Third molar impaction has been a concern for a very long time. Among all teeth, the most commonly impacted teeth are the third molars[1, 2] with the maxillary third molars having the highest rate of impaction. The etiology of impaction is variable but could be explained by space deficiency, inadequate skeletal bone growth, unfavorable direction of eruption, root configuration or the large mesiodistal width of the impacted tooth. The space available for their eruption usually depends on the remaining space available after the eruption of all teeth. This retromolar space is affected by the growth potential, the amount of crowding, and the mesial migration of the dentition.
In the maxillary arch, posterior arch lengthening occurs by bone apposition at the maxillary tuberosities. Peak maxillary growth is about eight to nine years in girls and ten to eleven years in boys. Growth slows to adult levels, on average, at about 15 years in girls and at about 17 years in boys. That growth could be attributed to further remodeling of the maxillary tuberosities. Vardimon et al in 2010 stated that the growth of the maxillary tuberosity continues to the age of 20 years and that it is directly related to dental development of the 2nd and 3rd molars.
Third molar impaction was not as frequent in ancient skeletons. This could be caused by interproximal wear attributed to the type of food they used to chew resulting in mesial drift of the dentition.[9, 10] Similarly, extraction of second molars have shown to have a favorable effect on the third molar eruption and the retromolar space.[11, 12] This is due to creating a greater space in the arch posteriorly for the third molar eruption.
Various treatment modalities in orthodontics might have variable effects on the available space for the maxillary third molar eruption. Premolar extraction, for example, might result in mesial migration of the 1st and 2nd molars, increasing the available space for the eruption of 3rd molars and decreasing the likelihood of impaction or eruption in an unfavorable angulation related to the occlusal plane. Moreover, researchers have found that premolar extraction increased the U6 to pterygoid vertical (PTV) distance by 3 mm and reduced the impaction rate of maxillary third molars.[14, 15]
With the theory of the retromolar space in mind, some orthodontic treatment options may negatively affect the available space for the third molar eruption, such as those causing distalization or inhibition of the mesial migration of the dentition whether this is achieved by the use of extraoral devices such as headgears or any intraoral distalizer.[16–18] Furthermore, this might be influenced significantly if there is minimal growth potential or lack of development of the tuberosities.
Regarding the effect of headgear, a study conducted by Ricketts in 1960 evaluated the effect of different orthodontic treatment modalities on the growing orthodontic patient, one of which were Class II cases treated with cervical headgear and compared them to a Class II control group. He found that in a 27 to 30 month period, the maxillary first molars were mesialized in the control group and distalized in the headgear group. A similar pattern was seen in another study, however, their sample at the follow up time point was around 17 years of age which is very early to judge the eruption of third molars. A recent study looked at the before and after records of orthodontic patients treated with headgear for the correction of Class II relationship. They stated that the third molar eruption space was less in patients treated with headgear than the control group. .
Although some studies evaluated the efficiency of different orthodontic appliances used for distalization and their effect on the U6 to PTV distance[18, 22–24], only a few looked at the effect of headgear on that space[20, 21, 25], and none have evaluated the long term effect after the average age of third molar eruption. Thus, the objective of this study was to examine the short and long term effects of headgear on the eruption space of the maxillary third molars and to examine the actual eruption status of maxillary third molars at the long term follow up.