Clinical anatomic considerations of the zygomaticus minor and depressor anguli oris muscle based on the morphology for botulinum toxin injection

Abstract

The zygomaticus minor muscle (Zmi) is involved with the formation of negative expressions; it raise the corner of the mouth and upper lip, then helps to curl the upper lip when showing contempt, smugness, and disdain. It also helps to deepen the nasolabial sulcus when showing sadness. Drooping of the mouth corner caused by hyperactivity of the depressor anguli oris (DAO) can also generate a sad, tired, or almost angry look in some patients. However, The details of the Zmi insertion pattern and morphology are not well described even in general anatomy textbooks and illustrations. In addition, it is difficult to inject botulinum toxin type A (BoNT) into the DAO because its medial border overlaps with the depressor labii inferioris, and its lateral border is adjacent to the risorius, zygomaticus major, and platysma muscles. The aim of this study was to clarify the morphology and insertion pattern of the Zmi, and to provide critical information for determining the safest and most effective BoNT injection site of the DAO.Fifty-four embalmed adult hemifaces (48 bilateral and 6 unilateral; 31 males, 23 females; age range, 45–48 years; mean age, 67.4 years) from 30 cadavers were used in Zmi study and forty-two hemifaces from Korean (20 hemifaces, 9 left and 11 right; mean age, 69.1 years) and Thai (22 hemifaces, 11 left and 11 right; mean age, 78.4 years) adult cadavers (in total, 20 left and 22 right; mean age, 73.9 years) were dissected for DAO study.The Zmi was classified into three types. Type A, in which the Zmi attached only to the upper lip, was observed in 63.0% (34/54) and was subdivided into two types: straight (A-1; 31.5%, 17/54) and curved (A-2; 31.5%, 17/54). Type B, in which the Zmi was attached to both the upper lip and the lateral alar region, occurred in 27.8% (15/54). In type C, there was either no or only undeveloped Zmi fibers (9.2%, 5/54). The location of the modiolus was 11.0±2.6mm laterally and 8.9±2.8mm inferiorly to the cheilion. The angle formed by the sagittal line passing through the modiolus (LV) and the line connecting the modiolus and P2 (the point which meet the lateral border of the DAO and the mandibular border) was 44.7±13.7°. The angle formed by LV and the line connecting the modiolus and the most concave point of the medial border of the DAO (LP3) was 31.8±8.5°.The present finding of Zmi fibers being attached to the alar region in many cases (27.8%) suggests that this muscle is involved in elevation of both the nose ala and upper lip. The fan-shaped area bounded by LP2, LP3, and the mandibular border is the safest and most effective injection site for DAO.