Abstract
___.Both orthodontists and ocularists, the specialists in making artificial eyes, intensively use acrylics which can leach. In the former case, the leaching occurs in the mouth. In the latter, the leaching occurs in a region rich in sensitive tissues. While few orthodontists showed interest in a simple method that could be used to evaluate leaching, the ocularists are eager to adopt it.
___.An indifference to this matter, however, may translate into malpractice suits. In the case of orthodontics, such leachings or elutions have justified the inclusion of textbook chapter entitled “Polymers in orthodontics: a worrisome present”.¹
Introduction
___.Around a year ago, Ortho-Cycle Co. was contacted by Mr. Leif Erickson, BCO, BADO, Chair, Standards and Research Committee of the American Society of Ocularists (ASO), inquiring further about Dr. C G Matasa’s method of detecting plastics’ leaching. His method, Reactive Gel Entrapment, published in World J. Orthod.² as well as in this newsletter,^3,4 seemed to be promising for detecting minute amounts of substances which can leach from artificial eyes. Without exception, all organic substances can be oxidized by potassium permanganate, which changes color as it reacts. The method is so sensitive that it is used in an ISO standard to detect fresh water impurities.⁵
___.Ocularists perform an important job as sixteen million people are blinded by cataracts worldwide,⁶ and more than 28 million Americans over age 40 have eye ailments that put them at risk for vision loss and blindness. Data from the U.S. National Center for Health Statistics estimate that nearly 2.4 million eye injuries occur in the United States annually. This report calculated that nearly one million Americans have permanent significant visual impairment due to injury, with more than 75% of these individuals being blind in one eye. The leading cause of blindness worldwide and the leading cause of poor vision in the United States are cataracts, affecting an estimated 20.5 million American adults. That number is expected to climb to 30.1 million in the next 20 years.⁷
___.The above mentioned detection method was applied to cured adhesives and retainers, Fig. 1 and even to latex gloves (where its usefulness has been appreciated by Prof. Gordon Christiansen, DDS, MSD, PhD, the “guru’ of dental biomaterials in the US:”I am delighted to see your progress in this important research. If the results can be related directly to actual epithelial reactions as well as overall allergenic potential, I think you have made a major contribution”).⁴
___.In making artificial eyes, ocularists switched long ago from glass to polymethyl methacrylate (PMMA). No polymerization yields 100% the desired polymer, and the finished product is accompanied by both oligomers (short-chain, or low molecular weight polymers) and the unreacted monomer, methyl methacrylate (MMA). Both leach, as do other ingredients (inks, adhesives, etc.). In vitro studies have indicated that MMA is absorbed through human skin. Acute dermal toxicity is reported for rabbit contact exposure in excess of 5000 mg/kg by weight (bw). The oral LD50 for rats and mice is 5000 (8700 by my source but you probably used MSDS) mg/kg bw. MMA has been shown to produce severe skin irritation when tested undiluted on rabbit skin.⁸
___.Management of pain is difficult in many eye diseases and in particular, in patients undergoing surgical procedures. Postoperative intractable pain is a major concern and is very difficult to manage.⁹ Indeed, the region is highly innervated, as shown in Fig. 2.
___.To explore one approach to avoiding this painful complication, specifically to detect and reduce artificial eyeball leached components that may cause local irritation, the ASO sent Ortho-Cycle samples of artificial eyes for testing, and after getting the results, invited C G Matasa to present his method and a course at their Mid-Year Meeting at Marriott’s Harbor Beach Resort & Spa in Ft. Lauderdale, Florida, May 20-25, 2006. The invitation was accepted, and the course was well received, see below.

Materials and method
___.The reagents were the same as those used for testing orthodontic polymers. The artificial eyes were received in separate, numbered envelopes referring to the manufacturing/treatment process used. In the first trial, we used the same method used for the polymers tested heretofore, see Fig. 3. It was observed, however, that this time, the amount of leaching was by far lower, and that in about two weeks of continuous exposure, the chemical environment needed for the reaction ended by reacting with the polymer itself. After the prolonged exposure mentioned, some of the plastic eyes adsorbed some manganese oxides. Their dark aspect remained even after rinsing them with hydrochloric acid, Fig. 4.
___.Realizing that the amount leached by PMMA eyes is significantly lower than that of the orthodontic appliances tested, the method had to be refined. Instead of using directly a pre- prepared standard gel which was the same for all the tested materials, the gel was first left for a day in the tray to be added with the samples to be tested. If the gel discolored (impurities in water, colloidal silica and the reagents and even clean tray walls contribute to reduce/discolor the permanganate), an additional amount of potassium permanganate was added and homogenized till a slight purple color remained for at least an hour. If the gel remains too colored (purple), a slight addition of oxalic acid (and again homogenized) helps to get back to the small excess of permanganate needed for differentiating the samples to be tested.
___.Using the new method, a test has been performed having as goal to find the best way to cure the PMMA artificial eyes. Four of these have been displayed in the tray shown in Fig. 5. Unrelated to the artificial eyes, in the last row (3) were also compared a piece of Veriflex™ (a memory shape polymer from CRG Industries, Dayton, OH 45440), with a Drufoplast one made by Raintree/Essix (Metairie, LA 70002). Both are, or can be used to make retainers, impression trays, etc.

Discussion
___.In Fig. 5, a comparison of the auras around samples c and d shows that boiling another ten minutes really helps reduce leaching, and that pressure cooking is the best method. Regarding the orthodontic plastics, the aura around the Veriflex sample indicates some leaching in contrast with the Essix sample. We believe that this refined method is now able to disclose even small differences in leaching, such as these between the orthodontic retainers Invisalign and the OrthoClear.
Conclusions
___.While the use of potassium permanganate to detect the leaching of acrylics has already been used by the FDA¹⁰ for the “determination of potassium permanganate oxidizable extractives”, the method described is tedious, highly specialized and requires special chemicals for comparing colors and an absorption spectroscope. In contrast, the method above is simple and uses common chemicals. Having devices leaching in the mouth instead of the sensitive tissue around an artificial eye should be for sure a comfort for orthodontists...
References
1. Matasa CG, “Polymers in orthodontics: a worisome present, Graber TM, Eliades T, Athanasiou AE (eds.). In : Risk management in orthodontics. Expert’s Guide to malpractice, Chicago, Quintessence, 2003
2. Matasa CG, Screening orthodontic polymers for leaching, World J. Orthod. 2003; 4: 157-161
3. Matasa CG, Reactive gel entrapment, a new, simple way to test leaching plastics, The Orthod. Mater. Insider 2004, 16 (1): 3-7 and
2004, 16 (3): 5-8
5. Water Quality, Determination of permanganate index, ISO 8467, 1993 Geneva
6. Charters, L., Ophthalmology Times, 2000, 25(15), 1-20
7. Newsletter, Hospital Materials Management, 2000, 25(9), 1-30.
8. http://www.chem.unep.ch/irptc/sids/OECDSIDS/80625.pdf
9. Intractable eye pain, May A, Gamulescu MA, Bogdahn U. Lohmann CP, Intractable eye pain: indication for triptans. Cephalalgia 2002; 22:195–196. London
10. FDA, U.S. Government Printing Office via GPO Access
Title 21CFR177.1010, pages 222-228.

A RENEWED INVITATION

___.In our December 2005 issue, we invited our readers to let us evaluate particular orthodontic products. The answer to a request that matched our capabilities will be presented below.
In the three decades since its inception, Ortho-Cycle’s Biomaterials Lab has grown enough to provide reasonable answers to some of the problems orthodontists face in this domain. While renewing this invitation in the hope to serve the profession, we feel obliged to remind those interested that we may not be able to answer to some questions because our equipment is not sophisticated. Nonetheless, when we provide an answer, it will be not only indicative, but also reproducible using do-it-yourself methods. Last but not least, such research may have cost the interested person thousands of dollars...

Dr. H. G., Germany: “I hate to waste time and money to have my broken acrylic-based appliances sent for repair. In addition, I don’t like to fix these by heating them under pressure with acrylic paste. Isn’t there an easier/ better way?”

Mending acrylic attachments

Abstract
___.For almost half a century, acrylics have been very useful for dental attachments. While resistant to water and easy to shape, their strength is low and repairs are often needed. The huge array of commercial products available for the purpose and the variables in their application warrants testing. As one of the forces contributing to failure is tensile strength, a bench tester, previously described, was used to pull combinations of three acrylics bonded with five potential adhesives. The results show that cold cured paste/powder systems designed specifically for repair provide good strength, in contrast with similar ones commonly used to make plates and retainers. The explanation may reside in the fact that the powder of the first resin system is comprised of methyl acrylate, and its liquid methacrylic acid and ethylene glycol dimethacrylate, a strong cross linking agent. In less demanding cases, gel cyanoacrylates and orthodontic adhesives can provide bonds almost as good as those obtained when using common self cured dental resin.
Introduction
___.In many countries, the process of”reuniting or replacing a broken or worn dental prosthesis” is taken very seriously. Thus, in Canada, this activity requires a special permit.¹ The candidates have to complete a specific training and should have at least two years of experience in the manufacturing and repair of dental prostheses in the five years preceding the application.
___.Acrylic orthodontic appliances break either within the polymer itself or at their interface with metal. Typically, in the first case the parts are sent to a licensed dental laboratory for repair, where the parts are usually joined using polymethyl methacrylate (PMMA). Depending on the workload of the dental technician and the location of the dental laboratory, the repair may take at least hours, increasing the cost.
___.When using heat, the appliance to be mended along with the necessary acrylic dough or paste, is placed in a pressure pot and kept at 30 psi for 30 minutes. While the device may warp during processing, the resulting appliance is usually stronger than the cold-cured one. If non-acrylic parts are involved (metal, ceramic), the latter are subjected to surface treatment methods such as etching,² silane coating^3,4 or/and high energy abrasion.⁵
___.Both etching and silanation are demanding operations, the first uses chemicals that may harm and the second is quite tricky. If in the last case an approximately monomolecular layer is not achieved, the bond fails. Too little silane will not cover enough the surface to be treated, diminishing the bond, while too much silane will generate an oily layer where the coupling agent is unable to bond to both substrates, but bonds only to itself.
___.Most appliances break due more to cracks and to forces exceeding their fracture strength than to shock. Opposed to these failures are the tensile strength and the toughness resistance. To measure the latter requires a rather complex machine (e.g. Instron’s Model 4444 bench top for the three points bending of composites, ASTM D-790). We have used only the first one using our “micro-universal testing machine” presented in our past issues^5,6. This is feasible as the tensile strength normal to the plane of a crack is indicative of the ability of the latter to propagate.
___.While there is a multitude of products that can be used to repair/unite acrylic parts, it is hard to find a study comparing them.
Materials and method
___.The principle of the installation used is shown in Fig. 1. As a difference from previous tests, we have added a large stainless steel container capable of being loaded not only with water but also with lead weights. Maintained at a constant level, the water kept in a cylinder flows in the above mentioned container. At one end of a wire laying on a system of pulleys hangs a container hangs, while at the other end are attached hooks or other pulling devices. At breaking point, the sum of the additional weights and that of the water are added.
Adhesives.
___.The adhesives used were Phase II, a two paste orthodontic adhesive (Reliance, Orthodontics, Itasca, IL 60143, marked with PH), Spofacryl (Spofa Dental, Prague, Czech R., with subsidiary in Frankfurt/M, Germany, marked with SP), a MMA-PMMA system in which the powder is added with methyl acrylate and the liquid with methacrylic acid and ethylene glycol dimethacrylate, Premium Denture, a heat-cured acrylic from Lang Dental (Wheeling, IL 60090, marked with (CD),) and Quick Gel, an ethyl cyanoacrylate (Elmer’s Products, Columbus OH 43215, (QG,, marked with QG))
Substrates
___.The acrylics used as substrates to be bonded were PMMA cubes (edge length 13.2 and 9.75 mm) and rods of 4 mm, or 3/16" diameter). The cubes were from Prospect Plastics (Ft. Lauderdale, FL 33334). Additional cubes and 4 mm rods were made in-house by curing in molds (see below) Premium Denture (CD), colored in blue with Ultramarine Blue E-770 (Akron Chem. Co., Akron, OH 44304) and an orthodontic adhesive composite, Rely-a-Bond (CR). Acrylate rods of 4 mm diameter were also made in house from both Rely-a-Bond (white) and Premium Denture (CD) (blue).
Molding cubes. To make cubes similar in size to these of the purchased PMMA, a low fusible metal was poured on several of these, as shown in Fig. 2. The Wood’s alloy (50% Bi, 26.7% Pb, 13.3% Sn, 10% Cd) becomes liquid at approximately 70° C (158°F). The resulting mold, Fig.3, was used to cure the other acrylics used as substrates.
___.To be firmly held for the tensile test, the cubes were embedded in the same low fusible alloy. After the surfaces to be bonded were abraded, the cubes were joined in different combinations and then subjected to tensile test. As in most cases the bond strengths exceeded the testing ability of the installation, the use of cubes was abandoned in favor of cylinders similar to the already purchased 4 mm PMMA rod.
Molding rods. Using a glass tube having an inner diameter of 4mm or 3/16", the rods were made by injecting into them pastes of a light- or chemically curable acrylic adhesives as shown in Fig. 4. Before bonding, the section to be bonded was flattened and abraded with sand paper (200 grit) and rubbed with 90% isopropanol to remove impurities. Rods in different combinations were pressed against each other, with adhesive in-between, in 4 mm glass tubes, as shown in Fig. 5. After cure, the glass tube was broken and the bonded rods freed. Samples of the bonded rods used for testing are shown in Fig. 6 and 7.
___.In a few experiments, the surfaces to be bonded were brushed with methylene chloride, a known solvent for PMMA. Other primers used were PVC cement (Oatey, Cleveland, OH 44135) and Weld-On (IPS, Gardena, CA 90246). In addition to methylene chloride, the first contains also methyl acetate, tetrahydrofuran and cyclohexanone, while the second contains methyl acetate and methyl methacrylate. After brushing, the samples were bonded after a waiting period of approximately an hour.
Testing. At least 24 h after curing, the samples were exposed to 100^oC in a laboratory oven. Then the Wood’s alloy embedded cubes and the rods were firmly held with the help of a clamp and a vise attached to a table grips the other end. The arrangement is shown in Fig. 8. Special care had to be taken to have the bonded samples vertically aligned and firmly held. As in some cases the tensile forces were quite high, the samples’ ends had to be dented to avoid their sliding. To prevent and/or protect these against collapsing under the pressure exerted by the vise, a protective layer of sand paper was added.
Results
___.As the debonding forces proved to be too large in some cases to reliably measure all the combinations, the testing of cubes and of the rods larger than 4 mm edge length or diameter were abandoned. In addition, holding the cubes with the help of the low fusible alloy proved to be impractical, as these kept sliding out from the holding device. After a number of tests, the brushing of the samples with methylene chloride and the other primers was abandoned as the solvent penetrated the polymer’s structure, weakening it.
___.The results obtained when substrates comprising combinations of rods of block polymerized methyl methacrylate (PMMA), the orthodontic adhesive Rely-a-Bond (CR) and the cold-cured Premium Denture acrylic, (CD), were bonded with the adhesives Phase II (PH), Quick Gel (QG), Premium Denture acrylic, (CD) and SPOFA (SP) are shown in Fig. 9-14. While each of the above substrates can act, of course before cure, also as an adhesive, only the cold cured acrylate Premium Denture has been used both ways and is of particular interest as an acrylic mender.

___.Examining the Adhesive Remnant Index (ARI), it was interesting to find out that between the substrates tested, the one which showed the lowest affinity was CR.
Discussion
___.Acrylic attachments are commonly mended with the help of cold-or heat cured systems. The first are known not to provide enough strength, while the latter are tedious (the attachment and the repair acrylic are placed in a pressure pot at 30 psi for at least 30 minutes). The polymer and the other components, metal or ceramic, are all abraded, while the last two have to be silanated to provide enough affinity for the mending resin.⁷
___.As the orthodontic treatment uses intensively an array of adhesives, it is reasonable to shorten the repair process using the in-house armamentarium in order to fill the space with the adequate acrylic.⁸ The selection of the latter was done irrespective to its color, bond strength being the decisive factor.
___.The tests above show limits, instead of precise, statistically processed results. The amount of time (in average five tests for each data point) and the simplified measuring system is in agreement with the variables encountered. A higher precision is unwarranted in view of the nature of the polymers involved (both as substrate and adhesive) and the thickness of the latter’s layer.
___.A “starving” or too “rich” layer of the same adhesive gives different results,¹⁰ as do variable porosity, closeness to the best setting time of the adhesive (see the Trommsdorff effect⁹) and the characteristics of the surfaces involved, etc. Indeed, while the chemical identity of the polymer involved may be the same, its molecular weight may significantly vary being determined especially by the ratio between the adhesive’s parts and the way to apply them.
___.Another important variable resided in the mounting of the samples to be tested. While perfectly normal tensile forces were expected, the firm holding of the rods (4 mm diameter) may have generated also the peeling and fracture of the adhesive layer.
___.A related phenomenon was the mismatch of the surfaces to be bonded. Despite the efforts taken to render the rods ends flat and normal to their length, slight deviations may have inherently occurred. This has been evidenced by the results obtained with a liquid cyanoacrylate (All Purpose Krazy Glue, also by Elmer) was used: in contrast, breaking the parts bonded with the cyanoacrylate Quick Gel (QG) required by far larger forces. Related to the position of the samples were the efforts to protect these from being fractured by the holders, a vise and a clamp: some attempts to firmly seize the ends of the bonded rods ended with failure as these broke at the glue line.
___.While PMMA can resist tensile forces up to 7000 PSI,¹¹ some unfilled samples were crushed before reaching even before 2000 PSI due to the pressure exerted by the vise despite of the measures taken to minimize damage.
___.The highest bond strength to bond PMMA to PMMA, Fig. 9, was given by the PMMA-MMA powder-liquid adhesive from SPOFA, SP, followed closely behind by the orthodontic adhesive PH and the cyanoacrylate QG. The weakest proved to be a similar system used to make dentures, base plates, etc. The difference may well be due, in the case of the first one, to the effect of the additional cross linking monomer and acrylic acid. The same pattern is followed when it comes to bonding PMMA to the similar, unfilled but cold-cured system CD, Fig. 10. Apparently weaker than the cyanoacrylate QG, in Fig. 11, the PMMA-MMA powder-liquid SP gave acceptable results when bonding PMMA to the highly filled composite CR, probably due to a lower affinity toward inorganic compounds. This was supported by the Adhesive Remnant Index (ARI) which showed a lower affinity for both PMMA and CR. In the last case, this can be explained by a partly cross linked structure, known to lead to lower solubility.
___.While third in strength whenever used for PMMA repair, orthodontic adhesives such as PH may be acceptable substitutes for the cold cured acrylics designed for the purpose. Indeed, while cyanoacrylates may provide strong bonds when the surfaces to be mended match closely enough, these could not be used for filling larger spaces.
___.The inter-bonding of the cold-cured CD and the orthodontic composite CR, Fig. 12-14 shows again that similitude in composition doesn’t go too far, as the added acrylic acid and dimethacrylate in the SP system provides better bonds, even if sometimes surpassed by the cyanoacrylate (the limitations of the latter have been discussed above). Exhibiting less affinity toward organic compounds, the aluminum oxide-filled orthodontic adhesive PH did not perform too well.
Conclusions
___.Acrylic attachments are repaired/reunited with the help of acrylic adhesives. The possibility of getting good results by replacing the higher temperature and pressure cure with a cold cure has its limits. Tensile tests of samples duplicating the attachments have shown that as far as bonding is concerned, the old adage similia similibus congregantur does not apply. In the most important case, the bonding PMMA to PMMA, using the basic powder/liquid PMMA, commonly designed for making plates, dentures and retainers proved to be significantly weaker than the one made specifically to repair them.
___.For matching surfaces, the use of gel cyanoacrylates is appropriate; their acceptance in medicine predates their use in industry. When it comes to filling spaces, cyanoacrylates’ bond strength decreases considerably. Up to a certain degree, i.e. when facing lower strains, orthodontic adhesives can be used.
References
1. Quebec, Statutes and Regulations, R.Q.c. C-26, r. 160.3
2. Sumithra N, Waknine S, Schulman A: Bond strength of etched porcelain denture teeth of PMMA. Quintessence Int 1986; 17: 745-748
3. Paffenbarger GC, Sweeney WT, Bowen RL: Bonding porcelain teeth to acrylic resin denture bases. J Am Dent Assoc 1967; 74: 1018-1023
4. Moffa JP, Jenkins WA, Weaver RG: Silane bonding of porcelain denture teeth to acrylic resin denture bases. J Prosthet Dent 1975; 33: 620-627
5. Marchack BW, Yu Z, Zhao XY, et al: Adhesion of denture tooth porcelain to heat-polymerized denture resin. J Prosthet Dent 1995; 74: 242-249
6. Matasa CG, A simple bond strength testing device and the Tromsdorff Effect, The Orthodontic Mater. Insider 2005; 17 (2): 6-8
7. Matasa CG, Acrylics-solvent: a promissing interaction. The Orthodontic Materials Insider 2005; 17 (4): 5-8
8. Ozcan M, A practical method for chairside repair of debonded porcelain denture teeth, J Prosthodont. 2006;15:47-50.
9. Matasa CG,, A simple bond strength testing device and... the Trommsdorff effect, The Orthod. Mater. Insider, 2005; 17 (2): 6-8
10. Matasa CG, Adhesion and its ten commandments, Am. J. Orthod. Dentofac Orthop 1989; 95: 355-356
11. http://www.polymerweb.com/_datash/pmma.html