Figure 2 illustrates the patient flow through the trial. Sixty-three patients were eligible, but one patient declined to participate. Thus, 62 patients consented to participate and were randomized (31 each in the control and test groups). One randomized patient in the control group did not start the trial and was eliminated. During the study, 3, 7, and 10 patients discontinued their participation at T1, T2, and T3 respectively, and were treated as dropouts.

Patient demographics are presented in Table 1. Globally, the mean age of the patients was 17 ± 4.4 years, and there were 18 males and 43 females. The LII value before the orthodontic treatment was 4.09 ± 2.32 mm. At bonding of lingual fixed retainer (T0), the two groups were homogeneous for all the parameters, except for a small difference in the CI (control group: 0.47 ± 0.51; test group: 0.77 ± 0.43; p = 0.017).

Over the study period (Figure 3), the primary endpoint (LII) did not significantly differ between the two groups. Although the mean LII at T0 was slightly higher in the test group (0.08 ± 0.14 mm) than in the control group (0.01 ± 0.06 mm), the difference (p = 0.016) did not reach the statistical significance level of 1.25% set in the power calculation. These findings were confirmed by applying a GLMM analysis to the data with no group effect (p = 0.25), no time effect (p = 0.32), and no interaction effect (p = 0.12). For the other dental cast parameters, including intercanine width, no significant differences within and between groups were demonstrated (Table 2).

On the panoramic radiographs, the degree of mandibular anterior tooth tipping did not show any intergroup difference at each time point. From T0 and T3, the IMPA and inter-incisor angles remained stable between the two groups (Table 3).

The overall periodontal parameters also remained unchanged from baseline until the end of the study in each group, except for the GI which was slightly, but significantly, higher in the test group than in the control group at T3 (control group: 0.23 ± 0.43; test group: 0.63 ± 0.76; p = 0.039) (Table 4).

Retainer integrity was categorized as total integrity in both groups at each time point. In assessments of retainer debonding over the study period, nine events were recorded in the control group and 10 events in the test group. The average number of debondings per patient showed no significant difference from T1 to T3 between the two groups. However, a significant increase in the quantity of debondings was noted over time in the two groups (time effect p = 0.022). Finally, the type of debonding (adhesive-enamel interface or wire-composite interface) did not show any difference between the two groups (Tables 5 and 6).

At the end of study, the level of satisfaction in terms of the final result was the same in the two groups (9.9 vs. 9.6, p = 0.23), similar to the level of discomfort for the tongue (0.3 vs. 0.6, p = 0.26).

After retainer bonding, the LII value remained stable over the study period in each group and between groups. Additionally, no difference was found for the anterior transversal and sagittal arch dimensions between the two groups, and the IMPA angle, whose average values correspond to an effective anterior guide, did not show any differences either. Therefore, while the stability of the six mandibular anterior teeth was ensured over time in both groups, the new CAD/CAD nitinol retainer did not demonstrate a better efficacy than the standard retainer.

Several explanations can be proposed to explain these findings. First, the positive effect of fixed retainers on the alignment stability of mandibular anterior teeth is well-known,¹⁷ although the measures for prevention of some undesirable occlusal changes remain unclear.¹⁷ Second, indirect bonding (applied in patients of both groups) is considered to be an accurate method of maintaining tooth alignment.¹⁸ Furthermore, the efficacy of the stainless-steel wire retention has already been demonstrated in several studies¹⁸ and a systematic review and meta-analysis.¹⁹ In an RCT, Gunay and Oz²⁰ compared the clinical success of 0.0175-in 6-strand stainless-steel wire with a 0.0195-in dead-soft coaxial wire (created directly on the patient’s mandibular arch without a dental cast) and considered the stainless-steel wire as being more efficient in tooth alignment retention. Moreover, a retrospective study by Renkema et al.⁷ demonstrated the stability of the LII in 90.5% of 200 patients over 5 years after active orthodontic treatment with stainless-steel flexible spiral wire.

Nevertheless, CAD/CAM retainers have some interesting characteristics; Schumacher¹⁰ and Kravitz et al.⁶ highlighted the wire shape manufacturing precision, and Möhlhenrich et al.²¹ showed in an in vitro study that CAD/CAM nitinol retainers had better mechanical properties in terms of permanent deformation resistance than twisted stainless-steel wires. Additionally, as investigated by Wolf et al.,³ CAD/CAM technology seems to provide a high level of correlation between the intraoral position of the retainer and the virtual setup position, which therefore induces a high level of predictability even in anatomically demanding regions. Nevertheless, to the best of our knowledge, no RCTs have already investigated tooth stability with CAD/CAM retainers. All things considered, according to our results and the literature cited above, hand-bent multi-stranded wires are comparable to CAD/CAM-designed and produced retainers in terms of anterior tooth stability, when using an indirect bonding for both wires.

No occurrence of retainer fracture was observed in either of the study groups. Indeed, Zinelis et al.²² have demonstrated that multi-stranded stainless-steel wires could maintain their elemental integrity throughout a 14-year intraoral exposure period. With respect to the CAD/CAM retainer, its properties, which include resistance to deformation and corrosion as well as flexibility,⁶ may explain the absence of fractures. Moreover, the fact that CAD/CAM wires are directly manufactured in the desired shape by laser cutting without wire bending could also reduce the risk of deformations or micro fractures.⁶ However, bonding failures were reported in both groups, with a higher occurrence in the first 6 months but similar rates of occurrence in the control and test groups. In fact, the occurrence of bond failure of lingual retainers is generally higher during the first 6 months, according to some authors.²³ The early failure peak can be explained by the fact that tooth mobility is higher just after the completion of orthodontic treatment than it is later in the retention period.²⁴ Interestingly, in an in vitro study, Aycan and Goymen⁹ investigated the type of debonding that occurred in a multi-stranded stainless-steel wire group when compared to a CAD/CAM nitinol wire group. They demonstrated, in contrast to our findings, that the higher flexibility of the CAD/CAM nitinol wire leads to more occurrences of enamel-adhesive interface debondings than wire-adhesive interface debondings.

Although fixed retainers are likely to increase gingival recession, plaque retention, and bleeding on probing,⁸ the results in the present study demonstrated that both retainers are equally compatible with good periodontal health. Whereas the CAD/CAM nitinol group showed a significant and slightly higher GI at T3 than the control group, the Silness and Löe scores remained, on average, inferior to 1 over the study period in both groups. In a recent systematic review, Arn et al.²⁵ confirmed that fixed retainers represent a good retention method in terms of respecting periodontal health. Moreover, patients were recalled at 1 month, 6 months, and 1 year, and toothbrushing recommendations were reiterated at each time point, since regular checkups are recommended in order to detect post-treatment complications at an early stage.⁷ Furthermore, in the present study, the inclusion criteria ensured that the patients had good oral brushing habits, since fixed retainers are restricted to patients who pay attention to their oral hygiene.²⁶ According to the literature, CAD/CAM retainer characteristics contribute to the decrease in periodontal events.^8,27 Firstly, CAD/CAM nitinol wires exhibit higher flexibility, which allows physiological dental mobility and also reduces the risk of further periodontal problems.¹⁰ Moreover, the CAD/CAM nitinol manufacturing procedure permits thinner manufactured retainers and integrates an electro-polishing step for the wire, both of which contribute to reducing plaque retention.²⁷ Finally, in the in vitro study by Knaup et al.,⁸ who analyzed biofilm formation on conventional lingual twistflex wires and computer-fabricated lingual nitinol retainers over 6 months, the CAD/CAM group demonstrated better results. Nevertheless, due to regular retainer checkups and patients who took care of their oral hygiene, CAD/CAM retainers did not clinically improve the overall periodontal health parameters compared to standard stainless-steel retainers in the present study.

The overall patient satisfaction rate was significantly high in both groups, and no difference in terms of tongue discomfort was noted between groups. Therefore, the CAD/CAM nitinol electro-polished surface seems to have no influence on patient comfort.

The study was designed as a single-center RCT, but it would be worthwhile to extend it to multiple centers in the future to take into account the wider experience of other sites. Secondly, while patients were blinded for the type of retainer, this was not possible for the orthodontist operator. Regarding patient characteristics, more female than male subjects were enrolled in the study due to the fact that more female undergo orthodontic treatment that is likely to improve their dental appearance. As for LII, limitations of the measurement technique have been reported.²⁸ The LII does not consider dental modifications such as interdental spaces or rotations when the contact point is intact; additionally, it does not differentiate several minor tooth displacements from a single high contact point displacement.²⁹ Nevertheless, the LII is considered to be a rapid and simple technique widely used in the literature, hence allowing comparisons with other publications.²⁹ The benefit-cost ratio between the two groups has not been evaluated. However, without any further investigations, the cost of the CAD/CAM nitinol retainer used here was three times greater than that of the standard retainer. Interestingly, in their prospective study, Hu et al.,³⁰ who compared titanium alloy retainers designed by the 3Shape software and manufactured by CAD/CAM with hand-bent conventional lingual retainers, found that the manufacturing of the conventional method retainers was twice as time-consuming. Moreover, the 12-month follow-up period is relatively limited in order to comprehensively explore the mechanical properties of CAD/CAM retainers over a much longer timeframe (e.g., the flexibility may become an advantage as the material ages) and in order to evaluate their lifespan. Finally, the trial was not registered and this could be defined as a limitation.