Introduction
___.Chemical research has led to the discovery and development of thousands of products. Materials science encompasses the natural and synthetic materials used in a wide range of products and structures in industry, ranging from integrated-circuit chips and fuel cells to pharmaceuticals. The rhythm of development in these fields is, however, uneven. After the government sponsored space research, considered to be the spearhead, industry closely follows. Aside the profit-making drug manufacture, which is stimulated both by competition and by an aging population, medicine lags behind. It still imports “as is” technology from industry, instead of playing its life-or-illness/death card. Having an even lesser impact, dentistry follows, having orthodontics at its fringe (low in consumption and less risky health-wise).
___.That even the medical devices are not a priority is reflected also in a report on nanotechnology made by NASA’s Ames Research Center entitled “Assessment of opportunities” which places the medical devices trailing behind many other fields. The dental -and especially the orthodontic biomaterials- are far from being in the forefront of science & technology. This can be easily assessed from the fact that most of the composites used today as adhesives are about the same as those which were developed almost half a century ago; the “nano” era has barely started there.
Composites = synergism
___.Composite materials are combinations of two or more components dissimilar in form and properties which exhibit clear boundaries between them, the purpose being to maximize advantages. The main categories are dispersion-strengthened, particle-strengthened, laminar and fiber-reinforced materials. Whatever the material, a composite is formed by a matrix (binder) of low-modulus material and reinforcing elements (often called fillers) with strength and stiffness properties 10 to 1000 times higher than those of the matrix. Both binders and fillers can be metals, polymers or ceramics.
Dispersion-strengthened composites. Their matrix of elementary substance or alloy hosts uniformly distributed particles sized from 0.01 to 0.1m and amounting to 1-15% by volume. Some dispersion-strengthened metal composites are well represented in orthodontics, as is the case with the main stainless steel alloy used for mini-brackets, PH 17-4 (the capital letters are abbreviations from Precipitation Hardened, indicating that it contains dispersed Cu and Nb). As these particles obstruct the motion of dislocations in the matrix, the material withstands deformation.
Particle-strengthened composites. Almost all the adhesives and restoratives used in orthodontics today contain as fillers particles whose size exceeds 1m and their volume fraction exceeds 25%. The load is distributed between the matrix and the particles, the latter generating a strengthening effect.
Fiber-reinforced composites. The strength due to fiber addition is significantly higher than that obtained with the help of particles, as shown in Fig. 2. Fiber-reinforced composite materials (FRC) are widely used in industry as their advantages include outstanding mechanical properties such as high strength, high stiffness, and unique flexibility in design capabilities. Besides, these are light weight, corrosion resistant, impact resistant and exhibit excellent strength to fatigue. Despite their advantages shown in books¹ papers^2-6 and tens of patents (shown here for adhesives quoting only the first, 1978,⁷ and the last, in 2005⁸), these are under-used. Compared with the metal ones, however, the FRC bridges have a better survival rate than traditional metal alloy resin-bonded bridges⁹. Following the pioneering efforts of C J Burstone and A J Goldberg¹⁰, such composites are used as structural members in various clinical situations (prosthodontic frameworks, posts retainers, splints and bars that join teeth to form either anchorage or active units), but only too seldom in adhesives and restoratives. However, an interesting attempt to introduce FRC in brackets has been made by Gestenko, Norway.
Laminar or sandwich composites. As their name indicates, these are made of layers of constituents, such as these found in the duplex stainless steels, Fig. 3, in ceramic/metal and plastic arch wires, Fig. 4, or in complex brackets (in self-engaging mechanisms or in ceramic or plastic ones that are added with mesh or metal-lining). Examples of the above can be found in the last issue of Orthodontics, Current Principles and Techniques.¹¹
___.While all of these composites have important advantages, the latter are primarily limited to in-plane properties. When out-of-plane or through-thickness properties are important, composites tend to perform poorly. This problem is being addressed by the development of 3D and “nano”-composites, domains seldom explored in orthodontics.
Nano-composites. As the name implies, these include constituents that are mixed on a nanometer-length scale. Conventional materials have grains varying in size anywhere from 0.1 mm to a few millimeters. A micron is a micrometer or a millionth (10-6) of a meter, while a nanometer (nm) is one thousand times smaller (10-9). A nanomaterial has at least part of its grains on the order of 1-100 nm. As the average size of an atom is on the order of 1-2 angstroms (Å) in radius and one nanometer comprises 10 Å, in one nm there may be 3-5 atoms, depending on their atomic radii.
___.Practically all nano-materials are composites whose features often have properties dramatically different from their bulk-scale counterparts examined above. As a result, the Biomaterials Network (www.Biomat.com), regularly quoted in J. Citation Report:(ISI), currently cites no less than ten “Nano” biomaterials oriented journals having in their name the word “Nano”.
___.Metallic nanoalloys, aside from improved properties, often exhibit unexpected performances (thus, while Fe and Ag are immiscible in bulk, these are easily mixed in clusters). When polymeric, nanoalloys offer improvements in several of the properties of thermoplastics including tensile strength, modulus, barrier properties as well as resistance to wear, erosion and corrosion. The nanoscale dispersion of sheet-like inorganic silicate particles in a polymer matrix allows it to exhibit a surprising optical clarity along with increased strength, stiffness, thermal stability, reduced permeability and flame retardancy.
___.Ceramic nanocomposites can often go beyond their normal flexural strength, a known weak point of their bulk counterparts. In many instances, nature provides the example; thus, GE has tried to duplicate the abalone shell (made chiefly of calcium carbonate organized into “tablets” glued by a rubbery polymer that serves as a cushion between the layers). These shells are resistant to breakage or shatter because when a micro-crack does form, the equivalent of bumps and bruises, they don’t easily break.
Economics
___.If nanocomposites could offer these improvements at no additional cost, then nanocomposites would quickly replace a large percentage of unfilled thermoplastics. Unfortunately, if the improved performance of a nanocomposite is compared, let say to a thermoplastic polymer, it comes with an increase in price. In spite of this, in some five years from now it is considered that there should be multi-million pound quantities of nanocomposites in both production and use in various applications. Thus, the US government has set up the Nanotechnology Initiative in 2000 and invested more than $700 million in the field in 2003. At a recent meeting on nano ethics held at the University of South Carolina, Michael C. Roco, Senior Advisor to the National Science Foundation, and Chair of the Nanoscale Science, Engineering and Technology Subcommittee (NSEC) of the National Science and Technology Committee (NSTC) has announced that there are already registered no less than 300 nanomachine/ nanosystem projects.
___.A European Nanobusiness Association report suggests that EU countries are spending twice the amount that the US is spending in this area. Japan, which has been spending much on nanotech research and technology since the mid-1980s, announced in early 2002 that it would spend up to $1 billion in this field in 2003 and 2004. South Korea has committed approximately $2 billion over 10 years (2001-2010) to nanotechnology. In India, synthetic bone substitutes are made by using nano-level fusion of biological substances, glass, and ceramic.
Composites in medicine & dentistry
___.To comprehensively address the technical issues involved in medical applications of molecular nanotechnology and medical nanodevice design, Robert A. Freitas Jr. has written four books (Nanomedicine, Landes Bioscience, Georgetown, TX 78626, available also from Amazon.com). The same author has published also an article entitled “Nanodentistry”¹² in which, in the part dedicated to orthodontics, he envisions nano-robots that could directly manipulate the periodontal tissues including gingiva, periodontal ligament, cementum and alveolar bone. This should allow a rapid and painless tooth straightening as well as fast rotating and vertical repositioning, in contrast to current molar uprighting techniques which require weeks or months to proceed to completion.
___.Three companies are already providing nano composites in which the “nano” is, for the time being, only the filler. One of these is ESPE/3M: its Filtek Supreme has a filler (20-75 nm in size) which forms “clusters” having a total size of 0.6 – 1.4 µ. An addition of zirconium oxide fillers (2-5 nm) renders it radio-opaque. The filler’s size, relative amount (72-78% by weight) and shrinkage rate are claimed to correspond to that of a commonly used hybrid composite. Pentron Clinical and its subsidiary Jeneric claim to have created the first nano-hybrid composite:^13,14 their Nano-Bond System contains silica-containing “cage-like” nano-particles. Dentsply Caulk’s TPH3 Micro Matrix Restorative and Esthet-X’s resin matrices¹⁵ are filled to 60% by volume (77% by weight) with both a barium-alumino-fluoroboro-silicate glass (0.6 to 0.8 microns) and silicon dioxide particles (10 to 20 nanometers): a similar composite by Kerr¹⁶ has been named PureNano™.
___.While designed for restorative purposes, similar composites should find a good use in orthodontics as the particle size of conventional composites is too large to penetrate deep into the tooth’s structure (1 nm to 10 nm in size).
Conclusions
___.Composites in medicine, dentistry and orthodontics are as different and varied as these are indispensable. Due to the fact that the demand is great anyway, too few manufacturers are trying to get out of the “me too” syndrome, i.e. manufacturing all too similar products. New ventures require a steady and intense effort which may offer an uncertain return. As there are relatively few related complaints, the clinicians are resigned to the present situation. With laws expanding California’s “Proposition 65”, according to which the clinician is considered responsible for the materials (s)he uses, things would have to change.
References
1. Freilich MA, Meiers JC, Duncan JP, Goldberg AJ, Fiber-reinforced composites in clinical dentistry, 1999, Quintessence Publishing Co., Chicago
2. Xu HHK, Schumacher GE, Eichmiller FC, Peterson RC, Antonucci JM, Mueller HJ, Continuous-fiber preform reinforcement composite restorations, Dental Materials 2003; 19, 523–530
3. Krause WR, Park SH, Straup RA. Mechanical properties of Bis-GMA resin short glass fiber composites. J Biomed Mater Res 1989; 23: 1195–211.
4. Bayne SC, Thompson JY. Mechanical property analysis of two admixed PRIMM-modified commercial dental composites. Acad Dent Mater Trans 1996; 9: 238.
5. Xu HHK, Martin TA, Antonucci JM, Eichmiller FC. Ceramic whisker reinforcement of dental resin composites. J Dent Res 1999; 78: 706–12.
6. Freilich MA, Meiers JC, Duncan JP, Eckrote KA, Goldberg AJ, Clinical evaluation of fiber-reinforced fixed bridges. J Am Dent Assoc 2002; 133: 1524–1534.
7. Lee HI, Jr.. Dental adhesive composites, U.S. Patent 4,107,845 1978.
8. Karmaker A et al. Dental bridges comprising fiber reinforced frameworks with fiber or particulate reinforced veneers, US Patent 6,872,076, 2005.
9. Vallittu PK. Survival rates of resin-bonded, glass fibre-reinforced composite fixed partial denture with a mean follow-up of 42 months: A pilot study. J Prosthet Dent 2004; 91 (3): 241-246.
10 Goldberg AJ, Burstone CJ, Orthodontic appliance system, US Patent 4,717,341, 1988
11. Matasa CG, Biomaterials in orthodontics, in Orthodontics: Current Principles and Techniques, 4th ed., Graber TM, Vanarsdall R, Vig R., editors, C.V. Mosby, St. Louis, MO, 2005
12. Freitas RA, Jr., Nanodentistry, J Am. Dent Assoc 2000; 131(November): 1559-1566
13. Prasad A, Karmaker A, High modulus hybrid fibers for dental restorations, US Patent 6,132,215, 2000
14.Mitra SB, Wu D, Holmes BN. An application of nanotechnology in advanced dental materials, J Am Dent Assoc. 2003; 134 (10):1382-1390
15. Davis N. A nanotechnology composite, J Am Dent Assoc 2003; 662-670
16. Angeletakis C, Nguyen MDS Dental composition containing discrete nanoparticles, US Patent 6,593,395, 2003.

Resin-based composites. Tomorrow.
Nano-reinforcers will become the matrix!

Introduction
___.The few examples given in the preceding article are far from offering a view of what will follow in the field of composites. In contrast with the belief that the filler component will be the most significant development in the evolution of composite resins,¹ matrices may offer more opportunities.
___.It is true; several uncommon fillers have been successfully tried in an area of great medical interest, bone cements. Fillers such as carbon fiber, Kevlar fiber, glass beads and titanium meshes have led only to minor improvements, but efforts are made to introduce micron-size filler particles of calcium phosphate, barium sulfate or ceramic particles such as zirconium or aluminum oxide. The most promising were found to be the nano tubes. Aside from arresting fatigue crack growth and allowing stress to transfer from the matrix to the reinforcement, carbon nano tubes (CNT) make also an ideal scaffold for the growth of bone tissue. A new technique could change the way doctors treat broken bones, as this may allow them to promote healing by simply injecting a solution of nanotubes into a fracture. A clear departure from the past will be, however, the matrix, as it will be made of ...reinforcers, i.e. from rod-like, polymerizable derivatives of liquid crystals (LC).
___.There are three common states of matter that most people know about: solid, liquid, and gas. Liquid crystal is a fourth “state” into which certain kinds of matter with particular structures can enter under the right conditions. If a LC substance becomes too cold, it reverts to an ordinary solid. If it becomes too hot, it reverts to an ordinary liquid. LCs are essentially more like liquids than they are like solids.
___.All the molecules that generate a LC phase are long and have a rigid central region (backbone), its ends being slightly flexible. Their part-crystal and part-liquid nature results in innumerable applications, such as in display technology (LCD), detergents, emulsifiers and high technological smart materials. Far from being ‘exotics’, LCs can be found in lecithin, DNA, cholesterol esters, lipids and gangliosids. In biological complexes such as membranes, the constituent rod-like molecules (such as the phospholipids) are organized perpendicularly to their surfaces. It is considered that biological membranes are a form of LC because the membrane is both fluid and elastic. A related LC material is the concentrated protein solution that is extruded by spiders.
___.As some LC materials include harmless natural products such as cellulose, sugars, chitin, xylans, etc., these derivatives can be by far less of a health-risk when compared with completely synthetic materials such as bis GMA, known to be toxic, cytotoxic, mutagenic, carcinogenic and oestrogenic.²
Nanotubes as fillers
___.By the time you may read this, the first international conference on the most promising composites may take place. Dedicated to “Carbon Nano Tube - Polymer Composites”, it takes place at the Technical University in Hamburg. The importance given to these “tubes of graphite” which are hundreds of times thinner than a human hair, is due to the important properties and the added strength these impart to materials. Due to their very small particle size (just a few nanometers in diameter and several microns long, see Fig.1), carbon nano tubes (CNT) are accepted in higher concentrations by resin systems. Their extraordinary mechanical properties are compared in the accompanying table.
___.While discovered in 1991, carbon nano tubes are available for $6/g in small amounts and 80% purity from several companies such as Nanolab, Newton MA 02458. While being black does not make them acceptable candidates for aesthetic purposes, these can be useful, along with a polymethyl methacrylate matrix, for bone cements.³ For orthodontic composites, Northwestern University researchers recommend off-white single-walled boron nitride (BN) nano tubes. Stronger and lighter than steel, these are used as hard coatings on gears to improve the efficiency of vehicles, or as an oxidation-resistant outer coating for airplane windows. More oxidation resistant than CNT, BN nano tubes are expected to exceed the mechanical and electronic properties of their well-known cousins, the carbon nanotubes.
Liquid crystals (LC) as matrix... and fillers
___.In liquids, molecules are randomly distributed (iso–tropy). In solids, the atoms exhibit both positional and orientational order. In liquid crystals, the molecules are constrained to point only in certain directions, see Fig.2, and to be in certain positions with respect to each other (anisotropy). As a result of this anisotropy, LCs exhibit some of the optical properties of solid crystals, such as birefrigerence.
___.The major difference between the LC derivatives and the other fillers is the fact that these can be at the same time the filler and the matrix itself. Before polymerization, while their rod-shaped molecules could move around to different positions, these tend to orient themselves along definite directions. This trend can easily be simulated by shaking a heap of toothpicks, as shown in Fig. 3. Rod-like, rigid and polarizable organic molecules behave the same way: these are free to move, but will line up in about the same direction. The “director” can be a magnetic field, electric current, an impurity or even an irregularity of the container. This liquid crystal behavior has received the name nematic (from nemos, in Greek = thread) with respect to thread-like textures observed under polarizing microscope. Interestingly, this assembling propensity is shown also where it is undesirable: in certain applications, nanotubes tend to clump together. In their formulations, these must be thoroughly blended to counteract the resulting lack of hommogeneity. .
___.To be rigid and rod-shaped, molecules have to be made of rigid cyclic units that are interconnected in such a way that only their limited bending can take place. A linear, planar conformation can be achieved in several ways, among which most important are resonance and steric hindrance. Resonance, commonly found in aromatic compounds, allows bonding electrons not only to be shared, but also to be delocalized. This situation can take place only if that particular structure (often a 6-membered cycle containing six p-electrons from three double bonds) is planar. If directly connected, these cycles allow the “electron cloud” to expand all along the molecule. Some linking units that contain multiple bonds, such as - (CH=N)-, -N=N-, -COO-, -(CH=CH)n-, -CH=N-N=CH-, can extend the area of electron delocalization by acting like bridges, and thus contribute to wider anisotropic polarization. In contrast, steric hindrance is a condition where the rotation/bending of a given group is restricted by the size of neighboring groups. As we have shown elsewhere,^4-6 the dimethylcarbenoid middle group hinders the rotation of bis phenol A derivatives (such as bis GMA), rendering them almost rod-like, Fig. 4.
___.Nowadays there are many nematic LC materials on the market. E. Merck Co. supplies a range of these at a price between $2.85 and $10.00 per gram, depending upon the type. On the market there is an impressive array of active matrix LC displays for computer, avionic, automobile, and consumer products from electro-optical displays to switchable gearboxes, surfactants and lubricants. Nematic liquids are widely used in alphanumeric liquid-crystal displays (LCDs), such as those found in digital wristwatches and many electronic devices. Like light-emitting diode (LED) and gas-plasma technologies, LCDs allow displays to be much thinner than the cathode ray tubes.
Liquid crystal polymers (LCP)
___.Highly ordered polymers, LCPs have their chains straight and inflexible. In solution and at low concentration, they behave much like ordinary polymers. However, when the concentration is increased, the chains become oriented by each other such that randomly situated domains of high orientation develop. Their typical properties are a low stretch or elongation, resistance to impact, cutting and wear. While when a liquid crystal system is disturbed (e.g. by heat), the orientation of the rods is lost and its “solid” properties vanish, behaving like a conventional liquid: when the liquid crystals become part of a polymer, their orientation remains and leads to an increased resistance to deformation. The above has been demonstrated with the help of a composite filled with glass fibers. As shown in Fig. 5, its tensile strength becomes twice as high when the fibers are oriented orthogonally (at a right angle among them) when compared with the random arrangement, and about five times larger when these point in a single direction.
___.In polymers, liquid crystal-containing monomers align themselves generating a self-enforcing effect which can be further enhanced by molding, extrusion, etc. Even in the absence of such methods, the alignment of the molecular chains often leads to extremely high strength and elastic modulus. Polycarbonates, which share with bis GMA the same bis phenol A moiety, are a vivid proof of this: they have the birefrigerence specific to liquid crystals. Polycarbonates’ backbone rigidity and crosslink density allow these to be used where high strength is needed, i.e. from anti-theft foils to compact disks. Another bis phenol A-based LCP is Xydar from Amoco Performance Products. Kevlar™ from DuPont, the strongest polymer known today to man, is also an LCP. To render them even stronger, over half of the LCPs sold today are reinforced with 30%-40% silanized glass fillers.
___.At the opposite end of the spectrum are the liquid crystal elastomers, prepared by linking the LC groups into slightly crosslinked polymer networks. These exhibit unexpected mechanical properties, such as drastical changes in their shape when interacting with light or heat, as the long chains become aligned.
___.Aside from all these qualities, many LCPs are also exceptionally inert. Even at elevated temperatures, they resist stress cracking when in the presence of most chemicals, including aromatic or halogenated hydrocarbons, strong acids, bases, ketones, and other aggressive industrial substances. Hydrolytic stability in boiling water is excellent.
Dental composites
___.As shown earlier,^4,5 both dental and orthodontic had a good start because about half a century ago, Rafael Bowen has adapted for the purpose the best epoxy resin from the market which was, accidentally, almost a molecular rod. Aside some electronic conditions, bis GMA molecules are not slender enough to form LCs (the shape condition is to have the molecule’s length at least three times long as it is wide). However, while bis GMA’s structure is not rod-like enough to display the characteristics of liquid crystals, it provides enough mechanical strength to render it preferred among the others available today.
___.In other words, “Bowen’s resin” has shown the way to go: the best monomers have to have a long and rigid backbone. The fact that liquid crystals are found in a multitude of living cells which interact with each other is reassuring, especially if we add to this that the same delicate cells can make up very hard tissues.
___.The opportunity to create composites that are at the same time very strong and non-hazardous has not escaped NASA and its affiliate or related organizations such as the Southwest Research Institute and the University of Texas Health Science Center (San Antonio, TX). A major difficulty they encounter is the need to imitate the way the products currently used in dentistry are handled. The liquid crystalline monomers should not be too viscous: their transition temperature should neither be too low, nor too high, as this may result in generating undesirable phases. Another problem to be solved is that most nematic liquid crystalline monomers do not polymerize efficiently: the poor conversion leads to leaching monomer.
___.In spite of al of this, a strong encouragement is the fact that LCPs can reduce resin shrinkage to 2%. In an interview, Dr. Henry R. Rawls from the Southwest Research Institute (San Antonio, TX) has explained the phenomenon this way: “Imagine a bowl of spaghetti. The cooked spaghetti take up more volume than uncooked spaghetti; the strands are all twisted and curled around each other, with gaps between them. Dry spaghetti, on the other hand, are aligned and pack together tightly. What we’re doing with these materials is making them more like dry spaghetti.” The institute mentioned strides to develop a super composite which will have not only an LCP matrix, but also inorganic nanofillers. The team, which comprises also B. K. Norling and S. T. Wellinghoff (University of Texas Health Science Center), has been awarded for the purpose a $3.4 million grant by the National Institute of Dental Research (NIH/NIDR). Using a combination of disciplines, these investigators are putting together their expertise and knowledge in the hope that they will develop a material for dental fillings which is “far superior to anything which currently exists”. Some of their publications and patents are referenced below.^7-14
___.Probably because reaching success may take years, only few companies have shown interest. Among these are Kerr Corporation¹⁵ (Orange county, CA), which along with Ormco is a subsidiary of Sybron, and De Trey, part of the German consortium Dentsply. The last company has started related research over ten years ago.^16,17
Conclusions
___.As we have shown in the preceding article, there are already on the market several composites which contain nano fillers: hopefully we will have also soon others in which the matrix will be based upon LCs. Once the difficulties to have these matching composites (which were entrenched for decades) are removed, the problem may be, once again, related to their health risks. Indeed, reading the related articles and patents, it becomes obvious that almost all the research is performed on polynuclear aromatic compounds. Or, just these are known to generate cancer. As early as in 1915, Katsusaburo Yamagiwa has succeeded in inducing skin cancers on rabbit ears by repeated painting these with coal-tar, a material known to be rich in polynuclear aromatic compounds. Would we then return to another, mechanically superior, bis GMA relative?
___.Resonance, today the most used phenomenon to generate LCs, is limited to aromatics and therefore associated to their health risks. In contrast, steric hindrance, the most important LC generator in nature, should be instead selected, as there is a multitude of already existing, harmless materials (e.g. carbohydrates) that can be transformed into suitable monomers.
References
1. Muselmann M. Composites make large difference in “small” medical, dental applications. Comp Tech 2003: 24-274
2. Matasa CG, Orthodontic polymers: a worrisome present? In: Graber TM, Eliades T, Athanasiou AE (eds.). Risk Management in Orthodontics: Expert’s guide to malpractice. Quintessence, Chicago, 2004.
3. Pienkowski DA, Andrews RJ, Polymethylmethacrylate augmented with carbon nanotubes, United States Patent 6,872,403, 2005
4. Matasa CG, Balaban AT, Challenges in medical in-situ biomaterial polymerizations, Conference proceedings, BiomMed D’2004 International Conference “Biomaterials & Medical Devices”, Nov.5-7, 2004, Bucharest, Romania; The Orthodontic Materials Insider 2004; 16(4): 2-8
5. Matasa CG, Balaban AT, To have great composites, you have to look...down, The Orthodontic Materials Insider 2005; 17(1): 4-7
6. Matasa CG, Balaban AT, In situ polymerizable resins and the “nano” era, Bull. Molecular Medicine 2005, 23, 19-32
7. NASA, Improved Polymeric Composite Materials for Dental Fillings, http://www.nasatech.com/Briefs/May01/MSC22842.html
8. Rawls HR, Wellinghoff VT, Norling BK, Leamon SH, Swynnerton NF, Wellinghoff ST, Low shrinkage resins from liquid crystal diacrylate monomers, Polymer Preprints 1997; 38:167–168
9. Rawls HR, J.K. Sparkman JK, Martinez CJ, Furman BR, Satsang N, Cardenas HL, Norling BK,Microleakage adjacent to low-shrinkage liquid crystal diacrylate (LCM) composites (IADR Abst.119, 2001)
10. Wellinghoff ST, Dixon, H, Rawls, Norling, BT, Methods of making functionalized nanoparticles, US 6410765, 2002
11. Wellinghoff ST, Dixon, H, Rawls, Norling, BT, Metal oxide compositions and methods, US Patent 6,417,244, 2002
12. Wellinghoff ST ,Dixon, H, Rawls, Norling, BT, Methods of dental repair using functionalized nanoparticles, US Patent 6,695,617, 2004
13. Wellinghoff ST, Dixon, H, Rawls, Norling, BT, Functionalized nanoparticles, US Patent 6,696,585, 2004
14. Wellinghoff ST, Dixon, H, Rawls, Norling, BT, Composites made using functionalized nanoparticles, US Patent 6,743,936, 2004
15. Qian X, Dental restorative compositions, US Patent 6,837,712, 2005
16. Klee JE, Frey H, Holter D, Mulhaupt R Liquid crystalline (meth)acrylate compounds, composition and method US Patent 5,998,499, 1999
17. Klee JE, Frey H, Holter D, Mulhaupt R, Liquid crystalline (meth)acrylate compounds, composition and method US Patent 6,339,114, 2002.

Orthodontic recycling: is it risky?

Introduction
___.“Before the era of disposable equipment, the reuse of medical instruments was the rule rather than the exception. The legitimacy and appropriateness of recycling and the reuse of single-use disposable medical devices has become a common contentious issue in many areas of medicine. As it will be shown, recycled orthodontic attachments pose the least risk on the risk scale of devices that are successfully reusable, being actually at its lower end, the upper being the cardiac catheters.
___.While by now there are tens of companies and perhaps hundreds of offices that recycle orthodontic attachments, the processes involved are just two, the difference residing in the way the adhesive is removed. Thus, the thermal process chars the adhesive subjecting the attachments to temperatures that alter both the structure and dimensions of the attachments. In contrast, the sophisticated dissolution of the acrylic polymer followed by burnishing does not alter the attachment’s structure without removing metal.
A risk comparison
___.Using the key words “reuse single-use instruments” on an Internet medical search engine, (PubMed,¹ developed by National Center for Biotechnology Information),² over one hundred medical articles were found debating the reuse of medical instruments that were designated by their manufacturers as “single use devices” (SUD). To assess the validity and clinical relevance of objections to recycled medical instruments in general, we also searched the medical literature for articles about the performance of the most potentially hazardous instruments, the plastic-containing invasive cardiac devices. While metals can withstand sterilizations without damage, plastics are by far more sensitive. Searching further for the key words “pacemaker reuse” and “angioplasty balloon reuse”, we found studies from different countries demonstrating that their reuse was proven to be “safe and effective”, the basic acceptance criteria used by the FDA.
___.Canada. If proper protocols are used, angioplasty catheters can be safely reused, saving on the average, after five reuses, approximately $5000.³ A reanalysis of previous studies showed that the balloon catheter reuse is not associated with an increased rate of in-hospital complications.⁴ The experience of two centers, one reusing balloon angioplasty catheters and the other using new catheters, led the authors to conclude that the rates of in-hospital adverse events were similar in both centers.⁵
___.Germany. After 25 years of heart catheter reuse in almost 100,000 interventions, there was no difference in complication rates when compared with catheters that were used only once.⁶
___.USA. The reuse of pacing catheters in 12 medical centers has been proven to be safe and cost-effective.⁷ In another study, 161 patients were treated with 426 new catheters; 152 patients with 384 multiple use catheters which were re-sterilized once or twice; and 101 patients with 325 multiple use catheters reprocessed several times. No infections or significant differences in outcomes were found.⁸ The reuse of coronary angioplasty catheters has been found to be safe and effective without detectable sacrifice in performance, with success and complication rates similar to those of new products.⁹
___.India. The efficacy of reused pacemakers was found to be comparable with that of new ones.¹⁰ When performing coronary angioplasty, reused catheters are as effective (similar angiographic success) and safe (similar rates of adverse events) as new ones.¹¹
___.Thailand. Reused balloon catheters were found to be safe for percutaneous transluminal coronary angioplasty, their success rate being high”.¹²

___.“While the invasive cardiac devices discussed above are considered by the organizations that control them as posing the greatest potential health risk, their reuse continues to steadily increase. The pressure from the Balanced Budget Act of 1997, the increasing prevalence of health maintenance organizations, and the Medicare program has caused some hospitals to turn to reprocessing SUDs as a way to reduce costs,¹³ practically forcing acceptance of reusable devices by the authorities that couldn’t find valid objections against these.
___.In contrast, orthodontic attachments are generally considered as posing the least potential health risk. Being both non-invasive and made of metal or ceramic, they withstand well both reprocessing methods and material fatigue, and can be easily decontaminated by heat. According to the American Association of Orthodontists counsel, Sally A. Bowers, “even if a bracket could become detached and the patient could swallow it, this will not result in any injury to the patient”, and “If reused brackets result in more breakage, longer treatment time with more office visits… we would see very few dentists using recycled brackets”.¹⁴
___.Moreover, the President of the American Association of Orthodontists, Dr. Daniel Poulton, has shown that recycled brackets are “Safe and effective”, the three key words for FDA’s acceptance. Indeed, according to the FDA, the physical characteristics and quality of the device should not be adversely affected and should remain safe and effective for its intended use. In addition, the Center for Disease Control (CDC) requires that devices should not be reprocessed or reused if the physical integrity and function are compromised in the cleaning, sterilization or disinfection process, and if the overall safety and effectiveness are affected. AORN, the Association of Peri-operative Registered Nurses has put it in an even more succinct way:¹⁶
___.a) If a device cannot be cleaned, it cannot be reprocessed and reused.
___.b) If sterility of a post-processed device cannot be demonstrated, the device cannot be reprocessed and reused.
___.c) If the integrity and functionality of a reprocessed single use device (SUD) cannot be demonstrated and proven as safe for patient care and/or equal to the original device specifications, the device cannot be reprocessed and reused.”
___All of these conditions are fulfilled by Ortho-Cycle Co...

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