The rat as a model for orthodontic tooth movement--a critical review and a proposed solution

The aims of this study were to perform a systematic review of the use of rats as a model for experimental tooth movement, to give a critical evaluation of the use of elastics as a force delivery system, and to describe a newly designed well-defined model for tooth movement in rats. The literature from 1981 to 2002 indicates that in 57 per cent of animal studies on orthodontic tooth movement, rats were used, but in many of these investigations the experimental set-up was poorly documented. Only three of the 159 studies fulfilled the inclusion criteria for a good model: a force magnitude of less than 20 cN; moving molar(s) mesially; an experimental duration longer than 2 weeks; and no extra experimental condition such as drug intervention. As more than one-quarter of the studies on tooth movement in rats used elastics to produce an orthodontic force, and as the forces they produced and their force decay during decompression are unknown, their mechanical characteristics on decompression were tested. Elastics stored under dry conditions or in water showed significant force decay from around 45 N to almost 0 N within the first 0.2 mm of decompression. With regard to the above-mentioned shortcomings of using rats as a model for tooth movement, a newly designed experimental appliance for tooth movement in rats was evaluated. It proved to be stable and simple and able to deliver a continuous and constant force as low as 10 cN on all three molars together during an experimental period of 12 weeks without interference in animal welfare, and was able to compensate for the effects of molar distal drift and continuous incisor eruption.