"Orthodontic Headgear" is a type of orthodontic appliance attached to the patient's head with a neck strap or a number of straps around the patients head. The device typically transfers the force to the teeth via a facebow or J hooks to the patients dental braces or a palatal expander that aids in correcting more severe bite problems or is used in retention of the teeth and jaws of the patient. 
Headgear is most commonly used to correct anteroposterior discrepancies. The headgear attaches to the braces via metal hooks or a facebow. Straps or a head cap anchor the headgear to the back of the head or neck. In some situations, both are used. Elastic bands are used to apply pressure to the bow or hooks. Its purpose is to slow or stop the upper jaw from growing, thereby preventing or correcting an overjet.
Other forms of headgear treat reverse overjets, in which the top jaw is not forward enough. It is similar to a facemask, also attached to braces, and encourages forward growth of the upper jaw.
Headgear can also be used to make more space for teeth to come in. In this instance the headgear is attached to the molars, via molar headgear bands and tubes, and helps to draw these molars backwards in the mouth, opening up space for the front teeth to be moved back using braces and bands. Multiple appliances and accessories are typically used along with the headgear, such as: power chains, coil springs, twin blocks, plates or retainers, facemasks, a headgear helmet (a headgear helmet is a cervical headgear with a cap that covers the entire head), lip bumpers, palate expanders, elastics, bionaters, Herbst appliances, Wilson appliances, other headgear, hybrid twinblocks, positioner retainers, and jasper jumpers. Many patients wear a combination of, or all of these appliances at any given time in their treatment. 
Headgear needs to be worn between 12–22 hours each day to be effective in correcting the overbite, typically for 12 to 18 months depending on the severity of the overbite, how much it is worn and what growth stage the patient is in. Typically however the prescribed daily wear time will be between 14 and 16 hours a day. 
Orthodontic headgear will usually consist of three major components:
Soreness of teeth when chewing or when the teeth touch is typical. Patients usually feel the soreness to 2 to 3 hours later, but younger patients tend to react sooner, (e.g., 1 to 1 1⁄2 hours). The headgear application is one of the most useful appliances available to the orthodontist when looking to correct a Class II malocclusion.
Facemask or reverse-pull headgear is an orthodontic appliance typically used in growing patients to correct underbites (technically termed Class-III orthodontic problems) by pulling forward and assisting the growth of the upper jaw (maxilla), allowing it to catch up to the size of the lower jaw (mandible). Facemasks or reverse-pull headgear needs to be worn between 12 and 22 hours per day, but typically 14 to 16 hours a day is effective in correcting the underbite. Overall wear time is usually anywhere from 12 to 18 months depending on the severity of the bite and how much a patient's jaws and bones are growing over this time.
The appliance normally consists of a frame or a center bars that are strapped to the patient's head during a fitting appointment. The frame has a section which is positioned in front of the patient's mouth, which allows for the attachment of elastic or rubber bands directly into the mouth area. These elastics are then hooked onto the patient's braces (brackets and bands) or appliance fitted in his or her mouth. This creates a forward pulling force to pull the upper jaw forward.
The orthodontic facemask typically consists of three major components:
In some cases surgery is required in conjunction with a facemask or reverse-pull headgear. Many parents and doctors recommend using early intervention (ages 7 to 13) by using a facemask to avoid costly and painful surgical procedures later.
The appliance is very effective in correcting Class III orthodontic problems in younger or adolescent patients that are still growing. Initially, it can be difficult for children to wear a mask or headgear, however most doctors and parents agree that children and adolescence adapt quickly to such changes and requirements. Parents should be aware that their child is often better-off wearing a facemask or headgear to avoid later surgery and the patient, friends and school peers normally get used to the new appliance after just a few weeks of wear. 
The overall treatment of skeletal Class III malocclusion is still regarded as a significant problem facing the orthodontic profession today. The Class III malocclusion can be caused by a retrognathic maxilla, a prognathic mandible or a combination of both. 
Sometimes surgical correction after the completion of growth phase is unavoidable in many cases, but most orthodontists will proceed with prescribing one of a number of different types or designed of reverse pull headgear or facemask.
Typically most patients with such maxillary deficiency (underbite), the use of a facemask for protraction of the jaw and maxilla is one of the most common treatment plans used. The Jean Delaire reverse pull facemask was introduced by Delaire in 1971. The Delaire facemask is unique in that it requires no additional headcap or straps to hold the appliance to the patients head, and it uses the direct force of the elastic bands into the child's mouth, attached to the braces using an intra-oral hook or a functional appliance, such as the Rapid Palatal Expander (RPE), to hold the mask or face frame in place.
The photo is of an orthodontic extra-oral appliance commonly known as a face-mask or a reverse pull headgear. This specific design is a Delaire type face-mask and is fitted to the patient's face, between 14 and 16 hours a day, during this treatment phase. The mask attaches to the patients upper jaw, typically, with braces fitted, using elastic bands (shown). This pulls the upper jaw forward while it is growing to correct a Class III malocclusion. The elastics also effectively hold the mask in place, on the patent's face.
The Delaire facemask is also unique in that it does not obstruct the child's vision or line of sight, or hinder glasses wear, as the frame wraps around the sides if the patients face. This is benefits daytime, evening wearing of the device. The drawback is that it somewhat limits the patient to lying on his or her back when sleeping, as the frame on the side of the face cause by pressure from the patients pillow. The treatment of Class III problem malocclusion is considered very effective when the patient is still at the age when bone developmental is prevalent (ages 7 up to 13). This is considered the optimal time for the orthodontist to prescribe and fit such a facemask. Typically then the treatment should start in the early mixed dentition phase if mandible growth is still continuing and will typically be worn for 12 to 18 months depending on the prescribed treatment plan. 
The second photo shows detail of the reverse side of an orthodontic extra-oral appliance commonly known as a face-mask or a reverse pull headgear. This specific design Delaire type face-mask and is fitted to the patients face, the padding that fits to the child's forehead and chin is shown. The appliance is typically worn for between 14 and 16 hours a day during this treatment phase. The mask attaches to the patients upper jaw, typically, with braces fitted, using elastic bands (shown). This pulls the upper jaw forward while it is growing to correct a Class III malocclusion. The elastics also effectively hold the mask in place, on the patents face. 
Using a facemask for Class III correction is however also known to cause problems as the forces due to maxillary protraction are normally applied to the upper jaw's teeth. This can cause some level movement of the upper teeth causing crowding. This can be mitigated by using various functional appliances to reduce the orthopedic treatment impact. The patient is required to become accustomed to wearing the appliance for relatively long periods of time although it has been shown that with regular wear, after a few days to a week most patients become accustomed to the facemask.
The treatment of a Class II malocclusion can require the specific inclusion of a J Hook Headgear to the treatment plan needing the patient's canine teeth retracted. A common approach for canine retraction is the use of headgear with J hooks. Since this approach incorporates extraoral anchorage (using a headcap or straps over the patients head), it is considered most effective in maximum anchorage cases.
This treatment plan will require the patient to have a head-cap strapped and fitted to his or her head.
Then the plan requires having metal hook inserted into the patient's mouth, typically hooking onto the patient's braces with a designed end hook and / or elastics. The opposite end of the hook is attached to the headcap through a safety tube, with elastics as shown in the photos. 
Sometimes multiple J hooks (2 upper and 2 lower ) will be used attaching to both the upper and lower canine teeth or used in combination with a facebow appliance headgear.
The photo is of an orthodontic J Hook appliance headgear. This specific design requires the headcap straps to be sized and fitted onto the patients head. The patient then wears the device strapped to his or her head. The orthodontist will have J Hooks are inserted into the patient's mouth, left and right, between 14 and 16 hours a day, during this treatment phase. Headgear J with J hooks attaches to the patients upper jaw, typically, with braces fitted, using elastic bands (shown). This can hold or pull the upper canine teeth and jaw backwards while the jaw is growing to correct a Class II case.
The orthodontic profession has constantly struggled with the improvement of Anchorage (Orthodontics) of both inter and intra oral appliances. In some treatment plans, the use of a full headcap or orthodontic helmet device was recommended.
The advantages of this application is that the anchorage of the intra oral face bows (in this case the patient has both an upper and lower facebow fitted) is significantly improved using such cap or helmet.
Other appliances and devices, such as a full rigid helmet headgear, have been designed and patented to fit onto the patient's head and provide a stable platform from which the orthodontist can attach various elastic bands, springs, metal facebows directly into the patient's mouth. According to the inventor Frank O Nelson (1967)  the helmet headgear of this type solves this long-standing problem of applying anterior or forwardly pulling forces to teeth, and is ideally adapted to correction of a variety of malocculsion forms. The helmet headgear is useful to reposition individual teeth or groups of several teeth which show, for example, overretraction and distoversion (tilting of the tooth axis from its normal orientation) after closing a natural or postextraction gap in the dental arch. The helmet headgear is equally useful in correcting interarch relationships where all the teeth in one arch must be shifted with respect to the other arch. The type is also sufficiently flexible that it may be used for simultaneous application of anterior forces to one group of teeth, and posterior forces to another group of teeth.
Summary the orthodontic headgear of this type comprises a substantially rigid helmet adapted to fit on a patient's head. The helmet includes a crown portion, and a pair of side portions depend from the crown portion and are formed to fit against the sides of the head. A rear portion of the helmet depends from the crown and is formed to fit against the back of the head, a front portion of the helmet depends from the crown and is formed to fit against the patients forehead. A member is attached to and extends forwardly from the helmet, the member having a forward portion spaced from and positioned ahead of the patients mouth when the helmet is on the head. Fastening means are provided on the forward portion of the member, and are adapted for attachment to an orthodontic appliance. The helmet is typically tightly held in position on patients head using a chin strap with chin cup and other straps around the base of the patients head. These helmet headgear appliances were experimented on in the 1970s and successfully prescribed to a number of patients at the time, but generally are only used in specific cases today.
Researchers have studied the long term effects of orthodontic headgear have found that it may flatten the face and prevent the chin from coming forward, pushing both the upper and lower jaw down and back, into the airway. In more technical terms, it inhibits the natural growth of the jaws and lead to a reduction in the SNA and ANB angles, which relate to the forward position of the maxilla and the mandible, and good indicators of the size of a person's airway. Reductions in the width of the airway, potentially causing obstructive sleep apnea (OSA), may also result from headgear.
In some cases, eye injuries have been reported, which is minimized with the use of safety release straps and safety facebows.
The need for headgear in orthodontics and its application by practitioners has somewhat decreased in recent years as some orthodontists use temporary implants (i.e., temporary anchorage devices) inside the patient's mouth to perform the same tooth movements however the headgear is still widely used and a very effective appliance used by orthodontists today. Soreness of teeth when chewing, or when the teeth touch, is typical. Adults usually feel the soreness 12 to 24 hours later, but younger patients tend to react sooner, (e.g., 2 to 6 hours). Adults are sometimes prescribed headgear but this is less frequent. The headgear is one of the most useful appliances available to the orthodontist, but many patients find it difficult to comply with daytime wear, so it is mainly worn in the evenings and when sleeping. A similar appliance is the reverse-pull headgear or orthodontic facemask, which pulls the patients teeth forward (rather than back, as in this case).