Do orthodontic treatments using fixed appliances and clear aligner achieve comparable quality of occlusal contacts?

Abstract

Design Non-randomised cohort study.

Cohort selection The inclusion criteria comprised patients who finished their comprehensive orthodontic treatment based on a camouflage non-extraction modality using self-ligating orthodontic appliances therapy (SPEED brackets, Canada or Empower brackets, American Ortho, Sheboygan, WI, USA) or clear aligner therapy (Invisalign, Align Technology, San Jose, CA, USA). Patients were recruited from a university orthodontic clinic and a senior orthodontist's practice. The exclusion criteria comprised patients presenting with hypodontia, microdontia, severe periodontal problems, heavily resorted teeth and patients who were using retainers with occlusal coverage.

Data analysis Included patients were assessed at three points in time: at the end of active orthodontic treatment and the start of retention phase (T0); at three months post treatment (T3); and at six months post treatment (T6). The following three assessments were done for each patient at T0, T3 and T6: a T-Scan 10 digital occlusal analysis recording into maximum intercuspation position (MIP); self-report about retainer compliance; and self-assessment of occlusal comfort using a visual analogue scale (VAS) ranging from 0-10 (0 = very uncomfortable; 10 = maximum comfort).

The digital occlusal analysis was performed using a 100μ thin, flexible, horseshoe-shaped Mylar sensor (Novus HD sensor, Tekscan Inc, S. Boston, MA, USA). This sensor contains 1,370 active pressure sensing cells, known as sensels, arranged in a compact grid, shaped as a dental arch. The patients were given instructions on how to bite on the sensor. With the sensor still in the patient's mouth, three consecutive self-intercuspated closure-into-MIP registrations were recorded. If there was a need to repeat the procedure, one minute was given as a rest. If several recordings were done for the same patient in the same visit, the most consistency between the three consecutive intercuspations was used for analysis. In addition, the same sensor was used in T0, T3 and T6, unless the sensor shown excessive wear.

The following outcomes were assessed using the digital occlusal analysis: 1) an estimate of occlusal contact surface area based on the activated sensels on the sensor at MIP; 2) the total surface of contacts and the area ratio between anterior (canine to canine) to posterior (premolar to second molar) contact surfaces; 3) the overall relative force distribution based on the by the position of the centre of force (COF); 4) the symmetry of contact distribution was expressed as the percentage of contacts on the right side to the left side (%R/L); and 5) the time-simultaneity of the closure into MIP contacts was calculated by the occlusion time measurement which is the duration between first contact and the time MIP was reached.

Results In total, 39 patients were enrolled in the study. The self-ligating fixed appliance group included 25 patients (mean age 18.7 ± 5.2; 6 women, 19 men) while the clear aligner therapy group included 14 patients (mean age 20.6 ± 7.3; ten women, four men). Both groups were matched in terms of age, Angle's classification, symmetry, retention protocol and total number of bonded lingual wires or facial type. However, they were not matched in terms of sex, with more women in the aligner group (p = 0.007). Moreover, eight patients (four in each group) were excluded from the study later on due to a change in the retention protocol or a missed visit. The results showed that self-reported compliance with a Hawley retainer was not different between groups. Occlusal comfort was similar in both groups at treatment completion, with a median score of eight in both groups.

The results showed that were no statistically significant differences between the two groups regarding all outcomes assessed using the digital occlusal analysis. Although the %R/L (normal range = 50% ± 5%) was not significantly different between both groups, neither treatment resulted in ideal occlusal balance (ie symmetry). Indeed, ten patients finished their treatment with subtle asymmetry (%R/L >50 ± 10%), especially in the self-ligating fixed appliance group (nine patients) showing side force differences (five right dominant, four left dominant) versus only one patient in the clear aligner group (left dominant).

The COF moved posteriorly in both groups from T0 to T6, in parallel with a decreased ratio of anterior to posterior surface area. A statistically significant difference was observed in the anteroposterior position of COF between sexes, being more anterior in women at all times (p <0.002).

Conclusions The quality of the occlusal contacts in MIP was comparable in both groups at T0, T3 and T6. Neither treatment resulted in an ideal occlusal balance (that is, symmetry). Ten patients finished their treatment with subtle occlusal force asymmetry (that is, asymmetric left-to-right side occlusal force distribution), especially in the self-ligating fixed appliance group. Most occlusal changes happened during the first three months of the retention phase, with more posterior contacts forces developing in both groups. In this study, female patients maintained more anterior COF when compared to male patients.