Line angles, commonly referred to as transitional lines, are the areas of the tooth where one surface transitions to another. For example, where the labial surface transitions to the proximal surface (as outlined in the pictures below).
The labial part of the tooth reflects light, whereas the interproximal parts of the tooth deflect light. This is significant, as it determines how our eyes perceive the dimensions of the tooth. Keeping the width of the tooth constant, the further apart the line angles, the wider the tooth will appear. Conversely, the closer together the line angles are, the narrower the tooth will appear. We will discuss how we can use this to our advantage when carrying out restorations in an upcoming tip!
As explained in the previous tip, pertaining to class III cavities, if a cavity includes the line angle (as demonstrated in the diagram below), it is best to bevel the cavity margins to ensure our restoration can be masked. Conversely, if the cavities do not encroach onto the line angles, we can achieve significant aesthetics without bevelling therefore conserving more tooth structure.
(Brown represents caries, grey represents the bevel, the black lines represent the line angles)
(UR2, 2mm bevel, UR1 no bevel)
(Bevels annotated)
(Post op)