Electrolytic processes in restorative dentisrty /certified fixed orthodontic courses by Indian dental academy

ELECTROLYTIC
PROCESSES IN
RESTORATIVE
DENTISTRY

INDIAN DENTAL ACADEMY
Leader in Continuing Dental Education
www.indiandentalacademy.com


CONTENTS
♦ INTRODUCTION
♦ HISTORY
♦ DEFINITIONS
♦ ELECTROFORMING
♦ ADVANTAGES
♦ MODE OF DEPOSITION
♦ FABRICATION OF INLAYS AND ONLAYS
♦ APPLICATION OF CERAMIC
♦ THE METAL CERAMIC BOND
♦ FLAWS IN ELECTRODEPOSITION


♦ ELECTROFORMING IMPRESSIONS
♦ COPPER FORMED DIES
♦ SILVER FORMED DIES
♦ BONDING TO PORCELAIN USING
ELECTRODEPOSITION
♦ ELECTROLYTIC ETCHING
♦ ELECTROLYTIC POLISHING
♦ CONCLUSION
♦ REFERENCES


INTRODUCTION
Materials used for restorations to remain in the
mouth over the long term period must meet
extraordinarily high requirements. They must be
able to withstand a variety of diverse mechanical,
chemical, thermal and osmotic changes without
undergoing any esthetic or functional alteration.

Electroforming or electrodeposition technology
provides treatment characterized by a good long
term prognosis while meeting esthetic and
biological requirements.


HISTORY
♦ The German scientist Jacobi laid the
groundwork in 1837 for all subsequent
industrial and dental electroforming.
♦ Electroforming techniques have been used in
dentistry for more than 50 years. In 1935,
Daimano and Viverihofi fabricated the first
electroformed dies from hydrocolloid
impression materials.
♦ Since 1950s , electroplating of mercaptan
rubber were performed.
♦ In 1962, Armstrong and Rogers produced gold
copings with a thickness of 250 ųm.

♦ The first ceramic veneered crowns were produced
by Rogers in 1979.
♦ In 1971, Weismann was awarded the patent for
fabrication of dental prostheses using dies coated
with silver lacquer.
♦ In 1989, Weiland Edelmetal Company (Germany)
introduced the first electroforming unit – the Auto
Galvano Crown Device.


DEFINITIONS
♦ ELECTROPLATING – This is the process in
which the workpiece (cathode) is plated with a
different metal (anode) while suspended in a bath
containing a water base electrolyte solution. Also
called electrodeposition
♦ ELECTROFORMING –is a highly specialized
process of metal part fabrication using
electrodeposition in a plating bath over a base
form or mandrel which is subsequently removed,
thus the coating itself becomes the product.
This differs from electroplating basically
because the skin is much thicker and can exist as
a self-supporting structure if the original matrix
is removed.

♦ ELECTOLYTIC ETCHING – The process by
which the surface metal is electrochemically
removed to create microscopic three dimensional
relief for micro mechanical retention.
♦ ELECTROPOLISHING –It is the controlled
removal of a layer of surface metal using a
combination of chemicals and electrical
current.This procedure is essentially opposite to
electroplating in which the metal part is the anode.
Used for polishing of base metal alloys.


ELECTROFORMING
♦ Electroforming is an alternative to cast metal
technology with significant advantages.
♦ It is an electroplating process where a thick
metal layer is deposited onto a mandrel, or
original to be replicated, and is then separated
from it.
♦ The part thus obtained is called an electroform.
The main advantage of electroforming is that it is
an atomic scale process assuring replication
fidelity that is unmatched by any other
technology.


ADVANTAGES OF
ELECTROFORMED
RESTORATIONS
♦ HIGH BIOCOMPATIBILITY
♦ PRECISION OF FIT
♦ PULP PROTECTION
♦ EASE OF LUTING
♦ ESTHETICS
♦ REASONABLE COST OF FABRICATION


♦ BIOCOMPATIBILITY–
a) Use of inert materials like ceramic and gold.
b) Does not require biologically questionable
unbound oxide layer for bonding as in metal
ceramic restorations.
c) Soldering is also avoided because solder
contains non precious metals subject to
corrosion.
d) Does not contain non homogenous lattice
structures resulting from heat, cold,inclusions
etc.
e) Minimal superficial flaws and impurities all of
which reduce the corrosion and release of toxic
by products.

♦ PRECISION OF FIT- Based on the technical
fabrication process, the precision of fit surpasses
that of other ceramic restorations and also that of
cast restorations made of base metal alloys or
precious metals.
♦ Hanning et al stated that there is less than 50 µm
marginal gaps in the electroformed inlays .
♦ Schafers reported marginal gap measurements
made at 375 points in electroformed inlays and
crowns. More than 80% of measurements were
less than 20 ųm.
♦ This is also attributed to burnishing or adapting
the ductile gold edges of the restoration.


♦ PULP PROTECTION – Due to their reduced
space requirement, it is hard tissue sparing (pulp
protecting) and is indicated for use in young
patients with vital teeth.
♦ The fine gold coping is 0.2 mm thick provides
adequate space for ceramic with appropriate
rigidity unlike cast metal framework of ceramo
metal restorations which is 0.7mm thick.
♦ All ceramic restorations, although esthetic, require
the largest amount of hard tissue reduction during
tooth preparation (1.5 – 2 mm).


♦ EASE OF LUTING – zinc phosphate cement is
the material of choice for luting unlike use of
technique sensitive resin materials for all ceramic
restorations.
♦ ESTHETICS – Major ceramic component and the
gold color of the coping result in the esthetic
appearance of the restorations.
♦ RESONABLE COST OF FARICATION – The
fabrication cost is less than that of the full cast
crown. Also the gold used in the baths is
recyclable depending upon the specific unit.
It offers broad indications for use in
inlays,onlays, crowns, fixed partial dentures,
prostheses with implants and electroformed bases
in complete denture prostheses.


MODE OF DEPOSITION
♦ A master cast of the tooth is prepared and coated
with a die spacer to facilitate separation of the
duplicating material.
♦ The dies are duplicated and a conductive silver
layer is applied to its surface.
♦ The die is connected to a plating head which is
the cathode.
♦ The gold bath is the decisive element in
deposition. Gold sulfite baths are commonly used
in dentistry.


♦ Earlier cyanide baths were used which can
release toxic cyanide vapor if acids were added
and hence were discontinued.
♦ Gold in the bath exist in the form of a dissolved
quaternary ammonium complex. The chemical
formula for the ammonium-gold-sulfite complex
is:
(NH4)3[Au(SO3)2].
Amine is added in the bath is for stability.
♦ This complex dissociates into a cathodic gold-
amine complex and a sulfite ion.


♦ The gold amine complex is destroyed on the
cathode surface, leaving a thin deposit of fine
gold, while the amine remains as a reduction
product in the bath solution.
♦ These reduction products are responsible for the
limited life of the electrolytic baths. Extension of
the bath’s life is possible by the addition of gold
concentrate.
♦ The gold remaining can be recovered from the
floor of the container and can be recycled,
sometimes up to 100%.
♦ Electrodeposition of gold on a surface of 1cm²
implies the deposit of 28 to 31 billion atoms of
gold per second.

♦ Such rapid formation of the fine gold layer does
not permit formation of ideal crystal lattices.
Thus, the hardness is increased approximately by
a factor of 4 through laminar strain formation of
crystal lattice.
♦ This results in the galvanically deposited gold
having a vickers hardness of 140 – 160.
♦ The deposited fine gold with its extraordinarily
fine lattice is recrystallized while being heated in
the ceramic furnace which reduces the internal
strains of the lattice structure.


♦ The gold weight required for inlays and onlays
varies between 0.2 and 0.9 g. The gold
deposited reaches 99.90% purity.
♦ The thickness of the gold copings or internal
restoration is around 0.2 to 0.4 mm deposited
for over 10 – 12 hours.
♦ The melting point of the deposited gold,
1063ºC, and the relative high thermal
coefficient of expansion, 15.2 x 10¯ 6/K in the
temperature range of 25ºC to 500ºC and its
firing stability makes it suitable for ceramic
veneering.


FABRICATION OF INLAYS
AND ONLAYS
♦ The dies are
duplicated with
silicone and dies can
be made with epoxy
resin or die stone.
♦ Special attention is
required to ensure
that undercut regions
and transverse
grooves are carefully
blocked out.


♦ Care has to be taken
against excess blockout
occlusally which may
cause fracture of the
onlay.
♦ Conducting silver lacquer
is applied precisely to the
preparation margin.
♦ To avoid deformation
caused by ceramic
shrinkage, layer thickness
of the inner wall should be
no less than 0.16 mm.


♦ The lacquered die is
then fitted for
electroforming.
♦ Various devices for
electroforming are
available over the years
from the small single
unit device to high
capacity versions.
♦ Capacity of the unit
varies from 5- 10 l, and
having a gold content
of 100- 150 g.


The most critical step in the
electroforming process is
the exact buildup and
cutback. For this, rubber
polishers of diverse sizes
are used.
♦ Beginning with the
proximal surfaces, the
electroformed margins are
ground at an angle of 90
degrees down to the
preparation margin with a
soft rubber polishing
wheel.


♦ The gold margin is now thinned starting at the
inside.
♦ Occlusally, excess material is first removed in the
direction of the preparation and the margins are
reduced precisely in the direction of the cusp
slope.
♦ It is important that the electroformed margins
have the exact anatomical shape approximally
and occlusally to avoid later grinding with
diamonds after ceramic firing which leads to
shearing and widening of margins.


APPLICATION OF CERAMIC
♦ After the inlay is
properly fitted on the
working die, the
margins are covered
with wax to protect
them during retentive
sandblasting with 150
µm grain size material.


♦ The wax is removed
and the bonding agent
is applied and firing is
performed.
♦ Next opaquing agent
is applied which is a
stiff paste that can be
thinly feathered down
to the margins.
Additional shades of
blue is applied at the
margins and orange in
the cavity and is fired
again.


♦ After firing the fit is
checked and
application of ceramic
begins.
♦ In the fissure region
orange color tint is
used.
♦ Below the gold
margin, blue tint is
used and is covered
with dentin ceramic
starting at the fissure.


♦ White incisor material
is added moving in the
direction of margins
and is covered with
transparent material.
♦ Partial drying is done
with a fleece.
♦ The still slightly moist
ceramic is then cut
with a very thin,elastic
blade in the regions of
the main fissures down
to the bottom.


♦ The result is a tension
free and homogenous
firing in the direction
of the margins.
♦ The ceramic is then
dried and the inlay is
carefully lifted from
the die and fired.
♦ Thereafter ceramic is
placed in the open cut
regions and corrective
firing is done.


♦ It is shown that an inlay
that fractures is
commonly due to
breaking open of cavity
caused by insufficient
closing of the gap.
♦ The inlay is placed on
the die, then in the
articulator for finish-
grinding of the occlusion
and proximal contact
points and is painted and
glazed.


♦ The inlay is finish-
fired, it is placed back
on the die and the
margins are polished
with a small brush and
diamond paste and is
cleaned with a steam
jet.
♦ Try-in is done and the
fit and the color is
checked & cemented.


♦ Plastic veneers can also be used instead of
ceramic for electroformed inlay. The difference is
in the bonding agent (rocatec) to achieve a
chemical bond with a synthetic material.
♦ An all ceramic insert e.g empress ceramic
material can also be combined with an
electroformed “cavity liner”.
♦ This technique is used when all ceramic inlay is
needed but the preparation in mesial or distal
region lies below enamel margin where adhesive
bonding is not possible.


♦ In this case, the
electroforming
technology can form a
precise base for
creation of an esthetic
ceramic inlay which
serves to fix the inlay
in position.


THE METAL CERAMIC BOND
♦ The theories of metal ceramic bonding assume
that an optimal bond is achieved by means of an
oxide film.
♦ As pure gold by itself has no tendency to oxidize,
a gold bonding agent containing ceramic particles
provides an intermediate step.
♦ The gold particles themselves consist of finer
gold spheres in which ceramic particles are
evenly distributed.


♦ The first step is cleaning and roughening of the
surface.
♦ Sandblasting with particle size larger than 125µm
and blasting pressure less than 2 bar is used.
♦ Sandblasting gives the surface its typical matte
appearance.
♦ Right-angled blasting avoids the formation of
gold lump and the danger of blistering the
ceramic.
♦ The gold bonding agent is applied to the clean
metal with a brush, dried in a kiln and then fired.


♦ During firing the gold particles fuse, flow
together, and completely or partially incise the
ceramic particles.
♦ The individual ceramic particles are partially held
by the fused gold particles and present their
surface to the opaquer of the dental ceramic so
that the ceramic can bond or mechanically
interdigitate.
♦ In the regions devoid of ceramic particles, the
confluent gold particles of the bonding agent
ensure a net like structure where the ceramic can
bond mechanically.


♦ Because of the small size of the particles, the
firing temperature of the gold particles in the
bonding agent is below the melting point of gold.
♦ Thus a close diffusion with the gold frame, a gap
free bond between the bonding agent and gold
coping is formed.


MECHANICAL BONDING
MECHANISMS
♦ The principle behind this bonding mechanism is
interdigitation between metal and ceramic at the
marginal surface of contact, produced by
roughening that surface.
♦ To this end, the metal surface is roughened with
sandblasting which cleans the surface through
abrasion, improves wettability and alters the
surface compositions through localized fusions.
♦ But dental porcelain does not require a
roughened area to bond to metal.


♦ In fact porcelain will fuse to a well polished
surface (Lacy et al 1977) but some surface
roughness is effective in increasing bonding
forces (Yamamoto et al 1985).
♦ A basic principle of metal ceramic systems is that
the thermal expansion of the metal should be
greater than that of ceramic.


♦ The difference in
thermal expansion
during cooling process
produces tension that
lead to the
stabilization of the
bond -compression
bonding.


VAN DER WAALS FORCES
♦ This refers to a bond by means of an electrostatic
attraction between two atoms that approach each
other within a space in which no chemical bond
is effective.
♦ Van der waals forces are achieved through dipole
attraction.
♦ They have little direct influence on the bonding
strength but are important because of their ability
to improve the surface wetting of a metal by the
viscous ceramic during firing.


CHEMICAL BONDING
MECHANISMS
♦ Chemical bonding systems are the primary forms
of bonding include atomic bonds, polar bonds
and metallic bonds.
♦ The ceramic materials form a crystal lattice
consisting of positively and negatively charged
ions, which are held together because of the
mutual attraction of variously charged particles.


♦ There exists a chemical
bond between the
ceramic and the oxides
on the subsurface of
metal which is the most
significant mechanism
♦ But in precious metals
like gold, the oxide layer
does not form and the
placement of the bonding
agent is the primary
mechanism of bonding in
electroformed
restorations.


ALTERNATIVES AND NEWER
TRENDS IN BONDING
♦ The conventional method for ceramic veneering
is by a bonding agent.
♦ However, the bonding agent can concentrate in
the corners of the restoration which is invariably
thick. This may cause the piling up of ceramic
particles suggesting the need for a ceramic bond
without the bonding agent.
♦ The fabrication of diamond burs with
galvanically attached diamond particles suggest a
similar way of anchoring the ceramic particles to
the surface of the electroformed restoration.

♦ In this, a dosing pump delivers a particulate
suspension into the bath leading to direct
electrodeposition of gold onto the surface.
♦ During firing this surface layer reconstructs itself
forming a tubular layer interspersed with cavities
and the ceramic meshes with this layer securely.


FLAWS IN
ELECTRODEPOSITION
1) FAULTS IN TOOTH PREPARATION
i. If the transition from the restoration to the
tooth is in antagonist contact regions, it will
cause cracks in the ceramic.
ii. A narrow isthmus design may cause breaking
apart at this spot during fitting or cementation.
iii. If the edges in the preparation are too angular,
marginal cracks may develop.


2) FAULTS IN
WORKMANSHIP
i. Improper application of
bonding agent will cause
bond failure.
ii. In the absence of fixation of
the particles of bonding
agent, attaching forces find
no resistance, will impede
optimal wetting of the gold
surface.
iii. Insufficient compression of
ceramic before firing causes
centrally located point like
defects in the ceramic inlays.

3) FLAWS IN DEPOSITION
i. Flaws that interrupt the performance of the
electroforming equipment are:
 Programming errors or incorrectly set values
 External interruption like power failures etc
 Improper placement of the dies
 Lack of conductive connection between contact
wire and silver coating
 Moist plaster dies leading to bubble formation


ii. Contamination by foreign substances results in
spongy surface.
iii. Reddish brown precipitate is due to the
formation of tiny spherical particles of gold
instead of formation of a single solid layer. The
reddish Color occurs as a result of light
refraction of the spherical particles.


♦ Low concentration of
gold in the plating
solution will cause
irregular deposition in
the form of globular
structures or streaks
and brownish black
color


CLINICAL EXPERIENCE AND
LONG TERM USE
♦ The durability of electroformed restorations are
comparable to that of other metal ceramic
restorations.
♦ Kaerschbaum et al stated that 5 year survival rate
rate was 99%.
♦ Leempoel et al found that the over a recall period
of 10 years, the rate of successful restorations was
89.7% .


ELECTROFORMED DIES


ELECTROFORMING
IMPRESSIONS
♦ The metal dies that are produced from
electroplated impression material have :
i. High strength – wax patterns and gold castings
can be easily burnished with little distortion to
the die.
ii. Adequate hardness- knoop hardness is 55- 80
iii. Excellent abrasion resistance
iv. Detail reproduction of 4 µm or less is easily
attainable.


♦ Electroformed dies are usually made from
polysulfide impression materials.
♦ Other impression materials that can be
electroplated are compound and silicones.
♦ Hydrocolloid impression materials are difficult to
electroplate and are not used.
♦ The electric current may be supplied by dry cell
storage batteries with a variable resistance and an
ammeter.


♦ A transformer and rectifier with a fixed resistance
can be used where alternating current of 110V is
converted to a direct low- voltage current suitable
for plating.
♦ A small container for the electrolyte with wire
electrodes are used and a bar of pure copper or
silver is the anode.
♦ Acidic copper sulfate solution is used for copper
plating.
♦ Alkaline silver cyanide is the electrolyte in silver
plating.


COPPER FORMED DIES
♦ The popularity of copper plated dies began in
1930s and later silver plated dies became more
popular.
♦ Copper formed dies are made by electroplating
compound or silicone impressions.
♦ The cathode is the impression to be plated.
♦ The anode is electrolytically pure copper and is
immersed in the plating solution . The area of
copper immersed is approximately equal to that of
the impression to be plated.
♦ The plating bath contains acid solution of copper
sulfate.

♦ Composition for solution for copper forming bath:
 Copper sulfate (crystals) - 200 gm
 Conc. Sulfuric acid - 30 ml
 Phenolsulfonic acid - 2 ml
 Distilled water - 1000 ml
♦ Sulfuric acid increases the conductivity of the
solution.
♦ Phenolsulfonic acid serves to assist the penetration
of copper ions into the deeper parts of the
impression-THROWING POWER.


♦ The surface of the impression is coated with a
conductor of electricity – METALLIZING.
♦ Impression compound is painted with a colloidal
dispersion of graphite and is allowed to dry.
♦ When the impression is a silicone rubber, finely
divided copper powder is brushed on the surface
to be plated.
♦ About 15 mA is used to plate a single tooth
impression.
♦ Once a thin layer of copper is formed, the current
is increased by 2-3 times the original.


♦ Plating is allowed to proceed for 12 to 15 hrs.
♦ During electrolysis, copper atoms give up 2
electrons (2e) and become Cu++ ions.
♦ The Cu++ ion is attracted to the cathode where it
gains 2e and is deposited as metallic copper.
♦ The metallic copper of the anode regenerates the
solution as the plating process occurs with the
removal of copper as cathode.
♦ ANODE: Cuº - 2e Cu++
♦ CATHODE: Cu++ + 2e Cuº


♦ The distance between the anode and the
impression to be plated is important, the more
greater the anode to impression distance, the more
even is the quality of deposit.
♦ About 6 inches is the suitable distance. Shorter
distance causes excess copper to be deposited on
the superficial surfaces leaving the deep areas
inadequately plated.
♦ The quality of deposit obtained with a freshly
made solution is not as good as the one achieved
when it has been in use for a short time.


♦ Loss of water from evaporation should be replaced
to maintain correct concentration of the
electrolyte.
♦ A sediment or sludge consisting of fine copper
particles accumulates in the floor of the bath and
the solution must be filtered.
♦ When anodes containing a trace of phosphorus is
used, the formation of sediment is considerably
reduced.


SILVER FORMED DIES
♦ With the advent of polysulfide impression
materials, silver forming became popular.
♦ The impression is made conductive by brushing
the surface with silver which adheres to the rubber
impressions.
♦ Various metallizing agents are available like
bronzing powder, aqueous suspensions of silver
powder and powdered graphite.
♦ The electroplating bath is a solution of alkaline
silver cyanide and the anode is pure silver.


♦ Composition of the silver bath is :
 Silver cyanide - 36 gm
 Potassium cyanide - 60 gm
 Potassium carbonate - 45 gm
 Distilled water - 1000 ml
♦ Addition of acids must be avoided to the cyanide
solution which causes release of poisonous
hydrogen cyanide vapor.
♦ The anode at least twice the area to be plated is
used.


♦ The greater the concentration of silver in the bath ,
faster the silver is deposited.
♦ The polysulfide impression is cleaned thoroughly
and dried and metallized.
♦ An electrical contact is made with the metallized
surface of the impression.
♦ A direct current is applied for 10 hrs using 5 to 10
mA/cm² of cathode surface.
♦ ANODE: Agº - e Ag+
♦ CATHODE: Ag+ + e Agº


♦ The impression is then filled with dental stone.
♦ When the stone hardens, it is mechanically locked
to the rough interior of the electroformed metal
shell.
♦ The impression material is then removed to
provide a die with greater surface hardness and
resistance to abrasion.


PROBLEMS IN SILVER
FORMING
♦ FAULTY CONDUCTION – The ammeter may
show a current flowing, but the impression does
not plate. This is caused by a short circuit through
the electrolyte because of the exposure of the
conducting wire to the solution.
♦ EXHAUSTED SOLUTION – Plating is slow and
the deposit is discolored. The solution must be
discarded and replaced.


♦ OVERCONCENTRATED SOLUTION – The
ammeter reading drops rapidly to zero after the
impression is placed in the bath. This is caused by
over concentration of the solution is rectified by
adding distilled water to it. An overconcentrated
solution softens the impressions and causes
discolorations of the cast.
♦ SILVER ANODE TOO SMALL – An anode
smaller than the impression leads to slow and
irregular plating.
♦ FRIABLE SILVER DEPOSIT – if the deposit is
friable and granular, indicates current setting is too
high.


BONDING TO PORCELAIN USING
ELECTRODEPOSITION
♦ Ceramic bonding to metal may require
electrodeposition of metal coatings and heating
to form suitable metal oxides.
♦ First, a layer of pure gold is deposited onto the
cast metal.
♦ Next, a short “flashing” deposition of tin is
done.
♦ Alloys of cobalt – chromium, stainless steel,
palladium – silver, gold alloys and titanium have
all been successfully electroplated and tin coated
to achieve satisfactory ceramic bonding.


♦ ADVANTAGES
A. Improves the wetting of porcelain onto the
metal.
B. Reduces the porosity at the metal – porcelain
interface.
C. The electrodeposited layer acts as a barrier
between metal casting and porcelain to inhibit
diffusion of atoms from metal to porcelain.
D. The gold color of the oxide film enhances the
vitality of the porcelain compared to the normal
dark oxides.


ELECTROLYTIC ETCHING
♦ The internal (tooth) surface of the cast restorations
can be subjected to electrolytic etching for
auxillary means of retention.
♦ This procedure is done in base metal alloys
leading to selective etching via dissolution of
grain boundaries.
♦ This leaves microscopic irregularities and the use
of a composite resin capable of wetting these
irregularities can increase the retention several
fold.


♦ The margins and the external surface of the
restoration is are covered with sticky wax.
♦ The restoration is used as an anode and stainless
steel is the cathode.
♦ A low voltage current is passed for 10 – 15 mins
to create microscopic irregularities.
♦ The tooth surface is also etched with phosphoric
acid to create an irregular surface for the
formation of resin tags.


♦ Resin bonded fixed partial
dentures are a more
conservative alternative to
conventional fixed partial
denture.
♦ MARYLAND BRIDGE is
a type of resin bonded
etched metal prostheses in
which electrochemical pit
corroding technique is used.
♦ Earlier, retentive resin rivets
extruding through the
perforated framework were
used (rochette bridge).

♦ Livaditis & Thomson postulated that these rivets
were exposed to increased stresses as well as
abrasion & leakage diminished their longevity.
♦ Thus they adapted electrolytic etching used by
Dunn and Reisbick & Tanaka et al to produce
pitting corrosion of metal for retaining acrylic
resin facings on metal framework.
♦ Livaditis et al used a 3.5% solution of nitric acid
with a current of 250mA/cm² for 5 mins. This was
followed by immersion in 18% hcl acid solution in
an ultrasonic cleaner for 10 mins.
♦ The metal framework is the anode and stainless
steel cathode is used.


♦ This will etch the internal surface of solid base
metal.
♦ The acid solution and technique were specific to
the non beryllium nickel chromium alloy.
♦ Subsequently, Thompson et al reported that 10%
sulfuric acid at 300 mA/cm² and same cleaning
procedures will produce similar results with
beryllium containing nickel chromium alloy.
♦ Sloan et al found that an electrochemically etched
surface was 2.9 times more retentive than a
perforated one.


♦ Mc Laughlin et al reported a much faster
technique for etching . This is by immersing them
in a combined solution of sulfuric and
hydrochloric acids placed in an ultrasonic cleaner
for 99 seconds while electric current is passed
through them.
♦ Electrochemical etching is a technique sensitive
procedure producing a gray matte surface.
♦ Over etching produces an electropolished surface
which is shiny and reflective.


ELECTROLYTIC POLISHING
♦ It is a technique for polishing base metal alloys
like cobalt- chromium etc which are very hard and
difficult to polish.
♦ After casting, sandblasting is done to remove any
surface roughness/ green layer of oxide.
♦ Then electropolishing is carried out. This
procedure is essentially opposite to electroplating.
♦ The rough metal surface is the anode in a bath of
strong acid electrolyte.
♦ A current is passed, causing the anode to ionize
and lose a surface film of metal.

♦ The first products of electrolysis will collect in the
hollows of rough metal surface and prevent further
attack in those areas.
♦ The prominences of the metal surface will
continue to be dissolved and in this way the
contours of the metal surface are smoothed.


CONCLUSION
♦ In view of the diversity of today’s alloys and
newer emerging technologies, a superior and
progressive treatment can be rendered to the
patients.
♦ Electrolytic processes presents with entirely new
dimensions of quality to dental restorations,
previously unattainable, hence a thorough
knowledge of these procedures are necessary for
every restorative clinician.


REFERENCES
♦ Electroforming in restorative dentistry – Wirz –
Hoffmann.
♦ Restorative dental materials – Craig
♦ Phillip’s science of dental materials – Anusavice
♦ Fundamentals of fixed prosthodontics- Shillinburg
♦ Introduction to metal ceramic technology – W.
Patrick Naylor
♦ Contemporary fixed prosthodontics – Rosensteil
♦ Applied dental materials- John F Mc Cabe


♦ Notes on dental materials – E C Combe
♦ Tylman’s theory and practice of fixed
prosthodontics - Malone
♦ Journal prosthetic dentistry – 1980, 1982, 1984,
1994


THANK YOU


Electrolytic processes in restorative dentisrty /certified fixed orthodontic courses by Indian dental academy

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