Facial Alloplastic Implants, Chin

Facial Alloplastic Implants, Chin

Introduction

Coburn et al state, "The chin, the nose, and the forehead are the three balancing masses of the face."¹ An aesthetically and visually pleasing chin provides balance, harmony, and symmetry to the rest of the face. As a prominent structure of the face, it affects the general appearance of other facial structures. The shape and size of the chin may create visual prejudices in others' eyes. A small chin, for example, may convey a sign of weakness, whereas a pointed and ptotic chin (witch's chin) may create negative connotations beyond those of unattractiveness. A prominent chin, as long as it is proportional, conveys the sense of a strong, determined, and decisive personality. One undesirable feature in a field of otherwise handsome structures may create disharmony. Conversely, a proportional and pleasing chin may improve the overall aesthetics of the face.

The aesthetic surgeon, in considering and evaluating the structures of the face, normally considers the face (particularly in profile view) as divided into thirds. The upper third consists of the forehead, brows, and upper eyelids; the middle third consists of the lower lids, cheeks, and nose; and the lower third consists of the mouth and chin. When deficiencies, significant disproportion, and disharmony are present among these 3 zones, correction by augmenting, reducing, or otherwise surgically modifying one or more of the zones is considered.

In the chin, minor deviations often can be corrected by the simple insertion of an implant. Cohen states, "In evaluating the chin from an aesthetic viewpoint, the absolute measurements of facial structures are not as important as the relative size and proportion of each structure in comparison with the others."² More significant malrelationships may require major surgical intervention that includes orthognathic or craniofacial surgery. In addition, occlusal relationships of the teeth need to be evaluated before augmentation mentoplasty is considered. However, consider augmentation of the chin in patients with a small or weak chin who have a normal or near-normal occlusion or when orthognathic surgery is not warranted.

History of the Procedure

Rubin et al mentioned the first alloplastic prosthetic materials for chin augmentation in 1948: "The important role the chin plays in facial appearance has been recognized since antiquity. Ivory, bovine bone, and a variety of alloplasts are but a few of the materials that have been used clinically with variable success to augment the contour of the chin."³ Millard wrote a significant article about chin implants in 1953. Eppley noted:

The use of implantable biomaterials and devices plays a potential role in most forms of reconstructive surgery. … The significant advances in materials science and engineering during the latter half of this century have made the internal use of synthetic implants an integral part of many surgical procedures. Their frequency of modern surgical use can also be traced to the simultaneous development of broad-spectrum antibiotics and the continuing refinement of sterile operative techniques.

Prior to the introduction of alloplastic implants, autogenous materials were used. This was particularly true where reduction rhinoplasties yielded autogenous graft materials in the form of the dorsal nasal hump bone and associated cartilage. Such materials placed as onlay grafts were generally unsuccessful and unsightly, since they were absorbed readily but unevenly. Their placement and positioning also were not part of a thorough understanding of the elements of "profileplasty."

Terino writes, "During the 1950s several varieties of chin implants composed of different materials were developed." Since their initial use, alloplastic chin implants have been made of a variety of materials (eg, silicone, Proplast I & II [synthetic porous composite of Teflon polymer and alumina], Mersilene, Teflon, Dacron, Gore-Tex, acrylic, polyethylene, methylmethacrylate, hydroxyapatite) and in numerous shapes. Some of the earliest implants were short and shaped like an extended oval. They were designed for placement on the central chin, a concept that was flawed. Except in a limited number of circumstances, these implants produced an oddly pointed chin shape.

In more recent times, they have been replaced by anatomic implants, which were created and developed by Ed Terino before their introduction by a commercial company. These more closely resemble the shape and dimensions of the chin and extend from just beyond the mental nerve on each side. Initial placement of the implant extraperiosteally evolved to the more satisfactory subperiosteal placement. Eppley states, "Alloplastic materials offer the potential for ‘off-the-shelf' solutions to reconstructive tissue needs which avoid donor scars and morbidity and typically simplify the procedure in terms of time and complexity of technique."

As more and improved injectable implant materials, including permanent injectable implants, are developed, they should be considered in the armamentarium of the aesthetic plastic surgeon.

Problem

The presence of a small or retrusive chin contour, whether alone or in the setting of large, disproportionate nasal contours, indicates consideration of augmentation mentoplasty with alloplastic material.

Frequency

The incidence of relative or absolute microgenia is not defined. However, the American Society of Plastic Surgeons reported in its 2004 statistical survey of Board Certified Plastic Surgeons that its members performed more than 15,000 augmentation mentoplasties in the United States. However, this number represents just a fraction of the chin augmentations performed as reported by the American Society of Aesthetic Plastic Surgeons, which reported more than 32,039 chin augmentation procedures for 2004 in the United States. Clearly, although not the most common cosmetic surgery, chin augmentations, most of which are by alloplastic implant, are quite common.

For information, including news and CME activities, on aesthetic procedures, see Medscape’s Aesthetic Medicine Resource Center.

Presentation

The workup can be divided into 2 distinct segments: (1) the initial evaluation of the need for mentoplasty and/or genioplasty and the magnitude of the problem and (2) the presurgical medical examination.

Initial evaluation

In the initial evaluation, the extent of the workup depends in part on whether the surgeon is a purely aesthetic (cosmetic) plastic surgeon or whether he or she also performs reconstructive procedures, including maxillofacial work.

The aesthetic plastic surgeon may reserve alloplastic mentoplasty for those patients with mild-to-moderate microgenia and/or retrogenia or retrusion, preferring to send patients with more severe retrusion and/or significant occlusive disorders to a colleague who concentrates on the reconstructive aspects of maxillofacial surgery. On occasion, even more severe retrusion with Class II or Class III malocclusion can be managed with an alloplastic mentoplasty alone.

For the aesthetic plastic surgeon performing routine alloplastic mentoplasty, workup of the problem can be limited to physical examination of the face, head, and neck region, including an evaluation of the patient's dental occlusion and accurate clinical photographs. In more severe situations, in which alloplastic mentoplasty still is considered, an oral surgical and orthodontic evaluation that includes dental models, panoramic radiographs, and cephalometrigrams is appropriate, as are accurate clinical photographs.

Generally, a decision to correct a small or retrusive chin contour usually is made on the basis of physical examination, photographic evaluation, and the plastic surgeon's "artistic impression." If, in profile, a "plumb line" dropped from the nasal spine to the chin projection does not graze the edge of the chin, sufficient indication exists to consider alloplastic mentoplasty.

Presurgical medical evaluation

Once the decision is made to proceed, the patient requires the same preoperative medical history and physical examination as for any individual undergoing surgery. This is the second stage of the workup.

Include a complete medical history; thorough physical examination; laboratory studies appropriate for the patient's age, magnitude of surgery, and associated medical conditions; and type of anesthetic planned. Such guidelines fall within the purview of the department of anesthesia and are individual to each hospital institution or approved/licensed outpatient center.

Indications

Under circumstances in which occlusion is reasonably normal (overbite and underbite are acceptable but not malocclusion) and the chin is retrusive or hypoplastic to either a minor or moderate degree, correction and/or improvement by placement of an alloplastic implant is indicated. The decision between a sliding genioplasty or alloplastic implant correction is an individual one made by the plastic surgeon and patient. Cohen writes, "A variety of authors have proposed aesthetic and cephalometric systems to evaluate the relative size and shape of the chin. None of these systems is absolute, and surgical decisions should be based on aesthetic relations and not cephalometric values."²

Relevant Anatomy

Cohen states:

The bony portion of the chin is the mandibular symphysis. Embryologically, two hemimandibular segments form independently from the first branchial arch system, with an ossification center appearing at about the sixth week of gestation. Intramembranous ossification continues to envelop and invade much of Meckel's cartilage, and the two mandibular bodies meet at the mandibular symphysis generally between the fourth and twelfth months after birth, as ossification converts the syndesmosis into a synostosis.²

Anatomically, the chin is defined as the region inferior to the labiomental fold, the groove that separates the lower lip from the chin. Several muscles cover the bony mandibular symphysis or chin. These include the mentalis, quadratus labii inferioris, triangularis, orbicularis oris, and some platysma fibers. Branches of the facial nerve innervate these muscles. The geniohyoid, genioglossus, and anterior bellies of the digastric muscle attach along the posterior and inferior surfaces of the mandibular symphysis (chin).

The sensory innervation of the chin area is from the mental nerve on either side, which is a branch of the inferior alveolar nerve, itself a branch of the mandibular nerve. All of these originate from the trigeminal (fifth cranial) nerve. The motor innervation is from the marginal mandibular branch of the facial (seventh cranial) nerve.

Contraindications

Contraindications include significant malocclusion, associated craniofacial abnormalities, magnitude of microgenia too great for implant correction, and the standard physical and psychiatric contraindications to surgery, particularly elective cosmetic surgery. These contraindications include a health condition that precludes any surgical procedure, uncontrolled hypertension, uncontrolled diabetes, tenuous cardiovascular or pulmonary condition, and terminal malignancy. Common sense dictates that most, if not all, of these situations take precedence over a cosmetic operation. In the psychological realm, contraindications range from gross and overt psychosis to the more subtle personality disorders manifested by unrealistic expectations, unrealistic motivation, and apparent inability to be pleased with any change or improvement in appearance.

While the surgical procedure may appear to be, and is, quite straightforward and technically simple, the criteria for patient selection, as for any surgery, must be applied.