Presentation on theme: "DHYG 113 Restorative Dentistry I. Objectives Define: Study model, cast, die Discuss differences between dental plaster, stone, & improved stone Explain."— Presentation transcript:
Objectives Define: Study model, cast, die Discuss differences between dental plaster, stone, & improved stone Explain initial and final setting times Give examples of how to increase and decrease setting times Discuss wet and dry strength Summarize pouring impressions
Gypsum Materials Supplied as fine powders Mixed with water Form fluid mass that can be formed and shaped Hardens into rigid stable mass Used for making positive reproductions of oral structures: Study Models – plan treatment & progress Casts – replica for building restoration or appliance Dies – working replicas of single teeth
Desirable Properties Accuracy Dimensional Stability Ability to reproduce fine detail Strength & resistance to abrasion Compatability with impression material Color Safety, ease of use, cost
Gypsum Made from gypsum rock (mineral) Ground into fine powder Heated to form variety of products Used in dentistry, medicine, homes & industry Heated to drive off part of the water of crystallization -- calcination
Chemical Composition Gypsum has 3 derivatives, each produced differently Beta Form (Plaster or Type II) Alpha Form (Stone or Type III) Alpha Modified (High-Strength/Improved Stone or Type IV) All are Calcium Sulfate Dihydrate
Beta Form (Plaster) First gypsum product available in dentistry Weakest, least expensive, and short setting time Powder consists of irregular porous crystals Relatively Soft: for rudimentary procedures, making models Usually white in color
Alpha Form (Stone) Stronger, more expensive, and longer setting time Powder consists of more regular, uniform shaped, and less porous crystals Used for making casts for diagnostic purposes and for complete and partial denture construction Usually yellowish in color
Alpha Modified (High Strength or Improved Stone) Strongest, most expensive, and longest setting time Powder consists of very dense, cuboidal-shaped crystals with less surface area Used mainly for making casts and dies for inlays and crowns Provides surface hardness
Type IV(High Strength) vs Type III (Stone) Qualities of Type IV Excellent working time Optimum strength when vacuum invested Good color contrast with other materials Reasonably inexpensive Qualities of Type III Easily fractures due to brittleness Susceptible areas can be easily abraded Less expensive than Alpha Modified
Setting Reaction Product of calcination (gypsum powder we use) Calcium sulfate hemihydrate + H 2 O = Calcium sulfate dihydrate (changed back to rock form) (gives off heat) CaSO 4 ٠ ½ H 2 O + 1٠ H 2 O CaSO 4 ٠ 2 H2O + heat
Water-Powder Ratio Plaster 45 – 50 ml/100 g Stone 28-30 ml/100 g Improved Stone 19-24 ml/100 g
Chemical Composition and Its Effect on Setting Expansion Minimal expansion desirable for most casts and dies Plaster expands most; high strength stone expands least Expansion reduced by Potassium Sulphate, Sodium Chloride, and Borax. Expansion increased by Immersion in or contact with water during setting.
Increased Setting Time (leads to slower-setting) Decreased Mixing Higher water/powder ratio – creates thinner mix Addition of retarders, such as Borax or Potassium Citrate Decreased Setting Time (leads to faster-setting) Increased mixing Lower water/powder ratio – creates thicker mix Addition of accelerator, such as Potassium Sulfate, Sodium Chloride
Chemical Composition and Its Effect on Strength and Surface Hardness Strength measured in terms of: Crushing or Compressive Strength Strength develops rapidly in 30-40 min after hydration is complete Strength depends on: Porosity of material Presence or absence of excess free water
Application to Dentistry Beta Form (Plaster or Type II) Study models, preliminary models Attaching casts to mechanical articulator Alpha Form (Stone or Type III) Making diagnostic casts Making casts for restorative or appliance fabrication Making casts for complete and partial denture construction Alpha Modified (High Strength Stone or Type IV) Making casts and dies for crowns, bridges, inlays, onlays
Application to Dentistry: Technique of Use Preferred method of mixing: Pour measured water into bowl Gradually add preweighed powder Mixing Hand: Use flexible bowl and stiff-bladed spatula; Mix until reaching smooth, homogenous, workable product free of air- bubbles Vacuum Mixing: Done mechanically with vacuum mixing and investing machine.
Application to Dentistry: Technique of Use Filling the Impression Mix needs to flow SLOWLY ahead of self Prevent air entrapment Accomplished with dental vibrator Vibrate after mixing to bring air to surface Good technique is essential to decrease chance of inaccurate results (air bubbles)
Application to Dentistry: Technique of Use Treatment Plan for Patient Depends greatly on the intended use of the models Appointment #1: Take Impressions After appointment, fill impressions with gypsum material of choice Allow material to set for at least 1-2 hours but preferably overnight in order to fully set Trim and design study models appropriately for intended use Appointment #2 and Further Appointments: Use study models for intended use; Additional appointments may be required for fitting and seating prosthetic appliances and crown and inlay fabrication.
OSHA and Safety Guidelines OSHA permissible exposure limit is “none”. Dental labs should be well-ventilated /mask to prevent dust particle inhalation. Can aggravate pre-existing upper respiratory conditions Prolonged exposure can cause lung disease Handling powder and trimming models produces dust Protective eye wear should be worn. Particles can cause irritation to eyes Gloves should be worn. For hygienic purposes Mixed material may develop enough heat to cause burns on skin while hardening
Precautions for Dental Hygiene Clinicians Gypsum materials are very stable and non- hazardous if handled properly. Follow OSHA guidelines by wearing a mask, gloves and protective eye wear. Wet grinders should be used while trimming models to reduce dust production. A lab coat may be worn to protect clothing from splatter while mixing and trimming.
Interview with Dr. Timmins Brands we found in Dr. Timmins’ office Beta Form (Plaster, type II): Orthodontic Plaster- Whip Mix Uses this brand because it is the cheapest. Mainly used for mounting casts on an articulator Alpha Form (Stone, type III):Hydrock- KerrLab Uses this brand because it is the most popular Mainly used to pour up alginate impressions for various uses
Interview with Dr. Timmins Continued Alpha Modified Form (High Strength, type IV): New Fuji Rock–GC Lab Technologies Uses this brand because, It has the highest level of hardness It is very abrasive resistant It is dimensionally stable It has very low setting expansion Mainly used to make models and dies for crown and bridge fabrication.
Other Companies that Manufacture Gypsum Materials Kerr Lab Whip Mix Modern materials – Heraeus Kulzer Patterson Brand GC Lab Technologies Dentsply Trubyte