Fundamentals of Tooth Preparation - 2019

YASSER ALI AL MORTADA AL WASIFI
BDS, MSC, DDS, PHD
C O P Y R I G H T © 2 0 1 9 , A L W A S I F I , Y . A . A L L R I G H T S R E S E R V E D
FUNDAMENTALS OF TOOTH PREPARATION
THE ROAD MAP FOR CAVITY PREPARATION
PRE CLINICAL
O P E R A T I V E
D E N T I S T R Y

F U N D A M E N T A L S O F T O O T H P R E P A R A T I O N : T HE R OAD M AP FOR C AVITY P REPARATION - C OPYRI GHT © 2 0 1 9 , A L W ASIFI , Y . A . 3
E X P E C T E D
O U T C O M E S
By the end of this presentation, you are expected to:
1. List the needs and objectives of tooth preparation
2. List the different concepts of tooth preparation
3. Differentiate between biological and mechanical concepts of tooth
preparation
4. List different steps that achieves both biological and mechanical tooth
preparation concepts
5. Protect the vital tooth structure during tooth preparation
6. Design a prepared cavity suitable for each type of restorative material
7. Assess and evaluate different designs of prepared cavities

C O N T E N T S
PART I
INTRODUCTION 9
NEEDS & OBJECTIVES 14
PRINCIPLES OF TOOTH PREPARATION 17
I. BIOLOGICAL PRINCIPLES 21
I.A. Pulp Protection
1) Against mechanical irritation
2) Against thermal irritation
3) Against chemical irritation
22
23
27
29
I.B. Prevention of Caries Recurrence 31
I.C. Aseptic Working Field 38
Expected Time: 60 Min

C O N T E N T S
PART II
II. MECHANICAL PRINCIPLES 41
II.A. Outline Form
1) Definition
2) Fundamental Rules of Outline Form Design
3) Factors Modifying The Outline Form Design
50
51
53
63
II.B. Resistance & Retention Forms
1) Definitions
2) Factors affecting resistance
3) Resistance forms
4) Types of retention
5) Means of retention
6) Factors affecting selection of retention forms
84
85
88
117
124
127
155

C O N T E N T S
PART III
II.C. Convenience Form
1) Introduction
2) Definition
3) Features
158
159
161
163
II.D. Removal of Remaining Carious Dentin
1) Routine cavity depth
2) Conditions at routine cavity depth
3) Instruments used to remove carious dentin
167
168
171
185

C O N T E N T S
PART IV
II.E. Finishing of Enamel Wall
1) Objectives
2) Requirements of enamel wall
3) Factors affecting inclination of CSA
4) Beveling of enamel wall
5) Instruments used for finishing of enamel wall
189
190
192
195
201
210
I.F. Cleaning & Toileting of the Cavity
1) Definition
2) Objectives
3) Techniques
212
213
215
217

PART I

I N T R O D U C T I O N

INTRODUCTION
Cavity:
• Defect of the calcified tooth structure (Enamel or Enamel & Dentin) due
to pathological lesion without definite shape or extension
Prepared cavity:
• Design or form created into a defective tooth according to bio-
mechanical basics to receive and retain restorative material

INTRODUCTION
• Our traditional, first, oldest and simplest mission is to convert the cavity into
prepared cavity

INTRODUCTION
CAVITY PREPARED CAVITY
AMALGAM
COMPOSITE RESIN
GOLD
GIC
LINER & BASE

INTRODUCTION

NE E D S & OB J E C T I V E S

NEEDS & OBJECTIVES
Teeth need a restorative intervention for a variety of reasons:
1. Repair a tooth after destruction from carious lesion
2. The replacement or repair of restorations with serious defects
3. Esthetic demands of patients is a reason for placing and replacing restorations
4. Restore form and function
5. Restoration of occlusion
6. Repair of fractured tooth
7. Tooth may be restored in a preventive sense

NEEDS & OBJECTIVES
The objectives of tooth preparation are:
1. To remove all defects and give the necessary protection to the pulp
2. Locate the margins of the restoration as conservatively as possible
3. Form the cavity so that the tooth or the restoration will not fracture under forces of
mastication and the restoration will not be displaced
4. Allow for the esthetic and functional placement of the restorative material

PRINCIPLES OF TOOTH
P R E P A R A T I O N

PRINCIPLES OF TOOTH PREPARATION
I. BIOLOGICAL II. MECHANICAL III. ESTHETICAL
Principles of Tooth Preparation Are:

Aims to:
• Preservation of the tooth vitality and
function and protect the supporting
structures
I. BIOLOGICAL
Aims to:
• Providing maximum resistance
against fracture to tooth structure and
restoration, and maximum retention
to restoration against displacement
with maximum conservation of tooth
structure
II. MECHANICAL

Steps:
a) Pulp protection against mechanical, thermal
and chemical irritation during cavity
preparation
b) Prevention of caries recurrence
c) Working in a completely aseptic field
I. BIOLOGICAL
Steps:
a) Obtaining the outline form
b) Performing resistance and retention forms
c) Convenience form
d) Removal of any remaining infected dentin
e) Finishing of enamel and external cavity walls
f) Cleaning and toileting
II. MECHANICAL

BIOLOGICAL PRINCIPLESI.

I. BIOLOGICAL PRINCIPLES
The steps that should be followed to achieve the biological principles of tooth
preparation are:
I.A. PULP PROTECTION
I.B. PREVENTION OF CARIES RECURRENCE
I.C. ASEPTIC WORKING FIELD

I.A. PULP PROTECTION 1. AGAINST MECHANICAL IRRITATION
• Avoid direct traumatic injury of the pulp
• Avoid unnecessary pressure and wrong direction of instruments
• Avoid cutting across the recessional lines of the pulp chamber
• Avoid over cutting of dentin and weakening of the tooth structure
• Avoid sharp line angles within the cavity, which act as stress concentration areas

I.A. PULP PROTECTION 2. AGAINST THERMAL IRRITATION
• Reduce and control heat generation during cavity
preparation to be within the thermal tolerance
zone of dentin
• Avoid working without coolant
• Avoid long time working

I.A. PULP PROTECTION 3. AGAINST CHEMICAL IRRITATION
• Avoid using of chemicals and caustics
for toilet of the cavity
• Avoid using air jet for long time

preparation are:

• Removal of all carious enamel and dentin

• The cavity outline should be extended to include all pits,
fissures and vulnerable areas to caries

• The cavity outline should be extended to include all pits,
fissures and vulnerable areas to caries
• Proper extension of cavity margins to self-cleansable
areas

• Removal of all undermined enamel
• Undermined enamel is that not supported by sound dentin,
which may fragment under force leaving a marginal ditch that
leads to food accumulation and recurrent caries

• Proper inclination of CSA suitable with the type of restoration to provide support to
the restoration and enamel at cavity margins that prevents their fragmentation and
recurrence of caries
CSA 90⁰ CSA < 90⁰
Fracture of Enamel
CSA >90⁰
Fracture of Amalgam

• Avoiding any sharp line angle

preparation are:

• Use sterile instruments
• Use disposable tools as much as possible
• Application of rubber dam to keep the fields clean and dry, and decrease the
chance of droplet infection
• Follow the instructions of ADA and WHO to control infection in dental office

PART II

MECHANICAL PRINCIPLESII.

II. MECHANICAL PRINCIPLES

G.V. Black, in 1891, was the first who described the
mechanical steps of cavity preparation using the principles
of mechanical engineering

F U N D A M E N T A L S O F T O O T H P R E P A R A T I O N : T HE R OAD M AP FOR C AVITY P REPARATION - C OPYRI GHT © 2 0 1 9 , A L W ASIFI , Y . A .
• Mechanical principles aim to provide:
1. Maximum resistance against fracture to tooth structure and restoration,
2. Maximum retention to restoration against displacement
3. With maximum conservation of tooth structure
45

• According to G.V. Black, the steps that should be followed to achieve the
mechanical principles of tooth preparation are:
II.A. Obtaining the Outline Form
II.B. Performing Resistance & Retention Forms
II.E. Finishing of Enamel & External Cavity Walls
II.F. Cleaning & Toileting of the Cavity
46

• In cases showing extensive carious lesion, removal of remaining carious dentin
should follow immediately the outline form:
47

48

49

50

II.A. OBTAINING THE OUTLINE FORM

II.A.1. DEFINITION

II.A.2. FUNDAMENTAL RULES OF OUTLINE FORM DESIGN

T A K E A N O T E
• With the development of adhesive restorative dentistry and the
increased popularity of bonded restorations as composite resin, no
need to open the weak ridge and connect the cavities approaching
each others
• This is because of the use of dentin adhesives provide the following
advantages:
1. Cross tying weak tooth structure with the strong one
2. Transfer stresses between all components of the restorative
system including restoration and remaining tooth structure
3. Reinforcement of weak tooth structure

T A K E A N O T E
• Trying to find excuse, many authors explained the mentioned disaster
of extension for prevention, as:
1. At the era or G.V. Black, there isn’t any diagnostic tool that could
discover early lesions, only extensive lesions could be diagnosed
2. Patients hadn’t enough knowledge about oral hygiene and and
how to practice home prophylactic measures
3. The burs manufacturers could not produce small sized burs like
that available nowadays
• Currently, involving pits, fissures and grooves within the prepared
cavity is considered as extension for prevention

T A K E A N O T E
• Extension of the cavity outline may be due to :
1. Extension for prevention (Not currently valid)

II.A.3. FACTORS MODIFYING THE OUTLINE FORM DESIGN
• Many factors may modify the design of outline form of prepared cavity, which
include:
1. Patient age
2. Oral hygiene
3. Physical properties of the selected
restorative material
4. Technique of construction of restoration
5. Esthetic needs
6. Convenience needs
7. Presence of weak cusp
8. Dimensions of the contact area

1. PATIENT AGE:
• Young aged patient shows increased liability for caries
recurrence due to prominent anatomical pits and
fissures
• So, the outline should be well extended to insure all
carious lesion and retentive area
• At the same time conservative due to increased liability
to remake the cavity as a result of caries recurrence

1. PATIENT AGE:
• Old aged patient shows decreased liability for caries
recurrence due to ill defined anatomical pits and fissures
as a result of attrition
• So, the outline should be limited to the carious lesion
only

2. ORAL HYGIENE:
• Patients with good oral hygiene, ultra conservative outline form could be
performed
• While in cases with bad oral hygiene, all caries susceptible areas must be included
in the cavity outline and the cavity should be extended to area self-cleansable

3. PHYSICAL PROPERTIES OF THE SELECTED RESTORATIVE MATERIAL:
• Brittle materials require depth bulk with minimal width to decrease the surface of
restoration exposed to occlusal force and to increase their strength
• So, the outline width should be minimal as much as possible
• In case of ductile materials, no bulk depth or width specifications are needed

4. TECHNIQUE OF CONSTRUCTION OF RESTORATION:
• Indirectly constructed restorations, such as cast gold restoration or esthetic inlays,
require further widening of the outline to allow:
a) Easy and accurate manipulation of impression, wax pattern
b) Casting of the metal

5. ESTHETIC NEEDS:
• In anterior teeth ultra conservative outline form should be performed in order to
preserve the natural esthetic appearance of the tooth structure as much as possible
• So, undermined labial enamel wall of Class III cavities could be left to preserve
the natural tooth appearance
• Gingival wall of Class V cavities should be parallel to the gingival contours and
Incisal wall should follow a graceful curve to provide optical illusion

5. ESTHETIC
NEEDS
F A C T O R S
MO D I F Y I N G T H E
O U T L I N E F O R M
D E S I G N
Undermined labial enamel left
for the purpose of esthetic
Cervical wall parallel to the gingival
margin
Graceful curve of the incisal wall

6. CONVENIENCE NEEDS:
• Areas of inaccessibility do hinder proper instrumentation and restoration.
• So, slight extension of the cavity outline is indicated
• Cutting occlusal portion to access proximal caries in Class II cavity preparation,
although it may be not invaded or undermined by caries, is considered as extension
for convenience
• The same when cutting a palatal portion to to access proximal caries in Class III
cavity preparation

6. CONVENIENCE
NEEDS
F A C T O R S
D E S I G N

T A K E A N O T E
2. Extension for convenience (currently limited)

7. PRESENCE OF WEAK CUSP:
• Weak cusp is that with its base measures less than its
height

7. PRESENCE OF WEAK CUSP:
• Weak cusp could be protected by reducing its height which increases the outline
form extension
• This differs according to the type of final restoration
1. AMALGAM 2. CAST GOLD

6. PRESENCE OF
WEAK CUSP
F A C T O R S
D E S I G N

T A K E A N O T E
• With the development of adhesive restorative dentistry and the
increased popularity of bonded restorations as composite resin, no
need to open the weak ridge and connect the cavities approaching
each others
• This is because of the use of dentin adhesives provide the following
advantages:
1. Cross tying weak tooth structure with the strong one
2. Transfer stresses between all components of the restorative
system including restoration and remaining tooth structure
3. Reinforcement of weak tooth structure

8. DIMENSIONS OF THE CONTACT AREA:
• The bucco-lingual width of the contact area determine the isthmus outline of
compound Class II cavities to ensure the cavity outline is located in area self-
cleansable, i.e. the embrasures
• The isthmus outline will follow one of the Ingerham¢s lines according to the width of
the contact area

6. DIMENSIONS
OF THE
CONTACT AREA
F A C T O R S
D E S I G N
Small Contact
Straight outline
Moderate Contact
Uniform outline
Broad Contact
Reverse Curve outline

6. DIMENSIONS
OF THE
CONTACT AREA
F A C T O R S
D E S I G N
Reverse Curve
Straight
Uniform
Uniform

• The reverse curve outline is always required in the buccal wall rather than the
lingual as the contact area is always shifted toward the buccal rather than lingual
• The reverse curve outline could only be performed in the lingual wall when the
caries extension is far lingual at the isthmus portion
• The reverse curve outline provides the adjusted required CSA at 90º and also
conserves much more tooth structure

• Also, the Occluso– Gingival height of the contact
area affects the position of the gingival floor of
compound proximal cavities
• It always necessitates to extend the cavity with a
proximal step to get the gingival floor at the
gingival embrasure and without endangering the
pulp

84

II.B. PERFORMING RESISTANCE & RETENTION FORMS

II.B.1. DEFINITIONS
• Resistance form is defined as that form given to prepared cavity to prevent fracture
of remaining tooth structure or/and the restoration
• Retention form is defined as that form given to prepared cavity to prevent
displacement or dislodgement of the restoration out of the prepared cavity
• Resistance and retention forms are two distinct but yet, inseparable and interrelated
steps
86

T A K E A N O T E
• Resistance and retention forms are two faces of the same coin:
1. Both of them constitute a cavity design that counteract the
destructive effect of the occlusal loading force
2. Each one of the two forms represents a major factor affecting the
other form design
3. Certain cavity feature may be designed mainly for resistance but
also provides retention and vise versa

II.B.2. FACTORS AFFECTING RESISTANCE
B) CAVITY DESIGNA) OCCLUSAL LOADING FORCE C) PHYSICAL PROPERTIES OF
RESTORATIVE MATERIAL
89

A) OCCLUSAL LOADING FORCE:
1. Magnitude:
• Differs from patient to another according to:
1. The action of the mastication muscles
2. The type of occlusion and inter-cuspation
3. The type of food
4. Age and sex
5. Varies from location to another in the same patient
6. Also, from time to another
90

2. Direction:
• It may be directed as compressive, tensile or shear and this depends on the form
and shape of the loaded surface as well as the inclination of the cavity walls at which
the stresses transmitted will be analyzed
91

3. Character:
• It may be:
- Static force in centric occlusion
- Dynamic in eccentric lateral movement
- Cyclic and repetitive during masticatory function
92

C O N C L U S I O N
• Cyclic force with different magnitudes and directions are present
during mastication producing fatigue of the restorative material and
enhance its fracture
• So, cavity preparation should be prepared to minimize the effect of
force on both remining tooth structure and restoration to be within their
tolerance

B) CAVITY DESIGN:
1. Walls & Floors:
• Direction, should be either parallel or perpendicular to the long axis of the tooth to
decrease the analysis of force into destructive tensile components
• Texture, Should be flat and smooth to avoid stress concentration and to provide
equal distribution of occlusal stresses
94

1. WALLS & FLOORS

B) CAVITY DESIGN:
2. Width & Depth:
• Both of them are responsible to provide bulk to the restoration
• Increasing the bulk of the restoration especially brittle ones, increase the strength of
the material and its resistance to fracture
• It is better to provide bulk to the restoration through depth (within limits to avoid
pulpal irritation) rather than width
98

B) CAVITY DESIGN:
2. Width & Depth:
• Increasing the width will:
- Weaken the remaining tooth structure
- Weaken the restoration as it leads to increased surface area of the restoration exposed to occlusal
force
• It is recommended to get a cavity depth of 0.5 – 1mm beyond the DEJ and a cavity
width of 1/4 - 1/3 the inter-cuspal distance
99

2. WIDTH & DEPTH

B) CAVITY DESIGN:
3. Conservation:
• Maximum conservation of remaining sound tooth structure as much as possible is
recommended to avoid its fracture
101

B) CAVITY DESIGN:
4. Line Angles:
• All axial line angles of the prepared cavity should be
rounded in the form of sweeping curves to avoid stress
concentration
102

B) CAVITY DESIGN:
4. Line Angles:
• For compound cavities, rounding or beveling of the axio-
pulpal line angle is recommended as it leads to:
- Decreased stress concentration
- Adding bulk to the restoration
103

4. L I N E A N G L E S
Sharp axio-pulpal line angle Beveled axio-pulpal line angle

4. L I N E A N G L E S
Rounded axio-
pulpal line angle
Beveled axio-
pulpal line angle
Rounded axio-
pulpal line angle

B) CAVITY DESIGN:
5. Cavo Surface Angle:
• It should have a correct angulation suitable with:
- The physical properties of the restorative material
- The direction of enamel rods
• For brittle materials such as amalgam CSA should have 90○ angles to:
- Get the strongest enamel wall
- Provide strength to amalgam at margins
• Any deviation from correct angulation may lead to fracture or tooth structure or
restoration at margin
106

5. CAVO SURFACE
A N G L E
CSA 90⁰ CSA < 90⁰
Fracture of Enamel
CSA >90⁰
Fracture of Amalgam

B) CAVITY DESIGN:
5. Cavo Surface Angle:
• For ductile materials such as gold:
• It should be 135○ to allow burnishing of the gold over the
enamel margin that provides protection
108

B) CAVITY DESIGN:
6. Amount of Retention:
• Adequate amount of retention for each part of the cavity increases the stability of
the restoration under stresses
• In compound or complex prepared cavities, each part should have its own ample
independent retention to avoid fatigue and fracture of the restoration at the isthmus
area
109

6. A M O U N T O F
R E T E N T I O N

B) CAVITY DESIGN:
7. Weak Cusp:
• Weak cusp is that with its base measures less than its
height
112

B) CAVITY DESIGN:
7. Weak Cusp:
• Weak cusp could be protected by reducing its height which increases the outline
form extension
• This differs according to the type of final restoration
113

1. AMALGAM 2. CAST GOLD 7. W E A K C U S P

C) PHYSICAL PROPERTIES OF RESTORATIVE MATERIAL:
1. Brittle Materials:
• The tensile type stresses are the significant stresses for the brittle substances as
amalgam, cements and porcelain because they have high compressive but very low
tensile and shear strength values
• These substances are especially sensitive to tensile stresses, i.e. they cannot
withstand high tensile stresses without fracture and cannot be finished to thin
margins otherwise ditching will occur
115

C) PHYSICAL PROPERTIES OF RESTORATIVE MATERIAL:
2. Ductile Materials:
• The strong and ductile gold is utilized for protection of the weakened tooth
structure
116

II.B.3. RESISTANCE FORMS
118

ADVANTAGES OF BOX-SHAPED PREPARED CAVITY:
1. The seat of the restoration (pulpal and gingival wall) is placed at a distinct right
angle to the direct of functional stresses
2. The tendency to split the buccal and lingual cusps of bicuspids and molars by
forces transmitted through the restoration is greatly diminished since the inverted
truncated cone shape prevents the wedging action of the restoration inside the
tooth
119

AD V A N T A G E S O F
B O X - S H A P E D
PREPARED CAVITY

AD V A N T A G E S O F
B O X - S H A P E D
PREPARED CAVITY
Dentin
Ledge
Dentin
Bridge

T A K E A N O T E
• No need to cut dentin ledge to add stability when using bonded
restoration such as composite resin, as the used dentin bonding agent
will provide the needed stability
• But the bond strength should be considered and should be more than
the limits of displacing forces

ADVANTAGES OF BOX-SHAPED PREPARED CAVITY:
3. It provides retention by friction due to relative parallelism of the axial walls
4. It allows access to and easy visualization of the interior of the cavity, which allows
for an easier and better instrumentation and filling
5. Restorative materials tend to adapt better against its plane surfaces
6. It allows the employment of retention features in dentin
123

II.B.4. TYPES OF RETENTION
According to the direction of displacing forces:
1. AXIAL RETENTION 2. LATERAL RETENTION
125

T A K E A N O T E
• Displacement; Dislocation, movement or instability of the restoration
within the prepared cavity
• Dislodgment; The restoration get out of the prepared cavity

II.B.5. MEANS OF RETENTION
Retention could be gained through:
1. Mechanical
undercuts
2. Frictional wall
retention
3. Gripping action of
dentin
4. Pin retention
1. Dove tail lock
2. Occlusal lock
3. Buccal and lingual
extensions
4. Extension for
retention
B) BY MODIFYING
THE CAVITY
OUTLINE
1. Proximal axial
grooves
2. Grooves in dentin
line angles
3. Reverse gingival
bevel or gingival
lock
C) BY MODIFYING
THE CAVITY
DESIGN
1. Dentin adhesive
systems for
composite resin
2. Dowel pin or post
retention
D) SPECIAL
FORMS
A) BY UTILIZING
DENTIN
128

A) BY UTILIZING DENTIN:
1. Mechanical Undercuts: through converging the cavity walls occlusally
2. Frictional wall retention: between parallel opposing cavity walls
3. Gripping action of dentin: due to visco-elasticity of vital dentin, stress relaxation of
dentin will grip the restoration. This is applicable only with gold foil (Direct gold)
restoration as it requires a hammering condensation force
4. Pin retention
129

M E A N S O F
R E T E N T I O N
A) BY UTILIZING
D E N T I N
Mechanical
Undercuts
Frictional Wall
Retention
Pin Retention

M E A N S O F
R E T E N T I O N
A) BY UTILIZING
D E N T I N
Gripping Action of Dentin

1. Mechanical
undercuts
2. Frictional wall
retention
dentin
4. Pin retention
1. Dove tail lock
2. Occlusal lock
extensions
4. Extension for
retention
B) BY MODIFYING
THE CAVITY
OUTLINE
1. Proximal axial
grooves
line angles
3. Reverse gingival
bevel or gingival
lock
C) BY MODIFYING
THE CAVITY
DESIGN
1. Dentin adhesive
systems for
composite resin
retention
D) SPECIAL
FORMS
A) BY UTILIZING
DENTIN
132

B) BY MODIFYING THE CAVITY OUTLINE:
1. Dove tail lock: in compound proximal cavities
2. Occlusal lock: in compound proximal cavities
3. Buccal and lingual extensions: to prevent proximal displacement
4. Extension for retention: performed by extension to the other side of the cavity
133

B) BY MODIFYING THE CAVITY OUTLINE:
• Extending the cavity outline to another surface to gain retention should be with step
to create the desired retention
• Extending the cavity outline to another surface to involve the remaining carious
dentin could be with or without step according to the level and extension of caries in
relation to the pulpal floor
• Creating a step in such a condition is to involve the carious lesion without
endangering the pulp
134

M E A N S O F
R E T E N T I O N
B) BY MODIFYING
T H E C A V I T Y
O U T L I N E

T A K E A N O T E
• For Premolars; Dovetail lock is considered as extension for retention
• For Molars; Dovetail lock is considered as extension for prevention that
provides retention

T A K E A N O T E
2. Extension for convenience (currently limited)
3. Extension for retention (currently limited)

1. Mechanical
undercuts
2. Frictional wall
retention
dentin
4. Pin retention
1. Dove tail lock
2. Occlusal lock
extensions
4. Extension for
retention
B) BY MODIFYING
THE CAVITY
OUTLINE
1. Proximal axial
grooves
line angles
3. Reverse gingival
bevel or gingival
lock
C) BY MODIFYING
THE CAVITY
DESIGN
1. Dentin adhesive
systems for
composite resin
retention
D) SPECIAL
FORMS
A) BY UTILIZING
DENTIN
140

C) BY MODIFYING THE CAVITY DESIGN:
1. Proximal axial grooves:
• Cut along the axio-buccal and axio-lingual line angles in expense of buccal and
lingual walls rather than axial wall in order to:
- Provide the required retention
- Be away from the pulp chamber
141

M E A N S O F
R E T E N T I O N
C) BY MODIFYING
T H E C A V I T Y
D E S I G N
Proximal Axial Grooves

• For amalgam cavities; it only extends from the axio-gingival line angle up to the
axio-buccal line angle
• For cast gold cavities; it extends from the axio-gingival line angle up to the CSA
143

M E A N S O F
R E T E N T I O N
C) BY MODIFYING
T H E C A V I T Y
D E S I G N
Proximal Axial Grooves

• They provides:
- Lateral retention by their position toward the buccal and lingual rather than axial
- Axial retention by their roof in case of amalgam as they are extended only up to the axio-
pulpal line angle and in case of cast gold, they increase the surface area of walls available to
provide frictional parallelism
145

2. Grooves in dentin line angles:
146

3. Reverse gingival bevel:
• For compound Class II cavity for gold inlay, slanting of the gingival floor to be in
the same direction of the gingival enamel rods creates unfavorable slop
• This slop tends to displace the restoration as it results in analysis of force in an
outward direction and the restoration will rotate around the axio-pulpal line angle
147

M E A N S O F
R E T E N T I O N
C) BY MODIFYING
T H E C A V I T Y
D E S I G N
Reverse Gingival Bevel

• Cutting a reverse gingival bevel in dentin of the gingival floor will create a favorable
slop that provides:
1. Favorable analysis of force in an inward direction
2. Stability of restoration as it prevents its rotation around the axio-pulpal line angle
3. Lateral retention by its design
4. Axial retention as it increases the length of axial wall
149

• It could not be used with amalgam cavities as it will lead to:
1. Stress concentration
2. Mercury accumulation at axio-gingival line angle
• All resulting in excessive weakening of amalgam
150

1. Mechanical
undercuts
2. Frictional wall
retention
dentin
4. Pin retention
1. Dove tail lock
2. Occlusal lock
extensions
4. Extension for
retention
B) BY MODIFYING
THE CAVITY
OUTLINE
1. Proximal axial
grooves
line angles
3. Reverse gingival
bevel or gingival
lock
C) BY MODIFYING
THE CAVITY
DESIGN
1. Dentin adhesive
systems for
composite resin
retention
D) SPECIAL
FORMS
A) BY UTILIZING
DENTIN
151

D) SPECIAL FORMS:
1. Dentin Adhesive Systems: for composite resin restoration
2. Dowel pin retention: i.e. post inside the root canal
152

M E A N S O F
R E T E N T I O N
D) SPECIAL FORMS
Dentin Adhesive System

Dowel Pin Retention
M E A N S O F
R E T E N T I O N
D) SPECIAL FORMS

II.B.6. FACTORS AFFECTING SELECTION OF RETENTION FORMS
1. Type of restorative material
2. Available amount of remaining tooth structure
3. Esthetic demands
4. Amount of retention needed
5. Pulp vitality; for non vital pulp will not provide gripping action of dentin due to
excessive dehydration
6. Type of occlusion; abnormal occlusion increases the magnitude of stresses
156

PART III

158

II.C. CONVENIENCE FORM

II.C.1. INTRODUCTION
Which of these cavities is easily seen, reached, instrumented, restored and
conservative?!!!
160

II.C.2. DEFINITION
• Convenience for is defined as the shape or form given to the prepared cavity to
make it easily:
1. Seen
2. Reached
3. Instrumented
4. Restored
162

II.C.3. FEATURES
1. Accentuation of point and line angles
2. Slight extension of cavity outline to facilitate insertion and condensation of the
restorative material
3. Roundation of axial line angles
4. Beveling of enamel wall with gold restoration to provide room for burnishing the
gold to protect marginal enamel
5. It is also performed with composite resin cavity preparation to increase surface area
for acid etching
6. Selection of smaller specially designed instruments that enable the operator to
prepare surfaces which are difficult to reach
7. Extension for convenience (limited conditions)
164

II.C.3. FEATURES
165

C O N V E N I E N C E
F E A T U R E S

167

II.D. REMOVAL OF REMAINING CARIOUS DENTIN

II.D.1. ROUTINE CAVITY DEPTH
• It is the process of removing remaining decayed
and decalcified enamel and dentin not involved in
the outline form of the cavity; and determining the
prepared cavity depth
• The routine cavity depth should be 0.5 – 1 mm
beyond the DEJ
169

II.D.1. ROUTINE CAVITY DEPTH
1. Avoid cutting at this sensitive area; i.e. DEJ
2. To get sure that there is no undermined enamel
resulted from the lateral spread of caries at DEJ
3. To detect lateral spread of caries at DEJ
4. To provide sufficient bulk of the restoration
5. To add retentive features
170

II.D.2. CONDITIONS AT ROUTINE CAVITY DEPTH
172
1. Hard Sound
Viable Dentin
2. Hard
Discolored
Dentin
3. Soft Dentin

1. HARD SOUND VIABLE DENTIN:
• Considered as the best condition
• Finish enamel walls and apply either varnish in case of
amalgam restoration or start bonding procedures in
case of composite resin restoration
173

2. HARD DISCOLORED DENTIN:
• Considered as sound dentin but the discoloration is
due to the chromogenicity of microorganisms
• In posterior teeth, it could be left and the final
restoration could be placed
• In anterior teeth, it must be removed as it appears
from enamel affecting esthetic or use composite resin
kit with opaquer
174

3. SOFT DENTIN:
• Which is painful, denoting presence of viable
protoplasmic processes, and may be discolored
(chronic caries) or not (acute caries)
• This layer constitutes the floor of moderately deep and
deep cavities
175

3. SOFT DENTIN:
• Such soft dentin must be removed since it is carious
and if left will extend to involve the pulp
• Removal of such carious dentin should be in a
selective pattern parallel to the pulpal recessional lines
• Dentin ledge should be created to provide stability of
restoration
176

3. SOFT DENTIN:
• If still, continue selective removal parallel to the pulpal
recessional lines for about 1.0 mm more in depth
• Keep or recreate the dentin ledge
• The cavity will be considered as moderately deep
cavity that needs a base of cement material
177

3. SOFT DENTIN:
• If still, continue selective removal parallel to the pulpal
recessional lines for about 1.0 mm more in depth
• Keep or recreate the dentin ledge
• The cavity will be considered as deep cavity that
needs a sub-base calcium hydroxide liner and a
base of cement material
178

T A K E A N O T E
• Cement base; provides thermal insulation to the pulp under metallic
restorations and could withstand the condensation force, but in deep
cavities it may be chemical irritant to the pulp due to its acidic nature
• So, in deep cavities, alkaline sub-base or liner material should be used
under the cement base to neutralize its acidity
• Calcium hydroxide sub-base or liner; alkaline material that chemically
protect the pulp against the acidity of cement base but its strength is
weak and could not withstand the condensation force of direct metallic
restorations as amalgam
• So, it couldn’t be used under amalgam restoration without a cement
base in-between
• It could be used under non-condensable direct restoration such as resin
composite without cement base in-between

3. SOFT DENTIN:
What if, the dentin still soft and carious and the shadow of the pulp showed off?!!
180

3. SOFT DENTIN:
• If there is still soft carious dentin at the floor of the cavity, it must be evaluated, either
acute or chronic caries
• If it is an acute caries, it could be left and sub-base and base are placed before
placing the final restoration
• Although it is a rapidly progressing caries but the acid penetration always preceding
the bacterial invasion, that is why the last layer could be left as it is only affected by
acid not infected by micro-organsims
• Apply calcium hydroxide as indirect pulp capping, as the last layer is sterile followed
by cement base
181

C O N D I T I O N S
A T R O U T I N E
CAVITY DEPTH
3. SOFT DENTIN
Last affected dentin layer

183
3. SOFT DENTIN:
• If it is a chronic caries, it should be removed even pulp exposure occurs, as the last
later is infected
• As being a long lasting slow caries progression, the acid penetration coincides with
the bacterial invasion, that is why the last layer couldn’t be left as it is both affected
by acid and infected by micro-organsims
• Although of the last layer is infected but the long time slow progress allows the pulp
to lay down a protective layer of dentin and a hard layer of sclerotic dentin could be
reached

C O N D I T I O N S
A T R O U T I N E
CAVITY DEPTH
3. SOFT DENTIN
Sclerotic dentin layer

1. HAND CUTTING INSTRUMENTS:
• Excavators with different shapes and sizes could be
used in a direction parallel to the pulp horns from the
cavity periphery to the center with scooping motion
• It provides no heat generation
2. ROTARY CUTTING INSTRUMENTS:
• Large round bur with low speed and without pressure
• Cutting should be performed under sufficient coolant
of water / air spray
II.D.3. INSTRUMENTS USED TO REMOVE CARIOUS DENTIN

T A K E A N O T E
• Always excavators are better to be used rather than round bur in
removing soft carious dentin as
1. Cutting is only limited to soft carious dentin without involving hard
sound dentin
2. Being a hand cutting instrument, there will be no heat generation So,
it couldn’t be used under amalgam restoration without a cement
base in-between

PART IV

II.E. FINISHING OF ENAMEL & EXTERNAL CAVITY WALLS

II.E.1. OBJECTIVES

II.E.2. REQUIREMENTS OF ENAMEL WALL

II.E.3. FACTORS AFFECTING INCLINATION OF CSA

F A C T O R S
A F F E C T I N G
INCLINATION
O F C S A
Direction of Enamel Rods

F A C T O R S
A F F E C T I N G
INCLINATION
O F C S A
Location of the Cavity Wall

T A K E A N O T E
• Undermined enamel is only left in the labial wall of class III because of:
1. Esthetic needs
2. It is subjected to minimal force produced from lip musculature
3. The destructive force is away from it as it is in a palato-labial
direction

II.E.4. BEVELING OF ENAMEL WALL
DEFINITION:
• Increasing CSA inclination more than 90°
TYPES:
1. Short bevel involving part of enamel thickness
2. Long bevel involving the full enamel thickness up to DEJ
3. Full bevel including enamel and dentin up to the pulpal floor
4. Counter bevel made against enamel rods of the cusp to make inlay with cusp
coverage, i.e. in-onlay

BEVELING OF
ENAMEL WALL
Short Bevel

BEVELING OF
ENAMEL WALL
Long Bevel

BEVELING OF
ENAMEL WALL
Full Bevel

BEVELING OF
ENAMEL WALL
Counter Bevel

BEVELING OF
ENAMEL WALL
Reverse Gingival Bevel

II.E.4. BEVELING OF ENAMEL WALL
FUNCTIONS:
1. Protection of weak enamel rods
2. Facilitate burnishing of ductile materials
3. Protection of cement line from solubility
4. Protection of weak cusps → counter bevel
5. Making the enamel wall parallel to the direction of enamel rods
6. Increasing surface area for acid etching needed for retention of composite
restoration
7. Better esthetic and decreased demarcation of composite restoration.
8. Add retention → reverse gingival bevel

T A K E A N O T E
• Only one bevel made in dentin, which is the reverse gingival bevel for
retention and stability of Class II gold inlay restorations

II.E.5. INSTRUMENTS USED FOR FINISHING OF ENAMEL WALL
1. HAND CUTTING INSTRUMENTS:
• Chisel and hatchets are the instruments of choice with no heat generation
2. ROTARY CUTTING INSTRUMENTS:
• Fissure bur – CUTTING NOT FINISHING

II.F. CLEANING & TOILETING OF THE CAVITY

II.F.1. DEFINITION
• The process of removing all debris from the prepared cavity,
e.g. cut dentin chips, blood, saliva and bacteria

II.F.2. OBJECTIVES
1. Increasing adaptation of the restoration to cavity walls
2. Prevents contamination of the restorative material
3. Enables the operator to examine properly all steps
4. The cavity should be clean and dry before insertion of the filling material

II.F.3. TECHNIQUES
1. Phenol:
• Leads to pulp necrosis
2. Silver Nitrate:
• Leads to:
a) Discoloration of tooth structure
b) Irritation of the pulp
c) Tarnish of amalgam
3. Alcohol:
• Leads to:
a) Dehydration of dentin
b) Irritation of the pulp
4. Hot Air Blast:
• Leads to dentin dehydration

II.F.3. TECHNIQUES
5. 3% Hydrogen Peroxide:
• Highly effective through its
effervescent action
• Should be washed immediately with
warm water to avoid thermal pulp
shock
6. Water Spray:
• The best as it is not a medicament
• Cotton pellet to dry the cavity to
reduce the use of air stream
• Removal of remaining water by air for
short time

C L E A N I N G &
TOILETING OF
T H E C A V I T Y

S U G G E S T E D
R E A D I N G S

@YWasifi
https://sa.linkedin.com/in/
yasseralwasifi
yasseramw@yahoo.com
yasseramw67@gmail.com
https://www.slideshare.net
/yasseramw
Yasser Al Wasifi

Fundamentals of Tooth Preparation - 2019

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