 mm is recommended
when the pulp chamber height is 2 mm or
less
When pulp chamber height is 4 to 6 mm in
depth, no advantage is gained from
extension into the root canal space
Natural undercuts in the pulp chamber and
the divergent canals- necessary retention
form.
The resistance form is improved by gingival
extension of the crown preparation
approximately 2 mm beyond the foundation
onto sound tooth structure. This extension
should have a total taper of opposing walls
of less than 10 degrees.

66. Resistance and retention forms
in cavity designs for DFG
05

67. • Line and point angles should be definite and within
dentin. If they are not sharp, it is necessary to
prepare starting points
• Pulpal floor should be made flat using a subbase,
base etc.
• Partial enamel bevel on the cavosurface-450 to
protect enamel margins from condensation energy
and to allow enamel margins to be covered with
durable gold.

68. • Class I Cavity with facial or
lingual extensions
• Extensions-//gm shape
• Mandatory partial enamel bevel
at cavosurface

69. In final shaping stage, Angle former- sharpens internal anatomy
Wedelstaedt chisel- partial enamel bevel

78. Resistance and retention forms
in cavity designs for Cast
Metal Inlay
06

79. • Uniformly tapered walls and smooth pulpal and gingival walls.
• No. 271 and the No. 169L burs
• The sides and end surface of the No. 271 bur - slightly rounded - to prevent
sharp, stress inducing internal angles.
• The marginal bevels - with a slender, fine-grit, flame-shaped diamond
instrument such as the No. 8862 bur
• Single path of draw with no undercuts
• Gingival-to-occlusal divergence of these preparation walls- from 2 to 5
degrees per wall from the line of draw.
• If the vertical walls are unusually short, a max 2 degrees occlusal
divergence is desirable to increase retention potential.
INITIAL TOOTH PREP

80. The tooth tructure and strength can be conserved by
(1) enameloplasty of the fissure ends, when possible, and
(2) the marginal bevel of the final preparation to include
(eliminate) the terminal ends of these fissures in the
outline form.
Dovetail retention form resists distal displacement of the
inlay.
Capping the weak remaining cuspal -adequate retention
and resistance form

81. • Shallow (0.3-mm deep) retention grooves
cut in the facioaxial and linguoaxial line
angles with the No. 169L carbide
bur(indicated especially when the prepared
tooth is short). The grooves are in sound
dentin, close to but not contacting, the DEJ.
Parallel axis

82. Final Preparation
• Base application with no undercuts
• With a calcium hydroxide liner , less
dentin is available for adhesive
bonding. In these circumstances,
small mechanical undercuts can
increase the retention of the glass
ionomer base.
• If suitable undercuts are not present
after the removal of infected dentin,
retention coves are placed with the
No. 1/4 carbide bur in the peripheral
dentin of the excavation and are as
remote from the pulp as possible

83. • Good resistance form-
pulpal wall should not be
formed entirely by a
cement base; rather, in at
least two regions, one
diametrically across
• Extensive caries- Cusp
reduction, skirts.

84. BEVELS AND FLARES
• After the cement base (where indicated) is completed, No. 8862 diamond is used to bevel the
occlusal and gingival margins and to apply the secondary flare on the distolingual and distofacial
walls resulting in 30- to 40-degree marginal metal on the inlay. This cavosurface design helps
seal and protect the margins and results in a strong enamel margin with an angle of 140 to 150
degrees.
• A cavosurface enamel angle of > 150 degrees is incorrect-Results in a less defined enamel
margin (finish line), the marginal cast metal alloy is too thin and weak if its angle is < 30 degrees.
• Conversely, if the enamel margin is < 140 degrees, the metal is too bulky and difficult to burnish
when its angle is greater than 40 degrees.
• The distolingual wall - two planes.
• The first - lingual primary flare; the second- lingual secondary flare.
• If the instrument is not tilted, the bevel is too steep, resulting in gingival bevel metal that is too
thin (<30-degree metal) and too weak. Although the instrument is tilted mesially, its long axis
must not tilt facially or lingually.
• The gingival bevel - 0.5 to 1 mm wide, should blend with the lingual secondary flare.
• The more slender No. 169L carbide bur may be used

86. The secondary flare is necessary for several reasons:
(1) Extends the margins into the embrasures-margins more self-cleaning and more accessible to finishing procedures
during the inlay insertion appointment, and does so with conservation of dentin.
(2) The direction of the flare results in 40-degree marginal metal. Metal with this angular design is burnishable;
however, metal shaped at a larger angle is unsatisfactory for burnishing;
metal with an angle less than 30 degrees is too thin and weak, with a corresponding enamel margin that is too
indefinite and ragged.
(3) A more blunted and stronger enamel margin
It is omitted for esthetic reasons on the mesiofacial proximal wall of preparations on premolars and first molars of
the maxillary dentition.
The width of the cavosurface bevel on the occlusal margin should be approximately one-fourth the depth of the
respective wall - Increases the strength of the marginal enamel and helps seal and protect the margins.
The diamond instrument also is used to bevel the axiopulpal line angle lightly. Such a bevel provides a thicker and
stronger wax pattern at this critical region.

88. Resistance and Retention forms
of cavity designs for cast metal
onlay
07

89. INITIAL TOOTH PREPARATION
• Cusp reduction
• Vertical walls oriented continually to a single
draw path, usually the long axis of the tooth
crown, so that the completed preparation has
draft (i.e., no undercuts)
• The gingival to-occlusal divergence of
preparation walls may range from 2 to 5
degrees from the line of draw, depending on
their heights.
• 0.5-mm occlusal step contributes to the
retention of the restoration and provides the
wax pattern and cast metal onlay with
additional bulk for rigidity

90. Final Preparation
• Counterbevels on the reduced cusps, gingival
bevels, secondary flares.
• The counterbevel should be of generous width
and should result in 30-degree marginal metal.
• The counterbevel - wide enough so that the
cavosurface margin is beyond (gingival to) any
contact with the opposing dentition.
• Sharp junctions between the counterbevels and
the secondary flares are rounded slightly
• The desirable metal angle at the margins of
onlays is 40 degrees except at the gingivally
directed margins, where the metal angle should
be 30 degrees.
• When properly positioned, the grooves are
entirely in dentin near the DEJ and do not
undermine enamel

91. Teeth grossly weekened by caries
Skirts-
• Thin extensions of the facial or lingual proximal
margins of the cast metal onlay that extend from
the primary flare to a termination just past the
transitional line angle of the tooth.
• Flame shaped fine-grit diamond
Indications
• If the facial or lingual of a proximal box is not
retentive for onlay
• Teeth with split tooth syndrome
• Proximal surface contour and contact are to be
extended more than the normal dimension to
develop a proximal contact.
• When improving the occlusal plane of a mesially
tilted molar by a cusp-capping onlay
• splinting posterior teeth together with onlays
Cons
• Esthetics

92. COLLAR PREPARATION
Indications:
• Weakened tooth for MOD onlay to cap all
cusps
Equipment-
• No. 271 carbide bur -0.8 mm–deep
shoulder (equivalent to the diameter of
the tip end of the bur) around the lingual
(or facial) surface to provide for a collar
about 2 to 3 mm high occlusogingivally
• 30 degree metal margin with fine grit
diamond

93. SLOT PREPARATION
Posterior most tooth. After cusp reduction- very little retention
form. The necessary retention -cutting a distal slot.
Pros
(1)Conservative
(2) the linear extent of marginal outline is less.
Technique-
• No. 169L carbide bur with its long axis parallel to the line of
draw.
• The slot is cut in dentin so that it would pass midway
between the pulp and the DEJ if it were to be extended
gingivally.
The position and direction of the slot thus avert
(1) the exposure of the pulp,
(2) the removal of the dentin supporting the distal enamel,
(3) the perforation of the distal surface of the tooth at the
gingival termination of the slot.
following approximate dimensions:
(1) the width (diameter) of the bur mesiodistally;
(2) 2 mm faciolingually; and
(3) a depth of 2 mm gingival of the normally positioned pulpal
wall.

94. Resistance and Retention forms
for an endodontic post
08

95. Post retention -
Ability of a post to resist vertical forces
Resistance
Ability of the tooth/post combination to
withstand lateral and rotational forces.
It is influenced by the presence of a
ferrule, the post’s length and rigidity, and
the presence of antirotational features.
Tapered NiTi rotary shaping files- very wide tapered and unretentive canal -Significant divergence from apical to
coronal- Longer posts
When teeth are protected by crowns with an adequate ferrule, longer posts do not further increase fracture
resistance. Posts designed with mechanical locking features in the heads and roughened surface texture-better
retention of the core
Active posts Passive posts
Primary retention directly from the root
dentin by the use of threads
Passively placed in close contact to the
dentin walls, and their retention
primarily relies on the luting cement
used for cementation
Potential for VRF during placement. Tapered or parallel
As the post is screwed into place-great
stresses within the root-wedging effect
A parallel post- more retentive but also
requires removal of more root dentin,
Less root fractures
Direct posts

96. Bonding fiber posts to root canal dentin can improve the
distribution of forces applied along the root- decreasing the
risk of root fracture, reinforcement of the remaining tooth
structure.
A well-adapted adhesively cemented fiber post is considered
the most retentive with the least stress generated on the canal
walls.

97. Indirect posts
One advantage of the cast post/core system is that the core
is an integral extension of the post and that the core does
not depend on mechanical means for retention on the
post. Prevents dislodgment of the core from the post
and root when minimal tooth structure remains.
Disadvantages-
• Valuable tooth structure must be removed to create a
path of insertion or withdrawal.
• Fracture of the metal at this interface under function
results in failure of the restoration.
• Most important, the cast post/core system has a higher
clinical rate of root fracture than preformed posts

98. Rotational resistance
• In areas where coronal dentin
has been completely lost, a small
groove placed in the canal can
serve as an anti rotational
element.
• The groove is normally located
where the root is bulkiest,
usually on the lingual aspect.

99. Ferrule
• The coronal tooth structure located
above the gingival level creates a
ferrule.
• Ferrule is formed by the margins and
walls of the crown, encasing atleast 2-
3 mm of sound tooth structure.
• A properly executed ferrule gives
fracture resistance by reinforcing the
tooth at its external surface and
dissipating forces that concentrate at
the narrowest circumference of the
tooth.
Didier D Avishai S: Restoration of the Endodontically Treated Tooth. In Cohen, S. Burns, RC,
editors: Pathways of the Pulp, 11th Edition.

100. • A dental ferrule preparation combines a shoulder
preparation with parallel coronal dentin walls. The tooth
tissue is enclosed by the crown improving the overall
resistance of the restoration.

101. Types of ferrule
• Core ferrule – part of a cast metal core.
For cast post & core a contrabevel is given, collar of metal that encircles the
tooth and serves as a secondary ferrule – independent of crown ferrule .
• Crown ferrule – the ferrule created by the overlying crown engaging
the tooth structure.
The ferrule must encircle a vertical wall of sound tooth structure
above the margin and must not terminate on restorative material.

102. Traditional Cements GI Luting cements Resin based luting cements Self adhesive cements
Compressive strength-100
MPa
Compressive strengths ranging
between 100 and 200 MPa
Compressive strengths around
200 MPa
Elastic moduli-relatively low (4
to 8 GPa), increase when a
dual-curing process is used
Elastic moduli are lower than
that of dentin-5 to 12 GPa
Young modulus is generally
about 5 GPa
Elastic moduli between 4 and
10 GPa
• Self cure- no
pretreatment, clinical
application- a single step
• recom.- all dual-cured resin
cements receive maximal
light to achieve superior
material properties
wherever clinically possible
Zinc phosphate cement-
cementing metal restorations
and posts;.
GI- metallic posts • Pretreatment-hybrid layer
• Resin polymerization
inhibited by oxygen rich
layer formed due to
irrigants like peroxide,
hypo., eugenol from
sealers
Film thickness is less
than 25 µm
Pros-ease of manipulation,
chemical setting, and ability to
bond to both tooth and post.
Dual cure-preferred due to
inaccessibility at apical root
canal.
Light cured ones-less flow,
more contraction stresses
Retention-mechanical means,
no chemical bond to the post
or to dentin but provide
clinically sufficient retention
Mechanically more resistant
than zinc phosphate cements,
and they can bond to dentin
with values ranging between 3
Successful bonding with fiber
posts-shorter posts
A broad spectrum protease
inhibitor-CHX-preservation of
Comparable Adhesion
performance to dentin but
bonding to enamel without
prior phosphoric acid etching

103. • Sturdevant's Art and Science of Operative Dentistry, 6th
edition
• Operative Dentistry: Modern Theory and Practice-Marzouk
• Cohen – Pathways of the pulp, 11th edition
• Shillingburg – Fundamentals of fixed prosthodontics, 4th
edition
REFERENCES

104. Any
Questions?