Impression procedures for removable partial dentures

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Impression procedures for removable partial
dentures
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What is an 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.

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TYPES OF IMPRESSION

• THERE ARE TWO TYPES OF IMPRESSION
• 1.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.

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TYPES OF IMPRESSION

• 2.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.

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Rigid materials

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

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Types of Rigid materials
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Thermoplastic materials

• Cannot record minute details accurately because
  they under go permanent distortion during removal
  from the tooth and tissue undercuts.

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Types of thermoplastic materials
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Elastic material

• Remain 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.

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Types of Elastic material
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Goals of Impression Techniques 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.

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Impression Techniques

• 1- Anatomic ridge form
• for tooth suppoted R.P.D. (Kenedys class III,
  short span class IV)
• so the edentulous ridges dont contribute to the
  support of the R.P.D.
• Single, pressure-free imp. records the teeth and
  soft tissues in their anatomic form .

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Impression Techniques

• 2-Physiologic or functional ridge form
• for tooth- tissue supported R.P.D. (Kenedys
  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.

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• The objective of any functional impression
  technique is
• to provide maximum support for the removable
  partial denture bases. This allows for
• maintenance of occlusal contact between both
  natural and artificial dentition
• minimum movement of the base, which would create
  leverage on the abutment teeth.

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Impression Techniques

• 2-Physiologic or functional ridge form
• for tooth tissue supported R.P.D. (Kenedys class
  I,II,long span class IV)
• The imp. must
• Record and relate the tissues under uniform
  loading.
• Distribute the load over as large an area as
  possible
• Accurately delineate the peripheral extent of
  the denture base.

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

• 1- Quality of soft tissues covering edentulous
  ridge
• 2- Type of bone making up denture bearing area
• 3- Design of partial denture
• 4- Amount of tissue coverage of denture base
• 5- Amount of occlusal forces
• 6- Anatomy of denture bearing area
• 7- Fit of denture base
• 8. Type and accuracy of the impression
  registration

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

• 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

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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 dense
• Cancellous bone with broad rounded
• crest and high vertical slops.
• Cortical bone can resist vertical forces better
  than cancellous bone.

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Factors influencing support from distal extension
bases

• 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.

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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

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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 patient
• It influences the amount of support required to
  stabilize the denture base..

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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.

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Factors influencing support from distal
extension bases

• 7- 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

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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

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Impression for distal extension R.P.D.

• At the imp. stage
• Mcleans and Hindels 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.

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

• 1. At the imp. stage

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

• 1- At the imp. stage
• McLeans 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.

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

• 1- At the imp. stage
• McLeans 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.

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

• 1- At the imp. stage
• Hindles technique (opened mouth)
• the same idea of McLeans technique but instead
  of the occlusion rims, use finger pressure
  through 2 circular openings in the posterior
  region of the hydrocolloid imp. Tray.

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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.

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

• 2. At the framework stage

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Altered cast method

• Steps
• 1- after the RPD frame work is constucted on
  anatomic imp.cast.it should be evaluated
• for any metal projections and sharp
• edges.
• 2-check the RPD metal frame
• work in the patients mouth

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Altered cast method

• 3-the impression tray is made
• using chemically activated resin, a
• the frame work with the
• attached impression tray is
• placed in the patients mouth
• and correct peripheral extension
• 4-border molding the impression
• tray using low fusing modeling
• plastic lt green or grey sticks gt

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Altered cast method

• 5-the final impression is made by
• using zinc-oxide euginol paste
• with the mouth opened and
• tripod pressure is applied on
• occlusal rests and indirect retainer
• 6-after the impression material is
• set, the tray is removed and
• checked for any discrepancies

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Altered cast method

• 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.

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

• 3- At the finished denture stage Functional
  relining method

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

• 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

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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

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• 4. Use thicker mix of Alginate
• 5. 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

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• 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.

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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.

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• The boxed method
• Boxing as we know is done using wax or plaster
  and pumice 21 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.

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Boxing the impression, separate the cast
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Possible causes of inaccurate casts

1. Distortion of the hydrocolloid impressiona) 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.

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Possible causes of inaccurate casts

1. Trapping of air, either in the mix or in pouring,
   because of insufficient vibration.
2. 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.
3. Premature separation of the cast from the
   impression.
4. Delayed separation of the cast.