The masseters and their treatment with botulinum toxin

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20 The masseters and their treatment with botulinum toxin

Greg J. Goodman

Summary and Key Features

• The masseter muscle may become hypertrophied distorting the mandibular angle

• Masseteric hypertrophy may be a cause of bruxism and facial pain syndromes

• Botulinum toxin may be used to treat both medical pain syndromes and to change facial shape by changing the mandibular angle

• Botulinum toxin may replace surgical and orthodontic techniques in selected patients in both medical and aesthetic indications

• Botulinum toxin may allow us to beautify patients by altering the lower facial width and converting the face to a more youthful triangular or heart-shaped face

Introduction

Botulinum toxin has been certainly an unusual drug. A drug does not usually find itself being continually reinvented, popping up all over disparate branches of medicine as a treatment for a seemingly never-ending number of indications. The story of botulinum toxin in the treatment of masseter hypertrophy may have started in the medical arena with the treatment of bruxism, but now has stepped well and truly into the cosmetic arena, establishing a central role in altering facial shape – arguably one of the most important determinants of facial beauty.

History

Following on from earlier experience by Jancovic & Orman in treating other facial dystonias with botulinum toxin, investigators including Lagueny et al and Van Zandijcke & Marchau turned their attention to the treatment of bruxism. Amongst the earliest use of botulinum toxin in the treatment of bruxism associated with masseteric (and temporalis) hypertrophy was by the latter group; this was an unusually severe case of a 32-year-old woman who, following a motor car accident and brain contusion, was beginning to communicate after a prolonged coma. Although she obeyed verbal commands, she had developed severe bruxism and ‘the teeth grinding was very loud and almost continuous causing damage to the teeth and annoyance to nearby patients’. In this case 25 units were injected into each temporalis and each masseter muscle, which is, somewhat surprisingly, a similar dose to an initial dosing in todays cases. The treatment of masseteric hypertrophy soon became more widespread and today is a mainstream treatment for this condition replacing surgery in many cases. In the treatment of bruxism it has a definite place alongside and in many cases replacing splints and orthodontic techniques in selected patients.

Evidence has begun to accumulate that the masseteric hypertrophy has other effects besides the bulk it directly confers because of the muscle volume. In line with theories of bone deposition and development, Wu found in 2010 that this muscle development may have effects on the underlying bone growth and volume. Bone growth and volume are under the influence and related to the activity of the muscles attached to the bone. A bone responds to the activity and strength of the muscles attached to it by thickening and the corollary is that bone will decrease when the muscular force acting on the bone is decreased, as reported by Tsai et al. Molina et al found that the masseter neuromuscular end plates are developed by 12 weeks’ fetal gestation. By restraining mouth opening, Habib and co-workers showed by the use of suturing that restricted fetal temporomandibular joint movement influences the process of endochondral bone formation of condylar cartilage. These effects on bone may explain the trimming effect that botulinum toxin has on the facial shape may be above and beyond the effects of what would be expected by a decrease in muscle bulk alone (Fig. 20.1). However, not all authors (e.g. Chang et al) believe that bony resorption occurs after botulinum toxin injection.

image image

Figure 20.1 (A) Pre-botulinum toxin injection for masseteric hypertrophy. (B) Result after two injection sessions. The trimming of the lower facial shape seems here to have produced more change than would be expected from just diminution of masseteric bulk and may involve some bony involution. This would be expected to be a temporary change with discontinued botulinum injection.

One of the great determinants of beauty is facial shape. In males a square facial shape is considered desirable, but this should not be a bottom- heavy face with a wider lower face than an upper face. In females this is more of an issue, with youth and beauty being related to an oval, triangular or heart-shaped face.

Anatomy

The bony framework

The background to the story of masseter (and temporalis) hypertrophy is their function as a set of masticatory muscles acting over the temporomandibular joint.

The temporomandibular joint is an engineering complex made up of bony and ligamentous structures. The bony parts are the anterior part of the mandibular fossa of the temporal bone, the articular tubercle of the maxilla, and the condyle of the mandible. The ligamentous structures include the articular capsule and disc, and a series of lateral ligaments.

Movements permitted the mandible by the temporomandibular joint include: depression or opening of the mouth from rest, elevation or closing of the mouth to rest, protrusion or pushing the mandible forwards from rest, retraction or pulling the mandible back to rest, and a small amount of side-to-side movement.

The muscles involved in closing the jaw (elevation) are the masseter, temporalis, and to some extent the medial pterygoid muscles. The temporalis muscle controls the first stage of the elevation action by initially lifting the mandible up and back, then the medial pterygoid muscle contracts slightly to lift the bottom of the mandible up. Lastly the masseter completes the elevation by contracting producing the final stage of mouth closure. Contraction can be seen in the muscle flowing from the anterior to the posterior portion of the muscle.

Masseter muscle

The masseter comprises three overlapping layers, all of which originate from the zygomatic arch and insert into the ascending ramus of the mandible thus allowing it to fulfill its function of elevating the mandible to close the jaw.

Specifically:

1. The superficial part arises from the zygomatic process of the maxilla and from the anterior two-thirds of the lower border of the zygomatic arch

2. The middle part originates from the deep surface of the anterior two-thirds of the zygomatic arch and from the lower border of the posterior one-third of the arch

3. The deep part arises from the deep surface of the zygomatic arch.

The three layers merge as the fibers pass downward and backwards to insert into the lateral surface of the angle, ramus and coronoid process of the mandible. Where these layers overlap is the thickest palpable region when the patients clench their teeth. This maximum bulk is usually inferiorly placed nearer the jawline than the zygoma.

The masseteric nerve from the anterior division of the mandibular nerve innervates the muscle and the vascular supply derives from the transverse facial branch of the superficial temporal, the masseteric branch of the maxillary artery, and the facial arteries.

Temporalis muscle

The temporalis muscle arises from the floor of the temporal fossa and from the overlying temporal fascia and its fibers converge towards their insertion at the apex, the anterior and posterior borders, and the medial surface of the coronoid process. In order to reach the coronoid process, the temporalis muscle runs beneath the zygomatic arch. Its function is to elevate the mandible (anterior or vertical fibers), but also by way of its posterior or horizontal fibers to retract it.

Surface anatomy of the masseter muscle

The muscle is easily outlined, with the superior border represented by the zygomatic arch and the inferior border being the inferior border of the mandible, The posterior border is the posterior border of the mandible, and the anterior border is easily seen in those with hypertrophy by asking the patient to clench their teeth. Anteriorly, along the jawline the masseter muscle will give way to a gap where often the arterial pulsation of the facial artery may be felt, and superior to this the hardness of the contracting masseter muscle may be contrasted with the softer connective tissue of the face anterior to the contracting muscle (Fig. 20.2).

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