Genioplasty
Introduction
Chin plays an important role in facial appearance
Indications
The absolute size is not as important as the relative size and proportion of each structure on the face
Facial harmony is the primary determinant of ideal facial appearance
The face is balanced when the upper, middle and lower thirds are of equal size and the structures within each segment are proportional in size and prominence
ANATOMY
The chin is in the area below the labiomental fold esp when viewed lateral.
The bony portion of the chin is the mandibular symphysis
Embryology
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
Anatomy
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
Mentalis:
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)
Two actions:
1protrusion of the lower lip
2elevation and wrinkling of the skin of the chin
Cleft 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
1)microgenia – small chin with an overall deficiency of bone on all three planes
2)retrogenia – the chin is positioned more posteriorly
- pure retrogenia exists when the occlusion is normal
- If there mandibular retrognanthia the retrogenia is secondary
3)macrogenia- large chin
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
Anatomical Considerations
Consider
1. soft tissue (lower lip, mentalis, labiomental fold)
2. bony structures (occlusion, mandible)
The lower lip is influenced by
1)The position of the maxillary and mandibular incisors
- The relative amount of over bite and over jet
2)tone of the orbicularis and the mentalis
- The hyperactivity of the mentalis can lead to increase in the vertical height. Underactivity, gravitational ptosis, iatrogenic injury will lead to lower incisor exposure
3)The bony chin projection
Soft tissue chin contour is affected by the
1)bony chin projection and length
2)mentalis muscle thickness
3)and the soft tissue chin depth
The morphological landmarks are major determinant of the vector and the movement of chin
the majority of pts who seek genioplasty for small chins actually had small mandibles and 51% had Class II skeletal deformity and further 40 % had had orthodontic treatment for excess over jeti.e. only 9% had class I occlusion
The class of bite will also affect the labiomental fold and i.e. class II leads to lower lip being pushed out and down accentuating the fold.
As the height of the lower face elongates the fold attenuates.
The labiomental fold deepens as the chin is advanced surgically.
The soft tissue of the chin advances on a 0.9:1 ratio with the underlying symphysisup to 8 mm. Beyond this length, muscular and soft tissue forces are thought to cause resorption. There is less predictability in vertical movements
PREOP ASSESSMENT
Eyeball assessment
1. lip eversion,
2. the anterior teeth
3. chin pad thickness
4. labiomental fold depth and height
5. dynamic chin pad motion with smile.
Need to assess the frontal, vertical and transverse facial planes
AP view
The lower face can be divided into equal thirds by lines passing through the subnasale, stomion, the labiomental crease and the menton(the inferior most point on the soft tissue chin)
Equal halves may be divided by lines passing thru the subnasale the muco cut jxn of the lower lip and the menton
Facial midline assessed and this is compared with the mand and maxillary dentalmidlines to assess for asym of the chin
Determine whether the cause of the decreased facial ht is secondary to vertical maxillary deficiency, chin deficiency or a combination of both
The best method of assessing for maxillary deficiency is to assess the amount of upper incisor show when the lips are in repose
Ideal incisor show is 2-3 mm and a pt with no incisor show with normal lip length is said to have a maxillary deficiency.
In the absence of vertical maxillary deficiency the reduced facial ht can be attributed to vertical deficiency of the chin. A large no of pts with class II occlusion pattern will have a vertical chin deficiency and if are not candidates for orthognathic surgery will benefit from vertical augmentation
Occlusion assessed
Profile view(lateral view)
The vertical reference line is drawn thru the subnasale and the nasofrontal junction
- The upper and lower lips should lie anterior to this and the lower lip slightly post to the upper
- soft tissue pogonion (most forward-projecting point on the anterior surface of the chin) of the chin should lie very close to this line
From Yaremchuk: The inclination of the facial profile as defined by a line from the glabella to the pogonion. The broken line is drawn perpendicular to the Frankfort horizontal at an inclination of 0 degrees. The solid red line represents the mean inclination of the study group. The shaded area encompasses 1 SD. (Left) The mean inclination in 100 young North American white men was -3 ± 3.4 degrees. (Right) The mean inclination in 100 young North American white women was -4.1 ± 3 degrees (after Farkas, L. G., Hreczko, T. A., and Katic, M. J. Craniofacial norms in North American Caucasians from birth (one year) to adulthood. In L. G. Farkas (Ed.), Anthropometry of the Head and Face, 2nd Ed. New York: Raven Press, 1994. Appendix A.). In both men and women, note that the chin rests slightly posterior to the lower lip and the lower lip lies slightly posterior to the upper lip.
Aesthetic systems
A variety of aesthetic and cephalometric analysis have been made to assess the relative and size and shape of the chin
1) Gonzalez dropping a line perpendicular to the frankforts horizontal thru the nasion should intersect the pogonion
2) rickets drawn a line from the nasal tip to the pogonion and the upper and lower lips should lie 4 and 2 mm away from this line – this however is significantly affected by the tip
3) fig E Holdaways related the nasion B point to form a angle of 7-9 degrees with the tangent to the upper lip and chin
Investigations
1)photographs- frontal lateral oblique and submental to delineate the soft tissue contour and facial relations
2)lateral cephalogram
3)OPG to visualize the teeth apices and position of the mental nerves
4)CT scan
5)Assess the nose
If class II and Class I occlusion consider orthodontic and orthagnathic referral
Operations
History
1942: Hofer first described the sliding horizontal osteotomy of the mandibular symphysis for microgenia
1950: Gillies – jumping genioplasty, external approach
1957: Trauner/Obwegeser coined the term genioplasty
1964: Converse/Wood-Smith horizontal osteotomy
McCarthy devised an algorithm based on the size and shape of the chin in those with normal occlusion
- Size was classified as small, large, and small/large
- Shape was characterized abnormal in the horizontal, vertical and horizontal vertical dimension
Goals
creating an aesthetically pleasing facial contour
establishing proportionate facial height.
Preserve mental nerve, mentalis function.
Contraindications:
When considering a mandibular reduction or a sliding osteotomy, carefully evaluate the teeth and the height of the mandible prior to surgery. Having long teeth with a short mandibular height is a relative contraindication for an osseous genioplasty or an aggressive bony reduction.
Principles
Intraoral/extraoral(submental) approach
Subperiosteal dissection
create pocket(mentoplasty)/osteotomy(genioplasty)
Rigid Fixation – plates better than wires
Protect mental nerve
Resuspend Mentalis
Osseous Genioplasty vs Alloplastic Mentoplasty
Autografts such as iliac crest and rib cartilage have been used more frequently for chin augmentation in the past. Nasal bone and cartilage have been used as well. Other options include autogenous or homologous (cadaveric) cartilage or bone, although these latter materials have a higher infection rate than is observed with autografts.
Historically, various materials have been used to augment the chin, including paraffin, ivory and methylmethacrylate.
Common Alloplastic implants includeSupramid, gortex, silicone, polytetrafluoroethylene, Mersiline and polyester mesh have gained a great deal of popularity through the years as a result of patient and surgeon satisfaction.
The depth of the labiomental fold may dictate which technique is used. Alloplastic implants tend to deepen the sulcus, which may be particularly unattractive in female patients. With osseous genioplasty, the fold generally increases with advancements and/or vertical shortening and becomes more effaced with vertical lengthening.
Alloplasts
-easy to place
-are less time consuming than a sliding genioplasty (can be done with LA)
-limited to the mild to moderately retruded chin, shallow labiomental groove
-primarily address deficiencies in the sagittal dimension at the pogonion and width deficiencies immediately lateral to the symphysis. No simple implant reliably changes the vertical dimension of the chin,
-complications
1)dehiscence,
2)bone resorption,
3)infection,
4)extrusion,
5)overprojection or underprojection,
6)asymmetry,
7)displacement/migration,
8)capsular contraction(smooth implants),
9)lower-lip retraction,
10)chin ptosis.
- Resorption occurs to some extent in many, if not all, patients. One study showed up to 5 mm of resorption at 48 months after surgery. Resorption has been attributed to subperiosteal placement of the implant. Tension in the soft tissue pocket due to pressure from the overlying skin or mentalis musculature has been thought to cause this pressure resorption. The overall soft tissue profile, however, is not usually affected by this bone resorption.
Osseous Genioplasty
-abnormalities in 3 dimensions can be addressed, making it a more versatile procedure (ie including vertical microgenia with and without retrogenia, vertical macrogenia with retrogenia, and prognathia)
-Complications
- Mental nerve injury, malunion, nonunion, irregularities, step-type deformities, lip drop, and overcorrection or undercorrection have been reported. Of note, undercorrection is better accepted than overcorrection in which the chin placed forward to the lower lip can yield a disharmonious profile.
Pattern of bone remodelling
Bone remodelling after genial segment advancement always occurs to a degree and thought to be related to the blood supply and the commonest location is of remodelling (ie absorption and deposition) is at the anterosuperior angle of the advanced segment
Zide’s Mentoplasty Principles
1. Standard Chin to Nose Analysis
A large nose will make the chin look smaller, and vice versa. Any augmentation that disrupts the chin-nose relationship by creating an excessively large chin may be more acceptable to men, but rarely to women. This is especially true when the labiomental fold is high. The key: always undercorrect women and realize that a sagittal augmentation will also make the chin look longer. Removal of a large implant (without replacement) makes the final configuration of the chin pad a matter of chance. Contraction of a prior pocket under the pad will yield unusual chin pad configurations, which are difficult to correct. Replace with smaller implant
2. Lower Lip Analysis
On profile examination, the lower lip should lie slightly posterior to the upper lip, and the most anterior lower lip white roll should be in the same anteroposterior plane as the soft tissue chin point. The advanced, new position of the soft-tissue chin point (that will exist after the placement of the implant) relative to the lower lip on profile view must be considered. When the chin pad soft-tissue thickness is normal (8 to 11 mm), the anterior surface of the implant should not project beyond the labial surface of normally positioned lower incisors.
Lower lip eversion may be due to a skeletal deep bite, lower tooth procumbency, or excess lip weight and bulk, as occurs with vascular enlargement of the lower lip. In addition, any anterior tooth position in which a large overjet occurs may also be associated with lower lip eversion. Eversion of the lower lip deepens the labiomental fold and may spuriously advance the lower lip white roll. Often, the bony chin is retropositioned but the existing lip position results in a white roll in close to proper position. Sagittal augmentation in these cases will only decrease the labiomental angle or deepen the labiomental fold. The harmonious relationship between the lower lip white roll and soft tissue pogonion is disturbed. The best operation in these cases is a mandibular advancement.
3. Labiomental Fold Analysis
The role of the labiomental fold is crucial to determine the appropriate treatment for microgenia/retrogenia. The labiomental fold is analyzed to determine its height, depth, and distinctness. Its position and depth do not really have normative values. A key point is to analyze what percentage of lower facial height is related to the lip-to-menton chin pad height versus lip-to-labiomental fold height. If the labiomental fold is high or close to the lower lip, the pad percentage is high, and vice versa.
In women with a high pad percentage (high or indistinct labiomental fold), an alloplastic chin augmentation can be an aesthetic disaster (Fig. 6). If the fold is very indistinct or high, the chin appears to consist primarily of soft-tissue pad after augmentation. This effect of augmentation is overwhelming, as it appears to enlarge the entire lower face, as opposed to just the chin pad. In contrast, only the chin pad, and not the entire lower face, will look larger after chin augmentation in patients with a very distinct lower labiomental fold. Thus, if the fold is located more inferiorly, augmentation will accentuate only the chin pad. In these cases, augmentation rarely looks too large.
4. Dynamic Chin Pad Analysis
It is important to ask the patient to smile. This is critical because ptosis of the soft-tissue chin pad can occur with smiling.
5. Dynamic Analysis of the Lip/Chin Appearance
Examine the patient during smiling and central lip elevation. This part of the physical examination focuses on the lip muscles as they affect chin appearance. Dynamic side view analysis of the patient will demonstrate the degree of soft-tissue effacement and even inferior displacement that may occur with smiling. A malpositioned implant will also be visible during effacement of the soft tissue over the chin prominence. In like manner, an extremely thin chin pad will reveal the underlying bony prominence.