Introduction
The objective of this study was to evaluate the characteristics of force degradation of latex elastics of 10 kinds of elastics over 48 hours, both in vivo and in vitro.
Methods
For the in vivo study, 10 different kinds of elastics were randomly chosen for investigation: 1/8-inch (2 oz); 1/8-inch (3.5 oz); 3/16-inch (2 oz); 3/16-inch (3.5 oz); 1/4-inch (2 oz); 1/4-inch (3.5 oz); 5/16-inch (2 oz); 5/16-inch (3.5 oz); 3/8-inch (2 oz); and 3/8-inch (3.5 oz). Ten volunteers (aged 22-24 years) were selected to wear personalized clear retainers, which were made to hold the elastics in the mouth and stretched to a specific length. Control samples of 1/4-inch (2 oz) and 1/4-inch (3.5 oz) latex elastics were stretched to the same length and held in dry air conditions (temperature = 25°C) and in artificial saliva (temperature = 37°C, pH = 6.7). Force value and percentage of force degradation were estimated 10 times over a 48-hour period in both the in vivo and in vitro groups. A 1-way ANOVA and t test were used to identify statistical significance ( P <0.05).
Results
The force degradation of the latex elastic in vivo is greater than in vitro. In the in vivo groups, during the first hour, the extension rate of all elastics decreased sharply about 13.16%-18.79%, then the rate of force degradation declined. The degradation of initial force was about 29.35%-39.94% after 48 hours. The extension range of 2.0-oz elastics reduced less than that of the 3.5-oz elastics in vivo. At the same time, with the same initial force, elastics with larger inner diameters decreased more slowly than the smaller elastics ( P <0.05).
Conclusions
The force degradation of latex elastic in vivo is much greater than that in both air and artificial saliva. In vivo, the force value of the orthodontic latex elastics decreased sharply in the first hour. The larger the inner diameter and smaller the setting force value were, the slower the force decay.
Highlights
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This study simulated force application both in vivo and in vitro.
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Degradation rates of 10 types of orthodontic latex elastics over 48 hours were measured.
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Force degradation of latex elastic was greater in vivo than in air or artificial saliva.
Intraoral latex elastics are used to help orthodontic mechanics in the delivery of force to the teeth. Generally an auxiliary method, latex elastics are characterized by high flexibility, relatively enduring force, and low cost. Patients can easily change the elastics by themselves and maintain good oral hygiene.
Latex elastic materials are made of natural macromolecule compounds. One of the inherent disadvantages of elastics is that force levels decrease over time. To achieve optimum orthodontic tooth movement, this force decay must remain within acceptable limits. At present, there are more than 20 kinds of latex elastics used during orthodontic treatment. Clinicians choose proper elastics to meet the need of force, personal experience, and habit. Elastics are said to exert the reported force at an extension of 300% of their diameter, but the validity of this claim has been seriously questioned, and the force levels may vary with the size and force level of the elastic. , Although elastic bands are used extensively, their mechanical properties are not well-defined. Particularly, these properties are influenced by factors related to the material, such as loss of elasticity, amount of force decay, and composition of the elastics and also environmental factors, such as the composition of saliva, intraoral pH, temperature variations, food texture, and pigments. Few studies have evaluated the changes on the force degradation of latex elastic in vivo.
Taking the limitations into consideration, it is important to conduct complementary studies on the degradation of latex elastics in vivo and in vitro. This study simulated the force application status in vivo and in vitro and analyzed the degradation rate of the 48-hour curve of 10 types of orthodontic latex elastics.
Material and methods
Latex elastics from 1 company (3M Unitek, St Paul, Minn) were used. Fifty samples in each of 10 sizes were tested. All the latex elastics were within their expiration dates and stored in sealed plastic packages in an appropriate environment. The force value of the 3.5-oz elastics was regarded as 100 g of force and that of the 2-oz elastics was regarded as 60 g of force. Force extension was measured in a ZHIQU testing machine (load sensor 500G/5N, accuracy of 0.01 mm; Dongwan ZHIQU company, Dongguan, China). Tensile readings were recorded in gram force with a duration of 10 seconds for each elastic. Two hooks were coupled to the machine, 1 at the upper connection point and the other at the bottom, aiming to the insertion of the elastics for their extension. The length of the setting force (100 or 60 g of force) was measured by electronic digital display (accuracy of 0.01 mm; Vernier, Germany Masterproof Company, Shanghai, China). The average length of each group was the distance of extension and is reported in Table I .
| Group | Environment | Size | The distance of extension (mm) mean (SD) | Reported force (g) | Actual initial force (g) mean (SD) |
|---|---|---|---|---|---|
| I | Vivo | 1/8″ 2 oz | 11.6 (0.40) | 60 | 58.92 (3.83) |
| II | Vivo | 3/16″ 2 oz | 17.2 (0.73) | 60 | 57.99 (2.58) |
| III | Vivo | 1/4″ 2 oz | 20.1 (0.57) | 60 | 60.62 (3.26) |
| IV | Vivo | 5/16″ 2 oz | 27.1 (1.24) | 60 | 61.34 (1.80) |
| V | Vivo | 3/8″ 2 oz | 31.2 (1.39) | 60 | 59.15 (1.60) |
| VI | Vivo | 1/8″ 3.5 oz | 13.2 (0.47) | 100 | 104.65 (1.87) |
| VII | Vivo | 3/16″ 3.5 oz | 20.8 (0.77) | 100 | 105.45 (3.65) |
| VIII | Vivo | 1/4″ 3.5 oz | 22.4 (1.23) | 100 | 105.66 (2.55) |
| IX | Vivo | 5/16″ 3.5 oz | 29.3 (1.28) | 100 | 102.72 (3.91) |
| X | Vivo | 3/8″ 3.5 oz | 34.1 (1.13) | 100 | 103.04 (3.09) |
| XI | Air | 1/4″ 2 oz | 20.1 (0.57) | 60 | 58.91 (4.96) |
| XII | Air | 1/4″ 3.5 oz | 22.4 (1.23) | 100 | 61.09 (4.08) |
| XIII | Artificial saliva | 1/4″ 2 oz | 20.1 (0.57) | 60 | 102.4 (2.85) |
| XIV | Artificial saliva | 1/4″ 3.5 oz | 22.4 (1.23) | 100 | 102.86 (5.41) |
The following inclusion and exclusion criteria were applied, and 10 volunteers were chosen: students of the School of Stomatology, Zhejiang Chinese Medical University; good physical and mental health, regular life, no history of orthodontic treatment, no bruxism, no dentition defect, no moderate or severe periodontal disease, and no dentition crowding. A silicon rubber impression was collected for each volunteer, and maxillary and mandibular gypsum models were produced. Personalized thermoplastic retainers for both jaws were made on a vacuum pressure film machine (China Guang ming Medical Device Company, Suzhou, China).
The distance of extension to produce initial force values of 60 and 100 g of force was represented by 2 buttons ( Table I ). Light solidification glue was used to fix the buttons on each quadrant of the clear retainers. Each volunteer was told to place the latex elastics on the buttons ( Fig 1 ).
Ten types of the latex elastics were used for the in vivo experiment. Timing started once volunteers began to wear the clear retainers. Each volunteer was required to test 5 elastics of the same types and wear the elastics for 48 hours (except during meals) without changing them. The volunteers were told to avoid eating food except the basic 3 meals every day to prevent breakage of the elastic bands. Force measurements were made at 10 intervals: 0, 1, 3, 6, 9, 12, 18, 24, 36, and 48 hours. To ensure the consistency of the tests, all measurements were performed by 1 person (LY). With a pair of tweezers, each elastic was carefully transferred from the volunteer to the ZHIQU testing machine in the sequence of time intervals. At a specific distance, the force value of the elastic showed on the test machine decreased rapidly at the initial stage, then tended to decline at a slower and more stable rate. The constant value in the test machine within 10 seconds was regarded as the force value of the elastic, and each elastic was read 3 times for an average value.
The latex elastics of 1/4-inch (2 oz) and 1/4-inch (3.5 oz) are frequently used in clinical treatment, thus they were chosen as the control groups for the in vitro experiment. Both sizes of latex elastics were stretched to the setting force in retainers as before. Instead of being worn in the volunteers’ mouths, the retainers were placed respectively in dry air conditions (constant temperature of 25°C) and artificial saliva (constant temperature of 37°C and pH = 6.7). Force measurements were made at the same time intervals as the in vivo experiment.
Statistical analysis
Force degradation curve fitting was performed with the relevant software (SPSS, version 22.0; IBM, Armonk, NY). The force degradation of each group was averaged, and the results were analyzed with 1-way ANOVA of variance and t test ( P <0.05).
Results
Means and standard deviations of force value and force degradation from the latex elastics at the measured time intervals in vitro and in vivo are shown in Table II .
| Group | 0 h | 1 h | 3 h | 6 h | 9 h | 12 h | 18 h | 24 h | 36 h | 48 h |
|---|---|---|---|---|---|---|---|---|---|---|
| I | 58.92 ± 3.84 | 47.75 ± 1.46 | 45.33 ± 1.47 | 43.68 ± 1.23 | 42.28 ± 1.05 | 41.29 ± 1.10 | 40.39 ± 1.47 | 39.59 ± 1.56 | 38.92 ± 1.58 | 38.22 ± 1.51 |
| 0 ± 0 | 18.79 ± 3.17 | 22.90 ± 3.24 | 25.71 ± 2.90 | 28.05 ± 3.46 | 29.74 ± 3.37 | 31.28 ± 3.47 | 32.65 ± 3.37 | 33.79 ± 3.51 | 34.97 ± 3.52 | |
| II | 57.99 ± 2.58 | 48.72 ± 2.19 | 46.13 ± 1.97 | 43.97 ± 2.18 | 42.70 ± 2.20 | 41.90 ± 2.35 | 40.92 ± 2.33 | 40.06 ± 0.97 | 64.42 ± 2.00 | 38.07 ± 1.89 |
| 0 ± 0 | 15.96 ± 2.22 | 20.42 ± 1.99 | 24.07 ± 4.58 | 26.25 ± 4.83 | 27.63 ± 5.07 | 29.32 ± 4.93 | 30.81 ± 4.88 | 32.50 ± 4.93 | 34.22 ± 4.58 | |
| III | 60.62 ± 3.26 | 50.48 ± 1.77 | 48.50 ± 1.82 | 46.88 ± 1.64 | 45.48 ± 1.45 | 45.48 ± 1.86 | 42.83 ± 1.83 | 41.98 ± 1.74 | 41.07 ± 1.60 | 40.06 ± 1.50 |
| 0 ± 0 | 16.54 ± 5.02 | 19.79 ± 5.35 | 22.52 ± 4.04 | 24.79 ± 4.61 | 24.72 ± 4.09 | 29.25 ± 3.18 | 30.66 ± 2.89 | 32.16 ± 2.77 | 33.83 ± 2.61 | |
| IV | 61.34 ± 1.80 | 51.8 ± 4.30 | 49.79 ± 3.86 | 47.81 ± 3.66 | 46.62 ± 3.42 | 45.83 ± 3.18 | 44.92 ± 2.81 | 44.33 ± 2.72 | 43.52 ± 2.72 | 42.41 ± 2.82 |
| 0 ± 0 | 15.62 ± 5.29 | 18.88 ± 4.82 | 22.11 ± 4.46 | 24.04 ± 4.13 | 25.33 ± 3.76 | 26.80 ± 3.35 | 27.76 ± 3.23 | 29.08 ± 3.23 | 30.89 ± 3.49 | |
| V | 59.51 ± 1.60 | 51.06 ± 3.01 | 48.08 ± 2.93 | 47.04 ± 2.07 | 46.01 ± 2.29 | 45.11 ± 2.37 | 44.04 ± 2.62 | 43.33 ± 2.65 | 42.59 ± 2.61 | 42.05 ± 2.78 |
| 0 ± 0 | 14.18 ± 4.85 | 19.20 ± 4.51 | 20.94 ± 3.20 | 22.68 ± 3.33 | 24.19 ± 3.62 | 26.00 ± 3.82 | 27.20 ± 3.84 | 28.44 ± 3.80 | 29.35 ± 3.99 | |
| VI | 104.65 ± 1.87 | 87.65 ± 1.22 | 81.18 ± 1.17 | 76.16 ± 1.86 | 73.97 ± 1.83 | 72.20 ± 1.59 | 69.48 ± 1.36 | 66.81 ± 0.97 | 64.42 ± 1.20 | 62.82 ± 1.48 |
| 0 ± 0 | 16.22 ± 1.69 | 22.41 ± 1.47 | 27.20 ± 2.18 | 29.30 ± 2.18 | 30.99 ± 2.02 | 33.59 ± 1.57 | 36.13 ± 1.78 | 38.42 ± 1.87 | 39.94 ± 2.14 | |
| VII | 105.45 ± 3.65 | 87.11 ± 3.76 | 82.5 ± 2.85 | 78.99 ± 2.28 | 76.46 ± 2.60 | 73.9 ± 1.64 | 71.17 ± 1.85 | 69.34 ± 2.31 | 67.02 ± 1.97 | 64.82 ± 2.45 |
| 0 ± 0 | 17.33 ± 4.06 | 21.70 ± 3.32 | 25.02 ± 3.21 | 27.42 ± 3.40 | 29.85 ± 2.71 | 32.43 ± 3.00 | 34.19 ± 2.74 | 36.38 ± 2.84 | 38.46 ± 3.26 | |
| VIII | 105.66 ± 2.55 | 90.54 ± 3.69 | 83.83 ± 3.32 | 80.41 ± 2.71 | 77.26 ± 2.82 | 73.95 ± 1.79 | 71.37 ± 2.53 | 69.62 ± 1.99 ∗ | 67.82 ± 2.83 | 66.56 ± 2.55 |
| 0 ± 0 | 14.28 ± 3.66 | 20.63 ± 3.51 | 23.85 ± 3.27 | 26.84 ± 3.23 | 29.98 ± 2.31 | 32.41 ± 3.10 | 34.07 ± 2.48 | 35.74 ± 3.88 | 36.94 ± 3.55 | |
| XI | 102.72 ± 3.91 | 88.44 ± 2.34 | 83.09 ± 2.67 | 79.96 ± 2.24 | 76.37 ± 2.61 | 73.75 ± 2.93 | 71.59 ± 2.86 | 69.87 ± 2.93 | 67.93 ± 2.74 | 66.03 ± 2.33 |
| 0 ± 0 | 13.84 ± 2.53 | 18.99 ± 4.42 | 22.05 ± 3.86 | 25.53 ± 4.42 | 28.08 ± 4.58 | 30.18 ± 4.68 | 31.84 ± 4.80 | 33.73 ± 4.77 | 35.59 ± 4.24 | |
| X | 103.04 ± 3.09 | 89.43 ± 3.80 | 83.94 ± 3.75 | 80.44 ± 3.13 | 78.04 ± 3.99 | 76.16 ± 3.53 | 74.16 ± 3.66 | 72.11 ± 3.365 | 70.85 ± 3.27 | 69.14 ± 2.50 |
| 0 ± 0 | 13.16 ± 3.86 | 18.52 ± 3.20 | 21.91 ± 2.64 | 24.27 ± 2.91 | 26.08 ± 2.83 | 28.03 ± 2.71 | 30.03 ± 1.93 | 31.25 ± 2.08 | 32.89 ± 1.57 | |
| XI | 58.91 ± 4.96 | 56.89 ± 4.81 | 55.46 ± 4.74 | 54.29 ± 4.35 | 53.25 ± 4.16 | 52.18 ± 3.43 | 51.12 ± 3.31 | 50.34 ± 3.08 | 49.69 ± 3.22 | 49.13 ± 3.12 |
| 0 ± 0 | 3.42 ± 1.33 | 5.86 ± 1.54 | 7.81 ± 1.43 | 9.56 ± 1.25 | 11.28 ± 2.96 | 13.06 ± 3.33 | 14.36 ± 3.50 | 15.49 ± 3.18 | 16.44 ± 3.20 | |
| XII | 102.4 ± 2.85 | 100.07 ± 2.90 | 95.54 ± 2.35 | 93.47 ± 1.85 | 91.12 ± 1.70 ∗ | 90.47 ± 1.87 ∗ | 89.57 ± 2.04 | 88.01 ± 1.68 | 86.87 ± 1.95 | 85.77 ± 1.85 |
| 0 ± 0 | 2.28 ± 0.51 | 6.68 ± 1.72 | 8.69 ± 1.82 | 10.96 ± 2.62 | 11.62 ± 1.81 | 12.50 ± 1.80 | 14.02 ± 1.99 | 15.14 ± 1.79 | 16.21 ± 1.84 | |
| XIII | 61.09 ± 4.08 | 56.22 ± 2.17 | 54.40 ± 2.44 | 52.80 ± 2.41 | 50.94 ± 2.62 | 49.75 ± 2.29 | 48.39 ± 2.77 | 47.46 ± 2.60 | 46.67 ± 2.58 | 45.98 ± 2.61 |
| 0 ± 0 | 7.79 ± 3.51 | 10.08 ± 3.32 | 13.42 ± 3.47 | 16.46 ± 4.14 | 18.40 ± 3.83 | 20.64 ± 4.52 | 22.17 ± 4.12 | 23.447 ± 3.99 | 24.60 ± 3.90 | |
| XIV | 102.86 ± 5.41 | 92.4 ± 6.19 | 89.88 ± 4.95 | 87.62 ± 5.07 | 85.46 ± 5.00 ∗ | 83.88 ± 5.18 | 81.8 ± 4.80 | 80.2 ± 4.68 | 78.06 ± 4.49 | 77.18 ± 3.79 |
| 0 ± 0 | 10.09 ± 5.27 | 12.57 ± 3.16 | 14.77 ± 3.34 | 16.88 ± 3.20 | 18.42 ± 3.54 | 20.43 ± 3.36 | 21.98 ± 3.32 | 24.07 ± 3.14 | 24.89 ± 3.03 |
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