Effect of Reversible hydrocolloid Impression Materials on the Dental Stone Model

Abstract

Taking an impression is an important clinical step in prosthetic dentistry because it links the treatment work and model technique together. Since the fabrication of the prosthesis requires indirect techniques, there are many high quality, newly developed materials available. However, hydrocolloid impression is one which has been used for quite some time. Compared with the others, the hydrocolloidimpression material has several advantages such as hydrophilia, better flow, and no requirement of pressure or mixing. But there are some disadvantages that have been pointed out: dimensional instability, weak gel strength, and impartation of a rough surface to dental stone. Many efforts have been made to alter its undesirable properties, yet still some remain. By using 4 types of hydrocolloid impression materials in two group-a) Rubberloid heavy bodied and 5/16 small stick (VAN-R) and b) Politube reqular and 3/8 stick white (SURGIDENT) and 12 types of model stones (9 dental improved hard stone and 3 hard stone), the following parameters were investigated: (1) dimensional stability, (2) accuracy, (3) effect of tray design, (4) detail reproduction, (5) surface character of stone model, and (6) the effect of fixation. According to the results obtained, the following observations can be made:
(1) The hydrocolloid impression material maintained in a supersonic humidifier (100% humidity) or in a wet box (85-90% humidity) had minimal weight change while those kept in the open room environment (50% humidity) or in water were unstable. But the dimensional stability of stone casts derived from the hydrocolloid impression of the original bridge die was poor under all condition used. This suggested that a short time interval between the impression work and stone pouring as well high humidity would give good results.
(2) When different stone types having different setting expansion values were used, there was no correlation between the degree of deformation and setting expansion values. Therefore dimensional changes can not be explained solely by setting expansion of the stone.
(3) Comparing the retention ability of hydrocolloid impression material with different tray forms, there were the same dimensional changes with both the rimlock type tray and that without the undercut. This suggests that there is no retentive effect in the horizontal vector. The retention due to vis or perforations was similiar. Therefore separation of the impression material from the tray after the impression work probably influences dimensional change.
(4) Concerning the reproduction of detail, all the hydrocolloid impression materials gave a good reproduction of a 10μm wide line, but there were differences with different dental stone. The stone crystal granule size giving good reproducibility was 5μm, and that giving the worst was 20μm. This was determined by SEM observa tion. It was suggested that the use of a granule size over 20μm would make it difficult to reproduce a 20μm wide line. Comparing the stone surfaces made from either Coltex or hydrocolloid impression material, the surface made from the hydrocolloid impression material revealed larger growing crystal granules which may be due to the surface roughness or hardness of the impression.
(5) Stones whose surface character may be affected by hydrocolloid impression material were Densite, Silkyrock, Velmix, Surstone, Tewe and Rapid stones, while those not easily affected were Mode-rock, Fuji-rock, Sunrock and New metal rock as well as New plastone and Hydro-gips. Clinically, the latter group seem to have less surface roughness.