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.