Location of the Modiolous and the Morphologic Variations of the Risorius and Zygomaticus Major Muscle Related to the Facial Expression in Koreans

Hu, Kyung Seok; Yang, Seung Jun; Kwak, Hyun Ho; Park, Hyun Do; Youn, Kwan Hyun; Jung, Han Sung; Kim, Hee Jin

Abstract

Location of the modiolous and morphological variations of the risorius and zygomaticus major muscles are related to the facial expression. The zygomaticus major, levator labii superioris, depressor labii inferioris, depressor anguli oris, risorius, orbicularis oris, buccinator and levator anguli oris muscles insert on the lateral border of the lip, forming the modiolus and mutually associating each other for functioning. The knowledge of the location of the modiolus and surrounding structures are essential to anatomy, prosthodontics, linguistic, physiology and computer simulation based on facial expressions. The authors examined the location of the modiolus, the morphological variations and anatomical relationship of risorius and zygomaticus major muscle to understand the features of the smile of Korean by dissecting 39 cadavers. The location of the modiolus can be showed as three types, according to their height related to the intercheilion horizontal line. Type A that modiolus locate at the intercheilion line was shown in 20 sides (26.0%), type B that modiolus locate above the intercheilion line was shown in 12 sides (15.6%), then type C that modiolus locate under the intercheilion line was shown in 45 sides (58.4%). Most modioli located at 10 ~20 mm lateral to the mouth corner and 0 ~10 mm below the intercheilion line. The risorius muscle was classified into five types by directions of muscle fibers. The depressor anguli oris -risorius type (type I) was observed in 31 sides (40.2%), the platisma -risorius type (type II) was observed in 30 sides (39.0%). Previously, it has been known that zygomaticus major muscle attaches to the modiolus mainly as one bundle. However, the results were clearly shown that two bundles of the zygomaticus major muscle attaches to the modiolus and the position of the mouth edge in 18 sides (23.4%). To sum it up, facial expression is of fundamental importance concerning the morphological variations and these results also can be considered for the facial reconstruction surgery and computer animation department.