Backgrounds and objectives: The incidence of mandibular third molar impaction has been reported to be between 18% and 59%. In 2000, their surgical removal was reported to cost the NHS in excess of £12 million per year. The most common and significant variable for impacted third molars is that of inadequate space in the retromolar region. Space for successful eruption of third molars can be provided by extraction of the second molar. This has previously been demonstrated by orthodontic research. The aim of this study is to determine how successfully the third molar can replace an extracted second molar in the developing dentition.

Methods: Records of 65 patients (mean age 15 years) who had second molars extracted were obtained from a dental practitioner in a stable community, and reviewed retrospectively. These records included panoramic radiographs and intra-oral radiographs. A total of 234 third molars were analysed (113 maxillary, 121 mandibular). Radiographic angulation, space available, and depth below the occlusal plane were measured before extraction of the second molar, and final angulation of the third molar was the main outcome variable.

Results: No third molars failed to erupt during the observation period. Of the 234 teeth analysed, 88.6% of mandibular third molars and 100% of maxillary third molars achieved satisfactory radiographic angulation (<35°) or better. The pre-extraction angle between the long axes of the crowns of the third molar and first molar (crown long axis angle) was found to be the most predictive variable of successful eruption.

Conclusion: With careful radiographic and clinical case selection, the third molar can replace the extracted second molar with high success rates.

Key words: wisdom tooth; third molar; impaction; second molar extraction