Biophysical (functional suction in complete dentures).

Atmospheric pressure - Vacuum

Atmospheric pressure has been claimed to be an important factor in complete denture retention. For atmospheric pressure to be effective, it must operate under condition of a pressure difference - (de) pressure. There must be a lower pressure beneath the dentures and only if vacuum were there the full effect could be felt.

Atmospheric pressure can act to resist dislodging forces, if the dentures have an effective seal around their borders. This is called ‘suction’ because it is the resistance to removal in a direction opposite to that of insertion. But there is no suction or negative pressure, except when another force is applied.

Under the assumption that vacuum could be generated by exerting a pull that tended to increase the volume beneath the base of the denture and the tissue, the lower pressure would have to be generated by external means and a perfect seal created and maintained around their entire borders for the lower pressure to be sustained.

Denture stability is a quality of a denture that permits it to maintain a state of equilibrium in relation to its tissue foundation and/or abutment teeth. Denture stability is the denture’s ability to remain securely in place when it is subjected to horizontal movements. This occurs during the functional force of chewing, talking, singing, whistling, etc.

-to be stable, the denture requires the following:

-good retention

-non-interfering occlusion

-proper tooth arrangement

-proper form and contour of the polished surfaces

-good control and coordination of the patient’s musculature

Retainers- prevent displacement of the denture during function

Direct Retainer (clasps).Clasp was introduced by Mouton in 1764.

Requirements of a Clasp

1. Retention

- Resistance to dislodgment in an occlusal direction .Provided by the clasp tip engaging the undercut when a dislodging force is Bracing (Stabilize)

2. Resistance to forces from a lateral (sideways) and anterior posterior direction. Provided by the arms and occlusal rest

3. Passivity

When a clasp is in place on a tooth, it should be at rest. Its retentive function is activated only when dislodging forces are applied

Clasp types

a. Cast metal - Fabricated using wax or plastic patterns

b. Wrought wire

- commercially supplied wire of various gauges

- adapted to cast by bending with pliers

Parts of a Wrought wire clasp (Fig.1)

a.Clasp arm

b.Shoulder

c.Tale

Indirect retainers

1.A part of a removable partial denture framework located on the opposite side of the fulcrum line from tipping forces; designed to counteract those forces

2. Distal extensions create most tipping forces

3. May be auxiliary rests, part of a clasp assembly, lingual rests under lingual plating, or major connector

Fig.1 Fig.2.

The arm (Fig.2-1) must lies in an undercut to grip the abutment tooth and provide retention. The arm must have enough length for flexibility to spring into the undercut. The arm must lie between an equator and gum, touch not gum, densely adjoins to the tooth.

The shoulder (Fig.2-2) is a part that lies above the height of contour on the proximal surface of abutment teeth adjacent to edentulous spaces. The shoulder connects the arm and the tail of clasp. The shoulder curve bucco-liqually. A shoulder of the wire clasp must be springy.

The tail (fig.2-3) lies in the base of denture above the alveolar ridge under the artificial teeth. The tail retains a clasp in the base of plastic RPD.

Clasps are divided

1. By the form - on round, half-round, flat (band).

2. By the method of making - wrought, cast, soldered

3. By the placing on the teeth and gum – dental, gingival and dental-gingival.

4. By the degree of scope of the tooth or teeth - one - arm, double - arm, ring-like.

5. By the function -retaining, rest.

A rest is a component of the metal framework that lies on the occlusal or incisal of an abutment tooth. The purpose of the rests is to provide vertical support for the RPD at each end of the edentulous space. Rests stabilize the PRD by controlling its vertical position. Rests prevent gingival displacement of the RPD.

A retentive clasp must lie in an undercut to grip the abutment tooth and provide retention.

6. By the material - from steel, gold, plastic.

7. By the method of connection with the base of denture - rigid, springy or semimobile, arthral or mobile.

There are such types of retention depending of the quantity of points of retention (by Wild).

1. Point - a 1 clasp is used.

1. Linear - 2 clasps are used. A pair of clasps creates a clasp axis. Line which connects 2 clasps in denture is called clasp axis. There are such types of clasp axis: diagonal, transversal and sagital.

Definition: FULCRUM LINE AXIS (sometimes referred to just as FULCRUM LINE) An imaginary line, connecting the most distal occlusal rests, around which a removable partial denture tends to rotate TOWARDS the tissue under masticatory forces.

Definition: RETENTIVE FULCRUM LINE AXIS – Movement of the base AWAY from the ridge around an imaginary line connecting the retentive clasp tips. (This is the axis relevant to indirect retention.)

Fig.3.

If a transversal (Fig.3 –line c,в) clasp axis is used, the long axis of the denture base is perpendicular to the axis of rotation. Occlusal forces tend to move the denture base in an arc almost parallel to the longitudinal axis of the residual ridge and towards it. The transversal clasp line is the best for the lower jaw.

If a diagonal axis is used (Fig.3 – line e), the axis of rotation is not perpendicular to the ridge. The resultant vector of force is directed in a buccal to lingual direction on the longer edentulous side and from a lingual to buccal direction on the shorter edentulous side as the denture base rotates eccentrically. The diagonal clasp axis is the best for the upper jaw.

A sagital axis (Fig.3- lines a,б) is an extreme situation where the axis of rotation is at an angle to the long axis of the ridge instead of parallel to it.

2. Plane retention- 3, 4 teeth are used.

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: 1384