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

Background: The botulinum neurotoxin Type A (BTX) injection into the masseter muscle often causes a change in the facial expression. There is as yet no precise anatomic evidence to support this etiologic factor of constrained facial expressions.

Objective: The aim of this study was to clarify the location and boundaries of the risorius muscle and its topographical relationship with the surrounding structures.

Materials And Methods: This study involved the dissection of 48 hemifaces. The locations of origin and insertion points of the risorius muscle were measured, and the masseter muscle was divided into 6 equally sized rectangular areas.

Results: Cases where the masseter muscle was covered by the risorius muscle were classified into the following 4 types: in Type A, Area III was partially covered by the risorius (17.8%); in Type B, Area VI was partially covered (20.0%); in Type C, Areas III and VI were partially covered (53.3%); and in Type D, Areas II, III, and VI were covered (6.7%).

Conclusion: These findings suggest that the medial part of the masseter muscle represents a hazard zone into which the injection of BTX may affect the risorius muscle, potentially resulting in iatrogenic unnatural facial expressions.