Tohru Hamano

Effect of different labio‐lingual spaces in tray designs on the displacement of and pressure against a mobile tooth

The present study aimed to examine the effect of custom tray designs on the displacement of mobile tooth and local impression pressures during the impression procedure, using partially edentulous simulation models with six anterior teeth containing a mobile tooth prepared in previous studies. The custom trays were designed by altering the thickness of the respective spaces on the labial and lingual sides of the remaining tooth arch. In previous studies, the mobile tooth was displaced in the labial direction and local impression pressures of the mobile tooth were greater against the lingual side than the labial side for all custom tray designs. Furthermore, the custom trays perforated with holes on the lingual side were effective to reduce mobile tooth displacement, labial and lingual impression pressures against the mobile tooth, and the differences between them. Therefore, the present study was performed focusing on the labial and lingual thickness of spaces in custom tray designs. It was found that mobile tooth displacement, labial and lingual impression pressures against the mobile teeth and their differences were less in trays with spaces>3.0 mm thick on both the labial and lingual sides, but markedly greater in trays with a 1.5 mm-thick space on the labial side. These results indicate that the thickness of spaces on the labial side in the tray should not be reduced to prevent mobile tooth displacement.

[Laboratory-made stud attachment. Report II. Effect of cycle of engaging and disengaging on fatigue]

A number of engaging and disengaging of retainers induce those fatigue and consequent decrease in retentive force. In this study on the laboratory-made stud attachment consisting of a male (Type IV gold) like a snap and a female with semi-loop and linear portion made by bending a wrought gold wire, the maximum buldges of males, the distances between the female tips, and the retentive forces were measured before and after each stage of engaging and disengaging of predetermined cycles (10: initial, 1, 000, 3, 000, 5, 000, 10, 000). The males had 0.25 mm undercut. The females were 0.8mm in diameter and 3, 5, 7 and 10mm in length from the contact points with the males at the semi-loop portion to the points fixed in the resin base at the linear portion.Results:1. The initial retentive force, the abrasion of male, the permanent deformation of female and the decrease in retentive force all increased with the shorter female.2. 5mm-length female kept the most amount of retentive force of approximately 380 to 190g during 1, 000 to 10, 000 cycles.