impression materials for partial denture

By Noha Taha

Impression materials for partial denture

Definition of impression

Types of impression

Impression materials

Impression methods

Factors influencing support of distal extension base

Outline

Defention

Types of impression

Primary impression

Secondary impression

Selection of stock tray

Primary impression

Stock tray try in

Modify of stock tray

With alginate material

Primary impression

Primary cast

Custom tray fabricaton

Impression materials

Rigid materials

Thermoplastics materials

Elastic materials

Rigid materials

It record tooth and tissue details accurately but it cannot be removed from the mouth without fracture.

Types of Rigid materials

Plaster of parisIt have been used in dentistry for over 200 years but now

elastic materials completely replaced it.

Metallic oxide pastNot used as primary impression materialsUsed for extension base edentulous ridge areas for RPD

Thermoplastic materialsCannot record minute details

accurately because they under go permanent distortion during removal from the tooth and tissue undercuts.

Types of thermoplastic materials

• Used mostly for border molding of custom impression tray.

Modeling plastic

• They have the ability to record border details accurately.

Impression waxes and

Natural Resins

Elastic materialRemain in an elastic state after they set and

removed from the mouth.

Used for making impression for RPD, when tissue undercuts and surface detail must be record with accuracy.

Types of Elastic material

• It is accurate for making master cast for RPD ,

• It is mainly used for duplication of cast

Reversible hydrocolloid (agar-agar)

• Are used for Making study cast and Master cast

Irreversible hydrocolloid

(alginate)

• Should not be used when several undercuts are present.

Mercaptan rubber –base

impression materials

• Provide good surface details and make them useful as border molding materials

Polyether impression materials

• More accurate and easier to use than other elastic impression material

Silicone impression

material

1- Anatomic ridge form: for tooth suppoted R.P.D. (Kenedy‘s class

III, short span class IV) so the edentulous ridges don´t

contribute to the support of the R.P.D. Single, pressure-free imp. records the

teeth and soft tissues in their anatomic form .

Impression Techniques

2-Physiologic or functional ridge form: for tooth- tissue supported R.P.D.

(Kenedy‘s class I,II,long span class IV) When the occlusal forces fall on tooth-

tissue supported R.P.D., the ridge contribute to support as well as teeth

This imp. recordteeth in their anatomic form and the ridge in its functional form under pressure.

Impression Techniques

The objectives of technique is :to provide maximum support for the

removable partial denture bases. This allows for:

1. maintenance of occlusal contact between both natural and artificial dentition

2. minimum movement of the base, which would create leverage on the abutment teeth.

Factors influencing support from distal extension bases

1- Quality of soft tissues covering edentulous ridge2- Type of bone making up denture bearing area3- Design of partial denture4- Amount of tissue coverage of denture base:5- Amount of occlusal forces6- Anatomy of denture bearing area:7- Fit of denture base:8. Type and accuracy of the impression registration:

1- Quality of soft tissues covering edentulous ridge

It should be firm, dense fibrous C.T. of even thicknessslightly compressible and firmly attached to the bone


2- Type of bone making up denture bearing area:

The ideal ridge would consist of:Cortical bone that covers denseCancellous bone with broad rounded crest and high vertical slops.Cortical bone can resist vertical forces better than

cancellous bone.

Factors influencing support from distal extension bases (factors influencing the amount of tissue displacement

3- Design of partial denture:The use of indirect retainer will control

rotational movement of distal extension RPD.


4- Amount of tissue coverage of denture base:

The broader the coverage of the edentulous ridge, the greater the distribution of the load & the smaller the force per unit area


5- Amount of occlusal forces:1- Number of artificial teeth. 2-Width of the occlusal table. 3- type of the opposing dentition 4-powerfull musculature of the patientIt influences the amount of support required to

stabilize the denture base..


6- Anatomy of denture bearing area:To distribute the forces of mastication to

the ridge most efficiently, the majority of force must be directed to the primary stress bearing areas, that are capable of withstanding that force.


Factors influencing support from distal extension bases7- Fit of denture base:Support is enhanced by intimate contact

between the mucosa and the fitting surface of the partial denture;

8. Type and accuracy of the impression registration

2- Physiologic or functional impression technique

which records the ridge portion of the cast in its physiologic or functioning form by placing an occlusal load on the impression tray as the impression is being made.

3-Selective tissue placement impression technique.

At the imp. stage: Mclean´s and Hindel´s methods = dual imp. Technique =

pseudo-functional imp. or Impressions with custom trays.

At the framework stage:Altered cast method either by functional imp.method At the finished denture stage:Functional relining method using fluid wax or zinc oxide

euginol or rubber base relining method.

Impression for distal extension R.P.D.

1- At the imp. stage: McLean‘s technique (closed mouth)

The technique consists of making an impression of the edentulous ridge in border-moulded denture base tray which is provided with occlusion rims.

Impression paste is used to record ridge areas under biting stresses

After setting of ZnO eugenol it is removed, tested, reinserted; overall alginate impression is made with the ZnO imp.seated in the mouth.

Imp. for Dis. Ex. R.P.D.

1- At the imp. stage: Hindle‘s technique (opened mouth)

the same idea of McLean‘s technique but instead of the occlusion rims, use finger pressure through 2 circular openings in the posterior region of the hydrocolloid imp. Tray.


1- At the imp. stage: Disadvantages

If the clasp action is sufficient to maintain the denture base in its intended position, This may result in compromised blood flow with adverse soft tissue reaction and bone resorption.

If clasp action is not sufficient to maintain that functional relationship of the denture base to the soft tissue, this will result in floating denture with premature contact and patient dissatisfaction.


Altered cast method :Steps:1- after the RPD frame work is constucted on anatomic

imp.cast.it should be evaluatedfor any metal projections and sharpedges.

2-check the RPD metal framework in the patient’s mouth

Altered cast method3-the impression tray is made using chemically activated resin, athe frame work with theattached impression tray isplaced in the patient’s mouthand correct peripheral extension4-border molding the impressiontray using low fusing modelingplastic < green or grey sticks >

Altered cast method5-the final impression is made byusing zinc-oxide euginol pastewith the mouth opened and tripod pressure is applied on occlusal rests and indirect retainer

6-after the impression material isset, the tray is removed andchecked for any discrepancies

7. The metal framework with the attached imp. is positioned on the master cast with all occlusal rests properly seated in their prepared recesses.

8. The entire assembly is boxed and poured in a different colored stone.

Altered cast method

3- At the finished denture stage: Functional relining method:

The finished denture is relined by applying for example ZnO eugenol imp. paste to the acrylic fitting surface of the distal extension saddle

the impression is made with the denture being seated by pressure on the occlusal rests and indirect retainers only.

No pressure is applied to the occlusal surface of the artificial teeth


Selective impression technique

impression materials for partial denture

Documents

impression registration

tooth tissue

noha taha impression

design of partial denture

functional ridge form

fit of denture base

tissue details

tissue undercuts