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.
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
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.
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.
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