Function and Effects of Neuromodulators on the Frontalis Muscle
Frontalis Muscle Function
The frontalis muscle is a key elevator of the eyebrows and forehead skin that creates horizontal forehead lines when contracted and plays a critical role in facial expression. The frontalis is part of the occipitofrontalis muscle complex and functions primarily to raise the eyebrows, elevate the upper eyelids, and create expressions of surprise, worry, or attention 1.
Key characteristics of the frontalis muscle:
- It extends from the galea aponeurotica to the skin and superficial fascia of the eyebrows and forehead
- Creates horizontal forehead lines when contracted
- Acts as the primary elevator of the eyebrows
- Exhibits bidirectional movement patterns during contraction:
- Lower forehead skin moves cranially (upward)
- Upper forehead skin moves caudally (downward)
- These movements converge at a horizontal line called the "line of convergence" or "C-line" 1
The shape of forehead lines varies among individuals and correlates with the underlying frontalis muscle morphology:
- Straight lines (41.9% of people)
- Wavy lines (58.1% of people) - associated with midline aponeurosis and greater muscle fascicle angles 2
Effects of Neuromodulators on the Frontalis
Botulinum toxin (e.g., onabotulinumtoxinA/Botox) blocks neuromuscular transmission by binding to nerve terminals, entering them, and inhibiting acetylcholine release, resulting in temporary chemical denervation and localized muscle weakness. 3
Mechanism of Action:
- Botulinum toxin binds to acceptor sites on motor nerve terminals
- Enters nerve terminals and cleaves SNAP-25 protein
- Prevents successful docking and release of acetylcholine from vesicles
- Results in partial chemical denervation of the muscle
- Causes localized reduction in muscle activity 3, 4
Clinical Effects on Frontalis:
When injected into the frontalis muscle, botulinum toxin:
- Reduces or eliminates horizontal forehead lines
- Decreases frontalis muscle activity in a graded manner
- Affects eyebrow position, with greatest vertical movement in the lateral (0.6-0.9 mm) and mid (0.5-0.7 mm) brow regions 5
- Can create a mild lateral arch of the eyebrow when properly administered
- Produces effects that last approximately 3-6 months before muscle reinnervation occurs 3, 4
Important Anatomical Considerations:
The bidirectional movement pattern of the frontalis muscle is clinically significant:
- Injections above the "C-line" (located at approximately 60% of forehead height) may reduce the risk of brow ptosis 1
- Understanding the shape of forehead lines (straight vs. wavy) helps predict underlying muscle morphology and optimize injection patterns 2
Clinical Applications and Precautions
Botulinum toxin is FDA-approved for temporary improvement in the appearance of:
- Moderate to severe forehead lines associated with frontalis muscle activity
- Moderate to severe glabellar lines associated with corrugator/procerus muscle activity 3
Dosing and Administration:
- Standard dose for forehead lines: 40 Units in 1 mL (when treating both forehead and glabellar lines) 3
- Treatment should not be administered more frequently than every 3 months 3
Potential Adverse Effects:
- Brow ptosis if injected incorrectly
- Spread of toxin effect beyond the injection site
- Rare but serious effects including generalized muscle weakness, dysphagia, dysphonia, and breathing difficulties 3
- Antibody formation with frequent injections or high doses 3
Contraindications:
- Known hypersensitivity to botulinum toxin
- Infection at the proposed injection site
- Neuromuscular disorders (relative contraindication)
Understanding the frontalis muscle's anatomy and function is essential for achieving optimal aesthetic outcomes while minimizing adverse effects when using neuromodulators like Botox. Proper injection technique and placement, particularly with respect to the "C-line," can help maintain natural eyebrow position while effectively reducing forehead lines.