Custom impression trays

Made by a technician for an individual patient and only used for that patient.

Custom trays are constructed by the technician on a preliminary model. The materials which can be used for the construction of a custom tray are as follows:

1. Acrylic resin – ‘self cure’ or ‘heat cure’

2. Pressure or vacuum formed themoplastic

Compounds

3. Light cured acrylic

Advantages of custom trays

1. Even thickness of impression material.

2. Controlled distortion of tissues.

• Muco-static: pertaining to the normal, relaxed condition of mucosal tissues covering alveolar ridges and denture related surfaces.
• Muco-compressive: pertaining to pressure on mucosal tissues covering alveolar ridges and denture related surfaces.

3. Even support of the impression material.

4. Control over the flow of the impression material.

5. Reduced discomfort for the patient.

6. Economic use of the impression material.

7. Reduced distortion during pouring of model due to support of impression material.

Factors determining the dimensions and accuracy of the edentulous impression

• Pressure applied to the impression
• Flow of impression material
• Setting time
• Accuracy
• Dimensional stability
• Reproduction of detail
• Contact Angles
• Deformation and Recovery of the denture bearing mucosa

To obtain an accurate record of the denture bearing mucosa the following should be considered:

• Mucosa should be firm and healthy
• Impression material should be of low viscosity
• Pressure used to seat and hold the impression material should be kept to a minimum

Functional impression.

The aim on each patient is to fashion the impression surface and the peripheral rolls of the dentures in such a way that support, retention and stability of the denture are maximized

• No pressure / mucostatic concept.
• Pressure / mucocompressive concept.
• Minimal pressure concept.
• Selective pressure concept.

Mucostatic theory (based on Pascal's law) sets out to record the mucosa in its static (supported by underlying basal bone), undisturbed form. This is possible only if the impression material is watery and virtually requires no pressure to place it against tissues. Such an impression will not cover enough area to afford retention, stability and esthetics of a denture.
Mucocompressive theory claims to record the tissues in their functional / supporting form so as to achieve stability in occlusal function. This concept is not very encouraging since it seeks to subject the tissues to a continuous pressure which is conducive to resorptive changes in basal tissues. In addition to this, displaced tissues tend to displace the denture in their attempt to return to their original form.
Minimal pressure theory is a compromise between the previous two. It advocates application of minimal possible pressure which is supposed to be little more than the weight of free flowing material.

Last but not the least is the selective pressure theory which is the most widely respected and accepted theory. Here, the idea is to vary the pressure over the denture seat (which is a single unit) depending on the displaceability of the supporting tissues and hence transferring the load over to the selected areas of the seat e.g. buccal shelf area (assuming that it is better suited for the purpose).
The method proposed for achieving selected pressure is by altering the spacer thickness and hence material thickness in selected areas.

Method for making impressions should be based on the basic principles of Maximum area coverage and Intimate contact so as to achieve the objectives of:

• Retention
• Support
• Stability
• Esthetics
• Preservation of ridge (supporting structures).

The denture-bearing surface comprises:

• Periphery - covered by lining mucosa
• Main seat - covered by masticatory mucosa

The aim of the functional impression is to maximize the rest area of the denture base taking the muscle movements into consideration.

In order to anchor the complete dentures on the edentulous jaw, a suction effect must be created between the denture base and the mucosal surface. This is achieved by means of the marginal accuracy of the denture base and the resulting cohesive and adhesive forces. In order to maintain the suction effect also during the speech and chewing functions, it is necessary to fully model the edges of the denture base, the inner and outer functional margins.

Before taking the impression, the future denture bearing tissue must be in a recovered state, i.e. that previous prosthesis must not have been worn for at least 24 hours. For the functional impression, individual impression trays are used, which are manufactured by the dental technician on the first working models, the anatomical models.

Before manufacturing these, information is required on the impression material with whichthe functional impression is to be made in order to manufacture accurately fitting impression trays in the case of a material with low viscosity, manufacture impression trays with a uniform space between them and the model in the case of a material with high viscosity.

The impression trays should ideally be made of clear acrylic in order to check their

accuracy of fit and make corrections if necessary. Furthermore, the impression tray

material should be sufficiently hard and torsion-free.

The expansion of the impression trays must be less than the surface of the future denture bearing tissue, and plenty of space should be left around the lip and cheek tendons as well as the tongue tendon.

Impression Materials: see Biomaterials

When the impression of jaw is made, technician makes models, outlines a border of the base of removable partial denture by a pencil and makes wax base with occlusion rims for centric occlusion registering.

Laboratory stagesin the plastic removable partial dentures making.

When the impression of jaw is made, technician makes models, outlines a border of the base of removable partial denture by a pencil and makes wax base with occlusion rims for centric occlusion registering.

MODELS

Models are accurate plaster reproductions of the teeth and their surrounding soft tissues.

Models consist of two parts:

1.Anatomic portion and

2.Artistic portion.

The anatomic portion is that part of the model which is the actual impression of the dental arch and its surrounding structures. This portion is usually made of stone plaster. The artistic portion of the model consists of a plaster base that supports the anatomic portion. This portion of the model helps in depicting the actual orientation and occlusion of the models and also gives a pleasing and symmetri­cal appearance to the models. In a well fabricated set of models the ratio of the anatomic portion to artistic por­tion should be 3:1.

The steps involved in the construction of model include:

1. Impression making

2. Disinfection of the impression

3. Casting the impression

4. Basing and trimming of the cast

5. Finishing and polishing

Materials for self-control:

A. Tests for self-control (tables, diagrams, drawings, graphs)

Materials for self-control:

A. Tests for self-control (tables, diagrams, drawings, graphs)

Task for self-control:

Draw a diagram of the structure of clinical examination methods.

Date: 2015-12-24; view: 1161