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Clear aligner therapy has undergone substantial improvements and its popular appeal has greatly increased since it was first made available in 1999 with the introduction of Invisalign® (Align Technology). 1Originally used to treat minor and mild malocclusions,2 aligner therapy has evolved over the years to address moderate and some more complex orthodontic cases. Currently, more thant 27 different clear aligner products are available on the market.2 Using clear thermoformed plastic aligners that cover many or all of the teeth, most clear aligners are relatively similar in their modality in terms of applying gentle, consistent pressure to the teeth, but products may differ in their range of application, construction, aligner thickness, use of resin-bonded attachments, wear time for aligner trays, and treatment sequence.2,3 For example, some aligners utilize trays contoured to the gingiva in a scalloped pattern and incorporate attachments to aid in movement, while other aligners cover the gingiva over a few millimeters in a nonscalloped pattern, requiring fewer engagers for overall tooth movement.
With both fixed orthodontics and clear aligner therapy, orthodontic tooth movement occurs when teeth undergo a mechanical force or pressure, which causes osteoclastic and osteoblastic cellular activity to be stimulated.4,5 Osteoblasts are responsible for bone apposition, by which new bone fills up the space that has been left behind after a tooth has been moved.6 By contrast, osteoclasts break down bone, allowing teeth to move to a new position. These processes occur continually throughout orthodontic treatment.6
In the clinical experience of the author, many adults prefer clear aligners to traditional fixed orthodontics, primarily because aligners are more esthetically pleasing.7,8 Marketing and advertising as well as increased awareness about clear aligners have led many patients to consider aligners to be "more esthetic, efficient, and safe for oral health."9
APPLICATION OF CLEAR ALIGNERS IN PROSTHODONTIC TREATMENTS
Patients with extensive tooth wear damage generally require a full prosthodontic work-up and comprehensive treatment planning to achieve best results. Tooth wear, defined as a loss of tooth substance that is not precipitated by dental caries or plaque accumulation, is not an abnormal phenomenon, but rather is a normal physiologic process that occurs throughout an individual's lifetime.10 However, when it becomes extensive or the rate of loss is rapid, tooth wear is considered pathological, and can cause functional, esthetic, and/or sensitivity problems.10 One in two adults have four or more teeth with moderate to severe wear trauma.11 The severity of tooth wear varies from patient to patient, and even within the same patient, depending on the teeth or the position on the tooth. Tooth wear such as that resulting in part from periodontal disease can eventually result in severely worn-down dentition, pulp exposure, and tooth mobility.11
Before beginning prosthodontic treatment planning, the practitioner should first understand the underlying causes for the patient's abnormal tooth wear damage, such as malocclusion. Because the ultimate goal of the prosthodontic treatment is to help the patient maintain healthy dentition over the long-term, the treatment strategy must go beyond simply restoring broken-down teeth one at a time or quadrant by quadrant without addressing the root causes. By incorporating aligner therapy into their patients' treatment plans, dental clinicians can move teeth into more stable positions to enhance the longevity of future restorations.12 For example, in a patient who has an end-to-end occlusion, feldspathic porcelain veneers on the maxillary and mandibular anterior teeth will likely be fractured over time, as those restorations are not in a protected or stable occlusion.12 Such risks can be reduced by applying clear aligner therapy at the beginning of a treatment plan, thereby creating a more uniform occlusal wear pattern and decreasing the likelihood of fractured, chipped, and broken dental restorations or teeth.13 Similarly, these risks can also be reduced by traditional orthodontics.
In addition to their usefulness in pre-prosthodontic workups, clear aligners may also help in better utilizing conservative preparations and restorations. If orthodontic intervention is not applied, prepping severely malpositioned teeth (eg, rotated teeth, tipped teeth, and/or misaligned teeth) may require more extensive tooth removal. Further dental treatment, such as root canal therapy and full-coverage crowns, may ultimately be necessary if orthodontics is not first used to create ideal tooth position. Taking the time to educate patients on the importance of proper tooth position is a necessary part of a successful treatment plan, whether it utilizes clear aligner therapy or traditional orthodontics to achieve that position.
One major application of clear aligner therapy is the correction of malocclusions. Defined by the World Health Organization as a "handicapping dentofacial anomaly," malocclusion can cause dental problems and compromised facial growth and development.14 In the experience of the author, many people with malocclusion problems, such as deep bite, crossbite, excessive overjet, midline deviation, and open bite, never correct these issues, often because they do not want to wear traditional braces that include brackets, wires, ligatures, and other orthodontic appliances.8,15 A lifetime of malocclusion may have detrimental effects, not only to the teeth but also to the overall health of the oral cavity.
Clear aligner therapy was originally designed to address simple malocclusions such as minor crowding and misalignments.16 Over time and with the introduction of improved technology, such as 3D digital scanners, computer-generated digital mock-ups, and auxiliaries and attachments, clear aligner therapy has become more applicable to moderate and some more complex malocclusions.17,18 Traditional fixed orthodontics is the most tried method for moving teeth, so there is currently more literature supporting it, but clear aligner therapy is evolving. However, as with any evolving treatment, clear aligner therapy has some limitations (Table 1). 7,15-19 For instance, it is less effective in controlling extrusion and rotation of teeth, particularly rounded teeth.20 Although these movements may ultimately be achieved with clear aligners, more modifications, attachments, and time may be required to do so.17 The predictability of these movements and their outcomes should always be considered by the dental practitioner, and it may be decided that clear aligners are not be the ideal treatment modality for some patients. Lombardo et al analyzed several studies regarding rotational movements, specifically of maxillary and mandibular canine teeth, and found varying degrees of rotational accuracy.21 The researchers concluded that rotational movement of mandibular canine teeth was the least predictable movement to achieve. Similarly, root movement is another less predictable movement with clear aligner therapy.22 Almost all the current literature on clear aligner therapy points to the need for continued research and larger sample sizes to further evaluate the efficacy of clear aligner systems.
The use of modifications (eg, power ridges, vertical and horizontal attachments) in combination with clear aligners may increase the efficacy of more complex movements.23 As with any dental procedure, proper case selection is vital. It is also important to discuss with the patient his or her preferences regarding orthodontic treatments.18Some patients may be more amenable to a longer treatment time, typically with traditional orthodontic braces, whereas others would prefer a shorter duration of treatment, such as offered by clear aligner therapy.18 All of these discussions should occur before treatment begins to ensure the proper orthodontic method is utilized for the patient.
Regardless of whether patients are treated with traditional braces or clear aligner therapy, patient compliance with oral hygiene regimens during treatment is challenging. However, while conventional orthodontic treatment can make dental hygiene especially difficult, owing to the fixed appliances,8 clear aligners are removable and can be cleaned, which facilitates easier maintenance of proper oral hygiene (brushing, flossing, rinsing) and results in an improvement in gingival and periodontal health.9,19,24 Studies such as systemic reviews by Rossini et al have shown that periodontal indices are improved and the quantity of plaque is reduced with clear aligner treatment compared with traditional fixed orthodontic therapy.16,19 Clear aligner therapy also has minimal effect on bacterial growth, compared with an increased bacterial growth when fixed orthodontic appliances are used.7 In addition, patients with tight interproximal contacts, rotated teeth, tipped teeth, and generalized malocclusion issues often have difficulty with their daily oral hygiene regimens, whereas properly positioned teeth facilitate easier cleaning of the oral cavity. Inflamed gingiva, bleeding, and plaque accumulation can also be reduced by aligning teeth into a more manageable position for daily oral hygiene.9,13, 24
Using 3D digital scans, practitioners can review the patient models with the patient to discuss the nuances of his or her mouth. With clear aligner simulation, patients can be shown how the repositioning of their teeth can enable improved periodontal health. During treatment, patients often notice when a rotated tooth is straightening because they can now see the interproximal stains that previously were not able to be cleaned off due to malocclusion. Visible improvements such as these, which testify to the ability of aligner therapy and traditional orthodontic treatment to enhance oral hygiene, can help keep patients motivated about their treatment.
ESTHETICS AND COMFORT
Because of its clear thermoplastic material, clear aligners are not easily noticeable and are more esthetically pleasing than fixed orthodontic appliances, and are therefore preferred by many patients7 (Table 1).
Although its removability is usually considered one of the advantages of clear aligners, it can also be a disadvantage for patients who do not handle tooth discomfort well. As the teeth are being moved by the force applied by the clear aligners, patients can remove those trays to alleviate discomfort. By removing trays, however, patient are negating any active movement that occurred. This is a factor that needs to be discussed with patients before treatment begins. Overall, however, clear aligners are reported to be associated with less pain than fixed orthodontics.15
Removal of the aligner for longer periods can interfere with the effectiveness of treatment. As discussed earlier, the application of orthodontic force results in the osteoclastic activity that allows for tooth movement. 4,5 This osteoclastic activity usually initiates within 2 to 4 days, but when pressure to the teeth is no longer applied, it can cease in as little as 4 hours.20 Hence, patient compliance is vital to successful tooth movements. Because the required wear time is at least 20 to 22 hours per day,3,25,26 there is not much "wiggle room" in any given day for patients to remove their clear aligners for extended periods of time or to make up for the "lost time" on the next day. This is especially true during the first week of a new clear aligner tray cycle, when the greatest amount of orthodontic tooth movement occurs.13
Clear aligner therapy has undergone substantial improvements since it was originally introduced more than 20 years ago. Over the years, it has grown from a popular new therapy used for improving minor and mild malocclusions, to an established orthodontic treatment used to address more moderate and even some complex cases. Although clear aligner therapy is still not ideal for some movements, such as bodily movement and torque,27 many such challenging movements can in fact be achieved with some type of modification or auxiliaries (eg, elastics, attachments, and/or ridges). Like other evolving therapies, clear aligner therapy is certain to continue to undergo technological improvements and to increase the variety and complexity of orthodontic cases that it can successfully treat. The value of optimized occlusion cannot be emphasized enough, and with the use of clear aligner therapy, more and more patients are experiencing the benefits of properly aligned teeth and stable occlusion, whether it be improved periodontal health, decreased occlusal trauma, or improved maintenance of their final restorations.
AUTHOR: Maria Jackson, DDS, FAGD; Private Practice General Dentist in Forney, Texas
Special Thanks to Dr. Esther Pedersen, DDS, FAGD of Overland Park, Kansas for her continual mentorship, guidance, and friendship in the dental field.
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