Despite published reports suggest that non-ideal contacts result following orthodontic treatments with aligners compared to fixed appliances, the results of this study found no difference in occlusal parameter quality between patients treated with aligners or fixed orthodontic appliances, with respect to contact surface area, overall force distribution and symmetry of contacts in MIP. Settling quality indicators, such as patient comfort and Occlusion Time (simultaneity), were also not statistically different between the two groups, suggesting that both therapies obtained similar closure contact occlusal outcomes.
It was a common finding at the end of treatment to see light contacts in the incisor area, and frequently, a lack of contacts on the lateral incisors, both of which should not be considered problematic. In a mutually protective occlusion, it has been advocated that the contacts should be stronger posteriorly, with molars anatomically shaped to sustain larger proportions of the total occlusal load, with light contact existing on the incisors in MIP.
The antero-posterior position of the COF six months after completion of orthodontic treatment stabilized in both groups in an identical position (0.55±0.11 vs 0.55±0.06), which corresponds with a position located qualitatively at the level of the mesial of the first molar, which has been shown to be its ideal position in patients with normal occlusions (27). The COF moved posteriorly in both groups from T0 to T3 (p=0.003), after which time, no statistically significant difference was noted. This suggests that most of the settling occurred within the first three months’ post-treatment, which corresponds with conclusions drawn in earlier studies (31). Interestingly, a statistically significant difference was observed in the anteroposterior position of COF between sexes, with it being positioned more anteriorly in female patients at all times.
A scientific basis for using the COF (equivalent to a barycenter of occlusal force) as an indicator of functional occlusal balance has been described by different research teams using different methods, that obtained similar overall results. In the early 1980’s, two studies determined the physiologic equilibrium point of the mandible, using electronic means (32, 33); when Class I patients clenched their teeth in centric relation with an applied force of 24 pounds the equilibrium point was estimated to lie within the mesial third of the mandibular 1st molar, while being close to the mid-sagittal plane. Using the Dental Prescale System™, another group determined that the Center of Force was not influenced by ethnicity, gender or age in a non-orthodontic sample (26).
Another unexpected finding of this study, was that an asymmetrical force distribution between the left and right sides remained in 1/3 of the treated patients after 6 months of settling. Right/Left balance appeared worse in fixed appliance group immediately after treatment completion and improved at T3. After 6 months of retention, less than 50% of all patients ended up with ideal symmetrical loading of the sensor, in the %L/R 50±5 range, and one third depicted a significant functional asymmetry. This study’s occlusal force asymmetry findings coincide with those reported by Qadeer et. al, where significant occlusal force imbalances were observed in 2 separate post orthodontic T-Scan studies (12, 13). Although not related to occlusal comfort or the appearance of any symptoms, this observed asymmetry, of unknown etiology, illustrated post-orthodontic uneven contact force distribution. This asymmetry could reflect the patient’s preference for chewing on a particular side, as a type of hemispheric laterality (34). One study showed significant and positive correlations existed between masticatory laterality, side differences in bite force, and side differences in occlusal contact area at MIP (35).
It is important to point out that the occlusal force asymmetry following the visual aligning of teeth to a predetermined ideal, appears to not translate into occlusal surface interactional force control. Although the teeth may fit together visually well, the visual ideal does not guarantee the functional interocclusal forces will be balanced, well-distributed, and of only low force intensity.
It is plausible that orthodontic appliances significantly interfere with the normal functioning of the masticatory muscles and that total neuromuscular recovery might vary between individuals. The reestablishment of normal function after orthodontic treatment was studied by Winocur et al. (36), who found that neuromuscular adaptability started immediately at bracket removal, with a maximal bite force increase of about 15%, followed by another 15.5% increase in the first 3 months, whereas only an additional 2% increase was noted after 6 months. Varga et al. using electromyography, reported weaker forces of mastication in females at the time of debonding, and their settling took longer than for male patients (31). The authors hypothesized that women, being cautious, avoided biting forcefully during their orthodontic treatment.
The findings of no difference in the quality of occlusal parameters in MIP after orthodontic treatment with aligners or conventional bonded brackets, still need to be interpreted with caution. These results may be specific to a university setting, where much care is conferred to obtaining ideal results, and may possibly not correspond to what can be seen following aligner therapy in a broader context. For the aligner group, on average, 3.9 modifications were made to each patient’s Clincheck™ (the virtual treatment planning with simulated movements software) prior to approval, 1.6 “refinements” were performed in the finishing stages, and the mean treatment time was 19.4 (±7.98) months, longer compared to the 12-18 months’ average treatment time reported by Align Technology.
Reduced posterior contacts in the course of aligner treatment has been often described, linked to different potential factors: the relative intrusion of posterior teeth due to the interocclusal thickness of aligners, premature anterior contacts (often due to a lack of maxillary incisor torque or insufficient overjet), and difficulty extruding posterior teeth with aligners to level the Curve of Spee (Figure 9). Several strategies have been suggested to avoid creating a posterior open bite including exaggerating the overjet and the anterior torque on maxillary incisors in the simulation software, using adequate attachments, enamel reduction interproximally when needed to help level the lower arch, and using inter-arch vertical elastics in the premolar area. The results this study suggest that with careful planning and proper knowledge of the aligner system’s limitations and how to counter them, ideal static occlusal objectives can be achieved with aligners.
Case selection might also have played a role, as these cases did not involve extractions, and in this sample no dolichofacial case was represented.
Another possible limitation of this study was the choice to use maxillary bonded lingual retainers in a number of patients, that may have prevented the incisors from moving vertically into definitive occlusal contact. This would likely have affected the aligner group, as more maxillary bonded lingual retainers were employed with these patients.
Last, the current study focused on assessing static occlusion, therefore dynamic movements were not evaluated. Although an ideal static occlusion may lead to a balanced functional occlusion, Morton and Pancherz demonstrated during the retention phase that settling, which improves static occlusion by increasing the amount of contacts, does not necessarily translate into better lateral and protrusive movements (37). This should be evaluated in another clinical study, with a larger sample of aligner cases.
A prospective design, with electromyographic measurements of the masticatory muscles before treatment, could establish the origin of the asymmetry of the measured forces, which could be present initially, or be caused by the treatment. The duration of follow-up could also be extended to 12 months or more.