Saturday, November 5, 2011

Jacket Crown,Dental Ceramic and Porcelain

Jacket crown
It is a type of crown that is formed by a tooth coloredmaterial. It is mainly used as a single unit in the anterior quadrant of themouth. It is the weakest  type  of crown  because  the tooth  colored  materials are  weaker and more brittle thanmetal. It can be divided into 2 types according to the material from which itis formed:
 
1.  Porcelain jacketcrown
2.  Acrylic jacketcrown 

It isn't a conservative type of crown because a buttshoulder finishing line  is  done all  around  and excessive  tooth  structure is  removed  to provide enough space for the acrylic orporcelain material in order to get a proper  shape  of the  crown,  to increase  the  rigidity of  material  and  toresist the fracture by increasing the thickness of the material.
The acrylic jacket crown may be used as a temporary crown orfor 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 toothstructure. 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 toothstructure)
3- Good tissue response
4- Lack of reinforcement by a metal sub structure permitslightly more conservative reduction of facial surface

Disadvantages :
1- Reduced strength of the restoration because of theabsence of reinforcing metal substructure.
2- Significant tooth reduction is necessary on the proximaland 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 awell-fitting margin, which can result in fracture because of the nature of Porcelain.
5- Proper preparation design is critical to ensuringmechanical success (90 degree  Cavosurface angle) thus a severely damaged tooth should not be restored withceramic crown.
6- All ceramic restoration do not tend themselves well touse as retainers for a fixed partial denture.
7- Wear has been observed on the functional surface ofnatural teeth that oppose Porcelain restoration.

Indications:
1- A high esthetic requirement exists
2- Proximal or facial caries that cannot longer beeffectively restored with composite resin
3- Because of the relative weakness of the restoration, theocclusal load should be favorablydistributed. Generally, this means that the centric contact must be in an areawhere the Porcelain is supported bytooth 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. (Increasedocclusal load and the reduced esthetic demand).
3-It is not possible to provide adequate support or an evenshoulder width of at least 1 mm circumferentially

Procedure of preparation
On The preparation of the tooth for a jacket crown resemblesthat 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 andPorcelain
The word ceramic is derived from the Greek word Keramoswhich means "burnt stuff" meaning a material produced by burning orfiring. It consists mainly of kaolin which blends with other minerals such assilica, and feldspar to produce the translucency and extra strength requiredfor dental restoration. A material containing these additionalimportant ingredients was given the name porcelain.

Composition of traditional dental porcelain
Thecomposition of the various types of porcelain is summarized in the table below.There are considerable differences in the composition between the dental porcelainsand decorative porcelain ex. dental porcelain contains little or no clay.
Kaolin is ahydrated aluminosilicate. The decorative porcelain is a mixture of thismaterial with silica, bound together by a binder (flux) such as feldspar (amixture of potassium and sodium aluminosilicates). Feldspar is the lowestfusing component which melts and flows during firing uniting the othercomponents in a solid mass. The fusion temperature of feldspar may be further reducedby adding to it other low- fusing fluxes such as borax.
Dentalporcelain is mainly divided (according to fusing temperature) to high fusingporcelain which fuses in the range of 1300-1400 C, and low-fusing porcelain whichfuses in the range of 850-1100 C.

Propertiesof porcelain
Esthetics. Porcelain  is  an almost  perfect  material for  the  replacement of missing tooth substance. It is available in a range of shades and atvarious 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 moretranslucent dentine material with a final coating of translucent enamelporcelain 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 excellentthermal insulator. This is very important when a gross amount of toothstructure is prepared and the layer of  dentin  may be  of  minimum thickness  to  act as  an insulator.

Resistance to chemicals. Porcelain is very resistantto any chemical and it
is unaffected by any variation in the pH in the oral cavity.

Biocompatibility. The outer layer of porcelain incoated by an oxide layer
(glazed porcelain) which is very smooth therefore it doesnot allow food adhesion on it more than the normal tooth structure. cervicallytherefore decreasing the translucency

Types of Porcelain
1- Opaque  porcelain: It  is  applied as  a  first ceramic  coat  and performs  two major functions:
  • It masksthe colour of the alloy (in metal fused to porcelain crown).
  • It isresponsible for the metal    ceramicbond.
2-Body  porcelain: This  porcelain  is placed  and  fired on  the  opaque layer.  It provides sometranslucency 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 togive the crown a translucent incisal third and the thickness of this porcelaindecreases as we go cervicallytherefore decreasing the translucency


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