19.1 Indications
Key words: alternative mandibular block rotation, augmentation and reduction, chin asymmetry, chin hyperplasia, chin hypoplasia, face center, functional and esthetic indication, horizontal and vertical corrections, lip closure insufficiency
The indication for chin correction can be for esthetic or functional reasons. It can contribute to the esthetics of the facial profile and the frontal view of the head, or to functional improvements such as to relocate the origin of the orbicularis muscle by shifting the chin apex, normalize muscle function, or enable involuntary lip closure.
The most frequent indication is hypoplasia with pincushion-like skin surface of the chin due to permanent tension of the mentalis muscles and protruding lower lip. The criterion for requesting surgery is usually an inadequate facial profile. Although patients themselves usually see only the frontal view of their face in the mirror, this is not as important as the profile view. A prerequisite for chin augmentation alone, however, should be a normal development of the mandibular arch and a regular anterior vertical overlap. Exclusively, chin augmentation in mandibular retrognathia is contraindicated. The correction of the malocclusion should be in the foreground.
A less frequent indication in the authors’ patient population is chin correction to reduce chin hyperplasia under the same above-mentioned conditions.
In addition to horizontal augmentation or reduction of the chin, the vertical dimension may also require correction if a vertically hypoplastic or a vertical and horizontal excessis present. The chin midline should communicate with the vertical facial midline as well with the dental midlines.
In most cases, chin correction in the horizontal and vertical direction is indicated. This can be combined with orthognathic surgical methods with careful planning. Chin correction has limits in the transverse extension, since only the anterior mandible can be changed until the region of the second premolar.
However, if the indication for chin correction involves not only the anterior region but also the lateral or even the entire mandibular margin, a chin wing correction is indicated instead of a genioplasty (see Chapter 20).
In cases of horizontal chin hypoplasia and pronounced protrusion of the mandibular anterior teeth, an indication for anterior mandibular block rotation may also be considered (see Chapter 22).
19.2 Surgical planning
Key words: chin bone resorption, chin displacement directions, face center setting, limitations of the chin osteotomy, planning with head photos/LCR/panoramic radiography, simultaneous chin and malocclusion correction, soft tissue thickness
The surgical planning is based on frontal and lateral photographs with relaxed lips, a lateral cephalometric radiograph (LCR), and a panoramic radiograph.
First, the center of the chin is adjusted to the center of the face for the frontal view exposure. This is determined via the median-sagittal vertical of the interpupillary line. Ideally, the nose and the maxillary dental midline also coincide with the center of the face. If this is not the case, the chin center setting must be discussed with the patient.
Chin tip adjustment is simulated on the LCR by defining soft tissue pogonion and pogonion in the horizontal direction and determining the anterior mandibular height (Ii-Gn = 45 mm on average) in the vertical direction. This results in the extent and displacement direction of the chin apex.
In principle, the chin rests can be adjusted by displacement in all directions: forward or backward displacement and cranialization or caudalization or, in almost all cases, a combination of these possibilities (Figs 19-1 and 19-2). Even asymmetrical chins can be corrected simultaneously by adjustment to the center of the face.
Figs 19-1a and 19-1b Systematics of horizontal and vertical chin augmentation with interposition of bone (black, initial situation; green, chin apex segment displaced anteriorly and caudally; dotted region is filled with bone graft). In most cases a combination of both segment movements is necessary. Planning in advance leads to more esthetic results. A thick soft tissue layer needs less bony advancement.
Figs 19-2a and 19-2b Systematics of horizontal and vertical chin reduction with wedge-shaped bone segment removal (black, initial situation; green, chin apex segment that is moved back and cranially; dashed region is removed). (b) The respective normal result (green). In most cases a combination of both segment movements is necessary. Attention must be paid to the thickness of the surrounding soft tissue layer. There exists the danger to create a double chin.
In the case of lip closure insufficiency, displacement of the chin apex anteriorly and cranially is performed to relocate the origin of the mentalis musculature. This can also contribute to the normalization of the function of the purely muscularly suspended orbicularis muscle, which can enable tensionless lip closure.
In the course of a simultaneous malocclusion correction, identical planning of the chin correction is performed, but only after the jaw displacements have been simulated on the LCR and the extent of the chin correction has been determined as part of the new facial profile (Fig 19-3).
Figs 19-3a to 19-3d Case 1: Description of simultaneous combined malocclusion and chin correction.
Figs 19-3a to 19-3d (a) Severe dentofacial malformation with tense mouth and chin muscles advanced (47 years, 7 months). (b) Facial profile with chin hypoplasia, tense mentalis muscles, and shortened mentocervical angle. (c) LCR shows mandibular retrognathia with narrowed oropharyngeal airway, Angle Class II dentition, and anterior open bite with relaxation splint in situ for centric adjustment of the condyles into the glenoid. (d) Cephalometric analysis: During the surgical simulation, the maxillomandibular correction with intrusion and forward displacement of the maxilla and forward displacement and CCW rotation of the mandible into neutral occlusion is performed first. Only then does the planning of a chin correction follow: reduction of the vertical mandibular height and augmentation of the horizontal height. All relevant osteotomies are coordinated and performed simultaneously.
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