Although clear aligner offers advantages of comfort, esthetics, and ease of oral hygiene care over its counterpart—fixed appliance [2, 3], its treatment efficacy has been of concern due to varying predictability of tooth movement [7], especially anterior retraction following premolar extractions [16]. From the perspectives of aligner biomechanics, mesial tipping of molars, distal tipping of canines, and lingual tipping and extrusion of incisors may occur since the retraction force (on anterior teeth) and protraction force (on molars) pass occlusally to their corresponding centers of resistance. Refinement or segmental archwire aiming to distalize molars and intrude incisors may be prescribed for this clinical situation [8]. Some clinical studies have demonstrated this phenomenon [11, 12]. As shown in our study, first molars were more mesialized by 2.2 mm with more mesial tipping of 5.4 degrees compared to the designed tooth movements. However, overtreatment (distal tipping) of first molars (2.9 degrees) was prescribed to counter the anchorage loss of first molars. Meanwhile, the distal cusp of the first molars remained stable in the vertical dimension, but the mesial cusp of the first molars was more intruded by 0.45 mm. This could explain the clinical phenomenon of buccal open bite in premolar extraction cases during clear aligner treatments [11, 12]. Thus, to avoid the anchorage loss of molars, more molar distalization may be prescribed to offset the mesial movement of molars.
Our results revealed that, compared to the designed tooth movements, canines were less distalized with more distal tipping, lingual tipping, mesial rotation, and extrusion. In particular, the lingual tipping of canines renders the canine roots to contact labial alveolar cortex and impedes canine root movement, resulting in distal tipping (if only root apices contact the alveolar cortex) or even no movement (if the whole roots contact the alveolar cortex) [22]. Moreover, severe lingual tipping risks canine bone fenestration. Thus, additional lingual root-torquing of canines should be prescribed to prevent these adverse effects.
As previous studies showed, unwanted lingual tipping and extrusion of incisors were encountered [8, 11]. This could lead to incisor interference and, together with mesial tipping and intrusion of first molars, result in anterior deep bite and posterior open bite, a phenomenon frequently encountered in clinical practice [23]. As mentioned above, as the overbite deepens, the incisor interference prevents the anterior teeth from being retracted, resulting in a greater molar mesial tipping if patients continue to change new pairs of aligners. In addition, we found that the actual mesial tipping of first molars was significantly associated with an overbite (β = 1.5). This indicates that additional 1.5 degrees of mesiodistal tipping of first molars will occur if overbite is increased by one millimeter. Furthermore, it suggests that deep overbite may lead to mesial inclination of molars owing to interference of anterior teeth and eventually results in failure of space closure. Thus, unwanted lingual tipping (torque loss) and extrusion of incisors should be avoided for extraction cases, and leveling the curve of Spee is of paramount importance to premolar extraction cases with clear aligners. Importantly, there are several variables that can affect the predictability of tooth movement, which we discuss below.
Crowding
As demonstrated in recent research, less space remains after the resolution of anterior crowding among patients with more anterior crowding [24]. Thus, the amount of en masse incisor retraction may be reduced for patients with more anterior crowding, thereby reducing the likelihood of molar anchorage loss since en masse incisor retraction requires greater molar anchorage [24]. Consistently, the multivariate analysis revealed that the actual mesial movement of first molars was significantly associated with crowding (β = − 0.25), which means that an increase in crowding by 1 mm would decrease the mesial movement of first molars by 0.25 mm. We found that crowding was negatively associated with lingual tipping of central incisors (β = − 0.47). This finding could be partly explained by the fact that more crowding is associated with less bodily retraction of incisors, resulting in less lingual tipping of the incisors. Meanwhile, we found that the actual distal movement of canines was positively associate with crowding (β = 0.13) and extrusion (β = 0.14) of canines. Therefore, this could be attributed to the fact that greater canine distalization was required for space gaining to solve anterior crowding, resulting greater canine extrusion.
Age and overjet
We also found that the actual extrusion of first molars was significantly associated with age (β = 0.96). These results indicate that a change from an adult patient to an adolescent patient will increase the extrusion of first molars by 0.96 mm, possibly due to the eruption of first molars among adolescents. Overjet was positively associated with distal movement and the distal tipping of canines, which could be explained by greater amounts of distal movement and subsequent distal tipping of canines required for correcting a larger overjet. Furthermore, a larger overjet requires a greater amount of incisor retraction and causes a greater amount of incisor extrusion, which likely explains the result that the extrusion of central incisors was positively associated with overjet (β = 0.16).
Mini-implant
The clinical effectiveness of orthodontic mini-implants has been well documented for preserving molar anchorage fixed appliances [25, 26]. Ironically, our results revealed that using mini-implants did not help preserve molar anchorage, likely due to the mode of mini-implant application and the influence of anterior interference. For clear aligner treatment, the molar anchorage is often augmented by wearing elastics between the precision cuts on canines and the mini-implants, with no direct force acting on posterior teeth for anchorage augmentation. Apart from that, mini-implants were placed when the mesial movement of the first molars had occurred. In this situation, molars are still susceptible to mesial movement and tipping even if mini-implants are used, especially when anterior incisor interference is present. Regardless of the magnitude of retraction force offered by the mini-implants, the incisor interference prevents the upper incisors from retracting and results in molar tipping if new pairs of aligners are worn [27]. Importantly, the results showed that mini-implant was positively associated with distal tipping (β = 11.53) of canines, which could be due to greater distalization force that was exerted on canines by mini-implants.
Attachment
It has been claimed that the Invisalign G6 system is beneficial for preserving molar anchorage due to optimized attachments on first molars that offer additional distalization force [1]. Additionally, it has been suggested to be beneficial for root control due to the additional anti-tipping moments provided by the optimized attachments on canines [1, 23, 27]. However, our results revealed that the mesial movement and extrusion of first molars did not differ among different types of attachments on first molars, except that the mesial tipping of first molars decreases with Invisalign G6 attachments (β = − 4.42, 95% CI − 8.96–0.12; p = 0.06 < 0.1). In contrast, compared to the vertical rectangular attachments, we found that canine-optimized attachments are associated with more distal tipping of canines (β = 4.92). Thus, we suggest vertical rectangular attachments on canines are superior to optimized attachments on canine root control among extraction cases.
Furthermore, the power ridge was invented to offer additional lingual root torque on incisors and is claimed to avoid lingual tipping and extrusion of incisors [23, 28]. However, the clinical effectiveness of the power ridge in achieving predicted lingual root torque was investigated in a clinical study by Simon et al. [29], where no difference in incisor torque was found with and without the power ridge. Likewise, we found that the power ridge did not influence the lingual tipping, extrusion, and retraction of central incisors on central incisors, which is in line with the previous study [30].
Interestingly, we found that unfavorable lingual tipping (β = 3.28) and extrusion (β = 0.94) of central incisors were associated with optimal attachments on canines (with vertical attachments on canines being the referent). These findings suggest that canines may be the principal teeth that offer anchorage for incisor control when retracting anterior, consistent with the result of Liu et al. [13]. As mentioned above, aligners tend to have incisal movement when designed for lingual root-torquing. Thus, adequate retention of clear aligners on incisors and canines is required to express these desired tooth movements. Due to the more significant bulkiness of vertical rectangular attachments, as compared to optimized attachments, the retention of clear aligners is more favorable with vertical rectangular attachments than with optimized attachments, resulting in better aligner retention on both the anchorage teeth (canines) and the teeth to be moved (incisors) [13]. This leads to a higher expression of lingual root-torquing and intrusion of incisors. Moreover, we were curious about the interaction effect between incisor power ridge and canine attachments. Even more so, as displayed in Figs. 3 and 4, when the power ridge was designed on incisors, undesirable lingual tipping and extrusion are less with canine vertical rectangular attachments compared to canine optimized attachments. Thus, we suggest the power ridge be accompanied by vertical rectangular attachments on canines to reinforce incisor intrusion and lingual root-torquing control.
As previous studies have shown that the medial 2/3 of the third rugae and the regional palatal vault dorsal might be stable regions to register 3D digital models for evaluation of orthodontic tooth movement in adult patients, we chose this region as a relatively stable structure to evaluate tooth movement [31]. Specifically, 4 unloaded miniscrews were used as stable structures to evaluate the reliable region throughout orthodontic treatment [31]. Although a skull region based on CBCT for superimposition models is considered to be more stable, its clinical application is limited by greater exposure to radiation. Meanwhile, a 3D CBCT examinations for model evaluation might be more suitable than a 2D cephalometric measurement. However, the difference in the vertical dimension should be interpreted with caution, especially in adolescents [17].
This study had some notable limitations. Firstly, more prospective studies with larger sample sizes are required for further analysis of valuable variables as this was a retrospective study with limited sample sizes. Secondly, the way mini-implants are used can impact the results. In our present study, mini-implants were placed when the mesial movement of the first molars had occurred, so molars were still susceptible to mesial movement and mesial tipping even if mini-implants were used. Therefore, the effect of mini-implant was possibly underestimated.