乳牙期和混合牙列期的后牙反(牙合)是较为常见的一种错(牙合)畸形,Kutin和Hawes报道这一畸形的发病率为8％,Hanson等报道此畸形在乳牙列期为12％,在混合牙列期为7.2％。许多学者认为这种畸形无法自行纠正,需早期进行矫治。对混合牙列期反(牙合)的矫治,需依赖于上颌牙弓的横向扩大。本研究的目的是评价比较 In the deciduous period and mixed dentition period, the posterior crossbite is the most common malocclusion. Kutin and Hawes reported that the incidence of this deformity was 8%. Hanson et al. Reported that this deformity was found in the deciduous teeth 12% for the period and 7.2% for the mixed dentition. Many scholars believe that this deformity can not correct itself, need to be treated early. On the mixed dentition anti (occlusal) orthodontic treatment, to rely on the horizontal expansion of the maxillary dental arch. The purpose of this study is to evaluate the comparison