Clear aligner attachment helps to increase the retention of the trays thus provide better control over tooth movements. Attachment loss during treatment may compromise such benefit and increase the need for refinement[13]. In this study, we identified several risk factors that may affect attachment loss rate. In summary, they can be separated into 3 groups:
1)Clinical Variables -age, gender, the number of attachments, the location of attachment (maxillary or mandibular, left or right, incisor/canine, premolar or molar), and the shape of attachment.
2)Operator-Related Variables - bonding protocol and bonding materials. Once bonding failure occurs, the attachment will detach from the tooth in a short time[14].
3)Patient-Related Variables - aligner removal frequency and method, aligner wear time, use of aligner tray seaters, having food with aligners, and masticatory habits.
In general,the patient-related causes of attachment loss, including aligner removal frequency and method, aligner wear time, use of aligner tray seaters, having food with aligners, and masticatory habits, accounted for 56.25% of total attachment loss. On the other hand, the tooth-related reasons that result in attachment loss was 6.73%, which is lower than the finding of Huang Rongcai et al[12] at16.41%.
Position
Our findings suggest the different locations of attachment, whether maxillary or mandibular, left or right, did not affect the attachment loss rate. However, the attachment loss rate may be influenced by different tooth positions (anterior, premolar and molar were 7.54%, 3.34%, and 11.49%, respectively). The highest rate of loss occurs with molar attachments, which may due to its special shape that is unfit for bonding attachments, in which the position of the height of contour of the buccal surface is far from the cervical [15, 16]. Such unique position of the molar increases the impacting forces delivered to the attachment in the process of wearing and removing the clear aligners. On the other hand, the operating space when performing molar attachment is limited, and it is difficult to keep the buccal surface isolated during bonding.
Having Food with Aligners Inserted
Our data showed that patients having food with aligners inserted had lower rate of attachment loss. The presence of the clear aligner may buffer the masticatory force on the attachments while eating as well as divert the direct impact of eating hard foods. In addition, the impact force of the food and chewing force of single attachment can be dispersed along the entire dentition, thus greatly reducing the force applied on individual attachment. Therefore, having food with aligner can reduce the probability of attachment loss. However, Mazyar, M et al.[17] suggests to avoid having food with aligners inserted to prevent white spot lesions (WSLs) during aligner therapy. The manufacturers of clear aligners also advise patients to remove aligners before eating, for both hygiene purposes and to protect the clear aligner material against food with high temperature which may reduce the lifetime of aligner. Other groups have shown that the softness of food did not directly affect the probability of attachment loss[18–20]. Future studies consist of larger population are needed to further investigate the influence of food intake on aligner therapy.
Using Aligner Tray Seaters
Aligner tray seaters, or Chewies, are a type of special appliance comprised of dense foam or plastic designed for clear aligner orthodontic treatment. The use of aligner tray seaters can make the aligners fit more closely with the tooth surface and achieve the intended orthodontic force system[21]. At the same time, the masticatory muscles are exercised with aligner tray seaters for patients who have had extracted premolars or molars, and the unjoyful appearance of “bracket face” can also be ameliorated [22]. However, our finding show that the use of aligner tray seaters will increase the probability of attachment loss, which is in contrast with the findings of Huang Rongcai et al[12]. Even though the clear aligners fit more closely with the attachment after using tray seaters, it may result in an occlusal force that being focused on a single tooth or attachment, which may then increase the attachment loss rate.
Aligner Removal Frequency
Each time the patient inserts or removes the aligner, it will produce an additional dislocation force on the attachments. When the dislocation force is greater than the bonding force, the attachment will separate from the tooth surface. The more frequently the aligner is removed and inserted, the more frequently the attachments are stressed [23, 24]. All 94 lost attachments in this study occurred during removal of the aligners. From the results of the study, the frequency of removing less than 5 times a day did not increase the probability of the attachment loss. However frequently removal ( > = 5 time a day) is a prominent risk factor of attachment loss.
Wear Time
Clear aligner manufacturers recommend keeping the aligners inserted at least 22 hours a day. However, we found it is difficult to ask our patients meet such requirements on our daily practice. Thus, we studied the influence of wear time on the attachment loss rate. The result showed that wearing the aligners less than 18 hours a day increased the probability of attachment loss. Tooth movement requires sufficient time and suitable forces to achieve desired results [25, 26]. Based on this result, the risk of attachment loss can be reduced by prolonging the duration of aligner wear [27].
Mastication Habit
Bilateral alternate mastication is considered healthy chewing habits under physiological conditions, which has a protective effect on the temporomandibular joint and is also conducive to the balance of occlusal relationship between bilateral posterior teeth [28, 29]. For patients with bilateral alternate mastication, the probability of attachment loss on either side should be similar, which is verified by our data. For patients with unilateral mastication, the occlusal function of the functional side is greater than that of the opposite side. When eating with the appliance inserted, the aligner on the functional side fits more closely with the tooth surface[30], and the difference between the actual tooth movement and the prescription value is smaller. Therefore, the attachment loss rate is also lower than opposite side. The results suggest that unilateral chewing should be avoided, and patients should establish the habit of alternating chewing on both sides.
Limitation of This Research
The data was mostly depending on patients' report, and multiple of these factors likely happened simultaneously without patient's acknowledgement. Therefore, in vitro tests need to be used to validate the factors that significantly contributed to attachment loss.