Morphological Variations of the Mandibular Premolars. Tooth Morphology and Identification - A Macro Analysis

Abstract

Background: In the context of teaching tooth morphology to dentist students, which is based on the anatomical and morphological hallmarks described in the literature, some of the teeth in the permanent dentition have shown to be harder to characterize than others. The aim of this project was to specifically focus on the premolars of the lower jaw. By studying a collection of extracted teeth and dental casts, we wanted to explore why the task of differentiating and deciding premolars remain troublesome, and look for novel morphological features on the lower jaw premolars. The knowledge gained here may therefore be useful in the teaching of dental morphology in general, or even interesting in the context of anthropological and developmental biological studies.

Material and methods: A total of 216 extracted mandibular premolars from University of Oslo and University of Tromsø – The Arctic University of Norway were selected and studied for quality, and then separated into three groups by four evaluators as: “typical 1st premolars”, “typical 2nd premolars” and “atypical premolars”. The teeth were then organized in subgroups and photographed from different dental aspects. In addition, 30 dental casts contributed from the Public Dental Service Competence Center of Northern Norway were measured and photographed.

Studying and analyzing the extracted teeth and dental casts led to a proposal of eleven different hypotheses. These hypotheses were then used one at a time to categorize the “atypical premolars” as a 1st or a 2nd premolar, by three independent evaluators. Significant agreements among the evaluators were then used as a measurement for “strong”, “medium” or “weak” morphological features.

Results and conclusion: Out of the eleven proposed hypotheses, we report four novel features from this study. The first feature states that when observed from an approximal aspect “the marginal ridge line ends centrally on the 2nd premolars, but it continues further towards the lingual aspect on the 1st premolars”. The second feature indicates that when observed from an occlusal angle, “the pits making up the central fissure will align on a centrally straight line placed (mesio-distal axis) in the 2nd premolars, but not in the 1st premolars”. The third feature describes a significant difference in the width/height-ratio of the two lower premolars when observed from a facial aspect. The fourth feature demonstrates that the observable facial aspect, when observed from an occlusal aspect, differs between the premolars.