Introduction

The mid-face is the region between the zygomatic arches superiorly and the intercommis ural line inferiorly. The mid-face has complex muscular anatomy demanding full anatomical knowledge and precise injection technique. The small muscles in this area are important in expressing emotion, elevating the upper lip, and smiling. These smaller muscles are more closely arranged than their upper facial counterparts, and functionally integrated through the superficial muscular aponeurotic system (SMAS). The upper lip elevators function to produce a smile and are essential for the accurate conveyance of positive emotion. Even minor dysfunction of these muscles can have dire social consequences. Injection in the mid-face area is not for the novice injector, but can deliver real and tangible benefit for the patient when applied correctly.

The smaller mid-facial muscles require smaller doses when compared with their upper facial counterparts, especially in initial treatments. The advent of multiple botulinum toxin formulations has added another layer of complexity to this already challenging application and it is important to be aware of proper conversions / dilutions for each toxin (Table 17.1). Small errors in absolute dosage make a large difference in the percentage of the intended doses in this area. The activity of onabotulinumtoxinA (Botox^®) and incobotulinumtoxinA (Xeomin^®) is measured in murine lethal units (U), with both agents available as 50 U or 100 U per bottle with unit equivalency. It is best to use a higher concentration (e.g. 100 U/mL for Botox^®) in the mid- and lower face to minimize toxin diffusion. Abobotulinum toxin A (Dysport^®) activity is measured in Speywood units (sU) with either 500 sU or 300 sU/bottle, with a typical dilution of 250 sU/mL. As such, onabotulinum / incobotulinum toxin units and abobotulinum units are not interchangeable, and the exact conversion factor is controversial. According to FDA-approved labeling for the treatment of the glabella, 1 U Botox^® is approximately equal to 2.5 sU Dysport^®. However, studies have found abobotulinumtoxinA : onabotulinumtoxinA unit ratios to be in the range of 2.5–5 : 1. Some studies such as that by Husseman & Mehta have found abobotulinumtoxinA (Dysport^®) to have greater diffusion than onabotulinumtoxinA, meriting even additional caution given the intimately associated nature of the muscles in this area.

Additional points to bear in mind when treating the mid-face are that it is critical to avoid initial overtreatment; a standard of low starting doses and upward dose titration as needed should be adopted. Exact placement of the neurotoxins is critical – and marking the injection site before inserting the needle is suggested. Photographic documentation of the marked sites can be a helpful part of the patient’s records. The expectation that botulinum toxin alone will achieve all of the aesthetic correction in the mid-face is fallacious. Fillers are often used in this area to correct age-associated volume loss. Botulinum toxins can improve the appearance of the mid-face and, with proper patient selection, rigorous knowledge of facial anatomy and precise injection technique, toxins can prove a useful tool in this area.

‘Bunny’ / nasal sidewall scrunch lines

‘Bunny’ lines or nasal sidewall ‘scrunch’ lines are angled, horizontal rhytides that traverse the nasal bridge. While not necessarily associated with aging, many patients find them distressing as they convey the impression of distaste. Bunny lines may appear in individuals who have had a number of BoNT-A treatments in the glabella. Bunny lines result primarily from contraction of superior nasalis (pars transversalis) fibers with accessory action from the levator labii superioris aleque nasii (LLSAN) and inferomedial orbicularis oculi.

Injection sites / dosage

Injection sites for bunny lines are highly individualizable, as are the patterns of the lines. In general, one should ‘inject the fold’ within the following parameters. Injection sites should be kept high on the lateral nasal sidewall (Fig. 17.1) and well medial to the nasofacial sulcus to avoid inadvertent LLSAN weakness. Injections need to be placed only superficially (i.e. intradermally, raising a bleb) to avoid increased diffusion in the low-resistance subnasalis plane. The typical dose ranges have been reported as 2–8 U for onabotulinumtoxinA on each side or 10–20 sU for abobotulinumtoxinA. Figure 17.2 shows a patient before and after bunny line treatment. If LLSAN has a large role in the formation of bunny lines, and the patient is a candidate for LLSAN treatment (as discussed below), that area can be treated concomitantly.

Figure 17.1 The nasalis muscle traverses the nasal bridge. The injection site for chemodenervation of this muscle is labeled with an ‘X’.

Figure 17.2 Bunny lines (A) before and (B) after 5 units of onabotulinumtoxinA injected per side. The patient’s glabellar complex was also treated with 6 units placed in the procerus and 8 units per corrugator muscle.

Pearl 1

Bunny lines are highly variable. Inject the fold for best results.

Levator labii superioris alequae nasii

This small, but long, muscle originates on the medial maxilla before bifurcating and inserting into the medial orbicularis oris and nasal ala. Contraction of this muscle has several sequelae. First, it works with the levator labii superioris to elevate the upper central lip. Hyperactivity of this muscle may lead to excessive elevation of the upper lip on smiling and producing upper lip gum show or a ‘gummy smile’. The LLSAN’s dermal SMAS connections contribute to the medial part of the melolabial fold It also flares the alar base superolaterally; therefore it can give the appearance of a ptotic nasal tip on smiling. Finally, maximal contraction of the LLSAN can contribute to ‘bunny lines’.

Treatment of this muscle is performed primarily for two desired outcomes. The first is to treat ‘gummy smile’ by lengthening the upper lip in order to reduce the full incisor and upper gum show, as demonstrated in Figure 17.4. Treatment of this muscle can also be used to attenuate the medial nasolabial fold (NLF) in patients whom the etiology is primarily muscular (young patients with a mild to moderate NLF). As for the latter indication, it is important to remember that lip lengthening will occur concomitantly. Most patients in whom dermatoheliosis and soft tissue descent are primarily responsible for the melolabial fold, and in whom a ‘gummy smile’ is absent, will be better served by filler injection for treatment of the melolabial fold.

Injection sites / dosage

The LLSAN should be injected at the pyriform aperture / apical triangle of the upper lip, 1 cm above the lateral extent of the ala (Fig. 17.3); inject 1–2.5 U onabotulinum toxin per side in the supraperiosteal plane. The effect can be long lasting (~ 6 months) making rigorous patient selection and injection technique paramount. Figure 17.4 shows a patient before and after botulinum toxin injection to the LLSAN.

Figure 17.3 The LLSAN is treated with a deep injection at the apical triangle of the upper lip.

Figure 17.4 (A) Before and (B) 2 weeks following gummy smile treatment, with 2 units of onabotulinumtoxinA injected per side in the apical triangle of the lip.

Zygomaticus complex

The zygomaticus complex is composed of the zygomaticus major and minor muscles (Fig. 17.5). The zygomaticus major originates laterally on the zygomatic arch deep to the orbicularis oculi muscle and inserts in the modiolus. It therefore pulls the lateral oral commissure superolaterally and dilates the oral aperture. Symmetrical movement of this muscle is perceived as smiling, whereas unilateral movement can give an impression of ‘sneering’. Additionally, dermal connection of this muscle through the SMAS may contribute to dimple formation.

Figure 17.5 Anatomical drawing with the mid-facial musculature labeled. The ‘X’ signifies the injection site for the zygomaticus minor, at the intersection of a horizontal line drawn through the alar base, and a line connecting the lateral canthus to the ipsalateral oral commissure.

The zygomaticus minor originates on the zygoma medial to the zygomaticus major and inserts on the superior fibers of the orbicularis oris. Like the LLSAN, the zygomaticus minor has dermal attachments through SMAS that may contribute to the middle third of the NLF. The zygomaticus minor may not be present in all patients.

Risorius

This muscle arises from the parotid fascia and inserts on the modiolus, functioning to pull the corner of the mouth laterally, as in smiling. Hyperactivity can cause a sneering expression and can also contribute to fine paracommissural wrinkles.

Injection sites / dosage

The risorious injection site should be identified by drawing a horizontal line through the oral commissure. Move laterally along this line away from the oral commissure for 2 cm. From this point enter the skin at a 30° angle with the needle pointed laterally, away from the oral commissure (Fig. 17.6). The typical risorius dosage is 1 U onabotulinumtoxinA.

Figure 17.6 The injection site for the risorious is located by drawing a horizontal line through the oral commissures. Move 2 cm lateral to the commissure along that line. The needle should enter the skin at a 30° angle moving away from the commissure, and remaining on the intercommissural line to avoid inadvertent paralysis of other perioral muscles.

Conclusion

The mid-face is an anatomically complex region with smaller, more intimately associated muscles than the upper face. These mid-facial muscles categorically have important functions for the conveyance of emotion and interpersonal communication in addition to their role in the pathogenesis of rhytides. As such patients must be carefully selected, and other corrective options, such as fillers, explored. An inexperienced injector should not attempt injection of these muscles.