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 ʹeasyʹ or ʹdifficultʹ. Taking Pell-Gregory class C as a predictor of a ʹdifficultʹ extraction, specificity was 88% but sensitivity was low at 15%. Taking Pell-Gregory class 3 as an indicator of ʹdifficultʹ, 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.