Aesthetics : The wire should be the least visible in the mouth. This property becomes very important when using ceramic brackets. While this is a very desirable property, there should be no compromise on mechanical properties.

Biocompatibility and environmental stability : Biocompatibility refers to resistance to corrosion and tissue tolerance to elements in the wire. The corrosive behaviour is tested according to the international standards ISO 10271. Environmental stability ensures that the desirable properties of the wire are maintained for extended periods of time. This property ensures that the wire is not harmful when in use in the mouth.

Biohostability : The ease with which a wire tends to accumulate bacteria, spores or viruses is called biohostability. An ideal arch wire should be a poor host.

Coefficient of friction : Friction is resistance to the motion of one material with respect to another closely approximated material. In orthodontics, friction describes the ease of movement of brackets over the wire. If the coefficient of friction is lower, sliding during alignment and space closure will be easier and will have less strain over the anchor segments.

Formability : This property describes the ease with which a material may be permanently deformed. Formability allows the wire to bend into desired configurations, such as loops, coils and stops, without fracturing the wire. This property is dependent on the number of slip planes in the crystal lattice of the alloy. The more the slip planes, the easier it is to deform the wire. Formability is measured by employing a 90-degree cold bend test. The wire which can be given the maximum number of bends without fracturing can be termed the most formable. This property is the opposite of springback or flexibility because the most flexible wire can be given the least number of bends.

Joinability : This denotes the ability to attach auxiliaries to orthodontic wires by welding or soldering.

Range : Range is the distance that the wire bends elastically before permanent deformation occurs. Clinically, this means the distance to which an arch wire can be activated without exceeding the limits of the material. To relate range, stiffness and strength, we can say that the range limits the amount the wire can be bent, and stiffness indicates the force required to reach that limit. The combination of both is the property of strength.

Resiliency : Resiliency describes the amount of energy stored in a body when it is elastically deformed. Resiliency is the area under the elastic portion of the stress–strain curve. The resilience of a wire represents the energy absorbing capacity of the material, which is a combination of strength and stiffness. A highly resilient wire will be able to exert force for a larger range and sustain the activation for a longer period. Hence, resilient wires give better control and need fewer wire changes in clinical settings.

Solderability : The ease with which attachments can be soldered to the wire.

Springback : Springback is a clinically applicable term for maximum elastic deflection, maximum flexibility, the range of activation, the range of deflection or a working range. It is the extent to which a wire recovers its shape after deactivation.

• Orthodontic wires should have a springback property, which means that the configuration will regain the original shape even after being greatly deformed. This would mean fewer wire changes.

Stiffness : It basically refers to the resistance of the wire to deformation. Stiffness also measures the force the wire is capable of delivering for a particular amount of deflection. Low stiffness or load–deflection rate implies that the wire will apply low forces, which will be more constant as the appliance deactivates. Burstone believes that the stiffness of a wire is related to both the material and cross-section. A stiff wire will not be easily deformed and hence used in situations where teeth need to be maintained in their positions. In contrast, wires with less stiffness which produce low forces for large range are required to facilitate tooth movement during active treatment.

Strength : A wire’s strength is defined as the force required to activate an arch wire to a specific distance.

• Stiffness and strength are often confused.

• Yield strength proportional limit and ultimate tensile strength are various measures of strength (see terminologies for definitions).

Toughness : Toughness is defined as the force required for fracturing a material. It can be measured as the total area under the stress–strain graph. Clinically, this property is seen when giving bends in a wire and when giving deflections.

Weldability : It is the ease with which the wire can be joined to other metals, by melting the work pieces around the bond. A filler metal may or may not be used in the process. Both the soldering and welding are included in the broad category of joinability. The ability to attach auxiliaries to orthodontic wires by welding or soldering provides an additional advantage when incorporating modifications to the appliance.

Zero stress relaxation : If a wire is deformed and held in a fixed position, the stress in the wire diminishes with time, but the strain remains constant. This is known as stress relaxation. At the molecular level, this occurs due to slippage of particles over each other when subjected to forces leading to loss of activation in the wire. Zero stress relaxation is the property of a wire to give constant light force when subjected to external forces. This property is desirable if a wire is to provide constant forces for a longer period of time, especially in springs and loops.

Annealing : It is the process of reversing the effects of cold working, such as strain hardening, distorted grains, etc. by simply heating the metal. In a clinical setting, a wire is considered annealed when it appears red hot.

Bauschinger effect : The phenomenon is named after German engineer Johann Bauschinger in 1886. Essentially, the Bauschinger effect describes the reduction in yield stress of a metal when the direction of deformation is reversed. If you permanently deform a metal in one direction, its yield stress is reduced in the opposite direction. The effect does not apply in the direction the metal has been strained. This can be used to advantage after wire bending because of residual stresses left in the material, improving its elastic properties in the direction in which the wire has been deformed.

Brittleness : This is considered the opposite of toughness. A brittle material cannot undergo plastic deformation.

Crack propagation : Crack propagation is seen especially in high tensile wires which have a high density of dislocations and lattice imperfections. The dislocations pile up and form a minute crack. The stress concentration at this point is very high, so that only small stress can result in crack propagation. The crack continues to propagate, and it appears at the surface some distance from the point where it is bent. This fracture appears as if the skin of the wire has been peeled off from the main wire.

Cyclic fatigue : If there is repeated cyclic stress of a magnitude below the fracture point of a wire, then fracture of the wire can occur. This is due to cyclic fatigue.

Elastic limit : It describes the greatest load to which the wire can be subjected, such that it returns to its original form. If 0.1% deformation is allowed, then it is quantified as the elastic limit. After this point, when the load on the wire is removed, it does not return to its original length.

Flexibility : When a material can undergo a large deformation (or large strain) with minimal force, within its elastic limit, it can be termed as flexible.

• Maximal flexibility is the strain that occurs when a wire is stressed to its elastic limit.

• Max flexibility = Proportional limit Modulus of elasticity

Heat treatment refers to a general process of using thermal energy to change the characteristics of metallic alloys, such as tempering, precipitation hardening or annealing. In a clinical setting, different wires are heat treated according to the manufacturer’s recommendations. A wire is considered heat treated when it appears straw coloured.

Hysteresis : The difference between the energy required to activate the wire by deflection and that released by it during deactivation is called energy loss or hysteresis.

Modulus of elasticity : According to Hooke’s law, the stress and strain are directly proportional to each other in the elastic portion of the graph. The ratio of the stress in the spring and applied force (but only within its proportional limit) is called the modulus of elasticity and is a constant for a given material.

Proportional limit : It is the point on the graph at which a permanent deformation is first observed. Although the definitions differ, the elastic limit and proportional limit, for all practical purposes, represent the same point.

Sensitisation : When stainless steels are heated up to temperatures between 800°F and 1200°F, carbon reacts with chromium to form chromium carbide; hence, chromium tied up, as the carbide cannot contribute to the corrosion resistance of the metal. This phenomenon is called sensitisation. The carbon inactivates the chromium at the grain boundaries opening them to corrosion.

Stabilisation : This is the process by which carbon is made unavailable for sensitisation. Steel that has been treated to reduce the available carbon is called stabilised steel.

Strain : It is the internal distortion produced by the load. It is defined as deflection per unit length.

• Strain = Change in length Original length

Strain, work hardening or cold working is the process of plastically deforming metal at a temperature lower than that at which it recrystallises new grains. This temperature is usually one-third to one-half of its absolute melting point. Cold working disrupts the normal atomic arrangement and incorporates strain across the grain boundaries.

Stress : It is the internal distribution of a load applied to a material. It is usually defined as force per unit area (F/A).

• Stress = Internal force Area of action

• Tensile stress is caused by a load that tends to stretch the body.

• Compressive stress is caused by a load that tends to compress the body.

• Shear stress is caused by a load that tends to slide one part of a body over another.

Ultimate tensile strength : The maximum force that a wire can withstand before it fractures is denoted as the ultimate tensile strength. This is always higher than the yield strength, and clinically, it is usually the indicator of the maximum force that the wire can deliver.

Yield strength : This denotes the amount of stress on the stress–strain graph that causes a certain amount of permanent deformation (usually 0.1%) is calculated. This is called the yield strength.

Formability:

• SS wires have excellent formability and can be bent into various designs without fracture within limits. These wires have the lowest coefficient of friction and are ideal for use during retraction mechanics.

Stiffness:

• SS is a relatively stiff material with low flexibility and a small range of action. Hence, it cannot withstand large activations or recover shape entirely from the initial deformation. A steep load–deflection rate means that forces delivered by the SS wires dissipate rapidly over a very short amount of deactivation.

Soldering:

• SS wires have ease of joining as they can be easily welded or soldered. Usually, the welded areas need a solder reinforcement.

Biocompatibility:

• In general, the SS wires have excellent biocompatibility and high corrosion resistance in the oral environment.

Low cost:

• The cost factor is also favourable for SS wires because they are quite inexpensive.

Sensitivity:

• Hypersensitivity reactions due to the leaching of Ni and Cr have been reported, but these are restricted to a minuscule percentage of the population.