Osseous Genioplasty

Mark E. Walker

Derek M. Steinbacher

DEFINITION

Osseous genioplasty is an operative technique wherein the surgeon performs an osteotomy proximal to the mental prominence allowing for the repositioning of the chin to improve facial harmony.
Mobilizing this segment enables the surgeon to address the complete spectrum of chin deformities including asymmetry, as well as vertical and sagittal deficiency, and excess.
Size, shape, and position of the chin can enhance or detract from normal facial harmony and symmetry.
Chin appearance influences social perceptions of masculinity and femininity.

ANATOMY

Anterior labial vestibule of the lower lip is the outermost layer forming the labiogingival sulcus.
Deep to the labiogingival mucosa, the two bellies and median raphe of the mentalis muscle insert onto and cover the chin.
Mentalis muscle is the sole elevator of the lower lip and chin providing major vertical support.
Mental nerve courses inferiorly (4.5 ± 1.9 mm) and anteriorly (5.0 ± 1.8 mm) within the canal prior to exiting at the mental foramen (FIG 1).

FIG 1 • Mental nerve courses inferiorly (4.5 ± 1.9 mm) and anteriorly (5.0 ± 1.8 mm) within the canal prior to exiting at the mental foramen.
Osteotomy is designed inferior to this path to avoid damage to the mental nerve.
Submental posterior insertion of the genioglossus muscle is responsible for blood supply to the bone flap following osteotomy.

PATHOGENESIS

Chin asymmetry and/or malposition can be congenital or traumatic in etiology.
Malocclusion and maxillomandibular discrepancy can contribute to chin malposition.

NATURAL HISTORY

In the setting of an open bite, progressive worsening of retrognathia, often perceived as microgenia, should be investigated for possible temporomandibular joint resorptive processes, such as idiopathic condylar resorption.

PATIENT HISTORY AND PHYSICAL FINDINGS

Inquire about any history of dentofacial trauma or prior maxillofacial surgery.
Assess patient for maxillomandibular discrepancy or obvious dentofacial abnormalities.
Simple visual inspection of the vertical and sagittal position can identify microgenia, macrogenia, and chin asymmetry.
- Microgenia, or small chin, is defined as some combination of vertical and sagittal deficiency (FIG 2A).
- Macrogenia, or large chin, is defined as some combination of vertical and sagittal excess (FIG 2B).
- Asymmetry assessed on frontal view can present with vertical and/or rotational components (FIG 2C).
Ideal chin projection is described in relation to the lips, nose, and midface.
Cephalometrics is a powerful tool to generate objective descriptors of chin position.
- Distance between soft tissue nasion to soft tissue chin (2 mm)
- Depth of the labiomental fold (4 mm)
- Cervicomental angle (acute vs obtuse)
- Ricketts S-line
  - Line that connects the soft tissue nasion with the menton, the distance from which measures 2 mm at the upper lip prominence and 4 mm at the lower lip prominence¹ (FIG 3A).
- Riedel plane
  - Line that connects but does not cross the most prominent portion of the upper and lower lips and connects with the soft tissue menton² (FIG 3B).
    
    FIG 2 • A. Vertical and sagittal microgenia. B. Vertical and sagittal macrogenia. C. Frontal asymmetry secondary to unilateral vertical microgenia and rotation.
- Facial convexity angle
  - On lateral view, this is the angle defined by the pogonion position in relation to a vertical line connecting the glabella and subnasale¹ (FIG 3C).

IMAGING

Panorex to assess dentition and mandibular growth
Cone-beam CT (CBCT) can be used for 3D assessment and surgical planning/modeling.
Three-dimensional digital photography can be used to evaluate chin volume, position, and symmetry.
- Simulation function can also be used for preoperative discussion/planning.
Assess position of mental foramen, canal pattern, and tooth root apices.

NONOPERATIVE MANAGEMENT

There are no conservative treatment measures available for correcting chin position.
Patients do have the option for no intervention should they not wish to pursue surgery.

FIG 3 • A. Ricketts S-line. B. Reidel plane. C. Facial convexity angle.

SURGICAL MANAGEMENT

Operative interventions to address chin aesthetics include the implantation of alloplastic materials (ie, porous polyethylene or silicone implants) and surgical osseous genioplasty.
Indications for surgical intervention include displeasure with one’s appearance, with the patient specifically identifying the chin as an area of focus.
The balance between lip, nose, and midface position must be addressed with the patient, as any deformity or asymmetry in these areas may either be contributing to the appearance of chin disharmony or may become more obvious after the chin has been addressed.
The risks of osseous genioplasty include bleeding, infection, injury to the mental nerve, paresthesias, injury to the mentalis muscle, “witch chin” deformity, mentalis muscle, pain, poor wound healing, scarring, persistent asymmetry, hardware malfunction, and the need for future additional operations.
Each skeletal configuration on the differential diagnosis list calls for a unique surgical solution to address the problem or some combination of these.
The surgical steps for correction vary primarily in the planning and execution of the osteotomies and positioning and fixation of the bony segment:
- Microgenia, vertical—lengthen (vertically)
- Macrogenia, sagittal—setback (usually wedge removed)
- Macrogenia, vertical—shorten (wedge removed)
- Asymmetry, frontal—rotate (mediolateral)
- Asymmetry, vertical—wedge reduction
- Combination—combination of the above