Function of the Levator Labii Superioris and Effects of Botulinum Toxin
The levator labii superioris (LLS) is a facial muscle that elevates the upper lip and contributes to nasolabial fold formation, and botulinum toxin (Botox) injections effectively weaken this muscle by blocking acetylcholine release at the neuromuscular junction, resulting in temporary paralysis that can improve aesthetic concerns.
Anatomy and Function of the Levator Labii Superioris
Anatomical Characteristics
- Origin: The LLS originates from the inferior orbital margin 1
- Insertion: Two main patterns:
- Lateral fibers insert superficially to the orbicularis oris muscle while deep fibers form part of the raphe at the corner of the mouth (70% of cases)
- Superficial fibers insert directly into the orbicularis oris muscle (30% of cases) 1
- Dimensions: Average length of 24.66 mm, thickness of 3.57 mm, width of 11.2 mm at insertion and 15.96 mm at origin 1
- Shape variations: The LLS can be classified into three morphological types:
- Rectangular (83% of cases)
- Fan-shaped (10% of cases)
- Trapezoid (7% of cases) 2
Relationship with Nasal Ala
- The medial fibers of the LLS attach to the deep surface of the alar facial crease
- These fibers intermingle with the alar part of the nasalis muscle
- Some deeper muscle fibers extend to the vestibular skin of the nasal lobule 2
Function
- Primary function: Elevation of the upper lip
- Secondary functions:
- Contributes to formation of the nasolabial fold
- Assists in facial expressions, particularly expressions of disgust, sadness, or crying
- The middle third of the nasolabial fold is specifically affected by the LLS 3
Mechanism of Botulinum Toxin on the Levator Labii Superioris
Pharmacological Mechanism
- Botulinum toxin is a zinc-endopeptidase protein consisting of:
- Heavy chain (~100,000 daltons)
- Light chain (~50,000 daltons) 4
- The toxin enters the vascular circulation and is transported to peripheral cholinergic nerve terminals 4
- At the neuromuscular junction, botulinum toxin:
- Binds to the neuronal cell via the heavy chain
- Undergoes receptor-mediated endocytosis
- Translocates to the cytosol
- Cleaves proteins involved in acetylcholine release 4
- This blocks acetylcholine transmission across the neuromuscular junction, resulting in flaccid paralysis 4
Clinical Effects on LLS
- When injected into the LLS:
- Temporarily weakens the muscle's ability to elevate the upper lip
- Reduces the prominence of the nasolabial fold
- Can improve a gummy smile by limiting upper lip elevation during smiling
- Effects are temporary, lasting approximately 3-4 months
Injection Technique
- EMG guidance may be used for precise targeting, particularly in complex facial musculature 4
- Careful injection technique is essential to avoid complications 4
- Botulinum toxin is considered the "treatment of choice" for certain movement disorders affecting facial muscles 4
Potential Complications and Considerations
Common Side Effects
- Potential complications include:
- Asymmetry of facial expression
- Difficulty with certain lip movements
- Temporary weakness of adjacent muscles
- Bruising or pain at injection site
Special Considerations
- Precise anatomical knowledge is crucial for optimal results
- The relationship between the LLS and surrounding structures (particularly the nasolabial fold) must be considered when planning injections 3
- Contraindications include allergy to botulinum toxin 4
Recovery Process
- Recovery from botulinum toxin effects occurs through sprouting of new nerve terminals
- This process typically takes weeks to months 4
- The temporary nature of the effect means repeated injections are necessary to maintain results
Understanding the precise anatomy of the LLS and its relationship with surrounding facial structures is essential for achieving optimal aesthetic outcomes when using botulinum toxin for facial rejuvenation.