BIOMATERIALS IN ODF
1. Introduction
The general principle of orthodontics is to move teeth in the three dimensions of space. These dental movements will be carried out in particular by the installation in the oral environment of an orthodontic device, most often metallic, comprising locks glued or sealed to the different teeth, one or more orthodontic wires in each arch depending on the technique used, as well as various auxiliaries. These devices are always composed of alloys .
2. Metals
2.1. Definition of metal
Metal is considered to be the union of millions of metal atoms.
These atoms can be identical: this is the case for pure metal.
But due to its weak properties, metal alloys are used which are presented as a mixture of several pure metals generally obtained after melting and cooling.
The atoms of these alloys are arranged on sites whose arrangement is described by a crystal lattice.
2.2. Stainless steel
Steel is any alloy whose main component is iron. Inexpensive, steels are all around us: metal bridges, automobiles, pliers and orthodontic rings… For biomedical use, they had to be made perfectly stainless so that they would not corrode in the oral environment.
Orthodontic steels are made stainless by the presence in the alloy of metals with particular characteristics: nickel and chromium .
- Stainless steel.
- Wear resistance.
Chromium:
- Increases hardness
- Gives shine to steels.
It remains very popular and widely used today. It is characterized by good malleability and is very easy to shape.
Stainless steel arches can be welded or brazed for the manufacture of orthodontic appliances.
These alloys consist of iron, chromium, nickel, and less than 2% carbon.
Single-strand stainless steel has a low coefficient of friction, meaning that it only opposes low frictional forces to tooth movement.
Multi-strand wires can be twisted in round or rectangular section, twisted around a coaxial wire or braided.
2.3. The Elgiloy
It is an alloy composed mainly of cobalt, chromium and nickel which was developed by the Elgin Watch Company in the 1950s.
The alloy comes in four different types depending on its resilience, differentiated by four different colors (Same composition, but their mechanical properties differ).
- Blue Elgiloy: which is the most popular, is not very resilient, recommended for significant bending or for welding.
- Yellow Elgiloy: is more resilient
- Green Elgiloy: is semi-resilient
- Red Elgiloy: is resilient and fragile.
Heat treatment allows the elastic limit to be increased and the resilience to be increased to a value close to that of stainless steel.
2.4. Brazing alloys
Tin-based solder – Silver-based solder – Gold-based solder
In the manufacture of orthodontic devices, parts composed of several parts can rarely be avoided and their assembly remains a problem.
2.4.1. Brazing
Brazing is the conventional means of assembly used in dentistry. It consists of joining metallic elements together by the interposition of a filler material whose composition is different and whose melting temperature is lower than that of the base alloy.
The orthodontist uses silver solder with a deoxidizing flux, incorporated or not, and with a gas torch.
Using a blowtorch involves all the disadvantages of overheating, imprecision, breakage and the effects of bimetallism including the risk of corrosion in the mouth.
Soldering is therefore a source of breakage, oxidation of devices and, in certain cases, allergies in the mouth;
Orthodontic wires have elastic properties specific to the use for which they are intended. It is therefore essential not to modify their elastic behavior and not to oxidize their surface.
2.4.2. Laser welding
Welding is the joining of two metal parts or certain synthetic products, carried out thermally.
The welder consists of a laser device, optical components to guide the beam and a work area to allow handling and positioning of the part to be welded.
Unlike brazing, with laser welding it is possible to obtain exact positioning of the elements, without the possibility of deformation, for a perfect fit.
It ensures a punctual fusion of the material, without heating the areas
Borders to ensure unmatched quality of welded parts.
In the mouth, no oxidation, therefore no release of metal ions following the effect of saliva.
2.5. Nickel-titanium alloys
Due to their flexibility and shape memory, these alloys are increasingly used and a wide variety of brands are commercially available. Nickel is combined with a more noble metal, titanium.
The mastery of metalworking processes partly explains the evolution of the properties offered by the different Ni-Ti alloys offered to orthodontists since the first Ni-Ti arches, marketed in 1972 under the generic name of Nitinol.
Nickel-titanium alloys thus exhibit pseudo-elastic properties linked to the existence of a phase transformation in the solid state.
This transformation takes place between so-called “low” and “high” temperatures, without diffusion, in a quasi-instantaneous manner and without modification of the chemical composition of the alloy. It leads to a displacive structural change by cooperative displacement of atoms over very short distances.
2.5.1. Nitinol
Historically , we find Nitinol ( Ni ckel, Titanium , Naval Ordnance Laboratory ), invented by Buehler in 1960. Composed mainly of nickel (52%), and titanium (45%) .
- Nitinol does not offer the characteristics of superelasticity and shape memory.
- Its very low modulus of elasticity allows it to withstand large elastic bendings.
- Nitinol can produce lower, more constant and more continuous force than steel of equivalent section.
- It has a less smooth surface than steel, which increases friction forces.
- Finally, Nitinol cannot be welded.
2.5.2. Chinese Nitinol
It is a Ni-Ti alloy developed by the team of Doctor Tien Hua Cheng, at the General Research Institute of Nonferrous Metals in Beijing.
Chinese Ni-Ti has greater elasticity than Nitinol.
2.5.3. Ni-Ti with shape memory and superelastics
They can undergo an apparently plastic deformation and then completely regain their initial shape by simple reheating.
The interest of these two properties, superelasticity and shape memory, is manifested during the stages of deactivation of the wires where the forces restored will be light and almost constant.
3. Metal locks
The orthodontic attachment or lock, bracket in English , is a device fixed to each of the teeth of an arch that can be glued to the tooth or welded to a ring and serves as an intermediary between the tooth to be moved or immobilized, and the mechanical effector that constitutes the active part of the orthodontic appliance. Friction is by definition a force that slows the relative movement of two objects in contact.
In orthodontics, it is determined by the nature of the bracket and the arch: the material, the quality of its machining, the surface finish, the width and depth of the groove, the inter-bracket distance, the method of ligature.
In recent years, many locks have been modified to reduce the surface area of the groove in contact with the wire in order to reduce friction forces.
3.1. Materials used
3.1.1. Stainless steel: The most commonly used material for the manufacture of metal brackets is stainless steel.
This alloy was proposed for dental prostheses in 1920. Initially, stainless steel was
Designed to resist corrosion rather than mechanical stress.
These steels contain, apart from iron, a certain percentage by weight of nickel, chromium, carbon, molybdenum, manganese, cobalt, niobium.
3.1.2. Titanium
Recently, the use of titanium brackets, manufactured by injection, opens new horizons. The mechanical properties are equivalent to steel, with better corrosion resistance and no nickel release.
3.1.3. Chromium-cobalt alloys
– This alloy is biocompatible, nickel-free.
– The bracket is constructed in one piece with integrated laser marked base.
– It does not present any galvanic corrosion.
3.2. Design and use
Means of fixing orthodontic brackets
- Sealed rings with welded attachment: The attachments are welded onto rings adjusted for each type of tooth.
- Glued fasteners: The bases are equipped with special retentions which can be mechanical or chemical.
3.3. Manufacturing methods
Fasteners are either cut metal, cast metal, or injection molded.
– Machining steels is difficult and expensive, especially if the alloy has a high hardness.
– The disadvantage of cast or injected brackets is that they are less resistant. The groove is less precise and less polished, which seems to increase friction. The base retentions are less effective than with a grid insert, which reduces the strength of the bonding
- CAD/CAM (Computer Aided Design and Manufacturing): Incognito type lingual brackets are manufactured from a high-precision impression and the digitalization of the models.
The brackets are completely designed using specific software.
4. Conclusion
Biomaterials are unanimously and widely used for the manufacture of orthodontic appliances . They thus constitute the bulk of the mechanical therapeutic arsenal.
As a user practitioner, the orthodontist is responsible for the equipment and materials used, and will be directly incriminated in the event of an abnormal reaction. He must therefore be aware of the regulations regarding the use of medical devices, and his obligations regarding materiovigilance (standards, traceability, etc.).
BIOMATERIALS IN ODF
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Clear aligners are a discreet alternative to braces.
Composite fillings are less visible than amalgam.
Interdental brushes prevent gum problems.
A diet rich in calcium strengthens teeth and bones.