انت هنا الان : شبكة جامعة بابل > موقع الكلية > نظام التعليم الالكتروني > مشاهدة المحاضرة
الكلية كلية طب الاسنان
القسم ترميم ومعالجة الاسنان
المرحلة 3
أستاذ المادة احمد غانم مهدي الهلال
4/27/2011 3:07:33 PM
Cementation of crown and bridge
There are several types of cements used for permanent cementation of bridge, these includes: zinc phosphate, zinc silicophosphste, poly carboxilate, glass ionomer and composite resin cement. Cements based on ZOE are not used for permanent cementation. There is no cement material has superior properties in all situations.
Zinc phosphate:
It’s first introduced in 1878; it has high compressive strength (96-100 MPa), with 3.5 pH at the time of cementation. Varnish can be applied before cementation to seal exposed dentinal tubules. It has low film thickness (25µm) and tensile strength (8 MPa).
Polycarboxilate cement:
It has higher tensile strength than zinc phosphate (10-12 MPa), it has low pH (4.8) but because of large particles of the poly acrylic acid, it can’t penetrate the dentinal tubules. Its compressive strength is (80 MPa) and it has high bond strength to enamel and dentine with film thickness of (15-17 µm).
Resin based cement:
It is composite composed of resin matrix e.g. bis-Gma or diurethan methacrylate and filler of few inorganic particles. They differ from composite primarily in there low viscosity.
Resin cement are insoluble in oral fluid, much stronger than conventional cement (200-250 MPa) and has high tensile strength (30-40 MPa), it’s present in three types:
1. Auto polymerized used under light blocking (metallic restoration).
2. Photo-cured cement.
3. Dual cure (light activated) used under translucent ceramic veneers and inlays (ceramic and composite) which allows light transmission.
It’s not used under full crown because of its film thickness (10-25 µm), and marginal leakage because of setting shrinkage and difficulty in removing the hardened excess resin from inaccessible margins (subgingival).
Hybrid ionomer cement:
Or resin modified poly amenotic acid, combines the strength and insolubility of resin with the fluoride release of glass ionomer. It differs from the remaining resin cement in that the glass particles react with the liquid during the hardening process.
Cementation:
• Regardless of the material used for the cementation, some problems caused by improper cementation technique (premature occlusion, pulpities, lessening of the restoration and recurrent caries).
• The factors that affect the complete seating of the cast are the viscosity of the cement, vibration, venting and seating force.
• Seating force must be adequate to ensure complete seating, excessive force of short duration may produce elastic strains in the dentine, creating a force that dislodges the restoration when the force relaxed.
• Venting full crown can be done by drilling a hole in the crown will facilitate escape of the cement from the crown and allow more complete seating. These holes can be sealed by direct filling material or by creating an internal escape channel (vertical groove) in the axial wall of the preparation or in the internal surface of the crown. This groove extended from the occlusal surface to slightly above the finishing line.
• Vibration is done by gentle tapping the side of the crown with a mirror handle. Vibration will produce more complete seating than the static force alone.
• The field must be kept dry during final seating of the restoration and hardening of the restoration of the cement.
• The solubility of zinc phosphate is greatly increased by premature contact with moisture.
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 .
|