Invisible orthodontics part 1: invisalign

Invisalign

Align Technology first introduced Invisalign in 1999. To date over 58,000 dentists and orthodontists worldwide are Invisalign certified, with an estimated one million patients treated so far.5 It involves wearing a series of customized clear plastic aligners (Figure 1) for a minimum of 20 hours per day. They are changed on a two-weekly basis. It is estimated that each aligner moves a tooth or group of teeth by 0.25 to 0.33 mm. CAD-CAM technology is combined with virtual 3D model correction software (ClinCheck) to stage tooth movements and correction of the malocclusion (Figure 2). The ClinCheck can be used for visualization of treatment results and sharing information with patients throughout the duration of treatment. A typical course of treatment takes approximately 25 aligners but varies according to the amount and complexity of required tooth movement.6 Concern has been raised about the potential release of bisphenol A from these plastic aligners but laboratory in vitro ageing studies have ruled out any potential cytotoxicity or oestrogenicity.7

Together with the indications listed in Table 1, Boyd has suggested that aligners can help to close a mild anterior open bite and correct cross-bites.8 Case reports also demonstrate that the Invisalign system can be an alternative to fixed appliances for the surgical set-up of orthognathic cases with skeletal Class III patients successfully prepared for surgery.8^,9 It is suggested that cases with minimal need for significant dental movements are best suited to Invisalign therapy.9 Recently, the Invisalign protocol has been modified to help address issues with managing more difficult cases. The effects of interarch elastics used to correct anterior-posterior discrepancies have been built into the software; interproximal reduction is timed to the stage when there is best access to the interproximal contacts and the threshold to trigger attachment placement has been lowered with more appropriate attachment designs. Optimized attachments (Figure 3) are now designed to improve extrusions of anterior teeth and canine rotations with aligner forces. These next-generation attachments are customized to a patient's unique tooth anatomy. Power ridges (Figure 3) now help to deliver lingual root torque by optimizing forces on upper incisors. They can be used to upright retroclined upper incisors, such as in Class 2 Division II cases. Invisalign Teen has also been introduced for adolescents, with the design compensating for incomplete eruption of the canines, second premolars and second molars. It also has a built-in blue wear indicator to gauge the amount of wear achieved. The advantages and disadvantages of Invisalign are listed in Table 2, with a classification system in Table 3. Initially, case series did suggest there was no measurable orthodontically induced inflammatory root resorption (OIIRR) associated with Invisalign, but a recent case report presented extreme OIIRR associated with the upper incisors.10

Clinical studies with Invisalign therapy have begun to quantify treatment efficacy, but to date no randomized controlled trials have been undertaken. The mean accuracy of Invisalign for all tooth movements was estimated at 41% in a recent prospective clinical study, but only anterior teeth were examined. The most accurate movements were lingual constriction of the mandibular canines and rotation of the maxillary central incisors, whilst extrusion of the central incisors and mesiodistal tip of the mandibular canine were least accurate.11 Clements et al, in their 2003 study, did comment that aligners were most successful in improving anterior alignment and least effective for buccal occlusion.12 Djeu et al's retrospective comparison of outcomes of non-extraction Invisalign and fixed appliance treatments, using the American Board of Orthodontics objective grading system, found a significant difference in the pass rate of Invisalign compared to Tip-Edge treatment (20.8%, 47.9%, respectively). Treatment time was significantly less for Invisalign at 1.4 years compared to 1.7 years for Tip-Edge treatment.13 It has been suggested that the highest incompletion rates are seen in premolar extraction cases.14

More research needs to be undertaken to quantify the effects of Invisalign further, particularly in comparison to fixed appliances so that patients can be fully informed of potential treatment outcomes.

The following two cases demonstrate the successful use of the Invisalign system for the treatment of Class II and Class III malocclusions, respectively.

Case 1

A 24-year-old female presented with the chief concern of crowding of her upper and lower teeth. She did not want to wear conventional fixed appliances. On examination, she presented with a Class II division 1 incisor relationship on a mild skeletal II base with an overjet of 7mm and an overbite that was increased and complete. In addition, the upper arch presented with mild crowding with proclined upper incisors. The lower arch presented with moderate crowding with mesio-labially rotated canines (Figures 4 and 5).

The pre-treatment OPT (Figure 6) shows the presence of all the permanent teeth with overall good alveolar bone density and good root morphology. The cephalometric radiograph (Figure 7) confirmed the mild II skeletal base with proclined upper incisors.

The treatment objectives were to align the arches and reduce the overjet by retroclining and retracting the upper incisors, intrude the lower incisors to level the curve of Spee and thus decrease the overbite. Upper and lower crowding was to be resolved by interproximal reduction. The buccal segment occlusion and Class I molar relationship was to be maintained with both fixed and removable retainers to maintain the treatment outcomes. Polyvinylsiloxane (PVS) impressions, occlusal bite, photographs, radiographs and the treatment planning form were sent to Align Technology for the creation of a ClinCheck (Figure 8). The ClinCheck allows the doctor to accept or revise the projected staging, tooth movements and finished treatment results prior to the manufacture of the Invisalign aligners (Figure 9). In this case, treatment involved 11 aligners for the upper arch and 24 aligners for the lower arch. Three attachments were bonded in the lower arch to permit three-dimensional control of tooth movement (Figure 10). The patient was asked to change the aligners at two-week intervals. Interproximal reduction of 1.5 mm was carried out in the upper labial segment from canine to canine and 2.5 mm in the lower labial segment from canine to canine. The patient did require refinement aligners, which involved three aligners in the upper arch and four in the lower arch. In addition, a further 0.4 mm of IPR was required between the upper incisors to reduce the small black triangular gaps. The total treatment time, including the refinement aligners, was 14 months.

At the end of treatment a successful outcome was achieved with a happy and satisfied patient (Figure 11). Both upper and lower arches were well aligned with complete reduction of overjet and overbite (Figure 12). Upper and lower fixed bonded retainers were fitted from canine to canine. In addition, the patient was given vacuum-formed retainers to be worn on a night-time basis.

Case 2

A 42-year-old male presented with Class III incisors on a mild III skeletal base with average lower face height and Frankfort mandibular plane angle (Figure 13). Intra-orally the incisor relationship was Class III with an overjet of -2 mm to the UR1. The overbite was decreased and incomplete with moderate crowding of his upper arch. The lower arch presented with mild crowding (Figure 14). The UR1 was of poor prognosis and had a long-standing post crown. The post crown had been loose a few times and the patient was told by his dentist that he required implant/bridge replacement of this tooth. The patient and his dentist both wanted to align his teeth fully before implant replacement of the UR1.

The OPT confirmed the poor prognosis of the UR1 (Figure 15). The cephalometric radiograph also confirmed his mild Class III incisor relationship and skeletal profile. The upper incisors were slightly retroclined and the lower incisors were proclined. This would help with the Class III incisor correction (Figure 16).

Treatment for this patient was carried out with Invisalign aligners. The aims of treatment were to relieve the crowding in both upper and lower arches and correct the Class III incisor relationship. Space in the upper arch was created by proclination of the upper incisors and in the lower arch by interproximal reduction. PVS impressions and full records were taken for development of ClinCheck (Figure 17 and 18). The treatment began with aligners. These were changed at two-week intervals. Attachments were bonded on all the canines, upper left lateral incisor and lower right first premolar to provide retention for the aligners (Figure 19). Treatment progressed well with the aligners. However, towards the end, the patient was still Class III incisally.

In this case, Class III elastics (Figure 20) were used with the aligners to help with correction of the Class III incisor relationship and achieve positive overjet and overbite. The patient had to wear the elastics on a night-time basis for a short period to achieve the desired result.

Treatment involved 15 aligners for the upper arch and 11 aligners for the lower arch and interproximal reduction of 2.5 mm between lower canine to canine. No refinement aligners were required in this case. Total treatment time was 8 months.

The extra-oral view (Figure 21) of the patient at the end of treatment shows improved incisal by proclining the upper incisors. Intra-oral pictures show a positive overjet and overbite has been achieved and the patient has Class I incisors with good buccal segment interdigitation. The positive overbite is going to contribute to the stability of the Class III incisor correction (Figure 22).

The patient is now in the process of having implant replacement of UR1. They were fitted with an upper vacuum-formed retainer and a lower fixed bonded retainer.

Discussion

A systematic treatment plan is crucial with the Invisalign system. The ClinCheck software can be used for diagnosis and treatment planning. It acts as a valuable tool to assess the need for expansion, extraction, interproximal reduction, distalization or proclination. This mode of treatment requires the clinician to plan out a reasonable sequential tooth movement for every tooth from start to finish.

Aligners are recommended over conventional fixed appliances for several reasons:

However, there are some limitations, which make the Invisalign approach unsuitable for treating certain advanced cases. Simple malocclusions have been treated successfully with Invisalign, but more complex cases do require fixed appliances or other treatment options for a perfect outcome. Patients who are undergoing premolar extractions may be unsuitable candidates for Invisalign treatment, as the aligners cannot maintain the teeth in an upright position during space closure. The patient's level of motivation is imperative for success, as the aligners must be worn for 22 hours a day.

Refinement aligners are often an important part of Invisalign treatment – as with fixed appliances, extra time is occasionally needed to finish a case. The same principle applies with Invisalign. In order to achieve the best results, refinement aligners are sometimes required, as described for Case 1.

One of the other drawbacks of the Invisalign technique is that, although the ClinCheck shows correction of the malocclusion with aligners, in some cases this may not occur and clinicians may have to use auxiliary techniques to achieve the best result. As in Case 2, Class III elastics were used with aligners for the complete correction of the incisor relationship.

Conclusion

The Invisalign system has opened up a contemporary area of adult orthodontics, helping patients who may not want traditional fixed appliances or where removable appliances may be ineffective. The treated cases demonstrate that the Invisalign system can be a valuable substitute for fixed appliances in mild to moderate malocclusions.

We still have a great deal to learn regarding the biomechanical feasibility and effectiveness of the Invisalign system. High quality randomized clinical trials are required to support the claims about Invisalign treatment. However, the key to success is appropriate case selection. Orthodontists will have to rely on the inadequate available evidence, expert opinions and their Invisalign clinical experience when using the system.