PCO5 Introduction to the Lab

Alginate and Study Model Materials

To be able to make any Prosthesis or Splint, we must make an accurate replica of the Patients mouth. To do this we take an Impression, and make a cast Gypsum model from the Impression.

Materials Included are -

  • ALGINATE – This is the usual material of choice, - accurate, easy to use and inexpensive.

  • DISINFECTANT - This is applied to all surfaces that have had any contact with the Patient. Confirmation of the disinfecting procedure must also be logged on the Laboratory Prescription Card.

  • GYPSUM – (Calcium Sulphate) This material is provided in five versions, they are selected by the Laboratory for their intended use. (accuracy, hardness and expense.)

DISINFECTANTS

It is stated in COSHH regulations that the Clinician is responsible for any Cross infection Issues with work supplied to the Laboratories. All impressions, (and surfaces handled with Clinical gloves) are disinfected in the Clinic.

The person who completed the procedure must sign the Prescription (Lab Card), this confirms the Disinfection of items - NO WORK WILL BE UNDERTAKEN WITHOUT THIS SAFEGUARD

The Impressions are completely immersed in PERFORM, a disinfectant, for 10 minutes.

CONSTITUENTS OF PERFORM – supplied as a powder and mixed with water

  • 0.2g Potassium Peroxomono Sulphate

  • 0.15g Sodium Benzoate

  • 0.1g Tartanic Acid

  • 5-15% Anionic surfactants

  • 5-15% Non ionic surfactants

  • Less than 5% soap, Phosphonate, and Fragrance

Surface Surfactants- (UNODENT) Supplied as an Alcohol-free spray, they are for use mainly on nonporous material. Fungicide and Bactericide

Kills:

  • Hepatitis B/ TB in 2minutes

  • Yeast and Fungus in 1 minute

  • Bacteria in 30 seconds

IDEAL PROPERTIES OF IMPRESSION MATERIAL

ALGINATE - Elastic Impression Material

The main Prosthetic Impression material

A powder turns to GEL when mixed with water and further converts into elastic material on manipulation, setting by SOL/GEL transition.

Used in conjunction with “FIX” Adhesive

  • Very accurate and reproduces fine detail

  • Poor long term stability

    • Water and component loss

    • Fluid can be absorbed - Imbibition

    • Fluid can be lost - syneresis

    • Constituents can also leach from the material

INGREDIENTS - NOTE, the Alginate container should be inverted before mixing, to disperse the constituents and ensure correct measures.

• Sodium alginate (from seaweed) very quickly reacts with Calcium sulphate dehydrate (CaSO₄ 2H₂O) to form a GEL

• K₂SO₄, silicates, Borates - counterbalance GEL effect (slows it down)

• Sodium Phosphate- retarder for Calcium/ alginate reaction

• Diatomaceous Earth - Main bulk/inert filler- 70% mass (also known as Kieselguhr, where it was first discovered)

Storage and Transport of ALGINATE IMPRESSIONS

To maintain accuracy, Alginate Impressions should be Cast as soon as possible after they have been Disinfected.

The material suffers from loss of moisture, (called SYNERESIS), so Impressions should be stored in a MOIST CONDITION, ie, wrapped in moist tissue and placed in a sealed Plastic Bag.

Do not use too much water as the Alginate material can also soak up any excess moisture,(called IMBIBITION).

If the Alginate loses or gains any liquids it loses accuracy.

If Alginate is left for too long a period before it is cast, it exudes some of its constituents which contaminate the Gypsum model.

Cast Gypsum Models

GYPSUM (Calcium Sulphate) is supplied as a Powder. It is mixed with water and then it is poured into the Impression. Then it will “SET” hard, forming the accurate replica of the mouth.

These Models are very accurate and are used by the Laboratory to construct Dental Appliances/Prosthesis.

It is supplied in different grades and the Laboratory will select the type of Gypsum appropriate to the use intended.

STUDY MODELS.

These can be cast in DENTAL STONE, or DIE STONE (more expensive).

The material usually employed is DENTAL STONE.

GYPSUM MATERIALS - CALCIUM SULPHATE- CaS0₄ 2H₂0

These are used to form accurate replica models of the mouth, Dental Moulds and Fixtures in the Lab

There are different types, all are chemically identical but differ in their morphology and physical properties due to differences in the manufacturing process

CAD/CAM STUDY MODELS

Where facilities for CAD/CAM are available –

The Patients mouth is electronically scanned, this eliminates the need for an Alginate Impression.

The information gathered can be stored in the Computer for long term reference, or used to make a physical model by the Computer Printing technique.

The information can be presented “On-Screen” for Patient education. Images from different periods of the Dental treatment can be screened together to illustrate and measure the progress made.

CAD/CAM Printed Models

Model Casting Technique

Mixing ratio

  • For one impression:

    • 400g dental stone

    • 120ml cold water

  • For two impressions:

    • 800g dental stone

    • 240ml cold water

Mixing

  • Measure the correct quantities of the dental stone and cold water

  • Sprinkle the dental stone onto the water

  • Do not add too much stone at once

  • Keep adding the stone until an excess forms on the surface

  • The stone will eventually float on the surface and begin to crust

  • After about 30 seconds the crust will absorb moisture and turn a darker yellow

  • The water and stone can now be mixed

  • A circular action should be used to avoid air bubbles being incorporated into the mix

  • Keep turning the bowl this ensures that the spatular scrapes the bottom and the sides of the bowl

  • This will include all the stone that has been added to the mix

  • Allow the plaster to drop off of the spatula in order to check the final consistency

  • The mix should resemble custard or double cream when mixed

Casting

  • The stone mix is now ready to pour into the impression

  • Add the stone in small increments so that air isn’t trapped in the anatomical portion of the impression

  • It is necessary to lightly tap the impression against the side of the bowl, to encourage the plaster to flow into the impression

  • Alternatively, a vibrating table can be utilised to achieve this effect

Basing

  • Once the anatomical area of the impression has been filled with stone, it is put to one side and plaster is placed on a paper towel

  • This is the beginning of the model base

  • The stone mix is gradually added to this mound until there is sufficient stone to form the model base

  • If the stone mix is too thin or the mound is created too quickly it will spread outwards rather than upwards

    • We are trying to achieve height rather than width

  • The impression can then be gently inverted onto the mound

  • Take care not to push the impression into stone which is starting to harden

  • If the mound is starting to set, then it is better to discard the base stone and make a new mix

  • The mound should be sufficiently stable to support the weight of the impression

  • The impression is then gently eased down into the base slightly

Trimming

  • The edges of the base mix are then trimmed and shaped and pushed up around the periphery of the impression

  • This is to ensure that all the anatomical areas are recorded

  • It is important to cover and support the heels and labial/buccal sulci

  • Make sure that there is an excess of base material around the full periphery of the impression

  • The stone base should by this time have consistency like plasticene and so can easily be trimmed, using a plaster knife, this is to remove any excess

  • The excess material is then removed and placed in a plaster bin

Cleaning

  • If you have not had chance earlier

  • Scrape any excess material from the bowl and discard the plaster in a plaster bin

  • Then wash the bowl and spatula ready for later use

Trimming

  • Allow the stone to set thoroughly before attempting to remove the impression

  • This will normally take at least 30 minutes

  • 1-2 hours is preferable if possible

Dental Gypsum Products

Dental Stone:

  • Colour yellow

  • Used for study models and working models

  • Mixing ratio 22-35ml water to 100g powder

Plaster of Paris:

  • Colour white

  • Used for flasking, mounting casts on articulator and basing casts

  • Mixing ration 50-60ml water to 100g powder

Ideal Properties

  • Dimensional accuracy and stability

  • Ease of use

  • Accurate reproduction of fine detail

  • No interaction between impression and model or die material

  • Good strength and hardness

  • Relatively inexpensive

  • The ability to resist abrasion

  • Good colour contrast

Dental Stone

(CaSO4)2.H2O

  • Alpha form, produced by heating gypsum under steam pressure in an autoclave at 120-150 degrees

  • It is composed of small regularly shaped prismatic particles

  • Particles with low porosity which pack together tightly

  • Initial set 5-20 minutes

  • Linear expansion 0.1-0.2%

Plaster of Paris

(CaSO4)2.H2O

  • Beta form is obtained by heating gypsum in an open vessel at 150-160 degrees

  • Part of the water of crystallisation is driven off

  • Possesses high porosity, a short setting time

  • Characterised by large irregularly shaped particles

  • Initial set 5-10 minutes

  • Linear expansion 0.3-0.4%

Casting Gypsum Models

1st Stage - Pouring casts/creating models

  • Dental stone powder and water are mixed, then applied and vibrated into the preliminary primary impression

  • A base is then created for each of the models

  • The stone is sculpted to form the rough outline of the base

  • When the stone has set sufficiently, the impressions are removed and trimmed (firstly using a model trimmer with water coolant)

  • This produces casts of a more manageable size whilst at the same time preserving the important peripheral area of the impression

  • Casts are then further trimmed using a plastic knife to remove excess amounts of stone

Bleaching Trays

Tooth whitening, Bleaching.

This can be achieved using Laser Light, or using a Bleaching Gel held onto the Teeth by a Thermoplastic Tray.

The Tray method is described here.

Tooth whitening can be used on an individual Tooth, or whole sections of the Dental Arch, (Aesthetic Zone).

Method of Manufacture

An accurate Alginate Impression of the Teeth and the surrounding Gum area is essential.

An accurate Dental Stone model is made.

The base of the Cast model is trimmed to suit the equipment being used, (Height and Shape).

Design for Bleaching Tray

Following manufacturers recommendations, the amount of BLEACHING Gel in contact with the Teeth can be varied.

THE CLINICIAN must prescribe the method of Tray manufacture.

The choice is between the use of -

METHOD ONE. The Tray being in intimate contact with the surface of the Tooth.

METHOD TWO. The Gel Reservoir

METHOD ONE. The Tray can be made directly onto the surface of the Model. A thin film of Gel will be in contact with the Teeth.

The Tooth surface is reproduced by the Dental Stone model.

The Bleaching Tray material is formed over the model, giving an exact fit. The viscous Gel will be held against the Tooth surface.

The Bleaching Tray is trimmed around the edges.

Palatal/Lingual areas are extended about 10mm beyond the gingival margin.

NO GEL IS PLACED ON THESE SOFT TISSUE AREAS. Mucosal irritation!

The Labial face of the Bleaching Tray is trimmed TO THE LABIAL GINGIVAL MARGIN.

The Gel is held against the Tooth Enamel –NOT THE SOFT TISSUE.

METHOD TWO.

Wax or Resin material can be added on to the surfaces of the teeth.

These additions, called RESERVOIRS, increase the amount of Gel material that is in contact with the Tooth surface.

The model is an exact replica of the Tooth surfaces.

Wax or Resin is applied to the surfaces to be Bleached. Approximately 0.5mm thick,(variable).

This forms a space under the Bleaching

The tray which will be filled with Gel.

It is important to shape the Reservoir so it does not extend onto the Gingival margin.

The Tray will form a seal against the Enamel and prevent the Gel from irritating the Gingival margins

The Bleaching Tray is formed over the model and Reservoirs. The edges are trimmed -

Palatal/Lingual extend about 10mm from the Gingival margin.

Labial edge is shaped to the Gingival Margin.

This forms a space under the Tray which can be filled with Bleaching Gel

The Bleaching Tray material is manufactured as thin sheets of Thermoplastic Blanks.

Various types are available.

There are different types of Thermo forming machines. Each manufacturer has developed their own Machine and their own Tray Blanks.

The Machines fall into two groups –

1) Machines which heat the Blank until soft ,then VACUUM FORM the blank onto the Model.

2) Machines which heat the Blank until soft, then High Pressure air is applied to the surface to push the blank onto the surface of the model.

The Soft splint is trimmed to shape using scissors and appropriate dental burs.

The completed splint is then returned onto the model and sent to Clinic

The Patient is taught how to apply the Gel and use the BLEACHING TRAY

FLUORIDE APPLICATION TRAYS

Essentially, these are made in exactly the same way as the Bleaching Trays.

The exception being – we do not form Reservoir areas on the Teeth. The viscosity of the Fluoride Gel is sufficient for surface application.

Other Splints.

Thermo forming Blanks are a common method to make Custom devices for the mouth.

These include –

Active and Passive Splints.

Sports Mouth Guards (Gumshields).

Anti-Snoring devices.

Prosthetic Laboratory Prescription Card

LABORATORY PRESCRIPTION CARDS

• A laboratory prescription card is a legal document.

• All details must be printed, clear and legible.

• Do not use felt tip pens, the card will be placed in a moist environment and it will smear.

• Do not enclose the card with damp impressions.

• All cards need valid signatures.

• Always disinfect clinical work and sign the relevant area;

WORK WILL NOT BE ACCEPTED BY THE LABORATORY WITHOUT THIS CONFIRMATION