Cephalometric evaluation of changes in vertical dimension and molar position in adult non-extraction treatment with clear aligners and traditional fixed appliances

ABSTRACT Introduction: Orthodontists have been using clear aligners to treat malocclusions, and one potential effect of treatment with orthodontic aligners is the intrusion and/or resists extrusion of the posterior teeth. This “bite-block effect” is primarily anecdotal due to the frequent occurrence of posterior open bites in patients after clear aligner therapy. Objective: The purpose of this study was to compare changes promoted by clear aligners and traditional fixed appliances in cephalometric measurements of the vertical dimension and molar position in adult patients with Class I malocclusion treated with non-extraction. Methods: Pre- and post-treatment lateral cephalometric radiographs of adult patients treated with either clear aligners (n=44) or traditional fixed appliances (n=22) were selected for retrospective analysis. Eight interval measurements and one nominal measurement were evaluated: anterior overbite (OB), mandibular plane angle related to cranial base (SN_MP) and related to Frankfort (FMA), lower molar height (L6H) and upper molar height (U6H), palatal plane to mandibular plane angle (PP_MP), lower facial height (LFH), total facial height (TFH), and posterior open bite (Posterior_OB). A single evaluator traced all cephalographs, and changes in select measures of the vertical dimension were compared within and between groups. Results: OB decreased (1.15 mm) and L6H increased (0.63 mm) in the traditional fixed appliance group. Mandibular plane angles (related to cranial base and to Frankfort) increased (0.43° and 0.53°, respectively) in the clear aligner group, but just FMA showed significant difference between groups (difference of 0.53°). LFH and TFH increased (ranging from 0.52 mm to 0.80 mm) in both groups, with no differences between treatment modality. Presence of visible posterior open bite significantly increased over the course of treatment. OB, FMA and L6H exhibited an interaction between treatment stage (pre- and post-treatment) and modality (clear aligner therapy and traditional fixed appliances), but no interaction among these three variables was found. Conclusions: The evidence does not support the theory that clear aligner therapy produces better vertical dimension control than traditional fixed appliances. Traditional fixed appliance therapy slightly extruded the lower molar, and clear aligner therapy produced a slightly mandibular backward rotation.


INTRODUCTION
Orthodontists have traditionally focused on anteroposterior dentoskeletal relationships, but many malocclusions are due to abnormal vertical development. Vertical control during orthodontic treatment has been a challenging problem in orthodontics. 1,2 Therefore, it is often an objective to maintain or decrease vertical dimension in orthodontic patients, especially in hyperdivergent partients. 1,3 Successful treatment depends on the orthodontist's ability to control vertical tooth movements, because the extrusion of the posterior teeth is the main etiology of the unwanted side effects, such as backward mandibular rotation. 1,4 In the past, orthodontists have traditionally addressed the vertical dimension of patients with high-pull headgear, both with and without extractions, but this approach appears to have little or no effect on the anteroposterior position of the mandible. [5][6][7] Today, some orthodontists may even attempt to reduce the mandibular plane angle, producing forward mandibular rotation using miniplates 8 and miniscrews. 1 Orthodontists know that fixed appliance therapy tends to extrude teeth, increasing the mandibular plane angle. 3 Recently, orthodontists have been using clear aligners to treat malocclusions due to esthetics, convenience, and comfort. 9,10 Additionally, as the materials and techniques advance, more Rask H, English JD, Colville C, Kasper FK, Gallerano R, Jacob HB -Cephalometric evaluation of changes in vertical dimension and molar position in adult non-extraction treatment with clear aligners and traditional fixed appliances 5 cases can be adequately treated with aligners. [11][12][13] One potential effect of treatment with orthodontic aligners is the "biteblock effect". In theory, the thickness of the aligner plastic combined with occlusal forces leads to intrusion and/or resists extrusion of the posterior teeth over the course of treatment.
Evidence for this "bite-block effect" is primarily anecdotal and substantiated by the frequent occurrence of posterior open bites in patients after clear aligner therapy. 11,14,15 Some practitioners even recommend clear aligner therapy for patients who present with anterior open bite tendency, due to this alleged benefit. 16,17 As demand for clear aligner therapy increases, it is imperative for the orthodontist to understand how they act on the oral system.
Both traditional fixed appliances and clear aligners work by applying forces to teeth. Despite utilizing the same principles, there are many differences between the treatment modalities.
A key difference is the ability to remove orthodontic aligners, which makes patient compliance imperative. Another significant difference is the appliance design. Aligners are polymer trays that fit snugly around the teeth, allowing force application from various directions, as opposed to traditional braces, which act primarily through the bracket on the buccal surface. 18 This difference leads to a number of advantages (e.g. patient comfort) 10 and disadvantages (e.g. limitations in amount of movement per aligner) associated with clear aligner therapy. 11 Rask H, English JD, Colville C, Kasper FK, Gallerano R, Jacob HB -Cephalometric evaluation of changes in vertical dimension and molar position in adult non-extraction treatment with clear aligners and traditional fixed appliances 6 It is important to investigate these claims and understand exactly how clear aligner therapy affects the vertical dimension in adult patients (the primary population requesting aligner therapy). If clear aligner therapy does limit changes in the vertical dimension or provides true intrusion, it could become a valuable treatment tool. However, if intrusion is not occurring, then the familiar post-treatment posterior open bite is due to other occurrences such as anterior interferences. It is the responsibility of orthodontists to understand the effects of their appliances, to provide the highest quality treatment results.
The aim of this longitudinal retrospective study is to compare changes in cephalometric measurements that represent the vertical dimension and molar position, before and after treatment in adult patients with Class I malocclusion treated with non-extraction, single-phase comprehensive treatment using clear aligners (Align Technology, Santa Clara, CA, USA) and traditional fixed edgewise appliances (Forestadent, Pforzheim, Germany). The null hypothesis was that there are no differences between treated groups. In addition, radiographs that were not of diagnostic quality

CEPHALOMETRIC ANALYSIS
Standard lateral cephalometric radiographs of the selected patients were recorded at two stages: pre-orthodontic treatment (T 1 ) and immediately after orthodontic treatment (T 2 ).   were seen on each lateral cephalogram (Fig 2). The poste-rior_OB was described as yes or no (lack of occlusal contact between maxillary and mandibular molars or contact between maxillary and mandibular molars, respectively).

STATISTICAL ANALYSIS
The evaluator randomly selected and retraced 10 sets of records (5 clear aligner and 5 traditional fixed appliance subjects) one month after the initial measurements were recorded.
Intraobserver systematic errors between the replicate scans were described as mean differences and statistically compared with paired t-tests. Intraobserver random error was estimated using intraclass correlation coefficients (ICCs) and method errors (√(Σd 2 /2n). 20 Sample size calculations were not performed prior to the study.

RESULTS
Systematic intraobserver reliability ranged between 0.02 mm and -1.01 mm ( Table 2). Five out of eight measurements (pres- ence of posterior open bite was identical between both replicates and therefore not included) were statistically significant ( Table   2). L6H showed the largest significant difference between the first and second replicates, with the first replicate 1.01 mm less larger than the second replicate (0.16 mm). Method error ranged from 0.25 mm to 1.14 mm and from 0.35° to 1.58° (Table 2). All measurements showed highly reliable interclass correlation (ICC), ranging from 0.978 to 1.000 (Table 2). Intraobserver errors were deemed to be within an acceptable range.
Invisalign group presented older patients than brackets group   Statistical analysis showed significant difference between groups only for one out of eight variables measured (Table 3). In comparison to the Brackets group, the Invisalign group initially had larger lower molar height (≈2.0 mm). In general, Invisalign group showed slightly larger dispersion than Brackets group (Fig 3).   Treatment modality produced significant group change differences (Table 4). OB decreased more in the Brackets group than   OB, FMA and L6H all exhibited a statistically significant interaction between treatment stage and treatment modality (Table 5 and Figure 5). Since L6H had a significant interaction between treatment modality over the course of treatment, and because extrusion is known to be more serious in high angle patients,

CONCLUSIONS
Within the limitations of this study, the following conclusions can be drawn: 1. Anterior overbite decreased more in the traditional fixed appliance therapy than clear aligner therapy.
2. Traditional fixed appliance therapy promoted a slightly larger lower molar extrusion than clear aligner therapy.

Both therapies (clear aligners and traditional appliances)
increased the total facial height and lower facial height. Overall responsibility:

HBJ.
Patients displayed in this article previously approved the use of their facial and intraoral photographs.
The authors report no commercial, proprietary or financial interest in the products or companies described in this article.