Answer:
A sliding horizontal osteotomy is done for the correction of pure horizontal microgenia. The osteotomy is done below the mental foramen nearly parallel to the inferior border of the mandible. The osteotomy should be at least 0.5 cm below the mental foramina and at least 0.5 cm below the roots of the teeth. The longest teeth, the canines, are 25.5 mm, so the distance from the incisal edge to the osteotomy should be no less than 30-31 mm. If one keeps the osteotomy at this level medially and about 5 mm below the foramina laterally, the osteotomy should be safe. The cut is angled caudally and posteriorly, and the osteotomized segment anteriorly and fixing it with a plate.
The disadvantages of an alloplastic genioplasty include:
- a higher chance of infection.
- rejection is a theoretical complication, seeing as there have been no reports in the literature of this.
- bone resorption under the implant will occur, but this also occurs with autogenous materials (cartilage), and the pattern of resorption is somewhat predictable, related to the size of the implant, the location, etc.
- capsular contracture has not been observed with a proplast implant, but is common with a silicone implant.
- the main disadvantage of alloplastic genioplasty is its limitations. The osteoplastic genioplasty will allow the caudal segment to be moved three-dimensionally, but the implant will allow augmentation in an anteroposterior dimension and only minimally in a cephalcaudad direction.
The advantages of alloplastic genioplasty include:
- less postoperative discomfort.
- less chance of injury to the mental nerve since there is no bony manipulation and less soft tissue dissection.
- the operation is easily reversible if the patient is dissatisfied.
- simplicity: the operation is short and can be done as an office procedure.
The advantages of the osseous genioplasty:
- more predictable and stable soft tissue response.
- improvement in the cervicomental angle that cannot be achieved with an implant.
- less prone to infection.
- the option of three-dimensional movement.
The disadvantages of the osseous genioplasty:
- higher incidence of paresthesia.
- the need sometimes for general anesthesia (though it can be done under local anesthesia).
- more post-op pain.
The complications of augmentation genioplasty are extrusion and rejection, infection, dehiscence, bone resorption beneath the implant, displacement, capsular contracture, lower-lip retraction, bleeding, and paresthesia.
The complications of osseous genioplasty are wound dehiscence and infection, hematoma, tooth devitalization, neurosensory loss, soft tissue chin ptosis, dental root exposure, asymmetry, irregularities, and step-type deformities, lower-lip lag, overcorrection or undercorrection.