Risorius muscle: anatomy and smile expression
- Muscle
Quick facts
- Origin
- Variable: fascia over the masseter and parotid, SMAS and, sometimes, fibers of the platysma.
- Insertion
- Modiolus, at the corner of the mouth.
- Plane
- Superficial, thin and horizontal, just under the skin of the lower cheek.
- Innervation
- Buccal branches of the facial nerve.
- Particularity
- An inconstant muscle: it may be absent, very thin or have more than one bundle.
The risorius is a thin, superficial muscle of the lower cheek that pulls the corner of the mouth sideways. It runs horizontally, from the area over the masseter and parotid to the modiolus, and takes part in the wide smile and in tension expressions. It is an inconstant muscle: in some people it is well developed, in others it is very thin or does not even show up in dissections.
Where is the risorius?
| Point | Description |
|---|---|
| Origin | Variable: fascia over the masseter and parotid, SMAS and, sometimes, fibers of the platysma |
| Course | Horizontal, across the lower cheek, just under the skin |
| Insertion | Modiolus, at the corner of the mouth |
| Plane | Superficial, over the masseter and the buccinator |
| Innervation | Buccal branches of the facial nerve |
Because its origin varies, the risorius is described sometimes as an independent muscle and sometimes as a continuation of platysma fibers. This variation explains why the same technique can have different effects in different people.
How does it act in the smile?
The risorius pulls the corner of the mouth sideways, almost horizontally. Together with the zygomaticus major, which pulls up and outward, it defines the width and shape of the smile. When it predominates, the smile is wider and flatter; when elevation predominates, the curve rises at the ends.
The name comes from Latin and refers to laughter, but the muscle also acts in the tense smile, the embarrassed or effortful one. The cheek dimple, sometimes attributed to it, is usually associated with variations of the zygomaticus major.
Why does it matter in HOF?
- Toxin in the masseter: the risorius passes over the masseter. Toxin applied in a superficial or very anterior plane can reach it and leave the smile asymmetric or "stuck" on one side. The topic is covered in botulinum toxin in the masseter and in asymmetric smile after toxin.
- Mouth corners: the risorius is one of the forces that reach the modiolus, and the balance among them defines the position of the commissure, as shown in mouth corner filler.
- Individual reading: because the muscle is inconstant, assessing each patient's smile counts for more than the general rule.
How does it show up in the assessment?
- Wide smile: how much the mouth corner is pulled sideways compared with the upward movement.
- Symmetry between the sides at rest, in the smile and in speech.
- History of toxin in the masseter and the lower third.
Also called: risorius, laughing muscle, musculus risorius.
Frequently asked questions
What is the function of the risorius muscle?
It pulls the corner of the mouth sideways, widening the mouth. It takes part in the wider smile and in tension expressions, such as a forced smile.
Can toxin in the masseter affect the smile?
It can. The risorius runs over the masseter, and toxin applied in a superficial or very anterior plane can reach it, leaving the smile temporarily asymmetric. Technique is the decision of the qualified professional.