The treatment effects of maxillary asymmetric mechanics with the use of the mesial-distalslider: A retrospective study

Introduction

This study had 2 objectives: (1) to evaluate the clinical efficacy of the Mesial-Distalslider (MD) appliance when used for simultaneous maxillary molar mesialization and distalization in patients with a maxillary asymmetrical relationship and (2) to compare the outcomes with those achieved using the unilateral Mesialslider (UM) appliance.

Methods

The sample included 40 subjects (25 females and 15 males) with a mean age of 22 years (range, 8-55 years). The patients were divided into 2 groups: 20 patients treated with an MD appliance (group 1) and a group of 20 patients treated with a UM appliance (group 2). Superimposition of digital dental models using regions of interest on the palate was performed to evaluate the 3-dimensional molar movements, the proclination or retroclination of the maxillary incisors, and the displacement of the maxillary midline.

Results

At the end of the treatment, the MD group’s total movements were 4.5 ± 2.2 mm (sagittal), −0.4 ± 2.4 mm (transverse), and 0.3 ± 0.9 mm (vertical) along the mesialization side and −2.4 ± 1.7 mm (sagittal), −0.5 ± 1.5 mm (transverse), and 0.2 ± 1.4 (vertical) along the distalization side. UM group total movements were 5.5 ± 3.6 mm (sagittal), −0.4 ± 2.7 mm (transverse), and 0.1 ± 2.0 mm (vertical). Incisor displacements were minimal. No statistically significant differences were found between the MD and UM groups for all the parameters.

Conclusions

The use of the MD enables controlled movements in the sagittal direction with negligible dental side effects in the other planes (transverse and vertical). Furthermore, it facilitates asymmetrical movements simultaneously in the desired direction without the patient’s compliance and provides good anchorage control.

Highlights

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The Mesial-Distalslider can be considered a reliable device for asymmetric mechanics.
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The study analyzes asymmetric mechanics by using the digital models superimpositions.
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The use of the Mesial-Distalslider results in predictable desired movements.

The orthodontic correction of dental asymmetries is often considered a complex and demanding process, primarily because of incorrect and poorly planned treatment mechanics, especially when conventional anchorage systems are employed. The management of malocclusion with some degree of asymmetry is usually difficult to perform because achieving a total correction of the malocclusion is necessary to use asymmetrical mechanics in the right and left quadrants of the dental arch. In case a precise and detailed analysis of the force systems has not been completed before the beginning of the therapy, relevant undesirable side effects of the mechanics may occur.

In the literature, ^, several methods suggest the use of unilateral or asymmetrical elastics, but potential side effects such as a mandibular midline shift, maxillary arch rotation, jaw discrepancy, and transverse occlusal canting should be taken into consideration. These side effects will depend on the magnitude and point of application of the force, as well as on the duration of wear of elastics.

Hence, predictable anchorage control is very important to preserve the mandibular midline as well as the buccal or lingual tipping of the maxillary and mandibular incisor during asymmetrical mechanics.

Over the last decade, the use of mini-implants has become a routine treatment modality because reliable anchorage has proved to be very helpful in reducing dental side effects. ^, The potential correction by using buccally inserted mini-implants is limited to about 1-2 mm because they are placed in the path of the moving teeth. To correct intraarch dental asymmetry through sliding mechanics, the anterior palate was shown to be the most appropriate area for skeletal anchorage because all of the teeth can be moved without interference.

The Mesial-Distalslider (MD) appliance, ^, attached to 2 coupled mini-implants in the anterior palate, has been suggested as a solution for the protraction and retraction of the maxillary dentition either unilaterally or bilaterally in asymmetrical treatment. It combines the mechanics of the Beneslider ^, and Mesialslider, ^, allowing simultaneous distalization and mesialization in the maxillary arch. However, there is no quantitative assessment of the treatment effects of the MD appliance.

This study aimed to evaluate the clinical efficacy of the MD appliance when used for simultaneous maxillary molar mesialization and distalization in maxillary dental asymmetry and to compare the outcomes with those achieved using the unilateral Mesialslider (UM) appliance.

Material and methods

Patients were selected from the Orthodontic Department, Faculty of Medicine, Heinrich Heine University Düsseldorf, Düsseldorf, Germany.

Inclusion criteria: patients with a maxillary midline deviation associated, in some cases, with unilateral missing teeth. Patients requiring mesialization on one side and distalization on the other were assigned to group 1 (MD), whereas those needing unilateral mesialization were assigned to group 2 (UM).

Exclusion criteria were (1) craniofacial syndromes, (2) systemic diseases or comorbidities, (3) moderate or severe periodontitis, or (4) pharmacotherapeutic exposure with possible effects on bone metabolism.

After the application of the inclusion and exclusion criteria, a retrospective single-center study was conducted with a sample of 40 subjects (25 females and 15 males).

At the beginning of the treatment, the mean age was 19.6 years (range, 8-49 years) in group 1 and 24.6 years (range, 9-55 years) in group 2. The total mean age was 22.1 years (range, 8-55 years) for both groups.

The patients were divided into 2 groups according to the type of malocclusion and the appliance used. Twenty patients presented with maxillary arch asymmetry along the sagittal axis with a deviation of the maxillary midline. Unilaterally missing teeth were treated with an MD appliance (group 1) ( Fig 1 , A and B ), and 20 patients with an initial indication for unilateral maxillary molar protraction were treated with UM appliance (group 2) ( Fig 2 ).

The median raphe was used to detect the maxillary arch asymmetry. Maxillary midline deviation was assessed by making linear measurements from the midpoint between the central incisors to the median raphe line and facial midline. On the maxillary digital model, 2 points—the first at the distal aspect of the incisive papilla and the second adjacent to the second rugae—defined the median raphe. An anterior reference line was constructed perpendicular to the median raphe through the midpoint between the central incisors using the horizontal default position of the software with grids. The position of the maxillary first molars in the anteroposterior direction was measured longitudinally as it was the distance from the mesiobuccal cusp of the maxillary first molars to the anterior reference line on the first and second quadrants. Asymmetries were calculated by determining the absolute difference between homologous measurements as related to the reference lines.

The posterior dentition was moved distally and mesially through sliding mechanics attached to the permanent molars. Nickel-titanium coil springs were mounted to the anterior or posterior part of the appliance’s rail. To achieve optimal treatment results, the nickel-titanium coil spring generated a force of 240 g. It was activated once a month to maintain a nearly constant force during the sagittal movements.

At the beginning of the treatment, written informed consent was signed by all patients after they had received detailed information about their future orthodontic treatment and research participation. The study was approved by the ethical committee of the Heinrich Heine University of Düsseldorf (no. 2023-2673) and was conducted according to its guidelines.

Three-dimensional (3D) scans were conducted with an intraoral scanner (3Shape Trios 3, Copenhagen, Denmark). The reconstruction and superimposition of the 3D digital models were performed using the open-source 3D modeling software Blender. The pretreatment (T0) dental models were aligned to the occlusal plane defined by the mesiobuccal cusps of the first molars and central incisors. Subsequently, a course alignment of the posttreatment (T1) models to the T0 models was performed by hand. The fine alignment was performed by matching the palatal structures (rugae and palatal slope) using the Iterative Closest Point algorithm. The alignment was restricted to an area in the anterior palate that is usually not affected by tooth movements ( Fig 3 , A and B ).

Landmarks were placed at (1) the mesiobuccal cusp of the maxillary first molars to assess sagittal, transverse, and vertical movements along the y-, x-, and z-axes; (2) the midpoint on the edge of the maxillary central incisors to assess the proclination and retroclination along the y-axis; and (3) the midpoint between the 2 central incisors to evaluate the maxillary midline displacement along the x-axis. The reference points were marked with blue dots (T0) and red dots (T1) on the digital models. The change in the translation of each tooth between the T0 and T1 models was measured in millimeters through lines connecting landmarks.

The y-axis indicates sagittal movements, whereas the x- and y-axis indicate transverse and vertical movements, respectively. A movement in the positive direction along the y-axis indicated mesial movement. Positive values in the x- and z-axes indicated expansion and extrusive tooth movements, respectively. Negative values in the x-, y- and z-axes indicated constriction, distal, and intrusion tooth movements, respectively ( Fig 4 , A and B )

Because of the nature of the intervention, it was not possible to blind the patients or the orthodontist. The researcher and the statistician who evaluated the data were blinded.

Statistical analysis

The Shapiro-Wilk test was used to verify the normality of the distribution of data. Continuous variables were presented as mean and standard deviation and categorical variables as count. Differences between the 2 groups were tested using the Mann-Whitney U-test and were presented in the table as median and interquartile range. Spearman’s rho coefficient was employed to analyze the correlation between the variables of 3D molar movements as well as incisor and midline displacement. A P <0.05 was considered statistically significant. All statistical analyses were performed using SPSS software (version 20.0; IBM, Armonk, NY).

Ten randomly selected sets of digital models were digitized and measured 2 weeks after the first set of recordings to calculate the reliability of repeated measures by 2 different investigators (M.E.D.F. and M.K.). The intraclass correlation coefficients showed good interindividual (mean of 0.90) and intraindividual agreement (mean of 0.92).

Results

The study group consisted of 40 patients, and the mean age at the start of treatment was 19.6 (MD) and 24.6 (UM) years. The duration of the active procedure was 2.4 years for the MD group and 2.1 years for the UM group from T0 to T1 ( Table I ).

Table I

Patient characteristics at baseline.

Appliance	Malocclusion			No. of participants			Age, y	Treatment duration, y
Appliance	Class I	Class II	Class III	Males	Females	Total	Age, y	Treatment duration, y
MD	9	8	3	7	13	20	19.60	2.4
UM	12	4	4	9	11	20	24.65	2.1

At the end of the treatment, the MD group showed average movement for molars of 4.5 ± 2.2 mm, −0.4 ± 2.4 mm, 0.3 ± 0.9 mm in the sagittal, transverse, and vertical direction along the mesialization side, and −2.4 ± 1.7 mm, −0.5 ± 1.5 mm, and 0.2 ± 1.4 mm along the distalization side, respectively. The UM group showed 5.5 ± 3.6 mm, −0.4 ± 2.7 mm, and 0.1 ± 2.0 mm for the sagittal, transverse, and vertical movements of the molars, respectively ( Tables II and III ).

Table II

Descriptive analysis of MD group.

Variables	Mean ± standard deviation	Minimum	Maximum	Percentile
Variables	Mean ± standard deviation	Minimum	Maximum	25th	Median	75th
Mesialization side
Molar movement (mm)
y-axis (anteroposterior)	4.54 ± 2.25	0.00	8.43	3.15	4.98	5.80
x-axis (transversal)	−0.40 ± 2.47	−5.01	4.30	−1.44	−0.19	1.17
z-axis (vertical)	0.39 ± 0.99	−1.90	1.65	−0.28	0.46	1.26
Incisor movement (mm)
y-axis (anteroposterior)	0.95 ± 1.77	−2.89	5.36	0.00	0.37	1.99
Distalization side
Molar movement (mm)
y-axis (anteroposterior)	−2.44 ± 1.76	−4.67	−0.03	−4.23	−2.43	−0.58
x-axis (transversal)	−0.58 ± 1.55	−2.91	1.46	−1.95	−0.61	1.00
z-axis (vertical)	0.20 ± 1.44	−2.04	2.44	−0.52	−0.16	1.63
Incisor movement (mm)
y-axis (anteroposterior)	0.63 ± 0.74	−0.42	1.82	0.16	0.36	1.30
Midline
Midline change (mm)
x-axis (transversal)	0.78 ± 0.44	0.09	2.05	0.47	0.72	1.02