This post is about a variant of the zygomaticus major muscle and its effects on dimples in smiling. Before getting into the details, it is necessary to clarify which interpretation of “dimple” I am referring to.
the dimple, the dimple – it’s not that simple.
FACS-fueled confusion & linguistic disillusion
If you’re familiar with FACS (Facial Action Coding System) terms, you will know that the “dimpler” action or AU14, is caused by the contraction of the buccinator muscle. When buccinator is activated, the corners of our lips pinch inward and push against our molar teeth, as demonstrated below:
Functionally, this action is important for chewing and keeping food in our mouths.
Outside of FACS, the word “dimple” almost always refers to an indentation of the skin (typically near the mouth or chin) caused by genetic muscle variation. For this reason, novice FACS learners often mistake any concave skin deformation as a sign of AU14; however, such assumptions are incorrect.
NOTE: I have been informed by FACS authorities that the next manual release will feature a re-naming of AU14 – one that is directly descriptive of the action itself. If my memory serves me well – it is to be called “lip corner pincher” – or something of that nature.
what everyone else means by “dimple”
The most frequently observed facial dimples are smile-based dimples. These dimples are thought to be caused by a variation of the zygomaticus major muscle (the muscle for AU12 – lip corner puller) – a variation that involves two insertion points rather than one.
NOTE: The word “smile” has many interpretations. In this post, I will use “smile” when referring to the contraction of the zygomaticus major muscle.
bifid zygomaticus major
variations on a diagram
If you’ve ever searched for anatomical representations of zygomaticus major, you may have noticed that some diagrams look like this:
(single insertion point)
. . . while others look more like this:
(two insertion points)
The latter is an example of a zygomaticus major with two insertion points – the suspected culprit behind dimpled smiles.
existing information on bifid zygomaticus major
There is currently not much in-depth information on bifid zygomaticus major; however, there is one good paper that describes its potential implications very well:
“ Clinically, the double or bifid zygomaticus major muscle may explain the formation of cheek ‘dimples.’ The inferior bundle was observed in several specimens to have a dermal attachment along its mid-portion, which tethers the overlying skin. When an individual with this anatomy smiles, traction on the skin may create a dimple due to this dermal tethering effect. “ – Pessa et al.
” There are two reports in the literature which describe the true double zygomaticus major muscle having an insertion above and below the corner of the mouth. The earliest report of this anatomical variation was in 1775 by Domenici Santorini. In his book Anatomici Summi, Santorini illustrated a double zygomaticus muscle with an insertion at the corner of the mouth, and an inferior insertion beneath the depressor anguli oris muscle (Fig. 3). Santorini stated that this muscle rarely divides, but that he had observed this double insertion in two individuals (‘‘nos bis observavimus’’).
A second report of this variation is from the plastic surgery literature, in which Zufferey (1992) found one case of a double zygomaticus major during cadaver dissection. From these two reports, and from the paucity of any other documentation, this anatomical variation would seem to be rare. In fact, from the present study, the double or bifid zygomaticus major muscle represents a relatively common variation. In this series, this variation was noted in 34% of the cadaver specimens.” – Pessa et al.
my research on bifid zygomaticus major
Because there is not a ton of in-depth information on bifid zygomaticus majors, I still have my own unanswered questions:
I will likely write a follow-up to this post, as there are still more examples to collect and papers to discover.
If you would like to do your own reading on the bifid subject, any info that was not personal observation was taken from the paper quoted earlier by Pessa et al. – “Double or bifid zygomaticus major muscle: anatomy, incidence, and clinical correlations .”