orthodontic Bracket materials /certified fixed orthodontic courses by Indian dental academy

ORTHODONTIC ORTHODONTIC BRACKET MATERIALSBRACKET MATERIALS

INDIAN DENTAL ACADEMY

Leader in continuing dental education www.indiandentalacademy.com

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 bracketsB) 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 steels4) Duplex steels5) Precipitation-hardenable (PH) steels6) Cobalt containing alloys7) 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 Crystalwww.indiandentalacademy.co

m

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) Sinteringthe 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 preparation2) Injection molding3) Debinding or debunking4) Sintering5) 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. www.indiandentalacademy.co

m

Injection moldingThe 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 debunkingThe "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.www.indiandentalacademy.co

m

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 misassembly2) Reversed Torques3) Incorrect angulations4) Improper base shapes5) Misleading permanent markings6) Improperly cut slots7) 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 engaged2) Brazing layer attaches foil to the bracket profile3) Foil metal piece of varying thickness to which mesh is attached4) 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 BASES1) Foil mesh base2) Mini mesh base3) Micro mesh base4) Laminated mesh base5) Dyna bond base6) Ormesh wide central7) Supermesh MB base

NON-MESH TYPE BASES1) Micro-loc base2) Dyna lock integral base3) Micro etch base4) Laminated perforated base5) Peripheral perforated base6) 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 BASESFOIL MESH BASE(DENTAURUM)

DYNA BOND MESH BASE (3M UNITEK)

SUPER MESH ME BASE (GAC)ORMESH BASE (100 gauge foil mesh)(ORMCO)


NON-MESH TYPE BASESMICRO-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 corrosionCREVICE 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. www.indiandentalacademy.co

m

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 stainlesssteel 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 brackets3) 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 brackets2) Fiberglass reinforced polycarbonate brackets3) Ceramic reinforced polycarbonate brackets4) Metal slot reinforced polycarbonate brackets5) Metal slot and ceramic reinforced polycarbonate

brackets www.indiandentalacademy.co

m

POLYCARBONATE BRACKETS Elation TM (GAC)

Polycarbonate-glass fibers1) Aesthetic Line2) Image

Polycarbonate-Composite1) DB fibre2) Elan

Polyurethane-Composite1) Envision

Thermoplastic-polyurethane1) 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 Alumina3) Polycrystalline Zirconia-Yttrium oxide Partially

Stabilised Zirconia (YPSZ)

CERAMIC BRACKETS


ATOMIC ARRANGEMENT OF CERAMICSCRYSTAL STRUCTURE OF ALUMINA

A l3+ A l3+

A l3 +

A l3 +

A l3 + A l3 +

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 2100C. 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 0.3m 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 1800C 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 0.2m) and 5wt% Yttrium oxide. A polycrystalline microstructure with an average grain size of about 0.5m 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 30m, 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 -methacryloxypropyltrimethoxysilane (-MPTS) has been used for promoting chemical adhesion between surfaces. The -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 ALUMINA1) Inspire2) Starfire TMB

POLYCRYSTALLINE ALUMINA1) Allure 7) Mxi2) Clarity 8) Signature3) Fascination 9) Virage4) Intrigue 10) CeramaFlex (with polycarbonate base)5) 20/20 11) InVu6) Lumina

POLYCRYSTALLINE ZIRCONIA1) Hi-Brace


MXi (TP ORTHODONTICS)

POLYCRYSTALLINE ALUMINA CERAMIC BRACKETS

MONOCRYSTALLINE CERAMIC BRACKETInspire Starfire


POLYCRYSTALLINE ALUMINA CERAMIC BRACKETSAllure (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.co

m

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 (c) 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 BRACKETS1) 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 flaming4) 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 m), 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 m 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.


Thank you

For more details please visit www.indiandentalacademy.com


orthodontic Bracket materials /certified fixed orthodontic courses by Indian dental academy

Education

Transcript of orthodontic Bracket materials /certified fixed orthodontic courses by Indian dental academy