# Genioplasty: Osseous Versus Alloplastic

## Overview

Genioplasty addresses chin position abnormalities that contribute to facial disharmony. The chin is one of the most important determinants of facial balance and profile aesthetics. Two primary approaches exist: sliding osseous genioplasty and alloplastic chin implants. Genioplasty is often performed in conjunction with orthognathic surgery or as an isolated procedure.

## Indications

The most common indication is microgenia or retrognathia (chin deficiency). Additional indications include macrogenia (chin excess), vertical chin excess (long chin) or deficiency (short chin), chin asymmetry with lateral deviation, combined deformities requiring multi-directional correction, and use as an adjunct to rhinoplasty for overall facial profile balance.

## Preoperative Assessment

Assessment begins with clinical facial evaluation in natural head position. Profile analysis evaluates chin projection relative to reference lines including the Ricketts E-line, Gonzales-Ulloa line, and Legan-Burstone line. The frontal view assesses chin symmetry and lower facial height proportionality. Cephalometric measurements include the distance from Pogonion to N-perpendicular, the SNPog angle, and the lower facial height ratio. Soft tissue assessment evaluates mentolabial fold depth, chin pad thickness, and lip competence. CBCT or lateral cephalogram is used for surgical planning, and dental evaluation confirms root positions relative to the planned osteotomy.

## Sliding Osseous Genioplasty

### Surgical Technique

The procedure is typically performed under general anesthesia or deep sedation. Local anesthetic with vasoconstrictor is infiltrated at the chin. An intraoral degloving incision is placed in the lower labial vestibule, 10-15 mm below the mucogingival junction, extending from canine to canine. A cuff of mucosa and mentalis muscle is left attached to the alveolus for later reattachment. Subperiosteal dissection exposes the chin, the mental foramina bilaterally (with identification and protection of the mental nerves), and the inferior border of the mandible.

The osteotomy is designed as a horizontal cut placed below the mental foramina and tooth root apices, with a minimum of 5 mm below the mental foramen. A reciprocating saw or piezosurgery is used, and the osteotomy extends from one mental foramen region to the other. The inferior segment is the mobilized chin segment, which remains pedicled on the genioglossus, geniohyoid, and anterior digastric muscles inferiorly. The segment is then repositioned as planned -- advanced, set back, shortened, lengthened, or shifted laterally.

Rigid fixation is achieved with a step plate, straight plate with monocortical screws, or lag screws. For advancement, one midline or two paramedian plates are used. For vertical change, plates are positioned to maintain the planned height, and an interpositional bone graft is placed if the lengthening gap exceeds 5 mm. Closure involves watertight layered repair with reattachment of the mentalis muscle to the periosteum. Failure to reattach the mentalis causes ptosis of the chin pad, known as the "witch's chin" deformity.

### Movements Available

Advancement is the most common movement, with up to 10-12 mm achievable. Setback reduces chin projection. Vertical reduction (shortening) involves removing a wedge of bone and closing the gap or overlapping the segments. Vertical lengthening opens the osteotomy gap and places an interpositional graft. Lateral shift corrects asymmetry by sliding the segment right or left. Combined movements are possible, such as advancement with shortening, advancement with lengthening, or advancement with lateral shift. A jumping genioplasty refers to large advancement exceeding 10 mm, which may require a double-slide technique or interpositional graft.

<image>Surgical illustrations demonstrating different movements achievable with sliding osseous genioplasty: horizontal advancement, vertical reduction with wedge removal, vertical lengthening with interpositional graft, and lateral shift for asymmetry correction, each showing plate fixation positioning</image>

### Advantages

Osseous genioplasty offers true three-dimensional repositioning capability in any direction. It uses autogenous tissue with no foreign body. It can correct vertical and transverse deformities, which implants cannot. Results are permanent and stable, and the procedure can be combined with orthognathic surgery in the same setting.

### Disadvantages

The procedure typically requires general anesthesia and is more invasive than implant placement. The mental nerve is at risk, with temporary paresthesia occurring in 10-30% and permanent injury in fewer than 5%. A step deformity may be palpable or visible at the osteotomy site, especially in thin patients. Operative time is longer than for alloplastic augmentation, and chin pad ptosis can occur if the mentalis muscle is not properly reattached.

## Alloplastic Chin Augmentation

### Implant Materials

| Material | Tissue Integration | Bone Resorption | Removability | Notes |
|---|---|---|---|---|
| Silicone (Silastic) | None (capsule forms) | Up to 60% | Easy | Most commonly used |
| Porous polyethylene (Medpor) | Tissue ingrowth | Less | Difficult | Can be contoured intraoperatively |
| PEEK | Minimal | Minimal | Moderate | Custom-milled, patient-specific from CT |
| ePTFE (Gore-Tex) | Minimal | Minimal | Easy | Soft, limited projection |
| Hydroxyapatite | Osteoconductive (bone integration) | Minimal | Very difficult | Risk of fracture |

Silicone (Silastic) is the most commonly used material. It is smooth, easy to place and remove, and does not integrate with bone. It forms a capsule and can cause bone resorption beneath the implant in up to 60% of cases over time. Porous polyethylene (Medpor) allows tissue ingrowth into its pores for fixation, is more difficult to remove if needed, causes less bone resorption, and can be contoured intraoperatively. Polyetheretherketone (PEEK) can be custom-milled with excellent biocompatibility and can be patient-specific based on CT data. ePTFE (Gore-Tex) is used less commonly, is soft, and provides limited projection. Hydroxyapatite is osteoconductive and can integrate with bone, but it is difficult to remove and carries a risk of fracture.

### Surgical Technique

The procedure can be performed through an intraoral approach using a lower labial vestibular incision similar to the genioplasty approach, or through a submental approach using a small incision in the submental crease, which offers shorter intraoral dissection and better visualization but leaves an external scar. A subperiosteal pocket is created precisely sized to the implant -- not too large, to prevent migration. The implant is placed and positioned at the midline with symmetry confirmed. Fixation with a titanium screw through the implant into bone is recommended, though suture fixation to periosteum is an alternative but less stable option. Layered closure with mentalis muscle reapproximation completes the procedure.

### Advantages

Alloplastic augmentation is a simpler procedure with shorter operative time. It can be performed under local anesthesia and is easily reversible, as the implant can be removed or exchanged. No bone osteotomy is required, and there is minimal nerve risk with careful dissection.

### Disadvantages

Alloplastic implants provide only horizontal augmentation (advancement) and cannot correct vertical or transverse deformities. As a foreign body, they carry infection risk (2-5%) and risks of extrusion and migration. Bone resorption beneath the implant is significant with silicone implants, occurring in up to 60% of cases. Projection is limited to typically less than 8-10 mm, and implant edges may be palpable in thin patients. Importantly, alloplastic augmentation does not address deep skeletal deformity and serves as cosmetic camouflage only.

<image>Side-by-side profile diagrams comparing the outcome of sliding osseous genioplasty versus alloplastic chin implant for chin deficiency, showing the bone osteotomy and plate fixation in the osseous technique versus the subperiosteal implant pocket in the alloplastic technique, with soft tissue profile changes annotated</image>

## Osseous vs. Alloplastic: Comparison

| Parameter | Osseous Genioplasty | Alloplastic Implant |
|---|---|---|
| Directions of movement | Any (3D repositioning) | Advancement only |
| Vertical/transverse correction | Yes | No |
| Anesthesia | General (typically) | Local possible |
| Operative time | 45-90 min | 20-40 min |
| Reversibility | Not reversible (revision possible) | Easily removable |
| Bone resorption | Minimal | Significant (up to 60% with silicone) |
| Foreign body risk | None | Yes |
| Temporary nerve injury | 10-30% | Lower with careful dissection |
| Long-term stability | Excellent | Variable (migration, resorption) |

When comparing the two approaches, osseous genioplasty allows movement in any direction (true 3D repositioning), while alloplastic implants are limited to advancement only. Only the osseous technique can correct vertical discrepancies and asymmetry. Alloplastic augmentation can be performed under local anesthesia in 20-40 minutes, whereas osseous genioplasty typically requires general anesthesia and takes 45-90 minutes. Alloplastic implants are easily removable, while osseous genioplasty is not reversible though revision is possible. Bone resorption is minimal with osseous genioplasty but significant with silicone implants. The osseous approach carries no foreign body risk, while the alloplastic approach does. Temporary nerve injury risk is higher with osseous genioplasty (10-30%) compared with alloplastic augmentation when careful dissection is performed. Long-term stability is excellent with osseous genioplasty but variable with implants due to migration and resorption.

## Complications

### Common to Both

Complications shared by both approaches include mental nerve injury with temporary paresthesia, infection, hematoma or seroma, chin pad ptosis from mentalis detachment, asymmetry from malposition, and unsatisfactory aesthetic results requiring revision.

### Specific to Osseous Genioplasty

Complications specific to the osseous technique include non-union or delayed union (rare with rigid fixation), step deformity at osteotomy margins, hardware irritation, and tooth root injury if the osteotomy is placed too close to the apices.

### Specific to Alloplastic

Complications unique to alloplastic augmentation include implant migration or malposition, implant extrusion through skin or mucosa, capsular contracture, bone erosion and resorption beneath the implant, and the need for implant removal or exchange.

## Clinical Pearls

For isolated horizontal chin deficiency of less than 5 mm, an alloplastic implant is a reasonable option, but for anything more complex, osseous genioplasty is preferred. Mentalis muscle reattachment is critical with both techniques, as failure leads to chin pad ptosis and an aged appearance. When performing osseous genioplasty with orthognathic surgery, the genioplasty should be performed last, after maxillary and mandibular positioning is finalized. Patients should always be informed that genioplasty changes the chin but does not change the jaw -- an underprojected mandible (Class II) requires a BSSO, not a genioplasty alone. The mentolabial fold deepens with advancement genioplasty, which may be aesthetically undesirable if the fold is already deep. For silicone implants, fixation with a screw is strongly recommended to prevent migration, as unfixed implants have a significantly higher malposition rate. In female patients, conservative advancement of 4-6 mm generally produces more harmonious results than aggressive advancement.

## References
- Guyuron B, et al. Chin augmentation. Plast Reconstr Surg. 2017.
- Shaughnessy S, et al. Osseous genioplasty: a review. Oral Maxillofac Surg Clin North Am. 2014.
- Peled M, et al. Alloplastic chin augmentation: a retrospective analysis. J Oral Maxillofac Surg. 2005.
- Ward JL, et al. Osseous genioplasty combined with Le Fort I or BSSO: outcomes. Am J Orthod Dentofacial Orthop. 2019.
- Eppley BL, et al. Chin implants: alloplastic augmentation. Clin Plast Surg. 1992.