RPD Impression

IMPRESSION Is a negative reproduction of dental structures from which a positive cast can be made.It is one of the most important steps in denture construction as all steps depend on it.

TYPES OF IMPRESSIONTYPES OF IMPRESSIONTHERE ARE TWO TYPES OF IMPRESSION

1.Primary impression1.Primary impression Used to make a reproduction of the teeth and

surrounding tissues. It is made in a stock tray for making a study

cast on which a custom tray is constructed.

TYPES OF IMPRESSIONTYPES OF IMPRESSION2.Final impression It is an impression made in custom tray Used for making the master cast on which

the denture is constructed. Used to make the most accurate

reproduction of the teeth and surrounding tissues.

Rigid materialsRigid materialsIt record tooth and tissue details

accurately but it cannot be removed from the mouth without fracture.

Types of Rigid materialsTypes of Rigid materials

Thermoplastic materialsThermoplastic materialsCannot record minute details

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

Types of thermoplastic materialsTypes of thermoplastic materials

Elastic materialRemain in an elastic state after they set and

removed from the mouth.

Used for making impression for RPD, immediate dentures, crowns, fixed partial dentures when tissue undercuts and surface detail must be record with accuracy.

Types of Elastic material

Goals of Impression Techniques Goals of Impression Techniques for RPD for RPD

Record hard unyielding tissues (teeth) as well as the soft yielding tissues (mucosa) and Surfaces that will contact the RPD framework

Delineate accurately Critical landmarks: preipheral extention retromolar pads, hamular notch, vestibular depths and edentulous regions.

1- Anatomic ridge form: for tooth suppoted R.P.D. for tooth suppoted R.P.D.

(Kenedy‘s class III, short span (Kenedy‘s class III, short span class IV)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 objective of any functional impression 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.

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

class I,II,long span class IV) The imp. must:

1. Record and relate the tissues under uniform loading.

2. Distribute the load over as large an area as possible

3. Accurately delineate the peripheral extent of the denture base.

Impression Techniques

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

1- Quality of soft tissues covering edentulous ridge

2- Type of bone making up denture bearing area

3- 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 thickness

slightly compressible and firmly attached to the bone

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

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:Knowledge of basic principles of

designs guides the management of functional forces.

The use of indirect retainer will control rotational movement of distal extension RPD.

Factors influencing support from distal extension bases

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

Factors influencing support from distal extension bases

5- Amount of occlusal forces:1- Number of artificial teeth. 2-Width of the occlusal table. 3- Efficiency of occlusal table. 4- type of the opposing dentition 5-powerfull musculature of the

patientIt influences the amount of support

required to stabilize the denture base..

Factors influencing support from distal extension bases

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 bases

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:

the majority of the force must be directed to portions of the ridge that are capable of withstanding the force

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.

In cases of soft displaceable mucosa

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 (fluid wax) or by selected pressure 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.

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

1. At the imp. stage:

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:

McLean‘s technique (closed mouth)

Since the tray used for the overall imp. is in contact with the occlusal rims, finger pressure is necessary to hold the original imp. in its functional position while the hydrocolloid material geles.

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.

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

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.

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

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

2. At the framework stage:

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 paste

with 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

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

3- At the finished denture stage: Functional relining 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

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

Gage reflex controlled by:1.Tell patient to relax and breathe

through their nose during the procedure.

2.All the instrument must be out of the sight of the patient and he must not see the mixing of impression material as these will initiate the gage reflex

3.Avoid touching the dorsum of the tongue with the back of the tray and seat the impression as quickly as possible

4. Use thicker mix of Alginate5. Set the patient in upright position 6.Carry out the impression technique using as

little material as possible.7. Desensitize the surface of the mucous

membrane with: phenol mouth washes Sucking a tablet making for this purpose Application of local anesthesia on the surface

8. The posterior border of the tray is shortened or post-damming is made.

9. Remove the viscous present on the soft palate.

10. Seat the tray posteriorly first.11. The patient's head should be

brought forwards and downwards.

Methods of forming casts:There are 2 methods to form a cast, either the

two-step inverted method or the boxed method.

The two step inverted method:The impression is poured with stone and left to

reach its initial set with the face up. A second mix of stone is made and placed on the bench top; then the impression with the hardened stone is inverted onto it and contoured while it is still soft.

This method is suitable for alginate impression.

The boxed method:Boxing as we know is done using wax or

plaster and pumice 2:1 “complaster”.Alginate impression should be boxed by

complaster because wax will not stick to alginate.

The complaster is mixed and placed on a clean, smooth surface, and the impression is partly embedded with its face up; form the cast shape and the tongue space with spatula.

Then after setting of the complaster, it is trimmed to suitable cast outline and wrapped in boxing wax which is sealed to the gypsum with hot wax.

The complaster land is painted with a separator and the cast is poured.

Boxing the impression, separate the cast

Possible causes of inaccurate casts1. Distortion of the hydrocolloid

impression:a) by partial dislodgment from the tray.b) by shrinkage caused by dehydration.c) by expansion caused by imbibition .d) by pouring the cast with too resistant stone.

2. High water powder ratio, results in a weak cast.

3. Improper mixing, results in a weak cast with chalky surface.

Possible causes of inaccurate casts4. Trapping of air, either in the mix or in

pouring, because of insufficient vibration.5. Soft or chalky cast surface resulting from

the retarding action of the hydrocolloid or the absorption of necessary water for crystallization by the dehydrating hydrocolloid.

6. Premature separation of the cast from the impression.

7. Delayed separation of the cast.