Role of Hyoid Bone Position in Maintaining Upper Airway Patency

Abstract

The hyoid is a mobile bone located at the base of the tongue. It is the point of insertion for many of the muscles that control the geometry and mechanical properties of the upper airway (UA). Consequently, the hyoid is likely essential in maintaining upper airway patency, particularly when exposed to inspiratory negative pressures. Recurrent collapse of the upper airway during sleep is characteristic of obstructive sleep apnea (OSA), a highly prevalent respiratory sleep disorder with serious health risks. An inferiorly positioned hyoid is the most consistently observed anatomical difference between people with OSA and healthy controls. Surgical hyoid repositioning procedures are being performed to treat OSA patients, but outcomes are highly variable, primarily because of the lack of knowledge regarding the precise influence of hyoid position on upper airway function. The aim of this thesis is to determine the role of hyoid bone position in maintaining upper airway patency. Seven anaesthetized, male, New Zealand White rabbits were tracheostomized and left to breath spontaneously via the caudal trachea (i.e. no UA airflow or muscle activity). The hyoid was repositioned within the mid-sagittal plane along cranial, caudal and anterior directions from baseline (0mm) to 5mm in 1mm increments, and along 30°, 45° and 60° cranial and caudal directions. These movement were performed using a custom-made device. The effect of hyoid position on upper airway patency was quantified using the upper airway closing pressure, Pclose (the negative pressure required to close the upper airway). On the basic premise that the more Pclose is negative, the less the upper airway is prone to collapse. Data were expressed as a change from baseline (0mm). Repeated measures ANOVA tests were performed to assess changes in key outcomes with hyoid displacement. Statistical significance was inferred for p <0.05. The baseline Pclose before any hyoid repositioning was -3.55 ± 0.95 cmH2O (mean± SD). The anterior displacement of the hyoid resulted in the greatest decrease in Pclose amongst all directions (p=0.002). Pclose decreased progressively with each increment of anterior hyoid bone displacement, going down by 3.98 ± 1.31 cmH2O at 5mm. In contrast, when the hyoid was moved cranially or caudally, Pclose did not change from the initial baseline value (p= 0.723 and p=0.352 respectively). When the hyoid was moved in anterior-cranial 45° and anterior-caudal 45° directions, Pclose decreased significantly (p=0.001 and p=0.004 respectively) and at similar magnitudes to the anterior direction (p>0.05). Also, the anterior-cranial 60° and anterior-caudal 60° directions decreased Pclose by 3.81 ± 0.78 cmH2O and 4.29 ± 1.68 cmH2O respectively, which is similar to the improvement obtained with the anterior direction. The anterior-cranial 30° and anterior-caudal 30° improved Pclose to a lesser extent (2.22 ± 0.58 cmH2O and 2.73 ± 0.85 cmH2O respectively). This is the first study to quantitatively examine the influence of hyoid bone position on upper airway patency. Study outcomes suggest that hyoid surgical interventions for OSA treatment and possibly mandibular advancement oral appliances should focus on increasing the amplitude of anterior hyoid repositioning as a main contributor to improved upper airway patency.