The risorius muscle: anatomic considerations with reference to botulinum neurotoxin injection for masseteric...

Area of Science:

• Anatomy
• Facial expression
• Botulinum neurotoxin Type A (BTX) injections

Background:

• Botulinum neurotoxin Type A (BTX) injections into the masseter muscle are known to cause changes in facial expression.
• Precise anatomical evidence linking BTX injections to constrained facial expressions is lacking.

Purpose of the Study:

• To clarify the anatomical location and boundaries of the risorius muscle.
• To determine the topographical relationship between the risorius and masseter muscles.

Main Methods:

• Dissection of 48 hemifaces.
• Measurement of risorius muscle origin and insertion points.
• Division of the masseter muscle into six equal rectangular areas for analysis.

Main Results:

• The risorius muscle partially or fully covered portions of the masseter muscle in all dissected hemifaces.
• Type A (Area III coverage): 17.8%
• Type B (Area VI coverage): 20.0%
• Type C (Areas III & VI coverage): 53.3%
• Type D (Areas II, III, & VI coverage): 6.7%

Conclusions:

• The medial masseter muscle is identified as a 'hazard zone' for BTX injections.
• Inadvertent injection into this zone can affect the risorius muscle.
• This may lead to iatrogenic, unnatural facial expressions.