When you pull your lower lip down or to the side, you activate your depressor labii inferioris. This facial muscle originates from the outer surface of the mandible (lower jaw) and stretches upward to attach to the skin of the lower lip. Working together with the orbicularis oris, the depressor labii inferioris moves the lower lip, making it one of the key muscles used to form expressions of sorrow, doubt, perseverance, and diligence.

The depressor labii inferioris, like other facial muscles, can be affected by many causes of facial muscle paralysis. These forms of paralysis can leave you with limited muscle function or total muscle paralysis.

• Bell's Palsy: Bell's Palsy is a type of paralysis that can affect the muscles controlled by the facial nerve, including the depressor labii inferioris. Facial muscles may become weak or, in some cases, may not be able to contract at all - affecting your ability to make facial expressions. Bell's palsy can also disrupt your sense of taste and make your smile appear asymmetrical or crooked.
• Head Trauma: While Bell's palsy is the most common source of facial paralysis, a study published in Craniomaxillofacial Trauma & Reconstruction notes that head trauma, as well as injuries arising from surgery, may damage the facial nerve and result in facial muscle paralysis. If the lower facial muscles are affected, you may have difficulty eating, drinking, and speaking.
• Tumors: Tumors that press on the facial nerve may also cause facial paralysis, interfering with the muscles' ability to function. If you're unable to move your lower lip, your depressor labii inferioris may be one of the muscles affected.

Dental professionals can help diagnose and treat problems that result in facial paralysis or the loss of muscle function. Mild cases of Bell's palsy may not require treatment, though a doctor may recommend medications like an oral steroid or therapeutic treatments for more severe cases. They may also recommend MRI or CT scans to diagnose the cause of facial paralysis if there's a possibility that a tumor is to blame.

If you're worried that your smile is asymmetrical or you experience facial paralysis of any kind, your dental and medical team can work with you to address the cause of the problem and help restore the normal function of your facial muscles. Be sure to speak with your dentist if you notice any numbness or loss of function in or around your mouth.

Oral Care Center articles are reviewed by an oral health medical professional. This information is for educational purposes only. This content is not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of your dentist, physician or other qualified healthcare provider.