To evaluate, inspect the lower face for a boxy or square-shaped appearance.

Palpate for masseter hypertrophy and rule out bony contours, overdeveloped parotid, and thick adipose tissue as the cause of the widened lower face.

Assess skin laxity, as injections in patients with poor skin tone may cause exacerbation of jowls for up to 2 months after treatment.

To assess muscle bulk, palpate the masseter during both relaxation and contraction.

Use standard frontal view photographs for preoperative and postoperative analysis, with optional lateral views to note muscle contraction changes.

The masseter is a three-layer masticatory muscle arising from the zygomatic arch and inserting on the lateral ramus and angle of the mandible.

Branches of the masseteric nerve are most commonly found in the lower middle third of masseter muscle.

The facial vein usually runs anterior to the muscle, and the facial artery usually runs anterior to the facial vein.

The parotid duct usually runs superior to a line drawn between the earlobe and oral commissure.

The risorius muscle may partially cover the anterior masseter.

Safety zone (Fig. 52.1):

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  Superior border: Imaginary line drawn between earlobe and oral commissure.

  
  
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  Inferior border: Mandibular border.

  
  
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  Anterior border: 1 cm posterior to the anterior edge of the masseter muscle.

  
  
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  Posterior border: Posterior masseter muscle.

  

Dosage of onabotulinum toxin A is divided equally among three injections per single masseter muscle.

Masseteric thickness determines the total dosage of onabotulinum toxin A per muscle:

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  Mild hypertrophy: 20 to 25 U

  
  
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  Moderate hypertrophy: 25 to 30 U

  
  
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  Severe hypertrophy: 30 to 40 U

Inject using a 30-gauge needle with a 1-mL syringe.

Injection sites:

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  Usually three injection sites per side (Fig. 52.2):

  
  
  
  
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    First injection: Thickest, central region of the muscle bulge, usually 1.5 cm superior to mandibular angle.

    
    
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    Second and 3rd injections: Distribute equidistant from the 1st across the bulging surface to form a triangle, staying inferior and posterior on the muscle to prevent inadvertent paralysis of the pterygoids and to prevent diffusion to the mimetic muscles.

  
  
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  For severe hypertrophy, total dosage may be divided into 4 + injection sites, distributed evenly to prevent post-treatment muscle bulging.

Technical tips:

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  To prevent inadvertent diffusion and paralysis of the medial and lateral pterygoids, stay below the sigmoid notch of the mandible.

  
  
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  Stay posterior on the muscle to avoid anterior diffusion of botulinum toxin into the risorius, the zygomaticus major and minor, and the levator anguli oris muscles to prevent facial expression and smile asymmetry.