IMPRESSION MATERIALS AND PROCEDURES IN REMOVABLE PARTIAL DENTURE

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IMPRESSION MATERIALSAND PROCEDURES IN REMOVABLE
PARTIAL DENTURE

• Presented by
• Dr. Kamleshwar Singh
• BDS, MDS, ICMR-IF(Japan)
• Assistant Professor
• Department of Prosthodontics
• King Georges Medical University, Lucknow

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• INTRODUCTION
• Impression
• A negative likeness or copy in reverse of the
  surface of an object imprint of teeth and
  adjacent structures for use in dentistry. GPT 8
• Partial denture impression
• A negative likeness of a part or all of a
  partially edentulous arch - GPT 8

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• An impression of partially edentulous arch must
  record accurately the anatomic form of teeth and
  surrounding tissues.
• Unless the cast upon which the prosthesis is to
  be constructed is an exact replica of mouth, the
  prosthesis cant be expected to fit properly and
  accurate cast can be obtained only from an
  accurate impression.

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• Impression trays
• A receptacle in to which suitable impression
  material is placed to make negative likeness
• OR
• A device that is used to carry, confine and
  control impression material while making an
  impression.

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• Impression trays can be classified broadly into
• stock trays
• and
• custom trays
• Stock trays for partially edentulous patients
  may be perforated to retain the impression
  material or they may be constructed with a
  rimlock for this purpose.
• Another type of stock tray designed for the
  reversible type of hydrocolloid is water cooled
  trays. It contains tubes through which water can
  be circulated for purpose of cooling the tray.

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• Modified stock tray (individual tray)
• Robert R Renners technique
• The stock tray can be modified with modeling
  composition and with wax to create an accurately
  fitting tray.
• This technique can be employed in class I and
  class II cases.
• Technique
• Softened modeling compound is placed in the
  stock impression tray in such a way that it may
  capture the edentulous areas of mouth and include
  one or two teeth adjacent to the space.

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• Modified stock tray (individual tray)
• Robert R Renners technique
• The stock tray can be modified with modeling
  composition and with wax to create an accurately
  fitting tray.
• This technique can be employed in class I and
  class II cases.
• Technique
• Softened modeling compound is placed in the
  stock impression tray in such a way that it may
  capture the edentulous areas of mouth and include
  one or two teeth adjacent to the space.

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• The tray is positioned in the mouth and
  compound is allowed to cool but it not permitted
  to harden completely, so that it is prevented
  from becoming hard when in contact with the
  adjacent teeth.
• When it is hardened sufficiently to contour it
  is removed from the mouth and thoroughly chilled.

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• The compound is trimmed so that it does not
  contact the adjacent teeth and surface of
  compound in the edentulous areas is scraped to a
  depth of 2 - 4 mm to provide space for a uniform
  layer of impression material.
• In maxillary impression the compound should
  cover the edentulous ridges and the palate and
  should accurately fit to post dam area.

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• Modification of the tray to make it adhesive
• If Impression material to be used is either
  alginate or agar, we can heat surface of compound
  with a flame.
• An alternate method Is to paint the surface
  of compound with a solvent such an chloroform to
  make it tacky and then to embed cotton fibers in
  it, the impression material will become enmeshed
  in cotton fiber. And if rubber base material is
  to be employed rubber adhesive is painted on the
  compound

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• Advantages over custom tray
• 1. Impression can be accomplished in one
  appointment.
• 2. Can be used inpatient with tendency to gag.
• Advantages over conventional use of stock stray
• Especially useful for mouth that is either
  exceptionally large or small or the one with
  anomalous contour which cannot be accurately
  fitted with conventional stock tray.

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• Disadvantages STOCK TRAY
• a. The peripheral borders cannot be accurately
  recorded.
• b. Considerably more bulkier than a custom tray.

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• Custom impression trays
• a. Peripheral borders can be precisely recorded
  in the impression
• b. Thickness of impression material can be
  controlled. This is important consideration when
  using rubber base type material, which should not
  exceed thickness of 2-4 mm because a section
  thicker than this is subject to distortion.

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• C. Well fitted tray will better support the
  impression in the palate, then avoiding even
  present danger of material slumping in vital
  areas.
• Custom trays are sometimes needed for mouths
  that are abnormally or of unusual configuration.

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Impression Materials
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• Factors that influence the selection of
  impression materials are
• Convenience of use
• Time of manipulation and set
• Cost
• Need for special trays
• Operator training and preference

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Impression Materials
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Non-elastic
Impression Materials
Agar (reversible)
Aqueous Hydrocolloids
Alginate (irreversible)
Elastic
Condensation
Non-aqueous Elastomers
Addition
OBrien Dental Materials their Selection 1997
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Reversible Hydrocolloid (Agar)

• Indications
• crown and bridge
• high accuracy
• Example
• Slate Hydrocolloid (Van R)

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Composition

• Agar
• complex polysaccharide
• seaweed
• gelling agent
• Borax
• strength

• Potassium sulfate
• improves gypsum surface
• Water (85)

OBrien Dental Materials their Selection 1997
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Manipulation

• Gel in tubes
• syringe and tray material

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Manipulation

• 3 chamber conditioning unit
• (1) liquefy at 100C for 10 minutes
• converts gel to sol
• (2) store at 65C
• place in tray
• (3) temper at 46C for 3 minutes
• seat tray
• cool with water at 13C for 3 minutes
• converts sol to gel

OBrien Dental Materials their Selection 1997
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Advantages

• Dimensionally accurate
• Hydrophilic
• displace moisture, blood, fluids
• Inexpensive
• after initial equipment
• No custom tray or adhesives
• Pleasant
• No mixing required

Phillips Science of Dental Materials 1996
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Disadvantages

• Initial expense
• special equipment
• Material prepared in advance
• Tears easily
• Dimensionally unstable
• immediate pour
• single cast
• Difficult to disinfect

Phillips Science of Dental Materials 1996
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Irreversible Hydrocolloid (Alginate)

• Most widely used impression material
• Indications
• study models
• removable fixed partial dentures
• framework
• Examples
• Jeltrate (Dentsply/Caulk)
• Coe Alginate (GC America)

Phillips Science of Dental Materials 1996
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Composition

• Sodium phosphate
• retarder
• Filler
• Potassium fluoride
• improves gypsum surface

• Sodium alginate
• salt of alginic acid
• mucous extraction of seaweed (algae)
• Calcium sulfate
• reactor

OBrien Dental Materials their Selection 1997
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Manipulation

• Weigh powder
• Powder added to water
• rubber bowl
• vacuum mixer
• Mixed for 45 sec to 1 min
• Place tray
• Remove 2 to 3 minutes
• after gelation (loss of tackiness)

Caswell JADA 1986
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Advantages

• Inexpensive
• Easy to use
• Hydrophilic
• displace moisture, blood, fluids
• Stock trays

Phillips Science of Dental Materials 1996
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Disadvantages

• Tears easily
• Dimensionally unstable
• immediate pour
• single cast
• Lower detail reproduction
• unacceptable for fixed prosthodontics
• High permanent deformation
• Difficult to disinfect

Phillips Science of Dental Materials 1996
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• RPD IMPRESSION Vs COMPLETE DENTURE
• The complete denture impression records the
  edentulous mucosa with underlying bone only,
  whereas partial denture impression records not
  only relative soft yielding tissues (the oral
  mucosa) as well as a hard unyielding substance
  (the remaining teeth).

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• Removable partial denture impression need to
  record the teeth that are irregular in contour as
  well as varying in their vertical relations to
  occlusal plane. The chosen impression material
  must be capable of recording the tissue contours
  as accurately as possible without distortion,
  which occurs as impression is withdrawn.

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• PRIMARY IMPRESSION
• Objectives
• To obtain an impression of all the standing
  teeth and denture - supporting tissues of each
  jaw from which study casts may be prepared.
• The purpose of the study casts are

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• To enable special trays and occlusion rims to be
  constructed if necessary.
• To examine the occlusion in detail on an
  articulator.
• By use of a surveyor, to plan the path of
  insertion of the proposed denture, arrive at a
  tentative design and plan any mouth preparation.

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• Checking Maxillary Tray For Correct Size
• Checking Mandibular Tray for Correct Size

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• Mixing Impression Material
• Alginate may be mixed by hand spatulation,
  mechanical spatulation, or mechanical spatulation
  under vacuum.
• The objective is to obtain a smooth, bubble-
  free mix of alginate. In hand spatulation a
  measured amount of distilled water at
  approximately 22 C is placed in a rubber mixing
  bowl The pre-weighed alginate powder is sifted
  from its container into the water.

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• The mixing should begin slowly using a stiff,
  broad - bladed spatula.When the powder is
  thoroughly wet, the speed of the spatulation
  should be increased The spatula should crush the
  material against the sides of the bowl to ensure
  that the material is completely mixed. The
  spatulation should continue for a minimum of 45
  seconds.

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• The strength of the gel can be reduced to 50
  if the mixing is not complete. Insufficient
  spatulation can result in failure of the
  ingredients to dissolve sufficiently. Then the
  chemical reaction of changing from sol to gel
  will not proceed uniformly throughout the mass of
  alginate. An incompletely spatulated mix will
  appear lumpy and granular and will have numerous
  areas of trapped air.

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• Complete spatulation will result in a smooth,
  creamy mixture. The mixing should be completed by
  wiping the alginate against the side of the bowl
  with the spatula to remove any trapped air. The
  most consistent method of making a smooth,
  bubble- free mix is mechanical spatulation under
  vacuum.

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• The pre-weighed powder is added to the
  pre-measured water in the mechanical mixing bowl
  .The powder is thoroughly incorporated into water
  by hand spatulation. The mix is then mechanically
  spatulated under 20 pounds of vacuum for 15
  seconds.

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• Longer spatulation will result in a greatly
  reduced setting time of the alginate and could
  affect the strength of the gel.

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• Loading the Impression Tray
• Small increments of the impression material
  should be placed in the tray and forced under the
  rim lock. Placing too large a portion of alginate
  at one time increases the possibility of trapping
  air The tray should be filled to the level with
  the flanges of the tray.
• Overfilling should be avoided.

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Making the Impression

• The mandibular impression is made first because
  it usually entails less patient discomfort
  patient confidence is increased when an
  impression has been successfully completed while
  holding the tray with the left hand the dentist
  uses the right hand to remove the gauze pads from
  the patients mouth.

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• The syringe is used to inject the impression
  material over the occlusal surface of the teeth
  and into the vestibular and alveolingual sulcus
  areas. The impression material will remain in
  place if the tissues are fairly dry. A tendency
  for the alginate to form a ball and not remain
  where placed indicates that the tissues are too
  moist and that voids are likely to be present in
  the impression.

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• There is not enough time to repack the mouth
  before gelation begins, so the impression
  procedure should be completed. The impression
  should be carefully inspected and if voids are
  present in critical areas, the impression
  procedure should be repeated. Packing the mouth
  with more or larger gauze pads and avoiding
  removal of the gauze until ready to apply the
  alginate will usually prevent this problem.

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The layer of alginate applied with the syringe
should be 3 to 4 mm thick If it is too thin, the
heat of the tissues of the oral cavity may cause
the material to set before the tray is seated,
resulting in a layered impression.
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• The fingers of the left hand that are retracting
  the right cheeks should depress the lower lip to
  provide good visibility. When the tray is
  correctly lined up over the teeth, the patient is
  asked to protrude the tongue. The tray is
  carefully seated so that its flanges are below
  the gingival margins of the teeth.

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• The tray should not be over seated because this
  could result in the cusps of the teeth contacting
  the tray, causing an inaccurate impression. Great
  care must be exercised in seating the tray if the
  patient has mandibular tori or other exostoses,
  or the making of this impression can be a very
  painful experience for the patient.

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• As the tray is being seated, the cheeks are
  pulled out to prevent the trapping of buccal
  tissues under the tray. The patient is asked to
  keep the tip of the tongue in contact with the
  upper surface of the tray during the gelation of
  the impression material.

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• The dentist must maintain the position of the
  tray during the entire gelation period. This can
  be accomplished most conveniently and effectively
  by placing the forefinger of each hand on the top
  of the tray in the premolar area and by placing
  the thumbs under the patient s chin.

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• The dentist through tactile sense can maintain
  an even amount of pressure on the tray even if
  the patient swallows or opens or closes the
  mouth. Any movement of the tray during the
  gelation period will result in an inaccurate
  impression.
• Allowing the patient or the assistant to hold
  the tray or leaving the patient unattended must
  be avoided.
• Within 3 to 4 minutes the alginate should be
  set.

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• For maxillary impression, the patients is
  prepared by using the rinses and placing the
  gauzes pads described for making the mandibular
  impression. While holding the loaded tray with
  the left hand the dentist uses the right hand to
  remove the gauze pads.

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• Alginate is injected onto the occlusal surfaces
  and in all vestibular areas as for the mandibular
  arch. In addition, a fairly large amount should
  be wiped onto the palate. Failure to accomplish
  this step will usually result in an impression
  with a large void in the palatal area.

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• The loaded maxillary tray is grasped by the
  thumb and forefinger of the right hand. As the
  right posterior flange of the impression tray
  stretches the right corner of the mouth, the
  dentist s left arm should be behind the
  patients head and headrest so that the thumb and
  index finger may grasp the left corner of the
  mouth and distend it slightly to allow the
  impression tray to enter the mouth in a straight
  line.

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• No attempt should be made to seat the tray until
  the tray is in its correct anteroposterior
  position. Once the tray is in the mouth, the
  thumb and forefinger of the left hand should
  raise the upper lip to allow the dentist to see
  the relationship between the labial flange of the
  tray and the anterior teeth or the residual
  ridge.

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• The tray must be centered and properly aligned.
  This position can best be verified by looking at
  the patient s face from above and observing the
  position of the handle of the tray.

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• It should protrude straight from the center of
  the mouth. After the proper position has been
  verified the tray is seated by using the fingers
  of both hands over the premolar areas. As the
  tray is being seated the cheeks must be lifted
  outward and upward to prevent the buccal tissues
  from being trapped under the flanges of the tray.

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• The lip must also be lifted up and out to allow
  good visibility and to avoid trapping the lip
  between the flanges of the tray and the anterior
  teeth. Care must be taken not to over seat the
  tray to avoid. contact between the tray and cusp
  tips of incisal edge of the teeth.

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• The tray should be stabilized throughout the set
  of the impression material by keeping light
  pressure over the premolar areas on both sides of
  the arch The alginate should set in 3 to 4
  minutes.

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• Effect of movement of tray
• Gelation of alginate occurs by a chemical
  reaction. When mixed with water, the sodium
  alginate and calcium sulfate in the powder react
  to form a lattice work of fibrils of insoluble
  calcium alginate. The heat of the oral tissues
  accelerates the chemical reaction, causing the
  alginate next to the tissues to gel first .

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• If the dentist exerts pressure or allows the
  tray to move during gelation of the remainder of
  the alginate, internal stresses are created that
  can distort the impression as it is removed from
  the mouth.

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• Removal of Impression from Mouth
• Clinically, the initial set of alginates is
  determined by a loss of surface tackiness. The
  impression should be left in the mouth for an
  additional 2 to 3 minutes to allow the
  development of additional strength. Early removal
  of the weak alginate may lead to unnecessary
  tearing of the impression.

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• The gel strength doubles during the first 4-
  minutes after initial gelation. No further
  strengthening is found after that time. In fact,
  Impression is left in the mouth for 5 minutes
  rather than the recommended 2 to 3 minutes after
  initial gelation exhibits definite distortion.

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• Most alginates improve their elasticity with
  time, providing a better opportunity for accurate
  reproduction of undercuts. Impressions removed
  too early after initial gelation produce a rough
  surface of the poured cast. These data indicate
  the alginate impressions should not be removed
  from the mouth for at least 2 to 3 minutes after
  initial gelation.

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• There are two reliable methods of determining the
• correct time for removal of the impression
• 1. A timer can be used to measure the 2 to 3
  minute period after initial gelation or
• 2. A small mound of the original mix of alginate
  can be placed on a glass or metal surface when
  this alginate will fracture cleanly with finger
  pressure, the impression is ready to be removed
  from the mouth.

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• Impression Methods
• There are basically two dual impression
  techniques. The physiologic, or functional,
  impression technique records the ridge portion by
  placing an occlusal load on the impression tray
  as the impression is being made.

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• For this dual impression a custom impression
  tray was constructed over a preliminary cast of
  the arch, a function impression of the distal
  extension ridge was made, and then hydrocolloid
  impression was made with the first impression
  held in its functional position with finger
  pressure.

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• The underlying s tissues will be displaced
  because displacement will normally occur under
  function.
• The physiologic impression techniques that
  discussed are as follows Mc Leans and Hindels
  methods, the functional relining method, and the
  fluid wax method.

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• The selected pressure impression technique not
  only equalizes the support between the abutment
  teeth and the soft tissue, but has the added
  advantage of directing the force to the portions
  of the ridge that are most capable of
  withstanding the force.
• This is accomplished by providing relief in
  the impression tray in selected areas and
  permitting the impression to be recorded.

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• The need for physiologic impressions was first
  recognized by McLean and others
• They realized the need of recording the tissues
  of the residual ridge that would eventually
  support a distal extension denture base in the
  functional or supporting form and then relating
  this functional impression to the remainder of
  the arch by means of a second impression.

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CONCLUSION
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• For the production of accurate master cast the
  impression technique far out weights the
  selection of the impression material.
• No available knowledge of the person making the
  impression material will produce results greater
  than the skill and knowledge of the person making
  the impression.

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MCQS
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• Which is true regarding maxillary residual ridge
• Crest is cortical bone
• Crest is cancellous bone with firm mucosa
• Crest is cancellous bone without firm mucosa
• Preesure placed on the crest result in irritation
  of these tissues

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• 2. In a maxillary denture, relief must be
  provided
• Crestal region
• Buccal slopes
• Palatal slopes
• Median palatal raphe

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• 3. Which is correct?
• A
• B
• Depends upon the residual ridge
• none

a
b
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• 4. For a distal extension base, which should be
  followed?
• Make a functional impression
• Make an anatomic impression
• Make an anatomic impression with stress breakers
• Both a and c

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• 5. Functional form of the residual ridge is not
  recorded by which of the following means?
• Under some loading, occlusal or finger
• By specially designed trays
• By soft impression material, such as ZnO, if the
  entire impressionn tray is uniformly relieved
• Maintaining the consistency of the recording
  medium

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• 6.In order to increase the amount of vertically
  directed forces on the residual ridge
• Move the rest posteriorly
• Move the rest anteriorly
• Reduce the number of rests
• None is correct

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• 7. Which is not outcome of reducing the occlusal
  table
• Reduces vertical and horizontal forces
• Increases vertical and horizontal forces
• Reduces stress on the abutment teeth
• Lessens stress on the residual ridge

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• 8. Which is true about anatomic impression
• It is a one-stage impression method using elastic
  impression material
• Places more masticatory load on the abutment
  teeth
• Places more masticatory loads on the residual
  ridge
• All of the above

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• 9. To evaluate complete seating of the framework
  several types of disclosing media are used. Which
  one is wrongly written?
• Rouge
• Chloroform
• Pencil correction fluid
• Waxes

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• 10. Altered cast impression technique is used in
• Kennedy Class I maxillary residual ridge
• Kennedy Class II mandibular residual ridge
• Kennedy Class IV maxillary residual ridge
• Kennedy Class III mandibular residual ridge

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• 1. b
• 2. d
• 3. a
• 4. d
• 5. c
• 6. b
• 7. b
• 8. d
• 9. c
• 10. b