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Sleep apnea is a sleep disorder generated by respiratory deficiencies based on anatomical and/or medical conditions. It is manifested by periods of breathing interruptions combined with shallow breathing, disrupting normal sleep patterns. Its symptoms vary from snoring, sleepwalking, and daytime sleepiness to irritability, difficulty concentrating, and morning headaches. Some causes for sleep apnea are obesity (which accounts for 60% of diagnosed patients), hormonal imbalance, medication, and lifestyle choices, such as smoking, drinking, etc. Recent statistics reported by the American Sleep Apnea Association suggest that more than 22 million Americans are suffering from sleep apnea, with as many as 80% undiagnosed, and between 1% and 5% of children of all ages experiencing symptoms of obstructive sleep apnea.
This sleep disorder can be categorized as central sleep apnea, obstructive sleep apnea, or a combination of the two. Central sleep apnea (CSA) is the most acute form of this medical condition because it is caused by the failure of the brain to send signals to the breathing muscles to maintain normal function. It develops in older patients and requires more invasive types of treatment, ranging from the use of continuous positive airway pressure (CPAP) units to surgeries. Obstructive sleep apnea (OSA) means that the airway is blocked during sleeping, either by the patient's neck and throat anatomy or as a consequence of poor health. Mixed sleep apnea is a combination of both central and obstructive sleep apnea. To determine the severity and the recommended treatment, patients should be diagnosed through accredited sleep centers.
In the past, the most prescribed treatment was the use of the CPAP machine, but recently the advancements in oral and dental research have led to the development of dental appliances for sleep apnea as a viable alternative treatment. Many patients' reports suggest that they prefer dental appliances over CPAP machines because the appliances are easier to wear and maintain, are cheaper, and allow for more comfortable sleeping positions.
The treatment of OSA using dental appliances is a growing sector in the dental industry, based on the increasing number of dentists interested in providing this service to their patients. According to a 2014 study, "The overwhelming majority of patients (82%) preferred the customized OA (Oral Appliance) at the end of the study. Hence customization to a patient's dentition is a key component of treatment success."1
This article discusses the different types of dental appliances used for treatment of OSA, including design, materials, and methods of fabrication. The type of dental appliance used for treating OSA depends on the specific application and can be customized for each patient's needs. In general, there are three major categories.
Mandibular Advancement Devices
Mandibular advancement devices (MADs) are the most popular option and are designed to bring the mandible forward, opening the airway. Because these appliances open the airway, they are also used to treat snoring. To determine the mandibular protrusion, the dentist will use a protrusive bite registration gauge to measure the most comfortable and effective jaw position to open the patient's airway. This position can vary from 25% to 100% of the maximum mandibular protrusion, which can be translated in millimeters. The maximum mandibular protrusion (100%) is a reference to potential side effects, and the distance that the mandible is moved forward represents the effectiveness in opening the airway. When using a MAD, it is inevitable that the vertical dimension will be modified due to the thickness of the dental appliance. Therefore, the dentist must register the lowest vertical dimension opening for the efficiency of the appliance.2
Based on their design, MADs can be customized to meet each patient's needs.
The one-piece (monobloc) MAD is composed of the upper and the lower part fused together. It is cheaper and easier to fabricate, and it should have an anterior airway passage (opening). It does not require a coupling mechanism and it is more rigid compared to the two-piece appliance.
Two-piece MADs keep the upper and the lower parts separate, allowing for some jaw movement through the coupling mechanism. There are many types of coupling mechanisms, including posterior nylon rods of different strengths and lengths to maintain the optimal mandibular protrusive distance; posterior rods made of dental base alloys with adjustable hinges that use keys or screwdrivers for adjustments; a combination of posterior rods and elastic bands; and an anterior coupling mechanism with a wire attachment on the upper part and a hook attachment on the lower part. This type of dental appliance is the most used because of its adaptability to different designs.2
Modifications of MADs
For patients suffering from bruxism, there are variations of the MAD monobloc or two-piece appliance that restrict clinching through either the use of a soft liner inside the appliance or attached coupling mechanisms with different lengths and strengths.
Another condition that calls for special modification is lateral deviation, which is a movement of the mandible to one side during the opening or moving forward of the jaw. It is caused by problems with the teeth, the temporomandibular joint, and/or the muscles of mastication.3 In these cases, the alignment of the upper and lower plates must take the patient's lateral deviation into consideration, and coupling mechanisms must allow for this movement.
Modifications can also be made for patients presenting interdental occlusal space in the posterior teeth during mandibular protrusion movement due to the incline of the temporal eminence, also known as Christensen's phenomenon. The design of the dental appliance should compensate by adding posterior stops on the appliance at the optimal mandibular protrusion.4 This design is especially important for patients with dentures; denture instability can result unless the denture design includes compensating curves for balancing movement during the lateral excursions.
When the patient wears complete dentures, the design of the dental appliance must compensate for the enlarged tongue and masseter muscle (bilateral) resulting from the missing dentition. This can be done by increasing the thickness of the appliance on both sides where it comes in contact with the muscle, expanding the tongue space, and creating an anterior vent/airflow passage in the appliance.5
For patients that have edentulous arches and are wearing implant-retained complete dentures, the manufacturing process is yet more complex because the MAD is retained by male and female attachments. The design of this type of appliance must be based on the design and fit of the dentures in order to be comfortable for the patient. It can be one piece or two pieces with a posterior coupling mechanism such as rods and elastic bands. To increase the appliances' vertical dimension, extra acrylic is added to the occlusal surfaces of the teeth, starting posteriorly from the canines, and an airway path is inserted in the anterior position.6
Tongue Retaining Devices
A tongue retaining device (TRD) works by utilizing the vacuum produced when the tongue is retained in a forward position. The TRD is a prefabricated silicone or polyvinyl appliance that comes in three sizes (small, medium, and large) to provide for the patient's comfort and reduce lacerations on the tongue. These devices are recommended for patients that cannot tolerate pressure on their teeth, because they either are periodontally compromised and/or are complete denture patients.7-9 The TRD is also used for treating patients that have TMJ problems as an alternative to the MAD, which can extend this condition.
When the situation calls for it, TRDs can be used in combination with MADs. These monobloc dental appliances have minimal mandibular advancement and maintain the tongue in an anterior position by suction, therefore opening the airway. The prefabricated TRD size is selected based on the patient's comfort and attached to the MAD.
This type of oral appliance is used for moderate OSA cases when patients can tolerate it. However, a 2012 study published in the Journal of Clinical Medicine states, "While the addition of a TD (tongue device) may benefit some patients with limited protrusion, some of our subjects may have had a better response with further protrusion rather than the addition of the relatively uncomfortable TD. A further limitation of TDs is that the amount of tissue in the bulb and the strength of suction is determined by the patient and cannot be controlled."10
Hybrid therapy consists of a customized MAD interfaced with a CPAP unit. The CPAP's nose-probes are connected to the MAD though a connection unit. This eliminates the CPAP's head straps, making the appliance more comfortable for patients, and can also lower the air pressure generated by the CPAP unit. The fabrication and customization of hybrid therapy devices require a good collaboration between the sleep medicine specialist or somnologist, dentist, and dental laboratory technician. This is an alternative treatment for patients suffering from moderate to severe OSA.
According to a study published in 2016 in the Journal of Dental Sleep Medicine, "Most complaints with conventional CPAP, such as stuffy nose, irritation of the mask, painful nose bridge, leakage, dry eyes, and dry mucous membrane of mouth and nose, became less of a problem when using hybrid therapy, while the swallowing of air and the presence of a headache appeared to increase slightly with hybrid therapy."11
Fabrication Methods and Materials
The treatment of obstructive sleep apnea using dental appliances is a growing sector of the industry that involves both clinical and technical knowledge working in tandem to design and manufacture appliances that will improve patients' health and well-being. Manufacturing methods can be analog or hands-on, make use of CAD/CAM systems, or combine both techniques.
The Analog Method
For the analog or hands-on method, the dental laboratory technician uses the impressions and bite registration provided by the dentist to fabricate the casts and to mount the case. The accuracy of the articulation is vital in designing a dental appliance that is comfortable and efficient in opening the blocked airway. An incorrect design can lead to either a short protrusion, which will not be efficient in reducing the apnea-hypopnea index (the average number of apneas and hypopneas that occur per hour of sleep), or a long protrusion, which can lead to temporomandibular joint problems. Additionally, an increased vertical dimension may have a negative effect on the upper airway opening.2
Following the prescription, the technician can decide the type of design, the coupling mechanisms, and any additional necessary attachments, such as an Adams clasps for posterior retention of the appliance, I-bar clasps for appliance retention, male/female attachments for fully edentulous patients, etc.
The materials used to fabricate dental appliances for sleep apnea using the analog method include PMMA, thermoplastic acrylate, polyurethane, polycarbonate, polyvinyl, silicon elastomers, cobalt-chromium alloys, and cobalt Crozat or stainless-steel wire, used for coupling mechanisms and other dental alloy attachments.12
The CAD/CAM Method
As digital dentistry progresses, new technologies are constantly being developed, and many dental software developers are now providing clinical and restorative CAD/CAM systems, including those specifically for the scanning, designing, and manufacturing of dental appliances for sleep apnea. When using this digital approach, it is important to select software programs that are compatible with each other and designed as open architecture digital systems, allowing for better integration with other CAD/CAM systems. The dental materials used for milling or printing oral appliances range from polyamide Nylon 12 (also known as PA 12) to PMMA for milling and photopolymer resin for 3D printing.
Choosing the Best Approach
In cases where a combination of different types of oral devices is critical, it is important to use the analog method because it allows better control of the outcome. On the other hand, in cases where the design is straightforward, the use of a CAD/CAM system is recommended because these systems can produce consistent quality and provide a faster manufacturing process. Following the industry trend toward digital dentistry, more dental laboratories are currently investing in this technology, and since these CAD/CAM systems can be customized to fit each business model, it is important to make use of their full potential.
One of the most important and advantageous aspects is the reduction of the amount of chairside time allowed for adjustments. The customization of oral appliances for each patient can reduce or eliminate chairside time for adjustments. Therefore, the patient's number of visits is dependent upon the quality of the dental appliance and its delivery time. However, if some adjustments are needed, these devices are relatively simple to modify due to the types of materials used in their fabrication. In evaluating the dental appliance manufacturing methods and materials, it is important to look at every aspect that is unique to each dental laboratory/practice, starting with establishing the workflow, training technicians, providing consulting, and advising services to dental professionals or partners Table 1, available at insidedentaltech.com/idt1282).
Regardless of the method used in fabricating oral appliances for sleep apnea, it is critical to use components, materials, and CAD/CAM systems approved by the FDA. According to the FDA's classification, dental appliances for sleep apnea and snoring must meet the criteria for 510(k) (premarket notification) Class II approval.13
Each dental laboratory that provides dental appliances for obstructive sleep apnea is responsible for upholding the ethical values and guidelines of the dental/medical profession by using FDA-approved dental materials to ensure the quality of oral devices and to preserve and enhance patients' wellbeing and health.
However, just using FDA-approved materials is still not enough. It is important to understand the properties of the materials used for fabrication of dental appliances and to select these materials based on the appliance's design and function (Table 2). For instance, a nylon appliance should not be made into a combination of MAD and TRD because its rigidity can cause the tongue retainer to be uncomfortable, and the bond between the prefabricated silicone TRD and the nylon MAD might be very weak, resulting in appliance failure.
Currently, the industry is expanding its research and development of more advanced appliances due to the growing number of patients being diagnosed with sleep apnea. It can be inferred that dental appliances for treating obstructive sleep apnea are beneficial in improving patients' health and wellbeing because they can be customized to patients' needs, are relatively easy to design and manufacture using biocompatible dental materials, and are inexpensive and therefore more widely accessible to those who need them.