Mandibular advancement devices in obstructive sleep apnea: an updated review
Obstructive sleep apnea (OSA) is the most prevalent sleep-disordered breathing in the adult
population and if untreated remains a significant cause of morbidity and mortality. Continuous
positive airway pressure (CPAP) therapy is still the gold standard treatment for OSA, but patient
acceptance and adherence are often poor due to a multitude of factors, thereby compromising
treatment success. Mandibular advancement devices (MADs) have been proposed not only as
a first line therapy for symptomatic snoring patients, but also for those suffering from mild to
moderate OSA, or those who refuse or do not tolerate CPAP. Yet, improved understanding of
MAD regarding design, construction, and mechanisms of action is an important requirement to
successfully implement MAD as a therapeutic tool. Therefore, the main focus of this paper is to
focus on the general concepts and mechanisms of action of MAD, while highlighting important
characteristics in the context of their use as a viable and effective treatment option for OSA patients.
Obstructive sleep apnea (OSA), the most prevalent clinical entity in the spectrum of sleep-disordered breathing
among adults and children, is defined by the presence of repeated episodes of complete or partial obstruction of the upper airways during sleep, despite ongoing respiratory efforts. Its high prevalence, currently estimated as affecting nearly 1 billion people worldwide, imposes significant morbidity and mortality, such that OSA has clearly emerged as a major public health issue, while leading to major financial and social burden on both healthcare systems and society in general.
The daytime consequences of OSA include a variety of symptoms, the most insidious being excessive daytime
sleepiness (EDS), neurocognitive and behavioral impairments, and mood disorders. In addition, OSA has clearly emerged as an independent risk factor for cardiovascular and metabolic diseases, such as hypertension, coronary artery disease, and endothelial dysfunction leading to myocardial ischemia, cardiac arrhythmias, stroke, as well as promoting or exacerbating dyslipidemias, insulin resistance, and type 2 diabetes mellitus.
The pathophysiology of OSA is complex, involving a multitude of genetic, craniofacial - anatomical, neuromuscular,
and inflammatory factors whose contributions differ from patient to patient13. Furthermore, OSA and insomnia may frequently co-occur challenging the usual standards of care14. In an effort to preserve the patency of the upper airway during sleep, various treatment modalities have been proposed. Continuous positive airway pressure therapy (CPAP), initially developed by Sullivan et al., in the early 1980s, is still considered the most efficient
therapeutic approach for OSA, since it stents the upper airway open throughout the respiratory cycle. Accordingly, CPAP has not only established itself as the gold standard and first line of treatment but has further shown its efficacy in a multitude of trials in which improvements in EDS, quality of life, and reductions in systemic blood pressure have clearly emerged. Despite these benefits, patient acceptance, and adherence of CPAP is often poor, with attendant consequences.
Oral sleep medicine field and mandibular advancement devices
Oral sleep medicine (OSM) is an area of dental medicine that investigates and diagnoses sleep-disordered breathing (SDB) and its oral and maxillofacial determinants, while also exploring its consequences on general health and sleep. Increased recognition of SDB has resulted in major expansion of maxillofacial surgical techniques and approaches aimed at correcting the underlying respiratory disturbance during sleep. In parallel, OSM has led to the development and to an incremental use of oral appliances (OA). Among the many types of OA, mandibular advancement devices (MAD), and tongue retainers (TR), have been incorporated as the most commonly used types of intraoral devices. These types of appliances underwent substantial developments over time and have been adopted as a standard therapeutic option in the clinical practice of sleep medicine. However, TR have not been extensively evaluated, are considered less effective than MAD and have been associated with lower patient adherence when compared to MAD. Notwithstanding, TR are still indicated for partial or total edentulous patients who use removable prosthetic dentures, which restrict MAD use, and may be also of benefit among those patients who have limited mandibular protrusion, with a large tongue, or those suffering from acute temporomandibular disorders, or from advanced periodontal disease. The primary focus of this paper is to discuss the general concepts and mechanisms of action that govern development and application of MAD as a treatment option for patients suffering from OSA.
MAD: concepts and mechanisms of action
MAD developed for treatment of sleep-disordered breathing are devices used in the oral cavity during sleep with the purpose of preventing the collapse between oropharyngeal tissues and the base of the tongue. In general, MAD are geared to generate mandibular advancement and stabilization during sleep by promoting anterior traction displacement of the mandible with subsequent increases in the tension of the genioglossus muscle and the supra and infrahyoid muscles, expanding the air space in the pharyngeal region.
A recent study evaluated skeletal/dental changes in a three-dimensional form using cone beam tomography during the use of MAD. It has been shown that mandibular protrusion promotes a linear vertical increase between the mandible and the maxilla and an anterosuperior displacement and rotation of the hyoid bone. Both of these features can assist professionals in deciding on the best candidates for this type of treatment. Treatment with MAD aims to maintain the upper airways open during sleep by decreasing its resistance as well as the frequency and/or duration of the apneas and hypopneas, arousals related to respiratory effort, and snoring events. In addition, MAD improve nighttime oxygenation at all levels of disease severity in adult patients26 with benefits upon the social and adverse health consequences of OSA and snoring (e.g., decreasing daytime sleepiness and improving quality of life).
Treatment of OSA with MAD improves both subjective and objective measures of excessive daytime sleepiness. Yet, the obtained subjective improvement may be also attributable to a placebo effect.
A meta-analysis of seven randomized controlled studies confirms a modest, but rather significant benefit of MAD treatment on blood pressure. The impact of treatment on other cardiovascular endpoints, such as cardiovascular events and mortality, remains unresolved.
Since the first commercially available oral devices were introduced in the 1980s there was a proliferation in
the development of several models of MAD but the lack of standardization for device design often hampered the
interpretation regarding comparisons in clinical use and research results. The main types of FDA approved MAD for use in adult OSA treatment. MAD can be made of distinct materials and may have different designs, some of them with the ability of a progressive mandibular advancement and lateral movements. MAD are classified as prefabricated and customized37-39 in a single block (monoblock) or two blocks, which can be adjustable with some freedom in lateral movements. MAD customization also involve material choice, which must be adapted to the oral structure and physical needs for each patient.
Prefabricated MAD tend to be bulky with some challenges regarding its retaining capacity on a stable mandibular
protrusive position during sleep. Therefore, this type of MAD are more prone to loss efficacy and lead to patient’s
discomfort. Nevertheless, a recent study demonstrated the efficacy of a titratable thermoplastic MAD in reducing OSA and related symptoms in patients with mild to severe disease presentation, but more studies are still needed to evaluate their efficacy in OSA treatment.
The use of a custom-made MAD (MADc) has been associated with increased patient-reported comfort, greater range of protrusive movement, and higher therapeutic effectiveness.
MADc are tailored to the patient’s dentition in a laboratory- controlled advancement. This kind of appliance can be adjustable (bi-block) or non adjustable (monoblock). Clinically, non-adjustable MAD are made in a fixed protrusive position remaining unchanged during treatment, while the protrusive position of the adjustable MAD allows progressive advances (e.g, titration process) aiming to increase the treatment efficacy and the patient’s comfort and quality of life.
In the design of a non-adjustable MAD there are a variety of coupling mechanisms such as elastic straps, lateral fins, bars, telescopic rods, springs, and tube connectors among others. The differences in the designs affect effectiveness and comfort. Devices with lateral coupling mechanism offer greater comfort than other systems because they allow mandibular lateral and protrusive movement while maximizing the space available for the tongue. Yet, the coupling mechanisms that allow a certain degree of opening and prevent the jaw from moving backwards at any time while opening the mouth are more effective. Mouth opening can be limited by
the integration of elastic bands in the design.
Although the range of protrusive movement differ between individuals, studies indicates that the ability to advance
the jaw is a key feature in MAD treatment success. This mostly depends on the amount of progression needed to reduce apnea-hypopnea index (AHI) and snoring. Nevertheless, a precise linear relationship between mandibular advancement and treatment success has not yet been shown. The mandibular protrusion mechanism must allow
advancement in increments of 1mm or less, with a minimal range of 5mm. Small increments allow the assessment of important parameters, pursuing to minimize, for example, the temporomandibular discomfort and to improve comfort. Current data support an initial position of, at least, 50% of the patient’s protrusive range, but there is no consensus as to whether this should be measured from an initial position of maximum intercuspation or maximum retrusive mandibular position. The mechanism should be reversible in order to adapt to the patient’s health related dynamics and to adequately manage side effects. Vertical adjustment has been another controversial subject in
the design of oral appliances25. Although with a lack of consensus, some studies have suggested that increased vertical dimension (inter-incisal distance) resulted in reduced patient acceptance. Indeed,
an increase in inter-incisal distance during mandibular protrusion with MAD was shown to result in a slightly more retrusive position caused by posterior rotation of the mandible. This finding lead to the standard of avoiding vertical increase as much as possible.
Conclusions and future perspectives
Current literature provides robust evidence that adjustable and personalized double-arch oral mandibular advancement devices are highly effective for the treatment of snoring and mild to moderate OSA. Although less effective than CPAP for improving AHI in moderate to severe OSA, several recent studies have found that oral appliances and CPAP were equally effective in improving daytime sleepiness, hypertension, neurocognitive function, quality of life, and cardiovascular mortality. Further studies are needed to establish the impact of different models of devices available on therapeutic success and patient compliance as well as related side effects.
A qualified MAD provider needs to have the necessary skills to choose, adjust and manage the side effects of the most appropriate device. MAD treatment protocol should only start after a medical evaluation based on standard clinical, physical and polysomnographic parameters. A careful candidate selection must be carried out by both the sleep specialist and qualified MAD provider in order to achieve a higher therapeutic success rate. Still, the development of tools to identify individual phenotypes and the combination of two or more therapies to obtain synergistic additive effects will be required for the adoption of an optimal personalized OSA treatment