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الكلية كلية طب الاسنان
القسم ترميم ومعالجة الاسنان
المرحلة 3
أستاذ المادة احمد غانم مهدي الهلال
4/27/2011 4:31:44 PM
Ceramics for ceramic-metal restorations :
The ceramics used for porcelain-fused-to metal restorations must fulfill five requirements:
(1) they must simulate the appearance of natural teeth.
(2) they must fuse at relatively low temperatures.
(3) they must have thermal expansion coefficients compatible with the metals used for
ceramic-metal bonding,
(4) they must withstand the oral environment.
(5) they must not unduly abrade opposing teeth.
The ceramic is carefully formulated to achieve these requirements.
These ceramics are composed of crystalline phases in an amorphous and glassy (vitreous) matrix.
Requirements for a ceramic-metal system
1.High fusing temperature of the alloy. The fusing temperature must be substantially higher (>100° C) than the firing temperature of the ceramic and solders used to join segments of a bridge.
2.Low fusing temperature of the ceramic. The fusing temperature must be lower than ceramic used for all-ceramic restorations so no distortion of the coping takes place during fabrication.
3.The ceramic must wet the alloy readily when applied as a slurry in order to prevent voids forming at the interface. In general, the contact angle should be 60 degrees or less.
4.A good bond between the ceramic and metal is essential and is achieved by the interactions of the ceramic with metal oxides on the surface of metal and by the roughness of the metal coping. 5.Compatible coefficients of thermal expansion of the ceramic and metal so the cross section of a ceramic-metal crown showing a gold alloy or base metal coping, the opaque body (dentin), and enamel ceramic layers. ceramic does not crack during fabrication. The system is designed so the value for the metal is slightly higher than for the ceramic, thus putting the ceramic in compression (where it is stronger) during cooling.
6.Adequate stiffness and strength of the alloy core. This requirement is especially important for fixed bridges and posterior crowns. High stiffness in the alloy reduces stresses in the ceramic by reducing deflection and strain. High strength is essential in the inter proximal regions in fixed bridges.
7.High sag resistance is essential. The alloy copings are relatively thin; no distortion should occur during firing of the ceramic or the fit of the restoration will be compromised.
8. An accurate casting of the metal coping is required even with the higher fusing temperature of the alloy.
9. Adequate design of the restoration is critical. The preparation should provide for adequate thickness of alloy and also provide enough space for an adequate thickness of ceramic to yield an esthetic restoration.
In some instances, a ceramic-metal restoration has an advantage over an all-ceramic restoration because less tooth structure needs to be removed to provide adequate bulk for the all-ceramic restoration. However, in cases of small, lower, anterior teeth, an all-ceramic restoration has an advantage with respect to esthetics, because with a ceramic-metal restoration it is difficult to remove enough tooth structure to provide space for the coping and the esthetic ceramic layer. The geometry of the shoulder should be flat with a rounded angle or a chamfer to allow enough bulk of ceramic and avoid fracture in this area. If full ceramic coverage is not used, the position of the ceramic-metal joint should be located as far as possible from areas of contact with opposing teeth.
Preparation of Ceramic metal Restorations:
The processing of the metal coping for ceramic metal restorations is much like that of all-metal crowns and bridges. One significant difference relates to the reuse of metal. when the metal is melted and cast, certain alloying elements can be lost, especially the elements that readily form oxides. These elements are important for bonding with noble metal alloys. To conserve metal, portions of the casting are commonly remelted. Each time the metal is remelted, some of these easily oxidized elements are lost. Therefore a certain portion of new alloy (usually half) should be added each time the metal is reused to replenish the lost alloying elements.
Surface treatment of the metal coping before ceramic application is important for good bonding. These treatments are used to roughen the coping surface and form surface oxides. The surface may be roughened by blasting with a fine abrasive (25 to 50 µm alumina); in some cases this results in a large increase in bond strength. In most cases the metal coping is heat treated either in air or under partial vacuum to produce a surface oxide to improve bonding. In some palladium alloys, the heat treatment
forms not only surface oxides but also internal oxides that penetrate the metal from the surface and effectively roughen the surface, thereby improving bonding. Some base-metal alloys tend to form excessively thick interfacial oxides, which weaken the metal-ceramic bond .With these alloys, the coping is heat-treated and then blasted to remove enough oxide to achieve higher bonding. If this process is not used, failure through the oxide may occur.
In ceramic application there are several important considerations. The first layer of ceramic is especially important with ceramic-metal restorations because it must hide the metal; special opaque ceramic must be used. After the opaque layer has been applied and fired the dentin (or body) ceramic, which contains less of the opaque oxides (such as SnO, and ZnO,), pigments, and fluorescing oxides, is applied and fired. Finally, once the correct contour has been established an essentially transparent glaze layer is applied and fired. The ceramic-alloy compatibility is another important consideration. As pointed out earlier, the thermal expansions must be matched and the porcelain firing temperatures must be low enough that the alloy will not sag; the manufacturer usually supplies compatibility information. Titanium alloys require special ceramics, otherwise processing of the ceramic is similar to that of the other alloys.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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