Sabtu, 05 November 2011

Jacket Crown,Dental Ceramic and Porcelain

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.

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 Keramoswhich 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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