Chin Implants

Updated: Apr 02, 2014
  • Author: Suzanne K Doud Galli, MD, PhD; Chief Editor: Arlen D Meyers, MD, MBA  more...
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Overview

Background

The chin has a prominent role in establishing facial symmetry and aesthetics. With the nose, it can determine facial balance, especially through its relationship to the face in profile. Surgeons have recognized this for many years, and combination procedures involving rhinoplasty and genioplasty are common. The chin defines the character of the face and, perhaps, the person; a long chin has been associated with strength and power, whereas a short chin portrays weakness.

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Before and after. Before and after.
Before and after. Before and after.
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History of the Procedure

Osteoplastic genioplasty has been the criterion standard in chin surgery. However, in comparison to alloplastic implants, the procedure is longer, technically challenging, and puts the mental and inferior alveolar nerves at risk. Chin augmentation is often performed in combination with other procedures, including rhinoplasty and rhytidectomy, to address the prejowl sulcus or a ptotic chin pad. Implants, which have been used intermittently over the last century, have regained popularity, especially with the development of alternative alloplastic materials.

Historically, various materials have been used for soft tissue and bony augmentation, including autogenous materials such as iliac and rib bone grafts and nasal cartilage. Aufricht described the successful use of the osteocartilaginous nasal hump for chin augmentation during combined plastic surgery of the nose and chin. [1] He developed this technique in 1928 and reported that in one patient, the autograft remained intact for at least 27 years. He advocated this technique not only because of the convenience of using an autograft but also because of the aesthetic benefit achieved by performing simultaneous rhinoplasty and genioplasty.

Alloplastic materials such as ivory, acrylic, and precious metals are now primarily of historic interest. Other materials such as polytetrafluoroethylene (PTFE) and silicone have also been popular. Newer materials have been developed, including soft materials (eg, nylon mesh, expanded PTFE) and harder materials (eg, high-density polyethylene, hydroxyapatite compounds).

Several general qualities contribute to the biocompatibility of an implant. Most importantly, those materials that do not elicit a chronic inflammatory response or foreign body reaction can be highly biocompatible. Lack of immunogenicity and carcinogenicity are also favorable implant characteristics. Implant material must be nondegradable, yet malleable, such that the shape and position are maintained over time.

For more than 40 years, silicone has been used in the human body. When elemental silicon (number 14 on the periodic table) is interlinked with oxygen, it can polymerize into silicone, which can exist as a liquid form (silicone gel) and a more solid form that is appropriate for facial augmentation. The solid form can be customized by intraoperative carving or by custom molding. A dense fibrous capsule forms around the implant that helps maintain the implant in its proper position, but no tissue ingrowth occurs after implantation. Silicone is easily sterilized, easily carved to the necessary shape, and easily removed in the event of revision or complication. Silicone implants are popular because they are not expensive and are comfortable.

PTFE, a material with a spongy consistency, is an ethylene monomer with 4 fluorine moieties attached. It is manufactured as implants under the trade names Teflon, Gore-Tex, and Proplast. Only Proplast and Gore-Tex have been used for facial plastic surgery. Similar to silicone, PTFE has been in use for more than 40 years and is nonimmunologic and noncarcinogenic.

The original Proplast (Proplast I) was black and consisted of Teflon (PTFE) and graphite. It was replaced by Proplast II, which, unlike the black Proplast I, is a white material composed of Teflon and alumina. It is a porous material with pores ranging from 50-400 µm in diameter, which allow fibrovascular ingrowth of the host's tissues. Proplast was discontinued after it was shown to delaminate under shear stress, form particles, and induce an inflammatory reaction. Gore-Tex is an expanded PFTE sheeting composed of fine expanded PFTE fibrils held together by solid expanded PTFE nodes. Proplast II is easily cut and trimmed for contouring and is porous, with pores ranging from 10-30 µm, which allow for limited fibrovascular ingrowth.

High-density polyethylene or Medpor is another alloplastic material, which, unlike PTFE, is not fluorinated or spongy in consistency. It is flexible, contouring with ease when heated in a warm-water bath. It is porous, with pores ranging from 125-250 µm, which allow for soft tissue and some bony ingrowth. It does not induce a foreign body response and is not degraded in the body. As demonstrated in a study contrasting the histologic response of Gore-Tex and Medpor implantation to experimental infection, the faster fibrovascular growth into high-density polyethylene seemed to protect against infection. This may be attributed to the larger pores and increased stability from host tissue ingrowth of the implant. However, fibrous ingrowth into porous materials does not guarantee implant stability.

Hydroxyapatite, another firm porous compound, is available as porous block hydroxyapatite. It is derived from marine coral and has been used in facial augmentation. Its pores allow for fibrous and bony ingrowth. It is applied with rigid fixation to the underlying bone and is associated with minimal bone erosion.

Mersilene mesh is a softer material composed of nonabsorbable polyester fiber sheeting that can be folded and constructed into an appropriate implant. It has been in use for 50 years and was first introduced for hernia repair. Unlike Supramid, the polyamide nylon mesh Mersilene does not degrade. It is cut easily without fraying or shrinking.

Silicone, expanded PTFE, and high-density polyethylene are available as prefabricated implants of variable sizes and shapes. These materials can be cut and carved to individualize them to the patient. Each alloplast has its own benefits and problems, and different surgeons report various experiences with these products.

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Problem

Surgeons have looked to art to establish an ideal model for the perfect chin. Indeed, analysis has proven that the chin has a central role in facial balance and beauty in a global sense. Chin projection is perceived as it relates to the rest of the face. Various methods of analysis and chin classifications have been described.

Chin augmentation is used to correct microgenia (ie, retrusion of the chin). This is distinct from micrognathia, which is a problem of the mandible, not the chin. When considering chin augmentation, the surgeon contemplates the chin not just as a bony protuberance, but rather, as a 3-dimensional prominence consisting of soft tissue components and bone. Additionally, the chin is considered in relation to the nose. This relationship is critical to facial balance. In a patient with a large nose and small chin, rhinoplasty may not be sufficient unless the chin is addressed.

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Epidemiology

Frequency

Chin deformities are the most common abnormality of the facial bones. Microgenia is the most common chin abnormality. However, because of a lack of associated functional deficit, microgenia is often left untreated. Most procedures are performed in combination with rhinoplasty.

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Presentation

A patient may present with microgenia or, more commonly, may become aware of microgenia during the process of rhinoplasty evaluation. Again, microgenia, or small chin, is distinctly different from micrognathia, which is mandibular hypoplasia. Microgenia is often a component of micrognathia, but microgenia takes into account all components of the chin.

The chin is located below the labiomental sulcus and consists of skin and subcutaneous tissues, the mentalis muscle, and the underlying bony structures from the mandible. Microgenia occurs and is addressed when there is acceptable occlusion. In situations of severely altered dentoalveolar relationships, orthognathic surgery is more appropriate than chin augmentation. However, chin augmentation by implant is appropriate for mild forms of microgenia.

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Indications

Gonzalez-Ulloa wrote extensively on profile plasty. [2, 3] With the basic tenet that the skeletal structure and proportions of the skull define beauty, he developed methods of analysis to understand facial relationships. By understanding that the bony skeleton supports the soft tissue and truly is the mechanism for achieving optimal aesthetics, the surgeon can plan the operation in a manner that will most likely result in a successful outcome.

The indication for a chin implant is mild microgenia. Gonzalez-Ulloa based his facial evaluations on the Frankfort plane and the 0° meridian. The Frankfort plane is the horizontal line from the upper external auditory canal to the infraorbital rim. The 0° meridian is drawn in the vertical plane perpendicular to the Frankfort horizontal plane and tangential to the nasion and the pogonion. If the chin is behind the 0° meridian, it is considered retruded. The intervention is determined by the degree of retrusion.

Many methods of analysis have been described and reviewed by several surgeons. Burstone's vertical line is tangential to the subnasale and the pogonion. The upper lip is 3.5 mm anterior, and the lower lip is 2.2 mm anterior.

Gibson uses soft tissue landmarks in the lower facial triangle to determine the ideal chin position with reference to lower facial height. [4] The 3 defining points are the tragion, subnasale, and chin. Goode drops a line perpendicular to the Frankfort horizontal plane at the alar crease to mark the ideal chin position. Legan's angle of facial convexity is 12° between a line to the glabella and subnasale and a second line tangential to the subnasale and pogonion.

Merrifield's Z angle is 80° (± 5°) at the transection of the Frankfort horizontal plane and a vertical tangent from the upper lip to the pogonion. Ricketts drew a vertical line tangential to the nasal tip and pogonion. [5] Posterior to this plane, the upper lip is at 4 mm, and the lower lip is at 2 mm. Steiner's nasal philtral S defines the anterior extent of the lips. A vertical line is dropped tangential to the pogonion, and an inverted S is formed by the lower nasal and upper labial profile.

Just as the Greek Parthenon was based on "the golden rectangle," the chin, in relationship to specific facial features, also follows the golden proportion, or phi. In the golden rectangle, the altitude is the proportion 1 and the base is 1.618 multiplied by the altitude. This relationship is phi.

The golden proportion is found in several of the height relationships of the face. The entire face fits the proportion, with the measurement from forehead to eye as 1 and from eye to menton as phi. The golden proportion is in reverse, with the measurement from menton to nasal ala as 1 and from nasal ala to forehead as phi. Again, the proportion is maintained from the eye to the chin, with the measurement from eye to nasal ala as 1 and from nasal ala to menton as phi. The proportion in reverse is measured from menton to stomion, with the measurement from stomion to eye as phi. Finally, the nose and chin are related in the golden proportion. With the measurement from nasal ala to stomion as 1, phi is measured from stomion to menton and from nasal ala to eye.

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Relevant Anatomy

The chin derives from the fusion of the 2 mandibular processes at the midline. The muscular portion of the soft tissues stems from the accompanying overlying mesoderm. Muscular fusion occurs at the midline but is often incomplete and separated into 2 halves by a fibrous median raphe. Soft tissue and bony skeleton make up the chin.

Located below the labiomental angle, the soft tissues include the skin and subcutaneous tissues and the muscle below. This chin pad is of variable thickness, especially when the mentalis muscle is hypertrophied. Typically, the mentalis muscle is small, has its origin on the mandible below the incisors, and inserts into the skin of the chin. Its innervation is via the seventh cranial nerve.

The mandible contributes the skeletal portion of the chin, specifically, the symphysis menti, which joins the 2 halves; the mental protuberance, which is an elevation above this junction; and the mental tubercles, which lie at the inferior margin. Projection of the chin is dependent on the mental protuberance and tubercles.

The chin is separate from the lip at the labiomental angle. However, the chin and lip have an intimate relationship. The depressor muscles insert on the mental tubercles and interdigitate with the orbicularis oris muscle. Any intervention on the chin can affect the lips.

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Contraindications

Severe microgenia is a contraindication to augmentation mentoplasty. Other contraindications include labial incompetence, lip protrusion, shortened mandibular height, severe malocclusion, and periodontal disease.

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