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.


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%).


CONCLUSIONS
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.
*Division in Anatomy and Developmental Biology, Department of Oral Biology, Human Identification Research Center, BK21 PLUS Project, Yonsei University College of Dentistry, Seoul, South Korea; †Department of Dentistry, College of Medicine, Inha Research Institute of Medical Science, Incheon, South Korea; ‡Department of Dental Hygiene, School of Health and Medicine, Namseoul University, Cheonan, South Korea; §Department of Dermatology, Modelo Clinic, Seoul National University College of Medicine, Seoul, South Korea; ‖Department of Anatomy, Chula Soft Cadaver Surgical Training Center, Faculty of Medicine, Chulalongkorn University, Bangkok, Thailand.
Jung-Hee
JH
Bae
Da-Yae
DY
Choi
Jae-Gi
JG
Lee
Kyle K
KK
Seo
Tanvaa
T
Tansatit
Hee-Jin
HJ
Kim
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D019274:Botulinum Toxins, Type A / Q000627:therapeutic use*; D002102:Cadaver; D004210:Dissection; D005149:Facial Expression; D005152:Facial Muscles / Q000033:anatomy & histology*; D005260:Female; D006801:Humans; D006984:Hypertrophy / Q000188:drug therapy; D007267:Injections; D008297:Male; D008406:Masseter Muscle / Q000473:pathology*; D008875:Middle Aged; D009465:Neuromuscular Agents / Q000627:therapeutic use*
D009465:Neuromuscular Agents; D019274:Botulinum Toxins, Type A