Biomechanics of Headgears

Detailed description involving biomechanics of extraoral orthopaedics headgear appliances

1. HEADGEARS Discussion Topic

2. Center of Resistance The center of resistance of a tooth is dependent on the root length and morphology, the number of roots, and the level of alveolar bone support. The exact location of the center of resistance is not easily identified. Analytical studies have determined that the center of resistance for single rooted tooth with normal alveolar bone levels is about one fourth to one third the distance from the cementoenamel junction to the root apex. Miki and Hirato found that the location of the center of resistance of the midface of the human skull was between the first and second upper premolars anteroposteriorly , and between the lower margin of orbitale and distal apex of the first molar vertically in the sagittal plane. It is distal to the lateral incisor roots for intrusive movements of maxillary anterior teeth.

3. BIOMECHANICS In 1971 Armstrong demonstrated the importance of the precise control of magnitude, direction, and duration of extraoral force to increase its efficiency and effectiveness in treating malocclusions in the late mixed dentition. Gould has shown how changes in the inclination of the facebow affect the direction of the force and ultimately the direction of tooth movement. Greenspan presented reference charts elaborating the different moments and forces produced with the various headgear designs .

4. Timing of Headgear Treatment The most optimum treatment time is between maturational stages SMI 4 to 7, a very high velocity period of growth. The next most desirable time to treat is during the accelerating velocity period between stages SMI 1 to 3 The least desirable time is during the decelerating velocity period between maturational stages SMI 8 to 11. This information is clinically useful for all growth related mechanics of treatment, retention, and ortho -surgical timing of therapy .

5. Force systems with cervical headgear . OB (Outer bow)-A lies along the LFO and therefore only vertical and horizontal forces will be produced, no M. The position of OB-B will produce an extrusive F, posteriorly directed F and counterclockwise M since it is above CR . Outer bows located below the LFO will produce posterior forces and smaller extrusive forces since they are closer vertically to the neckstrap and clockwise moments . CERVICAL HEADGEAR

6. The different moments and forces produced by the cervical headgear depend on the situation of the outer bow in relation to the LFO. By definition, when the outer bow lies along the LFO, no moment occurs, and the force system will be reduced to a bodily movement in a posterior and extrusive direction. If the outer bow is placed above this line (angle of above 20-30 degree above occlusal plane), the moment produced by the force will be in a counterclockwise direction. On the other hand, if the outer bow is adjusted below this line the moment created will be clockwise. However, the direction of the forces are the same – extrusive and posterior. It should be noted though that there is an exception to this rule .

7. If the outer bow is located below (angle of less than 20 degree to occlusal plane) the neckstrap , the resultant force will be a small intrusive one, instead of extrusive. Of course, a distal force and large clockwise moment will also be produced . The direction of pull provided by the cervical headgear is especially advantageous in treating short-face Class II maxillary protrusive cases with low mandibular plane angles and deep bites, where it is desirable to extrude the upper posterior teeth. Also, the clockwise moment that is so readily produced with this headgear is very effective in helping conserve anchorage in extraction cases . O uter bow is short -- steepen the occlusal plane O uter bow is long -- flatten the occlusal plane

8. Indications : Short face, class II maxillary protrusive cases with a low mandibular plane angle and deep bite. Anchorage conservation. Early correction of class II malocclusion as it helps to distalize the molar . Contraindications : Open bite cases. High MP angle. Long faces cases with an increase in lower anterior face height.

9. If the teeth are banded and stabilized, cervical pull appliance, produces a force below both center of resistance of maxilla and the dentition. The distances of the force vector to A and B determine the center of rotation (x ).

10. Cervical headgear ( neckstrap ) to the first molar. B odily movement is produced by an outer bow length and position that places the line of force (LFO) through the center of resistance of the molar; but with a lower direction of pull, the tooth is extruded as well as moved backward. Note that the outer bow of a facebow used with cervical traction nearly always is longer than the outer bow used with a high-pull headcap . If the line of force is above or below its center of resistance, the tooth will tip with the root or crown, respectively, going distally as indicated by the dotted arrows.

11. HIGH-PULL HEADGEAR Force systems with high-pull headgear . High-pull headgears produce intrusive and posterior forces. Locating the outer bow in front of the LFO (A and D) will produce a counterclockwise M while an OB behind (B and C) will create a clockwise M. An OB located on the LFO would of course produce no M.

12. The direction of the moment that is produced is dependent on the position of the outer bow. If the outer bow is placed anterior to the LFO (angulated > 45 degree to occlusal plane) moment produced will be counterclockwise . On the other hand, if the outer bow is placed posterior to this line (angulated less than 45 degree to occlusal plane), the moment produced will be in a clockwise direction . This force system would be beneficial in a long-face Class II patient with a high mandibular plane angle, where intrusion of maxillary molars would decrease facial height and improve the facial profile.

13. Indications : Long face patient to control further increase in facial height. Open bite cases. Anchorage control in vertical plane. To intrude and distalize molar. Contraindications : Short face class II malocclusion.

14. Short outer bow angulated high to create the headgear force line of action that is far anterior to the unit’s centre of resistance. This results in a force system at the unit’s center of resistance with a moment that tends to flatten the occlusal plane and distal and intrusive force components.

15. With long outer bow such that the headgear force’s line of action passes through the unit’s center of resistance and therefore no change in the cant of occlusal plane.

16. Long outer bow. The equivalent force system at the unit’s center of resistance has a moment that tends to steepen the occlusal plane and a force with intrusive and distal components. May be necessary for C lass II open bite patients.

17. High-pull headgear ( headcap ) to the first molar. To produce bodily movement of the molar (no tipping), the line of force (black arrow) must pass through the center of resistance of the molar tooth. This will produce both backward and upward movement of the molar. Note that the line of force is affected by the length and position of the outer bow, so that a longer outer bow bent up or a shorter one bent down could produce the same line of force. If bow length or position produces a line of force above or below the center of resistance (dotted red) , the tooth will tip with the root or the crown, respectively, going distally because of the moment that is produced.

18. This style headgear is a combination of the high-pull and cervical headgear, with the advantage of increased versatility. Depending on the force system desired, the orthodontist has the opportunity to change the location of the LFO. The prime advantage of this headgear is its ability to produce an essentially pure posterior translatory force. This is accomplished by placing the LFO through the center of resistance, parallel to the occlusal plane. Clinically , this means bending the outer bow to the same level as CR, and hooking the elastic to a notch at the same vertical level . COMBINATION-PULL or STRAIGHT PULL or INTERLANDI HEADGEAR

19. The straight-pull headgear is versatile in that the clinician has many optional LFO's. In this case, an OB located on the LFO would cause translation in a posterior and slightly superior direction. OB's above the LFO will produce posterior and extrusive forces and clockwise moments. Placing the outer bow along an LFO that Is parallel to the maxillary occlusal plane will produce a pure posterior translation.

20. The relation of the outer bow to the LFO dictates the direction and magnitude of forces and moments. Placing the outer bow above the LFO will produce a posterior force, counterclockwise rotation, and most often an intrusive force . However , if the LFO cants up anteriorly (attachment site of elastic is lower on headcap than at outer bow), an extrusive force will be produced. If the outer bow is below the LFO, the force produced will be posterior and superior, and the moment will be in a clockwise direction . The straight-pull is the headgear of choice in a Class II malocclusion with no vertical problems . It is also the headgear of preference when the main thrust of headgear wear is to prevent anterior migration of maxillary teeth, or possibly even translate them posteriorly .

21. Force’s line of action passes through center of resistance. No moment acting to change the cant of occlusal plane, and there is pure distal force passing through the center of resistance.

22. This configuration is typical for redirecting maxillary horizontal growth in class II patients and /or to move maxillary molars distally via translation . When a force is applied to a headgear with inner and outer bows, one side effect is buccal expansion component of forces, which act bilaterally . This side effect is often helpful in class II malocclusions because it is often necessary to expand the posteriors to maintain proper intercuspation as the buccal segment class II interrelationship is corrected . If such expansion is not required, it can be prevented by using a transpalatal arch .

23. The main purpose of this headgear is to produce an intrusive direction of force to maxillary teeth, with posteriorly directed forces. If the outer bow is hooked to the headcap so that the line of force is perpendicular to the occlusal plane and through the CR, pure intrusion may take place. Due to the multiple notches in the headcap , this headgear is also very versatile, as the LFO orientation may be changed. However, upon establishing the LFO, our principles of determining force systems produced remains unchanged. VERTICAL PULL HEADGEAR

24. The vertical-pull headgear is used primarily when a large magnitude of pure intrusion is needed. The outer bow must be located on the LFO to obtain pure intrusion (A). An OB located anterior to the LFO will produce an intrusive force and a smaller posterior force and a counterclockwise moment (B and C). Locating the OB posterior to LFO will cause intrusion a small anterior force and a clockwise moment (D and E ).

25. The head is divided into two components: the anterior component from the LFO forward and the posterior component located behind the LFO. If the outer bow is placed anywhere in the anterior compartment, the moment created will be counterclockwise, and the forces produced will be intrusive and posterior. If the outer bow is placed anywhere in the posterior section, the moment will be clockwise and the vertical force will be intrusive, but the horizontal force will be forward. If this latter force system is desired, it will require inserting the inner bow into the buccal headgear tube from the distal . The vertical-pull headgear is not as commonly used as are the others. However, it is very useful when pure intrusion of buccal segments is required, as in the Class I open-bite patient .

26. Extrusion of teeth and steepening of OP Cervical gear : OB even or low Extrusion of teeth and flattening of OP Cervical gear : OB very high Intrusion of teeth and steepening of OP Occipital gear : OB posterior to C Res Intrusion of teeth and flattening of OP Occipital gear : OB anterior to C Res Good distal force and flattening of OP Combi gear : OB above C Res Good distal force and steepening of OP Combi gear : OB below C Res Good distal force and no change in OP Combi gear : OB through C Res

27. Right versus left asymmetries can be corrected using transpalatal or lingual arches to correct asymmetric molar axial inclinations. The same mechanism can be used to correct asymmetric molar rotations . If buccal occlusion is asymmetric, e.g. Class I on one side and class II on the other side, without asymmetries either in molar axial inclinations or in rotations, then it is most logical to achieve the correction with asymmetric headgear. Distal forces exist on both sides, but they are three times greater on the long outer bow side than on the short outer bow side . ASYMMETRIC HEADGEAR

28. Lateral forces, directed toward the short outer bow side exist with this headgear. Crossbite development should be kept in mind. These are usually cervical or combination type. Suggestions to be noted with regard to the use of the asymmetric cervical gear : 1. The differential in length of arms of face-bow need not be great, only sufficient to alter the geometry so that the resultant bisector crosses the molar line closer to the more anteriorly positioned molar than to the other. Excessive difference in arm lengths could increase the lateral forces. 2. The diameter of wires can be increased for greater rigidity; it is suggested that the inner bow of 0.055 inch and the outer bow of 0.075 inch (the 0.075 inch face-bow is approximately five times as stiff as the 0.50 inch one). 3. The arms of the face-bow should clear the cheeks so as not to introduce more undesirable lateral forces .

29. From Haack DC, Weinstein S. The mechanics of centric and eccentric cervical traction. Am J Orthod 44(5):346. 1958

30. Rigorous force analysis of the several cervical gears of different design using elastic straps shows that the fundamental principle involved in the distribution of the forces to the right and left molars is the geometry of the direction of the right and left forces emanating from the cervical elastic band. If these forces are symmetrical with reference to the mid-sagittal line of the head, then the distribution of the reactionary forces at the right and left molars will be equal, irrespective of the design of the rigid portions of the appliance (or the point of attachment of face-bow to arch wire).

31. If the direction of forces from the cervical elastic band is asymmetrical with respect to the mid-sagittal line of the head, then the anterior-posterior components of the reactionary forces on the right and left molars will be unequal, the molar nearest the resultant of the two elastic band forces receiving the greater force. Small lateral forces on the molars are always developed by this eccentric design. These forces can be manipulated to cause all lateral reaction to occur on one side or the other by springing the labial arch inward or outward, respectively .

32. Headgear bracket-tube combinations can be attached to either lower first or second molar. If the bracket-tube combination is on the first molar, it is advantageous to place the headgear tube occlusally. First molar is preferred since, The lingual arch is on the first molar and gives better control. It is easier for the patient. Possible directions are: The posterior segments tend to move back. A positive moment will be produced, which will steepen the occlusal plane . HEADGEAR TO LOWER JAW

33. J-hooks to archwire A line of pull attached to the incisor region of the arch wire and passing occlusally to the center of the resistance will place a distally directed force upon the maxillary teeth, but will also tip the occlusal plane downwards at the incisor end of the arch. A line of pull through center of resistance will produce distal movement of the maxillary arch without undesirable rotational effects. A more vertical direction of pull, mesial and apical to center of resistance produces an anti-clockwise moment and an intrusive effect upon the incisor end of the arch wire. Disadvantage is that the flexibility of arch wire results in unavoidable deformations which subject the teeth near the attachment to diurnal reversals of force application as the extraoral appliance is attached or disengaged. Heavy arch wires minimize this rebound effect, but not eliminated . J-HOOK HEADGEAR

34. J-hooks to individual teeth If the center of resistance of a single tooth coincides with the centroid, the line of force of a J-hook headgear intended to produce upright bodily movement of an individual tooth should ideally pass through this center of resistance . Most authorities suggest an occipitally directed line of force to move maxillary canines distally .

35. Magnitude of Force When line of action is closer to center of resistance greater force of 450 – 500 gms /side may be applied for orthopedic changes. Forces away from center of resistance that produce a moment should be restricted to 50 – 150 gm /side as in J – hook headgear to prevent damage to periodontium during dentoalveolar changes that take place during a moment .

36. Duration of Force Intermittent force for 12 – 14 hrs /day in preadolescent age from early evening until next morning . Typical duration of treatment of about 12 to 18 months, depending on rapidity of growth and patient cooperation .

37. CLINICAL APPLICATIONS OF HEADGEAR FORCE There are four main uses of headgear force: Anchorage control Tooth movement Orthopedic changes Controlling the cant of occlusal plane

38. 1. Anchorage Control In Class II treatment, headgear force can play a major role in ensuring that buccal segment teeth do not move mesially when anteriors are retracted. Intraoral mechanics often result in eruption of teeth. Headgear produces a vertical force greater than the force of side effect. Inner and outer bows can be of any shape, and length. Only the angle and level of the final line of action after the strap forces have been applied to know exactly the force of headgear system.

39. The reaction force from headgear is dissipated against the bones of the cranial vault, thus adding the resistance of these structures to the anchorage unit. The only problem with reinforcement outside the dental arch is that springs within an arch provide constant forces, whereas elastics from one arch to the other tend to be intermittent, and extraoral force is likely to be even more intermittent. For first molar extraction cases - Interlandi headgear is suitable and well tolerated.

40. 2. Tooth Movement Intrusion in deep bite cases – 120 to 150 gm force is delivered. Distalization of molars – 300 gm force on each side.

41. 3. Orthopaedic Changes If the headgear is applied through the center of resistance of maxilla, which is in the posterosuperior part of maxilla. Determined clinically by dropping a line vertically 10 mm from the outer canthus of eye and above the apex of distobuccal root of maxillary 1 st molar .

42. Effects of orthopedic forces on maxilla Cervical traction produces stresses along the frontal process of maxilla, zygomaticofrontal suture, and the junction of the palatine bones, areas where high-pull traction produce no observable effect. Only the high-pull headgear produces stress at the anterior junction of maxillae (anterior nasal spine ).

43. Pterygoid plates of the sphenoid High stress develops upon activation. These stresses begin in the middle of the posterior curvature of the plates and just superior to their anterior junction with the palatine bone and maxilla. As the force increases, the stresses progress superiorly toward the body of the sphenoid bone.

44. Zygomatic arches Cervical and high pull HGs both produce similar stress. Starts at the inferior border of the zygomaticotemporal suture and proceeds posteriorly along the zygomatic process of temporal bone. Cervical force produces more intensity at lower load level .

45. Junction of the maxilla with the lacrimal and ethmoid bones Both cervical and high pull produce a stress concentration at the junction of the maxilla with the lacrimal bones and with the orbital plates of ethmoid . Maxillary teeth High stresses around maxillary molars with cervical traction. These located around the middle third of the mesiobuccal root and around distobuccal root at a position toward apex. Also distal to second molar .

46. Palate Cervical traction produces stress in posterior region developing in the horizontal portion of palatine bones. High pull has no effect. Anterior junction of left and right maxillae Only high pull produces forces below the anterior nasal spine and just lateral to the suture between the two maxillae. Frontal process of maxilla Stresses produced anterior to nasolacrimal foramen only with cervical pull .

47. Sphenomaxillary suture - large compressive stresses. Temporozygomatic suture - tensile normal stresses. Sphenozygomatic suture - large tensile stresses. Frontozygomatic suture - large compressive stresses. Frontomaxillary suture - large tensile stresses. Sphenomaxillary and sphenozygomatic sutures, in particular, resist the posterior displacement of the nasomaxillary complex. Stresses in the nasomaxillary sutures are varied by the direction of headgear force, and the force applied in the direction closest to that of the C Res may produce the most effective sutural modification for controlling maxillary growth.

48. Headgear treatment can have several side effects that complicate correction of Class II malocclusion. If the child wears the appliance, maxillary skeletal and dental forward movement will be restricted. Although this helps in correction of the Class II malocclusion, vertical control of the maxilla and maxillary teeth is important, because this determines the extent to which the mandible is directed forward and/or inferiorly. Downward maxillary skeletal movement or maxillary and mandibular molar eruption (all shown in dashed arrows) can reduce or totally negate forward growth of the mandible.

49. FABRICATION & DELIVERY OF HEADGEAR First the correct size and length of the facebow is selected. It is then adjusted and adapted to the maxillary cast such that, the inner bow is away from the maxillary teeth, and position comfortably to rest in between the lips at rest. The inner bow is slightly expanded to facilitate correction of constricted maxillary arch commonly seen in Class II. Molar stop is then made either with a U-loop or bayonet bend mesial to molar tube. Patient is made to sit in an upright position or standing position, then the facebow is inserted into the molar tube intraorally .

50. Mark the centre of resistance of the maxilla on patient face and then check the position of the outer bow with respect to the mark and then it should be bend in the planned direction of force and moment to be generated. The outer bow should be conformed to the cheeks. It should be at least 10 mm away from the cheeks. A safety release mechanism is recommended to prevent the facebow or J-hook springing back and injuring the patient’s eye or face if it is pulled away from the face while still connected to the attachment. Final position of the outer bow should be checked while patient is in an upright sitting position from the front side of patient and clearance from the cheeks from behind. Junction of the inner and outer bow should be away from maxillary anterior teeth and lie between the lips .

51. Once the clinician is satisfied with the final adjustment, it is best to demonstrate the placement and removal of the headgear to the patient with one of the parents present. This permits both the child and the parent to understand how to manipulate the appliance in a safe and an efficient manner. In the first week of wear it is necessary for the child to have some assistance. We should demonstrate how to carefully insert and remove the facebow without applying vertical forces which may loosen the band. Similarly insertion and removal of J-hook should be demonstrated in order to avoid deformation of the wire or debonding of the brackets .

52. Patient is shown how to safely connect and disconnect the headgear attachment to the facebow with an explanation and demonstration of the safety release mechanism. Instruction is given for safe use and avoidance of wear during most sports and other physical activity where child may vulnerable to blow to the face .

53. In order to determine the proper length needed for the outer bow , use the index fingers to apply pressure in the direction of the headgear selected. A, Pushing up and back in the direction of a high-pull headgear. B, Pushing down and back in the direction of a cervical headgear. As the fingers are moved from the anterior portion of the outer bow to the posterior portion, the position of the bow between the lips will change. C, If the bow moves up, the roots on the maxillary first molar will move distally. D, If the bow moves down on the lower lip the roots of the maxillary first molar will move mesially and the crown distally. E, If the bow does not move, the force is through the center of resistance of the maxillary first molar and the molar will move bodily and not rotate. These rules hold true for both high-pull and cervical-pull headgears. F, After the correct length is chosen and the outer bow cut with a pliers, a hook is bent at the end with a heavy pliers.

54. USES OF HEADGEAR To restrain the forward and downward growth of the maxilla and re- directioning maxillary growth. Molar distalization: Headgear may be used to distalize the maxillary molar to correct the Class II molar relationship or to gain space for relief of crowding. Headgears can be used to reinforce molar anchorage in high anchorage cases. Headgears should be worn for at least 10 hr / day with a minimum force of 300 gm per side. Headgear is an effective means of maintaining arch length by preventing mesial migration of molars. Molar rotation can also be brought about with the inner bow of the headgear .

55. LIMITATIONS OF HEADGEARS Headgears cannot apply force directly to the maxillary sutures. The orthopedic forces have to be applied through the dentition and therefore, dental changes are inevitably seen along with skeletal effects. Patient compliance is mandatory for headgear therapy to be successful. An adequate amount of mandibular growth is required to "catch up" while maxilla is restrained. However, this may not always be seen clinically .

56. HEADGEAR WITH FUNCTIONAL APPLIANCES Lehman appliance (Lehman activator ) : A combination activator-headgear appliance developed by R . Lehman . It consists of a maxillary acrylic plate that carries two rigidly fixed outer bows and a mandibular lingual shield. The acrylic plate covers the palate and it extends over the occlusal and incisal surfaces of the maxillary teeth, upto the occlusal third of their buccal and labial surfaces. Selective expansion of the maxillary arch is possible by appropriately activating the two transverse expansion screws (one anterior and one posterior) that are embedded in the plate.

57. Occipital traction is applied through a headstrap attached on the outer bows, which are fixed at the anterior aspect of the appliance. The mandibular lingual shield is connected to the maxillary plate by means of two heavy S-shaped wires. Unlike many activator type appliances which are constructed with the mandible in a protruded position, this appliance is made from a bite registration taken in centric occlusion. According to R. Lehman , the S-shaped wires are activated by approximately 2 mm every 4 to 6 weeks, to achieve a gradual advancement of the mandible.

58. Teuscher-Stockli activator/headgear combination appliance : A modified activator used in combination with a high-pull headgear. The appliance was introduced by U.M. Teuscher and P.W. Stockli as a means to avoid the detrimental profile effects of cervical traction while treating Class II malocclusions in growing individuals. Buccal headgear tubes are incorporated in the interocclusal acrylic at the level of the maxillary second premolar or first molar. The vector of the high-pull headgear force is directed through a point midway between the estimated center of resistance of the maxilla and that of the maxillary dentition .

59. In this way it is claimed that the best compromise is reached between a resulting counter-clockwise rotation of the maxillary occlusal plane and a clockwise rotation of the maxilla itself, possibly maintaining the inclination of the maxillary occlusal plane. The design includes reduced palatal acrylic coverage to provide more space for the tongue. The acrylic covers the occlusal and incisal surfaces of the maxillary teeth to distribute the headgear force over the entire dentition. The labial bow can be substituted by torquing springs to counteract palatal tipping of the maxillary incisors. Long lingual flanges extend from the lower portion of the appliance to enhance forward positioning of the mandible. In addition, Frankel-type lower lip pads may be added to enhance normal perioral muscle function. Finally, a jackscrew is added occasionally for controlled expansion.

60. Van Beek Appliance : A construction bite is taken in an end-to-end position, if possible, and with the teeth approximately 5-8 mm apart. The upper part of the appliance covers the front teeth upto the gingiva for torque control. The outer arms are short, ending just in front of the canines, and extend far enough laterally to avoid soft tissue irritation from the elastics. Molars and premolars are half covered, lateral grinding provides optimum expansion .

61. The Twin Block Traction Technique : In most cases, full functional correction can be achieved with twin block alone. In a minority of the cases, Severe maxillary protrusion. To control vertical growth pattern. The Concorde Facebow : A method was developed to combine extraoral and intermaxillary traction. Concorde facebow helped in restricting maxillary growth, at the same time encouraged mandibular growth in combination with the functional appliance. Patient comfort and acceptance was similar to the conventional facebow.

62. 1. W.J. Clarke. The twin block technique - A functional orthopedic appliance system. Am J Orthod Dentofac Orthop . 93 , 1988, 1-18. 2. W.J. Clarke. The twin block traction technique. European Journal of Orthodontics 4 (1982) 129-138.

63. The labial hook is positioned extraorally 1 cm clear of the lips. Traction component are worn only at night. Careful selection of case is very essential.

64. HEADGEAR INJURIES The results of headgear wear can be very beneficial to the patient, but documented reports of headgear trauma to the face and eyes demonstrate the potential for serious injury. In 1975, the American Association of Orthodontists (AAO) issued a special bulletin to its members urging them to take precautionary measures to eliminate accidental patient injuries. In that same year, the AAO also contacted manufacturers to explore the feasibility of making a safer headgear design. In 1984, the California State Society of Orthodontists highly recommended the exclusive use of safety facebows and breakaway headgear systems .

65. The incidence is very low, but when an eye injury does occur it can have serious consequences. Because wounds are contaminated by oral bacteria and are difficult to treat, eye injuries may result in impaired vision or even loss of the eye. Another serious consequence can be sympathetic ophthalmitis . Injuries from headgear can occur due either to recoil, where the appliance is actively pulled, or as a result of accidental disengagement, particularly during sleep. The dangers arising from accidents during play or incorrect use are known. Manufacturers now produce safety neck straps and anti-recoil headgear, but even so, anti-recoil devices alone do not make headgear totally safe. It has been reported that the greatest incidence of disengagement occurs at night and it is therefore essential when using headgear at night that the headgear is securely attached to the appliance .

66. Safety Mechanisms It is important that, where possible, all headgear has two safety mechanisms: one to prevent accidental disengagement and the second to prevent recoil injuries. It is the orthodontist’s responsibility to select the most appropriate combination of headgear safety features for each case. Safety Mechanisms and Fixed Appliances : Prevention of accidental disengagement : Rigid neck strap: Prevents the forward movement of the face bow, but must be correctly fitted to prevent the bow disengaging from the buccal tubes and the safety strap from the bow. This may be too tight for some patients to tolerate.

67. Locking mechanisms: These ensure that the face bow cannot be removed from the buccal tubes, but they may be difficult for the patient to fit and remove. Locating Elastics: Short strong elastics may be used between hooks on the inner bow and the buccal tubes to reduce the likelihood of disengagement. They may be variable in effect, and may present difficulties for the patent to fit and remove. Prevention of recoil injuries : Anti-recoil devices: These are designed to 'break-away' when excessive force is applied to the headgear. However, there is considerable variation in the amount of force required to activate the break-away mechanism and they do not prevent disengagement of the face bow from buccal tubes .

68. Rigid neck strap: prevents the forward movement of the face bow, but must be correctly fitted to prevent disengagement from buccal tubes - this may be too tight for some patients to tolerate. Other Safety Mechanisms : 'Safe ends': These do not prevent the accidental removal of the face bow from the buccal tubes, but provide a blunt end which may reduce the incidence of penetrating injuries

69. Safety Mechanisms and Removable Appliances : Integral face bow: When practical, consideration should be made to having the face bow constructed as an integral part of the removable appliance . Locking mechanisms: Mechanisms similar to those used for fixed appliances may be used to positively locate the face bow to the appliance. When face bows are used in conjunction with removable appliances, the patient should be instructed to fit the face bow with the appliance out of the mouth 1. Chaushu , G, Chausu , S, Weinberger, T: Infraorbital abscess from orthodontic headgear. Am J Orthod Dentofac Orthop . 112 , 1997, 364–366 . 2. Stafford , GD, Caputo, AA, Turley, PK: Characteristics of headgear release mechanisms: Safety implications. Angle Orthod . 68 , 1998, 319–326 .

70. Remove the headgear before the inner bow. Never remove or fit the headgear in one piece by pulling the headgear over the face/head. Do not wear headgear while playing sports or rough games. At night always ensure that the safety mechanism(s) are in place to prevent accidental removal of the headgear and face bow. If the headgear comes detached during sleep, stop wearing the headgear and contact your orthodontist. If any eye injury associated with the headgear occurs, it must be treated as a medical emergency. Attend your local Accident and Emergency Department for an ophthalmic opinion as soon as possible. Bring your headgear to each appointment and report any problems to your orthodontist.

71. Safety release headgear with spring mechanism which breaks apart when excessive force is applied .

72. Lee KG, Ryu YK, Park YC, Rudolph DJ. A study of holographic interferometry on the initial reaction of the maxillofacial complex during protraction. Am J Orthod Dentofacial Orthop . 1997; 111:623-32 .

73. REFERENCES Biomechanics in orthodontics. Michael R. Marcotte (1 st Edition). Orthodontics and Dentofacial Orthopaedics. James McNamara Jr and William Brudon (1 st Edition). Contemporary O rthodontics. William R. Proffit , Henry W. Fields and David M. Sarver (5 th Edition). Biomechanics in Clinical Orthodontics . Ravindra Nanda . 1001 Tips in Orthodontics and its Secrets . Esequiel Eduardo Rodriguez Yanez . Textbooks of Orthodontics. Gurkeerat Singh (2 nd Edition).