Relationship between initial crowding and interproximal force during retention phase.

This study investigated the change in interproximal force (IPF) in mandibular anterior teeth during retention and the relationship between the irregularity index before orthodontic treatment and the IPF. The effect that an erupting third molar had on IPF was also examined. Forty treated patients (40 with extraction of four bicuspids) were followed for 18 months during the retention phase. The irregularity index was determined from initial plaster casts. The total IPF was determined by measuring the interdental frictional forces at the mandibular anterior teeth by withdrawing a metal strip. The total IPF increased during the retention phase until 18 months, and there was a positive correlation between the irregularity index and total IPF 6 to 18 months after active treatment. An erupting third molar did not affect the total IPF. An increase in the total IPF may be an indication of relapse in mandibular anterior crowding. In conclusion, orthodontists should pay special attention to potential relapse in the lower anterior teeth 6 months after active treatment in cases with severe anterior crowding before treatment.