There is no exact definition of a gummy smile, however, it is generally recognised as referring to when 3-4 millimetres of the gum line is exposed, resulting in a disproportionate level of gum being visible when someone smiles. This is often influenced by the shape and height of the teeth, the way the lips move and the angle of the jaw. It could be a result of overactive muscles above the top lip.
The manner in which the teeth grow can also result in a gummy smile. For example, a condition called altered passive eruption can lead to a gummy smile as it causes the gums to cover an excessive amount of the teeth’s surface. In some cases, the gums might have grown too far, or become over erupted, something known as dentoalveolar extrusion.