OBJECTIVETo explore the factor of age related to protraction response.

METHODSFour 12-week mongrel dogs in the same nest were randomly assigned into two groups: the control (n = 1) and the experimental groups (n = 3). Four pairs of titanium bone markers were fixed on either side of the bone sutures of all animals. Distraction device was fitted in the experimental group, A forward elastic force was exerted through the device for 1 month. Midface skeleton protraction was applied to experimental group at different age. Dog 2 was started at 12 weeks of age. Dog 3 was started at 16 weeks of age. Dog 4 was started at 20 weeks of age. The protraction force was 600g. All the animals were sacrificed at their age of 9 months. The results were evaluated clinically, radiographically, and cephalometrically.

RESULTSAll the animals in the experimental group showed progressively forward movement of the maxilla at the end of the experiment. In the same condition, the younger age appeared to have more effective in treatment than the older age. After 3 approximately 4 weeks stagnation, the maxilla gradually recovered normal growth.

CONCLUSIONThe younger age lead more effective protraction than the older age. Persistent elastic distraction at the medium position of midface brought stable effects and no influence on facial growth.