Mentoplasty

Harry Mittelman

INTRODUCTION

While facial aesthetics has evolved over time, the balance of facial proportions is a common and universal aesthetic ideal. A straight and central mentum with smooth transitions to the lateral mandible defines an aesthetically pleasing and youthful chin. Augmentation and contouring of the chin and mandible are essential procedures in addressing age or congenitally related conditions. These procedures have become increasingly popular due to the improved understanding of changes in the mandible over time as well as the continued evolution of alloplastic implants. Many materials are used for augmentation including Gore-Tex, Medpor, Acrylico, Mersilene mesh, and solid flexible silicone. In my opinion, the silicone implants are by far the implants that are easiest to work with and least reactive. These implants are anatomically and artistically designed to deliver a significant improvement in what is commonly a relatively straightforward procedure. Alloplastic implantation is also completely reversible—a feature that may help broaden its appeal to prospective, but anxious patients. The size of the implant can also be adjusted to specific patient and surgeon desires.

The variety of materials that are available for augmentation of the mandible may, at first, be overwhelming. This perception is further heightened with the diversity of injectable fillers used to augment the facial soft tissues. It has become possible to achieve “surgical results” with the materials available, but such changes commonly require additional interventions and maintenance. With an erudite understanding of the morphologic differences between individual mandibles, and the aging process as it applies to them, permanent implant selection becomes much more clear-cut. More simply stated, a small number of alloplastic extended mandibular implants can fulfill the vast majority of the facial plastic surgeon’s clinical challenges. Few other procedures in the surgeon’s repertoire yield as much benefit for as little time and effort expended as augmentation of the mandible with a properly chosen alloplastic implant.

HISTORY

As with all patients pursuing cosmetic surgery, the history should begin by evaluating each patient’s motivation and emotional state to ensure that these are appropriate. Pertinent points in a patient’s history should include previous surgeries, facial trauma, dental/orthognathic procedures, bleeding problems, and anesthetic risk factors. Medical issues such as osteoporosis, previous cancer of the oral cavity, history of intravenous (IV) bisphosphonates, or radiation treatment should be evaluated and the results documented. Once a detailed medical history is completed, the surgeon should focus on the patient’s prior history of cosmetic procedures, including injectable fillers, as these are often directly or mistakenly omitted by the patient when completing standard preoperative questionnaires. It is critically important to identify any signs of functional mandibular problems, dysplasia, malocclusion, or temporomandibular joint dysfunction. These conditions are not directly addressed by chin augmentation and should prompt a referral to the appropriate specialist for further evaluation.

PHYSICAL EXAMINATION

Relevant Anatomy

While the basic anatomy of the mandible is familiar to the facial cosmetic surgeon, some points about the mental nerve are worth emphasizing. The mental foramen transmits the mental nerve, which exits in a superior direction and supplies sensation to the lower lip and chin. The expected location of the mental nerve is inferior to the second mandibular premolar on either side, although significant variability can exist in up to 50% of patients, with slight displacement of the mental foramen either anterior or posterior to this. In the typical young adult mandible, the mental foramen is located approximately halfway between the alveolar ridge and the inferior border of the mandible, and approximately 25 mm lateral to the midline, within a range of 20 to 30 mm. In children, the mental foramen lies closer to the inferior border of the mandible and slightly more anterior. During the aging process, atrophy of the alveolar ridge causes the foramen to lie in a relatively more superior position, since the distance to the inferior border of the mandible remains fairly constant. Even in the aging mandible, there is generally a distance of more than 8 mm between the mental foramen and the inferior border of the mandible at the site of the muscular attachments.

The surgeon must dissect carefully when creating a pocket for the implant that is below the mental foramen yet immediately above the muscular attachments at the inferior border of the mandible. Generally, one has approximately 10 mm of space in this area. Properly designed implants should have a vertical height of 6 to 8 mm when placed into this area. The facial nerve is unlikely to be damaged with the use of extended Silastic mandibular augmentation.

Care should be taken to avoid injury to the mentalis muscle. The mentalis muscle is a fan-shaped muscle separated in the midline by a firm septum. From an intraoral approach, one should take care not to strip the mentalis muscle from its origin.

Aesthetic Analysis

The basic tenets of facial aesthetic proportions have been summarized by Powell and Humphreys and include both a frontal and a lateral assessment. The frontal view of the face may be divided into thirds, with the lower third extending from the subnasale to the menton. This lower third can be subdivided so that the upper third is occurring from the subnasale to the stomion superius and the lower two thirds occurring from the stomion inferius to the menton. There is loss of the vertical height and anterior projection of the mandible with advancing age, resulting in a loss of the ideal proportions. Simultaneously, during this aging process, the soft tissues covering the mandible often display some atrophy as well as laxity. On lateral view, the method of Gonzalez-Ulloa may be applied to define a hypoplastic mentum. In this technique, a line is dropped from the nasion perpendicular to the Frankfort horizontal plane. The ideal chin projection should be at this line. However, when the chin is posterior to this line and the patient has a Class I occlusion, then a hypoplastic mentum is present. Another frequently used method is to simply drop a line from vermilion of the lower lip perpendicular to the Frankfort horizontal plane. Once again, the ideal chin projection should be at this line and a chin posterior to this line with Class I occlusion is considered hypoplastic. While a man’s ideal pogonion position is tangential to this line, a woman’s ideal position may lie 1 to 2 mm posterior to it.

Physical Examination

While assessing a patient for chin augmentation, the surgeon should be attentive to both the patient’s individual anatomy and his/her appearance in relation to that patient’s aesthetic ideal. More importantly, the surgeon should evaluate for any functional disturbance of the mandible (malocclusion, temporomandibular joint dysfunction). One must evaluate both the underlying skeletal structure as well as the overlying soft tissue envelope, as these elements contribute to the aesthetic lines of the jaw and cervicomental angle. The mentalis muscle and overlying soft tissue pad should be evaluated and palpated, and any pathology should be noted. Patients with a severely hypoplastic mentum and strong mentalis strain leading to lip incompetence should be considered for osseous advancement.

Although the development of a hypoplastic mentum is largely determined by genetic factors, the development of a prejowl sulcus is primarily the result of aging. However, the prejowl sulcus, or antigonion notch, may also be congenital and be present from childhood. A combination of progressive soft tissue atrophy and gradual bony resorption of the inferior mandibular edge between the chin and the remainder of the body of the mandible results in the development of the anterior mandibular groove, as named by the author in 1981. This area is known as the prejowl sulcus (Fig. 34.1). With continued aging, the prejowl sulcus may merge with the commissure-mandibular groove, or “Marionette line,” further accentuating the aging jawline.

FIGURE 34.1 Patient with a hypoplastic mentum and prejowl sulcus (black arrow) secondary to aging.

INDICATIONS

The main indication for chin augmentation is mild microgenia. As described above, there are a number of different methods to describe the appropriate relationship of chin position to the rest of the face. I prefer to use a straight line perpendicular to the Frankfort horizontal dropped from the vermillion to estimate appropriate anterior projection.

The presence of a prejowl sulcus is an indication for prejowl augmentation. While deficiency of volume in the prejowl area may be related to bony deficiency or to soft tissue deficiency alone, it is often multifactorial. Mild soft tissue deficiency in the prejowl area may suggest that the patient may benefit from injection of soft tissue filler injection only.

PREOPERATIVE PLANNING

Chin augmentation is frequently done in conjunction with other procedures, such as rhinoplasty or rhytido-plasty. Since chin projection is best viewed in profile, many patients are unaware of deficiencies when seeking consultation to improve their submental or nasal appearance. It behooves the astute facial plastic surgeon to always consider the importance of chin projection or irregular mandibular contour and explain the importance to the patient in order to obtain an optimal, balanced result (Fig. 34.2).

Preoperative photography with a minimum of frontal, lateral, and oblique views, ensuring that the patient is placed in the Frankfort plane, is essential for photodocumentation of the preoperative appearance and for implant sizing. Computer simulation is a useful tool to demonstrate the benefit of chin augmentation, especially for those patients who do not seek improvement in this area during their initial consultation (Fig. 34.3). Preoperatively, it is critical that the surgeon identify, document, and discuss with the patient any existing asymmetry, which otherwise may be noticed by the patient only after surgery. Also mandatory are preoperative assessment and discussion of occlusion with the patient, since alloplastic implants will not affect the patient’s occlusal status. Any desire by the patients to functionally improve their occlusion would be more appropriately addressed in orthognathic surgery.

It is most important to fully discuss the risks of surgery, especially paresthesia of the chin and lower lip. These are frequently resolved within 6 weeks of surgery but can persist for months. Rarely, some numbness of a portion of the chin and/or lip may be permanent.

FIGURE 34.2 Preoperative view of a patient presenting for cosmetic rhinoplasty, with a severely hypoplastic mentum in addition to a dorsal hump nasal deformity. She would likely be an excellent candidate for augmentation mentoplasty at the time of rhinoplasty.

Implant Selection

An ideal approach to chin augmentation requires an understanding of alloplastic implants, autologous augmentation materials (adipose tissue, fascia), and injectable fillers. Often, the use of a single modality can produce an acceptable outcome. However, taking advantage of the synergy between multiple modalities of chin augmentation can produce the most natural and long-lasting results. While a thorough discussion of adipose tissue grafts and their applications are beyond the scope of this chapter, adipose tissue grafts can be placed in such a way that they are long lasting, integrated, and natural in appearance. However, variable resorption, growth, and migration of injected adipose tissue grafts can lead to unpredictable clinical results with undesirable contours and bulges that are difficult to correct.

Hyaluronic acid (HA) fillers can be used alone to augment the mentum or to fill the soft tissue deficits of a prejowl sulcus, thus returning the contour of the jawline to a more youthful, straight, configuration (Fig. 34.4). The added safety profile of these fillers provided by the potential use of hyaluronidase to reverse, alter, or refine the HA injection provides a degree of safety not previously possible. Other injectable alloplastic materials, including poly-L-lactic acid (Sculptra, Dermik-Bridgewater, NJ) can be used to serially augment the soft tissues of the mentum and prejowl area to soften contour irregularity and increase projection.

FIGURE 34.3 (A) Preoperative photograph and computer-imaging photographs showing the potential changes to facial balance with (B) rhinoplasty alone or (C) rhinoplasty with chin augmentation implant.

FIGURE 34.4 A: Pretreatment photograph illustrating development of mild jowling and prejowl sulcus. B: Posttreatment photograph after treatment with 1 syringe of hyaluronic acid filler, with excellent improvement in prejowl sulcus and reestablishment of a youthful jawline contour.

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