The aim of this investigation was to assess the predictability of the crowding resolution, the efficacy of the different strategies to gain space, and their correlation to provide a suitable protocol to achieve predictable results.
The sample of this study presented an Irregularity Index mean of 7.26 mm in the upper arch and 8.13 mm in the lower one. These data indicated that the patients recruited had severe crowding in both arches, suggesting orthodontists' confidence in treating this issue with clear aligners.
At the end of digital planning (vT1), a residual crowding of 1.83 mm in the upper arch and 1.40 in the lower arch was still present. Therefore, it could be deduced that planning errors are often present and could affect the final outcomes.
At the end of the first set of aligners (T1), the crowding was statistically significantly reduced, but an Irregularity Index of 2.81 mm in the upper arch and 2.66 mm in the lower arch was still found. These data show a high value of predictability (87% in the upper arch and 81% in the lower one), confirming that the clear aligner treatment is efficacy to reduce crowding [24]. Unfortunately, single planning, even if very effective, is not enough to solve the crowding entirely.
Several studies compare, in clear aligner patients, the treatment outcomes that needed refinement, with their digital planning to evaluate the effectiveness of tooth movement. The results of these researches were similar to ours [12, 15, 23].
Therefore, additional aligners should always be considered to solve the crowding still present.
The predictability of crowding resolution with clear aligners is a multifactorial issue. Some are related to aligners, such as the protocol used, the features and the thickness of the aligner material, the planning software, and the different strategies to gain space planned.
Others depend on the operator, such as the experience of the clinician in case selection [19], the accuracy plan of the strategies to gain space, and the accuracy of IPR procedures [17].
As emerged in this paper, the single strategies to gain space reached different predictability, although they together, concur to solve crowding. Our results showed large variability of these data.
For example, transverse changes in the upper arch ranged between 59 and 83% depending on the form of the diameters considered.
This indicates that the aligners do not allow the planned expansion entirely as already reported in the literature [20, 21]. The least accurate transversal diameter change, as also reported in previous studies [15, 16, 20, 25], was for canine and the first premolar with the predictability of 59% and 60%, while the most accurate was for the first molar with the predictability of 83%. This could be related to the different extensions of the lingual surface and anatomy of each tooth related to the retention and fit of the clear aligner. Indeed, the slender and conical canine shape could not help to control the planned expansion movement.
In the mandibular arch, the predictability is even lesser, with the lowest value always found for the canine diameter of 49%. Mandibular canines have the longest root, a shape of crown with few undercuts, and a small lingual surface due to their usual more vestibular position; this reduces the retention of the aligner and the ability to push the tooth buccally.
The data regarding the sagittal incisor position (arch length) indicate that, on average, at the end of virtual digital planning (vT1), control of the arch length is planned [12, 26].
Instead, at T1, the slightly lingual movement of the incisors, through coronal tipping, was predictable with a value of about 70% both in the upper and lower arch.
Also, Kravitz reported [27], that the aligners are more accurate in retracting the incisor than in expanding them labially.
Obviously, arch length decrease or remaining stable is associated with the posterior expansion of the dental arch and it might be due to the maxillary arch form [28].
These results highlight that with the clear aligners, it is possible to plan crowding resolution without buccal tipping, thus avoiding side effects of the lower incisors proclination, which are often present with fixed appliances.
The clinicians should take in mind these outcomes to plan crowding resolution with less canine expansion and incisor proclination but primarily with IPR.
Regarding IPR, after aligner sequences, in the upper arch, less than half, 0.55 mm, was performed, while in the lower arch the result was better, 0.82 mm. These data attest to the accuracy of IPR at 49% in the upper arch and 42% in the lower one, values that confirm the results of the previous study [17].
These data indicate that in these patients, less than half of the planned IPR at the start of treatment with software was performed.
The predictability of IPR is a multifactorial issue. The amount of enamel removed depends, in fact, on several factors: some related to characteristics of the tooth such as the hardness of the enamel, the anatomy of the crown, or the position of the tooth itself. Others related, instead, to the operator, such as his experience, the technique used for the IPR, and the pressure exerted during the procedure [17].
Therefore, IPR is the least accurate strategy to gain space, and it requires greater accuracy. This could improve the predictability of crowding resolution, reaching the optimal values planned.
Moreover, there was not a high correlation between the irregularity index and the strategies to gain space; however, all the strategies concur to create useful space to solve the crowding.
In both arches, a high inverse statistically significant correlation was found between intercuspid diameters (Δ 3-3) and Δ Irregularity Index. This result indicates that crowding decreases as 3-3 diameter increases.
Incisor proclination (Δ arch length) was not correlated to Δ Irregularity Index, obviously because in digital planning there was no proclination to gain space.
Moreover, the correlation between Δ IPR and Δ Irregularity Index was positive, although not statistically significant. Indeed, with the reduction in enamel increasing, we registered a resolution of crowding improvement, although IPR has rather low efficacy rates. The result of this research showed that digital planning was not a predictor of final tooth position. Therefore, the virtual tooth position may not be the achieved final tooth position. So, it is important to know the limits of digital planning to overcorrect tooth movements when it is necessary to decrease the need for refinement [12]. Knowing the strengths and the weaknesses of clear aligners will help the clinician in selecting the best orthodontic appliance to treat a specific malocclusion.
Therefore, orthodontists play an important role in modifying the virtual digital plan with their clinical experience, programming the movements at the proper steps, and adding the features to improve the predictability of the tooth movement with the aligners [29].
To prevent the risk of selection bias, due to the retrospective nature of the study, patients were consecutively selected for each orthodontist. This study included only adult patients to avoid bias due to normal transverse growth of the jaws and because they currently represent most of the patients who request orthodontic treatment with clear aligners. Moreover, these patients generally show better compliance compared to adolescents, thus reducing a possible source of bias [9, 17].
However, the retrospective studies do not allow for estimating precisely patient cooperation. Thus, to verify the collaboration of the patients, their charts were reviewed at the end of the treatment to check if the aligners had been changed at regular intervals, but this may have been influenced by patient statements.
Future studies should add other measurements such as lateral cephalometric or volumetric 3-dimensional cone-beam to assess predictability of the different strategies to gain space. Such studies will allow for the evaluation of posterior tooth movement and address questions regarding root movement with clear aligners. Obviously, the possibility of developing prospective randomized clinical trials would allow emerging greater certainty regarding the predictability of treatment with clear aligners.