Classification of chin deformities. (a) Normal chin, (b) microgenia, (c) retrogenia, (d) pseudoretrogenia, (e) macrogenia, (f) pseudomacrogenia
The height of the lower face is important when evaluating the chin and selecting the appropriate surgical approach. For example, if the patient has a short lower face, elongation of the chin may be needed, while shortening is required in a long lower face. The assessment can be done with a proportionate analysis by dividing the face along anatomic landmarks such as the trichion, glabella, subnasale, and menton. With these landmarks, the face can be divided into three parts. When the three divisions are in equal thirds, the proportion of the face has been said to be ideal . However, especially in Asians, recently a ratio of 1:1:0.8 is considered to be more aesthetically ideal (Fig. 8.2).
Proportionate analysis of the chin. (a) The aesthetic facial proportion of 1:1:0.8. (b) In microgenia, the lower face is vertically deficient. (c) In retrogenia, the proportions may be the same as in a normal ideal face
Various analyzing methods were introduced to assess the facial profile, which the surgeon can use as a guide in evaluating the chin relative to the overall face, nose, and lips. For example, in the McNamara analysis, a line perpendicular to the Frankfort horizontal plane is drawn through the nasion, and the distance from the pogonion to the line is assessed (Fig. 8.3). The Arnett analysis is for soft tissue cephalometric analysis. Distance is measured between the soft tissue pogonion and a line placed through the subnasale perpendicular to the natural horizontal head position called the “true vertical line (TVL)” (Fig. 8.3) . Another method to analyze the chin position is by assessing the projection of pogonion from the N to B line (cephalometric line from nasion to B point) . The authors find the McNamara analysis and the Arnett analysis quite useful.
Profile analysis of the chin. (a) In the McNamara analysis, a line perpendicular to the Frankfort horizontal plane (line 1) is dropped through the nasion (line 2, nasion perpendicular line). (b) In the Arnett soft tissue cephalometric analysis, a line perpendicular to the natural horizontal head position is dropped through the subnasale (true vertical line)
Operations are performed under general anesthesia through orotracheal intubation. Local anesthesia solution containing 1% lidocaine and 1:100,000 epinephrine is infiltrated along the proposed incision line and dissection area, submucosally and also subperiosteally. A labial incision midway between the labial sulcus and the lower lip vermilion is made from the canine to the opposite canine, avoiding division of the frenulum. Subperiosteal dissection is then performed to expose the mid-symphyseal region. The dissection should not be done to totally deglove the inferior border of the mandible which may disturb blood supply to the bone segment. Great care should be taken to identify the mental nerves and to protect them during dissection. Dissection should be performed sufficiently posterior for adequate visualization and access for the osteotomy .
After dissection is completed, the symphyseal midline should be marked above and below the planned osteotomy. The osteotomy line should be designed at least 5 mm below the mental foramen, and the horizontality of the line should be double-checked. Then the osteotomy is completed with a reciprocating saw. As the distal bone segment gains mobility to allow manipulation, the segment should be advanced or set backed as planned according to preoperative profile analysis and rigidly fixed with plates and screws (Fig. 8.4) .
Operative procedures of genioplasty. (a) Labial incision is made at least 5–6 mm inferior to the sulcus avoiding division of the frenulum. (b) The mandible is exposed below the mental foramen and lateral along the inferior mandibular border. (c) Horizontal osteotomy is made with a reciprocating saw. (d, e) Distal bone segment is mobilized according to the preoperative plan. (f) Fixation is done with plate and screws
Meanwhile, a reduction or lengthening genioplasty can be performed to correct the chin deformity either with vertical excess or shortness. Usually when chin shortening is needed by a vertical reduction procedure, a horizontally parallel bone segment is removed from the chin (Fig. 8.5). After a reduction genioplasty, an additional marginal osteotomy or shaving is required as a step deformity can be noticed on each side of the chin. The steps may be approached through the same genioplasty incision with or without an extension, but an additional incision may be needed in cases with large steps. When the patient has a short lower face, a lengthening genioplasty may be planned to achieve ideal facial proportions . The osteotomy for the lengthening genioplasty procedure contains a horizontal osteotomy with two vertical osteotomies in an upside-down trapezoidal shape . While designing the line for the horizontal osteotomy, it is important to leave a small amount of the bone in the middle portion. The vertical height of the bony portion left in the center is in accordance with the amount that the chin is to be lengthened vertically (Fig. 8.6).
Operative procedures of reduction genioplasty. (a) Labial incision is done. (b) Mid-symphyseal exposure and design of osteotomy line. Note that the shaded part is to be removed and the midline is marked. (c) Removal of the middle bone segment. (d) Fixation is done with plate and screws
Operative procedures of lengthening genioplasty. (Left) (a) A horizontal osteotomy line and two vertical osteotomy lines are designed with a small segment left in the center. (b) Osteotomies are carried out with a reciprocating saw. (c) After removing the distal bone segment in the midportion, the two lateral bony segments are approximated in the center. (d) Bone segments are fixed with a microplate and screws. (Right) Illustration of operative desing. (e) Bony design. (f) Central bony segment is removed. (g) Lateral segment is approximated. (h) Fixation is done with microplastes
For correcting deformity of transverse excess and to make a slim lower face, a narrowing genioplasty is indicated [9, 10]. Horizontal osteotomy and two vertical osteotomies are designed as the amount of resection in the middle bony segment should be determined preoperatively. In cases with asymmetry, the center of the middle segment should be lateralized to the more prominent side. After the osteotomy is completed, the middle segment is removed, and the two lateral segments are fixed in the center (Fig. 8.7). To obtain a more natural-looking and smooth curvature in the lower border of the mandible, further osteotomy or shaving on the lateral steps is usually required . Resection of bony steps can be extended to the mandible angle when performed as a combination with the mandible contouring procedure .
Operative procedures of narrowing genioplasty. (a) A horizontal osteotomy line and two vertical osteotomy lines are designed. (b) Osteotomies are carried out with a reciprocating saw. (c) After removing the middle bone segment, the two lateral bony segments are fixed with a microplate and screws in the center. (d) Bone segments are removed during surgery
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Key Technical Points
During reduction genioplasty, the lower osteotomy should be done prior to upper osteotomy in order not to lose control of the distal segment. Also, as surgeons tempt to reduce more during the surgery, the chance of nerve injury might increase. It is important not to jeopardize the nerve in order to reduce more height .
The amount of lengthening during a lengthening genioplasty procedure is 2–3 mm in most cases . Far exceeding this average may cause problems such as bone instability, extra tension across the wound, lip tightness, lip eversion, or mouth closure disturbance.
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