Interceptive Maxillary Expansion to Relief Anterior Crowding in the Mixed Dentition

The goal of this clinical trial is to compare if it is better to treat severe anterior crowding in the early mixed dentition then later in the permanent dentition in children with anterior crowding of the upper teeth. The main questions this trial aims to answer are: * If there is a difference in the degree of anterior crowding between the experimental group and the control group after a five year follow-up period. * If there is a difference between the experimental group and the control group in regard to oral health related quality of life. * If there is spontaneous transversal adaption of the lower dental arch to upper dental arch after maxillary expansion and compare this to the control group. * If early maxillary expansion is more cost-effective than treatment in the permanent dentition. To be eligible for this trial the subject must be children between six and nine years old at the time for inclusion. The children must have crowding of the upper front teeth of six millimeters or more and the lateral incisors should not have erupted or just erupted through the gingiva. Subjects randomized to the experimental group will get an expansion treatment in the upper jaw with rapid maxillary expansion device to gain enough space for the lateral incisors to erupt. All subjects, also the subjects randomized to the control group, are followed until all permanent teeth have erupted. At the end of the follow-up period all subjects fill out a questionnaire to evaluate their oral health related quality of life. Researchers will then compare the subjects in both groups to evaluate if there is a difference in anterior crowding after the follow-up period. Differences in cost-effectiveness, morphological changes and the subjects oral health related quality of life will also be evaluated.