Background
Lingual orthodontics were introduced over three decades ago [
1], and in recent years the demand and thus provision of lingual orthodontic treatments have increased among patients seeking improved esthetic effects [
2]. A number of recent studies [
3,
4] have attempted to establish the advantages and inconveniences of lingual orthodontic appliances versus labial appliances.
Cephalometric analysis is used to study the craniofacial structures of the patient, and its results have an impact on treatment planning. Cephalometry is not a direct method for diagnosing the patient conditions, yet it offers details on the craniofacial structures of the patient and thus yields diagnostic information that is helpful in defining the orthodontic treatment strategy [
5]. With the advent of the computer age and our ever changing technological environment, digital imaging systems are becoming increasingly more popular than conventional film-based radiography. It is now possible to perform cephalometric tracing both through the use of digitizers and directly on screen-displayed digital images [
6]. Cephalometric analysis is widely used to evaluate the changes occurring after treatment with fixed appliances [
7‐
9], with the use of Herbst, Twin-Block [
10] or Frankel systems [
11], or to determine the cephalometric standards in a specific population [
12]. Cephalometric study is important for ensuring correct stability, for example through control of the position of the lower incisors with respect to point A and the pogonion (A-Po) and mandibular lines, and it must be taken into account that the movement of the lower incisors towards the A-Po line should not exceed ±2 mm from the original position [
13,
14]. Case reports in the literature have shown generally favorable clinical and cephalometric changes in patients treated with lingual appliances [
8,
15‐
18]. There have been several biomechanical and in vitro studies [
19‐
22] and case reports [
23] related to lingual appliances, but only a few clinical studies [
7‐
9] have compared their clinical outcome with that of labial appliances.
Apart from the undeniable esthetic advantages of lingual versus labial appliances, other biomechanical advantages have been described referred particularly to expansion, open bite or mass retraction [
24], torque, inclination and rotation of the teeth [
25,
26]. However, little evidence has been produced by clinical studies in this field, and a systematic review is therefore needed with the purpose of finding answers. Some systematic reviews and meta-analyses have been published comparing the adverse effects of labial and lingual fixed appliances [
27,
28]. However, to our knowledge, this is the first review to evaluate possible differences in treatment effects between the two techniques from the radiographic perspective, based on changes in cephalometric parameters.
Discussion
The present study offers a systematic review and exhaustive meta-analysis of the scientific literature, with the aim of answering the following question: Are there differences in treatment effects between labial and lingual fixed appliances? Cephalometric analysis confirmed that there was minimal change in the mandibular plane angle in the lingual group, though there was a slight tendency for the maxillary molars to extrude [
8]. Although from the cephalometric perspective the findings were similar in both groups, lingual orthodontics showed a tendency to increase the U1-L1/IIA angle and reduce the SN-U1 angle. Although there are some limitations, the results obtained offer useful clinical information that may serve as the basis for future investigations.
Some studies [
20‐
22] have compared the lingual orthodontic technique versus the labial technique. Most publications found to date have been in vitro studies comparing overall retraction between the two orthodontic techniques [
20], frictional resistance between the bracket lingual brackets and the labial brackets [
21], or the torque generated by the two techniques [
22]. Some authors [
37‐
39] have observed changes in the cephalometric measurements before and after labial orthodontic treatment, though few studies [
7‐
9,
31] have compared cephalometric parameters between the labial and lingual techniques used. One study [
40] analyzed four clinical situations (bite opening, incisor inclinations and torque control, incisor intrusion and soft-tissue profile) in 36 patients subjected to lingual orthodontic treatment. These authors found lingual appliances to apparently cause intrusion of incisors and extrusion of molars, resulting in clockwise mandibular rotation. Another study [
41] in 34 patients involved measurement of the angulation between the mandibular plane and the long axis of the mandibular central incisor, the distance from infradental level to the “D” point (bone height), and the distance from the incisal edge of the mandibular central incisor to the “D” point. The results obtained showed 57.6% of the cases to present an increase in labial alveolar bone height, while 30.3% exhibited a decrease in value, and 12.1% presented no change with the decrease in angulation between the long axis of the lower incisor and the mandibular plane (Go-Gn). The group presenting increases showed a significant prolongation of the distance “incisal edge to D point”, whereas this dimension decreased significantly in the rest of the cases.
Although it is more difficult to control incisor torque with the lingual orthodontic technique, a study [
42] has found retraction of the upper and lower incisors to be greater with lingual orthodontic treatment. A three-dimensional finite element study [
19] found lingual crown tipping to be more exaggerated with lingual appliances than with labial appliances. These results are consistent with those obtained in our systematic review, since lingual appliances tended to tip incisors by exerting lingual crown torque to a greater extent than labial appliances. However, once lingual crown tipping occurs, it is more difficult to correct with lingual orthodontics than with labial orthodontics [
19]. A study based on a mathematical model found the application of an intrusion force using lingual brackets to create clockwise rotation and lingual crown movement, while the application of an extrusion force using lingual brackets created counterclockwise rotation and labial crown movement. These findings have important clinical implications, since in malocclusions characterized by incisor retroclination (e.g., Class II, Division 2), intrusive force applied on a retroclined incisor using lingual brackets could aggravate the initial tooth position, making the tooth more retroclined. Clockwise rotation develops, aggravating the initial retroclination through labial root movement. The opposite occurs when extrusive force is applied with lingual brackets. An extrusive force on a retroclined tooth creates counterclockwise rotation, which may improve the inclination of the incisor [
43]. One study compared the dental-skeletal changes in 18 patients with lingual appliances versus 18 patients with labial appliances. The effectiveness of the Herbst appliance was evaluated in both groups by means of cephalometric analysis performed before and after treatment – the two techniques being found to yield similar results [
44].
With a prevalence of malocclusion of 45.6% among males between 18 and 21 years of age, it is easy to intuit that almost one-half of all evaluated patients will require orthodontic treatment [
45]. Incisor teeth crowding and misalignment of lower incisors are the most common types of malocclusions. In this systematic review, four publications [
7‐
9,
31] evaluated the cephalometric changes associated to the two orthodontic techniques, with very similar results referred to both orthodontic approaches. However, in the clinical setting, the practical implication may be that the tendency observed in lingual orthodontics to tip the incisor crown lingual and reduce the inter-incisor angle is favorable in certain types of malocclusion such as biprotrusion cases. On the other hand, what proves advantageous in cases of biprotrusion may prove inconvenient in situations requiring increased inclination of the upper incisors, as in Class II Division 2 malocclusions. This may be due to the biomechanical differences between the two types of orthodontic approaches – lingual brackets operating closer to the center of resistance. Furthermore, there is greater torque control, since the vertical position of the point of application of the force is taken into account. The point of application in the lingual arches is more gingival than in the labial arches, thereby allowing greater anterior tooth torque control. The lesser interbracket distance in the case of lingual orthodontics may be another implicated factor [
24].
Although the results of our study must be viewed with caution, they could represent a starting point for future research leading to the generation of a consensus document allowing selection of the type of orthodontic approach not only conditioned to the esthetic requirements of the patient but also considering the characteristics of the malocclusion. On analyzing the parameters measured by the different studies, only some of them appear to show some coincidence, for example referred to SNA and ANB [
8,
31], SNB [
8,
9,
31], and the angles U1-SN, L1-MP and SN-MP [
8,
9]. In contrast, in relation to the measured distances, none of the four articles cited in this study appear to coincide; the resulting arbitrariness therefore precludes comparison. This is due to the great variety of existing cephalometric analyses attempting to assess one same parameter but in different ways. As an example, assessment of the maxillary skeletal anomalies in the cephalometric analysis published by Steiner is based on measurement of the SNA angle, while in the analysis of Ricketts it is based on maxillary depth and facial convexity. In the same way that soft tissue analysis has been standardized in orthognathic surgery based on the analysis of Arnett and Bergman [
46,
47], international consensus would be needed referred to cephalometric studies in orthodontics, in order to prevent some articles from being neglected due to the impossibility of establishing comparisons with other publications – all because each investigator indistinctly uses one analytical approach or other.
Strengths and limitations of study
Our study has several strengths. A particular strength is that in order to minimize publication bias, we performed a systematic search of different international medical databases. Moreover, no language limitation or publication date was set in order to ensure inclusion of as many data as possible from appropriate studies. Two reviewers independently chose, extracted, and evaluated data quality in order to reduce bias and transcription errors. In addition, the statistical power of the study was 90%. To our knowledge, this is the first systematic review and meta-analysis to evaluate possible differences in treatment effects between labial and lingual fixed appliances. However, our systematic review also has certain limitations. A first limitation is the small number of articles available for review, which may cause penalization by a degree of type β error. Secondly, since fewer than 10 studies were included, funnel plots and Begg’s rank correlation test were not performed [
30,
36]. Lastly, we must mention the fact that the publications included in the meta-analysis were non-randomized studies, which are rated as having a lower level of evidence than randomized controlled trials (RCTs).
Conclusions
The present systematic review found no statistically significant cephalometric differences between the lingual and labial orthodontic techniques. However, there was a tendency to increase the interincisal angle and reduce the angle between the major axis of the upper central incisor and the sellar-nasion plane. These findings indicate that treatment with lingual appliances favors incisor tipping by exerting lingual crown torque. However, because of the small number of included studies, the results of this meta-analysis should be interpreted with caution. Future research is advisable, leading to the generation of a consensus document allowing selection of the type of orthodontic approach not only conditioned to the esthetic requirements of the patient but also considering the characteristics of the malocclusion. On the other hand, standardized international guidelines are lacking; the measurements of angles and distances therefore have to be unified with a view to future investigations.