Dental irreversible hydrocolloid

Literature review

Dental irreversible hydrocolloid (alginate) is a most important dental impression material used worldwide in many clinical procedures. On the other hand, alginate is dimensionally unstable and changes its dimensions after removal from the mouth. by means of storage time of more than ten minutes, alginate begins to distort, and after one to three hours (depending on the product and storage condition) cannot be used for many clinical purposes, especially fixed prosthodontics such as crown and bridges

To produce an ideal impression, the material used must fulfils certain criteria as laid down in:

  • Ease of manipulation and reasonable cost
  • Adequate flow properties
  • Appropriate setting time and characteristics
  • Sufficient mechanical strength not to tear or permanently deform during removal
  • Good dimensional accuracy
  • Acceptability to the patient
  • Safety ( not toxic or irritating )
  • No significant degradation of properties as a result of disinfection
  • Compatibility with die and cast materials
  • Good keeping qualities "no deterioration of unused material in the dental office"( Craig R G , 1996 )

ISO 1563:1990 specifies the International Standard for dental alginate impression material, but makes no reference to dimensional accuracy or stability.

ISO 4823:1992 the International Standard for dental elastomeric impression materials, does specify a requirement for dimensional accuracy and stability up to 24 hours( linear change must be less than 1.5%), and sets a method for their determination.

Composition irreversible hydrocolloid (dental alginate) impression material:

The formula of the powder component of a typical dental alginate is shown in Table 1. Soluble alginate, usually the sodium salt, reacts with the reactor, calcium sulphate, to form insoluble calcium alginate, which form the gel. A retarder, usually sodium phosphate, preferentially reacts with reactor to prevent the initiation of the reaction to enable the dentist time "working time" to insert the impression into the mouth prior to gelation begins. Once the impression is seated in the mouth, and the retarder is completely consumed, an accelerator (usually potassium titanium fluoride) takes over, and ensures the gelation reaction then proceeds swiftly to full set, to allow early removal from the mouth.

Dental alginates impression materials have high level of particulate filler to organize physical properties, together with viscosity, and which also have an effect on stability (Buchan & Peggie, 1966). A useful way to construct a more stable gel is to use particles whose surfaces in solution are charged, resulting in electrostatic stabilization. A lot of oxide particles, such as those of silicone and titanium, contain hydroxyl groups that can hydrolyses in aqueous media to form negatively charged oxide groups, which stabilize the suspension

Table 1: Composition of dental alginate "adapted from

Ideal preparation of irreversible hydrocolloid (alginate) impression material:

  • Proportion and mix material carefully
  • middle filled tray over arch prior to compressing over teeth
  • Work gently and distract patient at some stage in impression procedure
  • Leave in mouth one minute beyond set
  • Break suction prior to attempting impression taking away from mouth
  • Remove set impression with a single continuous pull
  • Clean and disinfect impression before pouring
  • Maintain appropriate humidity until impression is poured
  • position impression on flat surface tray side down

The Uses of irreversible hydrocolloid (dental alginate) impression material for example:

  • Study models( models for patient education)
  • Removal prostheses
  • Orthodontic appliances
  • Splints
  • Bleaching tray
  • Mouth guards

The advantages of irreversible hydrocolloid (dental alginate) impression material:

  • reasonably priced
  • Easy to use
  • Comparatively accurate at recording the size and shape of mouth structures
  • Completely bio-compatible
  • Used once and sets by a chemical reaction
  • High elastic recovery

The disadvantages of irreversible hydrocolloid (dental alginate) impression material:

  • Reacts irreversibly with water to form a gel
  • Has a thick, pasty consistency that possibly will reason gagging
  • Can be unsuccessful to record anatomy or be weakened by incorporation of air in mix
  • Has poor compressive and tensile strength
  • Can act in response with the environment to gain or lose water

The problems caused dimensional instability of irreversible hydrocolloid (dental alginate) impression materials:

Dental alginates, like all hydrocolloids, tend to distort with time, as they lose (by evaporation and syneresis) or gain (by imbibition) water, and thereby expand or contract. If irreversible hydrocolloid impression materials (dental alginate) expand or contract, the particular prosthesis made on the model produce will not fit in the mouth. Even when stored under condition of 100% humidity (to prevent dehydration), dental alginate impression will still (more slowly) contract, indicating that processes other than dehydration, including polymerization and syneresis are involved. Theoretically then, dental alginate impression materials should be poured immediately, before these factors can distort the impression materials. However, an unrelated factor, the (elastic deformation) of dental alginate, or the temporary distortion when it is withdrawn from undercut, prevent immediate pour. This is because it takes time (ten minutes) for the deformed dental alginate to recover the original proportions of the impression before removal from the mouth. Therefore, best results are obtained when dental alginate impression materials are poured after 10 minutes, to avoid distortion from initial expansion and elastic deformation but before one hour, to avoid distortion from alginate contraction or expansion due to water movement and syneresis

Of practical clinical interest from the point of view of possible imbibitions, is that modern 'accepted best practice' procedures required disinfection by immersion of dental impression in water-based disinfectant in order to reduce the risk of transmitting communicable diseases to dental laboratory. Fortunately, many workers have shown that, because the immersion times required to achieve sterility are relatively short, the dimensional changes resulting from disinfection were insignificant but nevertheless, worth reducing. Also, mixing techniques "mechanical versus hand mixing" have been investigated for their effect on dimensional stability, with results confirming there is no significant clinical difference

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