To access the clinical effect of orthodontic treatment in patients with extraction of mandible first molar.

Nineteen patients (three male and sixteen female) with extraction of mandible first molar because of caries, periapical disease were collected as experimental group. The age was 21.37 ± 5.07. Nineteen patients (seven male and twelve female) with extraction of four premolar as control group, The age was 20.42 ± 4.23. All the patients were treated with orthodontic space closure using MBT preadjusted appliance and sliding mechanics PAR index and cephalomertics that were compared before and after orthodontic treatment.

The two groups’ reduction in weighted PAR score and each of the PAR score differences were statistically significant (P<0.01). There was no significant difference between the two groups after treatment in PAR. There were significant changes in U1-NA (°)、U1-NA(mm)、U1-SN(°), but not in the rest of the value in experimental group (P<0.01). The L1-NB(°)、L1-NB (mm) was significant difference before and after treatment. Extraction spaces of mandible first molars were closed (P<0.01); Good occlusal relationships and soft tissue profiles were achieved in all cases.

If the mandibular first molars have serious caries or periapical lesions, removal of mandibular first molar, making full use of the mandibular third molar and keeping health premolars for orthodontic treatment could receive satisfactory results.