Prepared by: Mahmoud Kanan Mohsin BDS MSc Candidate in OrthodonticsSupervised by: Dr. Bayan Abdulla Hassan BDS, MSc, PhD
Orthodontic brackets are passive components of fixed orthodontic appliance, bonded to enamel which provide the means to transfer the force applied by the activated archwire to the tooth.
1. Width of bracket a. Mesiodistally narrow ex., Ribbon arch bracket, Begg bracket b. Mesiodistally wide ex., Edgewise bracket, Straight wire bracket2. Slot of bracket a. Horizontal slot ex., Edgewise bracket b. Vertical slot ex., Begg bracket3. Movement of tooth a. Tipping movement, Begg bracket b. Bodily movement, Preadjasted Bracket (Straight wire bracket, ROTH Brackets, MBT Brackets). c. Tipping & Bodily movement, Combination bracket, Tip- Edge bracket
4. Materials a. Metal – Stainless steel, Gold, Titanium, Nickel b. Plastics c. Ceramics d. Combination– Metal reinforced plastics, metal reinforced ceramic5. Ligation of bracket a. Conventional ligation – Edgewise bracket, Begg bracket b. Self-ligation- Edgelok bracket, SPEED bra
Type of Brackets According toMaterial fabricated: Metallic Brackets Aesthetic Brackets Ceramic Brackets Plastic Brackets
Metallic Brackets Stainless steel brackets have been used fordecades with highly successful clinical results.The morphology of the base, which is composedof a metal mesh, yields adequate adhesive bondstrength values to enamel that meet the demandsof in vivo orthodontic forces. Manufacturers haveincorporated a variety of mesh designs in theircurrently marketed products
Advances in Metallic Brackets Despite the clinically sufficient bond strength provided by conventional metal brackets, some attempts have focused on increasing the strength of the bracket-adhesive interface. The goal is to achieve bond longevity and integrity throughout treatment.
plasma-coated metal bracketbases having a variety of mesh designs as well as ceramic bracket bases. They reported that the enormously increased active surface area of the base resulted in much greater mechanical interlocking. For metal brackets, the non- mesh, plasma-sprayed bases had tensile adhesive bond strengths similar to those for unsprayed mesh bases. However, plasma- sprayed ceramic brackets did not yield similar results, and it was concluded that these brackets are not good candidates for this procedure.
AISI type 316L austenitic stainlesssteel alloy This alloy contains (values in wt%); Cr 16±18 % Ni 10±14 % Mo 2±3 % C maximum of 0.03 % the L designation refers to the lower carbon content compared to type 316 stainless steel, which contains a nominal maximum amount of 0.08 wt% C.
316L stainless steel contains somewhat more nickel, somewhat less chromium, and substantially less carbon than the AISI types 302 and 304 stainless steel archwire alloys, along with a small amount of molybdenum that is not present in the latter. Although the 316L stainless steel bracket alloy has performed well clinically, some corrosion of this material may be identified in the form of discoloration of the underlying adhesive layer
Corrosion attack of 316L stainless steel alloy identifiedfollowing debonding where corrosion products have diffusedinto the adhesive layer, causing discoloration
The corrosion resistance of stainless steel is provided by a passive surface film of chromium oxide; the corrosion resistance of titanium and its alloys is provided by an analogous passive film of titanium oxide. While the addition of molybdenum to the 316L stainless steel alloy provides further protection from crevice and pitting corrosion, the chromium oxide passive films are not as stable as their titanium oxide counterparts.
Advantage of Metallic Disadvantage of MetallicBracket Bracket1. They can be recycled. 1. Non-esthetic2. They can be sterilized. 2. Tend to get corrode3. They resist and cause staining deformation and of teeth fracture.4. They exhibit least friction at wire-bracket interface.5. They are not very expensive.
Aesthetic BracketsPlastic Brackets The first plastic brackets were manufactured from unfilled polycarbonate. Unfortunately, these brackets had a tendency to undergo creep deformation when transferring torque loads generated by archwires to the teeth. Ceramic reinforced, fiberglass-reinforced, and metal slot- reinforced polycarbonate brackets were subsequently introduced to alleviate this problem. Another problem was discoloration of the first-generation unfilled polycarbonate brackets during clinical aging. The reinforced polycarbonate brackets were also introduced in response to reports of enamel damage by ceramic brackets.
The primers that were used to prepare the base for bonding have gradually been eliminated so that this clinical step would be avoided during bracket placement on the etched enamel surface. These primers contain low-molecular-weight dimethacrylates and methyl methacrylate, producing a surface that facilitates micromechanical interlocking
Secondary electron images of the base of a polycarbonate bracket. (a) As-received.Original magnification (b) Following application of a methacrylate-basedprimer, depicting the dramatically increased roughness used to facilitatemicromechanical interlocking with the adhesive
While the metal slot-reinforced polycarbonate brackets appear to be capable of generating the desired torque on teeth under clinical conditions, problems have been reported with the integrity of the slot periphery. Some of the metal slots have a level of surface roughness that may significantly affect the archwire-bracket sliding friction.
Fig. 4 Secondary electron images of a polycarbonate bracket withmetallic slot, demonstrating increased roughness of the slot surfaceopposing the archwire. (a) Lateral view. Original magnification *60. (b)Lateral view. Original magnification *150. Note the incompleteadaptation of the metallic slot base to the bracket wall
A beneficial consequence of the relatively low elastic modulus of polycarbonate is that the load applied during debonding of the plastic brackets results in a peel-off effect, mimicking the debonding behavior of metal brackets (Fig. 4). Fig. 4 Polycarbonate bracket exhibiting bending distortion during debonding, where the pattern mimics the peel-off debonding of stainless steel brackets. Note the bending of the bracket wings.
Advantages of Plastic Disadvantages of PlasticBracket Bracket Available in tooth 1. Tend to discolor particularly in colored or transparent patients who smoke forms. or drink coffee 2. They offer poor dimensional stability 3. Their slot tend to distort 4. The friction between bracket and metal archwire is very high
Ceramic BracketsComposition and Structure1. Zirconia: Ceramic brackets fabricated from polycrystalline zirconium oxide .2. Alumina : Ceramic bracket made of high-purity aluminum oxide, and the brackets are available in both polycrystalline and single-crystal forms. The single-crystal alumina brackets contain less impurities than are found in the polycrystalline alumina brackets, which require the presence of sintering aids during manufacturing. Single-crystal brackets also have excellent optical clarity owing to the absence of internal grain boundaries. Physical properties of single crystal and polycrystalline alumina brackets are listed in Table 1.
Table 1 Mechanical properties of aluminum oxide
Single-crystal alumina has lower resistance to crack propagation than does polycrystalline alumina, where the advancing cracks follow irregular paths along grain boundaries. Figures. 5 and 6 are scanning electron microscope photographs of fractured tie-wings, illustrating the difference in crack propagation behavior for single crystal and polycrystalline alumina brackets.
Fig. 6 Secondary electron image of Fig. 5 Secondary electron image ofthe tie wing fracture site for the tie-wing fracturecompressive loading of a site for compressive loadingpolycrystalline alumina bracket. of a single-crystal alumina bracket
The strength of both single-crystal and polycrystalline alumina can be increased by eliminating surface flaws that can serve as sites of stress concentration and fracture initiation. Decreasing the grain size will also increase the strength of polycrystalline alumina. There has been interest in zirconia brackets because of the possibility of achieving much higher values of fracture toughness than are possible for polycrystalline alumina brackets.
Advantages of ceramic Disadvantages of ceramicbrackets brackets1. They are 1. They are very brittle dimensionally stable and therefore fracture or crack when undue and do not distort in forces are applied. the oral cavity. 2. To compensate it’s2. They are durable brittleness their size is and resist staining in increased which tend oral environment. to increase their bulk. 3. They exhibit greater friction at wire-bracket interface
Base Morphology The bonding mechanisms that have been identified may be classified into three major categories:1. Mechanical retention employing large recesses (Fig. 7) Fig. 7 Secondary electron image of the base of a polycrystalline alumina bracket employing large recesses for mechanical interlocking with the adhesive resin
2. Chemical adhesion facilitated by the use of a silane layer (Fig. 8)Fig. 8 Incident polarized light image of the base of a single-crystal ceramic bracketcovered with an island of a silane layer, illustrating a birefringence effect. Brightfield image, crossed polarizers.
3. Micromechanical retention through the utilization of a number of configurations, including protruding crystals, grooves, a porous surface, and spherical glass particles Fig. 9.Fig. 9 Bases of polycrystalline brackets employing various protrusion designs formicromechanical interlocking with the adhesive. (a) Base with spherical particles. (b) Crosssection of the base of the brackets shown in (a), illustrating the spherical particle protrusion.(c) Sharp-edged crystal protrusions.
A combination of two or more of the above mechanisms may also be employed to achieve further strengthening of the interfacial region between the base and adhesive and yield increased bond strength. Smooth bracket base surfaces should better distribute the shear stresses over the entire adhesive, while minimizing localized areas of stress concentration. In the presence of strong chemical adhesion, such localized stresses would promote cohesive resin failures, resin/enamel failures, and even cohesive bracket fractures. The irregular-shaped microparticles on the bases do not appear to offer sites for local stress concentrations to initiate failure, particularly when these particles are covered by the adhesive layer
The clinical importance of the designand structure of the bracket base1. The longevity and integrity of the adhesive bond depend strongly on the base.2. Studies have demonstrated the pivotal role of the base for enamel damage observed following debonding.
Optical Properties The amount of the photocuring light transmitted through a bracket may affect the properties of the light-cured adhesive and its curing efficiency. The optical transmittance of single crystalline is differed from polycrystalline. This difference arises from the presence of grain boundaries in the polycrystalline brackets, which cause light scattering and reduction in the intensity of the light beam reaching the adhesive paste.. It was proposed that under the usual conditions present in the orthodontic bonding environment, a critical value for light transmittance, ranging between 30 % and 40 %, must be attained to induce adequate polymerization of the adhesive.
Fracture Toughness 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 isto propagate a crack in the material.The alumina ceramics used in the manufacture oforthodontic brackets contain strong, directional covalentbonds that do not allow permanent deformation orductility by the movement of dislocations as found inmetals. When these ceramics are subjected to theirmaximum elastic stress levels, brittle failure occurs inwhich interatomic bonds at the tips of flaws rupture, andthe material fails by crack propagation.
Fracture Toughness Cont.,Alumina brackets are very susceptible to crack initiationat minute imperfections or regions where materialimpurities have accumulated.Crack propagation is relatively unimpeded in single-crystal alumina brackets compared with polycrystallinealumina brackets. Fracture surface energies are higher for polycrystallinealumina than for single-crystal alumina because of theirregular paths of crack propagation along the grainboundaries in the former.Acceptance of the zirconia brackets has not beenwidespread because of their inferior aesthetics (greateropacity and yellowish tint) compared to thepolycrystalline alumina brackets.
Tie-Wing Strength Tie-wing fracture of ceramic brackets is a major concern for the orthodontist, since the appliance becomes ineffective and ligation of the archwire to the bracket is no longer achievable. Debonding of the fractured bracket is required, followed by rebonding of a new bracket. Additional chairside time must be spent by the orthodontist to perform these procedures, in addition to the risk of enamel damage involved in removal of the adhesive resin. Studies have shown that the bases of the tie-wings are generally the locations of concentrated stresses when forces are applied to the bracket by the orthodontist. Tie-wing fractures have been much more common for the single crystal alumina brackets because of their lower resistance to crack propagation. Clinical procedures that may scratch or otherwise damage the surfaces of alumina brackets further reduce fracture toughness and predispose the bracket to eventual failure.
Tie-Wing Strength Cont., Studies have shown that the bases of the tie-wings are generally the locations of concentrated stresses when forces are applied to the bracket by the orthodontist. Tie-wing fractures have been much more common for the single crystal alumina brackets because of their lower resistance to crack propagation. Clinical procedures that may scratch or otherwise damage the surfaces of alumina brackets further reduce fracture toughness and predispose the bracket to eventual failure.
Bracket Slot - Archwire Friction The slot size and composition are critical elements for the clinical performance of the bracket. While the slot size and the angular offset in the three xyz coordinate planes are highly important parameters affecting the control of tooth movement, the slot surface is also involved in complex sliding friction phenomena with the engaged archwire. Scientific publications on archwire-bracket friction generally concur that metallic slot in contact with stainless steel archwires experience the least friction, while ceramic slots in contact with nickel- titanium and b-titanium archwires experience the greatest friction.
Bracket Slot - Archwire FrictionCont, Although some authors reported decreased friction for zirconia brackets, others found no differences in friction when zirconia brackets and several polycrystalline alumina brackets were compared. The origin of these observations lies in the surface roughness of the archwires and bracket slots. Much higher roughness is found for the nickel-titanium and b-titanium archwires, compared to the stainless steel and cobalt-chromium-nickel archwires. The stainless steel slots have lower roughness than the polycrystalline alumina slots, for which SEM observations show that pluck-out of grains occurs during bracket processing.
The Ligation Elastomeric modules, stainless steel ligature wires, and Teflon-coated ligatures have been used to ligate the archwire to the bracket in friction research. The problem with the use of an elastomeric module ring arises from the force relaxation in these polyurethane polymers. The force relaxation patterns of the elastomeric module products can vary significantly, with the consensus being that the 24-hour force loss may exceed 40 % under laboratory conditions.
The Ligation Cont, The use of stainless steel ligature ties has been shown to increase friction through a dual mechanism. There is a higher engagement force between the archwire and bracket, and additional friction is generated by the contact of the ligature surface with the archwire; however, elastomeric ligatures can induce the same effects. A practical conclusion from these studies was that self- ligating brackets showed less frictional forces, while the figure-of-eight ligature configuration increased friction significantly.
The Clinical Significance On a clinical level, the relationship of wire surface roughness to sliding friction has not been unequivocally defined. The majority of in vitro studies examining this issue have shown that friction increases with increased roughness of the wire and bracket surfaces. Those studies indicated that, in general, the b-titanium and nickel-titanium archwires and the ceramic brackets present increased friction, owing to their roughened surfaces arising from the manufacturing process.
The Clinical Significance Cont, Laboratory studies that use relatively clean sample surfaces do not simulate conditions in the oral environment where biofilms and calcified regions are present. Biofilms may reduce the coefficient of friction by producing a boundary lubrication effect through salivary protein adsorption and plaque accumulation. In contrast, the surface roughness and resistance to shear forces are expected to be increased at calcified.
References1. Orthodontic Material Scientific and Clinical Aspects, William A. Brantley and Theodore Eliades.2. Revolution of Orthodontic Brackets, Article, Dr. Tamizharasi and Dr. Senthil Kumar, Department of Orthodontics, KSR Institute of Dental Science & Research, Tiruchengode, Tamil Nadu.3. http://www.identalhub.com/article_what-are- orthodontic-brackets-163.aspx