A model cast denture is a removable partial denture (partial denture, partial prosthesis), the stabilizing base of which is manufactured from a cobalt-chromium-molybdenum alloy using the one-piece casting process. In the simple case, a model cast denture (synonyms: one-piece cast denture, cast-in denture, unitor denture) is anchored to the remaining teeth by means of cast clasps, in the more complex case by means of attachments incorporated in crowns. In general, a partial denture (partial prosthesis) is used to restore a gap situation. Various schemes are available for the classification of a gap denture, for example the classification according to Wild in:
| Class | Description |
| I | Free-end gap: distally (posteriorly) shortened dentition. |
| II | Switching gap: interrupted row of teeth |
| III | Combinations of shortened and interrupted tooth rows |
A simple cast model denture anchored only with clasps offers quite a few advantages. It is:
- Comparatively inexpensive
- Little costly
- Tooth gentle in the production, because the anchor teeth for the reception of clasps only little in the area of the tooth enamel must be prepared (ground). Thus, damage to the pulp (the pulp), which is a rare complication in the preparation for crowns, is excluded from the outset.
- Removable, making it easy to clean both the teeth of the residual dentition and the prosthesis itself.
- Extendable, if the extraction of another tooth should become necessary.
However, these favorable characteristics are offset by a number of disadvantages:
- Increased risk of caries, should the cleaning of the remaining teeth is inadequate and the denture is not regularly removed for cleaning.
- Visible clasps affect the aesthetics
- Compared to attachment constructions or bridges, lower splinting of the residual teeth
- Prosthesis statics or dynamics.
- Less comfortable than cost-intensive restorations such as bridges, telescopic prostheses, or superstructures on implants
Design principles
I. Cast clamps
Clasps are not only used to hold the prosthesis against tensile forces. Rather, they distribute the loads that occur among the clasp teeth and secure the prosthesis against horizontally acting shear forces:
- The hold against tensile forces is achieved by the so-called clasp lower arm, which engages in retentive (going under itself) areas of the tooth crown below its equator. To be able to snap into this position, the forearm must have a springy design.
- Clamp shoulder and upper arm act as rigid components against horizontal forces.
- The clasp support on the marginal bead of the occlusal surface distributes vertical forces largely in the axial direction of the clasp tooth on its periodontium (tooth-supporting apparatus).
Depending on the intended function and position in the overall structure, and depending on the shape of the clasp tooth and its visibility, different forms of clasps are used. The clasps are cast together with the denture base in one piece. As a cast piece, they have less elasticity than bent clasps made of drawn wire and fit the tooth precisely. Due to the physical enclosure of the tooth (- only if the tooth is enclosed at least halfway, it remains positionally stable -) and the high accuracy of fit, the mobility of the clasp on the tooth is greatly reduced. As far as the number of clasps required is concerned, more clasps are required in periodontally damaged residual dentition than in periodontally healthy dentition in order to distribute the load as evenly as possible. And even in the case of very shallow ridges, which cannot absorb horizontal forces, more teeth must be included in the clasp construction. II. denture saddle
This rests on the mucosa in the area of the tooth gaps. The metal framework of the model casting is covered by gum-colored PMMA plastic (polymethyl methacrylate), in which the denture teeth are anchored. For reasons of caries and periodontal prophylaxis (to prevent tooth decay and damage to the periodontium), the denture saddle is designed so that it does not touch the teeth and gingival margin.Depending on the gap situation, one speaks of a shift saddle or a free-end saddle. III. palatal plate and sublingual bar
For reasons of statics, but also of safety, a cast model denture is in principle anchored on both sides of a jaw. This means that even a unilateral switching or free-end situation is braced with additional clasps to two teeth on the opposite side. The rigid connection of both sides is made bypassing and protecting the remaining teeth:
- In the upper jaw by a flat-fitting palatal plate, about one to two centimeters wide, which leaves room for the tip of the tongue in the anterior (front) palatal area
- In the lower jaw by a sublingual bar, which, following the course of the arch of the teeth at a distance from them, sublingual (under the tongue) leads from one side to the other. Due to the spatial situation, it must be much narrower than the palatal plate, but drop-shaped in cross-section, designed.
IV. Storage of the prosthesis
The aim is usually a periodontal bearing: the periodontium (periodontal apparatus) of the clasp teeth should absorb as much as possible of the loads acting on the denture saddles, thus sparing the mucosa and bony support in the area of the gaps. This is the case with a straight saddle supported on teeth on both sides. Periodontal-gingival support: The situation is different, however, with free-end gaps or a large anterior gap running in an arch. Here, additional stress on the mucosa cannot be avoided as the distance from the clasp teeth increases. In order to achieve the most even distribution of force over the entire saddle, a saddle-remote support of the clasps and a wide expansion of the saddle are selected for a free-end situation. Gingival support: A prosthesis supported purely by the mucosa is generally not a cast model prosthesis. This type of bearing is found, for example, in complete dentures or dentures with curved clasps. V. Tiltable denture
Compared to the length of a free-end saddle, the retentive area in which the clasp forearm can provide support against tensile forces is very small. This results in a rotational movement that has the clamp forearm as the center of rotation – the saddle must inevitably tilt away from the jaw when chewing sticky food. This effect is reduced by clamp supports that are designed to be as far from the saddle as possible beyond the point of rotation and thus serve as a tilt moderator. The law of the tension clamp line should be mentioned here: as a connecting line between the retention areas (i.e. lower clamp arms) of the partial denture, it runs through the center of the denture base in the most favorable case. In this way, one half of the prosthesis acts in each case as a tilt-meider for the other half. The support bracket line, on the other hand, is different: it runs through the bracket rests and inevitably forms a rocking axis if there are combined periodontal-gingival supported denture areas on both sides. In this case, the periodontal support may have to be dispensed with completely or more elaborate attachment constructions may have to be used to avoid rocking.
Indications (areas of application)
- Bilateral switching gap – all teeth to be replaced are within the support polygon (within the support bracket lines), the saddles are periodontally supported. The indication takes effect, for example, when bridges are too costly or bridge spans are too long. A patient may also refuse the preparation of healthy teeth for crowns as bridge abutments.
- Bridge planning not indicated as an alternative due to risk abutment teeth.
- Bridge planning as an alternative due to overlong bridge spans not indicated
- Free-end situation
- Free end situation combined with switching gaps
Contraindications
- Severely reduced residual tooth stock – only one tooth or two adjacent teeth left.
- Prosthetic statics or dynamics – for example, gap situation that does not allow for planning of tilt avoiders
- Insufficient retention -lack of retentive areas on the natural tooth crowns where the lower arms of the clasps could find support against pull-off forces
Before the procedure
Before the procedure, planning is carried out taking into account the load-bearing capacity of the residual dentition and the design principles for the individual gap situation.Already in the planning phase, models of the jaws can be measured with the help of a parallelometer to obtain information about retentive areas – areas of the natural tooth crown between its equator (its strongest protrusion) and the tooth neck, in which the clasps find support against pull-off forces. If necessary, the planning must be extended to include more elaborate crown and attachment constructions.
The procedure
I. Dental practice
- Preparation of the clasp teeth – preparation (grinding in) of the rests and, if necessary, grinding corrections to improve the guide surfaces (clasp shoulder and upper arm).
- Impression of the jaw to be restored with alginate.
- Impression of the opposing jaw with alginate
- Fluoridation of the prepared enamel areas.
- Selection of tooth color and shape
II. dental laboratory
- Making working models (plaster models based on the impressions).
- Fabrication of registration templates for jaw relation determination.
III. dental practice
- Jaw relation determination – with the help of the registration templates, the positional relationship of the upper and lower jaws to each other is determined and fixed.
- Facebow installation – with the help of a facebow, the position of the upper jaw is determined in a skull-like manner and transferred to the laboratory. This is particularly useful if the jaw relation (distance of the bases of the upper and lower jaw to each other) is to be changed therapeutically.
IV. Dental laboratory
- Transferring the models into the so-called articulator (device for imitating the positions of the jaws and the movements of the temporomandibular joint).
- Master model – The models are measured with the parallelometer to determine the location of the retentive areas and the direction of insertion. The course of the metal framework is marked and areas that go under themselves are blocked out. Preparation wax is applied in the area of the subsequent denture saddles as a placeholder for the denture acrylic lying under the metal base. The plate boundaries are erased (little plaster is removed).
- Doubling the master cast – Gel or silicone is used to cast the master cast. The resulting hollow mold is poured with investment material. On this investment model, the future model casting framework is first modeled in wax with the help of wax prefabricated parts.
- Attaching wax casting channels to the wax modeling.
- Embedding the model in a casting muffle
- Burning out the wax in the casting furnace
- Pouring the resulting cavity with the previously melted cobalt-chromium-molybdenum alloy.
- Ausbetten of the cooled casting framework
- Cutting off the metal casting channels, finishing and polishing.
V. Dental practice
- Scaffold try-in – check for tension-free fit and occlusion problems (final bite and chewing movements).
VI. dental laboratory
- Setting up the denture teeth in wax.
VII. dental surgery
- Waking sample – minor corrections to the tooth set-up, if necessary.
VIII. Dental laboratory
- Converting wax modeling into denture acrylic based on PMMA (polymethyl methacrylate).
- Finishing and polishing
IX. Dental practice
Insertion of the finished model cast denture, if necessary, corrections of occlusion (final bite and chewing movements).
After the procedure
The patient is advised to attend regular follow-up appointments to have any necessary repairs or relinings that may be required due to the recession of the alveolar ridges caused by the stress of the denture.