by Stephen Yang, DMD, MS
An arch wire is the wire that attaches to your braces. It is called an "arch wire" because your top teeth comprise your top arch, and your bottom teeth comprise your bottom arch. An arch wire is like the engine that guides and moves your teeth. Without an arch wire to connect your braces, you would just be wearing braces for fun and your teeth would never move! Arch wires come in different sizes and have different material compositions.
Strength and Springiness
When you first start treatment with braces, your teeth are crooked. The wire that 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, yet exert a gentle force so that your brackets do not pop off when the orthodontist ties in your arch wire.
As your teeth get straighter, your orthodontist will exchange your wires for stronger wires that are usually less elastic and stiffer. These stronger, stiffer wires allow the orthodontist to have more control over tooth movements. Often times, these later wires have characteristics that allow the orthodontist to put permanent bends in the arch wire if he wants to move individual teeth.
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 and 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. This is because when considering arch wires made from identical materials, 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 are 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. In the beginning, the smaller rectangular wire may be like a small hand in a large glove. However, by the end of treatment, the rectangular wire you have may be more like a large hand in a large glove. By fitting snugly into the bracket, the rectangular wire controls tooth movement better than a round wire.
Arch Wire Materials
Now that you understand a little about why orthodontists like elastic wires in the beginning of braces treatment and stronger, stiffer wires at the end of treatment, let us consider what materials the arch wires are made of.
There are three main types of material compositions for arch wires: 1) Stainless Steel; 2) Nickel-Titanium (Ni-Ti); and 3) Beta-Titanium.
Stainless steel wires have been used for decades due to their high strength. In addition, stainless steel wires do not rust and can be adjusted many different ways by the orthodontist without breaking. However, stainless steel wires are not very elastic, meaning that if you bend these wires too much, they will assume the new position and will not return to their original position. In the beginning stages of treatment, it is important for the wires to be elastic so that the wires can bounce back to a nice smooth U-shape and carry the teeth with it at the same time. So as you can see, in the initial stages of aligning very crooked teeth, stainless steel wires may not be the best option.
Nickel-Titanium (Ni-Ti) wires are elastic and can return to their original shape when deformed. Therefore, in the beginning stages of orthodontic treatment, Ni-Ti wires are frequently used to put gentle forces on the crooked teeth to align them. A variation of Ni-Ti wires are heat-activated Ni-Ti (Copper Ni-Ti) wires. Heat-activated Ni-Ti wires can hold the deformed configuration at room temperate, but when the wire reaches the temperature of a patient’s mouth, the wire will return to its original shape. Heat-activated Ni-Ti wires are useful in the beginning stages of treatment. If the teeth are extremely crooked, the wire can be cooled so it can be tied into the brackets easier. Then after a few minutes, it will reach the temperature of the patient’s mouth, displaying its Ni-Ti elastic properties. The warm wire will want to assume its original U-shape and carry the teeth to their new, straighter positions.
Beta-Titanium wires were developed after Ni-Ti wires and offer an intermediate range of elasticity and strength, while also being able to be permanently deformed. This wire serves as a good intermediary wire between Ni-Ti and stainless steel. Some orthodontists will use this wire starting in the middle of treatment while other orthodontists do not use this type of wire at all.
Which Wire to Use?
So which wires will your orthodontist use? It is difficult to say because all patients are different and all orthodontists have their own technique for adjusting braces. Some orthodontists only use Ni-Ti wires while other orthodontists only use stainless steel wires. However, most orthodontists typically start with small Ni-Ti wires to align crooked teeth in the beginning, and progress to larger Stainless Steel or Beta-Titanium wires when more control of teeth is necessary.
Many patients think that their wires need to be changed at every visit. This is usually not true. With the wide assortment of wires these days, your orthodontist may only need to use three to five sets of wires during treatment. More wires may be needed for complicated cases.
Stephen Yang, DMD, MS, is a writer and Senior Editor for Bracesquestions.com.
Dr. Yang received his Bachelors degree in Psychology at the University of California at Los Angeles. Following his undergraduate studies, Dr. Yang attended the Harvard School of Dental Medicine, where he completed his Evidence-Based-Dentistry thesis and obtained his Doctor of Dental Medicine degree. After receiving his doctorate, Dr. Yang went on to study at the University of Southern California where he received his Certification in Orthodontics. Concurrently at USC, he completed his thesis on one phase versus two phase treatment in patients with large overbites, and obtained his Masters of Science in Craniofacial Biology.
Dr. Yang is a Board Certified Orthodontist, practicing in Antioch, California. He is a Certified Invisalign Provider, and is a member of the American Association of Orthodontists, Pacific Coast Society of Orthodontists, American Dental Association, and California Dental Association.