The physiological position of the mandible during sleep is slightly opened (1-5) mm and in patients with OSA is opened more than 5 mm and can reach up to 10 or 15 mm [19,20]. The opening of the mandible induces mandibular retrusion, which is associated with an increase in collapsibility of the upper airway [21] and the reduction of the efficacy of the MAD [22]. Following this though, this study wanted to analyze the different opening movement paths and their impact on the mandibular position according to the craniofacial morphology of the patient.
During normal sleep, the mouth is in a position known as the mandibular rest position or freeway space. The freeway space is described as the space between the maxillary and mandibular occlusal surfaces when the mandible is in the rest position and should be 1-5 mm [23], an opening of up to 5 mm for 88.9% of total sleep time [19]. Increasing oscillating lowering movements of the mandible in response to the airway collapse during obstructive apnea have been described [24-26]. In patients with OSA, the mouth opening is greater than 5 mm for 69.3% of total sleep time [26]. A common pattern characterized by a gradual opening followed by a quick closure of the mouth, generally after an arousal, has been described in normal patients and patients with OSA [19,24,26]. For this reason the disoccluding angle of 5 mm that measures the normal opening in healthy patients and 10 mm that measures the opening in patients with OSA were obtained. The mandibular position and related structures are influenced by and participate in patency of the pharynx and the complex mechanisms that lead to obstruction of the upper airway [16]. Mandible opening during sleep causes mandibular posterior rotation [27] and is associated with a reduced cross-sectional area of the lumen [16], reduced mechanical efficiency of the pharyngeal dilator muscles [24] and increased resistance and collapsibility of the upper airway [14,21,28]. All of which may contribute to sleep-related breathing abnormalities.
The findings of the present study show/suggest that the slope or angle of the mandibular movements are related to the craniofacial morphology with higher vertical anterior and shorter anteroposterior faces with a more horizontal path of mandibular movements than shorter vertical anterior and longer anteroposterior subjects who have a more vertical path (Fig 5). For an opening of 5 mm with an angle of 77º, the mandible retrudes 1.15 mm and with an angle of 63º is 2.55 mm. This is double for the same amount of opening at 5 mm. A similar scenario is present at the 10 mm opening where the retrusion at an angle of 75º is of 2.68 mm and at 61º is 5.54 mm which is close to a 3 mm difference.
The paths of mandibular movements are more horizontal in higher vertical anterior faces with a wider path of jaw opening on the horizontal axis; this finding has relevant clinical and biomechanical implications that can be applied in OSA patients and in the use of MADs. Low opening angles give more horizontal paths of mandibular movement, meaning that mandible retrudes more as the mandible opens than in those with more vertical direction. In patients with more horizontal paths, if we apply clinically the results of our study, the appliance selected should be designed to use a minimal increase in vertical opening and should control the direction and limit the amount of jaw opening once the appliance is in place and the patient falls asleep (Figure 6). All this is in order to reduce rotation and retrusion of the mandible.
Our results are similar to L’Estrange et al. [29] where they found a smaller effect on the oropharynx in subjects who had a reduced lower facial height. They found that in these cases, maximal mandibular protrusion had a minimal increase of vertical opening and the mandibular symphysis was further forward in relation to the posterior wall of the pharynx. The path of the mandible during aperture started in a position with the lower incisor much further forward and much closer to the cranium than in other subjects. Therefore, the mandible has a longer path to travel before it reaches the point where the airway begins to occlude. Subjects with higher vertical anterior faces and shorter anteroposteriors have an opening path beginning further backwards and downwards. As found in our study, any increase in the vertical dimension would quickly retrude the mandible and occlude the oropharynx [29].
MADs place the mandible in a determined anteroposterior and vertical position to improve the upper airway cross-sectional area, due to a combination of both their effect of the protrusion of the mandible, and for their capability to stabilize the mandible [30-32]. There are different MADs available in which control of the mandibular position with respect to the potential of mouth opening, especially in the supine position, depends on the MAD design [33]. A constantly larger mouth opening during sleep will reduce the efficacy of this treatment [22,28,34]. Two-piece appliance designs, which allow uncontrolled opening of the mandible, have shown a lower response rate in positional OSA [35], while appliances with limitation of mandibular opening are more effective in decreasing AHI [36]. Therefore, as shown in recent studies [18],the devices should incorporate vertical control in the design to not allow the jaw to move backwards at any time while opening the mouth, and as suggested by our study, consider the anatomic and kinematic of the mandible. Our results enhance recent studies that have identified the importance of phenotypic characteristics on treatment response, and suggest the relevance of MAD design features considering the kinematic behavior of the mandible as part of a personalized approach to treatment [18,30].
Limitations of this study are the inclusion of young adult population and not OSA patients. The use of young adult population allows us to determine normal range of craniofacial morphologies and their movements that can be applied in future studies with OSA patients. Another limitation is the use of lateral cephalometric radiograph for the analysis of mandibular movements. The radiograph was taken with the patient awake and in an upright position. The range of motion and the position of the mandible may be altered when the patient is asleep and should be consider when analyzing the changes in upper airway if measured with lateral cephalometric radiograph.