Extreme variations in the shape of mandibular premolars

Introduction: Human mandibular premolars (MnP) are highly variable in shape. An extreme variation, called here MnP tooth-shape deviation (MnP-TSD), is characterized by a squeezed faciolingual (FL) dimension and an enlarged mesiodistal (MD) dimension, reversing the typical premolar proportions. This is the first in-depth study of the MnP-TSD anomaly. Methods: Eighteen subjects with 29 MnP-TSD teeth were selected for odontometric analysis by using dental casts. For each affected MnP, an MD/FL proportional index and a representation of crown area (MD × FL) were calculated from maximum MD and FL crown diameters. Data were compared with reference values for MnP. Results: MnP-TSD was observed over 8 times more frequently in second premolars than in first premolars. Male expression was slightly greater (ratio, 1.4:1), and bilateral occurrence was observed in 59% of the subjects. Occurrence was noted among all ethno-racial groupings in the sample (16 whites, 1 black, 1 Asian). The MD/FL index of premolars with the MnP-TSD anomaly always exceeded 100 (103 to 148); this is significantly higher than normal values. In contrast, the crown areas of the affected teeth were remarkably similar to normal reference values. In addition to the squeezed tooth shape, subjects with pronounced MnP-TSD also had mesiolingual/distofacial twisting in MnP occlusal features. Conclusions: Extremely abnormal MD/FL proportions coupled with normal crown-area values provide reliable factors for differential diagnosis of the MnP-TSD anomaly. This uncommon MnP dysmorphology can be a challenge to orthodontic movement and achievement of ideal occlusal intercuspation.