A crooked smile is caused by a problem with the facial nerve. Such problems arise due to infection, disease, injury or circulation problems, according to St. Vincent Medical Center.
The most common infection to cause muscle weakness that could result in a crooked smile is Bell's Palsy, according to St. Vincent Medical Center. The condition causes the facial nerve to swell, creating pressure on the nerve. Treatment involves reducing the swelling and encouraging normal circulation. Herpes zoster oticus is a similar condition that can cause disruption to the facial nerve as well as hearing loss.
The existence or removal of an acoustic tumor or nonmalignant fibroid growth can also cause disruption of the facial nerve and result in a crooked smile.
Infection in the middle ear is another potential cause of a crooked smile. Swelling from the infection can create pressure on the nerve, resulting in weakness in the face that resolves when the swelling subsides.
Lastly, the disruption of circulation that results from a stroke can cause facial paralysis. A stroke is typically accompanied by several other symptoms.
A crooked smile resulting from facial nerve paralysis can be treated by medical or surgical procedures, St. Vincent Medical Center notes.