Time-Temperature Relations of Dental Instruments Heated in Root-Canal Instrument Sterilizers

¹ American Dental Association, Chicago, Illinois

The well-type sterilizer was designed to provide a simple, rapid method for the sterilization of small dental instruments. The molten-metal medium originally utilized in these instruments heated the inserted instrument to a high temperature within a few seconds. Other media, such as salt and glass beads, were later introduced to overcome some of the objectionable features of the molten-metal medium. These other [see figure in the PDF file] [see figure in the PDF file] media were less efficient as heat-transfer agents. The reduced ability of salt and glass beads to transfer heat rapidly to the immersed dental instrument became more important to the success of this method of sterilization as the size of the instrument to be sterilized increased.

The time required for the temperature of an immersed instrument to increase was directly proportional to the size of the instrument: the larger the instrument, the longer the heat-up time. The rate was also directly related to the heat-transfer characterstics of the medium, that is, the better the medium conducted heat, the more rapid the rise in temperature of the instrument. Thus, as the size of the immersed instrument increased from a small root-canal file to that of a mirror, contra-angle, or handpiece, both heating rate and final temperature decreased to a point where it was questionable that such a method could achieve the desired result.

It is important to determine the temperature of the bath at the depth of immersion of the instruments. While the temperature of the bath at this point will depend on the medium, the thermostat setting should be such that the immersed instrument will reach a temperature of about 218° C. (424° F.) in a time period practical in normal dental procedures. These temperatures have to be chosen with a view to the physical properties of the dental instruments to be used—for example, the temper of the metal and the melting point of the solder used in the junctions.