Mechanisms Of Action

The prevailing view has been that the primary mechanism of action of MAS arises from the anterior movement of the tongue, and the consequent increase in the anteroposterior dimensions of the oropharynx. It now appears that this is an overly simplistic view, based on a growing number of studies that indicate rather more

FIGURE 2 (See color insert.) An example of a duobloc (two-piece) mandibular advancement splint (SomnoMed MAS™, Australia), comprising separate upper and lower plates, with a unique "fin" coupling mechanism that permits the full range of mouth opening and closing, limited lateral movement, and titratable mandibular advancement.

FIGURE 2 (See color insert.) An example of a duobloc (two-piece) mandibular advancement splint (SomnoMed MAS™, Australia), comprising separate upper and lower plates, with a unique "fin" coupling mechanism that permits the full range of mouth opening and closing, limited lateral movement, and titratable mandibular advancement.

complex anatomical changes. Such studies have used a range of imaging modalities, including computerized tomography (6), magnetic resonance imaging (MRI) (7), and nasopharyngoscopy (8). Not surprisingly, airway volume increases with mandibular advancement. Of some surprise has been the consistent observation of an increase in cross-sectional area of the velopharynx, in both the lateral and antero-posterior dimensions, and increases in the lateral dimension of the oropharynx (Fig. 3). These changes are thought to be mediated through the intricate linkages that exist between the muscles of the tongue, soft palate, lateral pharyngeal walls, and the mandibular attachments. In particular, it has been proposed that the improvement in velopharyngeal dimensions is mediated through stretching of the palatoglossal and palatopharyngeal arches (9). Notably, it appears that there is interindividual variability in the airway configurational changes that occur with mandibular advancement, and this is likely to have major relevance to the variable clinical response associated with this treatment modality.

There remains uncertainty about the extent to which oral appliance effects are mediated through neuromuscular pathways. Whilst there are some studies indicating that oral appliances stimulate genioglossus muscle activity (10,11), the clinical significance of this has not been borne out by "placebo" controlled studies using inactive oral appliances, which have shown little change in sleep-disordered breathing parameters (12,13). This suggests that the primary mechanism of action is mechanical rather than neuromuscular. The mechanical effect results in greater airway stability, evidenced by reduced upper airway closing pressure during sleep (14). In a study of anesthetized OSA patients, Kato et al. (15) found a dose-dependent reduction in closing pressure of all pharyngeal segments.

The mechanism of action of TRD is likely to be a little different compared with mandibular advancement devices. The forward movement of the tongue out of the oral cavity tends to be greater than the tongue advancement achieved with a mandibular advancement device (Fig. 4), and this may produce more favorable anatomical changes in the retroglossal region (Fig. 3). In addition, it is possible that they counteract the effect of gravity on the tongue in the supine position.

A useful conceptual model for understanding the mechanism of action of an oral appliance is to consider the upper airway as a lumen, surrounded by soft tissue, and contained within a bony box (16). According to such a model, the shape and

FIGURE 3 (See color Insert.) Axial magnetic resonance imaging scans at the retroglossal level at baseline (A), with a mandibular advancement splint (B), and with a tongue retaining device (C) in the same patient, taken in the awake state, showing enlargement of cross-sectional area mediated by changes in both anteroposterior and lateral dimensions. Note the difference in airway changes between the two oral appliances.

FIGURE 3 (See color Insert.) Axial magnetic resonance imaging scans at the retroglossal level at baseline (A), with a mandibular advancement splint (B), and with a tongue retaining device (C) in the same patient, taken in the awake state, showing enlargement of cross-sectional area mediated by changes in both anteroposterior and lateral dimensions. Note the difference in airway changes between the two oral appliances.

FIGURE 4 (See color insert.) The effect of a tongue retaining device in holding the tongue out of the mouth by use of a suction cap.

size of the airway is determined by the surrounding tissue pressure, which in turn is determined by the amount of tissue contained within the box. Hence one would predict that mandibular advancement would reduce tissue pressure by enlarging the box. This has been observed in an animal study, which found that mandibular advancement reduced tissue pressure and upper airway resistance (17). In contrast, one would hypothesize that TRD reduce the amount of tissue in the box by pulling the tongue out of the mouth, thereby reducing tissue pressure.

Sleeping Sanctuary

Sleeping Sanctuary

Salvation For The Sleep Deprived The Ultimate Guide To Sleeping, Napping, Resting And  Restoring Your Energy. Of the many things that we do just instinctively and do not give much  of a thought to, sleep is probably the most prominent one. Most of us sleep only because we have to. We sleep because we cannot stay awake all 24 hours in the day.

Get My Free Ebook


Post a comment