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:

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).

Figure 20.2 Markings on the face showing the masseter muscle outlined and the position of the facial artery along the jawline anteriorly.

Bruxism

Masticatory muscle activities can be normal or abnormal in nature and were categorized by Ahrlberg et al as functional (e.g. chewing, swallowing, and speaking) or parafunctional (e.g. tooth clenching, grinding, and various oral habits). Bruxism (the nonfunctional gnashing and grinding of the teeth) is one such parafunctional activity commonly seen in humans. It had been primarily viewed as a dental phenomenon, but thinking has changed and it is now viewed as a psychophysiological disorder. It may be diurnal, where it has a prevalence of about 20% and is usually just jaw clenching, or nocturnal with a prevalence of clenching of about 10% and that of grinding of the teeth during sleep ranging from 8 to 16% (as reported by Lavigne & Montplaisir and Santamato et al). Bruxism appears to be equally common in males and females, but appears to occur more often in adults than in children.

Bruxism may lead to masseter muscle hypertrophy by the effect of recurrent action of the muscle over time. In this context, relief may be as simple as the use of adequate dosing of botulinum toxin over time to the masseter muscle.

Treatment of the masseter muscle for shaping and beautification of the lower face

Aesthetic recontouring of the lower face has been gradually developing as a concept initially with East Asian patients, but more recently also in Western patients. There has been a gradual increase in understanding of the use of botulinum toxin in the treatment of masseteric hypertrophy to promote a better facial appearance and enhance beauty. However, there is a definite gender difference in what is perceived to be attractive. Male facial shape is ideally full of sharp angles. The ideal facial female shape has been stated to be heart shaped, oval or base-up triangular or similar (Fig. 20.3).

Figure 20.3 ‘Heart-shaped face’, which is stated to have the appearance of youth.

There are many surgical procedures that have been described to decrease width of the lower third of the face, for example by Yang et al and Ying et al, and there is general agreement that attractiveness in female faces has as one of its components an aesthetically pleasing angles of the lower third of the face. In 2011 Liew looked at the similarities in truly beautiful people in different racial groups, and found that if one transposes a vertical line from the midline to intersect with the angle cast between the line drawn along the body of the mandible and tissue overlying the ascending ramus of the mandible it will form an acute angle (Fig. 20.4A–C). This angle is more acute (a lower angle) if the facial shape is square, such as one cast with masseter hypertrophy or parotid enlargement. It is less acute or greater if the facial shape is more triangular or the lower facial width is decreased. Hence, in Figure 20.3, one can see the improvement in attractiveness as one moves towards a greater ‘angle of beauty’, the ideal being 9–12°.

Figure 20.4 (A) Before treatment showing Liew’s ‘angle of beauty’ at 6°. (B) Interim result at 3 weeks following masseter injection with 25 units botulinum toxin with improved lower facial angle of 8°. (C) Final result with a more ideal angle of 10° at 15 weeks following injection.

Wu has also published results on the role of trimming parotid enlargement (which sits posterior to the enlargement produced by masseteric bulk) to further refine the cosmetic appearance of the lower face. Kwon et al reported that injection into the masseter has no effect on salivary flow rates, but injection into the parotid directly would certainly be expected to diminish salivary output. Wu noted that none of his patients suffered from symptoms of dry mouth after the parotids were injected for hypertrophy; the fullness shown by parotid enlargement may be missed as it may merge with the posterior border of the masseteric hypertrophy and be apparent only after masseter injection. Rarely, as Reddy et al found, masseteric hypertrophy may be associated with obstructive parotitis from obstruction of Stensen’s duct.

Injecting masseters

In the author’s method, 100 units of botulinum toxin (abobotulinum toxin) were dissolved in 2 mL of sterile normal saline, yielding a concentration of 5 units per 0.1 mL. The intended dose was drawn into a 1 mL syringe and the solution was injected with a 32 mm long ( inch) 30-gauge hypodermic needle.

The patient is asked to clench the jaw and the masseter palpated, outlining the anterior and posterior borders of the masseter muscle. Three to five injection points are used depending on the masseter bulk and markings placed at the site of maximum bulge, which is usually in the lower half of the muscle. The injection points are placed below a line drawn from the tragus to the angle of the mouth (see Fig. 20.2) and at least 1 cm from the borders of the muscles. Injecting outside these limits may interfere with other masticatory muscles, especially superolaterally, and the salivary glands posteriorly and inferiorly. The injections are placed deeply, as superficial placement may not reach the deeper heads of the muscle and produce n appearance of a chipmunk chewing on nuts when the patient is eating. Possibly this is due to the action of the functional deep head pushing against a treated and flaccid superficial head.

The dose depends on the bulk of the muscle being treated. In general Caucasian patients will require less than East Asian patients (Liew and Dart 2010) and males more than females. Most often, Caucasian females will require 15–25 units per side, whereas East Asian females may require 30–50 units per side. Males may need 10 units or so more than their female counterparts.

Masseter treatments may be repeated at 1–3-monthly intervals until the desired shape is attained (Fig. 20.5). Patients will readily report most often that their jaw clenching, morning headaches, sore teeth or other symptoms are better even after the first injection session. However, if symptoms are not improved or substantially settled this is a sign for reinjection of similar dose, otherwise often a lower dose than initially may be used.

Figure 20.5 (A–C) Treatment showing effective treatment of a patient in two stages – initially 20 units bilaterally then a top-up of 10 units to the right side only according to symptoms and palpation.

Quite often there will be asymmetry to the masseteric hypertrophy or even unilateral hypertrophy. Doses should be tailored to this event but, unless it is totally unilateral, often it is better to inject some on both sides but more obviously on the palpably stronger and visibly bigger side (Fig. 20.6).

Figure 20.6 (A–C) Significant asymmetry of masseteric hypertrophy: (A) treated with 10 units to the right side and 15 units to the left on the first treatment, then (B) treated with 15 units to the left and 5 units to the right and (C) the final result.

Whether or not one treats the temporalis muscle depends on a number of factors. One should observe the patient’s temporal fossa area when clenching or chewing for obvious signs of hypertrophy. If it is not obviously hypertrophic at rest with a marked convexity to the temple and bulging on mastication, I do not inject this at the first injection session. However, if it is then, similar to the masseter, it is injected according to its bulk in a number of injection points (usually three) in the area of maximum projection. However, unlike the change of facial shape produced by masseter injection, a hollow temple looks cadaveric and unattractive whereas a flat or slightly convex one is quite youthful. If a patient is complaining of bruxism and injection of apparent masseteric hypertrophy is insufficient to settle symptoms then injection of hypertrophic temporalis is indicated.

If there is a reversible element in the history such as a chewing habit (chewing gum, dried foods or other food oddities) then ceasing this and injecting the masseters a couple of times may be all that is required to change things in the long term.