GLOSARIO DE TERMINOS:

1. Determine the occlusal plane of the dentition. Once this has been determined, the base of the lower model is trimmed parallel to the occlusal plane (Fig. 8.lA). The occlusal plane is considered to be the highest three points of the erupted teeth.

2. Trim the back of the lower model perpendicular to the base (Fig. 8.18). This is done on a near symmetrical model by measuring the distance from the distal of the first molars or second bicuspids to

Fig. 8.1A: Trim lower base parallel 10 occlusal plane

Fig. 8.1B: Trim lower back perpendicular 10 base

the back of the model on both the left and right sides. On a model that is not symmetrical, compensation must be made to keep the back as near perpendicular to the midline as possible. The midline of the lower model should normally correspond favorably with the upper mid-palatal raphae. It is generally easier to locate the midline in the maxillary cast. The back is trinuned so that it is 900to the base of the model.

The posterior surface should not be trimmed too close to the posterior aspect of the mandibular teeth, as the hamular notch of the upper model

governs the distance, this surface 'should be from the teeth.

3. To trim the back of the upper model, occlude the models utilizing the wax bite. The wax bite not only maintains the correct interrelationship but will also help to prevent fracturing of the teeth. Carefully trim the posterior surface of the occluded models so that the upper model matches that of the lower model (Fig. 8.1C). The back of the upper and lower models should now be 90° to the base of the lower model. Trim the posterior surfaces of both models until you are just posterior to the hamular notch which should be several millimeters posterior to the last tooth.

4. Trim the base of the upper model so that it is parallel to the base of the lower model (Fig. 8.1D). Trim until the top base is flat, but leave several milJimeters of extra height so the occlusal plane of the models can be centered and the models finished to the specified dimension.

The occlusal plane should be centered between the top of the maxillary model and the base of the mandibular model, when the models are occluded without the wax bite, the models being viewed from the bucca I.

5. Now we have two bases parallel to each other and to the occlusal plane (Fig. 8.1£). The backs of both the upper and lower bases are at right angles to the bases, the occlusal plane and to the mid-palatal aponeurosis.

From here onwards, the upper and lower models shall be trimmed independently of each other.

The Lower Model

1. Make the buccal cuts on the edge of the vestibule at 60° angle to balk (Fig. 8.1F). The edge of the vestibule is taken as a point approximately 5 to 6 mm away from the most prominent point of the lower canine or first bicuspid tooth (whichever is more prominent). The model is trimmed to this point provided that this does not cut beyond the deepest point of the vestibule in the region of the first or second molar. If this guideline would place the cut beyond the depth of the vestibule, then it is advisable to use the depth of the vestibule to determine the depth of the buccal cuts.

2. The anterior segment of the lower a rch is trimmed into a curve, which should follow the arc of a circle

Fig. 8.1C: With models in occlusion, trim upper back so it is flush with the lower back

Fig. 8.1D: Place upper model (on its back) on the model trimmer. Trim until the top base is flat

Ftg. 8.1E: Occlude models. Check bases for parallelism, backs for flush plane

3. The heels of the lower model are cut at approxi- mately 115° to the back of the model (Fig. 8.lH). The floor of the mouth should be leveled and smoothed to form a flat surface.

The ideal set of models will have the art portion representing approximately l/3rd of the total height and the anatomic portion approximately 2/3rd of the height.

lower model

The Upper Model

1. Make the buccal cuts to the edge of the vestibule at an angle of 60° (Fig. 8.lI) to the back of the model. 2. Make the anterior cuts so that the ends are at the

midline and approximately in the canine area (Fig. 8.ln. This cut should be approximately 5 to 6 mm from the labial surface of the anterior teeth, with both sides of equal length and their intersection on the midline.

3. The heels of the upper model are formed by

Fig. 8.1 F: Make buccal cuts, at the edge of the vestibule60° occluding both the models and finishing them such to the baite of the model that they are flush with the heels of the lower models at 115° (Fig. 8.lK). The heels of the upper model may not be the same length as those of the lower model.

4. The finished height of the occluded models should be 7 cm (Fig. 8.lL).

Fig. 8.1G: Make a smooth curve fromcanine to canine (Fig. 8.lG). The radius of this segment of the circle shou Id represent the general curvature of the lower anterior teeth. The finished model should have this curve trimmed to within 5 to 6 mm of the anterior teeth.

FIg. 8.1H: Movemodeltrimmerguide to its extreme positionto make the heei

Fig.8.11:Make buccal cuts. at the edge of the vestibule 60° to back of the model

Fig. 8.1K Occlude models. Trim upper heels so they are flush with lower heels

Fig. 8.1J: Make anterior cuts. the ends of which should be at the midlineand the middle of each canine

Commercially available plastic bases are now available which can be used to base the upper and lower model in articulation. These bases offer the advantage that they have a standard size, made of plastic and hence are easy to store. The procedure involved for using pre formed plastic bases for basing of models is as follows:

•

Fig. 8.1L: Occluded models should have a sharp

90°angle between their base and back

1. Using a pencil and the mid-palatal raphe as a reference, mark the midline of the maxillary cast. Match the mid palatal plane to the symmetric scribe line on the positioning plate. Secure the model in this position against the specially provided hooks using elastics (Fig.8.1M).

Fig. 8.1M: The positioning plate stabilized using elastics

Fig. 8.10: Guides seated into the seats of the upper model plaster base with the rest point directed towards the model and the edge upwards

2. Now take the upper plastic model base and place the dove tails into the rear guide rails with the rest point outwards and the beveled edge upwards. The upper model base is easily recognizable by the anterior intercanine square shape. Pour sufficient quantity of well spatulated plaster into the upper model base and insert the position plate along with the stabilized upper

Fig. 8.1 N: The stabilized upper model placed in the upper model base loaded with plaster

Fig. 8.1P: Lower model stabilized in articulation using

elastics

model into the guide rails until the foot of the positioning plate touches the table top (Fig.8.1N). The excess plaster should be removed and the surface smoothened. The elastics can be cut and removed after the initial setting of the plaster has taken place. Once the plaster is completely set, the position plate and the guide rails can be taken off.

Fig. 8.1 Q: The based models in occlusion

3. The guides must be seated again into the seats of the upper model plaster base with the rest point directed towards the model and the edge upwards (Fig.8.10).

4. Place the lower model into articulation with the upper model and stabilize it with elastics (Fig.8.1 P). The lower base is now loaded with a suitable quantity of well spatulated plaster and seated over the lower model with the guide rails into the seats of the lower plastic model base, until they are in contact with the rest point.

5. The excess plaster can be removed and the models finished (Fig.8.1Q).

6. The rails can be replaced with the hinges which keep the models in occlusion and act as hinge joints for demonstration purpose (Fig.8.1 R).

Finishing the Models

The surface must be made smooth, remaining at the same time absolutely flat and at right angles to the bases of the models. The finishing process should not change the dimensions or any of the angulations of the models. In the carborandum stone technique, the model is rubbed over the stone with an even pressure under a stream of water until a smooth surface results. The method is to rub the model on a frosted glass surface. After the surfaces have been finished, and the

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Fig. 8.1 R: The based models with hinges act as joints for demonstration

exact dimensions achieved, the model is set aside to dry for 48 hours or dried overnight in an orthodontic oven. At this point the model should be labeled with the patient's name and date on the backs of both the upper and lower models.

The final glazing is put on the models by immer- sing them in a commercial gloss. The models are allowed to remain in this solution for one-half hour. Holding each arch under cold water, the models are polished and soap solution removed by buffing with cotton. The models are set on their occlusal surfaces to dry for another twelve hou rs, then buffed with a very light but rapid motion using cotton. The models should assume a high, even luster which will then resist soiling while handling.

The models should be placed on a flat surface with their backs down. They should be picked up together and always returned together. Individual handling of the models is more likely to result in damage to the models.

Cast Trimming to Indicate Occlusal Plane Relationships

The gnathostatic technique, was introduced by Simon in 1962. It reproduces the inclination of the occlusal plane with reference to the eye-ear (Frankfort horizontal) plane. Trinuned in this fashion, the casts

Fig. 8.2A: Sum of the mesiodistal widths of individual incisor Fig. 8.2C: Measured molar value (MMV) in the mandibular arch

teeth is added to calculate the arch width in the posterior region

Fig. 8.28: Measured molar value (MMV) in the maxillary arch

show the approximate inclination of the occlusal plane in the face of the patient. This information is valuable, but can be gained more accurately with the use of cephalogram. Also the casts so produced are not esthetically pleasing as they lack symmetry.

MODEL ANAL VSIS