Dominic Stone
The human face has 20 main facial muscles, or craniofacial muscles, which are essential for chewing and making facial expressions. One of these muscles, the frontalis, is the only muscle that raises the eyebrows. It spans the entire forehead from the eyebrows to the hairline and is necessary for conveying emotions and nonverbal communication. Injecting neuromodulators into the frontalis can smooth out horizontal forehead lines, but over-injecting can result in brow ptosis (brow droop) because the muscle can no longer counterbalance the downward pull of the muscles around the eyes.
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What You'll Learn
The frontalis muscle
The antagonist muscles to the frontalis muscle are the orbicularis oculi, corrugator supercilii, and procerus muscles. These muscles all come together at the glabella and the orbital rim, and their respective movements and forces on the skin can cause cutaneous rhytids (frown lines, smile lines, forehead lines, and horizontal nasal lines). The balance between these muscles determines the eyebrow position and shape.
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The corrugator supercilii muscle
The muscle receives its blood supply primarily from the ophthalmic artery, a branch of the internal carotid artery. It is innervated by the temporal branches of the facial nerve (CN VII), and venous blood is drained by the superior ophthalmic vein, which flows into the cavernous sinus.
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The orbicularis oculi muscle
The orbicularis oculi is a muscle in the face that surrounds the eye and is responsible for closing the eyelids. It is situated just beneath the eyelid skin and extends from the medial to the lateral canthal region, enhancing the eyelid's structural integrity and functionality. The main function of the muscle is to close the eyelids, and it also assists with tear drainage.
The orbicularis oculi is a sphincter-like muscle arranged concentrically around the upper and lower eyelids. It is a facial expression muscle that may be affected by conditions such as Bell Palsy and blepharospasm. The muscle is divided into orbital and palpebral sections, each further subdivided based on function. The orbital portion of the orbicularis oculi primarily facilitates the voluntary, forceful closure of the eyelids. The palpebral portion of the muscle is thin and pale, and it arises from the bifurcation of the medial palpebral ligament, forming a series of concentric curves.
The palpebral part of the orbicularis oculi muscle is crucial for voluntary and involuntary blinking and is subdivided into the preseptal, pretarsal, and ciliary sections. The preseptal section is split into deep and superficial heads. The deep head attaches to the fascia around the lacrimal sac and the posterior lacrimal crest, playing a role in the lacrimal drainage system. The pretarsal section also divides into a deep and a superficial head. The deep head, often referred to as the Horner muscle, attaches to the medial canthal tendon and the posterior lacrimal crest.
The marginal portion of the palpebral orbicularis muscle, also known as the muscle of Riolan or pars ciliaris, is adjacent to the eyelid margin. This muscle segment appears as a distinct coloured line along the length of the upper and lower eyelid margins. This feature, known as the grey line, is a critical surface anatomical landmark for the surgical repair of eyelid margin lacerations. The lacrimal part of the orbicularis oculi draws the eyelids and the ends of the lacrimal canals medialward and compresses them against the surface of the globe of the eye, thus placing them in the most favourable situation for receiving tears.
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The procerus muscle
The function of the procerus muscle is to depress the medial parts of the eyebrows and wrinkle the skin between them, known as the glabellar skin. This action enables frowning and contributes to the range of human facial expressions. The contraction of the procerus muscle produces the characteristic frowning expression shown in response to strong and bright light, during focusing, or when experiencing emotional distress. Over time, activation of the procerus muscle can lead to permanent wrinkles on the glabellar skin, often referred to as "bunny lines."
Due to its role in facial expressions, the procerus muscle is of clinical importance in cosmetic and surgical procedures. Injecting botulinum toxin (Botox) into the procerus muscle is a common anti-aging technique to reduce wrinkles. Botulinum toxin inhibits muscle contraction, preventing cutaneous furrows and associated wrinkling in the forehead region.
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Injecting neuromodulators
The frontalis is the only muscle in the forehead that lifts the brows. When you raise your eyebrows in surprise or open your eyes wide, you engage the frontalis. Injecting neuromodulators into the frontalis weakens its lifting action, smoothing out horizontal lines on the forehead. However, over-injecting can result in brow ptosis (brow droop) because the muscle can no longer counterbalance the downward pull of the muscles around the eyes.
Neuromodulators, commercially known as Botox, Dysport, Jeuveau, and Xeomin, are wrinkle-relaxing injections that use tiny amounts of a toxin produced by the bacterium Clostridium botulinum to treat wrinkles, frown lines, crow's feet, and other areas of the face and neck. A minuscule amount of the neuromodulator is injected directly into the carefully targeted muscles, causing them to relax, and gradually smoothing out the appearance of the overlying skin. The effects typically last about three to four months.
The forehead is a complex area where muscles that lift and muscles that pull down the eyebrows are constantly at odds. Injecting without considering this push and pull can lead to undesired effects like brow droop, an unnatural appearance, or even a frozen forehead. Understanding the functions of the frontalis, orbicularis oculi, corrugators, and procerus muscles is key to injecting neuromodulators correctly and achieving a natural result.
The corrugators are located at the inner part of the eyebrows and run diagonally toward the centre of the forehead. When activated, they pull the eyebrows inward and downward, creating vertical frown lines between the brows (often called "11 lines"). Injecting the corrugators with a neuromodulator softens these frown lines and reduces the downward pull on the brows. Proper placement of the injections in this muscle can create a more open, relaxed look without affecting the frontalis.
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Frequently asked questions
The frontalis muscle is the only muscle that raises the eyebrows.
The frontalis muscle covers the forehead, spanning from the eyebrows to the hairline.
The frontalis muscle is used for conveying emotions and non-verbal communication through facial expressions. It is also responsible for keeping the eyebrows out of one's field of vision.
Other muscles in the forehead include the corrugator, orbicularis oculi, and procerus muscles.
Injecting neuromodulators into the frontalis muscle weakens its lifting action, smoothing out horizontal lines on the forehead. However, over-injecting can lead to brow ptosis or brow droop.
