Low translational and rotational movements with 2-point stainless-steel retainers over a period of 1 and 3 years

One of the greatest challenges in modern orthodontics is the stabilization of treatment outcomes. Various studies have reported a relapse particularly of the lower incisors into their previous malocclusion [17, 29]. Although the reason for this relapse is not yet entirely understood, it appears to be a multifactorial problem caused, for example, by insufficient treatment planning, overexpansion or enormous changes of arch forms, an unfavorable skeletal growth pattern, a lack of stable occlusion, muscular dysfunctions and habits, or resilient Sharpey’s fibers [8, 18, 19, 22]. Patients’ expectations regarding preservation of a lifelong esthetic smile have increased in recent years. Securing the therapy result after years of orthodontic treatment and preventing a relapse of the anterior teeth toward the baseline malocclusion is of highest importance for patients [7]. To retain upper and lower incisors, both permanently fixed retainers and removable appliances are commonly used, e.g., Hawley-type retainers or vacuum-formed splints. In addition to fixed retainers, passive plates have an added impact on the prevention of tertiary crowding [18, 34].

Higher demands for long-term retention devices in recent years have led to increasing requirements on lingual fixed retainers in terms of material stability and economic aspects [30].

Considering suppositional patient’s adherence for long-term stability of treatment outcome, fixed lingual retainers appear to be superior to removable passive plates [7]. According to the literature and to clinical practice, there is evidence that patients with fixed retainers show less overall posttreatment changes after having completed multibracket appliance treatment [1, 5]. However, it is also known that the removal of a fixed retainer can lead to changes in tooth position even after a longer insertion period [10].

Over the last few decades, various retainer systems have been established in orthodontics. Since their introduction by Zachrisson in 1977, multistranded wire retainers are very commonly used and are considered the gold standard in posttreatment retention [6, 11, 35, 36]. Including a bonding site on each incisor, the multistranded wire retainer is beneficial for lower anterior tooth stabilization and preserves the intrinsic mobility of the teeth due to the elastic property of the wire [6, 15, 24, 34]. Multistranded wires can easily be adapted to the anterior tooth segment, but they need to be adjusted precisely to avoid gaps between the wire and the tooth surface. Larger gaps are bridged with composite filling material, which in turn might limit the patient’s comfort and possibly lead to premature contacts and increased plaque accumulation [16, 17].

Various studies reported a higher frequency of bonding side defects, when comparing twistflex to round stainless-steel retainers [24, 26]. In addition, possible residual stress in the wire and iatrogenic forces may lead to unwanted tooth movement, resulting in, for example, loss of torque control (x-effect or twist effect) [4, 6, 28]. Apart from these side effects, defects of bonding sites are often noticed by the patients only after relapse of anterior tooth positions into malocclusion [23].

These impacts led to the introduction of innovative computer-aided designed and computer-aided manufactured (CAD/CAM) retainers, such as Memotain (CA Digital GmbH, Hilden, Germany) or prime4me® RETAIN3R (Dentaurum, Ispringen, Germany). These high precision retainers are bonded to the lingual surfaces of the upper or lower incisors [13, 14, 33]. However, a high level of production complexity affects the cost-effectiveness of these high-precision CAD/CAM retainers.

In order to primarily maintain the intercanine distance, 2‑point stainless steel retainers are considered a cost-efficient alternative to CAD/CAM retainers [6]. In contrast to modern CAD/CAM and multistranded wire retainers, 2‑point stainless steel retainers are bonded to the canines only, which has three main advantages: first, by enabling interproximal cleaning, an improvement of oral hygiene can be observed [3, 34]. Oral hygiene is simplified by the fact that dental floss only needs to be passed under the retainer once and can then, due to the absence of further adhesive points, be easily moved to the next tooth. Second, the failure rate of bonding site defects was described to be significantly lower compared to the multistranded wire retainers, with the latter being bonded to all anterior teeth [3, 27, 31]. Possible defects of 2‑point retainers can be immediately noticed by the patients [23]. Third, though rotational movements of lower incisor were noticed [23, 31], no restrictions of intrinsic tooth mobility were observed due to the passive retention character [3]. On the other hand, the high wire diameter of the retainer could induce patient discomfort and tends to be unsuitable for the upper jaw [27, 31]. In addition, it remains unclear to what extent a retainer, which is only attached to the canines, provides sufficient retention for the incisors over time in order to provide an optimal posttreatment retention protocol.

Up to now, there are no studies analyzing the stability of 2‑point retainers on translational and rotational movements and there is no information whether posttreatment changes in tooth position occur early within the first years of retention or later. Thus, the aim of the present study was to systematically analyze tooth position stability of patients being treated with 2‑point retainers using a digital surface-matching approach. To address the question whether potential posttreatment tooth movements occur directly after retainer placement or later, we analyzed the translational and rotational movements of the lower anterior teeth during a retention period of 1 and 3 years [21].

To analyze the potential risk over time for posttherapeutic tooth adaption within the lower anterior teeth treated with a 2-point lingual retainer, analog impressions from 66 patients were digitalized, digitally superimposed and analyzed after 1 year (group 1) and 3 years (group 2) of fixed retention (Fig. 2). Patients of both groups (T1 and T2) still wore the Hawley retainers at the time of evaluation.

Twistflex retainers are the gold standard in fixed retention [36]. Nevertheless, developing technologies allow manufacturing of custom-fabricated retainers using digital CAD/CAM systems. Due to their more precise fit [13], few side effects [14], and their potential impact on oral health [11], they have moved into the focus of modern orthodontics. These reported advantages often go along with high manufacturing effort as well as economic aspects [14]. Therefore, systematically analyzed long-term data have to provide the basis to evaluate modern retention concepts compared to conventional versions. Within this context, the present investigation aimed to provide systematic data on the long-term stability of the widely used 2‑point retainers.

In this study, a digital and user-independent analysis tool which requires expectable stable structures for registration and superimposition was used. In contrast to the palatal rugae in the upper jaw, which are known to be rather stable, such stable nondental structures are not available in the lower jaw [21, 32]. For this reason, molars and premolars had to be used as a reference for superimposing the models. These may undergo vertical settling after debonding, which can influence the precision of the superimposition. To counteract this interference factor, three teeth per side were used in combination with a best-fit registration method. In this manner, individual teeth with increased vertical settling should have influenced the superimposition as little as possible.

Tooth movements in all three spatial planes were investigated regarding translation and rotation under retention in 2‑point retainer cases after short- and long-term retention. Based on recent data indicating that a total of 50% retention loss can be expected 2 years posttreatment [1], it remained unclear which are the most affected teeth, at what time point the development of misalignment could be observed, and whether the misalignment will increase over retention time. The present investigation aimed to address these points by analyzing posttreatment tooth movements after 1 and 3 years of fixed retention. According to the present data, we were able to show only slight changes in tooth position after 1 and 3 years in both translational and rotational tooth movements. For the rotational movements, the observed changes were relatively balanced for each axis. In contrast, the analyzed translational tooth movements showed the largest changes in the vertical direction. This could be explained by the fact that the 2‑point steel retainer does not directly secure the vertical position of the teeth. Overall, canines showed the least rotational and translational movements. This can be explained by the adhesive bonding to these teeth which leads to expectable higher stabilization compared to the nonbonded incisors.

According to data from other published studies on fixed retention systems, the observed posttreatment changes of tooth position for the 2‑point steel retainers were less than for other retainer modifications [23, 31]. A direct comparison of the present data with other retainer studies is limited because in most studies the Little’s irregularity index and not a precise digital measurement system was used [2, 16, 20]. The Little’s irregularity index is more inaccurate and does not allow a statement about all three spatial planes of space and it does not differentiate between rotational and translational tooth movements. This might explain why the studies that applied the Little’s irregularity index method showed larger changes during the retention phase compared to our data [23, 31]. Furthermore, the additional use of Hawley retainers may have led to decreased tooth movements compared to the sole use of a 2-point retainer which has to be mentioned as a limitation of this study. Another limitation represents the study design with two independent cohorts for the 1‑ and 3‑year period. For this reason, comparative statements between T1 and T2 cannot be made.

We assume that the combination of the passive stabilization character at the lateral and central incisors by a secured overjet as well as the reliable preservation of the intercanine distance was sufficient for high stability values of the mandibular anterior teeth [6, 12].

Compared to previous investigations of our group, which analyzed posttreatment changes of twistflex retainers, the overall observed position changes of the teeth were reduced in the present cohort treated with 2‑point retainers [32]. A possible explanation for this might be the material properties of the various wires used in the different retention systems. Reported side effects such as the X‑effect and the twist effect seem to occur more frequently with twistflex retainers. These twistflex retainers are less rigid, feature low torsional load transfer, and are discussed to store energy in the area between the composite bonding sites which may be transmitted to the teeth and result in unwanted tooth movement over retention time [9]. However, despite of the newest technologies and developments, a fixed retention concept without any side effects is still not available today [28].

Another important aspect of the current retention concept is a low loss rate, which supports a possible lifelong retention. According to our data, there was no breakage of any of the included retainers. This can be explained by its diameter, which is much larger in comparison with the twistflex retainer [9]. Higher failure rates of multistranded wire retainers compared to round stainless-steel retainers bonded to the canines were also reported by other authors [24‐26, 31]. These increased failure rates might be attributed to higher rates of retainer fractures and twisting, as well as to activation of the retainers caused by either deficient bonding or trauma [6, 28].

The present study demonstrated that also in the modern age of orthodontics with innovative digital technologies, the traditional 2‑point steel retainer system presents an effective retention method. In the first few years after treatment, only very minor occlusal adaption has to be expected.