Lower first permanent molars: developing better predictors of spontaneous space closure

First, first permanent molars (FPMs) of poor prognosis are often planned for extraction at an 'ideal time' so that second permanent molars (SPMs) erupt favourably to replace them. However for lower FPM extractions, timing is not an accurate predictor of success.

The aim of this study was to identify additional radiographic factors that could better predict the degree of spontaneous space closure of the lower SPM following FPM extraction.

Data from a previous study of 127 lower SPMs from 66 patients was re-analysed by incorporating additional radiographic factors. These included calcification stage of the bifurcation of the SPM, position of the second premolar, mesial angulation of SPM in relation to the FPM, and presence of the third permanent molar. Results were analysed using ordered logistic regression.

Only 58 per cent of FPMs extracted at the 'ideal time' (SPM development at Demirjian stage E) had complete space closure. The best outcomes resulted from a combination of SPMs not at Demirjian development stage G, together with the presence of mesial angulation of the SPM and presence of the third permanent molar, where 85 per cent of those cases had complete space closure.

Apart from extraction timing of the FPM, consideration must also be given to the presence of the third permanent molar and angulation of the SPM in order to ensure a reliable degree of spontaneous space closure of the lower SPM.