The Role of Hyoid Bone Repositioning and Mandibular Advancement in Maintaining Upper Airway Patency

Abstract

The hyoid is a mobile bone located at the base of the tongue. The mandible is the lower jawbone that hinges with the skull. Both bones are points of insertion of many upper airway (UA) muscles and are essential in maintaining UA patency. This is important as recurrent collapse of UA during sleep is characteristic of obstructive sleep apnea (OSA), a highly prevalent respiratory disorder with serious health risks. Indeed, OSA patients might have an inferiorly positioned hyoid and/or a retruded mandible. Surgical hyoid repositioning and/or mandibular advancement therapy(MAT) using a splint are viable options to treat OSA. However, MAT is only successful in ~50% of the cases. In fact, hyoid bone position may influence the outcome of MAT and may affect UA differently. Nevertheless, the precise influence of hyoid position and mandibular advancement altogether on UA function is unknown. The aim of this thesis is to determine the effect of hyoid position and mandibular advancement on UA patency. Twelve 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 mandible was repositioned anteriorly from 0 to 4mm with 2mm increments, and along 70° vector directions using a customized mandibular advancement splint (MAS). Moreover, the hyoid was repositioned also from baseline within the mid-sagittal plane (anterior, cranial and caudal) from 0 to 4mm with 2mm increments, and along 45° vector directions using a customized device. The effect of hyoid and mandibular repositioning on upper airway function was quantified using the upper airway closing pressure Pclose (the negative pressure required to close the upper airway). Repeated measures three-way ANOVAs were performed to assess changes in key outcomes with hyoid and mandibular movements. The test of within-subjects was used to evaluate changes resulting from independent variables. The pairwise comparison was used to assess which pairs of conditions significantly differ from one another. Statistical significance for all the above analyses was inferred for P less than 0.05. The interaction between mandibular advancement, hyoid repositioning direction and increment was statistically significant (p<0.001). The anterior movement of the hyoid and mandibular bones yielded the best outcome and the upper airway collapsibility decreased progressively when both bones were moved from 0 to 4mm. The more the hyoid and mandibular bones were advanced, the more P close was negative yielding a less collapsible upper airway. In contrast, when the hyoid bone was moved cranially or caudally, Pclose obtained was significantly higher at all incremental levels in comparison to Pclose obtained with the anterior component displacement. Mandibular advancement of 4mm combined with ant-cranial 45° hyoid repositioning decreased upper airway collapsibility the most in comparison with other interventions. This will be the first study to mechanistically examine the combined influence of mandibular advancement and hyoid bone repositioning on upper airway function. Final outcomes help understanding the role of the hyoid bone in MAT, and suggest potential combined therapy approaches to treat this serious health condition.