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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.
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 ).
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.
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 .
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).
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.
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.
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