Caso Clínico

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

 

J.V. Pascual Gil¹, M. Marqués Mateo², M. Puche Torres², L. Miragall Alba², M.E. Iglesias Gimilio²

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Abstract: The employment of the intermediate surgical splint in bimaxillary orthognatic surgery is a common procedure. The utility, aid and confidence which are provided are inquestionable. Anyway the classic procedure of attainment of the splint is complex. We propose to simplify the manufacturing of the splint without renouncing to it´s accuracy and reliability. We must question some concepts about the attainment method of this splint and consider the following basic concept: the vertical preoperatory craneomandibular dimension is the same as the vertical postoperatory craneomandibular dimension with the splint in position. To manufacture it we have designed a new device called: Maxillary Placet. Key words: Intermediate surgical splint; Bimaxillary orthognatic surgery.

Resumen: La utilización de la férula quirúrgica intermedia (FQI) en las intervenciones de cirugía ortognática bimaxilar es un procedimiento habitual. La utilidad, ayuda y confianza que aporta es indiscutible. Sin embargo, el procedimiento clásico de obtención de la FQI es complejo. Proponemos simplificar la fabricación de la FQI sin renunciar a su precisión y fiabilidad. Debemos cuestionar algunos conceptos del método de obtención de esta férula y reflexionar sobre el siguiente concepto básico: la dimensión vertical craneomandibular preoperatoria es igual a la dimensión vertical craneomandibular postoperatoria con la férula en posición. Para confeccionarla se ha diseñado un nuevo dispositivo denominado: Posicionador del maxilar. Palabras clave: Férula quirúrgica intermedia; Cirugía ortognática bimaxilar.

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1. Jefe de Servicio
2. Médico Especialista
Servicio de Cirugía Oral y Maxilofacial.
Hospital Clínico Universitario. Valencia, España.

Correspondencia:
Dr. José V. Pascual Gil
c/ Alemania, 6
46010 Valencia, España.

Introduction

The intermediate surgical splint is the dental occlusal splint used in bimaxillary orthognathic surgery, which places the upper maxilla that has been osteotomized into the new desired position, resting on the mandibular dental arch. The reliability of the ISS, that is to say: the accuracy of the information that it provides, should be very high; otherwise, at the most important moment of the surgical intervention (positioning of the maxilla and osteosynthesis) it would have an adverse effect, giving rise to doubts and uncertainties. The procedure for obtaining an ISS is tedious, complex and errors can frequently be made.¹

The method we are proposing is based on a fundamental concept: the vertical dimension between any point of the maxilla (situated above the osteotomy) and of the mandibular dental arch should be constant, in the preoperative central position of occlusion as well as in the postoperative position obtained, with the ISS in place. This could be summed up as: the preoperative vertical craniomandibular dimension is equal to the postoperative vertical craniomandibular dimension with the splint in place (Figs. 1 and 2).

If this affirmation is put into practice, relying on the fitting in an articulator (either anatomical or semi-adjustable) will not be necessary, as the concept of the relationship of space with the temporomandibular joints will not concern us, and condyle movement to an even lesser degree. Having some sort of fixture that, with the mandible in a constant position, permits the movement of the maxilla in an anteroposterior direction, and up and around, and which complies with what has been projected in the preoperative assessment, will suffice.

All this has motivated our interest - which is our objective - in obtaining an alternative method for manufacturing the ISS that will add simplicity without sacrificing reliability, which is essential. We have achieved this by using a specific device.

Material and Method

We have called this device: «Maxillary positioning device» (Fig. 3). It consists of two (upper and lower) platforms of poly-acrylic acid that are structurally independent.

The upper platform has a perforated canal so that a third, smaller platform can be incorporated into its lower surface, which will act as the support for the maxillary model. This can be moved along while attached to the platform above it, while positional changes can be made in the horizontal plane, along the anteroposterior axis as well as sideways. The perforated canal of the upper platform moves along the anteroposterior axis. It is positioned in the center and starts from a 0 position with a scale in millimeters which goes up and down from this point. The zero position is in the center of a cross which is marked on the platform, there being also a cross in the geometric center of the bracket which will support the upper model. Both crosses must be one on top of the other in the initial position as they will serve as a reference in order to carry out anteroposterior changes and a sideways motion (in the hypothetical case of there being a deviation from the midline).

The lower platform has three posts or columns with heights that can be altered as wished. They serve as a support for the upper platform so that it can be moved in the vertical plane (together with the maxillary model).

The columns allow vertical changes, either parallel to each other or with anteroposterior inclinations or lateral movements.

Movable millimeter scales at the level of the anterior incisors or last molar group, put into the grooves for this purpose, will indicate the number of millimeters, either up or down, which we have to impact + the maxilla.

Moving the three posts alternatively will be necessary so that the dimensions are adjusted adequately as angular measurements are involved, and on moving each one the height of the others is altered; two or three adjustments of each post will be needed in order to achieve the changes planned.

Description of the process

First we need dental models that are up-to-date and a register in centric occlusion. It is important that these are obtained a week prior to the operation.

Vinyl-polysiloxane putty should be used as impression material. This is putty that has two components. It sets on its own after being molded and it stains neither hands nor objects, and it has a very acceptable handling and setting time. It can easily be cut with a bistoury. It has a rate of dimensional variation that is practically nil, and it resists compression perfectly, especially if it is balanced. Once set it has considerable consistency and it hardens to such a point that it can loosen brackets.

Conventional trays that are used for making dental models have high vestibular and lingual sides that are designed precisely so that the material (which is generally alginate) completely encompasses all structures. This is exactly the opposite of what+ we require. For this reason we advise a modification of the trays which entails reducing the height of the sides to the minimum (1 or 2 mm) and giving them numerous retentive holes for the material. This is easy to achieve if we use trays made of disposable plastic, and if we cut the sides according to our requirements with cutting pliers (Fig. 4).

We put the putty, under direct vision, which has been mixed previously and which has a thickness of no more than 2 to 3 mm into the mouth pressing against the dental arch in such a way that it does not cover or is retained by the different orthodontic additaments. We obtain molds with a very flat relief at the borders that look like a «biscuit», which will be difficult to empty using plaster. In order to do this we construct a small strip with a little bit more putty that we put around the dental impression and, at the same time, we fill in what is missing of the palate, and then we proceed to empty the mold in the conventional way (Fig. 5).

The models we obtain will be questionable from the odontologic point of view; the anatomy relating to the dental crown will hardly be reproduced; they will not have a base or a vestibule and the final surface will be irregular. A record of the bite in centric occlusion is taken; that is to say: preoperative pathological occlusion, with the condyles in a centric relation (the physiological position furthest back, or posterior hinge axis) (Fig. 6). For this the putty previously mentioned is used.

We assemble the upper maxilla

We should consider that the movements that we are going to reproduce are of an independent type: that is to say: they are related only with the mandibular arch. Therefore, the assembly position of the maxillary model is of little consequence. Nevertheless, we will guide ourselves by anatomical criteria.

We know that, if we impact the maxilla or otherwise, or if we simply bring it forwards, there is a common circumstance in any surgical procedure: the inferior plane of the maxillary osteotomy will pass through the base of the nasal pit. This base of the nasal pit is what in cephalometry is called «palatal plane» (anterior nasal spine – posterior nasal spine). We know that, in addition to this, the palatal plane forms an angle with the occlusal plane, which according to Steiner was 3 to 4 degrees.

We will mount the maxilla into its support according to this angle (which will be different in each patient’s pathology). We could say that the upper support does the same job as the palatal plane of the base of the nasal pit. Once this angle is known, it is easy to place the putty between the upper model and the support and to press down on both of them, visualizing them from the frontal and lateral planes, in order to obtain the occlusal-palatal angle of the patient. The incisor midline of the model will of course be over the anteroposterior line of the support, or it will be further away depending on what occurs with the dental midline of the patient and his facial midline.

Once set, the support (with the model incorporated) is attached to the upper platform and, lining up both crosses we secure the unit using the screw for this purpose (Fig. 7).

The upper platform is placed on the three columns and both platforms are leveled down to point zero (they will be parallel), using the millimeter scale. The whole unit is turned over (the space position is inverted), the platform supporting the columns is removed, the preoperative records of centric occlusion are put into place, and we position correctly the lower model «over» the upper one (Fig. 8). Next, we put enough putty on the base of the lower model and we bring it down onto the lower platform in such a way that the columns fit into the grooves as planned. Once completely set, the positioning device is put upright. The mounting in the preoperative position has now finished (Fig. 9).

At this point what is supporting the upper part of the model to the upper platform is loosened and we can start making the horizontal type movements envisaged for the maxilla, which can be advancing it the millimeters needed, or displacing it the millimeters (in the vertical plane of the incisors) needed to place the midline in an adequate position.

We then make the changes planned in a vertical direction. We place the millimeter scales (two on each side) in the lateral transverse position of the incisor group and posterior molars, inserting them into the holes that we consider most adequate.

The «zero» position of the four scales should coincide with the level of the upper platform so that both platforms are parallel. We move as we wish the three adjustable posts until the scales indicate the number of millimeters that we have in mind to impact the maxilla, either in the anterior or posterior position, or parallel to itself.

If the clinical evaluation indicated that the maxilla needs only to be impacted, making horizontal movements with the maxilla support will not be necessary. If on the contrary, vertical movements (impactation) are not contemplated by the evaluation, the posts will not be moved and only the maxilla support will be moved as required. Once all the changes have been made regarding the positioning of the maxilla (Fig. 10), the ISS can now be made. We should at this point remember the two concepts that we stressed as being fundamental in the introduction:

• The craniomandibular dimension remains constant.

• The mandible has not been moved at all.

We raise the upper platform that contains the support with the model of the maxilla, we mix the putty and, molding a long cylinder in the form of a dental arch and we place it on the mandibular dental arch. We assemble again the upper platform with the maxilla, making the vertical columns coincide with their respective sockets, in such a way that the upper dental arch leaves an impression on the putty. We gently keep applying constant pressure until the mixture has set.

We remove the splint we have just obtained, cutting the overlapping edges, until we can see that the cuspids and the incisor borders fit perfectly into the impression base of the splint that has been placed into position (Figs. 11 and 12).

The ISS obtained can be subjected to cold sterilization perfectly, using the liquids designed for this, and it will resist and not be deformed by any compression it may be exposed to.

In the surgical act, once the maxilla has been suitably mobilized according to plan, we secure everything: maxilla- ISS-mandible with simple wiring (Fig. 13).

Next, moving the mandible to its posterior axis of rotation, we raise the whole unit obtained until it has adjusted to its new, predetermined position. If at this point the craniomandibular segments (which in the accompanying drawing we have called X and Y) coincide with the current position, and if the other references comply with what was projected, the ISS will demonstrate how good this surgical procedure is. It has ensured that what was planned has been achieved and -most importantly- it will allow a «repetition» of the same position providing a stability which will facilitate enormously the application of osteosynthesis.

Discussion

In bimaxillary orthognathic surgery the preoperative evaluation of the patient is absolutely indispensable, independently of the use of the ISS;^2,3 but the latter, even though perfectly made, can be extremely dangerous in the surgical information it provides, if the evaluation regarding the therapy to be carried out is not correct. The concept of the ISS goes back various decades and the use, help and confidence that it provides is undisputable.^4-9 Nevertheless 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.¹ We can therefore assert that the ISS it not indispensable but it is very convenient.

Unfortunately, obtaining an ISS through a laboratory is so complex that there may exist some uncertainty as to how practical it is.¹

To summarize, the normal procedure would, need to include:

• Dental models of the patient and wax bites in preoperative centric occlusion (first occlusal contact with the condyles in the centric relation position).

• Making an impression of the face bow for mounting on a semi-adjustable articulator.

• Mounting on the articulator, with a «high» upper element, (future marks and cut outs) and an incisal pin a few mm higher, if impacting is envisaged.

• Smoothing down and preparation of the surface of the upper element, vertical and horizontal reference marks; measurements of the distance of both axis.

• Cutting out lines of movement and losses of bone substance (plaster) in the upper element, according to the requirements of the case that have been studied.

• Relocation of the upper model to the predetermined position, according to different procedures

• Modification, or not, of the incisal pin (depending or not if there is to be surgical impactation).

• Preparation of the ISS with autopolymerizing acrylic.

Clinical recording the face bow is not an easy thing. It has to be done with great care; the position of the olives of the condyles is, at most, approximate; using the whole unit (arch, bite plate, infraorbital notch) is delicate, fragile and it is very easy for there to be small alterations that may be otherwise unnoticeable.

The same can be said for mounting the models on the articulator. At the best of times the appearance of the mounting table (once the process has finished) is quite deplorable, and the time involved in obtaining the mold is considerable, even if quick setting plaster is used.

The horizontal and vertical lines traced across the upper element are very uncertain and therefore liable to too much subjectivity; one way of avoiding this is to use procedures for moving the cast by means of mechanisms within the articulator, but this does not avoid any of the previous procedures.¹⁰

If the height of the incisal pin is modified, in cases where the maxilla is to be impacted, in order to achieve a reduction of the gap when in occlusion, we are playing with a dangerous concept: the axis of rotation of the condyle. We should not forget that we are working with a semi-adjustable articulator (Class II, type 3); we therefore cannot depend on a reliable hinge-like posterior axis. This would not have been the case had we used a dynamic face-bow with a pin with a «fixed swing» consistent with the condylar axis, or by means of pantographic tracings (adjustable articulator, Class 1). Therefore, any change that is made to the vertical dimension will be incorrect, and the reliability of the splint obtained will certainly be negatively influenced.^11,12 Lastly, making the splint in autopolymerizing acrylic will mean that, once cast, it will have dimensional changes which are too imprecise bearing in mind its parabolic shape. These changes will not take place if the mix we have recommend is used.

With the maxillary positioning device we can move the maxilla to any position indicated in the preoperative assessment. We can advance it a certain number of millimeters, we can turn it so that the anterior midline is displaced as we wish, we can impact it in either a uniform fashion or tilted either backward or forwards, and even impact it in an asymmetrical fashion (more on one side than on the other). It will only be unsuitable in those cases in which the maxilla has to be brought down as, on moving the mandibular position, the condyle relationship changes (very few cases).

We have carried out bimaxillary surgery moving the upper maxilla, either for vertical impactation (in a symmetrical or asymmetrical manner), either for protrusion, or for modification of the midline; we have made these alterations combining them in different ways and with different techniques. In all cases the ISS that we are recommending has provided total reliability and precision, providing all the steps described for making it have been followed faithfully.¹³

Conclusions

We have described a method for making an ISS that requires only a maxillary positioner, a material (vinyl-polysiloxane putty), a bistoury and not more than ten minutes work, of which more than half is setting time. The plaster will only be necessary for obtaining dental models.

The system is simple, easy, clean, fast and totally reliable.

References

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