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.

The study will be conducted on consecutively included subjects referred to the centers and then randomized into either an experimental group or a control group.

To be able to evaluate the amount of anterior crowding before the lateral incisors have erupted, low-dose Cone Beam Computed Tomography-scans are used to measure the width of the unerupted laterals. The anterior crowding is calculated by comparing the width of the laterals with measurements of the distance between the mesial surfaces of the deciduous upper canines on study casts. The eligible subjects are then randomized to either experimental och control group.

The subjects in the experimental group will be treated with a maxillary expansion appliance with bands on the second deciduous molars and arms to the deciduous canines. A small preparation on the palatal surface of the deciduous canines is performed to facilitate bonding of the arms. The screw gives a expansion of 0,20 millimeter per activation. The arms are bonded to the deciduous canines with light curing composite and the bands are cemented on the etched second deciduous molar with glassionomer cement.

Then the subject and the parents are instructed to activate the appliance once a day (1/4 turn) for 2 weeks and thereafter every second day until adequate space for the lateral incisors is obtained and the first permanent molars are approaching scissor-bite.

The expansion can be paused for 7-8 months after 25-30 activations if the lateral incisors are not yet erupted and the permanent molars are going in scissor-bite to allow for their spontaneous correction. The expansion is then continued until adequate space for the lateral permanent incisors is achieved.

During the expansion the subjects will be asked to complete a questionnaire on pain and discomfort.

When enough space is available for the crowding to be resolved the expander is blocked with light curing composite or a stainless steel ligature wire. The appliances is kept as a retainer until the laterals are fully erupted.

After the follow-up period the treatment effects will be evaluated by measuring of the study models. All measurements will be made digitally with the 3Shape OrthoAnalyzer Differences in palatal volume, transversal width of the maxillary and mandibular dentition between the start of the trial and at the end of the follow-up period will be measured. The questionnaires regarding pain and discomfort and PIDAQ will be analyzed.

The last part of the study is to perform a cost-effectiveness analysis for early treatment with maxillary expansion and a cost-effectiveness analysis for treatment in the permanent dentition and then compare them. The direct cost for both experiment and control group will be calculated by adding up material cost and chair-time cost. The indirect cost is defined as the cost for the parents regarding loss of income for the duration of the visits and the trips to and from the clinic. Due to the fact that there are substantial differences in traveling time and income for parents, the indirect costs of treatment will not be taken into account because the randomization will equalize the effect of the differences.

Inclusion Criteria:

• Healthy subjects without sucking habits
• Permanent maxillary central incisors and first permanent molars fully erupted. DS1M1.
• No severe crowding or protrusion in the lower arch considering the Leeway space and normal development of the arch.
• Permanent first molar in normal transversal relation.
• Crowding of the maxillary incisors with the lateral incisor just erupting ectopically or not yet emerged through the gingiva.
• Crowding greater than or equal to 6 millimeters between the upper deciduous canines.

Exclusion Criteria:

• Skeletal asymmetries, anomalies and syndromes.
• Previous orthodontic treatment

The outcome assessor will do measurements on study models. The study models will be masked when it comes to treatment modality. Surveys will be masked.