Orthodontic bracket materials

ORTHODONTIC BRACKETORTHODONTIC BRACKET
MATERIALSMATERIALS
www.indiandentalacademy.com

 INTRODUCTION
 TYPES OF BRACKET MATERIALS
 METAL BRACKETS
 STAINLESS STEEL BRACKETS
 TITANIUM BRACKETS
 GOLD-COATED BRACKETS
 PLATINUM-COATED BRACKETS
 PLASTIC BRACKETS
 CERAMIC BRACKETS
 COMPARATIVE BOND STRENGTH CHARACTERISTICS
OF BRACKETS
 COMPARATIVE FRICTIONAL CHARACTERISTICS OF
BRACKETS
 BRACKETS AND PLAQUE ACCUMULATION
 DEBONDING OF BRACKETS AND ENAMEL FRACTURE
 RECYCLING OF ORTHODONTIC BRACKETS
AND ITS EFFECTS
 CONCLUSION

INTRODUCTION
Orthodontic brackets bonded to enamel provide
the means to transfer the force applied by the activated
archwire to the tooth. Before Angle began his search
for new materials, orthodontic attachments were made
from noble metals and their alloys.
In 1887 Angle tried replacing noble metals with
German silver which were actually copper,
nickel, and zinc alloys that contained no silver.
The mechanical and chemical properties of German
silver were well below modern demands.
Stainless steel entered dentistry in 1919,
introduced by F.Hauptmeyer. By 1937 the value of
stainless steel as an orthodontic material had been
confirmed. However disadvantages like nickel
hypersensitivity, corrosion have also been reported.

Plastic brackets were introduced in late 1960s mainly
for esthetics but their tendency to undergo creep
deformation when transferring torque loads and
discolouration led to their unpopularity.
Ceramic orthodontic brackets were first introduced in
1987 as a more esthetic alternative to the traditional
stainless steel brackets. However, the most serious clinical
problems of ceramic brackets were brittleness, incidence of
enamel fracture during debonding and occasional tie-wing
fracture.
Owing to the allergic potential of Nickel that is released
from stainless steel brackets and corrosion of these
brackets, more recently metal brackets are coated with gold
and platinum. Further improvement led to the introduction of
titanium brackets, where titanium is known for its good
biocompatibility.

TYPES OF BRACKETS
(Based on materials)
A) METAL BRACKETS
1) Stainless steel brackets
2) Gold-coated brackets
3) Platinum-coated brackets
4) Titanium brackets
B) PLASTIC BRACKETS
1) Polycarbonate brackets
2) Polyurethane-composite brackets
3) Thermoplastic-polyurethane brackets
C) CERAMIC BRACKETS
1) Monocrystalline alumina (Sapphire)
2) Polycrystalline alumina
3) Polycrystalline Zirconia (YPSZ)

STAINLESS STEEL BRACKETS
Brackets made of stainless steel are alloys formulated
according to the American Iron and Steel Institute (AISI) in the
austenitic classes 303, 304, 316, and 317. According to this
classification, as the number increases, more alloying metals are
added to the iron, while its carbon content is lowered.
The steels, which have AISI numbers beginning with the
numeral 3, are all austenitic; the higher the number, the less non-
ferrous the alloy. The letter L signifies lower carbon content.
303 → has 17-19% Cr, 8-10% Ni and 0.6% Mo
316 →has 16-18% Cr, 10-14%, 2% Mn and 0.08% C
316L→ has 16-18% Cr, 10-14% Ni, 2.5% Mo and 0.02% C
SAF 2205→ has 22% Cr, 5.5% Ni, 3% Mn, and 0.03% C
The 2205 stainless steel alloy has a duplex microstructure
consisting of austenitic and delta-ferritic phases and is harder and
demonstrated less crevice corrosion than 316L alloy. (Oshida &
Moore)
STAINLESS STEEL – COMPOSITION AND STRUCTURE

Steels with a lower AISI number starting with 3 are rather soft
and easier to mill, but their corrosion resistance is low.
Relatively high Chromium content in SS→ favours the
stability of BCC unit cells of ferrite
Ni, Cu, Mn, N→ favours an FCC structure of austenite
Other additives are,
Silicon (Si) → if kept at lower concentration, improves
resistance to oxidation and carburization at
higher temperatures & to corrosion
Sulfur (S) → 0.015% sulfur content allows easy machining of
wrought parts
Phosphorus (P) → allows use of a lower temperature for
sintering
Manganese (Mn) → used as a replacement for nickel to
stabilize austenite

CLASSIFICATION OF STAINLESS STEELS
Stainless steels are classified according to the American Iron and
Steel Institute (AISI) system.
Various steels were,
1) Austenitic steels (300 series)
2) Martensitic steels (400 series)
3) Ferritic steels
4) Duplex steels
5) Precipitation-hardenable (PH) steels
6) Cobalt containing alloys
7) Manganese containing steels
These steels are solid solutions, which
offer better corrosion resistance. The FCC
crystal structure renders these steels
nonferromagnetic. Austenitic FCC structure
is unstable at lower temperatures, where it
tends to turn into the BCC structure known
as ferrite. If austenizing elements (Ni, Mn and
N) are added, the highly corrosion resistant
solid solution phase can be preserved even
at room temperature.
AUSTENITIC STEELS (300 SERIES)
FCC Crystal

Microstructure of these steels is the same as
that of iron at room temperature (BCC). Modern
“Super ferritics” contain 19% to 30 % chromium
and are used in several nickel free brackets.
These are highly resistant to chlorides and alloys
contain small amounts of aluminium and
molybdenum and very little carbon.
MARTENSITIC STEELS (400 SERIES)
In addition to carbon other elements were added to stainless steels to
stress their microstructure and thereby increase tensile strength.
FERRITIC STEELS
DUPLEX STEELS
It consists of an assembly of both austenite and ferrite grains. They also
contain molybdenum and chromium and lower nickel content. Their yield strength
is more than twice that of similar austenitic stainless steels. These steels have
been used for the manufacture of one-piece brackets (Eg: Bioline “low nickel”
brackets).
PRECIPITATION-HARDENABLE (PH) STEELS
These steels can be hardened by heat treatment, which promotes the
precipitation of some elements added.
PH 17-4 stainless steel is widely used for “mini” brackets.
PH 17-7 stainless steel is used to manufacture Edgelock brackets (Ormco).

Various methods used for manufacturing metal brackets were,
1) Milling→ one-piece attachment is milled on the lathe
(Eg: Dynalock bracket)
2) Casting→ where one-piece brackets are made by
casting
3) Sintering→the partial welding together of metal particles
below their melting point
4) Metal injection molding (MIM)→Metal and ceramic
injection molding are derivatives of powder metallurgy. Powders
can be shaped in a semi-fluid state, but after heating to high
temperatures the particles bond into strong, coherent masses. This
technique requires the use of computer-aided design, along with
computer-numerical controlled machines tools.
MANUFACTURING METHODS
FOR METAL BRACKETS

METAL INJECTION MOLDING (MIM)METAL INJECTION MOLDING (MIM)
MIM process include following
steps: 1) Feedstock preparation
2) Injection molding
3) Debinding or debunking
4) Sintering
5) Finishing procedures
Raw material preparation (feedstock)
The metal powder or "metal dust"(particle diameters are usually less
than 15 microns), is mixed with a large amount of binder to obtain a
homogeneous mix. For orthodontic components the breakdown is usually
55-65% metal dust and 45-35% binder. The binder is wax-based organic
material/polymer and special machines assure a good mixed composition.
This phase is essential to obtain quality in the final product. Metal brackets
are usually created from stainless steel powder (316L, 430L) obtained by
an atomization process and selected by grain size. The Ceramic Injection
Molding (CIM) process instead uses Al2O3, ZrO2, Si3N4, SiC, and Y2O3.

Injection molding
The next step is the molding process.
Certain devices are specifically
created; however, it is critical that all
parameters such as pressure
temperature, injection speed, are
extremely constant and controllable.
After molding, the part is called
"green body" and it is relatively
fragile. It is also 20% larger than the
final product.

Debinding or debunking
The "green" parts are exposed to
heat, solvent or a combination of the
two in order to remove most (at least
90%) of the binder material. At the end
the "brown" parts are approximately
the same size as the green parts but
are quite porous. This step is crucial
and the residual 10% will be removed
at the sintering phase. Several
debunking processes are required for
different binding materials.

Sintering
"Brown" parts are sintered in
vacuum type furnaces or with
controlled atmosphere furnaces.
This furnace reaches 1400 C° for
MIM and 2000 C°for CIM process
and they have sophisticated
regulation systems to optimize all
parameters and if necessary treat
the final product thermally.
Here, they shrink 17 to 22% to
nearly full density and are then
complete. The vacuum furnaces
are preferable to the others
because the final product does
not contain gas inclusions and it
is more compact because the
heat diffuses uniformly.

FLAWS IN BRACKET MANUFACTURING
Rhomboidal brackets in which
torques were reversed
Brackets and bases brazed in
reverse directions
Matasa et al (1990) described various
manufacturing defects in direct-bonding metallic
brackets like,
1) Bracket/Base misassembly
2) Reversed Torques
3) Incorrect angulations
4) Improper base shapes
5) Misleading permanent markings
6) Improperly cut slots
7) Brazing overflow

Cast brackets with uncut
slots
Brackets made with misleading or
missing markings
Brackets with excessive brazing
material clogging slots
Brackets with improperly stamped
bases
Central and lateral incisor brackets improperly assembled with
excessive angulation

PARTS OF A DIRECT-BONDING METAL BRACKET
PARTS OF A DIRECT BONDING METAL BRACKET
1) Bracket profile→ portion that bears the slot into which archwires are engaged
2) Brazing layer→ attaches foil to the bracket profile
3) Foil→ metal piece of varying thickness to which mesh is attached
4) Mesh → made of wires of different sizes which is attached to foil in either
horizontal and vertical or diagonal configuration

BRACKET BASE SHAPE
Rectangular, rhomboid, triangular (Glance by Unitek), oval,
round (Mini-Ultra-Trim by Dentaurum, Germany), in the shape of
a tooth (Sinterline by Lancer), or even of a cross (Comfort by "A"
Co.), they are all in use for reasons going from ease in machining
to patent infringement avoidance or to proper bracket matching.
MESH TYPE BASES
1) Foil mesh base
2) Mini mesh base
3) Micro mesh base
4) Laminated mesh base
5) Dyna bond base
6) Ormesh wide central
7) Supermesh MB base
NON-MESH TYPE BASES
1) Micro-loc base
2) Dyna lock integral base
3) Micro etch base
4) Laminated perforated base
5) Peripheral perforated base
6) Laser structured base
METAL BRACKET BASE TYPES

MESH TYPES
 The sizes of the wire mesh used in the manufacturing of the
various single mesh type bases were 40, 60, 80, and 100 meshes
(Dickinson 1980).
 The finest mesh used on metal brackets is 100 gauge, which
can accommodate up to a 155-micron particle size of filler (Paul
Gange 1995).
 In 1969 Mizrahi and Smith introduced the earliest technique of
welding mesh to stainless steel band material and directly bonded
the orthodontic attachments to the enamel.
 Nominal area of bracket base is measured by a method called
Planimetry where enlarged photographs of bracket base are
examined and mesh size is also calculated by counting wires per
linear inch (Dickinson 1980).

MESH TYPE BASES
FOIL MESH BASE
(DENTAURUM)
DYNA BOND MESH BASE
(3M UNITEK)
SUPER MESH ME BASE (GAC)ORMESH BASE (100 gauge foil mesh)
(ORMCO)

NON-MESH TYPE BASES
MICRO-LOC BASES
(GAC)
MICRO ETCH BASE
(3M UNITEK)
DYNA-LOCK INTEGRAL BASE
[3M UNITEK]
LASER STRUCTURED BASE
(DENTAURUM)

SPOT WELDING
 Originally, the strands within the mesh backing were
welded to each other and to the back of the bracket.
 Spot-welding appears to cause damage to the mesh
base where the mesh is completely obliterated by the
spot-welding, causing the wire to fracture and leaving
sharp areas exposed. Spot-weld damage not only
decrease the nominal area available for retention but
also produce an area of stress concentration which can
initiate the fracture of the adhesive at the adhesive-base
interface. Inadequate spot-welding may lead to
separation of the bracket from the base.

METHODS OF ATTACHING MESH TO
BRACKET BASE

BRAISING / BRAZING
 Instead of welding the mesh strands, they are united by a
special process called braising (brazing) that does not flatten
the wires (Sidney Brandt 1977).
 Brazing is a process where metal parts are joined together
by melting a filler metal between them at a temperature below
the solidus temperature of the metal being joined and the
melting point of the filler is above 840° F (450° C).
 The brazing layer usually contains a combination of silver,
gold or non-precious alloys such as AgCu, AuNi, or NiFeCu.
 Thus an attempt is made to maximize the area for
interlocking potential by making more room for the bonding
agent

Maijer and Smith (1983) improved mechanical retention of
metal brackets by fusing metallic or ceramic particles onto the
bracket base.
• Particulate-coated bases were prepared by sintering stainless
steel or cobalt-chromium beads of various mesh sizes onto the
bases at approximately 1,100° C for 4 hours in an inert
atmosphere.
• Ceramic coatings were applied by similar sintering techniques
or with a chemical bonding agent to the stainless steel.
• One advantage of a porous-coated base is that ready
penetration of bonding resin occurs through capillary action
and strong mechanical interlocking results, with concomitant
high bond strength if the porous coating is firmly bonded to the
base.
IMPROVEMENTS IN BRACKET BASE DESIGN
He also concluded that,
• The conventional mesh-base bracket showed failure at the mesh surface.
• The ceramic-coated base showed failure partly in the ceramic coating and partly at
the resin and bracket interfaces.
• The advantages of ceramic-coated base are the absence of corrosion at the resin
interface and the ability to incorporate releasable fluoride into the ceramic layer, thus
providing a local anticariogenic and possible remineralization effect.

Hanson and Gibbon (1983) used bracket bases coated
with porous metal powder and compared its bond
strength with foil mesh base.
 Stainless steel powder consisting of particles small
enough to pass through a 44 mm screen (— 325 mesh)
was used to coat the bracket base. A special sintering
process was used to fuse the particles to one another
and to orthodontic attachments to create strongly
cohesive coatings roughly 0.005 inch thick. The manner
in which the particles are joined creates highly irregular
pores varying in size up to 100 mm across their major
dimension.
 The large surface area and intricate microscopic
void network of the powder coating provide better
mechanical keying with orthodontic cement than does
mesh, with a corresponding significant increase in bond
strength.

Siomka and Powers (1985) used following methods to
improve bracket base retention.
 ETCHING→ An acid solution is used to roughen the
surfaces of the bases chemically to create a larger surface
area for mechanical retention of the adhesive
 SILANATION→ Process where a silane-coupling agent
dissolved in methanol to promote an increase in wetting of
the mesh base to allow better penetration of the resin into
undercut areas.
 SURFACE ACTIVAION→ It is an electrochemical
process used to remove oil, dust, and thin oxidation films
from alloy surfaces that might inhibit bonding.
 ETCHING PLUS SILANATION
 ETCHING PLUS SURFACE ACTIVATION

David Hamula (1996) introduced titanium brackets
whose retentive base pads were done by a computer-
aided laser (CAL) cutting process, which generates
micro- and macro-undercuts.
Olivier Sorel (2002) used a new type of laser
structured base retention (Discovery bracket,
Germany). The smooth surface of injection molded
single piece bracket base is treated by a sufficiently
powerful Nd: YAG laser, melting and evaporating the
metal and burning hole-shaped retentions.
The Supermesh type base consists of a pad
with a dense (200 gauge) mesh beneath a
standard (100 gauge) mesh. Double mesh or dual
mesh type bases have 80-gauge layered on a
150-gauge microetched foil mesh base. (Ovation
Roth bracket, GAC)
DUAL MESH
LASER STRUCTURED BASE

Orthodontic brackets undergo corrosion in oral
environment where saliva acts as an electrolyte. The most
notable effects of corrosion are the loss of metal weight and
the weakening of mechanical properties.
Matasa et al (1995) described that corrosion occurs in
several ways:
It is the most common form of
corrosion seen in orthodontic
attachments, which affects the
mechanical properties. It happens
when the attachment is made of
several parts or if it is improperly
treated, or if it contains impurities.
Localized or pitting corrosion
TYPES AND MECHANISM OF CORROSION

Microbial attack is directed mainly against
the bracket base and occurs especially in
non-aerated, sensitized areas such as the
junction between mesh and foil.
Microorganisms such as the sulfate-reducing
Bacteroides corrodens and the acid-producing
Streptococcus mutans are known to attack.
Most common attack can be observed as
symmetric, round craters in the metal.
Microbiologically induced corrosion
This occurs when the attachment is
in contact with plastic materials — an
adhesive, an acrylic prosthesis, or
elastic.
Crevice corrosion
CREVICE CORROSION at bracket-
adhesive interface

Intergranular corrosion
Stainless steels are particularly susceptible to sensitization,
which leads to intergranular corrosion. Heating between 400° and
900° C makes stainless steel more susceptible to intergranular
corrosion because chromium carbide separates at the steel grain
boundaries and consumes part of the protective chromium oxide
layer. The chromium carbide film is then readily attacked and
dissolved, with catastrophic consequences. This separation can
start at temperatures as low as 350°C, which means that
microstructural weakening can occur during brazing, welding, and
cold working as well as thermal reconditioning.
Jeffrey and Andres Guzman (1997) compared corrosion
behavior of 2205 duplex stainless steel with that of AISI type 316L
stainless steel and concluded that 2205 stainless steel exhibits
better corrosion resistance than 316L stainless steel
Catastrophic corrosion
A phenomenon that occurs if stainless steel is sensitized and then
exposed to chemical agents, where the pits on the metal surface later
transform into crevices, diminishing the mechanical properties.

CORROSION PRODUCTS
Maijer and Smith (1982) demonstrated that corrosion products
produce stains and staining was observed mostly in the anterior
teeth. Nickel and Chromium are mainly responsible for stains.
• Nickel oxide and nickel sulfide stains are black.
• Nickel hydroxide, nickel fluoride and nickel phosphate stains
are green.
• Chromium sulfide stain is black, and chromium phosphate
stain is violet.
• Hydrated chromium oxide and chromium fluoride stains are
green.

HYPERSENSISITIVITY TO CORROSION
PRODUCTS
Nickel and chromium are normally present in the foods consumed
by man. The dietary intake of nickel was reported to be 300 to 500 µg per
day, while chromium intake varied from 5 to more than 100 µg per day
(Underwood 1977).
Gwinnett (1982), Maijer and Smith (1982) described that corrosion of
orthodontic brackets releases heavy metals like nickel, cobalt and
chromium.
Nickel is the most common cause of metal-induced allergic contact
dermatitis in man and produces more allergic reactions than all other
metals combined. Second in frequency is chromium.
The incidence for nickel allergy was reported to be 1% in male
subjects and 10% in female subjects (Menne 1983). This higher
prevalence for females may be explained by an earlier exposure to nickel,
with earrings and other metallic clothing objects.

On the other hand, the incidence for chromium allergy is
estimated at 10% in male subjects and 3% in female subjects (Greig
1983).
Some of the manifestations of nickel allergy reported were allergic
contact dermatitis, utricaria, and asthma.
Bishara and Barret (1993) studied changes in blood level of nickel
during initial course of orthodontic therapy.
 Patients with fully banded and bonded orthodontic appliances did
not show a significant increase in the nickel blood levels during the
first 4 to 5 months of orthodontic therapy.
 Orthodontic therapy using appliances made of alloys containing
nickel-titanium did not result in a significant increase in the blood
levels of nickel.
 The biodegradation of orthodontic appliances during the initial 5
months of treatment did not result in significant or consistent
increase in the blood level of nickel.

Heidi Kerosuo (1997) investigated nickel and chromium
concentrations in saliva of patients with different types of fixed
orthodontic appliances and concluded that fixed orthodontic
appliances do not seem to affect significantly the nickel and
chromium concentration of saliva during the first month of
treatment.
Guilherme and Janson (1998) described that orthodontic
treatment with conventional stainless steel alloys does not induce
a nickel hypersensitive reaction.

TITANIUM BRACKETS
• The problems of nickel sensitivity,
corrosion, and inadequate retention have all
been solved with the introduction of a new,
pure titanium bracket (Rematitan-
DENTAURUM) in 1995. Its one-piece
construction requires no brazing layer, and
thus it is a solder- and nickel-free bracket.
• Titanium brackets were grayer in color and
rougher in texture than the stainless steel
brackets and imparts none of the metallic
taste as seen in stainless steel brackets.
• Titanium and titanium-based alloys have the greatest corrosion
resistance of any known metals.
• Titanium also has low thermal conductivity, and thus alleviates the
sensitivity to extreme temperature changes often experienced by
patients wearing metal appliances.

TITANIUM BRACKETS - COMPOSITION
A commercially pure (cp) medical grade 4 Ti
(designation DIN 17851-German standards)
was used as the basis for the manufacture of
titanium brackets. The chemical composition is
99+% Ti and reportedly less than 0.30% iron,
0.35% oxygen, 0.35% nitrogen, 0.05% carbon,
and 0.06% hydrogen.
The brackets were machined out of forged
and rolled profiles. The marking and the
structuring of the retentive base pads were
done by a computer-aided laser (CAL)
cutting process, which generates micro- and
macro-undercuts

GOLD-COATED BRACKETS
Recently gold-coated steel brackets
have been introduced and rapidly gained
considerable popularity, particularly for
maxillary posterior and mandibular anterior
and posterior regions. Brackets are now
available with 24 karat gold plating, plated
with 300 micro inches of gold.
Gold-coated brackets may be
regarded as an esthetic improvement over
stainless steel attachments, and they are
neater and thus more hygienic than ceramic
alternatives.
Patient acceptance of gold-coated
attachments is generally positive. Significant
side effects in the form of corrosion or
allergic reactions have not been observed
clinically.

COMMERCIALLY AVAILABLE GOLD-COATED
BRACKETS
Victory Series™
(3M UNITEK)
Forever Gold 24K Gold Plated
(American orthodontics)
ORTHOS GOLD (Ormco)

(GAC)
A process in which four layers of gold and a
select metal are ionically implanted into the
Stainless steel bracket surface manufactures
platinum-coated brackets.
The result is a bracket with five times the
abrasion resistance of gold.
A smoother, harder surface than stainless
steel for reduced friction and improved sliding
mechanics is achieved.
By combining platinum metal and an
exclusive implantation process, a barrier has
been created against the diffusion of nickel,
cobalt, and chromium. Platinum has been found
to be superior to all other known metals for the
manufacture of brackets and has been chosen
by the jewelry industry to comply with European
Directive EN1811, which dictates strict
standards on the emission of nickel.
PLATINUM COATED BRACKETS

( DENTAURUM )
NICKEL-FREE BRACKETS
Made of Cobalt chromium
(CoCr) dental alloy
One-piece construction (without
solder) by metal injection molding
technique
Laser structured bracket base for
retention

Ernest Schwartz (1971) used GAC
interchangeable plastic brackets and concluded
that
• Bonding of plastic brackets directly to anterior
teeth is both challenging and satisfying
• Larger flange of the bracket that extends
mesiodistally beyond the curvature of the tooth
results in bending of the archwire. This not only
results in increased debonding of the brackets, but
also inhibits free tipping and retraction of the
anterior teeth.
Morton Cohen and Silverman introduced the first commercially available
plastic brackets (IPB brackets), manufactured by GAC in 1963.
PLASTIC BRACKETS
The first plastic brackets were manufactured from unfilled polycarbonate
and esthetics was its main advantage.
Pure plastic brackets lack strength to resist distortion and breakage, wire
slot wear, uptake of water, discoloration and the need for compatible
resins.

Newman et al (1973) used other plastic brackets like nylon and the
polyolefins that repel water to a greater extent and are strong, but
they don't bond well.
Various plastic brackets were,
1) Polycarbonate brackets (E.g.Elation)
2) Reinforced polycarbonate brackets
3) Polyurethane-composite brackets (E.g.Envision)
4) Thermoplastic-polyurethane brackets (E.g.Value line)
PLASTIC BRACKET BASE TYPES AND BONDING
Some commercially available plastic bracket base types were,
• Mechanical lock base.
• MicroRock dovetail base.
Bonding mechanism of plastic brackets is mainly mechanical retention
type and utilization of plastic bracket primer such as methyl
methacrylate monomer improves the bond strength of adhesives to
plastic brackets.

DISADVANTAGES OF POLYCARBONATE BRACKETS
 Polycarbonate brackets undergo creep deformation when
transferring torque loads generated by arch wires to the
teeth
 Discoloration of first generation unfilled polycarbonate
brackets during clinical aging.
 They absorb water to a slight extent and tend to weaken in
the course of about one year (Newman 1973).
Most efforts are directed toward improving the strength of
polymeric brackets by reinforcing the plastic matrix.
Various reinforced polycarbonate brackets were,
1) Polymer fiber reinforced polycarbonate brackets
2) Fiberglass reinforced polycarbonate brackets
3) Ceramic reinforced polycarbonate brackets
4) Metal slot reinforced polycarbonate brackets
5) Metal slot and ceramic reinforced polycarbonate
brackets

POLYCARBONATE BRACKETS
ElationTM
(GAC)
Polycarbonate-glass fibers
1) Aesthetic Line
2) Image
Polycarbonate-Composite
1) DB fibre
2) Elan
Polyurethane-Composite
1) Envision
Thermoplastic-polyurethane
1) Value Line
COMMERCIALLY AVAILABLE PLASTIC BRACKETS

VogueTM
(GAC)
FIBER-GLASS REINFORCED COMPOSITE POLYMER
OysterTM
Bracket (GAC)
FIBER-REINFORCED POLYCARBONATE BRACKETS
Classic (American Orthodontics)Spirit®MB (ORMCO)

Ceramics used for the manufacturing of ceramic
brackets were Alumina and Zirconia. Both can be
found as tridimensional inorganic macromolecules.
TYPES OF CERAMIC BRACKETS
1) Monocrystalline (Sapphire)
2) Polycrystalline Alumina
3) Polycrystalline Zirconia-Yttrium oxide Partially
Stabilised Zirconia (YPSZ)
CERAMIC BRACKETS

ATOMIC ARRANGEMENT OF CERAMICS
CRYSTAL STRUCTURE OF ALUMINA
A l 3 + A l 3 +
A l 3 +
A l 3 +
A l 3 + A l 3 +
O 2 -
O 2 -
O 2 -
O 2 -
O 2 -
O 2 -
b 2 "
b 1
b '
b "
3
2
1
Aluminium oxide crystal structure
consists of a nearly Hexagonal
close pack (HCP) arrangement of
the larger oxygen anions (O2-),
with
smaller aluminium cations (Al3+),
located in two- thirds of the
octahedral interstitial sites in the
HCP structure.These octahedral
sites have six-fold coordination,
i.e.,each aluminium ion is
surrounded by six oxygen ions.
The crystal structure is determined by the ratio of the radii of the
aluminium and oxygen ions and the requirement of an
electrically neutral unit cell.

CRYSTAL STRUCTURE OF ZIRCONIA
Zirconium oxide can exist in three
different crystal structures: cubic,
tetragonal, and monoclinic, each of
which has a temperature range for
stability.
The crystal structure of zirconia at
room temperature consists of a
distorted simple cubic (monoclinic)
arrangement of the oxygen ions, with
the zirconium cations (Zr4+
) located in half of the available sites (Eight fold
coordination). Zirconia can easily undergo a martensitic transformation
from a tetragonal structure into a monoclinic one that is stable at room
temperature.
INTERATOMIC BONDING OF CERAMICS
Crystalline ceramic materials have a combination of covalent and
ionic bonding, with minimal dislocation movement at room temperature.
This strong interatomic bonding accounts for the advantageous chemical
inertness of dental ceramics.

MONOCRYSTALLINE (SAPPHIRE) BRACKETS
The first step in the manufacture of the single-crystal brackets is
the slow cooling of the molten high-purity aluminium oxide under
controlled conditions from temperatures above 2100°C. The
resulting bulk single crystal alumina rod or bar form is then milled
into brackets using diamond cutting, Nd: YAG lasers, or ultrasonic
cutting. The single-crystal brackets are also subsequently heat
treated to remove surface imperfections and stresses created by
the milling process.
POLYCRYSTALLINE ALUMINA BRACKETS
These brackets are manufactured by first combining a suitable
binder with aluminium oxide particles (average of 0.3µm size) so
that this mixture can be molded into the shape of a bracket. This
molded mixture is then heated (fired) at temperatures in excess of
1800°C to burn out the binder and achieve sintering of the particles.
MANUFACTURING METHODS FOR CERAMIC BRACKETS

Diamond cutting tools are then used to machine the slot
dimensions. Heat treatment is subsequently performed to relieve
the stresses caused by the cutting and to remove surface
imperfections resulting from the manufacturing processes.
Structural imperfections at the grain boundaries or trace amounts
of sintering aids can serve as sites of crack initiation under stress.
POLYCRYSTALLINE ZIRCONIA BRACKETS
The polycrystalline zirconia brackets are manufactured by
impression molding, followed by hot isostatic pressing. Yttrium
oxide-partially stabilized zirconia (YPSZ) can be obtained in bulk
form by sintering (without pressure up to 95% of the theoretical
density) a mixture of ultrafine powder (average starting particle size
of 0.2µm) and 5wt% Yttrium oxide. A polycrystalline microstructure
with an average grain size of about 0.5µm is obtained, and hot
isostatic pressing is subsequently employed to remove residual
porosity with limited additional grain growth.

 A very important physical property of ceramic brackets is
the extremely high hardness of aluminium oxide, so that
both monocrystalline and polycrystalline alumina have a
significant advantage over stainless steel (Birnie 1990).
 Swartz (1988) stated that ceramic brackets are nine
times harder than stainless steel brackets and enamel.
 Douglass (1989) gave a clinical report of enamel damage
found on the lingual surfaces of maxillary central incisors
that were in contact with poly-crystalline sapphire ceramic
brackets placed on the facial surfaces of lower incisors. This
is because, when natural tooth surfaces have opposing
contact with ceramic brackets in occlusion, due to the
hardness of ceramics enamel damage may occur.
HARDNESS OF CERAMICS AND
ENAMEL WEAR

 Fracture toughness is a measure of the strain energy-absorbing
ability prior to fracture for a brittle material. The higher the fracture
toughness, the more difficult it is to propagate a crack in the
material.
 Vickers hardness testing machine is used to test the fracture
toughness of ceramic brackets. Microscopic indentations are
placed on the surface of bracket and the associated cracks at the
tip of the indentation are evaluated.
 Fracture toughness of ceramics is 20 to 40 times less than that
of stainless steel (Scott 1988).
 Fracture behaviour is controlled by the influence of surface
cracks and other microscopic defects or internal pores. These are
called “Griffith flaws”.
 When ceramics are subjected to their maximum elastic stress
levels, brittle failure occurs in which interatomic bonds at the tips
of flaws rupture, and the material fails by crack propagation.
FRACTURE TOUGHNESS AND IMPACT RESISTANCE

 The alumina ceramics contain strong, directional covalent
bonds that do not allow permanent deformation or ductility by
the movement of dislocations as found in metals. Alumina
brackets are very susceptible to crack initiation at minute
imperfections or regions where material impurities have
accumulated.
 Polycrystalline alumina presents higher fracture toughness
than single-crystal alumina since fracture surface energies are
higher for polycrystalline alumina than for single-crystal
alumina.

 Single crystal brackets are noticeably clearer than polycrystalline
brackets, which tend to be translucent. The sintering process
produces a polycrystalline alumina microstructure with grain
boundaries, resulting in some translucency.
 There is loss of light transmission through the ceramic because of
a small variation in refractive index with crystallographic direction
within grains and scattering processes at the grain boundaries.
 Optical properties and strength are inversely related for the
polycrystalline alumina ceramics where, the larger the individual
grains in the microstructure the greater is the ceramic translucency.
However, when the grain size approaches 30µm, the material
becomes substantially weaker.
OPTICAL PROPERTIES

 The amount of the photocuring light transmitted through a
bracket may affect the properties of the light-cured adhesive
used for bonding.
 Experiments showed that the direct light transmittance at
the peak optical absorption wavelength (468nm) of the
photoinitiator camphoroquinone was found to range between
35% for a single-crystal alumina bracket and less than 5% for
several polycrystalline alumina brackets.
 The variation in direct light transmittance among the
different polycrystalline alumina brackets is due to differences
in bracket geometry and microstructural grain size, which
cause light scattering and reduction in the intensity of the light
beam reaching the adhesive paste.

TIE-WING STRENGTH
• Photoelastic studies and finite-element analyses have shown that
tie-wings are generally the locations of concentrated stresses when
forces are applied to the ceramic brackets. Tie-wing fractures have
been much more common for the single-crystal alumina brackets
because of their lower resistance to crack propagation.
• Sonneveld et al (1994) compared the breaking force in
compression for alumina and zirconia brackets and found that
zirconia brackets did not experience any tie wing fractures, but
instead underwent visually perceptible deformation prior to bulk
fracture.
• Research using finite element analysis has indicated that
brackets possessing an isthmus connecting the tie-wings
demonstrated better stress tolerance than those without this
feature.

BASE MORPHOLOGY OF CERAMIC
BRACKETS
Bonding mechanisms that have been identified for
ceramic brackets may be classified into three major
categories:
a) Mechanical retention employing large recesses.
b) Chemical adhesion facilitated by the use of a silane layer.
c) Micromechanical retention through the utilization of a
number of configurations, including protruding crystals,
grooves, a porous surface, and spherical glass particles.
(A) MECHANICAL INTERLOCK
Large grooves are cut in the base of the bracket where the edge
angle is 90° to provide mechanical retention. Further, there are
crosscuts to prevent the bracket from sliding along the undercut
grooves.

(B) SILANE COATING OF CERAMIC BRACKET BASES
 The coupling agent γ-methacryloxypropyltrimethoxysilane (γ-
MPTS) has been used for promoting chemical adhesion between
surfaces.
 The γ-MPTS is hydrolysed to the corresponding silanol. A
limited number of silanol groups per silanol molecule are
hydrogen-bonded to the water layer adsorbed on the base
surface.
 Side chain silanols are condensed, establishing a siloxane
network that stabilizes the structure.
 Owing to the silanol orientation toward the bracket base,
methacrylate groups are placed in a configuration that favours
cross-linking with the methacrlate-based adhesive.
Bonding arises from two mechanisms:
 Silanol groups of the hydrolysed silane adhere to the
hydration layer of the inorganic surfaces
 Methacrylate groups of the silane copolymerize with the
methacrylate resin matrix, forming covalent bonds

• Polycrystalline alumina brackets with a rough base comprised of
either randomly oriented sharp crystals or spherical glass particles.
These brackets provide only micromechanical interlocking with the
orthodontic adhesive.
• The different types of spheres found on the base of the bracket
may imply a different manufacturing process, perhaps involving the
spray atomization of melted glass that is fused onto the ceramic
base, generating the spherical shape as a result of surface tension.
(C) MICROMECHANICAL RETENTION OF CERAMIC BRACKETS
SEM Photomicrograph
of sharp crystals
Bright-field polarized-light photomicrograph
of spherical particles

COMMERCIALLY AVAILABLE
CERAMIC BRACKETS
MONOCRYSTALLINE ALUMINA
1) Inspire
2) Starfire TMB
POLYCRYSTALLINE ALUMINA
1) Allure 7) Mxi
2) Clarity 8) Signature
3) Fascination 9) Virage
4) Intrigue 10) CeramaFlex (with polycarbonate base)
5) 20/20 11) InVu
6) Lumina
POLYCRYSTALLINE ZIRCONIA
1) Hi-Brace

MXi (TP ORTHODONTICS)
POLYCRYSTALLINE ALUMINA CERAMIC BRACKETS
MONOCRYSTALLINE CERAMIC BRACKET
Inspire Starfire

POLYCRYSTALLINE ALUMINA CERAMIC BRACKETS
Allure (GAC)
CLARITY (3M UNITEK)
Micro-crystalline bonding surface
20/40
(American Orthodontics)

CERAMIC BRACKETS WITH METAL SLOTS
VIRAGE
(American Orthodontics)
CLARITY (3M UNITEK)

COMPARATIVE BOND STRENGTH
CHARACTERISTICS OF BRACKETS
Reynolds (1975) indicated that optimal bond strength of brackets to
enamel range between 5.9 and 7.8 Mpa.
STAINLESS STEEL BRACKETS WITH DIFFERENT BASE TYPES
James Lopez (1980) studied retentive shear strength of sixteen
commercially available stainless steel bracket bases.
 The solid bases with perforations around the periphery had lowest
mean shear strengths and are probably due to the lack of mechanical
retention in the center of the base.
 The solid base with perforations throughout the base slightly
increased the mean shear strength values.
 The Micro Lok or solid base with circular indents that serve for
retention was generally ranked in the intermediate bond strengths.
 The foil mesh designs proved to range from the most inferior to the
most superior shear strengths.
 Smaller foil mesh bases could be used without sacrificing significant
shear strength. www.indiandentalacademy.com

Dickinson (1980) evaluated tensile bond strengths for fourteen
direct-bonding bracket bases. The sizes of the wire mesh used in
the manufacturing of the various mesh type bases were 40, 60,
80, and 100 mesh.
 Statistically significant difference in tensile bond strengths was
observed between different brackets. Ultra-Trimline base and
Mini-mesh base had the highest values of tensile bond strength,
while Laminated mesh base and Peripheral perforated base had
the lowest values.
 Bond strength was independent of the nominal area and mesh
size for the bases tested.
CERAMIC BRACKETS WITH DIFFERENT BASE TYPES
Viazis (1990) compared the shear bond strength for two types
of ceramic brackets and concluded that,
The shear bond strength of silane chemical bonded ceramic
brackets is significantly higher than the grooved mechanical
bonded ceramic brackets.

COMPARATIVE FRICTIONAL
CHARACTERISTICS OF BRACKETS
Friction is a function of the relative roughness of two
surfaces in contact, and it arises when there is relative motion
or potential for it between the two surfaces.
 Static friction is the smallest force needed to start the motion
of solid surfaces that were previously at rest with respect to
each other.
 Kinetic friction is the force that resists the sliding motion of
one solid object over another at a constant speed.

CERAMIC BRACKETS VS STAINLESS STEEL BRACKETS
Pratten, Popli & Germane (1990) studied frictional resistance between
ceramic and stainless steel brackets using Nitinol and stainless steel
wires.
 Ceramic brackets provide significantly greater frictional resistance
than stainless steel brackets when they are used in combination with
either stainless steel or nitinol arch wires.
 Under all conditions, the stainless steel brackets had lower
coefficients of friction than the ceramic brackets.
The stainless steel wire generated less friction than nitinol, and friction
increased in the presence of artificial saliva in comparison with air
alone.
Omana and Moore (1992) compared static frictional properties of
metal and ceramic brackets and concluded that,
 Smoother, injection-molded ceramic brackets appear to create
less friction than other ceramic brackets.
 Wider metal or ceramic brackets create less friction than narrower
brackets of the same material.

TITANIUM BRACKETS VS DIFFERENT WIRES
Kusy and Whitley et al (1998) evaluated the static and
kinetic frictional coefficients of commercially pure titanium
brackets in the passive configuration in the dry and wet
states against stainless steel, nickel-titanium, and beta-
titanium archwires.
 The optical roughness of Ti brackets is greater than
conventional Stainless steel brackets
 With regard to frictional coefficients (µ), the Ti bracket
compares favorably against the conventional Stainless steel
bracket for all couples evaluated with Stainless steel, Ni-Ti,
and beta-Ti archwires at 34°C.
 Ti brackets may be substituted for SS brackets in order to
eliminate the potential allergen, Ni, from the oral cavities of
some patients.

CERAMIC BRACKETS VS CERAMIC BRACKETS
Keith, Kusy and Whitley (1994) evaluated frictional
characteristics between zirconia and alumina ceramic brackets.
In general, the frictional coefficients of zirconia brackets were
greater than or equal to those of the alumina brackets in either
the dry or the wet state.
Couples comprised of beta-titanium arch wires generally
produced the highest frictional coefficients

TITANIUM BRACKETS VS STAINLESS STEEL BRACKETS
Rupali Kapur and Pramod K. Sinha (1999) measured and
compared the level of static and kinetic frictional resistance
generated between titanium and stainless steel brackets. Both
0.018 and 0.022 inch slot size edgewise brackets were tested with
different sized rectangular stainless steel wires.
 Titanium brackets have different frictional characteristics
compared with stainless steel brackets using similar wires.
 Titanium brackets showed lower static and kinetic frictional force
as the wire size increased.
 Stainless steel brackets showed higher static and kinetic
frictional force as the wire size increased.

BRACKETS AND PLAQUE ACCUMULATION
Orthodontic brackets promote microbial colonization, thus inducing
plaque accumulation. The critical surface tension (γc
) of the substrate is
considered a key factor in modulating the attraction of species on the
surface. Critical surface tension may be defined as the maximum
surface tension of a liquid that will form a zero contact angle on a solid
substrate.
Adhesion of microorganisms to surfaces is a result of specific lectin
like reactions, electrostatic interactions, and van der Waals forces.
Gwinnett and Ceen (1978), using ultraviolet light and scanning
electron microscopy and Zachrisson (1978) evaluated hygienic
difference between mesh-type and perforated metal bracket bases.
 Perforated bases retained significantly more plaque than mesh-type
bases.
 Mesh-backed brackets were more hygienic and generally gave a
cleaner clinical impression and better gingival condition.

Eliades et al and Brantley (1995) investigated wettability and
microbial attachment on different bracket materials and concluded
that,
• Stainless steel brackets presented the highest critical surface
tension, indicating an increased potential for microorganism
attachment on metallic brackets.
• The lowest surface tension values obtained from the fiber-
reinforced polycarbonate and ceramic alumina brackets indicated
reduced plaque retaining capacity compared to stainless steel
brackets.

DEBONDING OF BRACKETS AND ENAMEL FRACTURE
The mechanical debonding technique of metal bracket removal
requires shearing or compression forces with the help of debonding
pliers.
Sheridan, Brawley and Hastings (1986) introduced an alternative to
conventional bracket removal. The technique is called Electrothermic
debracketing (ETD). ETD is the technique of removing bonded brackets
from enamel surfaces with a cordless battery device that generates heat.
DEBONDING TECHNIQUES FOR CERAMIC BRACKETS
1) Delaminating method (Swartz 1988)
2) Electrothermal debonding (Sheridan 1986)
3) Wrenching method (Bishara and Trulove 1990)
4) Ultrasonic debonding (Bishara and Trulove 1990)
5) Grinding method (Vukovich 1991)
6) Lift-off debracketing method (Reed and Shivapuja 1991)
7) Peppermint oil application (Winchester 1992)
8) Laser aided debonding→ using Nd: YAG laser (Strobl et al 1992),
XeCl excimer laser (Tocchio, Williams and Mayer 1993),
carbon-di-oxide laser (Rickabough and Marganoni 1996).

ENAMEL FRACTURE
Retief et al (1974) indicated that fractures in enamel could occur with
bond strength as low as 13.5 Mpa.
Fracture of enamel during debonding orthodontic brackets have been
reported by Swartz (1988), Strom (1990), Joseph and Roussow (1990),
Reid and Shivapuja (1991), Ghafari et al (1992) and Gibbs (1992).
Reid and Shivapuja (1991) compared ceramic and metal bracket
debonding effects on enamel and concluded that,
 Enamel damage is more likely from debonding ceramic brackets than
from debonding metal brackets, although it may only be apparent
microscopically.
Ceramic brackets using mechanical retention appear to cause enamel
damage less often than those using chemical retention.
Monocrystalline ceramic brackets display more enamel loss than
polycrystalline brackets.

RECYCLING OF ORTHODONTIC BRACKETS AND
ITS EFFECTS
Several in-office bracket-reconditioning methods have been
introduced since 1980, which include mechanical methods (e.g.
grinding, sandblasting), thermal methods (e.g. direct flaming or
heating in a furnace) and a combination of both mechanical and
thermal methods (e.g. the Buchman method).
METAL BRACKETS-RECYCLING METHODS
1) Grinding - Wright and Powers (1985)
2) Sandblasting - Millet et al (1993), Sonis (1996)
3) Direct flaming
4) Buchman method - Buchman (1980)
5) BigJane machine method - Buchman (1980)
GRINDING
A green stone operated on straight slow-speed handpiece at a
speed of 25,000 revolutions per minute for approximately 25 seconds.

SANDBLASTING
Sandblasting uses a high-speed stream of aluminium oxide
particles (50 µm), propelled by compressed air under 5 bars (72.5 psi)
line pressure for 20 to 40 seconds.
DIRECT FLAMING
The flame tip of a gas torch flame was pointed at the bracket base
for 3 seconds, during which the bonding agent started to ignite and
burn out. Then the bracket was immediately quenched in water at
room temperature and dried in an air stream.
BUCHMAN METHOD
A Bunsen burner flame was directed at the bracket base for 5 to 10
seconds until the bonding agent started to ignite and burn and then
quenched in water at room temperature. Then a laboratory sandblaster
with 50 µm aluminium oxide particles was used to sandblast the
bracket for 5 seconds. The third step was to electropolish the brackets.

BIGJANE MACHINE METHOD
The brackets were placed for 60 minutes in a furnace, which was
preheated to 850° F (454.4° C) until the bonding agent started to
ignite and burn and then quenched immediately in room temperature
cement solvent (ESMA-ORTHO liquid). This was followed by
ultrasonic cleaning for 10 to 15 minutes, rinsing in hot running water,
and drying in an air stream. An electropolishing step is then
employed to eliminate the remaining surface oxide layer.
Buchman (1980) concluded that as temperatures are increased in
thermal treatment, the hardness and tensile strength are decreased
and the microstructures illustrate corresponding susceptibility to
metallic intergranular corrosion.
Matasa et al (1989) described that heating method used for
reconditioning metal brackets causes intergranular corrosion. He also
enumerated the effects of heat on brackets like, structural metal
weakening, vertical slot obstruction, steel corrosion and base clogging.

CONCLUSION
Research works on orthodontic bracket materials has led to
the use of advanced manufacturing methods like injection
molding, improved bracket base designs for retention purposes,
use of titanium alloys in bracket manufacturing and coating
conventional metal brackets with noble metals to reduce
corrosion and hypersensitivity reactions.
Discomfort faced by the patients due to large profile of the
bracket led to the evolution of low profile brackets. In recent
years, manufacturers have reduced the size of stainless steel
brackets for esthetic reasons and for the mechanical advantage
of increased interbracket distances.
Future development in bracket materials relies on
orthodontist’s technical requirements, patient’s esthetic and
functional demands and manufacturer’s marketing needs.

Orthodontic bracket materials

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