Facial Alloplastic Implants, Mandibular Angle
Introduction
Over the years, techniques used in craniomaxillofacial reconstruction have become safe and standardized. These techniques are used to achieve an aesthetically pleasing correction of abnormal facial proportions or facial asymmetry. The treatment of these abnormalities requires the use of all applicable diagnostic aids. It also requires extensive presurgical planning to fully understand the 3-dimensional extent of the patient's defect and potential for correction. This article discusses the surgical correction of mandibular angle defects, specifically deficiencies in the posterior aspects of the mandible. The focus is implantation of alloplastic materials into the mandibular angle (see below).
History of the Procedure
The exact origin of maxillofacial prosthetics is difficult to identify. Popp (1939) claimed that artificial eyes, noses, and ears were discovered on Egyptian mummies. The Chinese also made artificial facial parts from wax and resins. Pare (1575) was probably the first surgeon to use an obturator to close cleft palates. He illustrated a prosthetic ear made of paper or leather and a prosthetic nose made of silver attached to the face with a string. In 1880, Kingsley described the use of artificial parts to repair defects of the orbit, nose, and palate. A few years later, Martin (1889) described prosthetic devices for the replacement of missing parts of the maxilla and mandible. These two latter figures were the pioneers of maxillofacial prosthetics.
In 1894, Tetamore described patients with loss of parts of the face, including the nose, which he had reconstructed with the use of prosthetics. The prosthetic material was made of a light plastic that was nonirritating and colored similarly to the skin. The prosthetics were secured to the face by spectacles. Tetamore is believed to have used cellulose nitrate to make these. In 1901, Upham used vulcanite to make prosthetic ears and noses. Kazanjian's contributions, in 1932, provided the initiative for maxillofacial, dental, and plastic surgeons to work together for the betterment of facially deformed or injured patients. Today, the techniques that have evolved in maxillofacial surgery have become safe. Further, these techniques have applications in situations where even minimal deformity is present.
Alloplastic materials
Dimethylsiloxane
Dimethylsiloxane (silicone) rubber implants with or without polymer fabric have been used in the augmentation of frontal, zygomatic, nasal, chin, parasymphyseal, paranasal, orbital, maxillary, malar, nasal dorsum, ear, and mandibular deficiencies.
Silicone rubber implants have been used for surgical applications since the 1950s. Silicone can be obtained preformed commercially or for custom shapes; room temperature vulcanizing silicone can be used. Silicone easily can be modified intraoperatively with a scalpel or scissors. It also can be fixated easily with a screw or suture to underlying tissues. This material has "memory," which demands adaptation to bone contour in the "relaxed" state, since bending may lead to extrusion or bone resorption.
These implants easily are sterilized using steam autoclave or irradiation without damaging the material. Surrounding tissues do not react adversely to silicone, and only a thin fibrous capsule forms without ingrowth of tissue. Porous silicone implants and silicone bonded to Dacron have been used to enhance stability. Use porous silicone implants in the presence of minimal or no tissue stress so that they do not tear or fracture.
Polytetrafluoroethylene
Polytetrafluoroethylene (PTFE) was marketed originally as Proplast during the 1980s. The Food and Drug Administration (FDA) withdrew the material because of fragmentation and foreign body reactions in a temporomandibular joint reconstruction as a glenoid fossa replacement. The material was brought back by Gore and Associates as Gore-Tex, an expanded microporous polymer of PTFE that has proven very useful in the augmentation of many soft tissue and bone defects.1,2 It is available in 1-, 2-, and 4-mm sheets that can be cut or shaped easily and placed under the soft tissues.
Gore-Tex causes a minimal foreign body reaction, which forms a fibrous capsule. Since this material has micropores, some but not much soft tissue ingrowth occurs. This allows for some stability, but the implant also can be removed easily if necessary. Due to its fibrillar composition, Gore-Tex has noninterconnected surface openings with pore sizes of 10-30 µm. This is conducive to microvascular ingrowth with minimal fibrous tissue encapsulation. This material also has become popular for subdermal implantation in various areas of the face, such as the lips and nasolabial folds.
Polyethylene
Polyethylene (Medpor) is a porous material that possesses high tensile strength. Its pore sizes are 125-250 µm. This material can be carved or contoured to fit a particular 3-D space. It produces a minimal foreign-body response and a resulting thin fibrous capsule. This encapsulation does not produce significant contraction. The material is difficult to sculpt. It is not really osteoconductive, although its porosity allows for some soft tissue and vascular ingrowth. Polyethylene has many applications and is being used in midfacial, chin, and mandibular reconstruction.
Polyester
This is one of the most widely used classes of compounds today. Nylamid, a polyamide mesh, has been used in facial augmentation with good short-term results but some hydrolytic decomposition was observed with resultant volumetric loss. The aliphatic polyesters currently are among the most commonly used polymers in surgery. Although aliphatic polyesters are not used for alloplastic implantation, they may be used in the future given their nonresorbable nature.
Acrylics
These materials are derived from polymerized esters of either acrylic or methyl acrylic acids. Polymethyl methacrylate (PMMA) resin has long been used in orthopedic surgery as bone cement for joint prostheses. It is made intraoperatively by mixing a liquid monomer and a powdered polymer. The polymers then cure in a few minutes through an exothermic reaction. The result is a rigid, almost translucent plastic. Models also can be made preoperatively, and from this model an implant can be fabricated, sterilized, and used in the operating room. This avoids the damage caused to the surrounding tissues by the exothermic reaction from mixing the components.
The material has been used frequently in cranioplasty procedures for full-thickness skull defects. Antibiotics also can be impregnated into the mixture to avoid bacterial inoculation and related complications. Metal mesh also has been incorporated to add strength and decrease the risk of fracture upon impact.
The disadvantages of PMMA are that its odor is difficult to tolerate and its fumes are teratogenic. Therefore, take precautions if women who are pregnant (including personnel) are present. Also, cool irrigation must be applied after placement of the material due to the potential for damage to the surrounding tissues from the material's high temperatures. Another disadvantage is that bacteria have a high affinity for this surface, thus it should not be placed in areas where opportunistic microbes reside (eg, paranasal sinuses, oral cavity).
A similar material, hard tissue replacement (HTR), is a composite of PMMA and polyhydroxyethylmethacrylate. HTR has interconnected pores, hydrophilicity, and a calcium hydroxide coat that gives it a negative charge. It is also quite strong. HTR has been used in the past for dental work and is now available as a preformed implant that is custom made from a CT scan of the patient's defect. It is useful in the replacement of large full-thickness defects in the cranium and other regions. Scant data exist documenting its use in mandibular augmentation.
Calcium phosphate ceramics
Hydroxyapatite and other calcium materials are known to interact with and can even incorporate into living bone tissue. Both porous and dense ceramic forms can be used for implantation. However, these materials are brittle and lack much strength, although they do not resorb. Their biocompatibility is excellent, and they appear to bond to bone by natural cementing mechanisms. This material is osteoconductive and allows for tissue ingrowth without the formation of a fibrous capsule. However, it is not osteoinductive. Block forms have been used as interpositional grafts in facial osteotomies. Nonceramic forms also exist and come as a powder that is mixed in the operating room to fill bony defects. The disadvantage is that due to their lack of strength and potential for fracture, they should not be used in load-bearing areas. This may limit their use in mandibular augmentation.
Other alloplastic materials exist, but those previously described are the most commonly used.
Etiology
Blunted mandibular angles may create a softer oval appearance of the lower face, which may be undesirable, especially in persons whose goal is a "strong, chiseled jaw" or a more masculine appearance. Reasons for seeking augmentation of the mandibular angle include congenitally small mandible or micrognathia reconstruction, reconstruction secondary to trauma or resection, and, more commonly, cosmetic augmentation of a normal anatomic variant.
Pathophysiology
The etiology of mandibular angle aesthetic deficiency can be attributed to hemifacial microsomia, micrognathia, normal anatomic variant, or a number of congenital abnormalities.
Presentation
Preoperative planning and workup
Presentation is variable and patient dependent, although cephalometric and photographic measurements may assist with localizing and quantifying the deficiency, including the width between the 2 gonion points.
As with all plastic and reconstructive surgeries, careful planning before the operation increases the likelihood of a good outcome. Communication with the patient is critical, particularly with respects to the patient's facial contour and the result he or she can expect. Ascertaining patients' expectations by any means possible, including photographs of famous people, models, or themselves (with drawn modifications), is important. In addition, identify and amend unrealistic expectations. The surgeon should never set out to achieve something he or she is not completely capable of accomplishing.
Preoperative planning should include a thorough evaluation of the patient's anatomy by zones to determine the degree of augmentation that is adequate for each region. Set aside time to measure and mark the patient prior to surgery to provide an excellent guideline for technical precision.
To analyze the face for aesthetic purposes, describe the typical face to provide a standard for comparison. This is a difficult task because normality depends on setting and culture. Farkas is credited with much of the research on facial topography and cephalometrics. According to Bartlett et al, the key to analyzing the facial form lies in the ability to analyze each region of the face separately for form and symmetry and to bring these parts together to form a whole. In front and profile views, the upper face height, midface height, and lower face height are approximately equal, as shown in the image below.
The most frequent variation is for the height of the lower face to be slightly greater than the height of the upper face. In the lower face, the distance from the subnasal point to the mouth comprises approximately one third of the distance. The remainder comprises two thirds. The widest part of the face transversely is through the malar midface complex. The bigonial and bitemporal distances, which are approximately equal, are approximately 10% less than the bizygomatic distance, as shown below.
In profile views, the relationship between the nose, lip, and chin must be assessed. A line that is dropped perpendicularly from the glabella should pass through the subnasal point and intersect the most anterior part of the chin. Also, the pogonion should lie approximately 2 mm from this line. The aesthetic plane of Rickett describes the relationship as follows: the lips should be on a line that joins the chin to the tip of the nose, as shown below.
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After assessing the entire face, the lower face region can be analyzed. The lower face is composed of the upper and lower jaw and the surrounding soft tissues. The vertical height of the lower face is approximately one third the height of the entire face. In patients with vertical mandibular excess, the lower lip is incompetent since the lower incisors are exposed. If the lips are forced shut, the labiomental fold becomes flat as the mentalis muscle contracts. The chin point is displaced cephalad. With vertical mandibular deficiency, the distance from the lip to the menton is shortened. The lower lip protrudes outward and closes over the incisors.
Obviously, the process of planning for surgery is highlighted by the physical examination of the face and its contours. Facial and dental measurements are made (cephalometrics and anthropometrics), and midline markings are photographed. Imaging studies can also be used in the assessment (see Workup).
Indications
Ramirez, a pioneer in the mandibular matrix implant system, has written that, as a result of aging, a mandibular skeletal soft tissue disproportion is created.3 He believes this disproportion is caused by the reduction of volume of the skeletal support and laxity with loss of soft tissue support of the overlying tissues. He describes the indications for mandible angle augmentation to include people with congenitally small mandible, edentulous individuals, and persons requesting facial enhancement.
As a person loses his or her teeth, the process is accelerated because gingival tissue is lost and the volume of bone below the mental foramen decreases. Patients may have congenitally small chins or mandibles, and these abnormalities can be made more obvious with aging. Also, trauma can result in deformities or asymmetries. The goal of surgery, according to Ramirez, is to achieve a smooth transition between the chin and the jowl area and between the latter and the mandible. Ramirez also stresses the importance of developing a "sharp step" transition between the lateral facial planes by developing a well-formed submandibular groove.3
Contraindications
Contraindications include present infection, teeth problems, thinning mandible bone stock, bleeding disorders, unrealistic expectations, a history of radiation, or a number of other health problems that contraindicate an elective surgical procedure.
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