LABORATORY STEPS IN FABRICATING COMPLETE DENTURE.

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LABORATORY STEPS IN FABRICATING COMPLETE DENTURE.

• Ojunta Ogechi N.

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Outline

• Introduction
• Lab. Stages in fabricating complete dentures.
• Diagnostic cast
• Primary cast
• Special tray
• Master cast or working cast
• Bite block occlusal rims

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• Setting up of teeth
• Processing and finishing
• Conclusion

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Introduction

• Lab. Stages in fabricating complete dentures
  include all the procedures done in the lab by the
  dental technician in other to produce the compete
  denture.
• In the fabrication of complete dentures the
  contribution of the dental technician can not be
  over emphasized.

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• The dental technician contribution is directed
  towards the careful translation of the prescribed
  denture design into the denture itself and also
  the accurate construction of the denture.

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Lab. Stages in fabricating C.D

• These stages include
• Diagnostic cast
• Primary cast
• Special tray
• Master cast or working cast
• Bite block occlussal rims
• Setting up of the teeth
• Processing and finishing.

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Diagnostic cast

• As a first step in preprosthetic surgery, a
  diagnostic cast is prepared and surveyed to
  determine the amount of surgery required.
• A diagnostic cast is made from the diagnostic
  impression.( a diagnostic impression is the
  negative replica of the oral tissues used to
  prepare a diagnostic cast).

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• The diagnostic cast is used for study purposes
  like measuring the undercuts, locating the path
  of insertion e.t.c and not for any fabrication
  purposes.

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Purpose of making a diagnostic cast.

• To measure the depth and extent of the undercuts.
• To determine the path of insertion of the
  denture.
• To identify and plan the treatment for
  interferences like tori.
• To determine the amount of preprosthetic surgery
  required.
• To evaluate the size and contour of the arch
• To get an idea about retention and stability
  offered by the tissues
• To determine the need for additional retentive
  features like over denture abutment, implant
  abutments, e.t.c.

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Pouring the diagnostic cast

• 1)The cast should be done within 15mins after
  impression making.
• 2)a base former can be used to make a proper
  base.
• 3)usually a diagnostic cast is poured using
  dental plaster because it is economical and
  reproduction of finer details is not of imp.
  requisite.

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• 4) the impression is placed and stabilized over a
  piece of cotton so that the ridge appears
  parallel on the table.
• 5) required quantity of water and powder are
  dispensed in a rubber bowl and mixed in a
  circular motion until it reaches a sufficient
  consistency.

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• 6)impressions are usually poured in three pours.
  The 1st pour should be of more liquid
  consistency. The plaster mix should be allowed to
  flow all over from the distal end. It should be
  placed on the vibrator to avoid the occurrence of
  air bubbles.
• The 2nd pour should be a little thicker in
  consistency. The 3rd pour or base is done usin a
  base former.

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• 7)The base should have a minimum thickness of
  10mm at the thinnest portion.
• 8)excess plaster should be trimmed away

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Finishing the diagnostic cast

• The diagnostic cast should be separated from the
  impression only an hour after its initial set.
• Care should be taken while removing the
  impression material from the undercut areas.
• Small nodules and projections on the impression
  surface should be removed

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• The cast should not be washed under direct water
  because the superficial surface of the plaster
  will dissolve and get washed away
• If a base former is not used during the 3rd pour
  then the base of the cast should be trimmed using
  a model trimmer and smoothened using sandpaper.

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Primary cast

• This cast is prepared after the primary
  impression is taken. Primary casts are used in
  the fabrication of special trays.

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Pouring and finishing the primary cast

• 1)the primary cast should be poured immediately
  after making the primary impression becos the
  impression compd tends to distort according to
  the environmental changes.
• 2)the impression is poured using dental plaster
  and using the same three-pour technique mentioned
  in the diagnostic cast.

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• 3)the cast should be separated from the
  impression about an hour after the initial set.
• 4)the poured impression is placed in warm, slurry
  water bath till the impression compound softens.
• 5)once softened, the impression can be easily
  removed.
• 6)if a base former was not used then the cast
  should be trimmed using a model trimmer. Then the
  primary cast is ready to fabricate a special tray.

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Special tray fabrication

• A special tray is a custom made device prepared
  for a particular patient which is used to carry,
  confine and control an impression material while
  making an impression.
• It is fabricated from the primary cast.

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• Fabrication of the special tray depends on the
  type of material used. Most commonly used
  materials include
• Shellac
• Cold cure acrylic
• Vacuum formed vinyl or polystyrene
• Vacuum formed thermoplastic resin
• Type II impression compd (tray compd)

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Fabrication of special tray from shellac

• The shellac material is thermoplastic. This means
  it is soften on heating and solidified on
  cooling. They are supplied in lower and upper
  shellac sheets. The lower is like horse-shoe
  shaped pattern while the upper is half rounded
  flat sheet. 
• Material needed
• (1) Primary cast.
• (2) A spacer (cotton gauze or wax spacer or tin
  foil).
• (3) Bunsen burner and flame.
• (4) Trimming machine and
• (5) Trimming scissors and knife.
• (6) Shellac tray materials.
•  

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• Outline with an indelible pencil the tray
  coverage area on the preliminary casts.
• Sock the primary cast in water for few minutes
• Adapt the spacer on the model.
• The thickness should depend on the impression
  material to be used .i.e.
• Zinc oxide eugenol impression material 0.5mm
  thick spacer
• Rubber based impression material - 0.5 -1.5mm
• Depending on viscosity - light 0.5mm
• -regular/ medium 1mm
• - heavy 1.5mm
• -impression plaster 1.5mm
• - alginate 2.0-3.0mm
• Gently flame the shellac tray material over flame
  until it softens, care must be taken not to melt
  it or burn.

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• Adapt the soften shellac on the primary cast
  with the spacer in between.
• Trim off the excess i.e over extension with
  scissors or trimming knife to the pencil outline.
• Construct handles from the scrap and attaches it
  at the midline.
• Perforate the trays so as to allow retension of
  the impression material and flow of excess
  impression material.
• Check for correct extension, fit and any rough
  edges.
• Smoothen the edges of the trays all round.

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Fabrication of special tray using cold cure
acrylic material

• It is also known as the auto-polymerising resin.
  The material sets by chemical reaction and hence
  it is irreversible.
• It is similar to the denture base resin used for
  the final fabrication of the denture.

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•  
• Materials
• Cold cure liquid and powder
• Vaseline
• Wax spacer or tin foil.
• Primary cast.
• Trimming knife.
• Trimming machine and burs.
• Cold mould seal.
•  

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Procedure

• The relief areas and borders of the special tray
  are marked by an indelible pencil on the primary
  cast. A wax spacer is adapted on the relief
  areas.
• Separating medium is coated on the entire cast
  and over the spacer.
• There are two major techniques commonly used in
  the fabrication of an acrylic special tray.

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• Sprinkle on technique
• Dough technique
• Sprinkle on technique
• The powder and liquid are loaded in separate
  dispensers.
• A small quantity of powder is sprinkled on a
  particular area over the cast and liquid is
  sprinkled over the powder.

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• Sprinkling drops of the liquid polymerizes the
  powder.
• This is continued till the entire ridge and the
  associated landmarks are covered.
• The advantage of this technique include its ease
  of use and minimal wastage of material.
• The disadvantages are even thickness cannot be
  obtained, too many porosities may form within the
  material, time consuming.

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• Dough technique
• The powder and liquid should be mixed in a
  mixture jar in the ratio of 31 b vol.
  porosities, excessive shrinkage and granularity
  may occur if this ratio is not maintained and
  insufficient monomer is used.
• After mixing the monomer and polymer the mix
  undergoes six different stages

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• 1)wet sandy stage
• 2)early stringy stage
• 3)late stringy stage
• 4)dough stage
• 5)rubbery stage
• 6)stiff stage

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• Manipulation is done in the late stringy and
  dough stages. The material is kneaded in the
  hands (vaseline is rubbed on the hands to avoid
  the dough sticking on the hands), to achieve a
  homogenous mix.
• The material is shaped into a 2mm thick sheet.
  Flattening the dough can be done using a roller
  or a plaster mould or pressing the material btwn
  two glass slabs.

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• Separating medium should be applied over the
  roller or the glass slads to avoid stickiness.
• The rolled acrylic is adapted over the cast from
  the centre to the periphery. This prevents the
  formation of wrinkles.
• The excess material should be cut out with a
  blade before the material sets.
• The material should be held in position as
  shrinkage and warpage may occur during
  polymerization.

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• The set material is then trimmed to obtain a
  smooth surface with smooth margins.
• The handle is fabricated using the excess dough
  material.

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Other materials used for making special trays

• Thermoplastic resins
• These are also very good special tray materials.
  They are adapted using a vacuum former.
• Vacuum-formed vinyl polysterene
• This is the fastest method for making special
  trays. Its only disadvantage is that it is very
  expensive

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• A vacuum-forming machine is required for this
  procedure.
• After fabricating the special tray, it should be
  stored in the cast till the next appointment.
• Acrylic special trays should be stored in water
  to avoid warpage.
• Shellac trays should be stored in a cool and dry
  place.

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Master cast or working cast

• The master cast is made after taking the
  secondary impression.
• The master cast should accurately reproduce the
  anatomy of the residual ridge hence, care must
  be taken to preserve the width and depth of the
  sulcus in the cast.
• The sulcus can be preserved by beading and boxing.

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Beading and boxing

• Beading is done to preserve the width and height
  of the sulcus in a cast. Beading waxes are
  generally blue in colour.
• Boxing is done to obtain a uniform, smooth,
  well-shaped base for the cast. Boxing waxes are
  white in colour.
• Other beading and boxing waxes include modelling
  wax, orthodontic tray wax e.t.c.

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Procedure

• The impression should be stabilized using soft
  wax or modelling clay to make the impression
  parallel to the floor.
• For a mand. impression, the tongue space should
  be covered with a sheet of wax.
• The beading wax is adapted 3-4mm below the height
  of contour of the impression flanges. The beading
  should at least 4mm wide and the width should be
  even all around the impression.
• The beading wax should be sealed on both sides
  (above and below) to the impression .

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• The beaded impression is positioned on the table.
  A strip of boxing wax about 15mm wide is heated
  and adapted around the beaded impression to form
  a base for the cast.
• Water should be poured into the boxed impression
  to check for leakages.

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• After beading and boxing the master cast is
  poured with dental stone (high strength, minimal
  expansion stone is preferred). A stone cast is
  superior to plaster cast bcos the finer
  particles make it stronger, denser and smoother.
• The three-pour technique for diagnostic cast is
  employed. After the cast is ready, it is mounted
  on the articulator.

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Making a master cast from a secondary impression
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Master cast
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Bite block occlusal rims

• Bite block is defined as occluding surfaces built
  on temporary or permanent denture bases for the
  purpose of making maxillomandibular relation
  records and arranging teeth.
• Bite blocks are used to record jaw relations.
  They are used to establish such a correct
  relationship of the mandible and maxilla after
  which teeth are arranged in the block for trying
  in at a later day.
•  
• They consist of 2 parts (1) The base plate (2)
  The Rim
•  
• The base plate can be of two types
• (1) Temporary base plate
• (2) Permanent base plate.
•  

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• (1) Temporary base plates that could be used are
  (i) wax (ii) shellac (iii) cold cure acrylic (iv)
  tin lead alloy.
• These temporary base plates are discarded at the
  flasking stage of denture fabrication.
•  
• (2) Permanent base plates This forms the denture
  base and part of the final product. The materials
  that can be used are heat cure acrylic resin,
  light cure acrylic resin, microwavable acrylic
  resins and metals.
•  

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• Requirement of bite blocks
• 1. The base must be adapted accurately to the
  underlying tissue
• 2. It must be sufficiently strong and rigid at
  mouth temperature to avoid distortion
• 3. It should be retentive and stable in the mouth
  to facilitate accurate recording.
• FABRICATION
• Bite blocks are fabricated on a working model
  which can be made from final impression.
•  

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FACTORS CONTROLLING THE FORM OF A BITE BLOCK

• Relationship of natural teeth to the alveolar
  bone.
• Relationship of the occlusal rim to the
  edentulous ridge.
• Standard dimensions used to fabricate an occlusal
  rim.

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Relationship of natural teeth to the alveolar
bone.

• It should be fabricated such that it is parallel
  to the long axis of the tooth to be replaced.
• The maxillary anteriors are labially inclined
  hence, the occlusal rim in that area should be
  labially inclined.
• All posterior teeth are placed vertically, hence
  the occlusal rim should also be fabricated
  vertically in this region

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Relationship of the occlusal rim to the
edentulous ridge.

• The residual ridge resorption changes the apex of
  the edentulous ridge.
• The bite blocks should be fabricated such that
  the midline of the occlusal plane coincides with
  the tip/apex of the edentulous ridge.

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Dimensions of the blocks

• For the upper bite blocks, the occlusal height
  anteriorly should be 22-24mm, posteriorly
  16-18mm. The width should be about 6-10mm.
• The anterior edge of the occlusal rim at midline
  should be about 8mm to 1cm away from the incisive
  papilla
•  For the lower bite block, the occlusal height
  anteriorly should be 16-18mm, posteriorly
  11-12mm. The width should be about 6-10mm

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Clinical guidelines for determining the shape of
Bite blocks

• Maxillary anterior edge should be 0-2 mm below
  the upper lip at rest. However varies with age.
• Maxillary occlusal plane should be 0.5mm below
  the parotid duct.
• Mandibilar incisal edge should be at the level of
  the lower lip and about 2mm behind the maxillary
  incisal edge.
• Canine eminence of the lower occlusal rim must be
  located at the corners of the mouth.

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• Posterior part of the lower occlusal plane should
  extend to two third the height of the retromolar
  pad.
• The anterior occlusal plane should be parallel to
  inter pupillary line
• The posterior plane should be parallel to alar
  tragal line.

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Fabrication

• Rolled wax technique
• Metal occlusal rim former. A prefabricated metal
  wax former is filled with wax to form the rim.
• Pre formed occlusal rim. Commercially available
  in standard sizes

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Rolled wax technique.

• The most common.
• A pencil outline marked with indelible pencil.
• The cast is soaked in water.
• A sheet of modelling wax or base plate wax is
  adapted to the cast after flaming it on a Bunsen
  burner.
• The modelling wax is trimmed to the pencil
  outline.
• One end of a sheet of modelling wax is soften
  over the flame and rolled to a width of 4mm

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• Care should be taken to flame the wax adequately
  and rolled carefully to avoid air entrapments.
• The procedure is done over and over until the wax
  is rolled to a thick cylinder of wax.
• The rolled wax is adapted over the base plate
  such that it follows the arch curvature
• The rolled wax is sealed to the base plate with
  wax knife

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• The depression created below the rolled wax and
  the base plate should be filled with wax.
• The lingual/palatal and buccal surfaces should be
  smoothened.
• Measurements are done and adjustments made
  accordingly.
• Polishing is done by swiftly flaming the occlusal
  rim followed by wiping with a wet cotton wool.

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CLINICAL SIGNIFICANCE

• To obtain maxillo mandibular relationship.
• To determine the level of occlusal plane.
• Help in selection of teeth.
• Help in setting of teeth
• Determine the lip and the cheek support.
• Help in determining and marking guidelines

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Bite blocks on articulators
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Fabrication of occlusal rims using metal occlusal
rim formers

• Scrap wax or base plate wax should be rolled to a
  cylinder as in the rolled wax tech.
• The cylinder of wax is then shaped using a metal
  occlusal rim former and stored for later use.
• The formed occlusal rims should be adapted on the
  denture base, sealed and finished.

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Preformed occlusal rims

• They are commercially available in standard sizes
  separately for the maxilla and mandible. They are
  fabricated the same tech. as that described for
  metal occlusal rim former.

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Setting up of teeth

• This involves the arrangement of the artificial
  teeth. It should be arranged according to certain
  principles to avoid deflection of any destructive
  forces towards the supporting tissues.
• An artificial tooth is set by softening the wax
  in the portion of the occlusal rim and
  positioning the tooth on it.

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•  
• The landmarks obtained during bite registration
  stage are usually used in construction of full
  denture i.e (i) occlusal plane (ii) midline (iii)
  canine line (iv) the vestibular shape of dental
  arch (v) various displacement of the mandible
  during registration.
• The construction of dental arches begins by
  setting up of the upper teeth. The teeth are set
  up in relation to the lower occlusal block which
  represent the occlusal plane.

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The upper arch

• (1) Central incisors the upper anterior teeth
  should be placed so that they give support to the
  vermillion border of the upper lip. The central
  incisors are set up symmetrically to the right or
  left of the tip of the midline (centric)
  indicator. Their incisal edges should touch the
  occlusal plane. They should be slightly
  vestibular to the alveolar axis (slightly
  proclined.)
• (2)The lateral incisors should follow the
  central incisors but the incisal edges should
  stand 1-1.5mm from the occlusal plane.

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• (3)The canines these are set in relation to the
  alveoli process like the other anterior teeth.
  They touch the occlusal plane with the apices of
  their bulges. The canines should be given a
  slight tilt towards the incisors.

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The upper masticatory teeth

• These are the molars and premolars.
• After the six anterior teeth are set up in place,
  the masticatory teeth are better arranged first
  on one side of the model and then on the other.
• The masticatory surface of the teeth should form
  the saggital and lateral occlusal curves with the
  curvature direction downwards (curve of Spee and
  Monson).
• the first premolar touches the occlusal plane
  only with buccal cusp while the palatal cusp is
  0.5mm short of occlusal plane it should be
  slightly rotated about its axis.

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• The second premolar touches the occlusal plane
  with both cusps.
• The first molar touches the occlusal plane only
  with mesio-palatal cusp while its other cusps do
  not touch the occlusal plane i.e mesio-bucal cusp
  0.5mm from the plane disto-buccal cusp 1.2-1.5mm
  from the plane and disto-palatal cusp 1mm from
  the plane.
• The second molar may not touch the occlusal plane
  at all. Its distal cusps should be about 2mm away
  from the plane.

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The lower arch

• The landmarks for setting up of lower teeth are
  the upper teeth relationship and the alveolar
  ridge of the lower jaw.
• All the teeth should be on the ridge and should
  allow enough tongue space.
• Setting up of teeth can be started with the
  anterior or masticatory teeth depending on the
  school of thought.

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• The setting up of anterior teeth begins with the
  centrals followed by the laterals and canines.
  Slight retroclination is allowed. Forward,
  lateral and side movements must be checked for
  interference during set up.
• The first molar is then positioned in such away
  that each one is in opposition to the second
  upper premolar and the first upper molar.
• The second premolar is also positioned in such a
  way that one is in opposition with the first and
  second upper premolar. Movement should be checked
  at all stages to ensure they are smooth.

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• After the second premolars are set up correctly
  the second molar and finally first premolars are
  seated in their place.
• It is important to check the accuracy of the
  tooth relationship not only in centric occlusion
  but also in lateral displacement.
• The lateral excursion should be smooth.

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• When all the teeth are set up they are fastened
  in place with hot wax and the denture (polished)
  surface are given the shape they will have in
  final denture.
• The trial denture is then disinfected before the
  next stage which is the wax try-in stage in
  which denture (wax stage) are removed from model
  and then checked in the patients mouth. 

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Compensating curves

• These are anterior posterior and lateral
  curvature of the occlusal surface of an
  artificial denture arbitrarily chosen to achieve
  balanced occlusion.
• These curves are
• Curve of Monson
• Curve of Spee
• Cartesian curves

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Curve of Monson

• This is an imaginary sphere 10cm radius described
  by Curges Monson with the glabella as center. All
  lower molar and buccal premolar cusp and the
  canine and incisors tips in perfect dentition are
  suppose to touch the surface of the sphere.
• The lateral cross section of this sphere in the
  molar region would touch the buccal and lingual
  cusps of a tooth in each side and will constitute
  a curve of monsoon.
• The occlusal surface of the molar (lower) will
  therefore face inwards and upwards towards the
  glabella.

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Curve of Spee

• Defined by Graf Von Spee as the arch of a circle
  of 6.5-7cm radius touching the occlusal surfaces
  of the premolars and molars of the lower jaw and
  when continued backward passing through the
  anterior aspect of the condyle.
• Its center lies behind the Crista Lacrymalis
  posterior. It is now know as the anterior
  posterior curve of posterior teeth.

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Cartesian curves

• The incisal edges of the anterior teeth follow an
  arch shape. The premolars follow a slanting line
  while the molars follow a straight line. This is
  referred to as the Cartesian curves in setting up
  teeth.
• Other guidelines for arrangment of teeth
• Key of occlussion (canine and molar key)
• Arch form
• Overjet and overbite
• Neutral zone

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Teeth set up
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Denture processing and finishing

• There are two techniques commonly used in dental
  processing namely compression moulding and
  injection moulding.
• Injection-moulding technique is not commonly
  followed.
• Compression moulding tech. is the most commonly
  used tech. in fabrication of dentures using
  acrylic resins.

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Steps in compression moulding technique

• Preparation of the trial denture
• Disaticulation
• Flasking procedure
• Dewaxing
• Application of separating medium
• Mixing of powder and liquid
• Packing
• Curing
• Cooling
• deflasking

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• After the try-in procedure, the trial denture and
  the cast are placed in the lower half of a metal
  flask filled with freshly mixed plaster.
• When set, the exposed surface of the plaster is
  coated with a suitable separating medium such as
  vaseline, cold mould seal etc.
• The inverted upper half of the flask is placed
  over the lower half of the flask and then filled
  with freshly mixed plaster. (Note that this
  procedure must be done over a vibrator).

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Boiling out

• After the plaster has set, the flask is heated to
  soften the wax and the flasks half separated.
• The artificial teeth remain in the plaster of the
  upper half of the flask. The wax is removed and
  final traces of wax eliminated with boiling
  water.

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Separating medium

• After removal of the wax, a space corresponding
  to the dimension of the required denture is left
  in the flask which has to be filled with acrylic.
  
• It is very important, however that the resin is
  protected from the gypsum, since water released
  from the gypsum would affect both the
  polymerization process and the properties of the
  cured resin.
• Monomer would also soak into the mould, again
  affecting the resulting resin, but also joining
  the gypsum in the two halves of the flask
  preventing them from being separated easily.

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• Therefore, a separating medium sometimes refered
  to as cold mould seal has to be used as a
  barrier.
• Usually, this is a solution of sodium alginate
  which is brushed onto the invested surface. Tin
  foil was at one time used extensively as a
  separating medium, being burnished onto the
  surface.
• This process was very time consuming and it is no
  longer used routinely.

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• However, since the alginate film is not
  completely water resistant, the surface of the
  cured acrylic may be slightly opaque as a result
  of the interaction with water.
• Therefore, if perfect clarity is absolutely
  essential, tin foil technique may still be used.

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Types of separating media

• Tin foil
• Cellulose lacquers
• Solution of alginated cmpds( Na alginate solns.
  Na phosphate, glycerine, alcohol, preservatives)
• Calcium oleate
• Soft soaps
• Sodium silicate
• Starches
• Evaporated milk

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Mixing of powder and liquid

• The powder/liquid ratio used is dependent upon
  the character of powder including the particle
  size and shape.
• It is always quoted by the manufacturer for a
  specific product and the instruction should be
  carefully followed.
• The critical factor is the ability of the liquid
  to wet all the powder completely and this is
  usually achieved with a liquid volume/powder
  volume ratio of 13.3 or 13.5 which correspond
  roughly to 12.5 by weight.

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• Excess liquid is undesirable as this gives a
  greater amount of polymerization shrinkage.
• Measuring devices are usually provided by the
  manufacturer.
• The mixing is done for a short time typically
  45seconds and then allowed to stand in a clean
  container to prevent evaporation of the volatile
  monomer for a length of time determined by the
  manufacturer.
• This is usually for about 10- 15min during which
  time the powder and the liquid interact.

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• The monomer first melts the powder and produces a
  coarse-textured material with a texture similar
  to that of wet sand (sandy stage).
• As the monomer starts to dissolve the surface of
  the polymer particles, the mixture becomes tacky
  (tacky stage) and then as it become saturated
  with more and more polymer in solution it looses
  it tackiness and forms a dough (dough stage).
• At this dough stage, it is ready to be packed
  into the mould.

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• The material should not be left longer than the
  dough stage before packing since it becomes too
  rubbery (rubbery stage) and eventually quite
  stiff (plastic).
• The dough time is influenced a number of factors
• (i) Powder liquid ratio. If a high powder/liquid
  ratio is used, it is reduced.
• (ii) Small powder particles and lower molecule
  weight of the polymer also aid rapid dissolution
  and give shorter dough time. The same effect is
  produced by raising the temperature and
  conversely refrigeration will slow the process
  down.

89
Packing

• The dough is shaped into a horseshoe form and
  placed in the upper half of the flask over the
  teeth. Sufficient resin should be used to ensure
  complete filling of the mould space.
• A polyethylene or similar sheet is place over the
  resin to prevent adhesion to the lower half of
  the mould during trial closure.

90

• This is achieved by pressing the two halves using
  a hydraulic or mechanical press, slowly together
  so that the dough spread out to fill the mould
  space.
• Any excess resin will flow out between the two
  halves of the flask producing a FLASH. This is
  removed and additional trial closure made until
  no appreciable flash is produced.

91

• The model is then repainted with cold mould seal
  and the protective sheet removed.
• The flask is reassembled and placed in a flask
  clamp so that curing may be achieved under
  pressure.
• The pressure of excessive flash leading to
  incomplete closure of the flask results in an
  increased vertical dimension in the denture and a
  corresponding reduction in the free way space.
  Some denture base resins flow readily at dough
  stage and a trial closure may not be necessary.

92
Curing (polymerization)

• In practice, curing cycles involving temperature
  between 60 and 1000c are used.
• There are both advantages and disadvantages in
  reducing the time needed for curing by increasing
  the temperature.
• The obvious advantage is that the total
  processing time is much shorter and with curing
  cycles that may last about an hour, the
  technician is able to provide a same- day
  service.

93

• On the other hand, there is the greater risk of
  porosity with rapid cure.
• This arises because the polymerization process is
  exothermic and the temperature of the resin will
  exceed the water temperature at some stage of the
  process the surrounding gypsum being a very poor
  conductor of heat.
• The boiling point of methyl metacrylate monomer
  is 100.30c. If the resin is heated rapidly, the
  exothermic reaction occurs early at a time when
  there is still a large amount of monomer.

94

• The vaporization of this monomer causes porosity
  which may reduce the strength of the denture and
  consequently microbial contamination.
• With a slower rise in temperature and hence a
  slower rate of reaction, the maximum temperature
  of the resin is reached later when there is less
  monomer available for vaporization.
• In addition a rapid cure may lead to warpage of
  the denture.

95

• The orthodox curing cycle involve heating the
  flask in a Pacco water bath to about 72 750c
  and leaving it at this temperature for at least 9
  hours usually overnight.
• This was thought to give best results, but much
  more than this, it is a convenient method.

96

• The problem associated with fast cure can be
  avoided if the rate of increase in temperature is
  kept reasonably low even if the final temperature
  is much greater than 750c.
• This allows sufficient time for most of the
  monomer to be used up before temperature reaches
  its boiling point.

97

• This is usually achieved by heating the flask
  either in air or water to about 700c for 1
  2hours before raising the temperature to 1000c
  for about an hour.
• There are variations in time and temperature for
  different products hence, manufacturers
  instruction must be followed.
• Many denture base resins are now supplied in a
  form suitable for even quicker setting.

98

• It is important that the flask is cooled in a
  controlled manner and that it reaches room
  temperature before it is opened.
• Rapid cooling and removal from the flask can lead
  to residual stresses in the denture, which in
  turn may result in distortion.
• Therefore, the flask is generally allowed to cool
  slowly in the air.
• Deflasking is also performed carefully to avoid
  damaging the denture, the plaster being removed
  with a saw.

99
Finishing and polishing

• Polishing is usually carried out wet for the same
  reason of avoiding a rise in temperature and
  subsequent warpage. Polishing procedures should
  not be carried out on the fitting surface of the
  denture.

100
Investment procedure
101
Deflasking and finishing
102

• Thank you for listening!!!!