Biomaterials 2

Denture Bases

Impression Materials

Successful Rem Pros Impressions

Definitions

Primary impressions:
Used for diagnostic and treatment
planning purposes and fabrication of custom trays

Secondary impression:
An imprint that records the entire functional denture-bearing area to ensure maximum support, retention and stability for the denture during use

Impression:
A negative imprint of hard and soft tissues in the mouth from which a positive reproductive cast/model can be formed

Boarder Moulding

Purpose:
correctly extended custom tray
flanges into the functional sulcus of the oral cavity
Technique:
achieved with hand
manipulation, and functional movements

Materials

White-wax

  • can be shaped w/o applying heat

Green Compound Wax

  • plastic when heated, rigid when cooled

Exahiflix - Tray Type Addition Silicone

Accuracy = increased w/ wax spacers/tissue stops

Tray Adhesive

  • Aids in retention of the impression material in the
    tray
  • Leave to dry in the impression tray for 10 minutes
    • nb: Additional retention is achieved by using
      perforated trays

Impression Technique

Reason for impression rejection:
1) bubbles/voids in critical areas
2) teeth contact w/ tray
3) peripheral under-extension
4) interproximal tearing
5) lack of detail

Acrylic Base Materials

Reline Materials

Ideal Properties

Hard Lining

  • Biological: non-toxic and non-irritant, chemically inert
  • Chemical: it should be unaffected by oral fluids
  • Mechanical: adequate mechanical properties (i.e. high elastic modulus)
  • Physical: High value of thermal conductivity, radioopaque
  • Aesthetic: aesthetically satisfactory.

Setting Reaction

Polymerization: activation, initiation, propagation, termination

Types

Ex. heat-cure,
Ex. auto-cure
Ex. light cure
nb: similar compositions, but differ in their initiation/activation mechanisms

Tissue Conditioner

Soft Lining

Types:

  • Type 1
  • Type 2
    • is less irritation to the oral cavity

Definition: Temporary/semi-permanent solution to ridge resorption, where materials are placed in the mouth and changes fabricated in the lab

Definition: help to relieve pain and increase patient adaptation which is done over a longer period of time

Types:

  • Temporary soft liner
    • When frequent replacement is difficult, to assess patient’s response to soft liners and to improve fitness before definite denture constructed
  • Permanent soft liner
    • permanent soft cushion for patients who cannot tolerate a hard denture

Successful Fixed Pros Impressions

Technique:

Clinical impression techniques can be classified as either one material-one step, two material-one step or two material-two step and are usually performed with custom trays.

Purpose: “Negative” likeness of a dental structure and accurate replica of intraoral tissues

Requirements:

  • should be:
    • fluid enough to adapt to tissues
    • viscous enough to be contained in the tray
    • remain stable until poured
    • biocompatible/cost effective
  • Not distort/tear when removed
  • Set rigid/rubbery in mouth

Summary

Impression Materials

Elastic

Elastomers

Polysulphides

Adv.

  • good detail reproduction (Hydrophilic)
  • flexible

Disadv.

  • Long setting time (indicated in Maxfac face impressions)
  • hydrophobic
    • smell

Addition Silicone (PVS)

Viscosities
Change w/ filler content

  • available as heavy, medium, light

Affected by the shear force(working) placed on the material, such as when they are extruded from a syringe, they will exhibit low viscosity. When placed in the impression tray with a spatula, they have a high viscosity (shear thinning)

Setting Reaction

Hydrogen can also be released in a secondary reaction. commonly due to impurities from the reaction, palladium is added to absorb the hydrogen (NOT A BYPRODUCT)
= recommended to wait at least 60 minutes before pouring

In the polymerization of addition silicone, hydrogen atoms from the siliane group move along the backbone structure onto the vinyl silicone. Following the platinum salt initiator, the units join as repeating units of dimethylsiloxane and terminate w/ vinyl group

Advantages/Disadvantages:

Advantages:
High accuracy
Good dimensional stability (up to 3-7 days)
Low setting shrinkage
Short setting time
Auto mixed & Dual-phase system
No hypersensitivity reactions

Disadvantages:
Inhibition by sulfur-containing materials
ie latex gloves
Relatively expensive
Actually hydrophobic, despite manufacturer claims

Polyethers

Adv.

  • Good surface detail/most accurate
  • Dimensional stable

Disadv.

  • Long setting time
  • Rigid set

Hydrocolloids

Alginate (irreversible)

Setting Reaction
Alginate & water (4:1) ---> sol ---> gel
Sodium alginate reacts with calcium sulfate to form calcium alginate forming crosslinks in solute. It then sets to form a gel.

Properties

  • syneresis
    = cross-linking reaction continues and water is extruded from the surface, resulting in a dimensional change
  • Imbibition
    = is the absorption of water by the material from the environemnt

Indications
= primary impression material but also has uses as secondary and impressions and duplicating models

Advantages of alginate:

  • Easy to use
  • Flexible
  • Smells nice
  • Cheap
  • Mainly its hydrophilic → therefore good surface detail in presence of saliva

Disadvantages:

  • Low tear strength
  • Dimensionally unstable
  • Low detail production

Trouble Shooting:
Early removal
Sticking to teeth
Improper tray placement/seating

Disinfect/Pouring
Placed in disinfection for 5 mins
It should be poured within 60 minutes of
taking the impression

Agar (reversible)


  • Used for duplication of impressions
  • Not really used in primary impression

  • long setting time
  • unstable out of the mouth
  • less permanent deformation than alginate
  • more hydrophilic than alginate

Non-Elastic

Impression Compound

thermoplastic materials that can be softened to their working consistency by working over a flame or in a hot bath

Types:

Type 1 - Impression compound(Lower fusing)
= sticks, are used for border moulding or impressions

Type 2 -Tray compound(Higher fusing)
= sheets, are used for secondary impression trays
(now been replaced by acrylics due instability)

Ideal characteristics

  • soft at a point above oral temperature (flow over tissues)
  • harden at mouth temperature (minimum flow to reduce distortion)

Adv.

  • Sufficient displacement of the lingual and soft tissues
  • good peripheral seal
    Disadv.
  • poor stability
  • low detail reproduction
  • mucocompressive
  • stiff

ZnOE

A metallic oxide paste
Uses:

  • secondary impression materials for complete full dentures
  • relining distal extension denture bases
  • non-undercut edentulous ridges

acid base/chelation reaction that forms a chelate, that is zinc eugenolate.
nb: Eugenol can be substituted for patients with allergie w/ lauric acid.

Setting

  • Delayed by
    • Cooling mixing slab or spatula
    • Adding small amount of retarder/old waxes
  • Increased by
    • Zinc acetate/additional accelerator
    • Drop of water in the paste before mixing

properties
inelastic, low shrinkage , stable, difficult to handle

Non-ionic surfactant added to the paste to render the surface of the impression hydrophilic (uses include RPD, FPD, F/F, Fi/Fi) ex. Exaflex

Condensation Silicone

produces ethanol as a reactionary by-product

reasonable setting time, high accuracy and elastic recovery, however their hydrophobicity and dimensional instability has made their use in dentistry relatively limited

with high filler content (putty) are rigid and may be used to make stents or customise a stock tray with the putty-wash technique

Properties

The impression must be poured within one hour to prevent distortion as a result of the evaporation of the by-product (water)

long working and setting time of 10 to 20 minutes

Highly flexible hence favoured for use in patients with periodontally compromised teeth or facial malformations

bad odour and hence not preferred by patients

properties

hydrophilic of the elastomers and exhibits excellent dimensional stability when stored in a dry environment

most rigid of the elastomers which may contraindicate in uses where presence of deep undercuts or periodontally-involved teeth

appreciable elastic recovery, high tear strengths and are extremely accurate (indicated for FPDs) ex. 3M Impregum

Agar is an excellent impression material and yields accurate impressions, it has been replaced by other impression materials due to its complexity and needing special equipment

Minimum Tray Thickness 2.0mm