Direct bonding metallic brackets: Where are they heading? Claude G. Matasa, PhD, DCE* Hollywood, Fla. E x a m i n a t i o n of the most successful brackets sold today shows that most of the orthodontists prefer a rhomboid-shaped, twin, miniaturized bracket, which is milled and made of several parts. Launching their highly successful Mini-Diamond series, Ormco of Glendale, Calif. t stated that despite the great number of products from which they can choose, clinicians treat one in four patients with Ormco Diamond brackets. Our statistics'- confirna their claim that these appliances are the single most popular bracket system in the world today. This does not mean, however, that tomorrow's preferences will remain the same. INCREDIBLE VARIETY
Shape and performance are supposed to be dictated by functionality, and this applies both in the realm of living beings, as well as among man-made items. Thus it is well-known that the body of a feline (the cheetah) can strongly resemble that of a completely different species, that of a canine (the greyhound), because they respond to the same need, speed in hunting. The phenomenon is called "convergence." On the other hand, 80 years ago a Mercedes was very different from a Ford; however, decades of improvements have refined
AtrrltOR'S I,;O1~:New information has become available since this article ,.,.'as originally accepted for publication. *Associate Professor,Oral Biomaterials. Ecologic University, Bucharest. President, Ortho-Cycle Co., ttollywood, Fla. 8 / ! 127837
their features, bringing both automobiles to almost the same streamline design. In our view, direct bonding brackets, these handles needed to straighten teeth as Angle called them, and on which modern orthodontics depends so much, are still in their infancy. Indeed, they are still in an early stage of development because they are far from converging toward the same shape and performance, In the last two decades, various bracket suppliers each have marketed at least a thousand different ones, and this inventory is still growing. This large inventory is due to different lines launched every few years, and to the various types, techniques, and prescriptions in existence. Their number is multiplied by the existence of two parallel slot sizes and by additional features or modifications that appear from one year to another. This contributes to confusion "even with the very people who are supposed to sell them, see Table I. IS THERE A WAY OUT OF THE CONFUSION?
It may sound presumptuous, but the only Theseus* who dares to extricate himself from this modern maze is the bracket reconditioner, who sells refurbished appliances, lle has to know thoroughly not only the brackets made through the years by some 30 manufacturers (Table II) but also the prescriptions and options in demand. Being exposed continuously to supply and de-
*Greek hero credited with killing the Minotaur~a Cretan monster,in a labyrinth. lte escaped from the maze by fonov.'ing a thread given him by Ariadne.
Table I. Number of different brackets released (minimal) Manufacturers (n = 10)
J
"'A" Company American Orthodontics GAC Lancer Masel Ortho-Organizers Ormco Rocky Mountain TP Orthodontics Unitek Total possibilities or number of
552
Types (n = 3) Begg
Lines (n = 3) Old (large)
Edgewise standard Standard (min.3) Preadjusted Mini
different direct bonding brackets: 12,600 brackets
I
Prescriptions (n = 3)
Andrews Alexander Burstone Creekmore Hilgers Level arch Minn. Integr. Ricketts Root Rolh
Teeth (n = 14)
Two centrals Two laterals Two upper canines Upper premolars Lower incisors Two lower canines Two lower premolars Two lower premolars
Special article
Volume 102 Number 6
mand, he has to steer his business wisely by buying only what he knows he can sell. This is not possible without a knowledge of the market and the use of statistics, specific charts and files on one hand, and the measuring tools that define torques, angulations, and the in-and-outs on the other.
553
Table II. Manufacturers of orthodontic brackets
Name "A" Company American Orthodontics
METHOD
Advanced Orthodontics
Recycling companies deal with a large number of practitioners the world over, the majority of which are in the United States. Whereas some recondition and purchase refurbished appliances, others are interested in selling their used appliances. Every batch is first evaluated. Then the appliances are subjected to adhesive removal and a first sterilization, after which they are counted, sorted, and identified. For the last purpose, some 200 characteristics are used, forming a complex profile code system. Thus, the type, manufacturer, line, slot size, and prescription, among others, are each represented through a specific number, which is carefully coded and entered in a computer. Although the appliances surveyed are generally about 2 years old, recycling representatives, who participate in most of the major meetings in the world, carefully note every new development and examine the newer appliances, which are then ordered through consultants. At one particular recycling company, Ortho-Cycle (Hollywood, Fla.), a collection of brackets from each manufacturer has been set up in a display mode of historical interest. Concurrently, rectangular arches, received with brackets attached to them, have been evaluated for a long time to correlate the number of bends found with the brackets, the type, line, and prescription. This study will probably be continued under the supervision of a university. Because of the competitive market conditions, part of the information thus gathered is kept confidential. Every spring, however, comparative percentages of the appliances received during the past years are released and discussed. Thus, on the basis of the annual total of brackets submitted for recycling, the frequency with which the appliances made by the major manufacturers were encountered in 1989"- is tabulated in Table Ill. Orthodontic meetings provide a variety of interesting sources of information concerning the demands and the opinions of the different appliances. The fact is few practitioners are able to keep up with the present multitude of new developments in the area, and even fewer know the latest releases. Because of the rapidly changing informational base, many practitioners seek the rapidly changing informational base, many practitioners seek the reconditioner's advice. Being immersed in the field and having no allegiance to any manufacturer, the reconditioner is often seen as a knowledgeable and impartial observer.
Centric Orthodontics Centro Europes de Ortodoncia Class One Orthodontics Dental Corporationof America Dentaurum GAC/Tomi Forestadent Glenroe Lancer Orthodontics Lee Pharmaceuticals Masel Orthodontics Morelli O.I.S. Orthod. Omega OREC Corporation Ormco Ortho Organizers Orthodontic S & E Orthorum Ortho Source Orthoval Oscar Rocky Mountain S.S. White Strite TP Orthodontics Unitek Corp./3M
I
Locatior, San Diego, Calif. Sheboygan, Wise. Brooklyn, N.Y. Marietta,Ga. Madrid Lubbock, Texas Rockville, Md. Pforzheim Central Islip, N.Y. Prorzheim Sarasota, Fla. Carlsbad, Calif. El Monte, Calif. Bristol, Pa. Sorocaba, SP Wilmington, Del. Wuppertal San Clemente, Calif. Glendora, Calif. San Marcos, Calif. Gaithersburg, Md. Montreal Hollywood, Calif. Sion Indianapolis, Ind. Denver, Colo. Holmdel, N . J . Cambridge La Porte, I n d . Monrovia, Calif.
I
Country United States United States United States United States Spain United States United States Germany United States Germany United States United States United States United States Brasil United States Germany UnitedStates United States United States United States Canada United States Switzerland United States UnitedStates UnitedStates Canada UnitedStates United States
large scale. On the other hand, it is unlikely that ceramic appliances will significantly replace the metallic ones in the next decade.
Likely future developments
CURRENT STATE OF THE ART The types
Using an historic perspective may help to understand today's appliances and predict their future developments, like that of the metallic bracket of the year 2000. Even if tooth movement will prove by then to be feasible with the help of electric, magnetic, chemical or biochemical stimuli, it will still take time before such advances are implemented on a
Presently, there are four types of brackets that have withstood the test of time: standard edgewise, preadjusted edgewise, Begg, and the combination edgewise/Begg, i.e., with vertical (or T-shaped) slots. Other types are used less and less (Fig. 1).
554
Matasa
AnL J. Orthod. Dentofac. Orthop. December 1992
T~N
AI~C~I
TRIPLE EI~E'dISE
BOWLES MULTIHIASE
FOCEL-HAG ILL
CIttJN IIOON
SI1APES OF BRACKETS NOW OBSOLETE
Fig. 1. Approximate distribution of main prescriptions.
Table III, Relative percentage of the brackets received by Ortho-Cycle from 1987 to 1989
a--
Year
"'A"-Co.
Orthodontics
1987 1988 1989 1990
19 20 17 18.6
18 16 15.5 12.4
I
1. Standard edgewise brackets are used less today because of the success of the preadjusted ones. Some practitioners, however, disillusioned by the arch wire bending they still have to do in the final stages of treatment, are inclined to pay less and purchase brackets without torque and angulation. 2. Preadjusted, that is pretorqued and preangulated brackets, are used today by most practitioners, several prescriptions and their modifications being used around the world. 3. Begg brackets as such are declining, with more and more clinicians switching to edgewise or to combinations of edgewise/Begg. Thus Begg brackets seems to be limited to a few countries where the technique is too well entrenched. The interest in the brackets combining standard edgewise with Begg is diminishing. Examples are the appliances developed by Unitek Corp., Monrovia, Calif. and Rocky Mountain, Denver, Colo. (brackets having a small vertical slot, 0.020 x 0.020 inch) and by the Broussard and
Unitek 16 14 17.2 12.5
I
Ormco
GAC
o 23 23 22.7 23.1
14 15 15 14.3
I
Others 10 12 12.6 19
the Minnesota Integrated System (brackets having a wider slot, 0.0185 • 0.046 inch). The combination preadjusted edgewise/Begg bracket with a 0.020 x 0.020-inch vertical slot is, however, highly successful. Single brackets such as Tip Edge and Straight Edge from TP Orthodontics (LaPorte, Ind.), twin brackets, such as "Mini-Diamond" from Ormco, Mini Master Series from American Orthodontics (Central Islip, N.Y.), and the V-slot Microarch from GAC are increasing in demand. While it is possible that the phenomenon is only temporary because of the transition to edgewise brackets by many former Begg brackets users, there are reasonable chances that the vertical slot will continue to remain as a feature in future brackets.
The prescriptions Two basic prescriptions, together with their various modifications, are now used almost worldwide, Fig. 2. While in the United States, Canada, and the southem part of South America, the Andrews prescription ("A" Company, San Diego, Calif.) (as such, or as rood-
Special article 555
Volume 102 Number 6
D ANDREWS, ROTH ~BEGG D
RICKETTS, HILGERS
Fig. 2. Worldwide distribution of main prescriptions.
ified by Roth of Unitek) is the most common, the Ricketts prescription (Rocky Mountains, Denver, Colo.), as well as its modification by Masel, are most often used in Europe. As a result, the 0.022-inch slot (recommended by Roth) is prevalent in North America, and the southern part of South America, whereas the 0.018inch slot (recommended by Ricketts) is commonly used in Europe and central and northern South America. A prescription that is gaining more and more followers is R. G. Alexander's Vari Simplex (Ormco, Glendora, Calif.), a system that.also uses the 0.018-inch slot. Examining the brackets received on arch wires as they are sent to our recycling company to be either sold or serviced, it Seems that more and more clinicians today are less faithful to a single prescription, a single brand, or even to a certain type of bracket. The number of bends foundon these arch wires is increasing instead of declining because of improvements. In our interpretation, m a n y clinicians have gone through a period of insecurity and now feel free to improvise according to the specific needs o f the patient and the individual requirements for an optimal finished result. Relying on wire bending and individualized creativity seems to be considered as a more efficient way to respond to the variety of cases encountered. What many Clinicians seem to retain from the pread; justed systems are the "in and out" of the slot and the torque; indeed, preset angulations are sometimes
replaced by "tailored" arch wire bending. This may eventually lead to a decrease of the bracket mesial-distal width, paving the way to further miniaturization and to a significant reduction in the inventory. The profile 9The days of piggy-back arch wires systems seem to be gone, and with them, the deep slots associated with high profiles. Patient comfort and ease in the treatment have diminished the height of the bracket to such an extent that in some instances the slot reaches the pad itself. Sometimes we find brackets where the vertical, T-slots are connected with the normal edgewise ones. Compound contoured bases, gingival extensions for the premolars and wedged profiles for the lower incisors have become standard features, and there are reasons to believe that they will remain ifi the brackets Of tomorrow. Rounded or chamfered contours are features that both clinicians and patients enjoy, and many practitioners use them as a criterion in bracket selection. They will not only remain, but will also become prevalent: Bracket base retention Today's favorite bases are die cut from laminates made of a fine mesh, pressed under heat to a foil that can have various thicknesses. T h e s e preformed pads are then brazed to the bracket body, leading to a four-
556 Matasa
Am. J.
Orthod.Dentofac.Orthop. December1992
Table IV. Composition of the main steels used in bracket manufacture
Designations ~ i UNS AISI
Cr (%)
303 304 304 L 316 316 L 317
S-30300 S-30400 S-30403 S-31600 S-31603 S-31700
14305 -14306 14401 14404 14438
0.15 0.08 0.03 0.08 0.03 0.08
2.00 2.00 2.00 2.130 2.00 2.00
1.00 1,00 1.00 1.00 1.00 1.00
17-19 18-20 18-20 16-18 16-18 18-20
630117-4 PH 631 / 17-7 PH
S-17400 S- 17700
14542 --
0.07 0.09
1.00 1.00
1.00 ! .00
15.5-17 16-18
I
Other
Ni (%) 8-10 8-10.5 8-12 10-14 10-14 11-15 3-5 6.5-7.5
(%)
0.20 0.045 0.045 0.045 0.045 0.045
0.15 0.03 0.03 0.03 0.03 0.03
0.6 Mo -2-3 iMo 2-3 Mo 3-4 Mo
0.04 0.04
0.03 0.04
4Cu, 3Nb 0.08-1.5 AI
The balance to 100% is iron. UNS,Unified number system; DIN,Deutsche Industrie Norman; C, carbon; Mn, maganese; Si, silicon; Cr,Chromium; Ni, nickel; P, phosphorous; S, sulfur; Mo, molybdemum; Cu, copper; Nb, niobium; AL, Aluminum.
part system (mesh, foil, braze layer, bracket) that is not only expensive to manufacture but also subject to human erroro.Improper brazing leads to poor joining, slot or under-tie-wing area clogging, as well as to incorrect bracket versus pad positioning, a Nonmesh bases; such as those obtained through chemical etching, sandblasting, or undercut milling, are not as widely accepted and are almost as expensive. A recent new approach, the multilayered mesh pad,"Super-Mesh" (GAC Corp., Central Islip, N.Y.), instead of simplifying the bracket construction, complicates it, thus increasing the cost. It also seems unnecessary as long as the practitioner uses common, retentive bases and the proper bonding techniques. (A simple way to determine the need for such bases is to observe if enough adhesive is left on the base after debonding.) On the other hand, the built-in retention of the less expensive cast or molded/sintered bases is limited by its very nature, the release from molds requiring the absence of undercuts. A promising forward step is the micromechanical retention, achieved by coating the bases with porous metal powder. ~ Unfortunately, the commercial scale-up as achieved by Orec, San Clemente, Calif., in their Speed brackets has been plagued with problems. While ceramic bracket silanation has b.een able to render even smooth bases so retentive as to lead to enamel fracture, the priming of metal bases has not been able to obviate the need for mechanical locking. The results of this treatment, mentioned first in 1982, have been described later.
Rotation As previously shown, the legacy left in rotation and uprighting by Begg and Broussard is alive in several modem brackets that are provided with vertical slots. Examination of the brackets sent for sale or service shows a decrease in the use of both Kobayashi loops and rotation wings. An exception are the Lang and
Lewis brackets used in the growing Vari Simplex technique, as recommended by R. G. Alexander. Confined only a few years ago to canine and premolar brackets, the addition of power arms has been extended to the upper laterals and lower incisors brackets, and occasionally to custom-modified upper centrals. Their use is spreading, and the demand is growing. It is of related interest to note that GAC has released ceramic canines endowed with double hooks, a feature allowing rotations in both directions. Brackets with hook-shaped and long power arms allow the application of stronger couples of forces, but because they are more likely to be irritating to patients, they have been now replaced by ball-ended designs. Resting under the currently impossible-to-hide metal arch wire, counterrotation features, such as the mesial-distal offsets as seen in Attract from "A"-Co., Mini Taurus from R.M.O., or Tip-Edge and StraightEdge from TP Orthodontics, do not alter the bracket aspect, and their use is likely to be expanded.
Bracket composition Almost all machined metal brackets are made of stainless steels 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, Table IV. The addition of the letter "'L" shows a further reduction in the carbon content than that corresponding to a particular class. Thus while the grade 303 has 18% chromium, 9% nickel and no molybdenum, 316L has 17% chromium, 13% nickel, 2.5% molybdenum, and 0.02% carbon. Steels with a lower AIS1 number starting with 3 are rather soft and easier to mill, but their corrosion resistance is low. That stainless steel rusts is not news; some brackets we receive have unsightly, typical stains both on them as well as on the adhesive. The
Voh~me 102 Number 6
use of higher grade steels improves the corrosion resistance, but at the same time increases hardness. To cut harder steels, even harder cutting tools have to be used, considerably increasing the machining difficulties. One of the highest grades, 316L, is considered too hard to be milled. It is likely that by launching their "Mini-Diamond" and the "Miniaturized" series using the precipitation hardening steel PH 17-4 (AISI 630), Ormco and Unitek have reached the limits for both twin bracket machining and for the selection of the stainless steels. Fortunately, casting, sintering, and metal injection molding (MIM) are not plagued by the machining difficulties related to the cold working, and therefore allow the use not only of harder and more corrosion resistant alloys like AISI 316L as recommended years ago, 7 but also of even stronger metals, a condition for further miniaturization.
Manufacturing Brackets made by milling are too expensive and at the same time too prone to human error, in comparison with those made with the help of molds. The dramatic step forward made by Unitek by milling a one-piece attachment on the lathe (Dynalock) has opened new horizons, several companies following suit and coming up later with one-piece brackets. This has opened the door to one-piece brackets made by casting (OrthoOrganizers, San Macros, Calif.), sintering (the partial welding together of metal particles below their melting point) and metal injection molding (MIM) (Fig. 3). It is interesting to note that while casting has long been a mainstay for "A" Company, allowing them to be the first ones to build-in identification marks and torque in the base, as well as to use more corrosion resistant alloys, the company has not extended the process to one-piece appliances, justifiably fearing retention problems. Indeed, building in a rough, high-surface area on the base side of the bracket while rendering the bracket side smooth and shiny is quite difficult, considering the minute size of the appliance. While sintering uses only metal powders, which are melted as such in molds, MIM adds to these various thermoplastic polymers, lubricants, and other additives, which are subsequently pyrolyzed and gasified. To compensate for the shrinking that occurs, the molds have to be larger than the parts; the operation is quite sophisticated and requires the use of computer-aided design, along with computer-numerical controlled machines tools. The resulting built-in accuracy and microstructural homogeneity, if not plagued by pores, leads to appliances having superior mechanical and chemical properties (brackets made of powders have been found to have a smaller corrosion rate than both the cast and the cold worked ones). 8
..
Special article
557'
Fig. 3. Cast,one-piece bracket with built-in base and power arm (Edgeway from Ortho-Organizers, San Macros, Calif.).
The cost
It is obvious that a bracket, made of several parts, each having to be machined and separately joined while the assembly has to be cautiously inspected, will be more expensive than the result of a molding process. To reduce the costs of such multipart brackets, some manufacturers had to resort to the use of cheaper, foreign labor. The expenses incurred by the use of molds in the processes of casting, sintering, and injection molding are initially high as a result of a more complex design, manufacture, and setup. These costs are, however, compensated later by the less expensive and less laborconsuming production, maintenance, and control. Again on the positive side are the better quality, the repeatable accuracy which can be achieved, the possibility to use a larger selection of alloys, as well as the ease in building in many valuable features, such as rounded surfaces, torque in the base, permanent markings, and scribe lines. Another cost-reducing feature is the possibility of directly obtaining one-piece brackets having both bases and power arms. Bracket shape
Today's direct bonding brackets exhibit an unlikely multitude of shapes with conflicting designs and structure. The bracket. Some extend along the mesiodistal plane, others have their length oriented along the gingival-incisal direction, whereas others are designed to be positioned diagonally, Fig. 4. The simultaneous existence of such a diversity makes one wonder how these brackets can be equally used to the best advantage, and why no outcries are heard concerning the opposite functioning principles. A case in point is presented by comparing the brackets Uni-Twin (Unitek) and Avant-Edge (TP Orthodontics), Fig. 5. Angle's brackets were single; to allow tooth rotation without the use of auxiliary eyelet ties, Swain 9 later introduced the Siamese or twin brackets, where two narrow edgewise brackets were joined together on a common base, at a distance equal to the width of one
558
Am. J. Orthod. Dentofac. Orthop.
Atatasa
December 1992 9
. . . . . . . .
i q w 9 * 9 w J * i 9 i * o i e a e , t a t%*a*a%%waIo,loo,.oo
.~176176176
::::::::::::::::::::: 9. : . . . . . : . , . . . . . . . : . . . -
.:o....:.:.....,......,'. . . . . . . . -.'.'.'. ..-.'.-.-........o*.......%
.,:.:.-.:...:.-.-.:.:.:.;..
9
*
i
9
J
*
9
9
9
w
!
*
*
i.
9
o*, o
J
~
7ti:i:i:il
*%%~
.alo o~.J
iiiii::iili
!iiii iiiiiiiiii /. 4
ACTIVA ("A"-Company)
HICRO-BONDS (American Orthod.)
ATTRACT ("A"-Company)
Fig, 4. Bracketsshowingdifferentorientations.
) j:,
UNITWIN (
UNITEK)
/
AVANT-EDGE ( T P LABORATORIES)
Fig. 5. Brackets showing opposite working principles.
of the brackets. At that time, such double brackets were welded to bands. The concept became especially popular with the advent of direct bonding, where a large base was necessary to increase retention, whereas a bulky appearance was undesirable. The trend toward miniaturization being all too obvious today, the gap between the two identical brackets has little value, especially when more and more practitioners count on rotating teeth by using power arms, vertical slots, and arch wire bending. Double brackets or twins will have to merge in singles, as we can already see in the design of many ceramic brackets where the gap has been eliminated to increase strength. The base. Rectangular, rhomboid, triangular (Glance by Unitek), oval, round (Mini-Ultra-Trim by Dentaurum, Pforzhering, 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 goingfrom ease in machining to patent infringement avoidance or to proper bracket matching. As the overall size of the bracket diminishes, the base will have to follow the exact shape of the bracket, preferably being built-in.
Bracket size In this respect, even if there is no ambiguity conceming the direction toward miniaturization, there is one concerning its extent. If one compares the brackets of the past with those of today, he cannot but wonder about the drastic size reduction that has occurred within a decade, Fig. 6. This trend is inescapable, but technology has its limits.
FUTURE DEVELOPMENT As already noted, to mill its Mini-Diamond brackets, Ormco has already departed from the common stainless steels. With structural engineering and a mechanical model simulating the torsion of the arch wire pressure exerted in the slot, '~ Figs. 7 and 8, it is possible to calculate the thickness of a bracket wall, c, needed to withstand different activation forces P produced by a rectangular arch wire with the help of the following formula: c = 6Phil6,, where b is the height of the arch wire in the slot, I the
Volume 102 Number6
Special article
1975 "A ' ~ Company
559
1990 Upl)er C e n t r a l s
Fig. 6. Bracket size reduction occurred within 15 years.
length of the wall along which the force is exerted, and SA the admissible unit load to bend a specific material. The necessary wall thickness for several presumptive slots into which a 0.022 • 0.025-inch arch wire exerts different activation forces is presented in Table V. The metal characteristics are taken from the literature and are noted as follows: SR, the tensile strength, Sc, the yield strength, and Sa, the acceptable strength. It is easy to see that as the wire fits higher in the slot (b) and the force P is larger, the wall thickness has to be greater. As the length of the wall on which this force is exerted (/), and the admissible load for bending for a certain alloy (Sa) is greater, this thickness can be reduced. By using this formula, one can calculate that a single, smaller bracket has better capacity to withstand larger forces than a larger twin. Thus, disregarding the difference in the steel used, one can find that the wall of the slot cut into a Mini-Diamond (Ormco) twin, upper central bracket is 1.8 mm wide (two times 0.9 mm) and 0.6 mm thick, and that the same dimensions in an Attract ("A" Company) single central bracket are 2.5 mm and 0.8 mm, respectively. Introducing these values into this formula and considering that both brackets are made of AISI 316L stainless steel, it shows that the smaller, single Attract bracket can withstand a force of 2.5 times larger than the larger twin Mini-Diamond one. Thus Ormco had to use a steel "four times harder than the standard stainless steels" for the new miniaturized twin brackets, t Knowing that the weight value of the "mini" brackets exceeds the weight value of gold or platinum several times, there is little doubt that if cosmetic reasons are paramount, more and more exotic alloys or metals may be used. Thus both the Udimet 500 alloy and molybdenum have the tensile and the yielding strength twice as high as those of the AISI 316L stainless steel. '1 As the overall size of the bracket diminishes, and
l,r~t.~e[uu~
~L;LIUn
Fig. 7. Distribution of~orces in bracket subjected to torsion.
B,,7
P y
//
/I I / / t I// / I I
I J
/ f '11111111
I I I
A J I_1
Fig. 8. Mechanical model of bracket slot wall subjected to bending.
especially its mesiodistal width narrows, single brackets will become generalized, and their bases will have to follow their exact shape, being oriented along the gingivoincisal direction. To provide for the higher "retention density" brought by miniaturized brackets, newer or better base production/treatments are needed. Thus powder-coated
560
Am. J. Orthod. Dentofac. Orthop. December 1992
Matasa
T a b l e V. M i n i m u m slot wall thickness per different metals Metal Torsional force p = Pll
Stainless steel AISI 316 L
I
I
Stainless steel AISI 201
I
Best standard
Ti stabilized
OR = 12 lbs/mm2 Oc = 6.5 Ibs/mm2 O^ = 0.70c = 45 Ibs/mm2
OR = 165 lbs/mm2 Oc = 86 lbs/mm'O^ = 60 lbs/mm2
Udimed 500 (Nonferrous)
Molybdenum (unnotched metal)
Ni, Ci, Co, Mo, Ti, A1 OR = 23 lbs/mm~ Oc = 171 lbs/mm2 O^ = 120 Ibs/mm2
OR = 23 Ibs/mmx Oc = 218 lbs/mm2 O^ = 152 Ibs/mm2
0.179 0.253 0.309 0.357 0.399
0.158 0.224 0.274 0.317 0.354
Mo
Wall thickness (ram)
1 lb/mm 2 Ib/mm 3 lb/mm 4 lb/mm 5 Ib/mm
0.289 0.409 0.501 0.578 0.646
0.253 0.357 0.438 0.505 0.565
Metal characteristics: O~, Tensile strength; Oc, yield strength; O,~, acceptable strength.
bases should be made more retentive and more reliable; controlled sintering or injection molding using different grades of powders could perhaps provide pores or locks at the base outer surface without compromising the bracket side, which has to be glossy. By borrowing methods long known in chemistry to generate high surface areas, topical roughness could be achieved. Thus, by incorporating finely divided aluminum in the corrosion-resistant metals (at the base surface), its dissolution by a solution of sodium hydroxide could leave, as in the case of the nickel Raney catalyst, a highly porous surface. CONCLUSION Today's preferences lean toward preadjusted, machined, four-component twin brackets. Such brackets are expensive to make and do not lend themselves to further miniaturization. According to the trends we see, the brackets of the year 2000 will be sleek, smaller, single, and therefore too narrow to provide enough rotation. They will be one-piece attachments, having built-in bases, vertical slots, or ball-ended power arms. New self-ligating systems will be found, as the present use of ligature wires is both unaesthetic and painstaking. The brackets will be provided with rounded forms, identification marks, scribe lines, as well as with torque, in-and-outs, and counterrotation aiding features. Their bases will be retentive not only because of macroundercuts but also because of larger surface areas, which could produced by chemical treatments, providing both microroughness and coupling affinity. The manufacturing process will be based on sintering, injection molding or casting; the metal used will be harder in the detriment of corrosion resistant. Being costlier, smaller, and therefore more delicate, new ways will probably be sought to debond tomorrow's brackets without damaging them o r / a n d harming
the patient. This is of increasing importance because for the same exerted forces, a smaller and smaller bonding area is available. This could increase the enamel strain well above the acceptable level. Trying to compare the brackets of tomorrow with those we are already familiar, they may resemble Attract or Straight-Edge, but without the tie-wings, which could be rendered unnecessary by detachable caps or other self-ligating means. Being at least partially cast or sintered in single one-piece units, their bases, power arms, and other features will be built in. REFERENCES 1. Ormco Corp. Print 070-5051. Glendora, California: Ormco Corp. 2. Matasa CG. Where do you stand? In: Phoenix without ashes. Hollywood, Florida: Ortho-CycleCo., 1990:8. 3. Matasa CG. Flaws in bracket manufacturing. J Clin Orthod 1990;26:149-52. 4. Hanson GH, Gibbon WM, Shimizu H. Bonding bases coated with porous metal powders. AM J OP.TtIOD1983;83:1-4. 5. Mascia VE, Chert SR. Shearing strengths of recycled directbonding brackets. Ar,l J ORT~tOD1982;82:212-60. 6. Siomka LV, Powers JM. In vitro bond strength of treated direct~ bonding bases. AM J ORTHOD1985;88:135. 7. Meijer R, Smith DC. Biodegradationof orthodonticbracket system. AM J ORTHODvoI. 90, nr. 3, p. 195, 1986. 8. Ballweber TC. Corrosion of orthodontic brackets: cast, cold worked and powdered metal. [Master's thesis] Loma Linda, California: Loma Linda University, 1985. 9. Lindquist JT. The edgewise appliance. In Graber TM, Swain BF, eds. Orthodontics, current principles and techniques. St. Louis: CV Mosby, 1985:569. 10. What limits bracket miniaturization.In: Phoenixwithout ashes. Hollywood, Florida: Ortho-CycleCo., 1990:6. 11. Taylor L. Properties and selectionsof metals. In: Metals handbook. 8th ed. Metals Park, Ohio: AmericanSociety of Metals, 1972. Reprint requests to:
Dr. Claude Matasa 2026 Scott St. Hollywood, FL 33020-2417
Shape and performance are supposed to be dictated by functionality, and this applies both in the realm of living beings, as well as among man-made items. Thus it is well-known that the body of a feline (the cheetah) can strongly resemble that of a completely different species, that of a canine (the greyhound), because they respond to the same need, speed in hunting. The phenomenon is called "convergence." On the other hand, 80 years ago a Mercedes was very different from a Ford; however, decades of improvements have refined
AtrrltOR'S I,;O1~:New information has become available since this article ,.,.'as originally accepted for publication. *Associate Professor,Oral Biomaterials. Ecologic University, Bucharest. President, Ortho-Cycle Co., ttollywood, Fla. 8 / ! 127837
their features, bringing both automobiles to almost the same streamline design. In our view, direct bonding brackets, these handles needed to straighten teeth as Angle called them, and on which modern orthodontics depends so much, are still in their infancy. Indeed, they are still in an early stage of development because they are far from converging toward the same shape and performance, In the last two decades, various bracket suppliers each have marketed at least a thousand different ones, and this inventory is still growing. This large inventory is due to different lines launched every few years, and to the various types, techniques, and prescriptions in existence. Their number is multiplied by the existence of two parallel slot sizes and by additional features or modifications that appear from one year to another. This contributes to confusion "even with the very people who are supposed to sell them, see Table I. IS THERE A WAY OUT OF THE CONFUSION?
It may sound presumptuous, but the only Theseus* who dares to extricate himself from this modern maze is the bracket reconditioner, who sells refurbished appliances, lle has to know thoroughly not only the brackets made through the years by some 30 manufacturers (Table II) but also the prescriptions and options in demand. Being exposed continuously to supply and de-
*Greek hero credited with killing the Minotaur~a Cretan monster,in a labyrinth. lte escaped from the maze by fonov.'ing a thread given him by Ariadne.
Table I. Number of different brackets released (minimal) Manufacturers (n = 10)
J
"'A" Company American Orthodontics GAC Lancer Masel Ortho-Organizers Ormco Rocky Mountain TP Orthodontics Unitek Total possibilities or number of
552
Types (n = 3) Begg
Lines (n = 3) Old (large)
Edgewise standard Standard (min.3) Preadjusted Mini
different direct bonding brackets: 12,600 brackets
I
Prescriptions (n = 3)
Andrews Alexander Burstone Creekmore Hilgers Level arch Minn. Integr. Ricketts Root Rolh
Teeth (n = 14)
Two centrals Two laterals Two upper canines Upper premolars Lower incisors Two lower canines Two lower premolars Two lower premolars
Special article
Volume 102 Number 6
mand, he has to steer his business wisely by buying only what he knows he can sell. This is not possible without a knowledge of the market and the use of statistics, specific charts and files on one hand, and the measuring tools that define torques, angulations, and the in-and-outs on the other.
553
Table II. Manufacturers of orthodontic brackets
Name "A" Company American Orthodontics
METHOD
Advanced Orthodontics
Recycling companies deal with a large number of practitioners the world over, the majority of which are in the United States. Whereas some recondition and purchase refurbished appliances, others are interested in selling their used appliances. Every batch is first evaluated. Then the appliances are subjected to adhesive removal and a first sterilization, after which they are counted, sorted, and identified. For the last purpose, some 200 characteristics are used, forming a complex profile code system. Thus, the type, manufacturer, line, slot size, and prescription, among others, are each represented through a specific number, which is carefully coded and entered in a computer. Although the appliances surveyed are generally about 2 years old, recycling representatives, who participate in most of the major meetings in the world, carefully note every new development and examine the newer appliances, which are then ordered through consultants. At one particular recycling company, Ortho-Cycle (Hollywood, Fla.), a collection of brackets from each manufacturer has been set up in a display mode of historical interest. Concurrently, rectangular arches, received with brackets attached to them, have been evaluated for a long time to correlate the number of bends found with the brackets, the type, line, and prescription. This study will probably be continued under the supervision of a university. Because of the competitive market conditions, part of the information thus gathered is kept confidential. Every spring, however, comparative percentages of the appliances received during the past years are released and discussed. Thus, on the basis of the annual total of brackets submitted for recycling, the frequency with which the appliances made by the major manufacturers were encountered in 1989"- is tabulated in Table Ill. Orthodontic meetings provide a variety of interesting sources of information concerning the demands and the opinions of the different appliances. The fact is few practitioners are able to keep up with the present multitude of new developments in the area, and even fewer know the latest releases. Because of the rapidly changing informational base, many practitioners seek the rapidly changing informational base, many practitioners seek the reconditioner's advice. Being immersed in the field and having no allegiance to any manufacturer, the reconditioner is often seen as a knowledgeable and impartial observer.
Centric Orthodontics Centro Europes de Ortodoncia Class One Orthodontics Dental Corporationof America Dentaurum GAC/Tomi Forestadent Glenroe Lancer Orthodontics Lee Pharmaceuticals Masel Orthodontics Morelli O.I.S. Orthod. Omega OREC Corporation Ormco Ortho Organizers Orthodontic S & E Orthorum Ortho Source Orthoval Oscar Rocky Mountain S.S. White Strite TP Orthodontics Unitek Corp./3M
I
Locatior, San Diego, Calif. Sheboygan, Wise. Brooklyn, N.Y. Marietta,Ga. Madrid Lubbock, Texas Rockville, Md. Pforzheim Central Islip, N.Y. Prorzheim Sarasota, Fla. Carlsbad, Calif. El Monte, Calif. Bristol, Pa. Sorocaba, SP Wilmington, Del. Wuppertal San Clemente, Calif. Glendora, Calif. San Marcos, Calif. Gaithersburg, Md. Montreal Hollywood, Calif. Sion Indianapolis, Ind. Denver, Colo. Holmdel, N . J . Cambridge La Porte, I n d . Monrovia, Calif.
I
Country United States United States United States United States Spain United States United States Germany United States Germany United States United States United States United States Brasil United States Germany UnitedStates United States United States United States Canada United States Switzerland United States UnitedStates UnitedStates Canada UnitedStates United States
large scale. On the other hand, it is unlikely that ceramic appliances will significantly replace the metallic ones in the next decade.
Likely future developments
CURRENT STATE OF THE ART The types
Using an historic perspective may help to understand today's appliances and predict their future developments, like that of the metallic bracket of the year 2000. Even if tooth movement will prove by then to be feasible with the help of electric, magnetic, chemical or biochemical stimuli, it will still take time before such advances are implemented on a
Presently, there are four types of brackets that have withstood the test of time: standard edgewise, preadjusted edgewise, Begg, and the combination edgewise/Begg, i.e., with vertical (or T-shaped) slots. Other types are used less and less (Fig. 1).
554
Matasa
AnL J. Orthod. Dentofac. Orthop. December 1992
T~N
AI~C~I
TRIPLE EI~E'dISE
BOWLES MULTIHIASE
FOCEL-HAG ILL
CIttJN IIOON
SI1APES OF BRACKETS NOW OBSOLETE
Fig. 1. Approximate distribution of main prescriptions.
Table III, Relative percentage of the brackets received by Ortho-Cycle from 1987 to 1989
a--
Year
"'A"-Co.
Orthodontics
1987 1988 1989 1990
19 20 17 18.6
18 16 15.5 12.4
I
1. Standard edgewise brackets are used less today because of the success of the preadjusted ones. Some practitioners, however, disillusioned by the arch wire bending they still have to do in the final stages of treatment, are inclined to pay less and purchase brackets without torque and angulation. 2. Preadjusted, that is pretorqued and preangulated brackets, are used today by most practitioners, several prescriptions and their modifications being used around the world. 3. Begg brackets as such are declining, with more and more clinicians switching to edgewise or to combinations of edgewise/Begg. Thus Begg brackets seems to be limited to a few countries where the technique is too well entrenched. The interest in the brackets combining standard edgewise with Begg is diminishing. Examples are the appliances developed by Unitek Corp., Monrovia, Calif. and Rocky Mountain, Denver, Colo. (brackets having a small vertical slot, 0.020 x 0.020 inch) and by the Broussard and
Unitek 16 14 17.2 12.5
I
Ormco
GAC
o 23 23 22.7 23.1
14 15 15 14.3
I
Others 10 12 12.6 19
the Minnesota Integrated System (brackets having a wider slot, 0.0185 • 0.046 inch). The combination preadjusted edgewise/Begg bracket with a 0.020 x 0.020-inch vertical slot is, however, highly successful. Single brackets such as Tip Edge and Straight Edge from TP Orthodontics (LaPorte, Ind.), twin brackets, such as "Mini-Diamond" from Ormco, Mini Master Series from American Orthodontics (Central Islip, N.Y.), and the V-slot Microarch from GAC are increasing in demand. While it is possible that the phenomenon is only temporary because of the transition to edgewise brackets by many former Begg brackets users, there are reasonable chances that the vertical slot will continue to remain as a feature in future brackets.
The prescriptions Two basic prescriptions, together with their various modifications, are now used almost worldwide, Fig. 2. While in the United States, Canada, and the southem part of South America, the Andrews prescription ("A" Company, San Diego, Calif.) (as such, or as rood-
Special article 555
Volume 102 Number 6
D ANDREWS, ROTH ~BEGG D
RICKETTS, HILGERS
Fig. 2. Worldwide distribution of main prescriptions.
ified by Roth of Unitek) is the most common, the Ricketts prescription (Rocky Mountains, Denver, Colo.), as well as its modification by Masel, are most often used in Europe. As a result, the 0.022-inch slot (recommended by Roth) is prevalent in North America, and the southern part of South America, whereas the 0.018inch slot (recommended by Ricketts) is commonly used in Europe and central and northern South America. A prescription that is gaining more and more followers is R. G. Alexander's Vari Simplex (Ormco, Glendora, Calif.), a system that.also uses the 0.018-inch slot. Examining the brackets received on arch wires as they are sent to our recycling company to be either sold or serviced, it Seems that more and more clinicians today are less faithful to a single prescription, a single brand, or even to a certain type of bracket. The number of bends foundon these arch wires is increasing instead of declining because of improvements. In our interpretation, m a n y clinicians have gone through a period of insecurity and now feel free to improvise according to the specific needs o f the patient and the individual requirements for an optimal finished result. Relying on wire bending and individualized creativity seems to be considered as a more efficient way to respond to the variety of cases encountered. What many Clinicians seem to retain from the pread; justed systems are the "in and out" of the slot and the torque; indeed, preset angulations are sometimes
replaced by "tailored" arch wire bending. This may eventually lead to a decrease of the bracket mesial-distal width, paving the way to further miniaturization and to a significant reduction in the inventory. The profile 9The days of piggy-back arch wires systems seem to be gone, and with them, the deep slots associated with high profiles. Patient comfort and ease in the treatment have diminished the height of the bracket to such an extent that in some instances the slot reaches the pad itself. Sometimes we find brackets where the vertical, T-slots are connected with the normal edgewise ones. Compound contoured bases, gingival extensions for the premolars and wedged profiles for the lower incisors have become standard features, and there are reasons to believe that they will remain ifi the brackets Of tomorrow. Rounded or chamfered contours are features that both clinicians and patients enjoy, and many practitioners use them as a criterion in bracket selection. They will not only remain, but will also become prevalent: Bracket base retention Today's favorite bases are die cut from laminates made of a fine mesh, pressed under heat to a foil that can have various thicknesses. T h e s e preformed pads are then brazed to the bracket body, leading to a four-
556 Matasa
Am. J.
Orthod.Dentofac.Orthop. December1992
Table IV. Composition of the main steels used in bracket manufacture
Designations ~ i UNS AISI
Cr (%)
303 304 304 L 316 316 L 317
S-30300 S-30400 S-30403 S-31600 S-31603 S-31700
14305 -14306 14401 14404 14438
0.15 0.08 0.03 0.08 0.03 0.08
2.00 2.00 2.00 2.130 2.00 2.00
1.00 1,00 1.00 1.00 1.00 1.00
17-19 18-20 18-20 16-18 16-18 18-20
630117-4 PH 631 / 17-7 PH
S-17400 S- 17700
14542 --
0.07 0.09
1.00 1.00
1.00 ! .00
15.5-17 16-18
I
Other
Ni (%) 8-10 8-10.5 8-12 10-14 10-14 11-15 3-5 6.5-7.5
(%)
0.20 0.045 0.045 0.045 0.045 0.045
0.15 0.03 0.03 0.03 0.03 0.03
0.6 Mo -2-3 iMo 2-3 Mo 3-4 Mo
0.04 0.04
0.03 0.04
4Cu, 3Nb 0.08-1.5 AI
The balance to 100% is iron. UNS,Unified number system; DIN,Deutsche Industrie Norman; C, carbon; Mn, maganese; Si, silicon; Cr,Chromium; Ni, nickel; P, phosphorous; S, sulfur; Mo, molybdemum; Cu, copper; Nb, niobium; AL, Aluminum.
part system (mesh, foil, braze layer, bracket) that is not only expensive to manufacture but also subject to human erroro.Improper brazing leads to poor joining, slot or under-tie-wing area clogging, as well as to incorrect bracket versus pad positioning, a Nonmesh bases; such as those obtained through chemical etching, sandblasting, or undercut milling, are not as widely accepted and are almost as expensive. A recent new approach, the multilayered mesh pad,"Super-Mesh" (GAC Corp., Central Islip, N.Y.), instead of simplifying the bracket construction, complicates it, thus increasing the cost. It also seems unnecessary as long as the practitioner uses common, retentive bases and the proper bonding techniques. (A simple way to determine the need for such bases is to observe if enough adhesive is left on the base after debonding.) On the other hand, the built-in retention of the less expensive cast or molded/sintered bases is limited by its very nature, the release from molds requiring the absence of undercuts. A promising forward step is the micromechanical retention, achieved by coating the bases with porous metal powder. ~ Unfortunately, the commercial scale-up as achieved by Orec, San Clemente, Calif., in their Speed brackets has been plagued with problems. While ceramic bracket silanation has b.een able to render even smooth bases so retentive as to lead to enamel fracture, the priming of metal bases has not been able to obviate the need for mechanical locking. The results of this treatment, mentioned first in 1982, have been described later.
Rotation As previously shown, the legacy left in rotation and uprighting by Begg and Broussard is alive in several modem brackets that are provided with vertical slots. Examination of the brackets sent for sale or service shows a decrease in the use of both Kobayashi loops and rotation wings. An exception are the Lang and
Lewis brackets used in the growing Vari Simplex technique, as recommended by R. G. Alexander. Confined only a few years ago to canine and premolar brackets, the addition of power arms has been extended to the upper laterals and lower incisors brackets, and occasionally to custom-modified upper centrals. Their use is spreading, and the demand is growing. It is of related interest to note that GAC has released ceramic canines endowed with double hooks, a feature allowing rotations in both directions. Brackets with hook-shaped and long power arms allow the application of stronger couples of forces, but because they are more likely to be irritating to patients, they have been now replaced by ball-ended designs. Resting under the currently impossible-to-hide metal arch wire, counterrotation features, such as the mesial-distal offsets as seen in Attract from "A"-Co., Mini Taurus from R.M.O., or Tip-Edge and StraightEdge from TP Orthodontics, do not alter the bracket aspect, and their use is likely to be expanded.
Bracket composition Almost all machined metal brackets are made of stainless steels 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, Table IV. The addition of the letter "'L" shows a further reduction in the carbon content than that corresponding to a particular class. Thus while the grade 303 has 18% chromium, 9% nickel and no molybdenum, 316L has 17% chromium, 13% nickel, 2.5% molybdenum, and 0.02% carbon. Steels with a lower AIS1 number starting with 3 are rather soft and easier to mill, but their corrosion resistance is low. That stainless steel rusts is not news; some brackets we receive have unsightly, typical stains both on them as well as on the adhesive. The
Voh~me 102 Number 6
use of higher grade steels improves the corrosion resistance, but at the same time increases hardness. To cut harder steels, even harder cutting tools have to be used, considerably increasing the machining difficulties. One of the highest grades, 316L, is considered too hard to be milled. It is likely that by launching their "Mini-Diamond" and the "Miniaturized" series using the precipitation hardening steel PH 17-4 (AISI 630), Ormco and Unitek have reached the limits for both twin bracket machining and for the selection of the stainless steels. Fortunately, casting, sintering, and metal injection molding (MIM) are not plagued by the machining difficulties related to the cold working, and therefore allow the use not only of harder and more corrosion resistant alloys like AISI 316L as recommended years ago, 7 but also of even stronger metals, a condition for further miniaturization.
Manufacturing Brackets made by milling are too expensive and at the same time too prone to human error, in comparison with those made with the help of molds. The dramatic step forward made by Unitek by milling a one-piece attachment on the lathe (Dynalock) has opened new horizons, several companies following suit and coming up later with one-piece brackets. This has opened the door to one-piece brackets made by casting (OrthoOrganizers, San Macros, Calif.), sintering (the partial welding together of metal particles below their melting point) and metal injection molding (MIM) (Fig. 3). It is interesting to note that while casting has long been a mainstay for "A" Company, allowing them to be the first ones to build-in identification marks and torque in the base, as well as to use more corrosion resistant alloys, the company has not extended the process to one-piece appliances, justifiably fearing retention problems. Indeed, building in a rough, high-surface area on the base side of the bracket while rendering the bracket side smooth and shiny is quite difficult, considering the minute size of the appliance. While sintering uses only metal powders, which are melted as such in molds, MIM adds to these various thermoplastic polymers, lubricants, and other additives, which are subsequently pyrolyzed and gasified. To compensate for the shrinking that occurs, the molds have to be larger than the parts; the operation is quite sophisticated and requires the use of computer-aided design, along with computer-numerical controlled machines tools. The resulting built-in accuracy and microstructural homogeneity, if not plagued by pores, leads to appliances having superior mechanical and chemical properties (brackets made of powders have been found to have a smaller corrosion rate than both the cast and the cold worked ones). 8
..
Special article
557'
Fig. 3. Cast,one-piece bracket with built-in base and power arm (Edgeway from Ortho-Organizers, San Macros, Calif.).
The cost
It is obvious that a bracket, made of several parts, each having to be machined and separately joined while the assembly has to be cautiously inspected, will be more expensive than the result of a molding process. To reduce the costs of such multipart brackets, some manufacturers had to resort to the use of cheaper, foreign labor. The expenses incurred by the use of molds in the processes of casting, sintering, and injection molding are initially high as a result of a more complex design, manufacture, and setup. These costs are, however, compensated later by the less expensive and less laborconsuming production, maintenance, and control. Again on the positive side are the better quality, the repeatable accuracy which can be achieved, the possibility to use a larger selection of alloys, as well as the ease in building in many valuable features, such as rounded surfaces, torque in the base, permanent markings, and scribe lines. Another cost-reducing feature is the possibility of directly obtaining one-piece brackets having both bases and power arms. Bracket shape
Today's direct bonding brackets exhibit an unlikely multitude of shapes with conflicting designs and structure. The bracket. Some extend along the mesiodistal plane, others have their length oriented along the gingival-incisal direction, whereas others are designed to be positioned diagonally, Fig. 4. The simultaneous existence of such a diversity makes one wonder how these brackets can be equally used to the best advantage, and why no outcries are heard concerning the opposite functioning principles. A case in point is presented by comparing the brackets Uni-Twin (Unitek) and Avant-Edge (TP Orthodontics), Fig. 5. Angle's brackets were single; to allow tooth rotation without the use of auxiliary eyelet ties, Swain 9 later introduced the Siamese or twin brackets, where two narrow edgewise brackets were joined together on a common base, at a distance equal to the width of one
558
Am. J. Orthod. Dentofac. Orthop.
Atatasa
December 1992 9
. . . . . . . .
i q w 9 * 9 w J * i 9 i * o i e a e , t a t%*a*a%%waIo,loo,.oo
.~176176176
::::::::::::::::::::: 9. : . . . . . : . , . . . . . . . : . . . -
.:o....:.:.....,......,'. . . . . . . . -.'.'.'. ..-.'.-.-........o*.......%
.,:.:.-.:...:.-.-.:.:.:.;..
9
*
i
9
J
*
9
9
9
w
!
*
*
i.
9
o*, o
J
~
7ti:i:i:il
*%%~
.alo o~.J
iiiii::iili
!iiii iiiiiiiiii /. 4
ACTIVA ("A"-Company)
HICRO-BONDS (American Orthod.)
ATTRACT ("A"-Company)
Fig, 4. Bracketsshowingdifferentorientations.
) j:,
UNITWIN (
UNITEK)
/
AVANT-EDGE ( T P LABORATORIES)
Fig. 5. Brackets showing opposite working principles.
of the brackets. At that time, such double brackets were welded to bands. The concept became especially popular with the advent of direct bonding, where a large base was necessary to increase retention, whereas a bulky appearance was undesirable. The trend toward miniaturization being all too obvious today, the gap between the two identical brackets has little value, especially when more and more practitioners count on rotating teeth by using power arms, vertical slots, and arch wire bending. Double brackets or twins will have to merge in singles, as we can already see in the design of many ceramic brackets where the gap has been eliminated to increase strength. The base. Rectangular, rhomboid, triangular (Glance by Unitek), oval, round (Mini-Ultra-Trim by Dentaurum, Pforzhering, 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 goingfrom ease in machining to patent infringement avoidance or to proper bracket matching. As the overall size of the bracket diminishes, the base will have to follow the exact shape of the bracket, preferably being built-in.
Bracket size In this respect, even if there is no ambiguity conceming the direction toward miniaturization, there is one concerning its extent. If one compares the brackets of the past with those of today, he cannot but wonder about the drastic size reduction that has occurred within a decade, Fig. 6. This trend is inescapable, but technology has its limits.
FUTURE DEVELOPMENT As already noted, to mill its Mini-Diamond brackets, Ormco has already departed from the common stainless steels. With structural engineering and a mechanical model simulating the torsion of the arch wire pressure exerted in the slot, '~ Figs. 7 and 8, it is possible to calculate the thickness of a bracket wall, c, needed to withstand different activation forces P produced by a rectangular arch wire with the help of the following formula: c = 6Phil6,, where b is the height of the arch wire in the slot, I the
Volume 102 Number6
Special article
1975 "A ' ~ Company
559
1990 Upl)er C e n t r a l s
Fig. 6. Bracket size reduction occurred within 15 years.
length of the wall along which the force is exerted, and SA the admissible unit load to bend a specific material. The necessary wall thickness for several presumptive slots into which a 0.022 • 0.025-inch arch wire exerts different activation forces is presented in Table V. The metal characteristics are taken from the literature and are noted as follows: SR, the tensile strength, Sc, the yield strength, and Sa, the acceptable strength. It is easy to see that as the wire fits higher in the slot (b) and the force P is larger, the wall thickness has to be greater. As the length of the wall on which this force is exerted (/), and the admissible load for bending for a certain alloy (Sa) is greater, this thickness can be reduced. By using this formula, one can calculate that a single, smaller bracket has better capacity to withstand larger forces than a larger twin. Thus, disregarding the difference in the steel used, one can find that the wall of the slot cut into a Mini-Diamond (Ormco) twin, upper central bracket is 1.8 mm wide (two times 0.9 mm) and 0.6 mm thick, and that the same dimensions in an Attract ("A" Company) single central bracket are 2.5 mm and 0.8 mm, respectively. Introducing these values into this formula and considering that both brackets are made of AISI 316L stainless steel, it shows that the smaller, single Attract bracket can withstand a force of 2.5 times larger than the larger twin Mini-Diamond one. Thus Ormco had to use a steel "four times harder than the standard stainless steels" for the new miniaturized twin brackets, t Knowing that the weight value of the "mini" brackets exceeds the weight value of gold or platinum several times, there is little doubt that if cosmetic reasons are paramount, more and more exotic alloys or metals may be used. Thus both the Udimet 500 alloy and molybdenum have the tensile and the yielding strength twice as high as those of the AISI 316L stainless steel. '1 As the overall size of the bracket diminishes, and
l,r~t.~e[uu~
~L;LIUn
Fig. 7. Distribution of~orces in bracket subjected to torsion.
B,,7
P y
//
/I I / / t I// / I I
I J
/ f '11111111
I I I
A J I_1
Fig. 8. Mechanical model of bracket slot wall subjected to bending.
especially its mesiodistal width narrows, single brackets will become generalized, and their bases will have to follow their exact shape, being oriented along the gingivoincisal direction. To provide for the higher "retention density" brought by miniaturized brackets, newer or better base production/treatments are needed. Thus powder-coated
560
Am. J. Orthod. Dentofac. Orthop. December 1992
Matasa
T a b l e V. M i n i m u m slot wall thickness per different metals Metal Torsional force p = Pll
Stainless steel AISI 316 L
I
I
Stainless steel AISI 201
I
Best standard
Ti stabilized
OR = 12 lbs/mm2 Oc = 6.5 Ibs/mm2 O^ = 0.70c = 45 Ibs/mm2
OR = 165 lbs/mm2 Oc = 86 lbs/mm'O^ = 60 lbs/mm2
Udimed 500 (Nonferrous)
Molybdenum (unnotched metal)
Ni, Ci, Co, Mo, Ti, A1 OR = 23 lbs/mm~ Oc = 171 lbs/mm2 O^ = 120 Ibs/mm2
OR = 23 Ibs/mmx Oc = 218 lbs/mm2 O^ = 152 Ibs/mm2
0.179 0.253 0.309 0.357 0.399
0.158 0.224 0.274 0.317 0.354
Mo
Wall thickness (ram)
1 lb/mm 2 Ib/mm 3 lb/mm 4 lb/mm 5 Ib/mm
0.289 0.409 0.501 0.578 0.646
0.253 0.357 0.438 0.505 0.565
Metal characteristics: O~, Tensile strength; Oc, yield strength; O,~, acceptable strength.
bases should be made more retentive and more reliable; controlled sintering or injection molding using different grades of powders could perhaps provide pores or locks at the base outer surface without compromising the bracket side, which has to be glossy. By borrowing methods long known in chemistry to generate high surface areas, topical roughness could be achieved. Thus, by incorporating finely divided aluminum in the corrosion-resistant metals (at the base surface), its dissolution by a solution of sodium hydroxide could leave, as in the case of the nickel Raney catalyst, a highly porous surface. CONCLUSION Today's preferences lean toward preadjusted, machined, four-component twin brackets. Such brackets are expensive to make and do not lend themselves to further miniaturization. According to the trends we see, the brackets of the year 2000 will be sleek, smaller, single, and therefore too narrow to provide enough rotation. They will be one-piece attachments, having built-in bases, vertical slots, or ball-ended power arms. New self-ligating systems will be found, as the present use of ligature wires is both unaesthetic and painstaking. The brackets will be provided with rounded forms, identification marks, scribe lines, as well as with torque, in-and-outs, and counterrotation aiding features. Their bases will be retentive not only because of macroundercuts but also because of larger surface areas, which could produced by chemical treatments, providing both microroughness and coupling affinity. The manufacturing process will be based on sintering, injection molding or casting; the metal used will be harder in the detriment of corrosion resistant. Being costlier, smaller, and therefore more delicate, new ways will probably be sought to debond tomorrow's brackets without damaging them o r / a n d harming
the patient. This is of increasing importance because for the same exerted forces, a smaller and smaller bonding area is available. This could increase the enamel strain well above the acceptable level. Trying to compare the brackets of tomorrow with those we are already familiar, they may resemble Attract or Straight-Edge, but without the tie-wings, which could be rendered unnecessary by detachable caps or other self-ligating means. Being at least partially cast or sintered in single one-piece units, their bases, power arms, and other features will be built in. REFERENCES 1. Ormco Corp. Print 070-5051. Glendora, California: Ormco Corp. 2. Matasa CG. Where do you stand? In: Phoenix without ashes. Hollywood, Florida: Ortho-CycleCo., 1990:8. 3. Matasa CG. Flaws in bracket manufacturing. J Clin Orthod 1990;26:149-52. 4. Hanson GH, Gibbon WM, Shimizu H. Bonding bases coated with porous metal powders. AM J OP.TtIOD1983;83:1-4. 5. Mascia VE, Chert SR. Shearing strengths of recycled directbonding brackets. Ar,l J ORT~tOD1982;82:212-60. 6. Siomka LV, Powers JM. In vitro bond strength of treated direct~ bonding bases. AM J ORTHOD1985;88:135. 7. Meijer R, Smith DC. Biodegradationof orthodonticbracket system. AM J ORTHODvoI. 90, nr. 3, p. 195, 1986. 8. Ballweber TC. Corrosion of orthodontic brackets: cast, cold worked and powdered metal. [Master's thesis] Loma Linda, California: Loma Linda University, 1985. 9. Lindquist JT. The edgewise appliance. In Graber TM, Swain BF, eds. Orthodontics, current principles and techniques. St. Louis: CV Mosby, 1985:569. 10. What limits bracket miniaturization.In: Phoenixwithout ashes. Hollywood, Florida: Ortho-CycleCo., 1990:6. 11. Taylor L. Properties and selectionsof metals. In: Metals handbook. 8th ed. Metals Park, Ohio: AmericanSociety of Metals, 1972. Reprint requests to:
Dr. Claude Matasa 2026 Scott St. Hollywood, FL 33020-2417
