FIXED PROSTHODONTICS

FPROSD 211 PROF. DR. ATEF FATHY

FACULTY OF DENTISTRY

MTI UNIVERSITY

Contents

Chapter 1: Terminology and classification ………………………….... 3

Chapter 2: Dental instruments…………………………………………28

Chapter 3: Principles of tooth preparation…………………………......48

Chapter 4: Finish line …………………………………………………65

Chapter 5: Complete metal crown………………………………………78

Evaluation sheet………………………….……………………..………………...84

Examination Chart…………………………………………………………………………..85

3

Chapter 1

Terminology and Classifications

Dentistry, dental medicine, oral medicine:

Dentistry is a branch of medicine that consists of the

study, diagnosis, prevention, and treatment of diseases, and

conditions of the oral cavity.

It is also defined as the science dealing with the prevention and

treatment of diseases and malformations of the teeth, gums, and oral

cavity, and the correction, and replacement of damaged, or lost parts.

Prosthesis:

Any artificial replacement of a missing or absent part of the human body.

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Prosthodontics:

The branch of dentistry concerned with the restoration of oral function,

comfort, appearance and health of the patient by restoring damaged

teeth and replacing missing teeth with artificial substitutes.

Teeth restored with metallic filling Missed tooth restored with a bridge

A Crown restoring destructed tooth

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Removable prosthodontics:

It is the branch of prosthodontics concerned with replacement of missing

teeth by removable artificial substitutes.

Complete denture Partial denture

Fixed prosthodontics:

The art and science of dentistry concerned with the replacement of

missing teeth by artificial substitutes that are attached (fixed) to natural

teeth or implants.

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Dental implant

It is a surgical component that combines with the bone of the jaw or skull to support a dental prosthesis such as a crown, bridge or denture.

A bridge supported by two implants. Denture can be supported by implants

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Restoration:

A broad term applied to any material or prosthesis that restores or replaces lost tooth structure, teeth or oral tissues.

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Crowns

Artificial restoration covering the coronal portion of the tooth to restore its anatomy, function and esthetics.

Indications of a crown: 1. Abnormalities in tooth structure

2. Abnormalities in tooth shape and / or position

3. Tooth wear

4. Cases which can’t be resorted by any other simple means (filling)

5. Fractured tooth.

(1) Enamel defects

Destructed tooth prepared tooth artificial crown final crown cemented

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(2) malformed teeth

(3) teeth wear

(4) badly decayed molar (5) fractured tooth

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Jacket Crown( all-ceramic crown):

• It is a non-metallic crown covering the coronal portion of anterior teeth to restore anatomy, function& esthetic.

• It is made of porcelain or acrylic.

Post Crown:

An artificial restoration that covers the whole coronal portion of the tooth and is retained in position mainly by a post inserted into the prepared root canal.

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Full Metal Crown:

It is a metallic crown covering the coronal portion of posterior teeth to restore anatomy & function.

Full Veneered Metal Crown:

It is a full metallic crown with all of its surfaces covered with porcelain.

Veneered Crown:

It is a full metallic crown with its facial surface covered with porcelain.

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Three quarter Crown:

It is a special type of crowns (partial coverage) in which the labial or buccal surface remains unprepared.

Another definition, it is a partial coverage crown covering all the

surfaces of tooth leaving the facial (labial /buccal) surface, unprepared

for conservative and esthetic reasons.

Reversed Three quarter Crown:

It is a modified form of the 3/4 crown, in which the Lingual

surface is left unprepared. (Restore all surfaces except the lingual

surface)

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One half Crown:

It is a modified form of the 3/4 crown, where half the tooth is

prepared and the other half is left unprepared.

The prepared surfaces involve one proximal surface, the

adjacent halves of the buccal and lingual surfaces and most of the

occlusal surface are covered, while the other half is left unprepared.

Mac Boyle Retainer:

It is a modified ‘3/4 crown’, with its retention derived from

grooves at the labio-proximal line angles.

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Seven Eighth crown:

It is a partial coverage crown with all surfaces prepared except the mesio-buccal cusp of upper molars (Restore all surfaces except the mesio- buccal cusp of maxillary molars).

Pin ledge Retainer:

Is a modified type of an anterior three quarter crown, with 3 pinholes prepared at the palatal surface.

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Laminate veneers

It is a thin restoration of cosmetic material (porcelain or composite) that restore the facial surface of the tooth for esthetic purposes.

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Classification of crowns

According to the material:

1. Metal.: e.g. full metal crown

2. Non-Metal: e.g. porcelain jacket crown

3. Combination: e.g. veneered crown

According to the number of covered surfaces:

I-Full coverage. II- Partial coverage.

I-Full coverage crowns

Classified according to mode of retention into:

A) Retained by complete encircling of the

preparation.

B) Retained by Post in the root canal.

A) Retained by complete encircling of the preparation:

1. Metallic crown: like full metal crown

2. Non-metallic crown:

-Porcelain jacket crown

-Acrylic jacket crown.

3. Combined:

-Veneered crown with acrylic or porcelain facing

-Full veneered crown porcelain.

B) Retained by post in the root canal (post crown):

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Components of the post crown system:

A. Post: inserted in the root canal. (Metallic or nonmetallic)

B. Core: rebuilding the lost coronal tooth structure

C. Extra-coronal restoration: crown covering the core

Crowns of this category are classified into two types:

1. Attached (one piece): where the post, core, and crown are fabricated as

one unit. example: post jacket crown

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Detached:

The detached system is classified into two types:

a) Two-piece system: where the post and core are fabricated as one

unit and later the crown is cemented over it.

b) Three-piece system: where the post is cemented at first in the root

canal, then the core is built up with filling material, and finally the

crown is cemented over it.

II- Partial coverage:

This category of crowns does not cover the whole coronal part, i.e.

leaving one surface or more unprepared for conservative reasons.

Types of partial coverage crowns:

1. Crowns retained by Grooves: examples:

▪ Three quarter crown: (mesial & distal grooves).

▪ Half-crown: (buccal and lingual grooves).

▪ Seven eighth crown: (buccal and mesial grooves) used in

maxillary first molar.

N.B: grooves are special preparations made during tooth

reduction (preparation) to increase the retention of the crown.

2. Crowns retained by Pins & Ledges: example: Pinledge retainer 3. Crowns retained by Combined means of retention: example: Modified Three quarter crown (made for anterior teeth where grooves are not sufficient for retention, so pinledge preparation is added to increase retention).

Preparation for three quarter crown with proximal groove

Groove

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Bridges

I t is artificial restoration restoring an edentulous space (missing one or

multiple teeth ) in the dental arch and attached at one or both ends to the

adjacent teeth.

Complications of unrestored missing tooth

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Components of a bridge:

1) Abutment tooth:

It is the natural tooth that supports and retains the bridge at one or both ends.

2) Retainer:

It is a restoration rebuilding the prepared tooth and connected to the pontic.

Retainers are cemented to the prepared abutment teeth.

3) Pontic:

It is that part of the bridge which substitutes the lost (missing) tooth.

4) Connector:

It is the part of the bridge that joins the retainer with the pontic.

It may be:

• Rigid connector: soldered or welded or casted type.

• Non-rigid connector: ex. occlusal rest or precision attachments.

Precision attachments 21

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Classification of bridges :

I- According to Retention

• Simple Bridge 1) Fixed-fixed. 2) Fixed-supported 3) Cantilever. 4) Spring cantilever 5) Removable. 6) Resin bonded (Adhesive or Maryland) bridge

• Compound bridge

Employing more than one type of the above.

Simple Bridges include:

1) Fixed-Fixed Bridge:

A bridge, which is cemented at both ends to the abutment teeth.

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2) Fixed-Supported Bridge:

• It is a bridge which is fixed by rigid connector at one end and non-rigid connector at the other end.

• It is not actually joined to one of the terminal abutment teeth but connected to it by means of a non-rigid connector allowing some individual movement of the abutment tooth.(Fixed-Movable, Fixed-semi-rigid bridge)

3) Cantilever Bridge: (Fixed Free): A bridge where the pontic is soldered (or cast) to the retainer at one end, while its other end is free (Unsupported).

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4) Spring Cantilever Bridge:

It is a cantilever bridge where the pontic is at the end of resilient curved arm deriving its support from an abutment away from the edentulous space

Compound Bridge:

A combination bridge composed of two or more simple bridges.

Compound Bridge: anterior fixed – fixed bridge & posterior fixed supported bridge

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II-According to Material:

A-Metallic:

o High noble alloys: ex. gold platinum alloys Au, Pt.

o Noble Alloy, e.g. Silver palladium alloys Ag Pd.

o Base metal alloy: cobalt chromium &nickel chromium alloys.

B- Non-metallic: o All acrylic.

o All ceramic.

C - Combined: o Metallic with labial or buccal veneers. o Metallic with full porcelain or acrylic veneer.

III- According to Site:

1) Anterior bridge: • Unilateral: doesn’t cross the midline • Bilateral: cross the midline

2) Posterior bridge:

3) Complex bridge: Anterior & posterior segments passing through canine

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Anterior bridge

Posterior bridge

Bilateral complex bridge

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Simple

According to retention

Types

Compound

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Chapter 2 Dental instruments

Dental instruments are classified according to their uses into:

1. Diagnostic instruments.

2. Cutting instruments.

3. Restoring instruments.

1. Diagnostic instruments.

Mirror , probe and twizer

Mouth mirror

Functions:

1. To provide indirect vision of inaccessible area (Fig.1).

2. To reflect light over the examined tooth. (Fig.2).

3. For retraction and protection of oral tissues (Fig.3).

4. For magnification (special magnifying mirrors)

Fig. 1 Fig.2 Fig.3

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Varieties of dental mirrors:

• Single-sided or double-sided

• Can be disposable or autocavable (can be sterilized)

• Plain or magnifying

Examination probe

Functions

1. Detection of defective pits and fissures. Fig 1

2. Detection of calculus. Fig 2

3. Detection of deficient margins of restorations. Fig 3

4. Detection of caries. Fig 4

Fig 1 Fig 2

Fig 3 Fig 4

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Varieties of probe

• Can be single-ended or double-ended

• Working ends may be straight or curved.

Tweezer

• Varieties:

o Locking and non-locking types

o Working ends can be straight, curved, serrated or smooth

• Functions:

• Placing and removing small objects from the mouth

• Locking type ‘lock’ to prevent dropping materials

Straight tweezer Curved tweezer Locking type tweezer

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Cutting instruments are classified into:

1. Hand cutting instruments.

2. Rotary (Powered) cutting instruments.

3. Laser equipment.

4. Other equipment.

(1)Hand Cutting Instruments

This group include a large number of instruments that are powered and held

by the hand. They are usually of light weight and manufactured from a

single piece of either:

a) Carbon steel: increased hardness, and decreased corrosion resistance.

b) Stainless steel: remains bright under most conditions.

c) Carbide inserts: to provide more durable cutting edges.

d)Other alloys of: Nickel, Cobalt, Chromium are used in the

manufacturing of hand instruments, rather than those used for rotary

cutting of tooth structure.

Hand Cutting Instrument

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Instrument design:

Most hand instruments regardless of their uses are composed of three main

parts:

1. Handle: It is the part used to hold the instrument and to direct the

instrument toward tooth structure.

2. Shank: it connects the handle with the working point and it is gradually

tapered from the handle to the working point. It is angled to facilitate the

access to the cutting edge.

3. Blade (working point): it is the working point of the instrument, it must

be made of hard metals and should not be brittle.

For non-cutting instruments the part corresponding to the blade is termed

the (nib) "working surface".

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(II) Rotary Cutting Instruments

The use of abrasive or bladed instruments held in rotary hand piece is usually

powered by compressed air.

Power source for dental units:

1. Electric driven (micro motor).

2. Air driven (compressed air).

Air driven hand pieces continue to be the most popular type of hand piece

equipment because of: simplicity of design, ease of control, and patient

acceptance.

Hand piece: is a device for holding rotary instruments and transmitting

power to them and for positioning them intraorally.

According to the shape there are two types of hand piece:

1-Straight 2 - Contra angle

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Rotary speed ranges:

The rotational speed of an instrument is measured in revolutions per

minute (rpm):

a) Low or slow speeds below 12,000 rpm.

b) Medium or intermediate speeds 12,000 to 200.000 rpm.

c) High or ultrahigh speeds above 200,000 rpm.

(a) Low speed cutting:

Properties:

a) Ineffective

b) Time consuming

c) Requires a relatively heavy force application which leads to heat

production at the operating site and vibrations of low frequency and

high amplitude.

Uses of low speed:

1- Preparation of grooves and pinholes during tooth preparation.

2- Cleaning of teeth

3- Caries removal.

4- Finishing and polishing procedures.

(b)High speed cutting is:

Properties:

1- Very effective ,so used for tooth preparation.

2- Less pressure is needed.

3- Better instrument control.

4- Better patient acceptance as it produces lesser vibration.

6- Faster, so many teeth can be prepared in the same visit.

Uses of high speed:

1. Tooth preparation

2. Removing of old restorations.

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Cutting Tools:

These are smaller tools, which are held in hand piece of various types.

Cutting tools: are divided into two major subdivisions:

1) Burs.

2) Abrasives: divided into:

a) Stones.

b) Discs.

Cutting tools design characteristics:

Rotary cutting instruments have certain design features; each instrument

consists of three parts:

1-Shank 2-Neck 3-Head

1-Shank:

It is the part that fits into the hand piece, and accepts the rotary motion from the

hand piece. The shank design and dimension vary with the hand piece for

which it is intended. They may have:

a) Long shanks for use in straight hand piece.

b) Short latch shanks for use in a latch contra angle.

c) Short friction grip shanks design for use in an ultrahigh speed

hand piece.

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Long shanks Short latch shank friction grip shank

2-Neck:

The neck is the intermediate portion of an instrument that connects the head

to the shank. It transmits rotational and transitional force to the head.

3- Head:

The head is the working part of the instrument, which perform the desired

shaping of tooth structure. The head of the instrument shows greater variation

in design and construction. They are classified according to the type of cutting

in to:

a) Type of cutting:

1- Bladed (burs) 2-Diamond( Abrasives)

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According to the Material of construction:

▪ Carbon steel

▪ Tungsten carbide

▪ diamond

▪ Sand

1-Dental Burs:

They are rotary instruments that have bladed cutting heads and

manufactured in various shapes and sizes.

Classification of dental burs:

According to its flute(blade) design into:

a) Plain. b) Cross-cut.

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According to their shape:

1) Round.

2) Inverted cone.

3) Fissure Tapered

6) End and side cutting.

7) End cutting.

8) Pointed.

2-Diamond abrasive instruments:

The second major category of rotary dental cutting instruments involves

abrasive rather than blade cutting.

Advantages

1. They have long life.

2. Great effectiveness in cutting enamel and dentine because of their

high hardness.

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Classification:

These instruments have many classifications because of the endless

variety of shapes and sizes manufactured.

• Classification according to diamond particles:

• Classification of abrasive diamond stones according to head shape:

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(1) The wheel stone:

o They are shaped like wheels of different sizes.

o They are supplied in the mounted or demounted form. The wheel stone is

used for:

a) Reduction of incisal surface of anterior teeth for jacket crown

preparation.

b) Reduction of lingual or palatal fossa of anterior teeth for jacket

crown preparation.

c) Reduction of occlusal surface of molars and premolars for all types of

crowns.

(2) Tapered stone: They are mounted stones applied in different sizes.

They are conical in shape with:

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a. Flat end: used for achieving shoulder finishing line.

b. Round end: used for achieving chamfer-finishing line.

(3) Cylindrical stone: They are available in the mounted form only and

shaped like cylinders and supplied in different sizes.

(4) Round and oval shaped stones: Supplied in mounted form only.

(5) Cup shaped stones: Could be supplied in the mounted and demounted type.

(6) Inverted cone stones: They are mounted stones supplied in different sizes

(7) Barrel shaped stones.

(8) Pear shaped stones.

(9) Bud shaped stones

(10) Root facer stone: Used for preparation of the root face in case of post

crown.

3- Discs

• They are used with conventional and standard speeds.

• Supplied either in the mounted or demounted form.

• The mounted form: the working point is permanently attached to the

shaft, so if the working point became dull it is should be discarded ( the

whole shaft & the working point).

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• The demounted form: the shank in this type is called mandrell which can

carry various changeable working parts so this type is economic.

Classification of discs:

1- According to the abrasive material used

1-Diamond

2- Carborundum

3-Metal (Laboratory work)

4-Sand paper (smoothening the preparation)

2- According to the size:

1- Small disc: 3/8 inch in diameter

2-Medium disc: 5/8 inch in diameter

3- Large disc: 7/8 inch in diameter

3- According to the side of abrasive material:

a) Safe sided: abrasive materials on one-side to prevent injury of adjacent teeth.

b) Double sided: abrasive material on both sides, and used when adjacent

tooth is missing.

4- According to the shape:

a) Flat shaped disc.

b) Cup shaped disc used for reduction of distal surface of lower

posterior teeth.

4-Finishing instruments:

Rotary instruments used to finish and polish restorations includes:

Finishing burs, mounted stones, Abrasive discs, Rubber cups, rubber discs.

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Laser Equipment

A device produces beams of high intensity light. The effect of the laser depends

on the power of the beam and the extent to which the beam is absorbed. The

laser ranges from long wave length (infrared) through visible wave length to

short wave length (ultraviolet).

At the present time carbon dioxide C02, and ND: YAG laser has shown the most

promise for dental applications. It is important to select the correct wave length

for absorption of energy and prevention of side effects from heat generation.

Air abrasion device

There are alternative methods of cutting enamel or dentine. Air abrasive

cutting was tested but there were several clinical problems that limits its general

acceptance:

1. The operator has no tactile sense associated with air abrasive cutting of tooth

structure.

2. The operator has difficulty to determine the cutting progress within the cavity

preparation.

3. The abrasive dust interferes with visibility of the cutting site and intends to

roughens the surface of the dental mirror.

4. Possibility of abrasive dust inhalation by the patient or office personnel.

At the present time air abrasive equipment is being promoted for:

1. Stain removal.

2. Debriding pit and fissures prior to sealing.

3. Micro mechanical roughening of surface to be bonded (enamel, metal alloys,

or porcelain).

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Sterilization of instruments

Infection may be transmitted in dental operation through:

1. Direct contact with: Blood, Oral fluids, other secretions.

2. Indirect contact with contaminated instruments.

Precautions to avoid infection:

• Sharp instruments and needles contaminated with patient's blood and

saliva should be considered potentially infective and handled with care to

prevent injuries.

• Used needles should never be capped.

• Used disposable syringes, needles and sharp items should be placed in

appropriate puncture - resistant containers.

Sterilization and Disinfection of Instruments

1. Instruments should be cleaned thoroughly by scrubbing with soap and

water or detergent solution or with mechanical device as ultrasonic cleaner

which increases the efficiency of cleaning.

2. Debris: During cleaning of instruments from debris, special gloves that

protect the skin from possible punctures or wounds is recommended.

3. All critical and semi-critical dental instruments that are heat stable should

be sterilized routinely between uses by steam under pressure (autoclaving),

dry heat or chemical vapor, following the instructions of the manufacturers.

4. After cleaning instruments, it should be packaged before sterilization.

The sterilization of instruments may be accomplished using the following

methods:

1. Steam Autoclave: It is the most effective method for destroying the

pathogenic organisms and the vegetative forms. It is achieved at 270°

Fahrenheit, at 27 PSI (pressure) for at least 10 minutes.

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Autoclave

2. Dry heat: It is achieved with special medical oven and temperature is

maintained at230°F, for approximately 2 hours; however, temperature

above 350°F may cause premature instrument failure.

Dry heat oven

3. Unsaturated Chemical vapor: It is effective for all metal instruments but is

best suited for carbon steel instruments because chemical vapors contain less

water which cause tarnish and corrosion in certain instruments than other

sterilization methods, it is achieved at 270°F, at 20 to 40 PSI for at least 20

minutes.

• It is a combination of heat and autoclave: the device produces hot

vapors of alcohol and formaldehyde that act as sterilizing agent.

• Chemicals used: alcohol, acetone, ketone, formaldehyde, distilled water

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Advantage:

• No corrosion of instruments

• Instruments are dry at end of cycle

Disadvantage: needs well ventilated area due to odor

4. Chemical Solution: It used only for materials that cannot withstand

heat sterilization process .To be effective, the solution must come in direct

contact with the organism for several hours.

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Chapter 3

Principles of Tooth Preparation

.

There are three principles must be considered the restoration of teeth: 1. Biologic considerations, which affect the health of oral tissues

2. Mechanical considerations, which affect the strength and durability of the restoration

3. Esthetic considerations, which affect the appearance of the patient

The optimum restoration should satisfy biologic, mechanical, and esthetic requirements.

Biologic considerations: I- Prevention of damage to:

1- Adjacent teeth.

2- Soft tissue.

3-Pulp

II-Conservation of tooth structure:

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I- Prevention of damage to:

1- Adjacent teeth

Damage to the adjacent tooth is a common error in dentistry. The damaged

surface becomes rough and attracts plaque accumulation. Even if a damaged

proximal contact area is carefully polished, it will be more susceptible to dental

caries than the original undamaged smooth tooth surface.so the dentist must

avoid any damage to the tooth adjacent to the prepared tooth during reduction.

To avoid damage to adjacent tooth:

There are two methods to avoid damage to adjacent tooth:

1. A metal matrix is used to protect the adjacent tooth: a thin matrix is adapted

over the unprepared tooth to protect it , hover the disadvantage of this

method is the possibility of perforation of the matrix as it is very thin. If the

matrix is perforated it should be discarded immediately and a new one is

used.

2. The preferred method is the use of a very thin tapered diamond stone to

prepare the proximal surface leaving a slight lip of enamel as a barrier

between the prepared tooth and the adjacent tooth. This lip of enamel will

51

protect the adjacent tooth till complete opening of the contact area.

2- Soft tissue Damage to the soft tissue of the tongue and cheeks must be prevented by

careful retraction with an aspirator tip, mouth mirror, or flanged saliva

ejector.

3-Pulp

Causes of pulpal injury during preparation:

a) Thermal irritation.

b) Chemical irritation.

c) Bacterial irritation.

Thermal, chemical, and bacterial irritation may cause irreversible pulpitis

particularly when they occur on freshly cut dentin.

Tooth preparation must take into consideration the morphology and size of

the dental pulp chamber which can be evaluated on a radiograph (x-ray).

(a) Thermal irritation

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Excessive temperature and heat generation during tooth reduction may

occurs due to:

1) Excessive pressure exerted during using the rotary instruments.

2) Ultra high speed rotary devices.

3) Working without water coolant.

4) The type, shape, and condition (dull or sharp) of the cutting

instrument (bur & stones).

To decrease heat generation the following must be considered:

Light touch with the least pressure must be applied during tooth preparation

and sharp unclogged stones must be used together with water coolant

(b) Chemical irritation:

• The chemical action of certain dental materials (bases, resins,

solvents, cements and cavity cleaners) can cause pulpal damage,

particularly when applied to freshly cut dentin through dentinal

tubules.

• Application of cavity varnish over the dentine will form an effective

barrier preventing the penetration of chemical irritants through the

dentinal tubules.

(c) Bacterial irritation:

Pulp infection by bacteria may occurs due to:

• Caries left (unremoved) under restoration.

• Leakage of restoration (open margin)

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Cross-section through dentin showing opened dentinal tubules

II-Conservation of tooth structure:

The thickness of remaining dentin is inversely proportional to the pulpal

response. This means that the greater dentine thickness, the lesser pulpal irritation

and vice versa (the lesser dentine thickness, the higher pulpal irritation). So,

conservation (little tooth reduction) of tooth structures is important during tooth

preparation.

Principals for tooth structure conservation:

1. The use of partial coverage rather than the complete coverage crowns.

2. Preparation with minimal taper.

3. Preparation of the occlusal surface following the anatomic planes of the

tooth.

4. Even (uniform) axial surface reduction.

5. Selection of conservative finish line.

6. Avoidance of unnecessary apical extension of the preparation (subgingival

finish line).

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Minimum taper over taper

Anatomical occlusal reduction(line1), flat non anatomic occ. Red. (line2)

Chamfer finish line shoulder finish line

(Conservative) (not conservative)

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MECHANICAL CONSIDERATION

To prevent deformation, fracture or dislodgment of restoration

I-Providing RETENTION form

II-Providing RESISTANCE form

III- Preventing DEFORMATION of restoration

I-RETENTION FORM

Def: it is a quality of the preparation that prevents the dislodgement of

restoration along the long axis of the tooth or the path of insertion.

Path of insertion:

It is an imaginary path along which the restoration will be placed onto or

removed from the preparation (the direction through which the restoration is

seated on the prepared tooth).

The three arrows represent the path of insertion.

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• It is determined before the preparation is started and all features of

the reduction should coincide with that line.

• It is especially important when preparing long bridge because multiple

paths of insertion must be parallel.

• This path should not encroach upon the pulp or adjacent teeth.

• For a full crown with proper contours its path of insertion would be

parallel to the long axis of the tooth.

A: correct path of insertion in normally oriented tooth, B: tilted long

axis of the tooth

C: wrong path of insertion interferes with adjacent tooth D: correct

path of insertion in tilted tooth

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1-Path of insertion of single Restoration:

Line of insertion:

It is the single direction through which the restoration could be precisely

seated on the prepared abutment.

Since it is a single direction, therefore, parallelism between opposing

axial surfaces is essential.

Range of insertion:

Multiple paths of insertion leads to the loss of parallelism between

opposing axial surfaces.

A: single path of insertion. B: Multiple (range) path of insertion

Retention is improved by limiting the numbers of paths along which a

restoration can be removed from the tooth preparation. Maximum retention is

achieved when there is only one path(single).

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2- Path of insertion of a bridge:

Common range of insertion:

the presence of a common path of insertion which coincide with the path of

insertion of each abutment is important otherwise the bridge will fail to seat .

Common range of insertion

Parallelism:

Parallelism between the opposing axial walls (buccal, lingual, mesial,

distal) is ideal, but cannot be achieved practically because:

1) Human hands cannot prepare exactly axial parallel walls.

2) Parallel walls will not allow complete seating of the crown during

cementation and will prevent the release of excess cement (hydraulic

pressure).

3) Parallel walls are impossible to be prepared in the mouth without

creating undercuts.

4) Parallel walls won't give enough space for restoration thickness and

durability.

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Therefore, the opposing axial walls of the preparation must taper (converge)

slightly to allow the restoration to seat.

To evaluate the preparation taper view it with one eye from a distance of

approximately 30 cm.

In this way it is possible to simultaneously see all the axial walls of a

preparation with adequate taper. An undercut as great as eight degrees

may not be detected if both eyes are used.

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Factors affecting retention:

1. Magnitude of the dislodging forces

2. Geometry of the tooth preparation

3. Roughness of the fitting surface of the restoration

4. Materials being cemented

5. Type of cement

1. Magnitude of the dislodging forces

Sticky food e.g. caramel or pulling action of dental floss can dislodge a crown or

even a bridge with poor retention.

3. Geometry of the tooth preparation

Geometry (outline) of the prepared tooth involves three elements:

• Taper

• Length (height)

• Preparation features (grooves, pinholes)

Taper:

Parallelism or slight convergence {2-6º} will provide perfect frictional retention.

The recommended convergence angle is 6 degrees. This is a very slight taper.

(The angle between the hands of a clock showing 12:01 is 5, degrees.)

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Length (height):

Crowns with long axial walls are more retentive than those with short axial

walls due to increased surface area

Preparation features {grooves, pinhole}:

Adding grooves or pinholes increase retention by increasing surface area and

limiting the path of withdrawal.

Types of grooves:

• V- shaped

• U-shaped

• Box-shapes

Grooves pinhole

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3. Roughness of the fitting surface of the restoration

• Roughness increase surface area for mechanical interlocking of cement.

• Roughness is made by sandblasting the fitting surface of the restoration

with 50 micron alumina particles, which increases retention by 64%.

4. Materials being cemented:

Experimental studies showed that base metal alloys are better retained than

high-gold alloys.

5. Type of cement:

Adhesive resin cements are the most retentive cement.

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RESISTANCE FORM

Def: a quality of the preparation that prevents the dislodgement of

restoration when subjected to horizontal or oblique forces.

During mastication, any restoration is subjected to horizontal or oblique

forces. These forces tend to displace the restoration by causing rotation

around the gingival margin. Rotation is prevented by any areas of the tooth

preparation that are placed in compression (called resistance areas).

The resistance area (RA) of a complete crown is placed under compression

when a lateral force (F) is applied. NRA, Non-resisting area.

Factors affecting resistance:

1- Magnitude and Direction of the Dislodging Forces

2- Geometry of the Tooth Preparation

3- Physical Properties of the cement

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1- Magnitude and Direction of the Dislodging Forces:

In a normal occlusion, biting force is distributed over all the teeth; most of it is

axially directed. However, if a patient has a biting habit such as pipe smoking

or nail biting, it may be difficult to prevent the large oblique forces from being

applied to a restoration. So, the prepared tooth and restoration must be able to

withstand these oblique forces.

2- Geometry of the Tooth Preparation:

• Taper

• Length (height)

• Preparation features (grooves, pinholes)

Long tooth preparations with large diameters were found to have great

resistance form. The U-shaped grooves and boxes provide more resistance than

V-shaped ones because the wall of the groove is perpendicular to the

dislodgment force.

U-shaped groove V-shaped groove

3- Physical Properties of the Luting cement:

Cements with high strength and physical properties have greater resistance

form than weaker cements with low strength.

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III- Preventing DEFORMATION of restoration

• Alloy Selection

• Adequate Tooth Reduction

1-Alloy Selection

Type I and Type II gold alloys are too soft for crowns and bridges, so Type III or

Type IV gold alloys and base metal alloys are chosen. These are harder, and their

strength and hardness can be increased by heat treatment.

2- Adequate Tooth Reduction

Sufficient tooth reduction to produce adequate bulk (thickness) for

restoration, otherwise perforation or fracture may occur.

ESTHETIC CONSIDERATIONS:

Patients prefer their dental restorations to look as natural as possible. However,

care must be taken that esthetic considerations are not pursued at the expense of

a patient's oral health or functional efficiency.

Esthetic principles:

1-Sufficient tooth reduction, otherwise the restoration will be bulky.

2- Proper color selection that matches natural teeth color.

3- Harmony between the restoration and adjacent teeth (alignment or contour).

4- Excellent marginal adaptation to prevent gingival inflammation.

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Chapter 4

Finish line

• Definition: it is the line of demarcation where the prepared tooth structure meets the

unprepared surface of the tooth.

• Another def.: it is the point at which a preparation terminates on the tooth. Or it is the

peripheral extension of tooth preparation.

• Whenever possible the finish line should be placed in an area where the margins

can be finished by the dentist easily and kept clean by the patient.

• In addition, finish lines must be placed so that they can be recorded by the

impression without tearing or deformation.

Finish line (margin)

Importance of Finish Lines:

1. It reflects of the amount tooth reduction.

2. It is used to evaluate the accuracy of impressions (if it is well accentuated

and clearly visible in the impression denotes a good impression).

3. A definite clear finish line helps in accurate die preparation.

4. Marginal adaptation of the wax pattern depends on a clear finish line.

5. A definite finish line helps in determining the proper seating of a restoration

during cementation.

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Good impression with clear FL Bad impression with ill-defined FL

Die preparation Adaptation of the wax pattern

Locations of finishing lines:

1. Supra-gingival.

2. Sub-gingival.

3. At the crest of the gingival margin.

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1. Supra-gingival F.L.

Golden rule:

It is always better to terminate preparations above the gingival margin i.e. supra-

gingivally.

Advantages of supra gingival margin:

1. Easy to prepare accurately without trauma to soft tissues.

2. Can be easily finished.

3. They are more easily kept clean.

4. Impressions are more easily made.

5. Restoration can be easily evaluated (checked).

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2 .Sub-gingival F.L.

Midway between the gingival crest and the base of the sulcus. Subgingival margins

may cause periodontal diseases or injury to the gingiva during preparation; however,

it is indicated in certain situation.

Indications of subgingival finish line:

1) Cervical caries, cervical erosion, and the presence of old restorations

extending subgingival.

2) The proximal contact area extends to the gingival crest.

3) Short occluso-gingival height (length).

4) In esthetic areas, where the margin of a metal-ceramic crown is to be

hidden below the gingiva.

5) Root sensitivity which cannot be controlled by conservative procedures.

3. at the crest of the gingiva

It is not recommended nowadays because the crest of the gingiva is the area of

plaque accumulation, so it is better to place the F.L which is rough and harbors

bacteria away from this area.

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Circumferential level:

The level of the F.L must follow the contour of the cement-enamel junction,

i.e. more coronal proximally than on the facial and lingual aspects.

Margin adaptation:

o The interface between a cemented restoration and the tooth always a

potential site for recurrent caries due to the dissolution of the cement and

inherent roughness.

o Rough, irregular or opened junctions greatly reduce the adaptation of the

restoration and lead to plaque accumulation.

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Types of Finish line:

1-Feather edge F.L (indefinite margin):

• It is not recommended nowadays and should be avoided.

• It is very shallow, ill-defined conservative margin.

• Although it conservative, its main disadvantage is the lack of well-defined

margin of sufficient depth, thus fails to provide adequate bulk of restoration

at the margin.

• Over contoured restorations occurs as a result of feather edge margins.

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2-Knife edge F.L. (Chisel edge):

It is a modification of feather edge margin; it is formed by increasing the amount

of reduction of featheredge margin.

Advantages:

Conservation of tooth structure.

Disadvantages:

1. Axial reduction fade out instead of ending in a definite finish line.

2. Difficulty of fabricating restoration with very thin margin. (Easily distorted).

Indications:

1- Cast metal restoration

2- Used in inaccessible area

3- Lingual surface of mandibular posterior teeth where minimum tooth reduction is

required.

4- Tilted teeth where the choice of another F.L. would result in excessive reduction

Instrument:

• Tapered diamond with pointed end

• Safe sided disk. (to open wide contact area)

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3. Chamfer F.L.:

Conservative margin with a thickness of 0.3-0.5 mm

Indications:

1-Cast metal crowns.

2-Lingual surface of metal ceramic restorations (metallic portion of metal

ceramic crowns).

Advantages:

• Exhibits the least stress exerted over the cement.

• Conservative & provides adequate bulk for restoration rigidity.

Disadvantages:

Care is needed to avoid the formation of unsupported lip of enamel (undermined

enamel) .Chamfer should never be prepared wider than half the tip of the diamond

otherwise a lip of enamel will form which will fracture when the crown is inserted.

Instrument:

Tapered diamond stone with round end.

4-Heavy chamfer FL:

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Description:

Modified chamfer which has large radius rounded internal angle that reduces

stress concentration by 50 %.

Advantages:

1. 50% reduction of stress concentration.

2. Rounded internal angles facilitate ceramic adaptation

3. Easier to prepare with precision.

Disadvantages:

• It provides better support for ceramic crowns than does a chamfer F.L, but it

is not as good as a shoulder.

• Metal framework with a chamfer design distorts more than a shoulder design

during porcelain firing because of the additional bulk provided by the

shoulder.

Indication:

Recommended for all ceramic restorations.

Instrument:

Large diameter tapered with rounded end.

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5-Bevel F.L.:

Its shape forms obtuse angle with the unprepared tooth surface

Advantages:

1. Removes unsupported enamel.

2. Allows finishing of metal and burnishing against the prepared tooth

structure.

Indications

Cast metal restorations especially if a ledge or shoulder already exists from

caries, cervical erosion or a previous restoration.

Instrument:

Tinker (torpedo) stone or bur.

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6- Shoulder F.L.:

Description:

It is a well-defined finish line which results in a butt joint between the

restoration and tooth. It forms 90 º degree angle. Its thickness is 1 mm in

average. (0.7-1.2 mm)

Advantages:

1) Provides adequate resistance to occlusal forces.

2) Provides bulk for healthy restoration

3) Provides bulk for proper contours and maximum esthetics

Disadvantages

1) Less conservation of tooth structure.

2) Sharp 90º internal line angle causes stress concentration that may lead to fracture.

Indications:

1) All ceramic crowns.

2) Facial margins of metal ceramic crowns.

Instruments:

1) Tapered with flat end.

2) Cylindrical stone

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7- Sloped Shoulder F.L.:

Description:

A modification of 90º degree shoulder finish line. The ledge created meets the axial

wall at an obtuse angle varying between 120º and 135 º degrees.

Advantages:

1- Provides bulk of material allowing metal thinning.

2- Sloped shoulders reduce the possibility of unsupported enamel lip.

Disadvantages:

Less conservation of tooth structure.

Indications:

Facial margins of metal ceramic crowns.

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8- Shoulder with bevel F.L.:

Description:

A bevel is added to an existing shoulder .The bevel is 0.2 to 0.3 mm wide and

forms an angle of 45º degree with the shoulder.

Advantages:

• Provides resistance against distortion of porcelain during firing.

• Provides bulk for the material.

Disadvantages:

Less conservation of tooth structure.

Indications:

1-Labial finish line of porcelain fused to metal restorations.

Instruments:

Cylinder stone to form shoulder. Bevel is added with tinker stone or flame

shaped bur or stone.

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Chapter 5

Complete Metal Cast Crown

The complete cast crown is a full coverage all-metallic restoration that restores functional

properties of the prepared tooth.

The terms (full metal crown), (complete metal crown), (full cast crown), and (full veneer

crown) can be used with complete metal cast crown.

Indications:

1. As a single restoration.

2. As a bridge retainer (part of a bridge).

3. Patients with high caries index and poor oral hygiene.

4. Presence of extensive caries.

5. Presence of large restorations.

6. Teeth with short clinical crowns.

7. Endodontically treated teeth to compensate for the loss of tooth structure.

8. Correction of axial contours of displaced or tilted teeth.

9. Support for a removable partial denture.

10. When esthetics is not a concern.

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Contraindications:

1. When a more conservative restorations can be attempted.

2. Patients with high esthetics demands.

3. When less than maximum retention and resistance are needed.

Advantages:

1. Greater retention and resistance to displacing forces than partial veneer restorations

(almost the double).

2. More conservative than other types of full crown such as porcelain fused to metal and

all ceramic crowns.

3. Allow for modification of axial tooth contour of malaligned teeth to provide support

for removable partial dentures.

4. Allow modification of occlusion as in cases of over-erupted tooth and when

occlusal plans need to re-establish.

5. Greater strength in comparison to partial coverage restorations.

Disadvantages:

1. Extensive removal of tooth structure can endanger pulp vitality or cause

inflammation of gingiva.

2. Less conservative than partial coverage restorations.

3. Lack of esthetic quality.

4. Electric pulp vitality test of the abutment tooth cannot be made.

Criteria of ideal tooth preparation for full veneer metal crown

1. Sufficient occlusal reduction to allow adequate thickness of restoration to avoid its

perforation.

2. Anatomical occlusal reduction (following the occlusal geometry).

3. Axial walls should be parallel to the long axis of the tooth and allow for the

recommended 6 degree taper.

4. The margin is a chamfer finish line, and ideally should be located supragingivally. It

should be smooth distinct and well demarcated.

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Steps of preparation:

1-Occlusal Reduction

Occlusal reduction is made to create a uniform space 1-1.5 mm between the occlusal

surface of the reduced tooth and the occluding surface of the opposing teeth.

Technique:

Depth grooves preparation

Depth grooves (guiding grooves) are grooves prepared in the surface of the tooth to act

as a guide or reference to determine the amount of tooth structure removed by

preparation. If the preparation is done without these grooves, under and over preparation

is possible, and more time will be spent by repeated checking of the preparation.

• Use a tapered diamond stone with round end to make depth cuts or guiding

grooves in the central , mesial and distal fossa, marginal ridges, buccal and lingual

developmental grooves The depth grooves should be 1.3 mm on the functional cusps

(buccal cusp of mandibular and palatal cusp of maxillary) and 0.8mm on the non-

functional cusps (lingual cusp of mandibular and buccal cusp of maxillary).Fig 1.

• Tooth structure remaining between the grooves is removed using tapered stone with

rounded end while preserving the occlusal configuration (anatomical reduction). Fig.2

Fig 1 Fig 2

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Functional Cusp Bevel

• Place depth grooves for the functional cusps across the facial-occlusal line angle of

mandibular molar, and across the lingual-occlusal line angle of maxillary molar of a

depth 1-1.5mm. Fig 3

• Tooth structure remaining between the grooves is removed using tapered stone with

rounded end creating a wide bevel 45 degrees to the long axis of the tooth. Fig 4

• The minimum recommended clearance is 1.5 mm on the centric cusps and 1 mm on

non- centric cusps.(Fig.3)

• Insufficient clearance leads to perforation of the restoration by wear or during finishing

procedures, while excessive reduction leads to bad retention due to reduced occluso-

gingival height (length).

• Adequate occlusal clearance is verified clinically by asking the patient to bite on two

layers of utility wax (Fig 5). If the wax shows thin spot or perforation, this indicates

insufficient occlusal clearance Fig 6

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Fig 3 Fig 4

Fig 5 Fig 6

2-Axial Reduction

1. Use the tapered diamond stone to create 3 depth grooves on the buccal and lingual walls,

parallel to the path of removal and with 6 degree taper (Fig. 7).

2. Remove tooth structure remaining between the grooves and create the chamfer F.L. by the

tapered diamond stone with rounded end. (Fig. 7).

3. The facial and lingual reduction are carried as far as possible into interproximal

embrasures.

4. Protect the adjacent teeth by matrix band and cut carefully inter proximally by a

thinner diamond tip

5. Place the cervical chamfer 0.5 mm wide to allow bulk at the margin to resist

distortion. The finishing line should be smooth and continuous.

6. The axial reduction should taper occlusally 3 to 5 degree and terminate cervically in a

chamfer finish line.

7. Inadequate axial reduction can cause thin restoration walls i.e. weak restoration.

8. Avoid over convergence as this will destroy retention and resistance of the final

restoration.

9. Finally, roundation of axial surfaces is made with smooth transition from surface to

another using a fine stone at low speed. (Fig. 8).

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Fig 7 Fig. 8

Fig. 8

3- seating groove

It is placed on axial surface with a tapered fissure bur. The groove should be cut to full

diameter of the bur and extended 0.5 mm above F.L. This groove will guide the crown

in place during cementation.

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Evaluation sheet

Student name:

Date/ Type of

preparation

Occlusal

reduction

Labial

reduction

Lingual

reduction

Proximal

reduction

Finishing Total Grade

1-

2-

3-

86

4-

5-

6-

7-

8-

9-

10-

Course director

Dr.Atef Fathy

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Examination Chart

Student name:

Student Name…………………………………….…………………..

I.D……………………………………………………………………..

Section…………………………………………….……………………

Supervisor………………………………..

Date ……/ ……./ …………

Comments

Course director

Dr. Atef Fathy

_______________

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