Classifying Synkinesis Smile Dynamics: The UCL Classification System

Patients with synkinesis struggle to emote through smiling, as their smiles tend to convey negative emotions rather than happiness. There is a wide variation in anatomic makeup and severity of smile asymmetry in patients with synkinesis. Despite this, a classification system does not exist to characterize these variations. This research presents a reliable, repeatable, and easy-to-use synkinesis smile classification system based on simple visual assessment: the "UCL System."

Two facial reanimation experts classified smile dynamics according to upper lip slope (U), paralyzed-side oral commissure angle (C), and lower lip symmetry (L). Automated facial expression software quantified emotions conveyed with smile and facial action units (AU) activated. Inter- and intrarater agreement were assessed via Fleiss' kappa value.

About 366 patients were classified into 12 UCL groups. Activation of facial AU 12 (zygomaticus major), AU 14 (buccinator), and AU 25 (depressor labii inferioris) were significantly different between groups (p = 0.011, 0.050, and 0.017, respectively). There were significant differences in perceived happiness and negative emotions between groups (p = 0.018 and 0.034, respectively). There was moderate interrater agreement (Fleiss' kappa = 0.481, p < 0.001) and substantial intrarater agreement (Fleiss' kappa = 0.670, p < 0.001).

The UCL synkinesis smile classification system is based on simple visual assessment and is reliable and repeatable. This system encourages clinicians to evaluate patients' smile dynamics and ultimately may help predict treatment outcomes.