DHYG 113 Restorative Dentistry I. Objectives Define: Study model, cast, die Discuss differences between dental plaster, stone, & improved stone Explain.

DHYG 113Restorative Dentistry I

ObjectivesDefine: Study model, cast, dieDiscuss differences between dental plaster, stone, & improved stoneExplain initial and final setting timesGive examples of how to increase and decrease setting timesDiscuss wet and dry strengthSummarize pouring impressions

Gypsum MaterialsSupplied as fine powders

Mixed with waterForm fluid mass that can be formed and shapedHardens into rigid stable mass

Used for making positive reproductions of oral structures:

Study Models – plan treatment & progress Casts – replica for building restoration or applianceDies – working replicas of single teeth

Desirable PropertiesAccuracyDimensional StabilityAbility to reproduce fine detailStrength & resistance to abrasionCompatability with impression materialColorSafety, ease of use, cost

GypsumMade from gypsum rock (mineral)Ground into fine powderHeated to form variety of productsUsed in dentistry, medicine, homes & industryHeated 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 dentistryWeakest, least expensive, and short setting timePowder consists of irregular porous crystalsRelatively Soft: for rudimentary procedures, making modelsUsually white in color

Alpha Form (Stone)Stronger, more expensive, and longer setting timePowder consists of more regular, uniform shaped, and less porous crystalsUsed for making casts for diagnostic purposes and for complete and partial denture constructionUsually yellowish in color

Alpha Modified (High Strength or Improved

Stone)Strongest, most expensive, and longest setting timePowder consists of very dense, cuboidal-shaped crystals with less surface areaUsed mainly for making casts and dies for inlays and crownsProvides surface hardness

Type IV(High Strength) vs Type III (Stone)

Qualities of Type IVExcellent working timeOptimum strength when vacuum investedGood color contrast with other materialsReasonably inexpensive

Qualities of Type IIIEasily fractures due to brittlenessSusceptible areas can be easily abradedLess expensive than Alpha Modified

Setting ReactionProduct of calcination (gypsum powder we use)Calcium sulfate hemihydrate + H2O =

Calcium sulfate dihydrate (changed back to rock form)

(gives off heat)

CaSO4 ٠ ½ H2O + 1٠ H2O CaSO4 ٠ 2 H2O + heat

Water-Powder RatioPlaster

45 – 50 ml/100 g

Stone28-30 ml/100 g

Improved Stone19-24 ml/100 g

Chemical Composition and Its Effect on Setting Expansion

Minimal expansion desirable for most casts and diesPlaster expands most; high strength stone expands leastExpansion 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 MixingHigher water/powder ratio – creates thinner mixAddition of retarders, such as Borax or Potassium Citrate

Decreased Setting Time (leads to faster-setting)

Increased mixingLower water/powder ratio – creates thicker mixAddition of accelerator, such as Potassium Sulfate, Sodium Chloride

Chemical Composition and Its Effect on Strength and Surface

HardnessStrength measured in terms of:

Crushing or Compressive Strength

Strength develops rapidly in 30-40 min after hydration is completeStrength depends on:

Porosity of materialPresence or absence of excess free water

Application to DentistryBeta Form (Plaster or Type II)

Study models, preliminary modelsAttaching casts to mechanical articulator

Alpha Form (Stone or Type III)Making diagnostic castsMaking casts for restorative or appliance fabricationMaking 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 bowlGradually add preweighed powder

MixingHand: Use flexible bowl and stiff-bladed spatula; Mix until reaching smooth, homogenous, workable product free of air-bubblesVacuum Mixing: Done mechanically with vacuum mixing and investing machine.

Application to Dentistry: Technique of Use

Filling the ImpressionMix needs to flow SLOWLY ahead of selfPrevent air entrapment

Accomplished with dental vibratorVibrate 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 PatientDepends greatly on the intended use of the modelsAppointment #1: Take Impressions

After appointment, fill impressions with gypsum material of choiceAllow 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 conditionsProlonged exposure can cause lung diseaseHandling powder and trimming models produces dust

Protective eye wear should be worn.Particles can cause irritation to eyes

Gloves should be worn.For hygienic purposesMixed 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’ officeBeta 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 popularMainly 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 hardnessIt is very abrasive resistantIt is dimensionally stableIt has very low setting expansion

Mainly used to make models and dies for crown and bridge fabrication.

Other Companies that Manufacture Gypsum

MaterialsKerr LabWhip MixModern materials – Heraeus KulzerPatterson BrandGC Lab TechnologiesDentsply Trubyte