Based on our current knowledge, this is the first systematic review that evaluates the influence of composite attachments on orthodontic therapy with transparent aligners and the possible differences between their configurations (shape, size, number and/or position). In order to obtain a more schematic analysis of the results, we separately assessed five groups of movements (anterior B-L tipping/root torque, intrusion, extrusion, rotation, and bodily movement in M-D direction). No article analyzed the effects of attachments on posterior B-L inclination and/or expansion movement.

While B-L tipping is considered an easier movement to be obtained [24,25], anterior root torque represents a challenge for treatments with aligners [26,27]. This review highlights the importance of auxiliary elements to achieve a better root control, a concept previously reported [12,28]. Simon demonstrated that torque [as well as bodily movement] can be obtained with aligners and auxiliary elements, such as attachments and power ridges, since they are able to release an adequate system of forces [21]. More specifically, the incisor torque is smaller with the use of horizontal ellipsoidal attachments in comparison with power ridges, which provide a force closer to the tooth neck, are easier to apply and more aesthetic and, finally, increase aligner resistance at the gingival third [21]. However, literature showed that attachments and power ridges may not be sufficient to ensure a right root control and hypercorrection or refinement might be necessary [21], as previously suggested by Kravitz [22] and more recently by Houle [29] and Khosravi [30]. The retraction of anterior teeth with a proper root control also depends on the achievement of a suitable posterior dental anchorage [16,23], which can be improved by adding attachments on a greater number of teeth [from canine to second molar] [13,16,31].

Literature shows that both intrusion and extrusion can be facilitated by the use of attachments [15,32,33,34]. Durrett [20] confirmed these findings analyzing the intrusion of incisors, canines, and premolars. In his study all the groups with attachments showed greater efficiency than the control group without attachments performing intrusion movement. He noticed no significant differences among the analyzed attachment shapes. The authors of this clinical trial highlighted that several possible limitations could have affected their results, so these findings should be confirmed in the future. Moreover, attachments could improve intrusion by enhancing the accuracy of the fit: some authors suggested to use attachments on premolars in order to enhance the retention of aligners during intrusion [33,34]. This effect could be useful in deep bite cases in order to improve levelling of the Spee curve [33]. The intrusion movement was also considered by Dai et al. [23]. However, the aim of the study and the analyzed teeth were different from Durrett’s study [20]; Dai compared predicted and achieved tooth movement of maxillary first molars and central incisors in extraction cases. The results of this trial showed greater intrusion of posterior teeth, compared to the predicted virtual tooth movement. As far as regarding the influence of attachments, the group with the optimized G6 attachments had the greatest difference between predicted and achieved tooth movement. These authors proposed that these treatment outcomes could be related to the occlusal splint effect that is created wearing the aligner and they suggest to consider heavy occlusal contacts on posterior teeth during setup, in order to prevent posterior open-bite [23].

Extrusion seems to be one of the most critical movements to be carried out by means of aligners [especially when referred to central incisors], due to the lack of elastic deformation of the aligner in the vertical direction [8,9,11]. Several authors in literature have highlighted this critical issue [8,9,11]. Unfortunately, Durrett’s study does not allow to obtain clear conclusions about this movement, due to the small sample size [20]. However, a recent experimental study demonstrated, through FEM analysis, that the use of one attachment bonded on the palatal side could improve incisor extrusion [35]: this is an encouraging sign for orthodontic research and future clinical trials could be useful to confirm these results.

Rotation is considered one of the most difficult movement to correct with transparent aligners, in particular when it involves conical teeth. Literature shows that the use of attachments could increase the effectiveness of derotation movement, creating undercuts and improving retention [2,36,37,38,39]. However, two of the five clinical studies included in this review do not highlight significant differences between the treated groups with and without attachments [21,22]. Kravitz attributed the absence of evident advantages to a large number of canines subjected to a rotation greater than 5° within the attachment group. Another factor that could justify the results reported in this article is the small sample size of the attachment group. It is worth considering that different patients’ compliance among the groups could affect the final outcomes [21]. Finally, a third study shows conflicting results [20]. In the latter work, attachments caused an improvement of rotation in the “reboot” group and a worse clinical outcome in the “non-reboot” group, compared to the control one without attachments [20]. However, the small sample size and other study limitations could have affected the study results, as also stated by the author of this clinical trial. The confounding factors of these studies do not allow us to draw clear conclusions about the efficacy of attachments in derotation and further clinical studies are needed to evaluate this. With reference to the number of attachments, Durrett showed that two attachments do not improve the magnitude of rotation, showing worse outcomes than the other attachment groups or resulting even less effective compared to the control group without attachments [20]. This result was confirmed by a subsequent experimental study by Momtaz [4], but further experimental and clinical studies are needed to confirm this evidence. As regards shape and size, larger attachments with sharper edges seem to perform better during derotation movements [20]. It is worth noting that there are other factors that must be taken into account during a treatment plan which includes rotation: the amount of total derotation movement [21,38], staging [degree of derotation per aligner] [21,37], interproximal reduction [IPR] [22], and the use of buttons with elastics [20] can in fact influence derotation efficacy.

Bodily mesio-distal movement is also considered difficult to achieve with aligners [27,40,41]. Nowadays, the introduction of new techniques and auxiliaries allows a better root control [13]; technological innovations allowed an improvement in orthodontic dental movement even with traditional and self-ligating multibrackets appliances [42,43]. Some evidence showed that aligners with attachments are able to release the necessary force system in order to achieve bodily molar distalization [16,21] and that staging plays an important role in achieving treatment success [21,36,44]. Attachments should be able to create a moment useful to counteract dental tipping: this moment seems to be determined by a complex force system on attachments active surfaces [3]. Some FEM analysis, for example, demonstrated that movements like canine distalization or incisor bodily movement during diastema closure can be improved by attachments [3,45,46,47]. However, clinical studies do not allow to draw clear conclusions about the attachment capacity to improve this movement. Simon et al. showed that the attachment group could be more effective than the group without attachments, but the differences do not seem to be clinically significant: the mean accuracy of movement obtained with and without attachments was 88.4% and 86.9%, respectively [21]. As far as concerning the number of attachments, it seems that the use of five attachments per quadrant [from canine to second molar] could improve posterior anchorage, molar bodily movement and it could enhance the amount of intrusion [16]. Posterior anchorage could also be influenced by the shape of attachments: optimized and rectangular horizontal attachments have shown the best results in molar anchorage, unlike the vertical rectangular ones, which were the least effective [23]. However, the sample size analyzed for vertical attachments was small: further studies are needed to compare different attachment shapes [23]. Considering the available evidence, further clinical trials are needed to evaluate the influence of attachments in the efficacy of molar distalization and posterior anchorage.