Cross-facial nerve transplants: why are spontaneous smiles not restored?

The technique of cross-facial nerve transplantation (CFNT), with or without the addition of vascularised muscle, has made it possible to achieve some voluntary movement of the paralysed side of the face in patients with unrecovered facial palsy. If normal faces are studied during conversation, it can be seen that there are two types of movement of the lips--those of emotional expression and those involved in the formation of vowel sounds and labial consonants. Smiles themselves can be classified roughly according to their length of muscle contraction--the longer "definitive" smile, and the shorter "flash" smile. It is the synergistic facial movement of the unpremeditated "flash" smile and the small movements in the formation of the vowel sounds and labial consonants which fail to occur in patients reanimated by revascularised, reinnervated muscle, despite the return of voluntary contraction and resting facial tone. This study was undertaken to try to determine why these synergistic facial movements of short duration are so difficult to achieve.