Introduction
Rubber elastics and polyurethane chains are widely used in orthodontics for intraoral purposes. Rubber bands can also be used extra-orally for headgear and facemask applications. Henry Albert Baker is credited with first using class II intermaxillary elastics to create orthodontic force, which became famous as ‘Baker’s Anchorage’ in the 1920s.
The rubber
Elastic materials for tooth movement can be sourced from natural rubber or synthetic polyurethane derived from the petrochemical industry. Both natural and synthetic rubbers share a common property of quickly returning to their original dimensions after being stretched to a limited deformation. This resiliency feature of rubber is utilised to generate a continuous force to be applied to a tooth or group of teeth, resulting in effective tooth movement.
Natural rubber comes from the sap of rubber trees, specifically Hevea brasiliensis . This material has been used by ancient Indian and Mayan civilisations. Manufacturing rubber involves purifying and mixing it with gum, ammonia, antioxidants and anti-ozone agents to make it suitable for various applications. High-quality rubber elastics can withstand the oral environment for sufficient time without losing their force levels, making them useful for intraoral use.
Latex polymer chains were introduced in orthodontics in the 1960s and have become an integral part of the orthodontic armamentarium. In various forms, these chains have multiple uses and have greatly simplified orthodontic treatment.
Synthetic latex elastics (poly rubber elastics) are made of an elastic polymer with a urethane linkage. They are synthesised by extending polyester or polyether glycol with a di-isocyanide.
Synthetic rubber elastics have superior properties to natural rubber. However, they tend to distort permanently and lose their elastic properties over time in the mouth.
Elastic bands
Elastic bands are produced by slicing rubber tubes of various lumen sizes and wall thicknesses. The force value of the elastic band, when stretched, depends on the lumen size and wall thickness.
Lumen dimensions
The lumen of the elastic band is usually expressed in parts of an inch. For example, a 5/16-in rubber band would mean that the lumen of the elastic band is 5 parts of the 16 parts of an inch.
Tube thickness
Elastic bands are made of rubber tubings with varying thicknesses, categorised as thin, medium or thick, depending on the force value. This means that for each category of lumen sizes, three force types of elastic bands are available: light, medium and heavy.
The orthodontic elastics can be grouped into the following categories ( Table 46.1 ):
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1.
According to use, that is as extraoral or intraoral.
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2.
According to force value (light, medium, heavy and extra heavy).
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3.
According to lumen size ( Fig. 46.1 ).
Figure 46.1 Size of elastics.
Orthodontic elastics are available in various sizes according to the inner diameter (lumen) expressed as a part of an inch. All of them are available as light, medium and heavy force values.
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4.
According to their use/placement in the arches (inter-arch or intra-arch).
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5.
According to colour.
TABLE 46.1
Classification of orthodontic elastics
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| Purple | 1/8 in. |
| Green | 3/8 in. |
| Red | 1/8 in., 1/4 in. |
| Blue | 1/4 in. |
| Orange | 3/8 in. |
| Yellow | 5/16 in. |
Intra-arch elastics are used within a dental arch to retract the anterior segment or close a space within an arch. Inter-arch elastics simultaneously act on the upper and lower arches.
Class II inter-arch elastics are utilised to retract maxillary anterior teeth, maxillary canine retraction and to counteract the labial vector of the torque force on maxillary incisors.
They are also used for a mesial shift of the lower buccal segment, that is, to correct the class II molar relationship to class I.
Elastics in class III fashion from upper molars to lower anterior arch segment are employed for mesial molar movement of the upper arch, retraction of the lower anterior teeth and anchorage preparation in the lower arch.
Box elastics: Elastics to close the dental open bite are applied in a box configuration between the upper and lower in the anterior region.
Oblique configuration: When employed obliquely, the elastics extend from the right of the maxilla to the left of the mandible or vice versa for the midline correction.
Class II elastics on one side and class III on the other are worn to correct mild midline asymmetry.
M and W elastics: Settling elastics can be used in various forms between upper and lower arch buccal segments, such as M or W ( Fig. 46.2 ).
Intraoral elastics are named according to the pattern of their placement.
(A) Class II elastics. (B) Class III. (C) Oblique elastics. (D) Cross-elastics is used to correct the cross-bite of molars. (E) Box elastics. (F) M elastics. (G) W elastics. (H) Triangular elastics.
Storage and dispensing of elastics
different manufacturers use either colour coding or children-friendly signs such as animals or birds to denote different elastics. this is very helpful for children to get them interested in using elastics and communicate with staff regarding their use, changing from one type to another or using two different kinds.
The elastics should be stored away from sunlight, and exposure to heat should be avoided. Sunlight and heat would cause the loss of the properties.
Manufacturers dispense elastic rubber bands in sealable opaque colour pouches to prolong their shelf-life and loss of force. A fresh batch of elastics should be used, and the ones with reputed manufacturing companies are more reliable for force values and their force degradation in the oral environment.
Instructions on wearing of elastics
The assistant or orthodontist should demonstrate elastic placement to the child and parent/guardian in the clinic. The wear instructions should be clear and supplemented with a note. The elastic dispensing pouch can graphically label the elastic locations in the mouth/appliance.
The elastic should be worn continuously except during meals if not indicated otherwise. When worn continuously, they exert a constant force and produce favourable tooth movement with minor discomfort to the patient and minimum tooth loosening and damage to the investing tissues. If the force applied is more than the recommended due to the inappropriate size or thickness of the band, it may cause undesired tooth movements, such as anchor loss or undue elevation of teeth. Lingual rolling, rotation, mesial and lingual displacement with possible soreness and looseness of teeth are usual complications on lower molars associated with using class II elastics.
On the other hand, if these elastics are worn intermittently part-time, the teeth will move more slowly, with loosening in their socket. Intermittent wear causes discomfort and sometimes pain even when wearing light elastics, which, therefore, prolongs the duration of the treatment.
Rubber elastics should not be stretched beyond three times their internal diameter. Research on rubber elastics has shown that synthetic rubber elastics’ first major force decay occurs within the first 24 h of their use in the mouth.
Extraoral elastics
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