Impression materials are used to record the shape of the teeth and alveolar ridges. There are a wide variety of impression materials available each with their own properties, advantages and disadvantages. Materials in common use can be classified as elastic or non-elastic according to the ability of the set material to be withdrawn over undercuts.
The main properties to consider are as follows:-
(1) Non toxic and non irritant
(2) Acceptability to the patient; Setting time, Taste & Consistency
(3) Accuracy; both for Surface reproducibility and Dimensional stability
(4) Use of material; Ease of mixing , Working time, Setting time & Handling of the material
(5) Compatible with model materials
(6) Economics of material; Cheap, Long shelf life & Accuracy (save redoing impression)
Impression materials, in the fluid or plastic state, are carried to the mouth in a suitably sized tray. Hardening of the material takes place either on cooling or through a chemical reaction.
There is no clear line between mucostatic and mucodisplacing impression materials.
Mucodisplacive : compound, high viscosity alginates, high viscosity elastomers
These viscous impressions materials are called mucocodisplacive impression materials. These record an impression of the mucosa under load. This results in a wider distribution of load during function, making it more stable it also compensates for the differing compressibility of the denture bearing area reducing the risk of a fracture due to flexion. However the retention of the dentures may be compromised as the soft tissues wish to return to their original position at rest.
Mucostatic; plaster, zinc oxide eugenol, low viscosity alginates
Materials which are more fluid displace the tissues less – these are known as mucostatic impression materials. These essentially record an impression of the undisplaced mucosa. This results in better retention of the denture because of closer adaptation to the mucosa at rest. However instability of the denture may occur during function as the tissues distort.There is no clear line between mucostatic and mucodisplacing impression materials.