Pell-Gregory classification is unreliable as a predictor of difficulty in extracting impacted lower third molars.

We present a study of 166 extractions of impacted lower third molars, all vertical and all extracted by the same surgeon. Each tooth was classified according to the Pell-Gregory scales of position for the occlusal plane (scale A-C) and the ascending ram us of the mandible (scale 1-3). The extraction was subsequently rated as 'easy' or 'difficult'.Taking Pell-Gregory class C as a predictor of a 'difficult' extraction, specificity was 88% but sensitivity was low at 15%. Taking Pell-Gregory class 3 as an indicator of 'difficult', sensitivity was somewhat better (50%), but at the expense of specificity (62%). Likelihood ratios for the individual classes also indicated that the scales are of little value for predicting a difficult extraction.