Laboratory Procedures in Orthodontics.ppt

LABORATORY
PROCEDURES
DR SUGAREDDY
PROFESSOR HOD, DEPT OF
ORTHODONTICS
NAVODAYA DENTAL COLLEGE,
RAICHUR

Brazing :
-joining of metal parts to a suitable temp. below
the solidus temp. of the parts.
-filler metal having liquidus temp above 450°C
-filler metal flows by capillary attraction between
the parts.

Soldering:
-joining of metal parts to a suitable temp. below
the solidus temp. of the parts.
-filler metal having liquidus temp below 450°C.
-filler metal flows by capillary attraction between
the parts.

Ideal requisites of dental solders :
1. Exihibit excellent tarnish & corrosion resistance in
oral environment.
2. Fusion temperature of the solder should be 50o C -
100 o C
3. Solder should be free flowing & should adequately
wet the metal parts, so that good adhesion is
achieved.
4. Strength should be similar to that of metals being
joined.
5. Colour should match with that of the parts to be
soldered.

 Gold , silver , copper , zinc , tin & nickel are
common dental solders used.
 Copper gives yellow colour but its substitution
by nickel gives more white colour.

Types of Soldering
 Investment soldering
 Free Hand soldering
Investment soldering : it is carried out whenever the
area of contact between the metallic parts being
joined is large & whenever precision is needed in
joining the metals.
The procedure involves embedding
of the metallic parts in an investment leaving a gap of
about 0.13 mm between metals.

 Free Hand soldering
soldering without use of investment is
called as free Hand soldering. Most commonly
used procedure by orthodontists.

Flux
- purpose of flux is to remove oxide layer on the
substrate metal
- Flux dissolves any surface impurities
- Flux reduces melting point of dental solder
Composition :
-silica
-boric acid
-borax glass
-boric anhydride
-KF for dissolving passivating film formed by Cr.

ANTIFLUX :
Material used to confine the flow of the molten
solder over the metals being joined.
Eg: graphite lines, iron rouge & pencil markings.

Heat source
 Hydrogen gas
-flame temp.2660°C
-low heat content indicates heating would be slow
 Natural gas
- flame temp.2680°C
-gas is available in non uniform composition
-has water vapors which cools the flame & uses
some of the heat content of the gas.

 Acetylene
- flame temp.3140°C
- highest flame temp.
- the variation in temp. from one part of the flame
to another may be more than 100°C
-with this variation, the positioning of torch is
critical.

 Propane
-flame temp.2850°C
-highest heat content
-readily available gas
Butane is most commonly used.

Flame
 It is divided into four zones
-cold mixing zone
-partial combustion zone (unburned gas )
-reducing zone
-oxidizing zone ( burned gas )
 Reducing zone produces the most efficient
burning process & most heat.

Steps in Soldering
1. Cleaning the surfaces to be joined
2. Assembling the parts to be joined
3. Selecting the right solder & the flux
4. Selection of proper joint
5. Application of flux
6. Heating & introduction of solder
7. Quenching

Practical considerations in soldering :
1. Use the reducing flame of the soldering torch
2. Use wet cotton & asbestos to limit the spread
of heat
3. Soldered joint should not be polished as it
weakens the joined
4. Anti flux should be used to prevent excessive
spread of solder.

Welding:
-joining of two or more metal parts by applying
heat, pressure or both.
-filler metal is not used.
-a localized union takes place across the interface
through fusion or diffusion.

Types of Welding
 Cold Welding
involves application of pressure
 Hot Welding or Spot Welding
involves both heat & pressure

Principles of Spot Welding
 Heat & pressure are used
 Electric current is made to pass through a step down
transformer to obtain a low voltage & high amperage
current that is conducted through two copper
electrodes on either sides of the metals being joined.

 Passage of current is not more than 1/10 seconds.
 If more, it results in weld decay due to precipitation
of carbides from the metal.
Welding of stainless steel depends upon:
- current flowing through circuit
- time duration
- mechanical pressure applied at joint.

Procedure of Spot Welding
 Broad electrode to be used for thinner material & narrow
electrode for thicker material
 Surface of electrodes to be cleaned to remove carbides.
 When electrodes are together, it should be in tight
contact.
 The welder timer should be set
 Metals joined to be placed between the two electrodes.
 The electrode pressure to be maintained for few seconds
to obtain good joint.

Applications of Welding
 Joining of metal strips during banding
 Fixing attachments such as brackets & molar tubes
onto bands.

ORTHODONTIC STUDY MODELS
Ideal requisite of study models :
1. Models should accurately reproduce the teeth,
alveolar bone & their surrounding soft tissues.
2. Models should enable instant identification of
asymmetries in arch form
3. Models should accurately reproduce
occlusion
4. Study model should have a nodule free
surface

STEPS INVOLVED IN CONSTRUCTION OF
STUDY MODELS :
1. Impression making
2. Disinfection
3. Casting the impression
4. Basing & trimming of the cast
5. Finishing & polishing

Uses of Study Models.
 1)To assess and record the dental anatomy ,
intercuspation , the arch form .
 2)To carry out various space analysis.
 3)To assess and record the curves of occlusion.
 4)To evaluate occlusion with the aid of articulators.
 5)To measure progress during treatment.
 6)To detect abnormalities. (eg: localized
enlargements,distortion of arch form)
 7)To provide a record before ,immediately after, and
several years following treatment for the purpose of
studying treatment procedures.

Parts of the Study Models.
 ANATOMIC portion.
 ARTISTIC portion.

Anatomic portion.
 Anatomic portion is that part of the study
model which is the actual impression of the
dental arch and its surrounding structures.
 Usually made of stone plaster.
 Must be preserved while trimming the model.

Artistic portion.
 The artistic portion of study model is a plaster
base that supports the anatomic portion.
 Helps in depicting the actual orientation and
the occlusion of study models.
 Gives a pleasing and symmetrical appearance
to the models.

In a well fabricated set of study models
the ratio of the anatomic portion to
artistic portion should be 3:1.

Steps in construction of study
models.
 Impression making.
 Taking a wax bite.
 Disinfection of the impression.
 Casting the impression.
 Basing and trimming.
 Finishing and polishing.
 Care of the study models.

Impression making.
 Irreversible hydrocolloids(alginate) are widely used.
 Advantages of alginate:
1)easy to manipulate.
2)comfortable for the patient.
3)relatively inexpensive as it does not require elaborate
equipment.
4)has pleasant taste.
5)able to displace blood and saliva.
6)they are hydrophilic.
7)they are compatible with stone so it is easy to pour and
retrieve the cast.
8)they can be used with stock trays.

Orthodontic impression trays.
 Orthodontic trays are used as their rim in labial
region is about ¾ inch (20 mm) and their edges are
beaded.
 Strips of soft utility wax or mortem can be added to
tray periphery.

Disinfecting the impression.
 First rinse with water.
 Immerse it in a disinfectant solution approved
by O.S.H.A such as Biocide or Chlorhexidine.
 Be sure to rinse it again with water to remove
the residual disinfectant solution.

Taking a wax bite.
 It should always be taken as it helps the dentist
to relate the upper and lower casts in
occlusion.
 Also helps to hold the casts flushed with each
other while trimming.

Taking a wax bite.
 Phillip Adams:
Wax bite should consist of only a bar of
moderately softened wax across the premolar
region.
 Graber:
Two layers of soft base-plate wax roughly
shaped to arch form and warmed in water may
be used.

Casting the model.
 The models may be cast in
Plain dental plaster.
Stone plaster.
Mixture of plain plaster and stone plaster.
Or the anatomic portion may be cast in stone
plaster and and bases in plain plaster.

Cross Section of Plaster Casts in Rubber
Mould Base Formers:
 Occlusal plane should
be parallel with top and
bottom cast surfaces.
 The back surfaces of the
casts should be related
in exactly the same
plane ,perpendicular to
top and bottom cast
surfaces.

Broussard cast former.
(Rocky Mountain Dental Products Co.)

STUDY MODEL TRIMMING.
(Lawson)

American Board of Orthodontics
requirements for study models.
(American Board of Orthodontics: Specific Instructions for
Candidates, St Louis,1998, The Board)

American Board of Orthodontics
requirements for study models

Study model trimming…
 Accurate detailed
impressions are essential to
the production of high
quality study models.
Before the mandibular cast
is poured the tongue space
must be filled in with
alginate.

 Casting material is carefully
proportioned according to
the manufacturers
instructions.generally , the
ratio of 30 ml of water to
100g of stone will be
sufficient to pour one
model. This should be
spatulated for 20 to 30
seconds by a vacuum mixer
to produce a smooth, dense,
bubble free mix.

 Vibrate the mix into the
impression so that it flows
slowly from one posterior
end around to the anterior
and then the other posterior
end. Avoid incorporating air
bubbles. Fill its base in a
like manner. Carefully
invert the impression onto
its base and add stone to the
heel areas so that they
extend smoothly into the
model base surface. Repeat
this procedure for the
opposing model.

 Cast are allowed to dry
thoroughly .
 Now one electric plaster-
trimming machine with a
medium grit,carborundum
wheel,grit no. 60 is used for
trimming.
 Trimmer has dual wheels.

 A template(or degree
plate) on the model
trimmer platform is
used as a reference for
holding the casts at a
proper angle while
trimming.

 In addition to no.600
wet and dry sand
paper,small and medium
scrapers,lab knife and
some special equipment
will be used.
 Angle guide.
 Squaring tool.

 Allow the models to dry
for one hour after
pouring.remove the base
formers and then the
impression.
 Inspect.

 The casts are soaked in
water for atleast 10
minutes before they are
trimmed. Never attempt
to trim a dry model.

 Trim the maxillary cast first.
Remove excess stone from
the heel areas so that the
incisors and terminal molar
cusp tips will be able to
touch the flat plane of the
squaring tool
simultaneously.

 Position the occlusal
plane against the
squaring tool .
 Check to be sure proper
contact is being made.

 Turn on the water supply before
switching on the model trimmer.
 While holding the occlusal plane
of the cast firmly against the
squaring tool, slide the whole unit
toward the rough cut wheel. Trim
away the excess base material so
that the remaining base portion is
about one third the total cast
height.

 Draw a line over the
median palatine raphe.

 A perpendicular line is
drawn, about ½ inch distal
to the hamular notches.trim
the back of cast to this line.
 The back of the cast forms
right angles with the median
palatine raphe.

 The buccal segments are
trimmed at a 70 degree
angle to the back of the cast.
 To accomplish this, rotate
the degree plate and slide
the angle guide into place.
 Press the cast against the
angle guide and trim to
about 5mm from the buccal
surfaces of teeth
 Reset the degree plate to
trim the other side.

 Use even, steady
pressure against the
wheel,but let the model
trimmer do the work.
 Both buccal segments
have been trimmed at a
70 degree angle.

 Change the degree plate
to indicate a 30-degree
setting and reposition
the angle guide.
 Trim the maxillary
anterior portion.
 Carefully preserve the
mucobuccal fold area.

 Both anterior angles are
complete.
 Note that each angle
begins at a point at the
middle of the canine.
 The two angles intersect
at a point that is an
extension of the median
palatine raphe line.

 The maxillary posterior
segments are trimmed at
130-135 degrees angle.
 They should be about ½
to 5/8 inches wide.

 Occlude the maxillary and
mandibular casts, trimming
excess stone from the
mandibular heels if
needed.
 The wax bite registration
wafer should be in place
when trimming occluded
models because the
vibration may abrade the
tooth surfaces.

Use the squaring tool to
trim the mandibular
base so that it is parallel
to the maxillary base.
The height of the base
should be about 1/3 rd
the total height of the
cast.

 With the casts occluded,
trim the back of the
mandibular cast so that it is
parallel and flush with the
maxillary cast.
 This will allow the casts to
remain in occlusion when
placed on their backs on a
flat surface.

 The backs of the
maxillary and
mandibular casts are
trimmed evenly.
 The mandibular
posterior corner
segments are trimmed to
130-135 degree
angles,flushed with their
maxillary counterparts.

 The buccal segments are
trimmed at 65-degree
angles to the deepest
part of the buccal
vestibule.
 Do not trim closer than
5mm from the buccal
surfaces of the teeth.

 Make a semicircular
mark from the center of
one canine to the center
of the other. This will
indicate the anterior
trim line.
 It should be no closer
than 5mm from the
labial tooth surfaces.

 Carefully trim the
anterior segment to the
mark.
 It should have a
smooth,symmetrical
shape.

 Gently carve any excess
material from sulcus
areas.
 Use a laboratory knife
and scraper to remove
blebs and define detail.

 Use extra-fine no.600
sandpaper to smooth
sulcus areas by hand.
 The tongue area must
also be sanded smooth.

 Use the fine sanding
wheel on the model
trimmer to smooth
scratches left by the
rough wheel.
 Fill any small voids in
the models with a thin
mix of orthodontic
stone.
 Lightly sand again if
necessary.

 Allow the models to dry
for 24 hours before putting
them in finishing soap
solution. They should be
soaked for about 20
minutes but not more than
30, or the models will
dissolve.
 Rinse off the excess soap
and buff with a soft cotton
cloth.

 After the models have
dried they must be
labelled.
 Use an indelible marker
to print the patients
name and the date the
impressions were taken.

 The study model set is
complete.
 Note the sharp,
symmetric angles.
 The surfaces are sanded
and polished to a
smooth,attractive luster.

 The base of each model
is parallel and flush with
the base of the other.
 The set will remain in
occlusion when resting
on the heels, or on the
right or left posterior
corners.

Care of study models. (White and Gardiner)
 Record casts should be kept in boxes holding
5-6 sets of models,each pair held together by a
light elastic band.
 A square of thin plastic foam should be kept
between the occlusal surfaces of the teeth.
 The wax bite should not be left between the
teeth, however it should be stored as it will be
of help in finding the occlusal relationship at a
later date.

Orthodontic model boxes.
(J. of Cl. Ortho.,1991).
 Orthodontic model boxes can be used for systematic
storage, identification and coding of study models.
 BARGER model box: lid style. Stores 8 models per
box.
 FUNT model box: drawer style. Stores 4 models per
drawer, 8 models per box.
 ARTICULATED model box: drawer style. For tall
mounted models.
 Space maintainer lab, Indiana.
North Western paper box company,Washington.
The Board foot, Texas: Model display cabinets.

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