NEET MDS Lessons
Dental Materials
Pit-and-Fissure Dental Sealants
Applications/Use
Occlusal surfaces of newly erupted posterior teeth
Labial surfaces of anterior teeth with fissures
Occlusal surfaces of teeth in older patients with reduced saliva flow (because low saliva increases the susceptibility to caries)
Types
Polymerization method
Self-curing (amine accelerated)
Light curing (light accelerated)
Filler content
Unfilled-most systems are unfilled because filler tends to interfere with wear away from self-cleaning occlusal areas(sealants are designed to wear away, except where there is no self-cleaning action a common misconception is that sealants should be wear resistant)
Components
Monomer-BIS-GMA with TEGDM diluent to facilitate flow into pits and fissures prior to cure
Initiator-benzoyl peroxide (in self-cured) and diketone (in light cured)
Accelerator-amine (In light cured)
Opaque filler-I % titanium dioxide. or other colorant to make the material detectable on tooth surfaces
Reinforcing filler-generally not added because wear resistance is not required within pits and fissures
Reaction-free radical reaction
Manipulation
Preparation
Clean pits and fissures of organic debris. Do not apply fluoride before etching because it will tend to make enamel more acid resistant. Etch occlusal surfaces, pits, and fissures for 30 seconds (gel) or 60 seconds (liquid) with 37% phosphoric acid . Wash occlusal surfaces for 20 seconds. Dry etched area for 20 seconds with clean air spray. Apply sealant and polymerize
Mixing or dispensing
Self-cured-mix equal amounts of liquids in Dappen dish for 5 seconds with brush applicator. Light cured-dispense from syringe tips
Placement
-pits, fissures, and occlusal surfaces --> Allow 60 seconds for self-cured materials to set.
Finishing
Remove unpolymerized and excess material .Examine hardness of sealant. Make occlusal adjustments where necessary in sealant; some sealant materials are self-adjusting
Properties
Physical
Wetting-low-viscosity sealants wet acid etched tooth structure the best
Mechanical
Wear resistance should not be too great because sealant should be able to wear off of self-cleaning areas of tooth
Be careful to protect sealants during polishing procedures with air abrading units to prevent sealant loss
Clinical efficacy
Effectiveness is 100% if retained in pits and fissures .Requires routine clinical evaluation for resealing of areas of sealant loss attributable to poor retention .
Sealants resist effects of topical fluorides
CLEANING AND PICKLING ALLOYS
The surface oxidation or other contamination of dental alloys is a troublesome occurrence. The oxidation of base metals in most alloys can be kept to a minimum or avoided by using a properly adjusted method of heating the alloy and a suitable amount of flux when melting the alloy . Despite these precautions, as the hot metal enters the mold, certain alloys tend to become contaminated on the surface by combining with the hot mold gases, reacting with investment ingredients, or physically including mold particles in the metal surface. The surface of most cast, soldered, or otherwise heated metal dental appliances is cleaned by warming the structure in suitable solutions, mechanical polishing, or other treatment of the alloy to restore the normal surface condition.
Surface tarnish or oxidation can be removed by the process of pickling. Castings of noble or high-noble metal may be cleaned in this manner by warming them in a 50% sulfuric acid and water solution . . After casting, the alloy (with sprue attached) is placed into the warmed pickling solution for a few seconds. The pickling solution will reduce oxides that have formed during casting. However, pickling will not eliminate a dark color caused by carbon deposition
The effect of the solution can be seen by comparing the submerged surfaces to those that have still not contacted the solution. the ordinary inorganic acid solutions and do not release poisonous gases on boiling (as sulfuric acid does). In either case, the casting to be cleaned is placed in a suitable porcelain beaker with the pickling solution and warmed gently, but short of the boiling point. After a few moments of heating, the alloy surface normally becomes bright as the oxides are reduced. When the heating is completed, the acid may be poured from the beaker into the original storage container and the casting is thoroughly rinsed with water. Periodically, the pickling solution should be replaced with fresh solution to avoid excessive contamination.
Precautions to be taken while pickling
With the diversity of compositions of casting alloys available today, it is prudent to follow the manufacturer's instructions for pickling precisely, as all pickling solutions may not be compatible with all alloys. Furthermore, the practice of dropping a red-hot casting into the pickling solution should beavoided. This practice may alter the phase structure of the alloy or warp thin castings, and splashing acid may be dangerous to the operator. Finally, steel or stainless steel tweezers should not be used to remove castings from the pickling solutions. The pickling solution may dissolve the tweezers and plate the component metals onto the casting. Rubber-coated or Teflon tweezers are recommended for this purpose.
Denture Teeth
Use-complete or partial dentures
Type
a. Porcelain teeth
b. Acrylic resin teeth
c. Abrasion-resistant teeth (microfilled composite)
Structure and properties
1. Porcelain teeth (high-fusing porcelain)
Only bonded into denture base mechanically. Harder than natural teeth or other restorations and abrades those surfaces. Good aesthetics.Used when patients have good ridge support and sufficient room between the arches
2. Acrylic resin teeth (PMMA [polymethyl methacrylate])
Bonded pseudochemically into the denture base. Soft and easily worn by abrasive foods . Good initial aesthetics
Used with patients with poor ridges and in cases where they oppose natural teeth
3. Abrasion-resistant teeth (microfilled resins)
Bonded pseudochemically into the denture base.Better abrasion resistance then acrylic resin teeth
Principles of cutting, polishing, and surface cleaning
- Surface mechanics for materials
Cutting-requires highest possible hardness materials to produce cutting
Finishing-requires highest possible hardness materials to produce finishing, except at margins of restorations where tooth structure may be inadvertently affected
Polishing- requires materials with Mohs ./ hardness that is 1 to 2 units above that of substrate
Debriding-requires materials with Mohs hardness that is less than or equal to that of substrate to prevent scratching
- Factors affecting cutting, polishing. and surface cleaning
- Applied pressure
- Particle size of abrasive
- Hardness of abrasive
- Hardness of substrate
- Precautions
- During cutting heat will build up and change the mechanical behavior of the substrate from brittle to ductile and encourage smearing
- Instruments may transfer debris onto the cut surface from their own surfaces during cutting, polishing, or cleaning operations (this is important for cleaning implant surfaces)
Reaction
a. Calcium sulfate hemihydrate(one-half water) crystals dissolve and react with water
b. Calcium sulfate dihydrate(two waters) form and precipitate new crystals
c. Unreacted (excess) water is left between crystals in solid
DISTORTION OF THE PATTERN
Distortion is dependant on temperature & time interval before investing .
To avoid any distortion ,
Invest the pattern as soon as possible .
Proper handling of the pattern .
PREREQUISITES
Wax pattern should be evaluated for smoothness , finish & contour .
Pattern is inspected under magnification & residual flash is removed .
COMPOSITE RESINS
Components
- Filler particles-colloidal silica, crystalline silica (quartz), or silicates of various particle sizes (containing Li, AI, Zn, Yr)
- Matrix-BIS-GMA (or UDMA) with lower molecular weight diluents (e.g., TEGDMA) that correct during polymerization
- Coupling agent- silane that chemically bonds the surfaces of the filter particles to the polymer matrix