Anatomy of the Smile
In 1979 differentiation was made between MMN palsy and
‘Pseudo-paralysis’ of the mandibular branch of the facial nerve,
described by Ellenbogen after platysmal face-lift4.
However, there is often confusion between clinicians, and the
literature, distinguishing between the characteristics of individual
palsies, and diagnosing a MMN injury9. The reason is
not only the variable and overlapping innervation, but also the variable
function in the peri-oral musculature.
Furthermore, individuals have predominantly different smile types,
characterised by Rubin (figure 2). These smile types are associated with
different musculature predominance10, impacting the
effect of palsies in different individuals. Finally, possible
arborisation between branches of the MMN and cervical nerve over the
mandible have been noted, and so there may be cervical nerve supply to
the lower lip depressors. However, this overlap is inconsistent and
rare8 therefore the significance is disputed.
Rubin classified smiles into three types (Figure 2). In the ‘Mona Lisa’
smile (a) the zygomaticus major muscle predominates pulling on the
corner of the mouth. In the ‘canine smile’ (b) the dominant muscle is
the levator labii superioris. Finally in the full denture smile (c) all
muscles are equally activated. Therefore the distinguishing
characteristic of the ‘full denture smile’ is activation of the lip
depressors; depressor labii inferioris, depressor anguli oris (MMN) and
the platysma (cervical branch). Not only does platysma co-function with
the depressors3, but it is also structurally
interweaved with them4. It makes an important
contribution to movement at the mouth corners during opening and
smiling. The platysma is supplied by cervical branches of the facial
nerve. A patient with this type of smile will have an exaggerated
asymmetry with any weakness of depressor function.
The MMN, in addition to innervating the lip depressors, innervates
mentalis. The mentalis produces a pout, characterised by eversion of the
lower lip9. This is perhaps the most helpful
distinguishing feature of MMN due to less overlap with other perioral
musculature. The obicularis oris may function to aid pursing of the
lips. However, the action will be noticeably weaker on the ipsilateral
side in MMN injury.
The weakness of lip eversion on the affected side is therefore a clear
indicator of a true MMN palsy, whereas asymmetry with retained lip
eversion will more likely be related to platysmal dysfunction.