Orthodontic treatment of substituting third molars for missing permanent molars

Introduction: This study aimed to evaluate the orthodontic effect and efficiency of substituting third molars for missing first or second permanent molars systematically. Methods: Forty-six patients (69 third molars total) with missing permanent molars replaced by third molars were selected. The angulation, crown-to-root ratio, and periodontal condition of the third molars before and after treatment were compared. The American Board of Orthodontics Objective Grading System was used to evaluate the alignment and occlusion of third molars after treatment. The duration of orthodontic treatment and third molar replacement therapy were also recorded. Results: The average orthodontic treatment time was 33.9 +/- 5.6 months, and the average angulation change of third molars during treatment was 49.8 +/- 29.8 degrees. The average height of mesial alveolar bone increased by 4.8 +/- 0.5 mm in patients whose third molars were mesially inclined or horizontally impacted. The root length of adult patients decreased by 0.72 +/- 0.02 mm on average, and the average gingival recession was 0.10 mm, both of which were not statistically significant. The average score for each third molar evaluated by the American Board of Orthodontics Objective Grading System was 1.8 +/- 0.5 points. Conclusions: If the indications and timing of treatment were well-controlled, third molars would be excellent substitutes for missing first or second permanent molars through the orthodontic method.