Full metal crown construction

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• Full metal crown construction
• Casting procedures

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• Definition
• Casting procedure is the process of converting
  the wax pattern into a metal restoration ready
  for cementation.
• -After preparation is completed, tissue
  retraction is done and an impression is taken. A
  stone working cast will be obtained. Wax pattern
  and spuring is carried out on the removable die.
• -Investing, burning out of wax and casting of
  metal, then finishing and polishing of the cast
  crown will be the last step in construction of
  all metal crown.

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• I- Wax pattern
• Definition
• It is the process of rebuilding the prepared
  teeth, on the removable die, by wax using direct
  and\or indirect technique.

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Methods of wax pattern construction

• There are three methods for construction of wax
  pattern
• 1)Direct wax pattern
• -Indicated in case of post crown and inlay
  retainer.
• -Done and adjusted and carved in the patients
  mouth.

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• 2) Indirect wax pattern
• -Indicated in all types of retainers.
• -Done on the die and working cast.
• 3)Indirect-direct wax pattern
• The wax pattern is constructed on the die and
  then the margins are refined and rechecked in the
  patient mouth.

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Steps of wax pattern construction (indirect
method)

• 1-lubricate the die by any separating medium.
• 2-using inverted cone bur, a circumferential
  groove might be cut around the die, 0.3 mm below
  the preparation margin.
• N.B. this process is called Ditching of the die
  it act as

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• a) A guide in carving the wax pattern.
• b) Checking the seating of the crown on the die.
• c) Finishing of the cast crown at the margin.
• 3-the wax is softened and placed on the die
  increment by increment or by dipping the die in a
  container that contains molten wax several times.

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• 4- after hardening of the wax, carving is carried
  out by blunt and slightly hot wax carver. This is
  important to minimize the internal stresses
  inside the wax
• 5-the buccal and lingual surfaces should be
  carved to reproduce proper contour and convexity
  mesio -distally and occluso -gingivally.
• 6-the mesial and distal surfaces should have
  their proper contour and contact restored with
  the adjacent teeth.

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• 7- the occlusal surface
• a) Its proper carving without any high cusps.
• b) Its proper intercuspation with the opposing
  teeth.
• c) After complete carving and checking,
  smoothening of the wax pattern is done by a piece
  of silk cloth.
• d) The pattern is then sprued and invested as
  soon as possible to prevent stress relaxation and
  deformity of the pattern

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Preparation of the die prior wax pattern
construction

• 1) Trimming the die
• If the die has a representing free gingiva at a
  higher level than the finish line, trimming away
  must be done with a stone to define the margin.

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• 2) Correction of defects
• There are may be small effects in the die
  resulting from caries or previous restorations
  that are under cut with respect to the path of
  withdrawal of the new restoration. It is
  convenient to block them out on the working die
  with zinc phosphate cement.

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• 3) Shaping of the die handle
• Shaping of the die handle by using an acrylic
  bur, then ditching the die below the finish line
  by 0.3mm using inverted cone bur, this ditching
  is a circumferential groove act as a guide during
  wax pattern construction at the gingival margin
  and help also during finishing of the cast
  restoration.

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• 4) Marking the margin
• The finish line of the preparation should be
  colored with color bright red pencil without any
  scratches and to facilitate the carving of the
  wax at this margin accurately.
• 5) Provision of an adequate cement space
• Die spacer ( similar to model airplane paint) is
  used to maintain good marginal adaptation. The
  produce space (20-40 micron) is necessary to
  provide room for the luting agent and permit
  complete seating of the restoration during
  cementation.

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• After that the die is thoroughly lubricated with
  a water-soluble lubricant to allow the die to
  soak in for several minutes, remove the excess
  lubricant with gentle stream of compressed air.
  The lubricant will reduce the surface tension and
  allow the wax to flow more freely.

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The casting wax requirements

• 1) Has different color than the used die
  material.
• 2) Flow readily
• 3) Easily carved without shipping or flaking
• 4) Accept addition and carved easily and
  precisely.
• 5) Evaporate without leaving any debris or
  residual ashes, which can contaminate the
  produced cast restoration.
• 6) Take the fine and sharp details of the pattern
  and preserve it till investing without any
  distortion.

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• 7) Has a minimum rate of stress relaxation after
  carving and before investing to prevent any
  distortion.
• 8) Can be burnished and polished.
• 9) Can compensate part of the metal shrinkage
  during its solidification.
• 10) Has acceptable strength and rigidity after
  cooling.

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Different methods of wax pattern construction

• 1) Dipping method to develop a thin, uniform and
  adapted layer of thin wax on the die. This is
  done by dipping the die into wax that has been
  thoroughly melted

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• 2) Addition method
• To obtain a bulk of the wax pattern, then carving
  is carried out. This is done by melting the wax
  and dropping it on the die using a heated carver
  until complete bilding of the pattern, and then
  carved by sharp carver. The main drawback of this
  technique is due to strain collected from
  multiple addition of wax tends to release with
  time and subsequently distort the wax pattern.

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Wax instrument

• P.K.Thomas wax instrument consist of
• 1- wax addition.
• 2- wax carving.
• 3- wax burnishing.

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• P.K.Thomas no.1 and no.2 are wax addition
  instruments.
• P.K.Thomas no.3 is aburnisher for refining
  occlusal anatomy.
• P.K.Thomas no.4 and no.5 wax carver.

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• Wax is added by heating the instrument in the
  bunset flame touching it to the wax and quickly
  reheating its shank in the flame. wax flows away
  from the hottest part of the instrument, so that
  if the shank is heated, a bead of wax will flow
  off the tip. PKT no.1 instrument is used for
  large increment, the smaller no.2 is used for
  lesser additions. No.7 A waxing spatula is useful
  for adding large amount of wax.

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Electric wax instrument
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Wax spatula instrument
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Internal Surface. Forming a closely adapted
internal surface is the first step in waxing. The
wax must reproduce all retention features of the
restoration.
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The complete and accurate wax pattern

• A- Proximal surfaces
• 1) A proximal surface of the pattern cervical to
  the proximal contact should be flat or slightly
  concave for interdental papilla.
• 2) Contact area located more occlusally and
  buccally leading to wide lingual embrasure,
• While between the first and second molars is
  centrally placed

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• 3) Too narrow contact allows fibrous food to
  wedge between teeth, while wide contact do not
  deflect food.
• 4) To ensure that the finished restoration with
  enough bulk and slight pressure in contact with
  the adjacent teeth, without creating an open
  contact, grind small amount of stone from the
  proximal surface of the adjacent teeth on the
  cast of both sides

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Buccal and lingual surfaces

• 1) The surfaces corresponding to surfaces of the
  adjacent teeth in the arch.
• 2) Height of contour lies the cervical 1/3 except
  on the lingual surfaces of the mandibular
  posterior teeth, which occur at the middle 1/3
• 3) Height of contour extends 0.5mm. Away from the
  level of cementoenamel junction except the
  lingual height of lower second premolar (0.75mm)
  and lingual of the lower molar (1mm)

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• 4) The line between the base of gingival sulcus
  and height of contour described as emergence
  profile, which is a straight line to faciliate
  cleaning by toothbrush. This creating a space
  between a) 0.7mm in ceramic restoration b) 0.35mm
  in metallic one.
• 5) Over contouring produces gingival inflammation
  As it promotes the accumulation of food debris
  and plaque, also hyperplastic changes of the
  gingiva may occur, while under contouring may
  cause gum recession due to the forced deflection
  of food direction or in many cases no significant
  changes.

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Occlusal surface morphology

• 1) During centric closure of normal dentition,
  lingual cusp of upper and buccal of lower called
  functional cusps, which occluded to fossa or
  marginal ridges. While other cusps (non
  functional) act as rim to prevent over flow of
  food and protect the cheek and tongue away from
  the functional cusps.

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• 2) Occlusal anatomy and cusps follows the curve
  of wilson and anterioposterior curves (curve of
  spee).
• 3) Small concavities placed on the distal
  inclination of buccal cusp of upper premolars on
  lateral movements.
• 4) Two types of grooves are found, the primary
  grooves which represent the union of the lobes
  and form the major part of the main pits and
  groove pattern, while secondary grooves are to be
  characterized by its shallower position and not
  concerned with tooth development. The secondary
  grooves located at the inclined planes, increase
  the cutting and shearing efficiency during
  mastication.

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3) Cervical margin

• 1) The margin should be checked and corrected.
  Some prefer to readapt the margin by burnishing
  immediately before investing.
• 2) The margin is not over extended may affect the
  gingival after applying the restoration leading
  to gum recession, or under extended which may
  cause caries around the margins. The bulge margin
  also is biologically not accepted.

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