Response of erupting human premolars to force application.

To test the hypothesis that human tooth eruption requires a critical time period during which no force is applied to the erupting tooth, the eruption of a maxillary second premolar in the prefunctional stage was recorded during the afternoon or evening hours in eight children, using an optical device based on the principle of Moiré magnification, while intermittent loads of 300-400 mN were applied to the teeth. When a tooth was erupting actively, light force applications either had no discernible effect or decreased the eruption rate for 2 min or less. When a tooth was intruding spontaneously rather than erupting, a varied response was observed, but the rate of intrusion never increased after force application. Often intrusion showed or halted, and especially during the evening, eruption was likely to occur after a force application during an initial period of intrusion. The conclusion is that, although light force applications significantly displace an erupting premolar, they have little or no effect on net eruption, and that a critical time period without force application is not necessary for eruption to occur.

Accuracy of movement tracking with a miniaturized second-generation Moiré device.

The availability of a Nanoform 600, a machine used in the contouring of optical components, with movement capabilities of 12.5 A, made it possible to determine the sensitivity, accuracy and movement tracking ability of a high-resolution measuring device using the principle of Moiré magnification. The Nanoform 600 was programmed to make a series of movements measured by the Moiré device. Comparison of the Moiré measurement with the programmed movements verified that the Moiré device is capable of reliably tracking movements as small as 0.05 microm. Increasing the rate of movement had little effect on linearity. Tracking movements over a 350 microm distance at rates of up to 340 microm per second resulted in a 0.23% error. Intentionally defocusing the Moiré signal did not appreciably affect sensitivity.