Invited Review Paper Orthognathic Surgery Biomechanics of the upper airway: Changing concepts in the pathogenesis of obstructive sleep apnea S. M. Susarla, R. J. Thomas, Z. R. Abramson, L. B. Kaban: Biomechanics of the upper airway: Changing concepts in the pathogenesis of obstructive sleep apnea. Int. J. Oral Maxillofac. Surg. 2010; 39: 1149–1159. # 2010 International Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved. S. M. Susarla 1 , R. J. Thomas 2 , Z. R. Abramson 1 , L. B. Kaban 1 1 Department of Oral and Maxillofacial Surgery, Massachusetts General Hospital and Harvard School of Dental Medicine, Boston, MA, USA; 2 Department of Medicine, Division of Pulmonary, Critical Care & Sleep, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, MA, USA Abstract. Obstructive sleep apnea (OSA) is a disorder characterized by repetitive, episodic collapse of the pharyngeal airway. Over the last two decades, understanding of the pathophysiology of sleep disordered breathing, which includes OSA, has improved. Once thought to be predominately related to anatomic constriction of the maxillomandibular complex, central nervous system regulation of breathing is now recognized as a significant contributor to the pathogenesis of OSA. Ventilator control, the central response to chemoreceptor phenomena, has important implications for oral and maxillofacial surgeons who treat OSA, particularly for patients who appear refractory to treatment with maxillomandibular advancement (MMA). The purpose of this article is to review the biomechanics of the upper airway as it relates to the pathophysiology of OSA, to discuss emerging concepts of ventilator control mechanisms in normal sleep versus sleep-disordered breathing and to discuss the concept of complex sleep apnea, a new category of sleep disordered breathing with both obstructive and central features. Keywords: obstructive sleep apnea; airway dynamics; maxillomandibular advancement; orthognathic surgery. Accepted for publication 13 September 2010 Available online 27 October 2010 Obstructive sleep apnea (OSA) is a well- recognized disorder that affects at least 4% of the population in the USA 36,99 . It is characterized by repetitive collapse of the upper airway, commonly defined as the soft tissue region bounded by the nasopharynx superiorly, the epiglottis inferiorly, the maxillomandibular com- plex anteriorly and the spinal column pos- teriorly. Collapse of the upper airway decreases its intra-luminal diameter and increases airway resistance, in accordance with Pouiseille’s Law (Fig. 1). This increased resistance to flow results in reductions in ventilation (hypopneas) or complete cessation of ventilation (apneas), with corresponding metabolic disturbances (respiratory acidosis due to hypoventilation) and sleep fragmentation. Compensation for metabolic disturbances occurs primarily in the form of alterations in respiratory effort, which is the primary determinant of arousals from sleep. While brief transitions to the wake state provide ventilator compensation to correct blood gas abnormalities, this comes at the cost of further destabilizing respiratory control at the sleep–wake interface. Concomitant activation of the sympathetic nervous sys- tem results in increases in blood pressure, which, over the long term, have been linke to increased risk for sudden death, stroke and myocardial infarction 5,12,19,49,58,59,95 . Sleep-event related arousals produce fragmentation of sleep patterns, resulting in excessive daytime somnolence, neu- rocognitive impairment and increased risk for accidents related to sleep deprivation 36,38,61,98 . Int. J. Oral Maxillofac. Surg. 2010; 39: 1149–1159 doi:10.1016/j.ijom.2010.09.007, available online at http://www.sciencedirect.com 0901-5027/1201149 + 011 $36.00/0 # 2010 International Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved.