NEET MDS Lessons
Dental Materials
Mouth Protectors
Use - to protect against effects of blows to chin, top of the head, the face, or grinding of the teeth
Types
o Stock protectors-least desirable because of poor fit
o Mouth-formed protectors-improved fit compared with stock type
o Custom-made protectors-preferred because of durability. low speech impairment, and comfort
I. Components
a. Stock protectors-thermoplastic copolymer of PYA-PE (polyvinyl acetate-polyethylene copolymer)
b. Mouth-formed protectors-thermoplastic copolymer
c. Custom-made protectors- thermoplastic copolymer, rubber. or polyurethane
2. Reaction-physical reaction of hardening during cooling
3. Fabrication
Alginate impression made of maxillary arch. High-strength stone cast poured immediately. Thermoplastic material is heated in hot water and vacuum-molded to cast .
Mouth protector trimmed to within ½ inch of labial fold, clearance provided at the buccal and labial frena, and edges smoothed by flaming. Gagging, taste, irritation. and impairment of speech are minimized with properly fabricated appliances
4. Instructions for use
a. Rinse before and after use with cold water
b. Clean protector occasionally with soap and cool water
c. Store the protector in a rigid container
d. Protect from heat and pressure during storage
e. Evaluate protector routinely for evidence of deterioration
Properties
1. Physical-thermal insulators
2. Chemical-absorbs after during use
3. Mechanical-tensile strength, modulus, and hardness decrease after water absorption, but elongation, tear strength, and resilience increase
4. Biologic-nontoxic as long as no bacterial, fungal, or viral growth occurs on surfaces between uses
Acrylic Appliances
Use - space maintenance or tooth movement for orthodontics and pediatric dentistry
1. Components
a. Powder-PMMA powder. peroxide initiator, and pigments
b. Liquid-MMA monomer, hydroquinone inhibitor, cross-linking agents, and chemical accelerators (N, N-dimethyl-p-toluidine)
2. Reaction
PMMA powder makes mixture viscous for manipulation before curing . Chemical accelerators cause decomposition of benzoyl peroxide into free radicals that initiate polymerization of monomer . New PMMA is formed into a matrix that surrounds PMMA powder. Linear shrinkage of 5% to 7% during setting. but dimensions of appliances are not critical
Investment Techniques
Single step investing technique:
The investing procedure is carried out in one step either by brush technique or by vacuum technique.
a). Brush technique:
The accurate water-powder ratio is mixed under vacuum. A brush is then used to paint the wax pattern with mix then the casting ring is applied over the crucible former and the ring is filled under vibration until it is completely filled.
b). vacuum technique:
• The mix in first hand spatulated, and then with the crucible former and pattern is place, then ring is attached to the mixing bowl.
• The vacuum hose is then attached to the assembly. The bowel is inverted and the ring is filled under vacuum and vibration
Two-step investing technique:
The investing procedure is carried out in two steps:
• First, the wax pattern is painted with a thick mix andis left till complete setting, the set investment block(first cost) is immersed in water for about tenminutes . the casting ring is then applied over the crucible former and filled with the properly mixedinvestment (second coat) till the ring is completely filled and the mix is left to set.The two-step investing technique is recommendedwhenever greater amount of expansion is required. Thistechnique also minimizes the distortion of the waxpattern and provides castings with smoother surfaces.
• The investment is allowed to set for the recommendedtime (usually one-hour) then the crucible former is removed. If a metal sprue former is used, it is removedby heating over a flame to loosen it from the wax pattern. Any loose particles of investment should beblown off with compressed air should be placed in a humidor if stored overnight.
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Silver |
Tin |
Copper |
Zinc |
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Strength |
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Durability |
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Hardness |
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Expansion |
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Flow |
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Color |
Imparts |
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Setting time |
Decreases |
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Workability |
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Composition of Acrylic Resins.
· Powder. The powder is composed of a polymethyl methacrylate (PMMA), peroxide initiator, and pigments
· Liquid. The liquid is a monomethyl methacrylate (MMA), hydroquinone inhibitor, cross-linking agents, and chemical accelerators (N, N-dimethyl-p-toluidine)