Jacket crown
It is a type of crown that is formed by a tooth colored
material. It is mainly used as a single unit in the anterior quadrant of the
mouth. It is the weakest type of
crown because the
tooth colored materials
are weaker and more brittle than
metal. It can be divided into 2 types according to the material from which it
is formed:
1. Porcelain jacket
crown
2. Acrylic jacket
crown
It isn't a conservative type of crown because a butt
shoulder finishing line is done
all around and
excessive tooth structure
is removed to provide enough space for the acrylic or
porcelain material in order to get a
proper shape of
the crown, to
increase the rigidity
of material and to
resist the fracture by increasing the thickness of the material.
The acrylic jacket crown may be used as a temporary crown or
for crowning a tooth
of a patient
under 18 years
of age, until
full eruption finishes to the
tooth,
and then a
final crown (full
veneer crown or porcelain jacket crown) All ceramic
crowns are some
of the most
esthetically pleasing prosthodontic restorations
. Because there
is no metal
to block light transmission ,
they can resemble
better in terms
of color ,
translucency than any other restorative option can natural tooth
structure. There chief disadvantage is their susceptibility to fracture ,
although this is lessened by use of A resin
bonded technique.
Advantage :
1- Superior esthetic
2- Excellent translucency (similar to that of natural tooth
structure)
3- Good tissue response
4- Lack of reinforcement by a metal sub structure permit
slightly more conservative reduction of facial surface
Disadvantages :
1- Reduced strength of the restoration because of the
absence of reinforcing metal substructure.
2- Significant tooth reduction is necessary on the proximal
and lingual aspects due to the need for a shoulder-type margin circum ferentially.
(less conservation).
3- Porcelain brittleness
4- Difficulties may be associated with obtaining a
well-fitting margin, which can result in fracture because of the nature of Porcelain.
5- Proper preparation design is critical to ensuring
mechanical success (90 degree Cavo
surface angle) thus a severely damaged tooth should not be restored with
ceramic crown.
6- All ceramic restoration do not tend themselves well to
use as retainers for a fixed partial denture.
7- Wear has been observed on the functional surface of
natural teeth that oppose Porcelain restoration.
Indications:
1- A high esthetic requirement exists
2- Proximal or facial caries that cannot longer be
effectively restored with composite resin
3- Because of the relative weakness of the restoration, the
occlusal load should be favorably
distributed. Generally, this means that the centric contact must be in an area
where the Porcelain is supported by
tooth structure (i.e in a middle third of a lingual wall)
Contra indications :
1- When a more conservation restorative can be used.
2- Rarely are they recommended for molar teeth. (Increased
occlusal load and the reduced esthetic demand).
3-It is not possible to provide adequate support or an even
shoulder width of at least 1 mm circumferentially
Procedure of preparation
On The preparation of the tooth for a jacket crown resembles
that of the preparatifor full veneer except that the jacket crown needs:
1. A uniform
1 mm reduction
is done all
around the crown
(labial lingual and proximal surfaces).
2. A butt (90)
shoulder finishing line is done all around the tooth.
Dental Ceramic and
Porcelain
The word ceramic is derived from the Greek word Keramos
which means "burnt stuff" meaning a material produced by burning or
firing. It consists mainly of kaolin which blends with other minerals such as
silica, and feldspar to produce the translucency and extra strength required
for dental restoration. A material containing these additional
important ingredients was given the name porcelain.
Composition of traditional dental porcelain
The
composition of the various types of porcelain is summarized in the table below.
There are considerable differences in the composition between the dental porcelains
and decorative porcelain ex. dental porcelain contains little or no clay.
Kaolin is a
hydrated aluminosilicate. The decorative porcelain is a mixture of this
material with silica, bound together by a binder (flux) such as feldspar (a
mixture of potassium and sodium aluminosilicates). Feldspar is the lowest
fusing component which melts and flows during firing uniting the other
components in a solid mass. The fusion temperature of feldspar may be further reduced
by adding to it other low- fusing fluxes such as borax.
Dental
porcelain is mainly divided (according to fusing temperature) to high fusing
porcelain which fuses in the range of 1300-1400 C, and low-fusing porcelain which
fuses in the range of 850-1100 C.
Properties
of porcelain
Esthetics.
Porcelain is an
almost perfect material
for the replacement
of missing tooth substance. It is available in a range of shades and at
various levels of translucency
giving an almost
natural appearance. The
inner layer of the
porcelain crown is
constructed from a
fairly opaque core material. This is covered with a more
translucent dentine material with a final coating of translucent enamel
porcelain forming the outermost layer.
Rigidity and
Brittleness. Porcelain
is a very
rigid, hard, and
brittle material whose strength
is reduced by
the presence of
surface irregularities or internal voids and porosity
Thermal conductivity. Porcelain is an excellent
thermal insulator. This is very important when a gross amount of tooth
structure is prepared and the layer
of dentin may
be of minimum
thickness to act
as an insulator.
Resistance to chemicals. Porcelain is very resistant
to any chemical and it
is unaffected by any variation in the pH in the oral cavity.
Biocompatibility. The outer layer of porcelain in
coated by an oxide layer
(glazed porcelain) which is very smooth therefore it does
not allow food adhesion on it more than the normal tooth structure. cervically
therefore decreasing the translucency
Types of Porcelain
1- Opaque porcelain:
It is applied
as a first
ceramic coat and
performs two major functions:
- It masks the colour of the alloy (in metal fused to porcelain crown).
- It is responsible for the metal ceramic bond.
2-Body porcelain:
This porcelain is
placed and fired
on the opaque
layer. It provides some
translucency and contains oxides that aid in shade matching.
3-Incisal porcelain:
This type of
porcelain is more
translucent than the
above types of porcelain. It is placed mostly in the incisal third to
give the crown a translucent incisal third and the thickness of this porcelain
decreases as we go cervically
therefore decreasing the translucency
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