Discusión

Intermediate surgical splint in bimaxillary orthognathic surgery:
A simple method for obtaining it
Férula quirúrgica intermedia en cirugía ortognática bimaxilar:
Un método simple de obtención

One of the most sensitive aspects regarding treatment planning of dentofacial deformity concerns model surgery. In this procedure, the objective is to transfer to the articulator the consensual movements of the skeleton from the clinical and radiological analysis. These movements will allow the surgeon to reproduce subsequent surgical developments and produce the splints that will contain the information regarding the movements planned. Various well-known devices have appeared over the past years such as «surgical simulators» that allow the maxilla and mandible, once articulated together and to the skull base, to be displaced tridimensionally, and in a differential manner, as a result of micrometric screws.¹

All these devices, presuppose the previous assembly of the maxilla and mandible in a semi-adjustable articulator using the «face-bow transfer».^2,3

This transfer entails adopting a common reference plane, which allows all the elements of the analysis to be integrated in a precise and reliable way. The Frankfort plane, which is defined as porion and orbitale, and corrected according to the natural position of the head, serves as a reference plane for planning and modifying the maxillomandibular position with according to a stable reference.

The models mounted in the semi-adjustable articulator, simulate the facial structure of the patient from the functional and structural point of view, using the previously mentioned Frankfort plane as a reference. That is to say, maxilla and mandible -the elements to be modified- are related with the base of the skull -which will not be modified-.

This type of assembly allows the correlation of movements that have been planned clinically and radiologically with those performed, in such a way that the skeletal elements that are changed (maxilla and mandible), are displaced in relation to this reference plane.⁴

All authors acknowledge that this method requires experience and reproducibility on transferring of the face-bow. The precision of this method is related directly with the precision in assembling the articulator, and with the adequate transfer of the radiological planning to the reference plane of the articulator -upper arm-.⁵

The authors of the article that concerns us, try to simplify the repositioning movement of the maxillary model, using as a reference not the horizontal plane (Frankfort, upper arm of the articulator), but the mandible, assuming that the position of the latter is stable during the simulation process. They indicate that «the assembly position of the maxillary model is of little consequence.» This is precisely where the error lies as to how the system has been devised. What is of particular importance in repositioning surgery of the maxilla – either of the models or in the operating theatre – is the position of the latter in relation to the rest of the normal face, and not with regard to the mandible, which in a large proportion of cases will be badly positioned and will also change during the procedure.

This fact becomes especially relevant in cases where there is transverse canting of the occlusal plane. Here what is of particular importance is levelling the maxilla -segmented or not- in relation to the face -horizontal plane, upper arm of the articulator, face-bow- and not in relation to the mandible which in these cases will also have been altered.

The authors indicate that «the maxillary movements that we are going to reproduce are of and independent type» and that «Therefore, the assembly position of the maxillary model is of little consequence» If there is anything of prime importance in the planning of maxillomandibular movements, it is the position of the maxilla -or of the mandible when inverted surgery is carried out- regarding the rest of the face. The objective in maxillofacial skeletal surgery is not only to normalize intermaxillary relationships, but also the maxillamandible relationship with regard to the rest of the face. Reaching the second objective would be difficult if in the planning [procedure] the face and maxillo-mandibular complex have not been related.

The authors indicate that, with regard to their device, «the upper support acts as the palatal plane or the base of the nasal pit». The model of the maxilla is fixed to the support with the adequate angle according to cephalometry, in such a way that the palatal plane remains adequately related to the occlusal plane of the maxilla. The error in this point consists in taking the palatine plane as a reference and not the Frankfort plane, when it is the former and not the latter that remains unmodified. In addition to this, the palatal and occlusal planes form part of the maxilla and they will move with it. It is therefore obvious that the palatine plane is not a stable reference.

The reference plane has, in short, to be the same in the facial analysis, in the STO or graphic simulation of the skeletal movements, in the model surgery and finally in the execution of the surgery. That is to say if we want to level the maxilla in relation to the face, we cannot leave it aside on carrying out model surgery, and making the midlines coincide as indicated by the authors is not enough, as they are just one element in the planning [procedure].

In the discussion of their article, the authors indicate that: «we know that without the use of the splint the operative results will be correct if the movements regarding changes in position are duly checked intraoperatively». We cannot agree with this assertion. Intraoperative verification of tridimensional movements, in addition to prolonging surgical time unnecessarily, significantly increases the time for accumulating errors.⁶

In the near future, the planning and execution of skeletal movements will be done combining 3D skeletal soft tissue information with navigation systems.^7,8 But, even in this case, the navigator will need to have three reference points that are both constant and stable during the complete process.

Federico Hernández Alfaro
Cirujano Oral y Maxillofacial
Centro Médico Teknon, Barcelona, España