## 5 Discussion  
The use of virtual CAD/AM techniques allows achieving an accurate and realistic vision of the surgical plan in orthognathic surgery and helps the surgeon to evaluate in a precise and accurate way how to perform the surgery itself. The added value of the VE (virtual environment) as an environment in which CAD/AM techniques have to be used provides the capability to create a new orthognathic surgery methodology by integrating and/or modifying the conventional surgical procedures with a useful reduction of probability of error.  
Thanks to the proposed interactive methodology, the surgeon can carry on the surgery without using dental splints that need to be managed and manually held in the correct place during all segments' repositioning. The virtual environment is able to provide the surgeon an easy way to set their operative field and to plan the best way to conduct the surgery; on the other hand, the easy way of interaction allows engineers to design and realize the best template for performing the surgery, avoiding the errors of manual surgery procedures.  
This implies a lighter procedure with less probability of errors and, at the same time, requires even the integration of high-resolution scans with the computer tomography of the bones.  
As demonstrated, the errors that can be committed while using correctly the procedure were always less than one millimeter, thus being better than the most precise manual surgery that can be performed by an expert surgeon.  
On one hand, modeling software, like Rhinoceros, presents some limitations regarding the solid modeling of complex structures because it cannot manage the Boolean operations, like mathematical/logical intersection with solids having complex shapes. This is one of the biggest limitations of this procedure and is closely linked to the dichotomy between a performing (in terms of graphics and of time of elaboration) use of CAD systems (light mathematics, few geometries, good representation) and a very good representation of complex shapes (high ranked mathematics, very high number of geometrical entities, high resolution in representation). Additionally, the right setup for AM systems could be a \"noise factor\" that can cause errors and cost increase; this aspect has to be taken into account when following the whole proposed workflow.  
Another limitation has to be highlighted in the use of a special kind of plate: despite some authors [37] considering that the best configuration for the stability of the plate and screws is the one having an inverted L configuration, our methodology proposes only the use of linear (with aligned holes) plates. However, this can be easily improved.  
Another advantage related to this procedure is the exact evaluation of the transversal intercondylar distance variation after mandibular arch advancement/set back because of the triangular shape of the mandibular arch and the telescopic mechanic of the sagittal split osteotomy. By three-dimensional reconstruction of the mandibular shape and bone fragment movements after segmentation, a precise amount of bone fragments overlapping, in the case of mandibular arch advancement, or diastema in case of set-back, can be calculated. Following this three-dimensional evaluation, proper surgical planning can be developed, selecting bone segment movements compatible with surgical and anatomical limits.