Participants

The samples used in this study were collected from 2008 to 2013 at the Peking University School and Hospital of Stomatology. Detailed information regarding their recruitment process, miniscrew implantation, and orthodontic treatment has been previously published [20, 21]. The study included 20 patients (14 females and 6 males) aged 21–41 years, with a mean age of 24 years. The sample comprised 13 Angle Class I and 7 Class II malocclusions, including 11 skeletal Class I and 9 skeletal Class II malocclusions. The average overbite (OB) was 2.9 mm, with one case presenting with an open bite, and the average overjet was 4.1 mm. Ethical approval for this study was obtained from the Biomedical Ethics Committee of Peking University (approval number: IRB00001052-09010). Informed consent was obtained from all participants.

To clearly investigate the impact of miniscrews in the process of space closure, this study specifically focused on patients with mild crowding who primarily sought relief from protrusion. Individuals with craniofacial growth and development issues or pathological conditions were excluded to minimize the influence of confounding factors. The inclusion criteria were as follows: (1) age > 18 years, (2) significant protrusion of the upper teeth, (3) extraction of the maxillary first premolar, (4) utilization of the miniscrew sliding technique for space closure, and (5) overall good health without chronic diseases or disabilities. Exclusion criteria were as follows: (1) upper dentition crowding > 4 mm, (2) congenital loss of maxillary permanent teeth (except for the first premolars and third molars), (3) history of cranial or facial trauma, and (4) cleft lip and palate or syndromic conditions.

Treatment procedure

During the treatment, we used two types of self-drilling miniscrews (diameter: 1.6 mm, length: 11 mm; Ci Bei Corp., Zhejiang, China). Two of these miniscrews were loaded and inserted into the buccal interradicular space between the maxillary second premolar and the first molar on either side. This location offered a clear operating field and minimized the risk of injuring tooth roots. Four additional miniscrews were placed but left unloaded. The unloaded miniscrews were inserted between the lateral incisor and canine in the anterior region and between the first and second molars in the posterior region.

All patients underwent treatment with McLaughlin-Bennett-Trevisi (MBT™) straight wire appliances. Nickel-titanium wire was used in the initial aligning and leveling stage. For space closure using the miniscrew sliding technique, we employed a 0.019 inch × 0.025 inch stainless steel wire along with free traction hooks (4 mm in length) placed between the maxillary lateral incisors and canines. This wire size was selected for its adequate strength to resist arch wire deformation. A traction force of approximately 150 g per side was applied using powerchains for space closure. Aside from the miniscrews, no additional devices were used for anchorage enhancement. On average, the duration of anterior teeth retraction using miniscrews was 11.8 months.

Data acquisition and measurement

Maxillary impressions were taken at two key time points: before treatment (T0) and after space closure using the miniscrew sliding technique (T1). These impressions were captured using silicone rubber and subsequently used to create plaster models. To digitize the maxillary models, we employed a 3D laser scanner (3Shape R700, 3Shape A/S, Copenhagen, Denmark) with a scanning accuracy of ± 0.02 mm. For Cone Beam Computed Tomography (CBCT) scans, the same technician and machine (DCT Pro, Vatech Co., Yongin-Si, Korea) were used at both T0 and T1. The scan parameters were as follows: a scanning field of 20 cm × 19 cm, a tube voltage of 90 kV, a tube current of 7 mA, and a scanning duration of 15 s. During the scans, patients were positioned in their natural head posture with the mandible in the intercuspal position.

Three-dimensional displacement of maxillary first molars and maxillary central incisors

The three-dimensional displacement of the first molars and maxillary central incisors was measured on digital dental models using Rapidform2006 (INUS Technology Inc., Seoul, Korea). The measurements and their definitions were shown in Table 1.

Table 1 Variables and their definitionsMaxillary digital dental model superimposition

Maxillary digital dental models at T0 and T1 were superimposed using stable miniscrews (Fig. 1).

Fig. 1

(a) Maxillary digital dental model with 6 miniscrews before treatment (T0); (b) Maxillary digital dental model with 6 miniscrews after space closure (T1); (c) The result of miniscrew superimposition. All 6 miniscrews overlapped well and showed a perfect superimposition result

Landmark transfer

Mesiobuccal cusps of the maxillary first molars and midpoints of the central incisor edges were marked on the T0 digital models. The landmarks were transferred from the T0 model to the T1 model through individual tooth registration. Errors from tooth landmark identification were excluded (Fig. 2).

Fig. 2

The three-dimensional local reference coordinates used in this study

Local reference coordinate establishment

The occlusal plane, based on the mesial buccal cusps of the maxillary first molar and midpoints of the central incisor edges, was defined as the transverse plane. Two points were marked on the palatal suture as points A and B, and projected onto the occlusal plane to obtain points A’ and B’. We set point B’ as the origin, B’-A’ as the x-axis, and B’-B as the z-axis to establish a three-dimensional reference coordinate.

Tooth movement measurement

Using the local reference coordinate system, we assessed the three-dimensional displacement changes in the maxillary first molars and maxillary central incisors using specialized software. All measurements represent the averages of replicates from independently conducted landmark location and digital cast superimposition procedures. The changes observed on the right and left sides were averaged to ensure accuracy and consistency of assessment.

Vertical changes in the mandibular plane

To measure changes in the mandibular plane, we utilized CBCT scans and defined the mandibular plane in three-dimensional space as a plane formed by the gonion (Go) and menton (Me) points on either side. Initially, the patient’s computed tomography (CT) data were saved in Digital Imaging and Communication in Medicine (DICOM) format and managed using an interactive medical image control system (MIMICS 10.0, Materialise, Leuven, Belgium). This system was employed to construct a three-dimensional surface model of craniofacial hard tissues. The resulting data were then exported in STL format and imported into Rapidform 2006. Stable anatomical regions, such as the cranial base, frontal bone, and cheekbones, were used as reference points for superimposition. To quantify the change in the mandibular plane, we measured the angle between the mandibular planes at T1 and T2, denoted as ΔMP (Fig. 3). We adopted the landmark transfer technique, specifically referencing the plane transfer technique where a reference plane is established using three marked points. Three landmarks were located on the mandibular surface of the model at T0 and then transferred to the surface model at T1 through the superimposition of stable areas within the mandible (Fig. 4). Two observers, both possessing similar clinical experience, underwent simultaneous calibration training encompassing various aspects, including software operation, landmark definition, selection of stable regions, and other relevant procedures. Subsequently, the measurements were carried out twice by the trained observers following the same standardized procedure, and the results were averaged for accuracy and consistency.

Fig. 3

(a) Stable areas, such as the cranial base, frontal bone, and cheekbones were used for superimposition at T0 (gray) and T1 (blue); (b) Mandibular planes are shown in white (T0) and green (T1) colors in the T0 model (gray) and T1 model (blue); (c) Rapidform calculation of the angle between the two planes, which indicated the change in the mandibular plane, is shown

Fig. 4

(a) Mandible at T1; (b) Mandible and mandibular plane at T0 that were bonded with each other; (c) By mandibular superimposition, the mandibular plane on the T0 model was transferred to the T1 model, which excluded the errors from tooth landmark identification at different timepoints

Statistical analysis

Data analysis was performed using SPSS Statistics (version 23.0, IBM Corp., Armonk, NY). P-values < 0.05 were considered statistically significant. The statistical hypothesis was that maxillary miniscrew sliding technique would allow the maintenance of the original sagittal and vertical factors, which could be quantified through changes in upper first molars, central incisors, and the mandibular plane. Based on this statistical hypothesis, one-sample t-tests were conducted to compare changes observed between T0 and T1 with a reference value of 0. This allowed us to assess alterations during the period of space closure using the maxillary miniscrew sliding technique. A statistically significant result indicated that a vertical or sagittal change occurred during space closure with the maxillary miniscrew. Conversely, a lack of statistical significance indicated that the positions of the upper first molars, central incisors, and mandibular plane remained stable throughout the treatment. To evaluate the reliability of measurements obtained from digital dental models and the 3D mandibular plane change measurement method, we calculated intraclass correlation coefficients (ICCs) between the two observers. ICC values > 0.9 were considered to indicate excellent reliability in the measurements.