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In this video, we are going to focus on the region that lies between the angle of the jaw, the apex of the chin, and that runs inferiorly to the clavicle. There are several key structures in this region that we need to be aware of. In terms of muscles, we find here the masseter, the depressor anguli oris, the depressor labii inferioris, and the platysma. We also see the parotid gland and duct, the mandibular retaining ligament, facial artery, facial vein, inferior labial artery, and the mental artery.
The jaw can be divided into four main areas, the chin area, the prejowl sulcus, the area of the jowl, and the post jowl area that runs up to the angle of the jaw. When we begin to examine the key muscles in this area, we need to be aware of their origins, insertions, and actions. The mentalis is a paired muscle that arises from the base of the chin and inserts into the fibres of the orbicularis oris. Excessive contraction causes a cobblestone appearance to the chin.
The platysma is a large, flat cutaneous muscle which stretches from the upper chest all the way to the mid cheek. It arises from the pectoralis muscle with the fibres ascending upwards. At the angle of the jaw, some of the fibres insert into the bone of the mandible. Interestingly, this is the only bony insertion. Some of the other fibres blend into the modiolus, the depressor angularis oris, the lower lip, and the depressor labii inferioris. Other fibres ascend above the mandible into the SMAS. This muscle is absent in the midline of the neck, except where the fibres intertwine underneath the chin. When it contracts, it depresses the skin of the lower face with the medial fibres in posterior bands contributing to lowering the corners of the mouth. When both the upper and lower parts contract, they pull the skin towards the centre of the muscle, creating prominent platysmal bands.
The masseter is the muscle of mastication, or chewing. It is a thick muscle and is made up of three layers. It arises from both the zygomatic arch and zygomatic process of the maxilla. It then inserts into the lower portion of the border of the mandible. This is a powerful muscle which varies in thickness. Its contraction is associated with the chewing action. Hypertrophy of this muscle can cause the jaw to have a squared appearance.
The depressor anguli oris originates from the mandible and inserts into the modiolus. Its action causes the corner of the mouth to be pulled down. The depressor labii inferioris arises from the oblique line of the mandible and inserts into the skin of the lower lip. When contracted, it causes lowering of the bottom lip.
We need to be aware of a number of anatomical structures that are at risk when we treat this region. In the chin, the at risk structures are the facial artery and vein, the inferior labial artery, and the mental artery. In the jaw, we need to avoid the parotid gland, parotid duct, and the risorius muscle, which inserts from the modiolus into the superficial fascia surrounding the masseter muscle. Ageing results in numerous changes within this area. Firstly, the mandible can shorten, which leads to the formation of jowls. Secondly, a retrognathic chin can develop, and in some patients the masseter can become hypertrophic. Finally, contraction of the platysma can cause blunting of the jawline and the development of platysmal bands.
Using our knowledge of the anatomy of this area, we can determine our points of injection and the relevant depth. For the treatment of the mentalis with botulinum toxin we focus on a midline approach with a deep injection. This is to avoid migration to neighbouring muscles. When we treat the masseter muscle with botulinum toxin, we should remember that it is a thick muscle which comprises of three layers. We need to target the deepest layer and inject towards the lower end of the muscle. This is to avoid the anatomical structures lying close to the muscle at the superior end, including the parotid duct and the risorius muscle. When injecting dermal filler into the chin, we focus on injection into the periosteum to reshape this area. Finally, when we treat the jawline, we target a superficial plane. This is to avoid vascular structures such as the facial artery, which can be located at the border of the mandible.