Fixed appliances can have a greater mechanical control of the teeth and the treatment outcome is greater with the use of fixed appliances. Fixed appliances are, for example, used to rotate teeth that don't fit the arch shape of the other teeth, to move multiple teeth to different places, to change the angle of teeth, or to change the position of the root of the tooth. It is not preferable if the patient has poor oral hygiene (as that can result in decalcification, tooth decay, and other problems), if the patient isn't motivated (as treatment lasts several months and commitment to oral hygiene is required), or if the malocclusions are mild. Braces are usually placed on the front side of the teeth, but may also be placed on the side facing the tongue (called lingual braces). Brackets made out of stainless steel or porcelain are bonded to the center of the teeth using an adhesive. Wires are placed in a slot in the brackets which allows for controlled movement in all three dimensions. Apart from wires, forces can be applied using elastic bands, and springs may be used to push teeth apart or to close a gap. Several teeth may be tied together with ligatures and different kinds of hooks can be placed to allow for connecting an elastic band. Clear aligners are an alternative to braces, but insufficient evidence exists to determine their effectiveness. The time required for braces varies from person to person, depending on the severity of the problem; the amount of room available; the distance the teeth must travel; the health of the teeth, gums, and supporting bone; and how closely the patient follows instructions. On average, however, once the braces are put on, they usually remain in place for one to three years. After braces are removed, most patients will need to wear a retainer all the time for the first six months, then only during sleep for many years. Orthodontic headgear—sometimes referred to as an "extra-oral appliance"—is a treatment approach that requires the patient to have a device strapped onto his or her head to help correct malocclusion—typically used when the teeth do not align properly. Headgear is most often used along with braces or other orthodontic appliances. While braces correct the position of teeth, orthodontic headgear—which as the name suggests is worn on or is strapped onto the patient's head—is most often added to orthodontic treatment to help alter the alignment of the jaw, although there are some situations in which such an appliance can help move teeth, particularly molars.