Extra-oral appliances in orthodontic treatment

Extra-oral unit

This part of the appliance provides the anchorage for the extra-oral force in a form of headcap (Figure 1), neckpad/strap or chin cup (for retraction headgear) or a facemask (for protraction headgear). Retraction headgear normally includes the delivery force system in the headcap/neckpad/strap.

Force delivery system

This can involve either a spring-loaded device (Figure 2) or heavy-force, extra-oral elastic. The former is used with retraction headgear and chin cup appliances while the latter is commonly used with protraction facemask appliances.

Intermediate/connecting component

This transmits the force to the teeth and underlying skeleton and connects the extra-oral and intra-oral components. With retraction headgear, the intermediate component is the outer part of the facebow that is welded/soldered to the inner part (inner bow) of the extra-oral component. The inner bow should also incorporate a robust safety mechanism to prevent an accidental disengagement^2,3 (Figure 3a, b).

Intra-oral component

Retraction headgear is attached via the headgear facebow to either fixed, removable or functional appliances for either anchorage reinforcement or molar distal movement. With a fixed appliance (Figure 4), the inner bow inserts into molar bands using welded headgear tubes which are either gingivally placed (close to the centre of tooth rotation) or occlusally positioned (for better access during insertion and removal). With removable appliances (Figure 5), the facebow either inserts into tubes soldered to the molar clasps or to coils wound as part of the clasps (Figure 3b). Headgear tubes can also be incorporated into the acrylic of a functional appliance for orthopaedic force delivery. The only headgear appliance that has no intra-oral component is the chin-cup appliance.⁴ When using a protraction facemask, the intra-oral component is either a fixed or removable appliance with many operators simultaneously treating the patient with slow maxillary expansion (using a quad helix appliance) or rapid maxillary expansion (RME).^5,6

Anchorage reinforcement

Retraction headgear can be used for anchorage reinforcement antero-posteriorly and vertically. Although headgear is considered to be a good method of anchorage reinforcement, TADs have been shown to provide equivalent results with minimal patient co-operation.⁹ For anchorage reinforcement, force levels of 250–300 g per side are required for a minimum of 10 hours/day, on average.^{10,11,12,13,14}

Molar distalization

Retraction headgear can distalize the molars by up to ½ a molar unit (approximately 5 mm) which can be useful when used for correction of a Class II molar relationship and relief of crowding on a non-extraction basis or reduction of an increased overjet.^15,16,17,18 Molar distalization using headgear in conjunction with extraction of the deciduous canine in order to provide space for ectopic permanent maxillary canines has been shown to have a success rate of 80%, in comparison to 50% where the deciduous canine was extracted as an isolated measure.¹⁹ However, the Cochrane review by Parkin et al has shown that there is no high-quality evidence to justify the interceptive effect of extraction of primary canines.²⁰ Headgear can also be used to upright impacted upper first molars and to regain lost space after premature loss of primary teeth.²¹ Force levels of 400–500 g/side used over a period of 12–14 hours/day are necessary for distalization.¹⁰ Headgear use can tip and extrude the molars, depending on the magnitude and direction of the applied force as well as the duration of headgear wear.^11,12,13

Canine retraction/labial segment movement

J-hook headgear has historically been used to intrude upper anterior teeth, retract maxillary canines individually or, rarely, retract lower canines.²² Force levels of 250–300 g per side are required for a minimum of 10 hours/day, on average.¹⁰ Owing to the risk of ocular trauma, and as canine retraction is more easily achieved with sliding mechanics when using contemporary fixed appliances, J-hook headgear is no longer used.

Differential (asymmetric) tooth movement

Asymmetric movement of the molars can be achieved using asymmetrical headgear (AHG).²³ There are many designs for AHG but the main principle of action is Castagliano's Theorum, which involves greater movement on one side due to a longer outer bow or heavier force on this side. The variety of designs of AHG include the power-arm facebow, soldered-offset facebow, swivel-offset facebow and spring-attachment facebow.^24,25,26 One of the disadvantages of AHG is the tendency to produce a crossbite on the side of the long arm and an increase in difficulty when fitting the appliance.²⁷ Force levels of 250–300 g per side are required for a minimum of 10 hours/day, on average.^11,12,13

Growth modification

Anterior open bite

High-pull headgear combined with a twin block, monoblock or Dynamax appliance can be used to treat high angle Class II malocclusions.³⁵ However, the evidence for the effect of headgear on the vertical dimension is weak as Oliveira et al found only a limited improvement in anterior open bite when treated with a palatal crib and headgear.³⁶ Force levels of 400–500 g/side used over a period of 12–14 hours/day are recommended for a skeletal effect.¹⁰

Retraction headgear

Low-pull traction (Figure 6a)

This type of headgear is frequently referred to as cervical traction and is the most commonly used headgear appliance.³⁷ Cervical traction is used mainly for the correction of low angle Class II malocclusion by restraining the forward growth of the maxilla.³⁸ Cervical traction is believed to have a reciprocal effect on the growth of the mandible as well as extrusion of maxillary molars.³⁹ This latter effect results in a clockwise mandibular rotation, thus cervical headgear is indicated mainly for growing children with a deep overbite.^39,40

J-Hook

Protraction headgear (Figures 7, 8)

This is also called a facemask or reverse headgear. Facemask therapy is an effective orthopaedic appliance for growing children in order to correct a Class III malocclusion through forward movement of the maxilla.⁴¹ The results of treatment are better in the early mixed dentition than in the late mixed dentition.⁴² In addition to the skeletal changes, reverse headgear can result in dental compensation to assist with the correction of a reverse overjet or Class III malocclusion.^14,43

Chin-cup

Chin-cup therapy is used to correct Class III malocclusions in growing patients using forces ranging from 250–600 g per side for at least one year. Some researchers have found that chin-cup therapy has dental effects as well as redirecting, inhibiting or slowing condylar growth.^44,45 However, as no forces are applied to the maxilla, cases with maxillary hypoplasia will not have any upregulation of maxillary growth when treated with a chin-cup appliance. As a result, chin-cup therapy is seldom used in the UK.

Retraction

The first step in fitting the retraction headgear involves selecting the correct facebow size. The inner bow should be 1.13 mm while the outer bow is 1.45 mm for maximum rigidity. The facebow should be set parallel to the occlusal plane with slight expansion. The centre of bow should be slightly away and above the central incisor edge. The inner bow should follow the contour of lip and cheeks but not actively displacing them. Each side of the facebow must be adjusted to fit into the fixed appliance molar bands or removable appliance tubes/colis at one side at a time. If the molars are not severely rotated then toe-in bends should be placed. The length of the outer bow, relationship to the centre of rotation, the direction of pull should be carefully selected and adjusted to minimize distal crown tipping. All these factors are important in determining the force vectors and the net force. In particular, if the outer bow is positioned at the trifurcation point of molars, the result will be distal translocation of the molars.^10,46

Protraction

With protraction headgear, the fitting procedure starts with adjusting the cams (Figures 7, 8) using the Allen key until they are 15 degrees below the occlusal plane. The elastics are attached to the intra-oral device and should be heavy enough to apply the required force at 30 degree angulation. It is preferable for the elastics to cross-over to avoid lip irritation.³¹

Monitoring progress with headgear appliances can be undertaken by asking the patient/parent about compliance, using compliance charts,⁸ assessing the ability of the patient to insert/remove the appliance, checking for physical signs of wear and tear, identifying positive tooth movement in comparison with pre-treatment study models/cephalometry and detecting molar mobility.