The two hemimandibular segments form independently from the first arch with the ossification centres appearing about the 6th week of gestation
Intramembranous ossification continues to envelop and invade the much of Meckel’s cartilage and the two mandibular bodies meet at the mandibular symphysis between 4th -12th month after birth as ossification converts syndesmosis into a synostosis
Between the ages of 10 and 13 all the permanents erupt in the region of the mandibular symphysis and thus genioplasty should not be undertaken until after 15.
Sensation of the lower lip and chin and lower incisors is via the mental nerve
The mental foramen lies on the same vertical line defined by the pupil, infraorbital foramen and the second bicuspid tooth.
After eruption of the permanent dentition the roots of the teeth can be expected to lie above the mental foramen and thus the bone below this level may be used for genioplasty
Horizontal osteotomy should be placed 4- 6mm below the mental foramen and this will prevent injury to the mental nerve/ inf alv nerve
The genioglossus, geniohyoid and anterior belly of digastric attach to the post portion of the inferior border of the symphysis and the genial tubercle is the point of tendinous attachment
As the periosteum is usually stripped off anteriorly during genioplasty preservation of these muscle attachments provides a blood supply to the lower fragment after the horizontal osteotomy
arises from the incisive fossa on the anterior aspect of the mandible. It inserts into the skin of the chin.
It is innervated by the mandibular branch of the facial nerve (CN VII) (superficial surface)
protrusion of the lower lip
elevation and wrinkling of the skin of the chin
Historically, chin implants attempted to form a cleft chin by placing a groove in the central portion of the implant. This was not successful. Furthermore, chin implants placed under existing chin clefts tend to efface the natural cleft by adding bulk to the soft tissue underlying the cleft. This occurs because of the anatomy underlying a cleft chin. A cleft chin is thin centrally at the cleft, with thicker soft tissue on each side of the cleft.
In the cleft chin, the cleft is always the thinnest part of the chin pad, and it is a measure of central muscle deficiency. Normally, the paired mentalis muscles arise at the sulcus to pass transversely (at the fold) and inferoobliquely to insert into the skin of the chin pad. The septum between these muscles usually fades out, and the muscles converge or fuse in the midline to form the single chin pad. Thus, in most people, no central muscle deficiency, or cleft, exists.
The mentalis muscles are analogous to two "megaphones" of muscle that usually fuse centrally as they proceed toward the chin pad. The megaphones fuse as they enlarge distally. When the muscles do not fuse centrally, a deficiency occurs or cleft develops (i.e., a muscle-free zone), not unlike that seen between the frontalis muscles. Thus, the cleft chin is the result of a lack of fusion of the insertions of the mentalis muscles or the result of a persistent septum preventing muscle fusion. The anatomy can produce varied forme frustes of chin clefting, ranging from a dimple to a vertical line. Furthermore, the cleft may be partial, involving only the superior or inferior aspects of the mentalis .
Terms used to describe abnormal anatomy of the chin microgenia – small chin with an overall deficiency of bone on all three planes
retrogenia – the chin is positioned more posteriorly
pure retrogenia exists when the occlusion is normal
If only a hypoplasia of the mandible exists, the term micrognathia is more accurate and should be used. When no skeletal malformation is present, the terms for a recessed chin include retrogenia, microgenia, retruded chin, hypoplastic mentum, and horizontal mandibular hypoplasia.
Macro and microgenia can be associated with mandibular prognathism
Vertical abnormalities such as increased or decreased ht may also exist
Abnormalities with chin sym localized exostoses and a variety of more unusual transverse chin abnormalities may also be present