LABORATORY ASSESSMENT OF IMPRESSION ACCURACY BY CLINICAL SIMULATION

Some laboratory tests of impression material accuracy mimic the clinic al situation (simulatory) while others attempt to quantify a material' s individual properties. This review concentrates on simulatory testin g and aims to give a classification of the numerous tests available. M easurements can be made of the impression itself or the resulting cast . Cast measurements are divided into those made of individual dies and those made of interdie relations. Contact measurement techniques have the advantage of simplicity but are potentially inaccurate because of die abrasion. Non-contact techniques can overcome the abrasion proble m but the measurements, especially those made in three dimensions, may be difficult to interpret. Nevertheless, providing that care is taken to avoid parallax error non-contact methods are preferable as experim ental variables are easier to control. Where measurements are made of individual dies these should include the die width across the finishin g line, as occlusal width measurements provide only limited informatio n. A new concept of 'differential die distortion' (dimensional differe nce from the master model in one plane minus the dimensional differenc e in the perpendicular plane) provides a clinically relevant method of interpreting dimensional changes. Where measurements are made between dies movement of the individual dies within the master model must be prevented. Many of the test methods can be criticized as providing cli nically unrealistic master models/dies or impression trays. Phantom he ad typodonts form a useful basis for the morphology of master models p roviding that undercuts are standardized and the master model temperat ure adequately controlled.