Mandible

Indications

Three groups of patients benefit from implant augmentation of the mandibular body, angle, and ramus. These include patients with normal, deficient, or surgically altered anatomy.

Normal dimensions

Most patients who desire mandible augmentation have lower face horizontal dimensions that relate to the upper and middle thirds of the face within a normal range ( Figs. 12.1 and 12.2 ). These patients, usually males, perceive a wider lower face as desirable. This group benefits from implants designed to augment the ramus and posterior body of the mandible and, in so doing, increase the bigonial distance. Other patients desire more definition and angularly to the mandibular border. Implants that augment the anterior mandible back to the ramus can achieve this goal.

Fig. 12.1
Normal values in millimeters for upper, middle, and lower facial width in white North American adult men (ages 19 to 25) ( n = 109) and young white adult women ( n = 200). The bigonial distance (go–go) is measured from gonion (go) to gonion, which is the most lateral point of the mandibular angle close to the bony gonion. Note that the male lower face is both relatively and absolutely wider than that of the female.

Fig. 12.2
Normal values for mentocervical angle (in degrees), height of the mandible ramus (in millimeters; go–cdl), and depth of the lower face (in millimeters; go–gn) in white North Americans: ( A ) young adult men and ( B ) young adult women. The menton (or gnathion; gn) is the lowest median landmark on the lower border of the mandible. It is identified by palpation and is identical to the bony gnathion. The gonion (go) is the most lateral point of the mandibular angle close to the bony gonion. The mentocervical angle is formed by the upper contour of the chin and the surface beneath the mandible.

Skeletal deficiency

Another group of patients who benefit from mandible augmentation are those with skeletal mandibular deficiency ( Fig. 12.3 ). It has been estimated that approximately 5% of the total US population has skeletal mandibular deficiency resulting in a class II occlusal problem. The majority of these patients (approximately 80%) can have their dental relationships normalized through orthodontic tooth movement. The remaining 20%, or 1% of the total population, have mandibular deficiency that is so severe that surgical mandibular advancement would be needed to correct it.

Fig. 12.3
Soft tissue contour and skeletal configuration of a patient with mandibular deficiency and corrected occlusion. Note the obtuse mandibular and mentocervical angles with steep mandibular plane.

The classic method of correcting class II dental malocclusion in patients with significant mandibular deficiency, in addition to preoperative and postoperative orthodontic treatment, includes sagittal split ramus osteotomy and sliding advancement genioplasty, with possible Le Fort I maxillary impaction. This combination of procedures can provide a class I dental relationship while normalizing the skeletal contour. In patients with mandibular deficiency who have had their malocclusion corrected through orthodontics alone, mandibular osteotomy would disturb these dental relationships, requiring extensive perioperative orthodontic treatment. This type of treatment is both costly and time consuming. Alloplastic augmentation of the mandible can provide a visual effect similar to and, in my opinion, superior to that of sagittal osteotomy with advancement. Implant augmentation involves a less surgical procedure that avoids any further dental manipulation. Mandible implants can address the obtuse mandible angle with steep mentocervical angle, as well as the decreased vertical (go–cdl) and transverse ramus (go –gn) dimensions. An extended chin implant will camouflage the poorly projecting chin.

Surgically altered anatomy

A third group of patients who may benefit from alloplastic augmentation of the mandible are patients who have had their class II dental malocclusion due to mandibular deficiency corrected by sagittal split osteotomy with advancement of the distal segment. This procedure splits or separates the tooth-bearing symphysis and adjacent bodies from the non-tooth-bearing but articulating rami. Requisites for positioning the resultant anterior and posterior segments to improve occlusion, allow bone healing, and continue joint function may result in a displeasing postoperative contour. The advancement of the distal, or tooth-bearing, segment is dictated by occlusal relations. It inevitably creates a contour irregularity at the site of the body osteotomy and distal segment advancement. This area of narrowing may be visible, and even disfiguring, in certain individuals. Positioning of the posterior segment requires that the condyle be seated in the glenoid fossa and that there be sufficient segment contact to allow bone healing. Fulfillment of these requisites dictates the position of the ramus and its angle. This may result in an aesthetically displeasing position of the ramus angle with insufficient height, insufficient width, or asymmetry ( Fig. 12.4 ). Mandible implants can be used to improve contour in these patients. Computer-aided design and manufacture (CAD/CAM) implant augmentation has proven to be a more precise method to treat the contour irregularities and asymmetries in this group of patients. The use of CAD/CAM implants is presented in Chapter14, Chapter15 .

Fig. 12.4
Illustration (A) showing skeletal contour after sagittal split osteotomy and advancement of the distal segment, as well as sliding advancement genioplasty of the deficient mandible. Note that there is an indentation along the mandibular border at the sites of osteotomy and advancement. Acrylic models ( B , C ) obtained from CT scan data of a patient who had undergone sagittal split and horizontal maxillary osteotomies. Note the contour irregularities along the mandibular border at the sites of mandibular osteotomies. Note also the asymmetries in the posterior segment position. These will lead to discrepancies in lower face width and height.

Mandibular body and ramus augmentation in women with normal-dimensioned mandibles should be conservative with the intention of providing definition. The anthropometric measurements provided by Farkas ( Fig. 12.1 ) help in understanding our visual interpretation of male and female patients with normal mandibular morphology and dimensions who undergo mandibular augmentation. These data show that all transverse facial dimensions are greater in men than in women and that the bigonial distance is the transverse facial dimension that has the greatest difference between sexes. In other words, the lower third of women’s faces tends to be absolutely and relatively narrower than that of men. Hence, when normal male mandibles undergo augmentation, they may be perceived as stronger. However, when normal female mandibles undergo aggressive augmentation, they may be perceived as less feminine.

Pearl

Aggressive augmentation of the female mandible risks its masculinization.

Evaluation and Planning

Physical examination

Physical examination is the most important element in preoperative assessment and planning. Reviewing life-size posteroanterior and lateral photographs with the patient can be helpful when discussing aesthetic concerns and goals. To allow the patient to understand the scale and scope of augmentation, it is useful to have sample implants available to demonstrate on a model skull and to apply these implants to the relevant area of the patient’s face.

X-rays

Posterior–anterior and lateral cephalograms provide data that help the surgeon determine how the dimension of the implant might be altered to best suit the patient. Three-dimensional computed tomographic (CT) scans and the models obtained from their data can be invaluable when attempting to correct asymmetries associated with congenital, posttraumatic, or postsurgical deformities.

In general, however, the size and position of the implant are largely aesthetic judgments. An approximately 1:1 ratio of augmentation to resultant projection is anticipated where soft tissue coverage is relatively thin, such as the inferior border of the mandible. Masseter muscle coverage over the ramus decreases the amount of projection provided by the implant.

Surgical Anatomy ( Fig. 12.5 )

Skeleton

The mandible consists of the tooth-bearing body and the ramus that extends upward from the angle. The ramus, including the angle, is covered on its external surface by the masseter muscle. The gonion is the tip of the outer surface of the angle. The outer surface of the angle may be embellished by a masseteric tuberosity denoting the insertion of the masseter muscle. (Its inner surface may be raised by a pterygoid tuberosity where the medial pterygoid muscle inserts.) The upper aspect of the ramus ends posteriorly in a condylar process and anteriorly in a coronoid process separated by the mandibular notch. The oblique line runs forward and downward from the anterior border of the ramus. It affords attachment for the depressor labii inferioris and depressor anguli oris. The platysma is attached near the inferior border of the mandible. The buccinator muscle is attached to the outer lip of the superior border of the mandible as far forward as the first molar tooth. The mental foramen lies approximately at the interspace between the two premolars and about midway in the height of the dentulous adult mandible. As the body proceeds anteriorly toward the midline, it swells out to form the mental protuberance.

Fig. 12.5
Mandible anatomy. ( A ) Surface topography. ( B ) Muscle origins and insertions.

Musculature

There are four muscles predominantly responsible for the movement of the mandible: the masseter, the temporal, the medial pterygoid, and the lateral pterygoid. Only the masseter and, indirectly, the medial pterygoid, as part of the mandibular sling, are encountered during mandibular augmentation. The masseter muscle consists of superficial and deep portions. The larger superficial portion arises from the zygomatic process of the maxilla and from the anterior two-thirds of the inferior border of the zygomatic arch. Its fibers pass inferiorly and posteriorly to insert into the angle and inferior half of the lateral surface of the ramus of the mandible. The masseter and the medial pterygoid muscle are so positioned that they form a sling around the inferior border of the mandible and therefore suspend the angle of the mandible. When placing a mandibular angle implant it is necessary to separate the superficial portion of the masseter and its sling component from the mandible in a subperiosteal plane. Tearing the periosteum at the inferior border may disrupt the sling mechanism and allow the freed masseter to ride up, causing a depression in the soft tissues overlying the angle. This depression is exaggerated when the muscle is contracted. The smaller deep portion of the masseter arises from the posterior third of the inferior border and from the whole of the medial surface of the zygomatic arch. This portion of the muscle passes anteriorly and inferiorly to insert into the superior half of the ramus and the lateral surface of the coronoid process.

The buccinator is another muscle that is encountered during exposure of the mandible during mandibular augmentation. The buccinator is the principal muscle of the cheek. It forms the lateral wall of the oral cavity. It occupies the area between the mandible and the maxilla and originates in part from both structures. It originates from the outer surface of the alveolar processes of the maxilla above and the mandible below, adjacent to their three molar teeth. The other origin of this quadrilateral muscle is the pterygomandibular raphe, which stretches from the medial pterygoid process to the inner surface of the mandible. Its fibers pass forward to become continuous with the orbicularis oris.

Inferior alveolar nerve

The inferior alveolar nerve is a branch of the mandibular nerve (V3). It enters the mandibular canal with the inferior alveolar vessels through the mandibular foramen, which is located in the inner aspect of the ramus of the mandible approximately halfway between its anterior and posterior borders. It is important to visualize the path of the inferior alveolar nerve when placing screws to immobilize mandibular implants.

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Feb 9, 2020 | Posted by in Oral and Maxillofacial Surgery | Comments Off on Mandible
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