Force Decay Evaluation of Thermoplastic and Thermoset Elastomeric Chains - A. Mechanical Design Comparison

Transcription

1 Force Decay Evaluation of Thermoplastic and Thermoset Elastomeric Chains - A Mechanical Design Comparison BY AHMED I MASOUD B.D.S, King Abdulaziz University 2008 THESIS Submitted as partial fulfillment of the requirements for the degree of Master of Science in Oral Sciences in the Graduate College of the University of Illinois at Chicago, 2013 Chicago, Illinois Defense Committee: Dr. Ana K. Bedran-Russo Dr. T. Peter Tsay Dr. Ellen BeGole

2 ACKNOWLEDGEMENTS Dr. Ana K. Bedran-Russo: Thank you for your guidance and support. Thank you for giving me my own space in your lab. You are one of the nicest people I have ever met. It was a pleasure working with you. Dr. Ellen BeGole and Mrs. Grace Viana: Thank you for keeping your office doors always open. I appreciate all your patience and guidance during our long discussions. Dr. T. Peter Tsay: You are one of the most dedicated people I know. Thank you for reading my thesis numerous times. I wanted to thank you not for just helping me with my research, but for also being there in the clinic and teaching me most of what I know. Dr. Carla Evans: Thank you for accepting me into the program and giving me this opportunity to work in this field. I really enjoyed my years here and will probably enjoy my next year or two. I would also like to express gratitude to American Orthodontics and Ormco for their donation of the power chains used in this study. ii AM

3 CHAPTER TABLE OF CONTENTS PAGE I. INTRODUCTION A. Background... 1 B. Aims.. 3 C. Null hypotheses II. LIERATURE REVIEW A. Force decay in elastomers. 4 B. Comparing thermoplastic and thermoset polymers... 5 C. The effect of the manufacturing process on force decay.. 5 D. The effect of the initial force on force decay. 6 E. Force decay in open and closed chain F. The effect of pre-stretching on force decay G. The effect of different ligating designs on force decay. 7 H. The effect of space closure on the remaining force.. 8 I. The effect of pigmentation of force decay. 8 J. The effect of the test media on force decay... 9 K. the effect of the environment on force decay 10 L. Force magnitudes required for tooth movements.. 10 M. Artificial saliva composition and properties 11 III. MATERIAL AND METHODS A. Sample B. Testing Apparatus Stretch jig Media ph meter Containers Digital force gauge Digital caliper Transfer jig 20 C. Measurement acquisition. 21 D. Design Description of subgroups Determination of stretching required to reach selected 23 magnitudes of force.. 3. Grouping the sample Time points.. 25 E. Measurement protocols Baseline measurements One hour measurements Daily and weekly measurements iii

4 CHAPTER TABLE OF CONTENTS cont. PAGE F. Statistics. 29 IV. RESULTS V. DISCUSSION A. Elongation percentages B. Comparison thermoplastic and thermoset power chains C. Comparison between the differences in force decay between the 51 four subgroups American Orthodontics thermoplastic power chains American Orthodontics thermoset power chains Ormco thermoplastic power chains Ormco thermoset power chains D. The effect of time on force decay American Orthodontics thermoplastic power chains American Orthodontics thermoset power chains Ormco thermoplastic power chains Ormco thermoset power chains. 61 A. Force magnitudes and percentage. 62 B. Comparison between thermoplastic and thermoset elastomers. 63 C. Comparison between the differences in force decay when using 64 single and loop power chains D. Comparison between the differences in force decay when using 65 different force levels..... E. The effect of time on force decay.. 66 F. Force magnitudes required for tooth movements.. 67 G. Limitations H. Strengths I. Future studies 69 J. Clinical significance.. 69 VI. CONCLUSION CITED LITERATURE VITA iv

5 LIST OF TABLES TABLE PAGE I. REQUIRED STRETCHING FOR SINGLE CHAINS II. REQUIRED STRETCHING FOR LOOP CHAINS.. 24 III. ELONGATION PERCENTAGES OF SINGLE CHAINS IV. ELONGATION PERCENTAGES OF LOOP CHAINS V. AMERICAN ORTHODONTICS THERMOPLASTIC VS THERMOSET FOR SUBGROUP VI. AMERICAN ORTHODONTICS THERMOPLASTIC VS THERMOSET FOR SUBGROUP VII. AMERICAN ORTHODONTICS THERMOPLASTIC VS THERMOSET FOR SUBGROUP VIII. AMERICAN ORTHODONTICS THERMOPLASTIC VS THERMOSET FOR SUBGROUP IX. ORMCO THERMOPLASTIC VS THERMOSET FOR SUBGROUP X. ORMCO THERMOPLASTIC VS THERMOSET FOR SUBGROUP XI. ORMCO THERMOPLASTIC VS THERMOSET FOR SUBGROUP XII. ORMCO THERMOPLASTIC VS THERMOSET FOR SUBGROUP XIII. PAIRWISE COMPARISONS BETWEEN SUBGROUPS OF AOTP. 52 XIV. PAIRWISE COMPARISONS BETWEEN SUBGROUPS OF AOTS. 53 XV. PAIRWISE COMPARISONS BETWEEN SUBGROUPS OF OrTP XVI. PAIRWISE COMPARISONS BETWEEN SUBGROUPS OF OrTP AT WEEK 2, 3, 4, 5 AND XVII. PAIRWISE COMPARISONS BETWEEN SUBGROUPS OF OrTS.. 58 XVIII. TIME POINTS SHOWING SIGNIFICANT DECREASES IN FORCE DECAY v

6 LIST OF FIGURES FIGURE PAGE 1. Total sample comprised of 4 power chain groups From top to bottom; AOTP, AOTS, OrTP, and OrTS Stretch jig Closed container with label on lid Digital force measurement gauge (IMADA DS2-11) mounted on a vertical manual wheel operated test stand (IMADA HV-110) Digital caliper (TRESNA Series EC16, ID: B) Transfer jig Left; AOTP single chain. Right; AOTP loop chain Each power chain group had 4 subgroups Forty AOTP specimens used for one hour measurements Forty AOTP specimens used for the daily and weekly measurements Eleven independent samples t tests were done for subgroup 1 of AOTP vs. subgroup 1 of AOTS Repeated Measures ANOVA of AOTP One way ANOVA for OrTP at weeks 2, 3, 4, 5 and Comparison of AOTP and AOTS subgroup Comparison of AOTP and AOTS subgroup Comparison of AOTP and AOTS subgroup Comparison of AOTP and AOTS subgroup Comparison of OrTP and OrTS subgroup Comparison of OrTP and OrTS subgroup Comparison of OrTP and OrTS subgroup Comparison of OrTP and OrTS subgroup 4 51 vi

7 LIST OF FIGURES cont. FIGURE PAGE 23. Force decay of all 4 subgroups of AOTP Force decay of all 4 subgroups of AOTS Force decay of all 4 subgroups of OrTP Force decay of all 4 subgroups of OrTS vii

8 LIST OF ABBREVIATIONS ANOVA AO AOTP AOTS Or OrTP OrTS ph RMO Analysis of Variance American Orthodontics American Orthodontics thermoplastic American Orthodontics thermoset Ormco Ormco thermoplastic Ormco thermoset Power of hydrogen Rocky Mountain Orthodontics viii

9 SUMMARY Orthodontic power chains are clinically used for short periods of time, typically being replaced every 3 to 4 weeks. They are replaced at this interval, since the literature has found that power chains become inactive after 3 weeks. Since the 1970 s, several studies have been published about the force decay of elastomers. The results of these various studies have shown a wide range of force decay. One of the reasons such a wide range was observed is that researchers and clinicians are equally unaware that there is a significant difference in the material behavior between the two types of elastomeric power chains, thermoplastic and thermoset. Therefore, both chain types were included in published results without any differentiation. To date, no study has compared the force decay of both types of power chains whether in vivo or in vitro. Therefore, this study aimed to compare the force decay of thermoplastic and thermoset chains in vitro, while simulating intraoral conditions. Furthermore, the effect of different elastic elongation was investigated. The factors such as doubling the power chains and starting with lower initial forces are believed to decrease the force decay by many clinicians; therefore, these variables were also studied. Unlike previous studies which tested for a period of six weeks, this study extended to 8 weeks to provide a better understanding of force decay in power chains to clinicians using self-ligating brackets. The present findings showed that contrary to the interchangeable use of thermoplastic and thermoset power chains in published literature and in clinical practice, they perform differently ix

10 under stress and that a clear distinction should be made. Thermoset elastomers exhibited less force decay over time compared to thermoplastic elastomers and required more stretching to reach the desired forces. Single and loop chains behaved similarly in the thermoset power chain groups. However, loop chains did behave favorably in OrTP suggesting that loop chains might be superior when using thermoplastic elastomers. There was no difference in light or heavy initial forces when examining single or loop chains separately. Additionally, all chains products shared the same general pattern of force decay. The greatest force decay occurred in the first hour and first day. The results showed that significant force decay occurred until the first week in the Or chains and until the second week in the AO chains. The force decay then reached a plateau until week 7 where the chains then exhibited a significant decrease in force decay at week 7 and 8. x

11 I. INTRODUCTION A. Background: Elastomers have been widely used in orthodontic treatment since the 1960 s. In addition to their capability to return to their original form after elastic deformation, elastomers are easy to use and economical (Bousquet et al., 2006). There are two types of elastomers used in orthodontics. The first type is natural elastomers, which are used in inter-arch mechanics and are usually referred to as ELASTICS. The second is synthetic elastomers, which are used as power chains, ligatures, elastic threads, or non-latex inter-arch elastics. Synthetic elastomers are usually referred to as ALASTIKS (Andreasen and Bishara, 1970; Bishara and Andreasen, 1970). The exact composition of synthetic elastomers ALASTIKS is kept proprietary by orthodontic manufacturers, but they are made mainly from polyurethanes. Polyurethane rubber is a generic material manufactured from polyester or polyether glycol, or from polyhydrocarbon diols with a diisocyanate (Halimi et al. 2012). Synthetic elastomers ALASTIKS can be either thermoplastic or thermoset. Thermoplastic materials are materials that can be made plastic and are moldable at high temperatures. On the other hand, thermoset materials are irreversibly cured during the process of manufacturing, are not remoldable, and will burn at high temperatures. Thermoplastic materials have weak dipole or Van der Waal bonds between their polymers, while thermoset materials 1

12 2 have stronger covalent chemical bonds. Each material has its advantages and disadvantages. Thermoplastic materials have an attractive surface finish, emit less volatile organic compounds (VOC s), are recyclable and cost less to manufacture. However, they are softened with heat. On the other hand, thermoset materials are better suited for higher temperatures, but they lack a perfect surface finish, release more VOC s and are more expensive to manufacture (Nondestructive Testing, 2012). One of the biggest shortcomings of either type of elastomers is the rapid decay of force over time. Clinically, orthodontics power chains are used for short periods of time and replaced at 3 to 4 weeks based on the idea that power chains become inactive after 3 weeks (Andreasen and Bishara, 1970; Bishara and Andreasen, 1970; Lu et al., 1993). Power chains are used in orthodontics to retract canines, retract incisors or en masse retraction of all anterior teeth, protract teeth, close small spaces, or correct rotations in addition to any other movement that requires pulling. Some companies make power chains in both thermoplastic and thermoset materials. To date no study has compared the force decay of both types of power chains whether in vivo or in vitro. In addition, studies on elastomers in orthodontics often do not differentiate between thermoplastic and thermoset elastomers and their samples are usually a mix of both. For example, in several studies a commercial brand (Rocky Mountain Orthodontics, Denver, Colorado (RMO)) energy chains maintained a higher percentage of their initial forces at the end of the study (Ferriter et al., 1990; Lu et al., 1993; Josell et al., 1997). After some investigation, it was noticed that RMO energy chains are made from thermoset materials.

13 3 B. Aims 1. To compare force decay between thermoplastic and thermoset power chains within the same company over a period of 8 weeks. 2. To compare force decay between single and loop chains over a period of 8 weeks. 3. To compare force decay between using different force over a period of 8 weeks. C. Null hypotheses 1. There is no difference in force decay between thermoplastic and thermoset power chains within the same company. 2. There is no difference in force decay when using single and loop chains. 3. There is no difference in force decay when using different force levels.

14 II. LITERATURE REVIEW A. Force decay in elastomers Andreasen and Bishara are among the first to study force decay of both Alastiks and Elastics in the early 1970 s. Their studies concluded that: 1. Most of the initial force decay occurs during the first day. The percentages were 74.21% ± 5.8 and 41.6% ± 4.1 for Alastiks and elastics respectively. However, Alastics had more initial force, so still had a greater force for the next three weeks. 2. The greatest percent of force decay per unit time occurred during the first hour. They were 55.7% ± 6.7 and 26.4% ± 6.4 for Alastiks and elastics repectively. 3. After the first day there is a reasonably constant force remaining throughout three weeks. 4. If one wishes to apply 100 grams of force, an Alastik with an initial force of 400 grams should be used (Andreasen and Bishara, 1970; Bishara and Andreasen, 1970). Since that time, material properties have improved. In the early 90 s Lu et al. (1993) reported the effective force for canine retraction (184 grams) can be maintained for 3 weeks, after which the power chains need to be replaced. Stevenson and Kusy (1994) reported that as much as four times the desired load may be required at insertion, potentially causing excessive discomfort for the patient. Josell et al. (1997) evaluated chains from six orthodontic suppliers in 4

15 5 artificial saliva at room temperature. They showed force decay at 28 days to range from 15-70%. Kim et al. (2005) reported mean force decays of 27.9% and 53.4% for one hour and 28 days respectively. More recently, Balhoff et al. (2011) found mean force decays of 17% and 42.38% respectively for the same two time points. B. Comparing thermoplastic and thermoset polymers To date, the only study comparing thermoplastic and thermoset elastomers was conducted by Cabanlit et al. (2007). They compared thermoplastic and thermoset shape memory polymers (TP-SMP, TS-SMP) for use in vascular stents in plasma-free blood. TS-SMP were less stimulatory to macrophages and more biocompatible than TP-SMP. However, on the TS-SMP discs there was more protein adhesion which can elicit vascular occlusion. It is hard to relate the results of this study to elastomeric usage in the oral cavity. One of the questions this current study will attempt to answer is if the different reaction of these two elastomers will significantly affect their usage in the oral cavity. C. The effect of the manufacturing process on fore decay Both thermoplastic and thermoset power chains can be manufactured by either injection molding and die cut stamping. Two studies compared force decay between die cut stamped and injection molded power chains. In an in vitro setting, Hershey and Reynolds (1975) found that injection molded power chains showed more force decay compared to die cut stamped ones. Bousquet et al. (2006) compared both methods in vivo. They found that both types of elastomeric chain behaved similarly and that there was no statistical significance. The materials used in the present study are die cut stamped.

16 6 D. The effect of the initial force on force decay Studies on the effect of initial force on force decay are controversial. De Genova et al. (1985) reported that polyurethane chains producing higher initial forces underwent less force decay after 21 days than did modules producing lower initial forces. On the other hand, Wong, A.K (1976) and Lu et al. (1993) found that the greater the initial force the greater the force decay of all chains. Likewise, Huget et al. (1990) showed that chains stretched to 50% of their length lost less force than chains stretched 100% or 200% of their length. Alternatively, Hershey and Reynolds (1975) found that the percentages of force loss were similar for modules stretched at high or low initial forces. They used a mean of 573 grams for high initial forces and a mean of 284 grams for low initial forces. E. Force decay in open and closed chains The configuration of power chains namely closed (without an inter-modular link) and open (with an inter-modular link) can affect the behavior of elastomeric chains (Baty et al., 1994; Eliades et al., 2004). Eliades et al. (1999) used optical transmission microscopy to investigate morphological variations and residual stress gradients within the modules of open and closed chains. They found that in open chains there was an increased concentration of residual stress at the inter-modular link region. This was in contrast to closed power chains where the strain at the modular ring was much higher. Studies by Ash and Nikolai (1978) and Eliades et al. (2004) showed no statistical difference in force decay between open and closed power chains. On the other hand, Young and Sandrik (1979) reported that open power chains retained 12.8% more of their initial force after the 24 hours (their final time point) when compared to the closed power chains.

17 7 F. The effect of pre-stretching on force decay Brantley et al. (1979) studied the effect of pre-stretching on force decay of elastomeric chains. They pre-stretched power chains 100% for 24 hours and 3 weeks. They showed 5% less force decay in pre-stretched groups and reported no meaningful difference (within 10 grams). In a study by Young and Sandrik (1979) the chains were pre-stretched quickly by hand, and were returned immediately to their original length. They showed that pre-stretching had no significant effect on chains where the helices were spaced (open chains). It did however have an effect on chains where the helices were not spaced (closed chains). The pre-stretched groups retained % more of their initial force when compared to the control closed chain group after 24. More recently, Kim et al. (2005) pre-stretched samples for 1 hour, 24 hours, 2 weeks, and 4 weeks. They concluded that the pre-stretched modules had significantly lower initial forces compared to the control group. However, after 4 weeks similar remaining forces were found between the prestretched and the control groups. G. The effect of different ligating designs on force decay Canine retraction can be done by single chains or double chains. A single chain can either be a chain stretched from the molar hook to the canine hook (6-3), or a chain stretched from the molar hook, ligating to the premolar bracket, and attached to the canine hook (6-5-3). A double chain or a chain loop was described by Balhoff et al. as stretching the chain from the first molar hook, looping around the canine hook, and attaching back to the molar hook. Balhoff et al. (2011) evaluated the percentage force decay of elastomeric chains utilizing these three different methods of canine retraction. They reported that the single chain stretched from the molar hook to the canine hook had the smallest mean percentage force decay compared to the other two

18 8 designs. They noted, however, that the initial force values for the chain loop were approximately two times the initial force values of the other two designs and stated that method 2, the chain loop design, should be repeated utilizing more units of elastomeric chain to determine the proper number of units which will generate appropriate physiological forces. H. The effect of space closure on the remaining force Spaces decrease between the canine and the bicuspid brackets when retracting canines. To simulate this, the distance between the pins can be decreased 0.5 mm per week as in the studies done by De Genova et al. (1985) and Lu et al. (1993) or 0.25 mm per week as in the study by Balhoff et al. (2011). Hershey and Reynolds (1975) considered the effect of stimulated tooth movement on force loss by decreasing the stretch dimension across adjustable frameworks. They had 3 groups: no closure, closure at a rate of 0.25 mm per week, and closure at a rate of 0.5 mm per week. They found that after 4 weeks, modules retained 40% of their initial force in the no closure group, 32% in the groups closed at a 0.25 mm rate and 25% in the groups closed at a 0.5 mm rate. They concluded that space closure increased the rate of force loss. In the present study the distance was kept constant. I. The effect of pigmentation on force decay Clear and grey power chains were originally marketed. The color instability of clear elastomeric chains led to the predominant clinical application of the grey power chains. Companies have then introduced colored chains, which appeal to the younger patient market (Renick et al., 2004). A study was undertaken by Baty et al. (1994) to examine the force delivery behavior of a variety of colored power chains and to determine their dimensional stability when

19 9 immersed in a fluid environment. They concluded that in general, the colored chains of a particular manufacturer behaved similarly to the grey chain. The exception to this is Ormco purple and green chains, which needed to be stretched more to reach the desired force. Renik et al. (2004) stated that the force delivered by elastomeric products is related to their molecular structure. Insight into this structure can be obtained by measuring glass transition temperature. Hence, they investigated the glass transition temperature of elastomeric chain products in the asreceived condition and after orthodontic use to determine differences between brands and pigments. Their results indicated that pigments had no significant effect on the glass transition temperature with Ormco and RMO products, and thus would not be expected to alter mechanical properties. On the other hand, significant differences were found in the mean glass transition temperature for the pigmented chains of the G&H brand after orthodontic use. The mean glass transition temperature was considerably higher for the purple G&H chains compared with the grey and red chains, thus suggesting that the former might deliver significantly higher forces in vivo. J. The effect of the test media on force decay A research was undertaken by Ash and Nikolai (1978) to obtain the relaxation patterns in vivo and to compare the intraoral data with those obtained in air and in water. The force decay in vivo was significantly greater than in air. However, significant differences in relaxation in vivo versus simulation by in a water bath at oral temperature do not appear until the alastiks have been loaded continuously for 1 week. At the end of the 3-week period, the difference in force level exceeded 43 grams with more force decay found in vivo. Similarly, Kuster et al. (1986) showed more force decay in chains placed in vivo compared to chains stored in air. Different

20 10 storage media have been used to study elastomeric chains in-vitro. The most often used are distilled water and artificial saliva. Von Fraunhofer et al. (1992) showed that chains in artificial saliva required more stretching to reach the required force after immersion for 28 days as compared to distilled water. The least stretching was needed when the chains were stored in air. K. The effect of the environment on force decay Other environmental factors can also affect the force decay of elastomers. Taloumis et al. (1997) reported that moisture and heat had a pronounced effect on force decay and permanent deformation of elastomers. Teixeira et al. (2008) immersed elastomeric chains in light coke, phosphoric acid and citric acid and found no significant difference in force decay as compared to artificial saliva. Ferriter et al. (1990) found more force decay in a ph of 7.25 than in a ph of 4.95, concluding that the more basic the environment the more force decay will occur. L. Force magnitudes required for tooth movement Forces in the range of grams are suggested as optimal forces for canine retraction by some authors (Storey and Smith, 1952; Reitan, K, 1957), while Begg stated it to be 300 grams instead (Hixon et al., 1969). Hixon et al. (1969) found that higher forces, up to 1,000 grams, produced more rapid tooth movement than those under 300 grams. Boester and Johnston (1974) applied forces ranging from 57 to 312 grams. They found that 57 grams of force yielded less tooth movement compared to 142 gm, 227 gm and 312 gm. They did not find any significant difference between 142 gm, 227 gm and 312 gm of initial force in space closure.

21 11 M. Artificial saliva compositions and properties There is currently no universally accepted artificial saliva composition used for studies. Ferriter et al. (1990) buffered distilled water with sodium phosphate monobasic solution (NaH 2 PO 4 H 2 O), sodium phosphate dibasic heptahydrate solution (NaHPO 4 7H 2 0) and sodium chloride (NaCl). The solution was titrated with hydrogen chloride (HCl) and sodium hydroxide (NaOH) until the desired ph was reached. Others used Oralube as their artificial saliva, which is an artificial saliva solution containing electrolytes in concentration and viscosity modified to stimulate natural saliva (Von Fraunhofer et al., 1992; Baty et al., 1994). Some studies on bonding use calcium (Ca) and phosphate (P) in a buffer solution of Tris buffer (Hara et al., 2004). Esterase has been added in recent studies in an attempt to mimic the oral cavity even more closely. Phua et al. (1987) report that enzymes are substrate specific and it is not normally supposed that enzymes are able to cause degradation of synthetic polymers. However, they can reduce the activation energy of chemical reactions. A degradation process, which usually takes place only at high temperature or in the presence of ultra violet light, may take place under physiologic conditions in the presence of proper enzymes. Research is unclear on which enzyme would have that effect. They also mention that polyurethanes are degraded by fungi which have protease activity. Common enzymes in the oral cavity include pseudocholinesterase (Ryhänen et al., 1983), acetylcholinesterase (Ng et al., 2009), cholesterol esterase (Armstrong et al., 2006), collagenase (Armstrong et al., 2006), amaylase (Brand et al., 2006) and enolase. The effect of adding these enzymes to artificial saliva on elastomers has not been tested.

22 12 The temperature and the ph also have to be adjusted to mimic those in the oral cavity. According to Wunderlich (1871), the normal body temperature for a healthy adult is approximately 37 C which is used by most studies (Bishara and Andreasen, 1970; Hershey and Reynolds, 1975; Stevenson and Kusy, 1994; Taloumis et al., 1997; Balhoff et al., 2011). As for the ph, Brawley (1935) found resting saliva ph of 6.75 in 3405 subjects, while Kleinberg and Jenkins (1964) found it to be 6.77 in a sample of 85 first and second year dental students. In patients undergoing fixed orthodontic treatment, the ph fluctuates. According to Peros et al. (2011) salivary ph significantly increased after the first 6 weeks, continued to increase during the next 6 weeks, and between week 12 and week 18 decreased toward baseline. In their study it was concluded that the 6 th and 12 th week of orthodontic therapy is a period of very intensive salivary functions and physiologic response. The mean ph at week 18 was 7.30.

23 III. MATERIALS AND METHODS The study was an in vitro, laboratory study, in which elastomeric chains were stretched to predetermined distances and the force decay was measured at different time points. Factors studied were: the chain type 2 levels (thermoplastic and thermoset), the attachment type 2 levels (single chain and loop chain), and the force magnitudes 2 levels (200 grams and 350 grams). A digital force gauge was used to measure the remaining force magnitudes at each time point. A. Sample American Orthodontics (Sheboygan, Wisconsin) and Orthodontic Research and Manufacturing Company (Ormco, Glendora, Calif) manufacture power chains made from both thermoplastic and thermoset elastomers. Hence, four groups were selected for evaluation: 1. American Orthodontics thermoplastic power chains (AOTP) 2. American Orthodontics thermoset power chains (AOTS) 3. Ormco thermoplastic power chains (OrTP) 4. Ormco thermoset power chains (OrTS) Two spools were ordered for each group, each from a different batch having a different lot number. Eighty power chain specimens for each group were used; 40 from spool a and 40 from spool b. The total number of chains used was 320; 80 for each of the 4 chain products. 13

24 14 Figure 1. Total sample comprised of 4 power chain groups. The spools were ordered no more than two months before the experiment. Once received, the power chains were stored in a drawer at room temperature away from light and extreme humidity. Grey power chains were selected to exclude any effect pigmentation might have on elastic behavior. Closed chains were selected since this design showed minimal variation between companies when lengths of specimens containing the same number of modules were compared (Figure 2). The specimens were cut using a ligature cutter (RMO i-551) immediately before testing. An extra module was left at each end of the specimen to assist in transferring the specimens and to make sure the last module tested on either end was not cut too short.

25 15 Figure 2. From top to bottom; AOTP, AOTS, OrTP and OrTS B. Testing Apparatus 1. Stretch jig A metal stretch jig made of two halves joined by a jack screw with 10 pins on each half was fabricated by an engineer at the University of Illinois at Chicago (Figure 3). The distance between the adjacent pins on each half was 4.445mm. This distance was chosen in order to allow power chains to loop around the pins. The length of each pin is 5 mm and the diameter was 1.57 mm. Each jig had a knob with markings on it. When the knob is turned 1/8 of a turn, the platform separated 0.1mm. The chains were stretched over the pins with the jack screw opened to the distance required for that specific chain group and subgroup. Sixteen jigs were made so that the testing of 160 chains can be done simultaneously.

26 16 Figure 3. Stretch jig. 2. Media Artificial saliva was formulated from 20 mmol Hepes (4-(2-hydroxyethyl)-1- piperazineethanesulfonic acid), mmol Calcium Chloride Dihydrate (CaC 2.2H 2 0), mmol Potassium Phosphate (KH 2 PO 4 ) and mmol Potassium Chloride (KCl). Three liters of artificial saliva were made weekly. To get accurate weights a digital analytical balance was used (Denver Instrument Company A 160 Digital Analytical Balance, Arvada, Colorado). A ph of 6.75 was selected and adjusted using 4M NaOH was added until the desired ph was reached.

27 17 3. ph meter A ph meter was used to measure the ph (SevenMulti Channel ph/ion Meter, Mettler Toledo, Toledo, Ohio). The ph was measured at three occasions: when formulating the artificial saliva to get a ph of 6.75, immediately before inserting the containers into the incubator, and right after taking them out. The ph meter was calibrated at the beginning of each measuring day using buffer solutions (Buffer Solution ph 7.00 Colored Yellow Solution Tampon and Buffer Solution ph 4.00 Colored Red Solution Tampon). 4. Containers Four sealed plastic containers were used to contain the media in which the stretch jigs were fully immersed in the media during the period the chains were tested. A label listing the group and the distances required between the pins of the jigs for each subgroup was attached to the lid of each container. The containers were placed in an incubator (Lab-Line Incubator model 403, Melrose Park, IL) to maintain the temperature at 37 C. Weekly, fresh artificial saliva was formulated. Each container was emptied weekly and 0.75L of fresh artificial saliva was added in the container. All power chain products had different chemical compositions, so each plastic container had one power chain group to avoid possible contamination from chemicals released into the media.

28 18 Figure 4.Closed container with label on lid. 5. Digital force gauge To measure the force, a digital force measurement gauge (DS2-11, IMADA, Tokyo, Japan) with load capacity 50.00N and a specifity of 0.01N mounted on a test stand (HV-110, IMADA, Tokyo, Japan) was used. The chains were stretched between two hooks: one attached to the gauge and the other on the base of the test stand. The test stand had a manual wheel to allow distance adjustments. A digital distance meter on the test stand was used to determine the distance when the power chains were stretched between the hooks.

29 19 Figure 5. Digital force measurement gauge (IMADA DS2-11) mounted on a vertical manual wheel operated test stand (IMADA HV-110). 6. Digital caliper A digital caliper (Series EC16, ID: B, TRESNA, Guangxi Province, China) was used to measure the length of the unstretched specimens and to recheck distances measured by the digital distance meter on the force gauge stand. The digital caliper had a total range of 200mm with 0.03mm accuracy.

30 20 Figure 6. Digital caliper (TRESNA Series EC16, ID: B). 7. Transfer jig To prevent power chain relaxation while transferring the power chains from the stretch jig to the force gauge, a modified divider was used as a transfer jig. An adjusting screw between the legs of the divider was used to set the distance between the two tips of the divider according to the test group being tested. The divider tips that came with the divider were replaced with longer segments of inch (0.9144mm) stainless steel wire. The tips of these segments were pointed using rotary instrument. These longer segments made it easier to transfer the specimens.

31 21 Figure 7. Transfer jig C. Measurement acquisition The specimens were cut from spools a and b using a ligature cutter (RMO i-550) and stretched over the hooks on both the force gauge and the base of the test stand. The specimens were allowed to remain stretched for 15 seconds before the force was recorded on an Excel spread sheet. The transfer jig was opened and the tips were inserted in the extra modules to transfer the specimens from the force gauge to the designated stretch jig. The transfer jig was also used for all proceeding measurements to transfer the specimens from the stretch jig to the force gauge and vice versa. A relaxation period of 15 seconds was chosen since the continuous force decay was at a much slower rate after 15 seconds. The ph and force magnitudes were measured at 11 different time points: one hour, 24 hours, 3 days, 1 week, 2 weeks, 3 weeks, 4 weeks, 5 weeks, 6 weeks, 7 weeks, and 8 weeks.

32 22 D. Design 1. Description of subgroups Each of the 4 power chain groups was subdivided into 4 subgroups to test 2 canine retraction designs and 2 force magnitudes as follows: 1. Single chains light force 2. Single chains heavy force 3. Loop chains light force 4. Loop chains heavy force Two force magnitudes were selected to represent light forces (200gm), and heavy forces (350gm). Some suggest that initial forces extending beyond 300gm can affect the force delivered by the elastomeric chain and the force decay (Wong, A.K., 1976; Von Fraunhofer et al., 1992; Bousquet et al., 2006). This was also tested in this study in regards to thermoset and thermoplastic power chains. Single chains were chosen to be stretched between two pins representing a 6-3 canine retraction design. For the single chain groups each chain sample contained 6 modules. When these power chains were stretched to 50% and 100% of their initial length, the stretched power chains had an average length of 22.9mm and 30.6 respectively. The average distance between the midpoints of the first molar and canine brackets in a normal dentition prior to space closure was between 25 and 28 mm (Josell et al., 1997; Balhoff et al., 2011). An extra module was left at both ends of the specimen. Loop chains were stretched from one pin around the second pin and

33 23 back again to the first pin. The specimens for the loop chain groups contain 14 modules. They had double the number of modules from the the single chain groups plus 2 extra modules to allow the specimen to turn around the pin. Again, an extra module was left at both ends of each specimen ( ) (Figure 8). Figure 8. Left; AOTP single chain. Right; AOTP loop chain 2. Determination of stretching required to reach selected magnitudes of force Measurements were obtained to estimate how long each power chain group needed to be stretched to reach the desired light and heavy forces for the single and loop chains. Specimens from AOTP were cut from spools a and b. For subgroup 1 (single chains light force) these specimens were extended on the hooks of the force gauge and the base of test stand so that the force magnitude recorded was 200 grams (after 15 seconds of being stretched). The average

34 24 stretch distance between the hooks from these specimens was used as the distance between the pins on the stretch jig for AOTP subgroup 1. Once used, these specimens were discarded and no specimens were reused. The same process was repeated by using new specimens for each of the remaining 3 subgroups of AOTP. The whole process was repeated for the 3 remaining power chain groups (AOTS, OrTP, and OrTS). The following distances between the pins were selected for every power chain group for single and loop chains: Power chain group TABLE I REQUIRED STRETCHING FOR SINGLE CHAINS 0 grams Light force Heavy force (200gm) (350gm) AOTP 15.30mm 19.10mm 26.30mm AOTS 15.45mm 23.13mm 44.68mm OrTP 15.15mm 20.04mm 31.30mm OrTS 15.12mm 22.23mm 45.00mm Power chain group TABLE II REQUIRED STRETCHING FOR LOOP CHAINS 0 grams Light force (200gm) Heavy force (350gm) AOTP 38.16mm 19.50mm 21.64mm AOTS 37.69mm 21.00mm 24.24mm OrTP 37.20mm 19.85mm 22.00mm OrTS 37.80mm 20.50mm 23.54mm

35 25 3. Grouping the sample A total of 16 stretch jigs were used in this experiment. Four stretch jigs were used per power chain group, one for each of the four subgroups. Each stretch jig had 10 specimens, 5 from batch a and 5 from batch b. Figure 9. Each power chain group had 4 subgroups 4. Time points There was a total of 12 points in the study: 0 hours, 1 hour, 1 day, 3 days, 1 week, 2weeks, 3 weeks, 4 weeks, 5 weeks, 6 weeks, 7 weeks, and 8 weeks.

36 26 One hour measurements were done separately since setting up the zero hour measurements for all four subgroups of one of the power chain groups would take about an hour, and measuring it after an hour would take an hour too. Therefore, it would be difficult to do all one hour measurements for all 4 power chain groups simultaneously. There were 80 power chain specimens per power chain group. Forty were used for the 1 hour measurements and 40 for the daily and weekly measurements. As an example of how these specimens were divided, AOTP is demonstrated: Figure 10. Forty AOTP specimens used for one hour measurements.

37 27 Figure 11. Forty AOTP specimens used for the daily and weekly measurements. D. Measurement protocols Three different protocols were established for measuring: initial measurements, one hour measurements, and daily and weekly measurements. 1. Baseline measurements The ph calibration was done before measurements were performed. At time point 0, the hooks on the gauge and the base of the test stand were adjusted to the needed distance for subgroup 1 of one of the power chain groups. A jig was also opened so that the distance between the pins matched that distance. The specimens were cut and stretched over the hooks on both the force gauge and the base of the test stand. The power chains were allowed to remain stretched for 15 seconds before the force magnitude was recorded on an Excel spread sheet. The transfer jig was used to transfer the specimen from the gauge to the jig. Measurements were obtained for all 10 specimens of subgroup 1. For subgroup 2, single chains heavy force, the distance between the

38 28 hooks was adjusted. A jig was adjusted to the same distance and the force was recorded for 10 specimens in a similar manner to group 1. Once this was done for the remaining 2 subgroups of that power chain group, artificial saliva was added to a container, ph was recorded, the 4 jigs were immersed in the media, and the container was sealed and inserted into the incubator to maintain the temperature at 37 C. The whole process was repeated for the remaining 3 power chain groups and all 0 hour measurements were obtained. 2. One hour measurements Obtaining 0 hour measurements for the 1 hour measurements was a little different than it was for the daily and weekly measurements. Rather than preparing all 4 jigs and then dispersing the artificial saliva in the container to immerse the jigs, an empty container was filled with artificial saliva and the container was put in the incubator. After the zero hour measurements were recorded for the subgroup 1, the time was recorded and the jig was immediately immersed in the media. The 0 hour measurements were then recorded the same way for the second, third and fourth subgroups of that power chain group. After an hour had passed from the time the first jig was put in the incubator it was taken out and the 1 hour measurements were obtained for the first jig. The divider was used to transfer the first specimen from the jig to the hooks on the gauge and the stand. The specimen was left stretched on the hooks for 15 seconds before the force magnitude was recorded. After the force was recorded the specimen was discarded. The same was done for the remaining 9 specimens of that jig. When an hour has passed from when the second jig was put in the incubator, the jig was taken out and the measurements were recorded and the specimens were discarded. The same was done for third and fourth jig. The ph was recorded when the container was filled with artificial saliva before any specimens were put

39 29 in the container. The ph was recorded again after the one hour measurements were obtained for subgroup Daily and weekly measurements Daily and weekly measurements were obtained at 10 different time points: 1 day, 3 days, 1 week, 2 weeks, 3 weeks, 4 weeks, 5 weeks, 6 weeks, 7 weeks, and 8 weeks. At each time point the ph meter was calibrated before measuring. The first container was taken out containing all 4 subgroups of the first chain product and the ph was recorded. A container containing distilled water was used to rinse the jigs before measuring took place. After rinsing the jig the digital caliper was used to make sure the distance between the pins did not change and matched the distance on the lid. The distance between the hooks was then adjusted to match that distance using the digital distance meter on the stand and then rechecked with the digital caliper. The transfer jig was used to transfer the specimens from the stretch jig to the force gauge. The specimens were allowed to rest for 15 seconds before the force was recorded on the Excel spread sheet. The force was recorded for the 3 remaining jigs in the same manner. An additional step for the weekly measurements was that the container was emptied and new artificial saliva was dispersed in the container. The ph was recorded for the new artificial saliva before the container was returned to the incubator and the next container taken out for measurement. E. Statistics Each power chain group had two separate Excel spread sheets to record the ph and the remaining force in grams. The first was for the 1 hour measurements and the 0 hour

40 30 measurements used for them. The second was for the 10 daily and weekly measurements with the 0 hour measurements used for them. Once all the data were recorded for all the chain products, the remaining forces in grams from both spread sheets were then converted into percentages by dividing the remaining forces by the initial forces. A new spread sheet was then created that had the remaining forces of all specimens shown in percentages with the means and standard deviations. The data were then exported into IBM SPSS Statistics 19 for analysis. To test the difference in force decay between the thermoplastic and thermoset chain products of the same company, independent samples t-tests were used. The data for all 10 specimens in each subgroup and each time point from AOTP was compared to the same subgroup and time point from AOTS for a total of 44 independent samples t-tests for AO. The same was done for OrTP and OrTS making it 88 independent samples t-tests for both companies combined. Figure 12 shows 11 independent samples tests that were done for subgroup 1 of AOTP vs. subgroup 1 of AOTS. Significant differences were noted at α=0.05.

41 31 Figure 12. Eleven independent samples t tests were done for subgroup 1 of AOTP vs. subgroup 1 of AOTS. Repeated Measures Analysis of Variance (ANOVA) was used to compare the differences in force decay between the 4 subgroups and the effect of time on force decay. Each time point was compared with the time point that preceded it to evaluate significance. Scheffé was used for pairwise comparisons. Significant differences were noted at α=0.05.

42 32 Four Repeated Measures ANOVAs were done, one for every chain product. Figure 13 illustrates the Repeated Measures ANOVA of AOTP. Figure 13. Repeated Measures ANOVA of AOTP. When the Repeated Measures ANOVA showed significant differences between the subgroups, One Way ANOVA was used to analyze the differences between the subgroups at week 2, week 3, week 4, week 5, and week 6 (Figure 14). Week 2 through 6 were chosen since clinicians do not usually keep power chains in the oral cavity for more than 6 weeks. Scheffé was used for pairwise comparisons. Significant differences were noted at α=0.05.

43 Figure 14. One way ANOVA for OrTP at weeks 2, 3, 4, 5 and 6. 33

44 IV. RESULTS A. Elongation percentages To determine the elongation percentages, the first step was obtaining the elongation in millimeters. This was obtained by subtracting the unstretched lengths from the required stretch distances of each subgroup from Tables I and II. The elongation in millimeters was then divided by the unstretched length of the specimens to determine the elongation percentage. Tables III and IV show the elongation percentages. TABLE III ELONGATION PERCENTAGES OF SINGLE CHAINS Chain Product Light force Heavy force AO-TP (200gm) 24.84% (350gm) 71.90% AO-TS 49.71% % Or-TP 32.28% % Or-TS 47.02% % TABLE IV ELONGATION PERCENTAGES OF LOOP CHAINS Chain Product Light force Heavy force AO-TP (200gm) 10.17% (350gm) 21.80% AO-TS 19.93% 37.12% Or-TP 14.60% 26.88% Or-TS 17.12% 33.02% 34

45 35 B. Comparison of thermoplastic and thermoset power chains within the same company The results of the independent samples t-tests showed that the thermoplastic subgroups from American Orthodontics had significantly (p 0.001) more force decay when compared to the corresponding thermoset subgroups at all the time point. The average mean differences for subgroups 1, 2, 3 and 4 were 20.57%, 20.21%, 18.80% and 19.40% respectively. Tables III through VI show these mean differences. Similarly, Ormco thermoplastic subgroups had significantly (p 0.001) more force decay when compared to the corresponding thermoset subgroups at all the time point. The average mean differences for subgroups 1, 2, 3 and 4 were 24.74%, 23.58%, 15.35% and 16.77% respectively. Tables VII through X show these mean differences. Combining all subgroups, AOTP had 19.75% more force decay compared to AOTS while OrTP had 20.11% more force decay compared to OrTS.

46 36 TABLE V AMERICAN ORTHODONTICS THERMOPLASTIC VS THERMOSET FOR SUBGROUP 1 Mean Std. Pair # Time points Mean t value P value difference Deviation 1 AOTP one hour * AOTS one hour AOTP one day * AOTS one day AOTP three days * AOTS three days AOTP one week * AOTS one week AOTP two weeks * AOTS two weeks AOTP three weeks * AOTS three weeks AOTP four weeks * AOTS four weeks AOTP five weeks * AOTS five weeks AOTP six weeks * AOTS six weeks AOTP seven weeks * AOTS seven weeks AOTP eight weeks * AOTS eight weeks Average mean difference *significant at p 0.05 n = 10

47 Figure 15. Comparison of AOTP and AOTS subgroup 1. 37

48 38 TABLE VI AMERICAN ORTHODONTICS THERMOPLASTIC VS THERMOSET FOR SUBGROUP 2 Mean Std. Pair # Time points Mean t value P value differenc Deviation AOTP one hour * AOTS one hour AOTP one day * AOTS one day AOTP three days * AOTS three days AOTP one week * AOTS one week AOTP two weeks * AOTS two weeks AOTP three weeks * AOTS three weeks AOTP four weeks * AOTS four weeks AOTP five weeks * AOTS five weeks AOTP six weeks * AOTS six weeks AOTP seven weeks * AOTS seven weeks AOTP eight weeks * AOTS eight weeks Average mean difference *significant at p 0.05 n = 10