NEET MDS Lessons
Dental Materials
Properties of Acrylic Resins.
- They have a low thermal conductivity. These resins are not easily washed out by the acids of the oral cavity (low solubility). Acrylic resins are also resilient, which allows them to be used in stress-bearing areas.
- Acrylic resins exhibit a moderate shrinkage of from 3 to 8 percent. This shrinkage and low marginal strength can lead to marginal leakage. Acrylic resins have a low resistance to wear. Acrylic resins cannot be used over a zinc oxide and eugenol-type base because eugenol interferes with the acrylic curing process.
- Mixing. Insufficient mixing will cause an uneven color or streaks in the mixture. Overmixing will cause the material to harden before it can be placed
- Poor distortion resistance at higher temperatures, therefore dentures should not be cleaned in hot water
- Good resistance to color change
- Absorbs water and must be kept hydrated (stored in water when not in mouth) to prevent dehydration cycling and changes in dimensions
- Not resistant to strong oxidizing agents
- Low strength; however, flexible, with good fatigue resistance
- Poor scratch resistance; clean tissue-bearing surfaces of denture with soft brush and do not use abrasive cleaners
RINGLESS INVESTMENT TECHNIQUE
Used for phosphate bonded investments .
This method uses paper or plastic casting ring .
It is designed to allow urestricted expansion .
Useful for high melting alloys .
Components
a. Fillers added to most to control shrinkage
b. Matrix
COMPOSITE RESINS
Applications / Use
- Anterior restorations for aesthetics (class III, IV, V, cervical erosion abrasion lesions)
- Low-stress posterior restorations (small class I, II)
- Veneers
- Cores for cast restorations
- Cements for porcelain restorations
- Cements for acid-etched Maryland bridges
- Repair systems for composites or porcelains
Polymerization--reaction of small molecules (monomers) into very large molecules (polymers)
Cross-linking-tying together of polymer molecules by chemical reaction between the molecules to produce a continuous three-dimensional network
Spruing Technique:
Direct Spruing:
The flow of the molten metal is straight(direct) from the casting crucible to pattern area in the ring. Even with the ball reservoir, the Spruing method is still direct. A basic weakness of direct Spruing is the potential for suck-back porosity at the junction of restoration and the Sprue.
Indirect Spruing:
Molten alloy does not flow directly from the casting crucible into the pattern area, instead the alloy takes a circuitous (indirect) route. The connector (or runner) bar is often used to which the wax pattern Sprue formers area attached. Indirect Spruing offers advantages such as greater reliability & predictability in casting plus enhanced control of solidification shrinkage .The Connector bar is often referred to as a “reservoir .
Armamentarium :
1 . Sprue
2 . Sticky wax
3 . Rubber crucible former
4 . Casting ring
5 . Pattern cleaner
6 . Scalpel blade & Forceps
7 . Bunsen burner
CASTING: casting is the process by which the wax pattern of a restoration is converted to a replicate in a dental alloy. The casting process is used to make dental restorations such as inlays, onlays, crowns, bridges and removable partial dentures.
Objectives of casting
1) To heat the alloy as quickly as possible to a completely molten condition.
2) To prevent oxidation by heating the metal with awell adjusted torch .
3) To produce a casting with sharp details by having adequate pressure to the well melted metal to force into the mold.
STEPS IN MAKING A CAST RESTORATION
1. TOOTH PREPARATION
2. IMPRESSION
3. DIE PREPARATION
4. WAX PATTERN FABRICATION
5. SPRUING
Root canal sealers
Applications
Cementation of silver cone gutta-percha point
Paste filling material
Types
Zinc oxide-eugenol cement types
Noneugenol cement types
Therapeutic cement types
properties
Physical-radiopacity
Chemical-insolubility
Mechanical-flow; tensile strength
Biologic-inertness
Gingival tissue packs
Application-provide temporary displacement of gingival tissues
Composition-slow setting zinc oxide-eugenol cement mixed with cotton twills for texture and strength
Surgical dressings
1.Application-gingival covering after periodontal surgery
2. Composition-modified zinc oxide-eugenol cement (containing tannic, acid. rosin, and various oils)
Orthodontic cements
Application-cementation of orthodontic bands
Composition-zinc phosphate cement
Manipulation
Zinc phosphate types are routinely mixed with cold or frozen mixing slab to extend the working time
Enamel bonding agent types use acid etching for improved bonding
Band, bracket, or cement removal requires special care