Arch wires Dr Ayaz Ahmad Khan

What is an Arch Wire An arch wire is the wire that attaches to your braces It is called an "arch wire" because it engages in the brackets in the upper and lower arches It releases energy as mechanical work that correctly aligns your teeth

Arch wires can be considered as beams supported on both ends spanning between the attachments An arch wire is just like an engine that guides and moves your teeth. Without an arch wire to connect, you would be wearing braces just for fun and your teeth would never move!

When you first start treatment with braces, your teeth are crooked. The arch wire attaches to your braces must be able to return to its original shape when it is deformed or bent. The force that returns the wire to its original shape is what moves your teeth. The wire should be springy and exert a gentle force so that your brackets do not pop off when the orthodontist ties in your arch wire.

For orthodontic purposes, three major properties of beam materials are critical in defining their clinical usefulness: Strength Stiffness Range

Strength It is a measurement of force applied and deflection produced as a result. A graph drawn between stress and strain shows the maximum load that a material can resist. It is measured as the elastic limit which is the point at which a deflection of 0.1% is produced. The new titanium alloys have much more ultimate tensile strength than steel wires. Strength is measured in strength units gm/cm2

Stiffness Stiffness is a measure of wire resistance to bending. Stiffness can indicate at the rate at which it can exert force. There must be a presence of lower stiffness for the arch wires which helps for the lower and constant force for the movement of the teeth as needed.

Range It is the measure how far the wire can be deflected within its elastic limit before permanent deformation. It is measured in millimetres. If the wire is deflected beyond its yield strength, it will not return to its original shape.

Thus a wire should possess High strength High range Low stiffness High formability: it helps to make the arch bend into desired configurations like loops, coils etc. Resilience: the measure of the maximum force an arch wire can withstand. Joinability: The arch wire must have the ability to get soldered or welded so that hooks and stops can be incorporated in it

Friction: There must be least friction at the arch wire-bracket interface. Least friction shows greater efficiency Biocompatibility and Environment Stability: This is one of the important feature of the orthodontic arch wire since the orthodontic treatment normally takes a year or two so it should be resistant to tarnish and corrosion and should also be non-toxicity. It should be reasonable in cost

Composition of arch wires Three main types of compositions for arch wires : Stainless Steel Nickel-Titanium (Ni-Ti) Beta-Titanium. Precious Metal alloys of Platinum and palladium with gold and copper compositions were initially used in the 20th century. The Crozat appliance is still made from Gold exclusively.

Nickel-Titanium (Ni-Ti) Nitinol wire is made from an alloy of Nickel, Titanium and Cobalt. The Austenite form exists at higher temperatures and called A-Niti which is a preferred material for orthodontic wires where a long range of activation with relatively constant force is needed e.g coil springs. The Martensite form exists at lower temperatures and called M-Niti used in the later stages of treatment when flexible but stiffer wires are used. It is available in preformed arch wire shape and in straight lengths.

They are used in the beginning stages of orthodontic treatment to put gentle forces. It is more expensive, is brittle and has low formability so available in commercially preformed shape where a certain shape is set while the alloy is maintained at a higher temperature. The Niti offers the greatest working range and is more resistant to distortion in the mouth Friction forces are lowest but it has a greater tendency to fracture. Orthonol has similar strength and springiness but better formability.

They have two important properties Shape Memory Super Elasticity Shape Memory is the property to “remember” and return to their original shape when deformed.

Heat Activated NiTi Heat Activated Nickel Titanium (NiTi) wires comfortably level and align tough cases with easy engagement and low forces. They exert lowest forces for gentle movement. They become dead soft when chilled below 20°C and achieve optimum activation at 37°C. Their centerline is etched. They are ideal for easier bracket engagement.

Stainless steel wires Stainless steel wires have been used for decades due to their high strength It is resistant to rust because of its high chromium content A typical formulation has 18% chromium and 8% nickel, thus referred as 18-8 stainless steel. Steel may be softened by annealing and hardened by cold working. Fully annealed steel wires are soft and are highly formable. This dead soft wire is also used to make steel ligatures.

They deliver a perfect finish High force values ideal for finishing cases. Made of medical grade stainless steel It is also made in braided form, with up to 6 or more separate wires making a multi strand arch wire. They have reduced stiffness and find their application early in treatment when there are marked tooth displacements within the arch.

Beta- Titanium Beta-Titanium, Beta-Ti/TMA arch wires are composed of alloy of Titanium, Molybdenum, Zirconium, and Tin. These produces forces which are 40% stronger than that of NiTi, but have only 42% of the stiffness of SS. They offer an intermediate range of elasticity and strength and serve as a good intermediary wire between Ni-Ti and stainless steel. They have a good formability. They may be used at the start of the treatment to produce initial alignment. In larger sizes, it can be used as a finishing arch wire towards the end of the treatment and is useful for final detailing of tooth positioning.

Elgiloy Made up of Cobalt, Chromium, Nickel alloy. It can be used in the soft state to apply light forces and can be hardened by heating to increase its stiffness. After heat treatment, it becomes equal to stainless steel in properties. The inconvenience of heat treatment is a disadvantage keeping in view of the availability of a variety of wire types available.

Composite Plastics There has been a corresponding increase in demand for more esthetic orthodontic appliances By adjusting the ceramic/polymer proportions, these wires can be manufactured in a wide range of clinically relevant levels of elastic stiffness. Allergic reactions to nickel are also averted with composite materials. With further developments, in the near future, fiber reinforced composite materials are expected to replace metals as the material of choice for orthodontic arch wires.

Size of Arch Wires When orthodontists talk about the “size” of an arch wire, they are referring to the cross-section or thickness of the wire. Considering arch wires made from identical materials, the smaller the cross-section, the more elastic and less stiff the wire will be. Wires come in two types of cross-sections: 1) Round 2) Rectangular. Round wires are obviously round in cross-section. Rectangular wires can be square or rectangular in cross section.

In the beginning stages of treatment, round wires are typically used to level and align the teeth because round wires are more elastic and so the orthodontist will be able to engage all your teeth into the wire without popping off brackets. If he uses a wire that is too stiff and tries to tie the arch wire to a really crooked tooth, the wire will put too much pressure on the bracket, and the bracket may break off from the tooth.

After the teeth get straighter, orthodontists usually advance to rectangular wires. Because the wire slot of the bracket is rectangular, a rectangular wire fits into the bracket like a hand fits into a glove. By fitting snugly into the bracket, the rectangular wire controls tooth movement better than a round wire.

Arch wires for initial alignment: At the beginning of the treatment, the aim is to reduce individual discrepancies of teeth by achieving initial resolution of labio lingual displacement, rotation and apical displacement to begin the process of “levelling”. Nickel titanium and multi stranded stainless steel wires are used Achieve full engagement of arch wire in all the brackets before moving to next size

After preformed Niti 0. 012, next wire can be 0. 014 or 0 After preformed Niti 0.012, next wire can be 0.014 or 0.016 in Niti, or straight length 0.0175 in coaxial multistranded stainless steel. All of them allow too much tipping of anchorage units because of the torque. Initial alignment is usually completed within 3 months of the commencement of the treatment.

Mid treatment archwires The flexible arch wires are replaced by “working” arch wires of greater stiffness offering more control over tooth movements. Round single stranded arch wires are used such as 0.014 progressing to 0.016 and 0.018. Elastics can be used safely with SS 0.016 wires and above. In the early stages, Overbite and over jet are corrected and spaces are closed. When the principal tooth movements have been achieved, final detailing of tooth position and then retention are done.

Step of treatment Choice’s rules of wire First choice Second Third Initial step •leveling •tipping •rotation Low force Wide activation range Low elastic modulus Low friction Nitinol HA ThermoActiv Nitinol SE Nitinol Classic Intermediate step •spaces closure •correction of arch •leveling occlusal plane Intermediate force Intermediate activation range Intermediate elastic modulus Intermediate malleability Beta III Titanium Perma chrome Finishing step •vertical details •individual rotations •finishing •interdigitation •contention Short activation range High elastic modulus High malleability Flexiloy