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March 2009 | Dental Lab Products
Kreyer on Removables


Edentulous model analysis

Proper model analysis ensures quality control throughout the treatment process.

By Robert Kreyer, CDT



Designing and creating stable denture bases requires that dental technicians understand the area of support, which is the foundation for a complete denture prosthesis.

Support is defined: “Resistance to vertical components of masticatory force in a direction toward the basel seat or mucosal tissue surface.” Stability is defined: “The quality of a denture to be firm, steady, and constant in position when forces are applied.”

Support and stability are the two most important factors to consider when designing a removable prosthesis. If minimum support exists, then stability becomes increasingly important, and if maximum support exists, then stability becomes less important and is easy to achieve. The goal of a model analysis is to understand the supporting surfaces, to assess the inclined planes (which contribute to destabilization of the denture base) and to evaluate the inter-residual ridge relationships for establishing a proper occlusal scheme.


THE TECHNIQUE IN PICTURES: SLIDESHOW
(refer to slideshow for figures)

Model Articulation

To conduct an analysis during the technical design and treatment stage, the models must be mounted on a dental articulator according to average value. A preliminary centric relation and occlusal vertical dimension must be recorded by the clinician to properly articulate the edentulous models.

A Centric Tray (Ivoclar Vivadent) preliminary record is taken and used to mount the models on an average value articulator (Ivoclar Vivadent) (Fig. A).

This preliminary record is similar to a triple tray bite registration. The blue registration material used in the maxillary and mandibular sections of the green Centric Tray is Virtual Putty (Ivoclar Vivadent), an intraoral polyvinylsiloxane.

The models are poured and mounted (Whip Mix Hard Rock Ivory and Mounting Plaster) using the magnetic double split cast technique. After the models are mounted, they are easily separated from the magnetic plaster bases.

The Centric Tray preliminary record is removed and the inter-residual ridge relationships of the edentulous maxillae and mandible are ready for analysis. In Fig. B we can see the mounted edentulous model relationship, but oral structures must be examined on the stone model to understand and verify the supporting structures that exist.

Analyzing the maxillary

The models are removed from their plaster mountings, and the maxillary model is analyzed first. The incisive papillae is outlined in blue and the midline drawn from palatal median raphe up through the middle of the incisive papillae and over the ridge to extend onto labial land area of the model. A V-groove notch is placed 4- to 5-mm to the side of the labial frenum and carved in the land area. The notch extends to the deepest point of the vestibule in the maxillae and serves as a reference for inter-vestibular measurement (Fig. C).

Fig. D shows the midline transferred down to the mandibular model. A V-groove notch is placed 4- to 5-mm from the midline, which should be directly opposing the maxillary land area notch. The mandibular notch extends into the deepest point of the mucolabial fold or labial sulcus and serves as a point of reference for measurement with a Boley Gauge.

The maxillary model is removed from the mounting again, and the first most prominent rugae is marked in the anterior palate. The point of rugae should extend to the canine position (Fig. E). Note: The U-shaped markings outline the rugae in canine position. The labial surface of a canine is typically 9 mm labial to the point of this first most prominent rugae.

A red line is drawn on the anterior crest of the maxillary ridge from canine to canine. A blue posterior line is drawn from the distal of the canine position through tuberosity to the land area, which represents the most buccal position of the maxillary posterior teeth. This helps us understand the curvature of the anterior ridge for placement of anterior teeth.

Analyzing the mandibular

The mandibular model is now analyzed by outlining the retromolar pads in blue (Fig. F), bisecting the pads with a horizontal line two-thirds up the pad and extending to the land area of model. This line represents the height of our mandibular occlusal plane.

In the mandibular anterior residual ridge crest, a red line is drawn from canine to canine position (Fig. G). From the distal of this canine position, a blue line is drawn to the first molar position. The line extends from the distal of the first molar in red up to the crest of the residual ridge to the retromolar pad. Depending on residual ridge resorption, this line will vary from case to case and may be lateral to the retromolar pad in some mandibular edentulous models.

Final analysis

The maxillary model analysis now is completed by outlining the right and left tuberosities as shown in green. Outlining the tuberosity aids in understanding the medial extent for posterior palatal seal placement and in the visual inspection of the space between retromolar pads and tuberosities.

A red line is now drawn straight from the hamular notch extending to the crest of the posterior ridge anterior to tuberosity. The blue buccal line on tuberosity represents the most buccal position of the posterior teeth and the red line on tuberosity the most palatal position of posterior teeth. If the posterior denture teeth are set too far to the buccal surface, this contributes to excessive lateral forces and will fracture the denture base palate in the midline region.

To understand the denture peripheral border horizontal dimension, a red line is drawn on the internal aspect of the land area of the edentulous model, which acts as the margin for waxing and finishing the denture border. To achieve a good peripheral seal, the vestibule must be completely filled to its biofunctional limit.

The maxillary and mandibular models are placed again on their magnetic mountings (Fig. H). Note the base of the mandibular model shows the crest of ridge outline made with a Profile Ridge Compass (Candulor USA). The vertical blue mark on the model base represents the most distal placement of posterior teeth, and the red line denotes the first molar position.

The area on the posterior residual ridge that has the deepest resorption is usually in the second pre-molar or the first molar position. The V-grooves for intervestibular measurement are lined up, and the “42” on the base represents this millimeter measurement. The 42 mm will then be split, making the maxillary and mandibular occlusal rims 21 mm each in height from the base of the V-groove notch. This will be a starting point if no other measurement exists to go by in creating the occlusal rim.

The edentulous models as seen in Fig. H are now coming to life with information. In Fig. I, the residual ridge relationship becomes evident as we can visualize a skeletal Class II and can contour the occlusal rims accordingly. The extra time it takes to complete a preliminary CR/VDO record and mount the edentulous models will save the clinician at least 15 minutes in contouring the occlusal rims chairside during the final records appointment. With this information, we also know approximate tooth placement for proper occlusal scheme.

A posterior model analysis (Fig. J) shows the posterior ridge relationship, which could be a possible crossbite occlusion. This verification and posterior view is important to developing the proper occlusal scheme.

For information on courses or consulting by Robert Kreyer Jr., CDT, go to www.PersonalizedDenture.com.