An Osseous Approach to Chin Deformities

S. Anthony Wolfe

Saoussen Salhi

DEFINITION

Osseous genioplasty was introduced in 1942, when Otto Hofer described a horizontal osteotomy of the mandibular symphysis through an external approach to correct the deficient chin on a cadaver.¹^,² However, we owe the first description of a genioplasty performed on a living patient to Gillies and Millard in 1957. They described a jumping genioplasty, through an external approach, where the lower border of the mandibular symphysis was osteotomized, advanced superiorly, and positioned on top of the upper mandibular segment.²
In 1957, Richard Trauner and Hugo Obwegeser described the intraoral approach to the osseous genioplasty,¹^,²^,³^,⁴ and later, in 1964, Converse and Wood-Smith expanded on the possible variations of the procedure.¹^,² In their description of the surgical technique, the osteotomized caudal segment of the mandibular symphysis was stripped of all of its muscular attachments before advancing it, thereby converting it to a free bone graft. This resulted in significant bone resorption, which prompted these surgeons to preserve the blood supply to the caudal segment.⁴
Despite its description more than half a century ago, osseous genioplasty remains less frequently performed than alloplastic chin implants.³ In 1950, Converse described the use of intraoral bone grafts for chin augmentation. These quickly fell out of favor because of their variable resorption, donorsite morbidity, and the successful introduction of alloplasts.¹ In 1953, Safian introduced the first silicone chin implant.¹

ANATOMY

When performing a genioplasty, pertinent anatomy relates to the mental nerve.
The mental nerve is the terminal sensory branch of the inferior alveolar nerve, which is itself a branch of the mandibular division of the trigeminal nerve.
The mental nerve provides sensory innervation to the lower lip and the chin.
It exits the mandible through the mental foramen at the level of the second premolar.³
The inferior alveolar nerve courses inferiorly prior to heading cephalad toward the mental foramen.⁵ For this reason, the horizontal osteotomy of the mandibular symphysis performed during a genioplasty has to be placed 5 to 6 mm below the mental foramen.⁵

PATIENT HISTORY AND PHYSICAL FINDINGS

Physical examination starts by dividing the face along the trichion, glabella, subnasale, and menton.³ When these landmarks divide the face into equal thirds, the face is considered ideal.²^,³
The lower third of the face, delimitated superiorly by the subnasale and inferiorly by the menton, is further divided by the oral commissure into an upper third, the upper lip, and lower two-thirds: the lower lip, labiomental sulcus, and chin.² This allows assessment of the volume of the chin relative to the overall facial profile.
The chin pogonion projection is assessed relative to the facial plane, the zero meridian, which is a line started at the nasion and dropped perpendicular to the Frankfurt horizontal. Ideally, the chin pogonion should be at or near the zero meridian, being more projected in men than in women. It should not be beyond the lower lip border.³
Chin deformities are commonly divided into two categories: macrogenia and microgenia. Guyuron et al. distinguish seven different chin deformities based on the volume of the chin:
Class I: macrogenia or chin excess
Class II: microgenia or chin deficiency
Class III: combined excess in one dimension and deficiency in another
Class IV: asymmetric deformity
Class V: witch’s chin characterized by ptotic soft tissue and a deep labiomental groove
Class VI: pseudomacrogenia or chin excess secondary to soft tissue excess overlying normal bony anatomy as evidenced by a normal cephalogram.⁶
According to Guyuron et al., class II microgenia is the most commonly encountered chin deformity, followed by class II macrogenia and class III, a combination of both.⁶ It is important to note that the microgenia or macrogenia can be present in a number of planes: vertical, horizontal, or both.³
The chin can also be characterized by its spatial position. It can be positioned posterior to its ideal position at the zero meridian, which is referred to as retrogenia.
When the retrogenia is secondary to mandibular retrognathia (class II malocclusion) or vertical maxillary excess leading to a clockwise rotation of the mandible, this is referred to as pseudoretrogenia.³
Osseous genioplasty alone cannot address this issue, and the patient likely needs orthognathic surgery to correct the underlying malocclusion.
Often, patients who present complaining of a deficient chin have mandibular hypoplasia with or without chin deficiency.
For this reason, all patients should undergo an intraoral examination including an evaluation of the skeletal relationship of the maxilla and mandible to exclude any underlying malocclusion.³
Finally, the experienced surgeon needs to rely on his or her eye and the patient’s desired result.³

IMAGING

In most patients, clinical examination and photographic analysis are sufficient to determine if the chin shows any abnormality in both its position and its volume.³
A lateral cephalogram can provide quantitative confirmation of these abnormalities.³
A frontal cephalogram can be helpful in planning the genioplasty osteotomy aimed at correcting an asymmetrical chin.³
A panorex is used to identify the mental foramen and the dental roots and to plan the osteotomy and hardware placement to avoid injury to these structures.³