Horizontal Chin Deficiency with or without Vertical Discrepancy

Contemporary maxillofacial surgeons now recognize that individuals who are seeking a “stronger chin” (i.e., horizontal projection) frequently also have disproportions in the vertical dimension of their face.23,33,45,59,61,77,88,89,92,116,120,123,127,131 Studies suggest that approximately 40% of individuals with horizontal (sagittal) chin deficiency have a decreased lower anterior facial height (i.e., short face growth pattern) (Figs. 37-4, 37-6, and 37-7). It is also known that another 25% of these will have the other extreme of an increased lower anterior facial height (i.e., long face growth pattern) (Figs. 37-8 through 37-11). In either case, the bony chin dysmorphology will also result in soft-tissue abnormalities (distortions) of the labiomental fold. This often includes a deep labiomental angle in those with a short face growth pattern and a flat labiomental fold in those with a long face growth pattern. Fifty percent of individuals with a horizontally deficient chin will have an Angle class II malocclusion, which is indicative of mandibular retrusion. Forty percent will have previously undergone orthodontic camouflage treatment in an attempt to mask their micrognathia (see Fig. 37-1). Incisor flaring will have resulted from the orthodontic camouflage approach that attempted to neutralize a Class II excess overjet malocclusion as an alternative to a more biologic correction of the mandibular deficiency through orthognathic surgery. Some of these individuals will have also undergone chin implant augmentation, often with a suboptimal result (see Fig. 37-1, 37-7, and 37-9).

A pleasing labiomental fold is an essential component of optimal lower face aesthetics. A strong correlation exists between the lower anterior facial height and the labiomental fold morphology. Individuals with decreased lower anterior facial height in combination with a retrusive-appearing chin tend to have an exaggerated and excessively deep labiomental fold with acute angulation between the lower lip and the soft-tissue chin pad (i.e. primary mandibular deficiency or maxillomandibular deficiency; see Figs. 37-4, 37-6, and 37-7). Individuals with increased lower anterior facial height in combination with a horizontally deficient chin are likely to have a labiomental fold that is flattened and effaced, with poor fold definition, excess mentalis strain, and difficult lip closure (i.e., long face growth pattern). The long face growth pattern classically consists of vertical excess of the maxillary and mandibular alveolar regions. This is combined with mandibular retrognathia and an anterior open-bite Class II malocclusion (see Figs. 37-8 and 37-9).

Rosen and others have pointed out that many individuals with mandibular deficiency and decreased lower anterior facial height who have a functional occlusion are unwilling to undergo an orthognathic correction and may opt for a compromise approach to at least achieve some aesthetic advantage (Figs. 37-12 through 37-17).^88–93An osseous genioplasty to simultaneously vertically lengthen and horizontally advance the chin will generally achieve a degree of improved lower facial height and chin projection while reducing the deep labiomental fold. When this compromise is undertaken, no improvement in any baseline malocclusion or any significant opening of the airway should be expected. From an aesthetic perspective, it is essential to prevent an operated look when choosing a chin-only approach. This aesthetic error can be limited by avoiding the temptation to fully correct a moderate to severe mandibular deficiency through an isolated chin procedure (see Figs. 37-13 and 37-15).

In the individual with a functional occlusion, a retrusive chin and an increased lower anterior facial height (i.e., long face growth pattern), the surgical camouflage approach will be different than what is used for the patient with a short face growth pattern, as described previously. As an aesthetic compromise, it will be an advantage to vertically shorten and horizontally advance the chin (see Fig. 40-9). This will at least marginally decrease the mentalis strain to improve the ease of lip closure and also to enhance the appearance of the lower face (see Fig. 37-11).

To review, when evaluating a “weak” profile, it is important to consider not only the horizontal (sagittal) position of the chin but also the vertical dimension (excess or deficiency). In addition, it is essential to analyze the maxillo-mandibular proportions of the face (i.e., any existing jaw deformity) and the alignment of the teeth in the alveolar bone (e.g., dental compensation such as incisor flaring). Only after all facial and dental aspects are evaluated will the ideal treatment be understood and any reasonable compromise options be rationally considered in the light of their shortcomings.

Horizontal Chin Prominence and Associated Vertical Discrepancy

Individuals who present with a prominent pogonion are likely to have disproportions of the lower anterior facial height (i.e., the vertical dimension), which in turn affect labiomental fold morphology.6,64,99,135 When an individual presents with a prominent pogonion and asks for a chin reduction only, he or she will likely not be pleased with the aesthetic result if this limited request is honored.

A commonly seen pattern of jaw deformity is maxillary deficiency in combination with relative mandibular excess (i.e., skeletal Class III growth pattern; see Chapter 20). This often presents in association with a vertically long (i.e., as measured in millimeters from the mandibular incisor edge to the menton height) and protrusive (i.e. forwardly projected) chin (Fig. 37-18). These individuals generally benefit from Le Fort I advancement with or without simultaneous vertical lengthening and clockwise rotation. Mandibular ramus osteotomies are also carried out with limited or no set-back at the incisors but with clockwise rotation to diminish projection at the pogonion. In this case, lower-face aesthetics are often further enhanced via an osseous genioplasty with vertical reduction and minimal sagittal change.

A second frequent pattern of presentation is an individual with a chin that appears prominent in profile because the mandible is overly closed (i.e., short face growth pattern). In these patients, the vertical height (in millimeters) from the mandibular incisal edges to the menton is deficient. With this dentofacial deformity, both horizontal and vertical deficiency of the maxilla and the mandible (i.e., decreased lower anterior facial height and limited facial projection) are present (see Chapter 23). A “pseudoprotrusion” of the chin will result. A patient who asks for a chin set-back only to manage his or her short face (i.e., maxillomandibular deficiency) is unlikely to be pleased with the aesthetic result if this request is honored. In this case, reestablishing the preferred vertical and horizontal dimensions of the face requires a Le Fort I osteotomy with horizontal advancement, vertical lengthening, and, often, clockwise rotation. The mandible—which is also horizontally and vertically deficient—requires sagittal split ramus osteotomies and reorientation to match the new maxillary position. The chin region also benefits from a change in shape. This is accomplished through an oblique osteotomy of the chin with clockwise rotation (i.e., downward and backward positioning of the pogonion) while maintaining contact and pivoting at the posterior osteotomy line. The interpositional gap is grafted with allograft, autograft, or bone substitute, and the chin segment is fixed in place with plates and screws (see Figs. 37-6 and 37-7).

All of these facial aspects must be considered before embarking on any treatment of the chin region if a favorable result and a satisfied patient are to be expected.

Chin Asymmetries

Asymmetries of the chin relative to the face are most frequently encountered in association with specific syndromes and other conditions such as hemifacial microsomia, hemimandibular elongation, hemimandibular hyperplasia, or a condyle fracture sustained during childhood with resultant ipsilateral mandibular undergrowth (Fig. 37-19). For individuals who present with the chin shifted to one side, aesthetic assessment of the entire face, including the soft tissues and the skeletal structures, should be undertaken. Consideration is given to both the lower facial skeleton (i.e., the maxilla, the mandible, and the chin region) and the upper facial skeleton (i.e., the cranial vault, the zygomas, the orbits, and the nasal bones). The chin region may be asymmetric or disproportionate, or it may simply appear to be so as a result of maxillary or mandibular malposition. In several of the conditions mentioned, the extent of actual chin asymmetry is much less than that of the mandible and maxilla. When indicated, a Le Fort I osteotomy and sagittal split ramus osteotomies of the mandible with three-dimensional repositioning will greatly improve the chin symmetry. A separate osteotomy to reposition the chin region is also generally required to simultaneously correct disproportion of the lower third of the face.

• Attention is turned to the facial vestibule of the chin. The lower lip is stretched outward to allow for visualization of the mental nerve through the mucosa on each side. In the depth of the vestibule, an incision is made with a Bovie electrocautery device or a knife (no. 15 blade) just through the mucosa from cuspid region to cuspid region, stopping just short of the visualized mental nerve on each side (Fig. 37-20, A). The center two thirds of the incision is next extended down to bone. A full cuff of mucosa and muscle is maintained adjacent to the attached gingiva of the anterior teeth; this should allow for adequate layered wound closure of the muscle and mucosa without periodontal sequelae.

• Dissection with an elevator exposes the anterior surface of the chin although not completely to the inferior border of the central chin. The dissection continues laterally and remains inferior to the mental nerve, with lateral exposure to the inferior border of the mandible. There is no need to dissect above the mental nerve on either side. If 360-degree dissection around the nerve is completed, it is more likely that the nerve will be excessively stretched or avulsed.

• A small S-shaped Tessier chin retractor is placed on each side inferior to the mental nerve and laterally to the inferior border. A sterile pencil is used to mark the location of the oblique osteotomy ((see Fig. 37-20, B). This should be planned sufficiently below the dental roots and the mental foramen on each side. The exact location of the osteotomy will depend on the presenting chin morphology and the planned reconstruction (i.e., vertical reduction or lengthening and the extent of horizontal advancement).

• With the use of an oscillating saw (i.e., a thin fan blade on a long shaft), a vertical groove is made in the midline across and perpendicular to the proposed horizontal osteotomy. This will help to maintain vertical orientation after the osteotomy and before fixation (see Fig. 37-20, C).

• A drill hole is placed in the midline within the distal chin (see Fig. 37-20, C). Later, a screw (1.7 mm in diameter and 8 mm in length) will be partially inserted and used as a retractor. The screw is usually not placed in the distal chin until the osteotomy is completed. It will then be held with a wire twister that is used as a retractor to facilitate the repositioning and orientation of the distal chin unit.

• The oblique osteotomy of the chin is initiated in the central portion with the use of an oscillating saw (i.e. wide fan blade on a short shaft). The use of this saw and blade to initiate the osteotomy helps to maintain orientation and to limit canting (see Fig. 37-20, D).

• Using a reciprocating saw with a short blade and then with a longer blade is helpful to complete the lateral aspects of the osteotomy and to go through the depth of the bone on each side (see Fig. 37-20, E). An osteotome may be inserted with a twisting motion to complete the osteotomy separation (see Fig. 37-20, E).

• When vertical shortening of the chin is planned, there are two options. If only limited shortening is required in combination with horizontal advancement, it may be preferred to use a rotary drill with a watermelon bur to remove bone height at the osteotomy line. If a more significant vertical reduction is planned, a second parallel osteotomy (for ostectomy) is completed through the proximal chin (below the dental roots) using a reciprocating saw with a straight blade on a long shaft. Any lateral–inferior bony irregularities are also removed using a rotary drill with a watermelon bur to limit palpable or visible step-offs.

• The distal chin is held in the desired position by the assistant to the surgeon with the use of the wire twister secured to the positioning screw. The surgeon contours each titanium plate across the osteotomy site and then secures each with screws (1.7 mm in diameter and 4 mm in length). Typically, either a three- a four-hole straight plate is contoured with the ends placed on either side of the osteotomy. The superior screw is usually placed in between and inferior to the lateral incisor and canine on each side (see Fig. 37-20, F). With the chin secured in its new location, the positioning screw is removed.

• If significant vertical lengthening of the chin is planned, an interpositional graft (e.g., autograft, allograft, bloc hydroxyapatite) is crafted and placed in the gap. The graft fills the central gap between the fixation plates. I do not find it necessary to place graft in the lateral aspect of the gaps. An additional plate and screws are generally placed vertically in the midline across the osteotomy site and directly over the graft (see Chapter 18 and Fig. 37-20, G).

1. When assessing a chin that is considered to be either retrusive or protrusive, place the patient in the natural head position and the mandible in the rest position (i.e., with the condyles seated and with normal freeway space between the teeth in each arch). The lips should be relaxed and separated. By assessing the face in this way, you are likely to unmask any baseline dentofacial deformity (e.g., short or long face growth pattern) and to gain a more accurate impression of any true chin disproportion.

2. The experienced surgeon will rely heavily on the direct visual examination to evaluate overall facial proportions and any notable mirror-image asymmetries that involve (but are not limited to) the chin region. Without protractors or rulers, the absence of Euclidean proportions can usually be discerned. Confirmation of the clinician’s aesthetic impressions may then be clarified through either anthropometric surface measurements or cephalometric radiographic analysis, as indicated.

3. The theoretic advantage of an alloplastic (chin implant) genioplasty is that it is a procedure that is seemingly easier to execute and often considered less invasive by the patient. It has the disadvantage of being primarily effective only for a mild anteroposterior deficiency of the chin, and it generally requires an external facial scar. It will have a higher incidence of infection than a chin osteotomy, and such an infection may even occur years later.34,35,41,49,82,121,125 In addition, there may be underlying bony erosion from the implant over time, and the individual can never be without at least some concern about trauma to the chin. Alternatively, the osseous genioplasty is versatile in all three planes and has proven and long-lasting results without concerns about infection, bony erosion, a shift in position, or the need for limited physical activities after initial satisfactory healing has occurred.

4. When completing the intraoral incision and the soft-tissue dissection to expose the surface of the bony chin, avoid subperiosteal dissection above the mental neurovascular bundle to limit the extent of nerve contusion and stretching and the chance of avulsion from the mental foramen. The mucosal incision should be deep in the vestibule to leave a full cuff of mucosa and muscle for ease of two-layer closure and to limit periodontal sequelae.

5. When lengthening the chin, an oblique osteotomy with the interpositional placement of a graft (e.g., allogenic or autogenous bone, porous bloc hydroxyapatite) is generally preferable to the placement of a chin implant (as mentioned in point 3; see Fig. 37-20, G).

6. When vertical reduction and posterior set-back are indicated, the vector of reduction and set-back should generally follow the long axis of the chin prominence. This will avoid step-offs at the junction of the chin osteotomy and the inferior border of the mandible and limit suboptimal aesthetics as a result of the “bunching” of the soft tissues (see Fig. 37-18).

7. When using a reciprocating saw to complete the posterior (lateral) aspect of the oblique chin osteotomy, the inferior border may unknowingly remain partially intact. When this happens, there is a tendency for the uncontrolled fracture of the chin to be more posterior than desired. This can result in an uneven aesthetic result, and it is avoided by carefully completing the lateral and posterior osteotomy before the “splitting” of the chin (see Fig. 37-20, E).

8. When completing a vertical reduction and advancement genioplasty, limit significant step-offs at the junction of the chin osteotomy and the inferior border of the mandible to avoid a palpable ridge or an unattractive “hourglass” appearance to the inferior border of the mandible. A continuous inferior border of the mandible is less likely to accentuate jowling. A significant ridge at the osteotomy site may be prevented by extending the chin osteotomy more posterior (i.e., as far back as first molar); by not “overadvancing” the chin (i.e., in profile, the soft tissue of the chin should not be anterior to the lower lip); and by recontouring any significant step-offs using a rotary drill with a watermelon bur before securing the chin in its new location.

9. When an alloplastic chin implant has failed and requires removal (i.e., as a result of suboptimal aesthetics, infection, bony erosion, or a shift in implant position), chin morphology is generally best restored with a vascularized osseous genioplasty rather than another avascular implant. Unfortunately, the implant will have stretched the overlying soft tissues, resulting in ptosis and possible muscle dysfunction when it is removed. If the chin implant was composed of non-porous material (e.g., Silastic), then a fibrous capsule will have formed, which adds to the residual soft-tissue deformity. Reassessing the overall maxillofacial morphology is essential before proceeding with further chin surgery. A baseline maxillo-mandibular dysharmony may be present and require reconstruction to best achieve patient satisfaction (see Figs. 37-1, 37-7, 37-9, and 37-10).

10. When the surface contour of the chin is severely dysmorphic (i.e., overly pointed or bifid), direct bur recontouring using a rotary drill with a watermelon bur is the preferred approach. This does not negate the simultaneous completion of an oblique osteotomy for the three-dimensional repositioning of the remaining chin unit. Adequate circulation to the distal chin is maintained through the genioglossus and geniohyoid muscle attachments to the genial tubercle.

11. When completing an oblique osteotomy of the chin, the temporary placement of a screw in the distal chin (i.e., one that is only halfway screwed in) that is then clamped with a wire twister (i.e., to be use used as a handle or retractor) is helpful for ease of positioning of the chin before osteotomy fixation (see Fig. 37-20, C and F).

12. Marking the midline of the bony chin with a small vertical notch perpendicular to the proposed horizontal osteotomy helps with the maintenance of chin symmetry before the placement of plate and screw fixation (see Fig. 37-20, C).

13. Adequate fixation of the distal chin across the osteotomy site is reliably carried out with a limited volume of small titanium plates and screws. The type and extent of screws and plates used will vary in accordance with patient-specific fixation needs and surgeon preferences (see Fig. 37-20, F and G).

Infection

Infection occurs among less than 1% of patients who have undergone an osseous genioplasty. The routine use of prophylactic antibiotics to cover both oral flora and skin organisms is useful to limit this complication. When an avascular implant is placed to augment the chin (rather than a chin osteotomy being performed), a higher rate of infection would be expected. This is especially true for non-porous implants that are not stably fixed to the underlying bone.

Altered Sensibility

The mental nerve is at risk of injury in association with an osseous genioplasty. The mental nerves will always be stretched when the soft-tissue dissection and retraction is carried out for exposure of the chin. The observed loss of sensibility to the lip–chin region early after surgery is usually temporary. The potential for cutting the mental nerve during the soft-tissue vestibular incision is limited by taking care during the lateral aspect of the incision. The nerve can also be avulsed from the mental foramen during retraction at the time of osteotomy. When determining a location for the osteotomy, remember that the nerve dips below the mental foramen posteriorly. According to the literature, as many as 10% of patients may experience a degree of permanent loss of sensibility in the distribution of the mental nerve on one or both sides as a result of genioplasty.

Wound Complications

The osseous genioplasty is completed through an intraoral vestibular incision. To limit wound-healing complications, the incision is made in the depth of the vestibule. Maintenance of the attached gingiva and a full cuff of mucosa and muscle adjacent to the cervical margins of the teeth best ensures a relaxed wound closure in two layers (i.e., the muscle and the mucosa). This will minimize wound dehiscence, muscle dysfunction, and chin ptosis and limit mucogingival problems. Placing the soft-tissue incision too close to the teeth can negatively affect wound healing and result in scar bands and periodontal sequelae (e.g., mucogingival pull).

Injury to the Teeth

The location of the chin osteotomy should be based on the patient’s chin morphology. The review of the patient’s radiographs (e.g., lateral cephalometric, Panorex) will confirm the anatomy of the anterior teeth and the location of the mental foramen. Injury to the teeth at the time of osteotomy is not common, but it can occur. Dental injury may also occur as a result of the placement of the fixation screws, but this is also not common. If dental injury is suspected, evaluation by the patient’s dentist or referral to an endodontist is recommended. Discussions among the surgeon, the dentist, and the patient are essential, because root canal therapy or other forms of treatment may be indicated (Fig. 37-21).

Patient Dissatisfaction

Patient dissatisfaction after an osseous genioplasty will generally revolve around facial aesthetics. This is best avoided via the following means:

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