49. Genioplasty

Ashkan Ghavami, Bahman Guyuron

RELEVANT ANATOMY

MUSCLES (Fig. 49-1)

Fig. 49-1 Relevant muscular anatomy for genioplasty.

■ Mentalis

• Conelike, vertical fibers from incisive fossa to overlying skin

• Can cause wrinkling, and if hyperdynamic, may be visible under lower lip

• Midline void between fibers seen when chin dimple present

■ Orbicularis oris (lower fibers)

■ Depressor anguli oris

■ Quadratus (depressor) labii inferioris

■ Geniohyoid, genioglossus, mylohyoid, and anterior belly of digastric

• Attach to lingual (posterior) aspect of chin

BONY LANDMARKS

■ Mental foramen

■ Digastric fossa

■ Mental protuberance

■ Mental spines

■ Submandibular fossa

NERVE SUPPLY

■ Inferior alveolar nerve and mental nerve (terminating branch exiting mental foramen)

• Mental nerve: Located at base of first or second bicuspid

• Inferior alveolar nerve

► Risk of injury during genioplasty procedures

► Osteotomies should be 5-6 mm below mental foramen to prevent injury to nerve branches or tooth apices.

SIGNIFICANT CEPHALOMETRIC POINTS (Fig. 49-2)

Fig. 49-2 Significant cephalometric points. (A, A-point; ANS, anterior nasal spine; Ar, articulaire; B, B-point; Ba, basion; FH, Frankfort horizontal plane; GN, gnathion; Go, gonion; Me, menton; MP, mandibular plane; N, nasion; OP, occlusal plane; Or, orbitale; PNS, posterior nasal spine; Po, porion; Pog, pogonion; PNS, posterior nasal plane; S, sella.)

■ Pogonion (Pog): Most projecting portion of mandible. Denotes chin excess or deficiency in relation to other structures (i.e., nasion and lip position)

■ Menton (Me): Lowest (most caudal) portion of chin

■ Subspinale (A): Columellar-labial junction

■ Supramentale (B): Deepest point between pogonion and incisor

■ Nasion (N): Nasofrontal junction

INDICATIONS AND CONTRAINDICATIONS

OSSEOUS GENIOPLASTY

■ Indications

• Horizontal asymmetries of any magnitude

• Excess deficiency or excess in both vertical and sagittal planes

► Moderate to severe microgenia

• Secondary cases after osseous or alloplastic genioplasty

• Adjunct to formal orthognathic surgery

► Alloplastic genioplasty is rarely, if ever, combined with formal orthognathic lower or upper jaw surgery.

■ Contraindications

• Inadequate bone stock (i.e., elderly patients, bone pathology)

• Abnormal dentition or significant dental pathology

• Patient preference to not have osteotomy

■ Contrary to common belief, can be a relatively simple and efficient procedure

■ More versatile procedure versus alloplastic augmentation

• Allows multidimensional chin correction, including reduction

■ Caveat: Aesthetic surgery patients seem to prefer alloplastic augmentation, and tend to shy away from osteotomies.

• Facelift/necklift procedures often include a submental incision that can easily be used for placing a chin implant.

• Popular media has shown a bias toward alloplastic augmentation and present any “cuts in the bone” as very “invasive.”

■ Malocclusion requires consideration of orthognathic surgery and a more extensive workup (cephalometric analysis, occlusion models) and possible collaboration with an oromaxillofacial surgeon.

■ Significant microgenia usually requires an osseous genioplasty, because a very large implant can appear awkward.

PREOPERATIVE EVALUATION

■ Medical comorbidities

• Diabetic and immunosuppressed patients: Not good candidates for alloplastic chin implantation

► Osteotomy site(s) may heal poorly.

• Age: Higher-age patients may have osteopenic bone—not good candidates for osseous genioplasty

■ Occlusion type (Fig. 49-3)

Fig. 49-3 Occlusion types. A, Angle class I. B, Angle class II. C, Angle class III.

• Normal occlusion (Angle class I)

► Mesiobuccal cusp of maxillary first molar occludes into the buccal groove of the mandibular first molar (Fig. 49-3, A).

• Angle class II malocclusion

► Mesiobuccal cusp of maxillary first molar occludes medial to the buccal groove (Fig. 49-3, B).

► Mesiobuccal cusp of the maxillary first molar occludes distal to the buccal groove of the mandibular first molar (Fig. 49-3, C).

■ Dentition

• Before 15 years of age, permanent dentition may not be fully erupted.

► Greater risk of injury during osteotomies

• Elderly patients may have retruded alveolar ridge (if edentulous), which contributes to chin pad ptosis.

► Presence of little bone stock

► May be better candidates for alloplastic augmentation

• Patients with poor dentition or infected dentition are very poor candidates for any form of genioplasty until fully treated.

LIFE-SIZE PHOTOGRAPHS²

■ Bilateral sagittal view, frontal views, and bilateral oblique (three-quarter) views

MIDFACE HEIGHT

■ Vertical maxillary excess: Especially important when accompanied by a deep labiomental fold

• Patient better served by formal orthognathic correction, with or without a genioplasty

NOSE-CHIN-LIP EVALUATION (Fig. 49-4)

Fig. 49-4 Nose-chin-lip evaluation.

■ Nasofacial harmony is linked with chin dimensions and vice versa.

■ Chin projection should be 3 mm posterior to nose-lip-chin plane (NLCP).⁵

■ Nasal length: Two thirds of midfacial height and exactly equal to chin vertical length⁵

Symmetry of Lower Third of Face

■ Right-to-left asymmetries of the mandible and chin may require multiple osteotomy configurations to centralize chin or canting of the osteotomy line and differential plate bending.

■ Difficult to correct with alloplastic augmentation alone

SOFT TISSUE ANALYSIS

■ Soft tissue pad: Normally 9-11 mm thick

• Palpated at pogonion and off midline with patient in repose and then when smiling

• Soft tissue contribution can predict effects of augmentation.

■ Stomion: Junction between upper and lower lip in repose

■ Upper/lower lips: Lower lip eversion from deep bite, excess lip bulk, or excess overjet may deepen labiomental fold.⁴

■ Labiomental fold

• Indentation or crease between lower lip and lowest point of mandible (menton) best seen on sagittal view

• Fold aesthetics dependent on vertical proportion of mandible and facial length³

► Example: Deep fold may look good on longer faces.³

• Evaluate for height (when stomion-to-menton is divided into thirds, fold often falls at junction of upper and middle third).

► If fold is too low, augmentation may only address chin pad.⁴

• Depth

► Fold depth approximately 6 mm in men and 4 mm in women⁶

► If deep, horizontal vector, chin augmentation may result in an awkward exaggerated deep fold and an overprojected chin

► If shallow, may be further effaced by vertical augmentation

■ Riedel line: A line drawn vertically down facial plane on sagittal view, tangential to anterior upper and lower lip (Fig. 49-5)

Fig. 49-5 Riedel plane is a simple line that connects the most prominent portion of the upper and lower lip, which on a balanced face should touch the pogonion.

• Lower lip should be 2-3 mm posterior to upper lip projection.

• Pogonion should never project beyond this line and should be slightly posterior to it (or just touching it).

DYNAMIC AND STATIC CHIN PAD ANALYSIS4

■ A thin chin pad on smiling: Potential for increased pad effacement with increased bony prominence (i.e., native or from augmentation)

• Burr reduction or osteotomy setback may be required.

■ A thick pad may increase submental soft tissue fullness and worsen the cervicomental angle if bony setback performed

WITCH’S-CHIN DEFORMITY

■ Definition: Ptosis of soft tissue caudal to menton and an exaggerated submental crease

■ Correction requires soft tissue/muscle resection and/or repositioning.

■ Augmentation can exaggerate deformity.

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