A novel guide for minimally invasive genioplasty

Mentoplasty or genioplasty is a simple procedure to enhance facial aesthetics. While usually considered simple, it might present certain difficulties during execution, especially when a minimally invasive approach is preferred. Drilling and cutting guides are a valuable tool to overcome such challenges but usually require larger incision and dissection to allow adequate positioning and bone rigid fixation. In this article, we describe a novel guide design that permits adequate bone repositioning while preserving minimally invasive protocols.


Introduction
Mentoplasty or genioplasty is a simple procedure used to alter chin position.While usually considered simple, it can greatly affect facial aesthetics and therefore its technical details and difficulties should not be minimized [1][2][3].
Some aspects can make the procedure particularly difficult: Dense bone to perform osteotomies, branches of the inferior alveolar nerve that can be injured during surgery, and the difficulty to perform a symmetrical osteotomy.Also, the repositioning of the osteotomized segment may pose a challenge, since it can be relocated in the three planes of the space, particularly when asymmetric movements are necessary.To overcome such difficulties, surgical guides present as a useful tool, but usually requires a larger incision and dissection [4][5][6].
"Minimally Invasive Surgery" (MIS) and "Fast-track Protocols" have gained popularity in several medical specialties, including Maxillofacial Surgery.In the orthognathic field, a minimally invasive approach includes modified osteotomies, reduced incisions, and specific bone fixation, allowing for a faster procedure and reduced recovery time [7][8][9].
Under such philosophy, difficulties of genioplasty are greatly improved due to a reduced surgical access, which limits adequate visualization and forbids the use of the standard genioplasty guides.The aim of this article is to present a design modification that allows the use of surgical drilling and cutting guides while maintaining MIS principles.

Technical description
Virtual surgical planning is performed in the usual manner, with particular attention to bone surface quality since adequate adaptation relies on the correspondence between the virtual planning and the surgical field.Once the DICOM files are segmented, head position is adjusted and dental casts STL files are positioned over the occlusion to create the composite skull.The osteotomies are then simulated, and the necessary movements performed.
To place the guides in the exact position without the need of a large bone contact area, our preference is to first use an occlusal-supported drilling guide (G1) (Figs. 1 and 2).The use of a drilling guide allows to minimize incisions and dissection.The drills performed by G1 allow adequate positioning of a cutting guide (G2) (Figs. 1 and 3) that is fixed in position with monocortical screws.To maintain a reduced surgical approach, G2 is segmented in the midline.A fifth drilling is performed, by the midline, as demarcated by G2 and then the osteotomy is performed with use of a reciprocating saw, using the superior portion of G2 as a guiding plane.To preserve the MIS concept, guides only demarcates the main region of interest, since small alterations at the final portion of the osteotomy will not have any clinical significance (Fig. 3).
Once osteotomies are completed a T-shaped guide (G3) repositions the distal segment using the previous drilled holes and monocortical screws, while keeping a lateral inferior hole free (Figs. 1 and 4).A long bur completes the drilling at the proximal segment and a bicortical screw is then inserted.Using the same bur, the contralateral inferior part of the guide is fractured, and another bicortical screw is inserted, securing the chin at the planned position (Figs. 4  and 5).

Discussion
Mentoplasty is a valuable technique to enhance facial aesthetics.While frequently indicated, certain aspects of the surgery can make adequate repositioning of the chin difficult, such as bone density, adequate visualization, and difficult bone repositioning [1][2][3].
The use of drilling and cutting guides greatly reduces repositioning error.However, such appliances usually require larger incision and dissection to be adequately used.This is necessary to ensure an adequate positioning in the bone surface and place for plate positioning [4][5][6].To maintain MIS concept, the usual design had to be modified into a shape limited to the central portion of the chin.The greatest challenge at this approach is to keep the distal segment adequately positioned and permit rigid bone fixation, while maintaining a small surgical access.To reach this goal, the authors utilize bicortical screws and a breakable guide, and to the best of our knowledge, this is the first report of such a solution.
Recently, patient-specific guides and fixation plates has been reported with several advantages on its use, such as splintless surgery, absence of vertical reference and  improved control of the condylar segment.Also, implants and guides can be modified to respect MIS principles [7,10].MIS techniques have greatly gained popularity due to a faster recovery period, less swelling, and minimal scarring, allowing to natural lip function.However, the use of patient specific implants is not wildly available and greatly increases surgery costs, which may pose a problem, especially in developing countries.
In conclusion, the authors consider that MIS principles in genioplasty greatly reduce post-operatory discomfort and scarring while also preserving lower lip mobility.The use of a modified drilling and cutting guide is an adequate solution for surgeons seeking to ensure symmetric mies, adequate bone repositioning, and a minimally invasive approach, allowing the same bone stability obtained through more complex and aggressive techniques.

Fig. 1
Fig. 1 Drilling and cutting guides used

Fig. 3 Fig. 4
Fig. 3 Use of a cutting guide (G2), fixed in the inferior holes performed by G1