Chin





Chin augmentation with implants is the most frequently performed facial implant surgery. It has long been considered a simple procedure, often done in the office setting, and one that always gives gratifying results. Unfortunately, outcomes after chin augmentation are often less than ideal. It is not uncommon for the augmented chin to be asymmetric, to be too large (especially in women), to have a poor transition with the native mandible, or to negatively impact on the adjacent and overlying soft tissues. Optimization of the aesthetic results after alloplastic chin augmentation requires careful preoperative analysis as well as refinements in surgical technique and implant design.


Evaluation and Planning


Effective preoperative planning requires: understanding of the patient’s desires, recognition of the impact of previous facial surgery or orthodontic treatment on the patient’s appearance, and careful facial examination. When chin augmentation is considered as an adjunct to rhinoplasty or neck rejuvenation, the preoperative analysis should be as rigorous as it is when chin deficiency alone is the presenting complaint.


Physical examination


Physical examination must include analysis of both the skeleton and its soft tissue envelope. The lower one-third of the face must relate appropriately to the upper two-thirds. Within the lower one-third, the position of the lips and depth of the labiomental sulcus are impacted by the projection of the chin.


X-rays


Unlike orthognathic surgery, where cephalometric X-ray studies are intrinsic to the preoperative plan, for implant surgery, preoperative X-rays can be helpful but are not mandatory. Plain X-rays may be particularly helpful during revision surgery after previous osteotomies.


Three-dimensional computed tomography (CT) scans and the models that can be generated from their data can be useful for planning surgery to ameliorate facial skeletal asymmetries. Custom implants can be made using these data (see Chapter 14, Chapter 15 ).


Anthropometric data


Normative data are helpful in analyzing the chin, not only in its sagittal but also in its vertical and horizontal projections. These dimensions and their potential change with chin augmentation should all be considered in the context of lip projection and the depth of the labiomental angle. Although these data derive from large numbers reflecting a heterogeneous population, and cannot be applied to every individual, average values of the normal face should not be discounted and should be used in the evaluation process.


Sagittal projection (profile)


The anthropometric studies of Farkas et. al. showed that the mean inclination of the facial profile as defined by a line from the glabella to the pogonion was −3 ± 3.4 degrees in men and −4.1 ± 3.0 degrees in women ( Fig. 11.1 ). These data show that men’s chins project more than women’s and that “normal” chin projection is considerably less than that determined “ideal” by subjective criteria. This does not mean that augmentation beyond one standard deviation would be unattractive in any given individual, but that it is may look too large, especially in a woman.




Fig. 11.1


The inclination of the facial profile as defined by a line from the glabella (g) to the pogonion (pg). The broken line is drawn perpendicular to the Frankfort horizontal at the inclination of 0 degrees. The solid red line represents the mean inclination of the study group. The shaded area encompasses 1 standard deviation (SD). (A) The mean inclination in 100 young North American white men was −3 ± 3.4 degrees. (B) The mean inclination in 100 young North American white women was −4.1 ± 3 degrees. 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.

Data from Farkas et al. 1994, with permission.


Lip–chin relations


Suggested ideal relationships between the chin and the lips based on normative cephalometric data by different authors are in consensus that the chin should rest slightly posterior to the lower lip and the lower lip posterior to the upper lip ( Fig. 11.1 ).


Labiomental angle


The inclination of labiomental angle must be evaluated when considering an increase in the sagittal projection of the chin. There is a considerable variability in this inclination. In general, the inclination is more acute in men (113 ± 21 degrees) than it is in women (121 ± 14 degrees) ( Fig. 11.2 ).




Fig. 11.2


Labiomental angle has considerable variability. The mean inclination is more acute in men at 113±21 degrees (A) , than it is in women at 121 ±14 degrees (B) .

Data from Farkas et al. 1994, with permission.


When the angle is already acute, chin augmentation will make it more acute, thereby deepening the labiomental angle. Such deepening is usually dysaesthetic. Certain patients with retrognathia have their upper central incisors abutting on their lower lip, thrusting it forward, and creating a deep sulcus ( Fig. 11.3 ). Depending on the occlusal status, these patients may be better served with repositioning of the entire mandible by sagittal split osteotomy.




Fig. 11.3


In patients with severe mandibular deficiency and overbite, the upper incisors may thrust the lower lip forward resulting in an acute labiomental angle. Chin augmentation in such patients will further increase the labiomental angle. (A) Artist’s depiction. (B) Lateral X-ray.

After Flowers 1991.


Subjective criteria


Subjective criteria for an ideal profile are at variance from those obtained by objective analyses. When McCarthy and Ruff and Farkas et al. compared the profile of young North American white adults to classic art and medical illustrations, they found that artists usually portray a larger chin than normally exists in reality. A frequently cited ideal chin projection is adapted from Gonzalez-Ulloa. His ideal facial plane was a vertical line that extended downward from the nasion to meet the Frankfort horizontal (which extends from the upper margin of the external auditory meatus to the lower orbital ridge). This was determined from his analysis of “contemporary beautiful faces and faces known as beautiful through history.” It is redrawn in Fig. 11.4 .




Fig. 11.4


Gonzalez-Ulloa’s ideal facial plane was a vertical line that extended down from the nasion to meet the Frankfort horizontal. The anteriormost projection of the lips and chin meets this line. This profile is determined by subjective analyses and does not agree with data obtained from normal individuals.


This ideal is beyond the upper limit of normal for women as determined by the data of Farkas et al., does not take into account sexual dimorphism, and is at variance with ideal lip–chin relations derived from cephalometric studies.



Pearl


A large chin implant in a woman may masculinize her lower face.



Vertical projection (height)


Fig. 11.5 shows average dimensions of both attractive male and attractive female faces. On average, the distance from the base of the nose (subnasion; sn) to the mouth opening (stomion; sto) is half the distance from sn to gnathion (gn). In other words, the height of the mandible in the midline should account for approximately two-thirds of the lower face height (sn–gn).




Fig. 11.5


Average dimensions of both male and female faces. The distance from the base of the nose (subnasale; sn) to the mouth opening (stomion; sto) is half of the distance from the subnasale to the gnathion (gn). In other words, the height of the mandible in the midline (sto–gn) should account for two-thirds of the lower face height (sn–gn).


Increasing the vertical height of the chin with implants can be accomplished in three ways. An illusion of increased length can be obtained by moving the most projecting part of the chin inferiorly by implant positioning ( Fig. 11.6 ). However, real increases in vertical projection require CAD/CAM implants or horizontal osteotomy with a spacer of desired height between the proximal and distal chin segments.




Fig. 11.6


A 7-mm, in projection, implant was repositioned at the inferior border of the chin to visually increase the vertical height of the chin. (A) Preoperative. (B) Postoperative.


The senior author’s preference is to place a porous polyethylene block between the segments. Because the entire lower jaw is small and to eliminate a step-off between the intact mandible body and lengthened chin, a mandible implant is placed to create a smooth mandibular border as well as increase the overall size of the lower jaw.


Horizontal projection (chin width)


The width of the chin should be appropriate for the vertical projection. This is the rationale for extended chin implants that allow a transition between the augmented chin point and the anterior mandibular body. Because women’s mandibles are disproportionately narrower (relative to overall body size) than men’s, care should be taken not to over-augment either the anterior or posterior mandible in a woman. Fig. 11.7 shows the difference between male and female lower facial width.




Fig. 11.7


(A) Male and (B) female skulls demonstrate the difference in chin and overall lower facial width.

From the Atkinson Skull Collection, University of the Pacific, Dugoni School of Dentistry, San Francisco, CA.


Predicting the soft tissue response to skeletal augmentation


Objective analyses have shown that the soft tissue–to–hard tissue changes in patients undergoing alloplastic augmentation of the chin averaged between 77.7% and 90%. One would anticipate variations in the soft tissue response because of the variability in the thickness of the chin soft tissue envelope. The thicker the overlying soft tissue envelope, the lesser its surface response to the underlying skeletal augmentation. The thickness in the soft tissue envelope overlying the chin varies within the individual, between individuals for any given area, and is usually thicker in males than in females.




Surgical Anatomy


Structures susceptible to iatrogenic injury during chin augmentation include the mental nerve as well as the lower lip retractors and elevators. Their anatomy is depicted in Fig. 11.8 .




Fig. 11.8


Shows the anatomy of the lower lip elevators and retractors as well as the path of the mental nerve relative to the mandible. These structures are susceptible to injury during chin augmentation. (A) Frontal view with facial musculature and position of infraorbital and mental nerves. (B) Lateral view with facial musculature origins and path of inferior alveolar nerve.


Mental nerve


The inferior alveolar nerve is a branch of the mandibular nerve (V3). It enters the mandibular canal with the inferior alveolar vessels through the mandibular foramen, which is located in the inner aspect of the ramus of the mandible. The nerve, together with vessels, courses obliquely from the ramus to the body where it exits as the mental nerve through the mental foramen, which is located at the level of the first or second premolar. Hwang et al. documented the course of the inferior alveolar nerve in Korean cadavers. They found that the foramen was located below the second premolar tooth in approximately two-thirds of the cadavers studied. Vertically, the mental foramen was located almost halfway from the tip of the alveolar process to the lower border of the mandible. They also found that the terminal mandibular canal is an average of 4.5 mm under the mental foramen, advances 5 mm anteriorly, loops, and ends at the foramen. The potential location of this nerve is important to recognize when placing screws or performing horizontal osteotomies of the mandible. Any cuts or holes made in the bone should be placed at least 6 mm beneath the foramen. After exiting the foramen, the mental nerve divides into two or three branches to supply sensation to the mucosa and the skin of the lower lip and chin.



Pearl


The inferior alveolar nerve courses at a level inferior to the mental foramen.



Mentalis muscle


Zide has defined the anatomy of the mentalis muscle and the repercussions of iatrogenic injury to this structure. The mentalis muscle is an elevator of the central lower lip. It arises from the mandible at the level of the root of the lower lateral incisor and therefore defines the inferior limits of the sulcus intraorally. It fans inferiorly as a truncated cone whose base inserts on the skin and therefore dimples the skin when elevating and protruding the lower lip ( Fig. 11.9 ).




Fig. 11.9


Cadaver dissection showing mentalis muscle anatomy. Frontal view with chin soft tissues split in midline.


This is the most frequently damaged muscle during chin surgery. If it is divided and improperly repaired, or stripped from its origin and allowed to descend to a more inferior position, the result is inferior malposition of the lower lip with increased lower incisor show, deepening of the sulcus, and inferior displacement of the chin pad ( Fig. 11.10 ).



Pearl


The mentalis muscle is the most frequently damaged structure during chin surgery. A submental surgical approach and extended subperiosteal dissection avoid damage to the mentalis muscle.




Fig. 11.10


Lower lip descent and a deep labiodental sulcus in a patient whose mentalis muscle is divided or stripped and not attached to the mandible during the intraoral approach for chin implant placement. (A) Frontal intraoral view lower lip posture at rest. (B) Intraoral view of deep sulcus reflecting descent of mentalis muscle origin.


The depressor anguli oris (triangularis), the depressor labii inferioris (quadratus), and platysma are all depressors of the lower lip. The depressor labii inferioris and depressor anguli oris arise from the oblique line of the mandible to merge with orbicularis oris and skin of the lower lip. The platysma muscle sends fibers that originate from the mandible beneath the oblique line and merge with the lip depressors. Subperiosteal dissection separates these muscles as a continuum from the bone. Failure to stay in the subperiosteal plane may cause damage, usually short lived, to these muscles. Clinically this injury mimics that of marginal mandibular branch palsy since the marginal branch innervates the lower lip depressors. Temporary palsy to this musculature may last 1 to 2 weeks after primary surgery and 2 to 3 months after revision surgery ( Fig. 11.11 ).


Feb 9, 2020 | Posted by in Craniofacial surgery | Comments Off on Chin
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