After an accident, a patient is unable to close her mouth completely due to certain  facial injuries. Which muscle is paralyzed most commonly?
The facial muscles, or the muscles of facial expression, are subcutaneous muscles. They bring about different facial expressions. These have small motor units.
Functionally, most of these muscles may be regarded primarily as regulators of the three openings situated on the face, namely the palpebral fissures, the nostrils and the oral fissure. Each opening has a single sphincter, and a variable number of dilators. Sphincters are naturally circular and the dilators radial in their arrangement.  These muscles are better developed around the eyes and mouth than around the nose.
Key Concept:
Orbicularis oris closes lips and protrudes lips, numerous extrinsic muscles make it most versatile for various types of grimaces.
Orbicularis oris
Zygomaticus major
Levator anguli oris
Buccinators