Material and methods

The protocol of the study was approved by the ethics committee of Nippon DentalUniversity (number T2014-34).

This was designed as a retrospective cross-sectional study. The sample size was estimated to have an effect size of 0.973, which was determined based on a previous study using the G-power statistical program (version 3.1; Heinrich Heine Universitat Dusseldorf Experimentelle Psycologie, Dusseldorf, Germany). Power analysis indicated that the required minimum sample size for each group was 18 subjects to detect this effect size with 80% power and a significance level of 5%.

Approximately 100 male and female subjects with normal occlusion who were initially selected from a total population of about 4000 students at Nippon Dental University and other related schools were evaluated by 2 orthodontists. The following inclusion criteria were used in the initial selection process.

• 1.

  Age 18 years or older without previous orthodontic treatment or congenital craniofacial anomalies such as cleft lip or palate.

• 2.

  Straight profile without strain on the lips or mentalis muscle.

• 3.

  Normally erupted permanent teeth from second molar to second molar in the maxillary and mandibular arches with an Angle Class I relationship without anterior or posterior crossbites, and no obvious asymmetric continuous dental arch.

• 4.

  No teeth with abnormal crowns (including fused teeth, macrodont or microdont teeth, severe tooth wear, fractured teeth, minimum restorations that cover incisal edges or cusp tips that obstruct the measurements for this study), no rotated, displaced, impacted, transposed teeth or prolonged retained deciduous teeth.

• 5.

  Healthy periodontal tissues without gingivitis or gingival recession.

Based on these inclusion criteria, 69 women were selected. The purpose of this study, the protocol, and the potential risk due to radiation exposure during cephalographic imaging were explained in writing, and 58 subjects agreed to participate and provided informed consent. Dental casts and cephalograms of these subjects were taken. The dental casts were measured using a digital caliper (NTD12-15C; Digimatic, Mitutoyo, Kawasaki, Japan), and subjects with the following criteria were further selected: overbite and overjet, +1.0-3.0 mm; curve of Spee, <1.5 mm in the mandibular dental arch; and arch length discrepancy, ± 2.0 mm in the maxillary and mandibular arches. Furthermore, lateral cephalograms were analyzed by a software program (version 11.5; Dolphin Imaging, Chatsworth, Calif), and subjects with the following were selected: ANB angle, 2.5° ± 2.0°. Accordingly, 33 women (mean age, 20.7 ± 2.3 years; range, 18-29 years) were selected as the control group.

For the MxAC group, dental casts, facial and intraoral photographs, lateral cephalograms, and treatment records of approximately 4200 patients diagnosed at Nippon Dental University Hospital between July 1998 and December 2015 were reviewed by 2 evaluators in the Department of Orthodontics to select female patients aged 18 years or older with the following inclusion criteria.

• 1.

  No previous orthodontic treatments or congenital anomalies including cleft lip or palate.

• 2.

  Straight facial profile without signs of lip incompetence.

• 3.

  Completely erupted permanent teeth from second molar to second molar in the maxillary and mandibular arches except for third molars (subjects with incomplete eruption of a maxillary canine were excluded).

• 4.

  Crowding (arch length discrepancy, <−4.0 mm) in the maxillary dental arch and at least 1 maxillary canine displaced in the facial direction from the dental arch (ie, the tip of the canine must be displaced on the facial side from the line between the center of the lateral incisor edge and the buccal cusp tip of the first premolar observed in the occlusal plane).

• 5.

  No abnormal crowns (eg, fused teeth or macrodont or microdont teeth), supernumerary teeth, transposed teeth, prolonged retained deciduous teeth, severe dental wear, occlusal attrition, fractures, minimum restorations that cover incisal edges, or cusp tips that obstruct the measurements for this study (except for the third molars).

• 6.

  Healthy periodontal tissues without gingivitis or gingival recession.

Accordingly, dental casts and lateral cephalograms of 174 patients were evaluated. Those with Angle Class I first molar relationships without posterior crossbite were selected. Additionally, patients with tooth rotation greater than 45° were excluded based on observation of the maxillary dental cast in the occlusal plane. The dental casts of the selected patients were measured using the digital caliper, and then subjects with overbite and overjet within +1.0 to 4.0 mm were selected.

Maxillary dental casts of subjects in the control group were scanned using a 3-dimensional (3D) laser scanner (Surflacer model VMS-100F; UNISN, Osaka, Japan), and reference points were identified at the center of incisor edges, cusp tips of canines, and buccal cusps of premolars using 3D point cloud evaluation software (Inspect version 7.5; GOM, Braunschweig, Germany). Three-dimensional coordinates were then converted to a file for Microsoft Office Excel 2013 (Microsoft, Redmond, Wash). Next, the fourth-order polynomial equation was fit using the least squares method to create a curve through the reference points at each tooth excluding both lateral incisors and canines in the occlusal plane. The horizontal distances between the curve and the reference points at the lateral incisor and canine were calculated as facial-palatal displacement of the lateral incisor and canine, respectively ( Fig 1 ). The distance toward the facial direction was defined as a positive value, and palatal displacement was defined as a negative value. At this point, the subjects in the control group were considered unilaterally, and palatal displacement of lateral incisors was subtracted from facial displacement of the canines for 66 sides of the 33 subjects. These data were pooled, and the means and standard deviations (0.86 ± 0.72 mm) was calculated. To evaluate facial-palatal displacement of the lateral incisors and canines of the 174 patients initially selected for the MxAC group, 2 SD was added to the mean value obtained in the control group and determined as the standard value (2.31 mm) for the facial-palatal positional relationship between the lateral incisors and canines. Thus, patients having values that exceeded this standard value were selected as the most severe MxAC subjects.

A, Reference point locations and B, dental arch measurements of 3D cast images.

Consequently, patients with an ANB angle within 2.5° ± 2.0° were selected by the cephalometric analysis, and 33 women (mean age, 23.3 ± 3.8 years; range, 18-31 years) were selected as the MxAC group.

All subjects in both groups were selected from the same ethnic population in Japan. We set the minimum age as 18 years because we considered patients aged 18 years or older to have completed growth. Patients in the MxAC group were selected to match the age range of the control group as much as possible.

Mesiodistal tooth crown diameters for the maxillary central incisors, lateral incisors, canines, first and second premolars, and first molars were measured using the digital caliper, and the bilateral measurements were pooled. The median and interquartile range (IQR) with means and standard deviations were then calculated for both groups.

For the central incisor relationship, overjet and overbite were measured on dental casts using the digital caliper, and medians and IQRs with means and standard deviations were calculated for both groups.

Tooth size-arch size relationship was measured using the segment arch method with the digital caliper. Facial-palatal displacements of the lateral incisors and canines were measured ( Fig 1 ). Medians and IQRs with means and standard deviations for these measurements were then calculated for both groups.

Dental arch dimensions were measured by the 3D cloud evaluation software. Dental arch widths were measured as the distances between reference points on bilateral canines, first and second premolars, and first molars (midpoint of mesiodistal buccal cusps of the first molars). Dental arch depths were measured as the distances between the midpoint of the center of the edges of bilateral central incisors and the midpoints of bilateral reference points at the canines, first and second premolars, and first molars ( Fig 1 ). Medians and IQRs with means and standard deviations for these measurements were then calculated for both groups.

All cephalograms were taken with the same machine (CX-150SK; Asahi-Roentgen, Kyoto, Japan), and magnification was automatically adjusted to 1:1 by the cephalometric analyzing software program. Based on previous studies, 14 skeletal and 10 dental cephalometric measurements were conducted using the analyzing software program, and medians and IQRs with means and standard deviations were calculated for both groups ( Fig 2 ; Tables I and II ). All reference points were identified by 1 evaluator (M.I.). Articulare was used as the reference at the most posterior point of the cranial base, which is considered to be a more biometrically reliable point than basion.

Cephalometric measurements.

In addition, all x-y coordinates for each patient were exported as text data and imported into the Microsoft Office Excel program. The coordinates were superimposed at sella as the origin and on the sella-nasion line at 7° above the horizontal line for each group. Medians for each reference point were calculated for each group, and a graph was drawn ( Fig 3 ).

Comparison of the results of cephalometric measurements in the MxAC and control groups. Medians of reference point locations in the x-y coordinates for each group were superimposed at sella and the S-N line.

To calculate and examine intraexaminer errors with Dahlberg’s formula, 10 subjects were randomly selected from both groups (total, 20 subjects). The same cephalometric measurements were taken twice with a minimum of a 2-week interval by 1 evaluator (M.I.) and once by another evaluator (a member of the orthodontic department) using the same general clinical procedure without a calibration session. The maximum intraexaminer and interexaminer error values for the cephalometric analysis were 0.55 and 0.63 mm for linear measurements and 0.61° and 0.79° for angular measurements, respectively. For mesiodistal tooth crown diameter measurements on dental casts, the same 20 subjects were evaluated. The maximum intraexaminer and interexaminer error values were 0.17 and 0.36 mm, respectively.

In addition, to analyze intraexaminer and interexaminer errors for dental cast measurements obtained by the 3D laser scanner, 5 dental casts from both groups (total, 10 casts) were also randomly selected. All measurement landmarks were identified twice with a minimum of a 2-week interval by 1 evaluator (M.I.) and once by another evaluator with a careful calibration session, since we do not use this system clinically. The maximum intraexaminer and interexaminer error values for the 3D dental cast measurements were 0.10 and 0.12 mm, respectively.

Results

Mesiodistal diameters of tooth crowns of the MxAC group were significantly larger than those of the control group for all teeth except for the first molar ( P <0.01).

Overbite was significantly smaller in the MxAC group compared with the control group ( P <0.01) ( Table III ).

Table III

Comparison of the dental cast analysis in the maxillary arches between the groups

Measurement (mm)	Control group (n = 33)				MxAC group (n = 33)				Statistical analysis
	Median	IQR	Mean	SD	Median	IQR	Mean	SD	P value
Mesiodistal tooth dimension ‡
Central incisor	8.42	0.56	8.47	0.41	8.63	0.70	8.73	0.50	0.000	†
Lateral incisor	7.05	0.51	7.13	0.43	7.48	0.94	7.59	0.56	0.000	†
Canine	7.80	0.28	7.80	0.19	8.14	0.70	8.17	0.40	0.000	†
First premolar	7.30	0.52	7.30	0.39	7.67	0.58	7.74	0.40	0.000	†
Second premolar	6.89	0.45	6.84	0.34	7.08	0.47	7.09	0.33	0.000	†
First molar	10.47	0.62	10.41	0.50	10.60	0.91	10.53	0.55	0.110	NS
Central incisor relationship
Overjet	2.52	1.18	2.53	0.82	2.76	0.95	2.65	1.01	0.387	NS
Overbite	2.05	0.78	2.16	0.57	1.69	1.27	1.72 ‖	0.98	0.005	†
Tooth size-arch size relationship
Arch length discrepancy	0.01	1.00	−0.05	0.77	−9.71	7.51	−10.00	3.94	0.000	†
(1) Displacement of lateral incisor ‡	−0.30	1.71	−0.19 ‖	1.55	−2.72	2.69	−2.86	1.94	0.000	†
(2) Displacement of canine ‡	0.44	1.13	0.48	0.95	3.03	2.45	2.99	1.64	0.000	†
Sum of the displacements for 1 side ‡ §	0.76	0.79	0.86	0.72	5.61	2.74	5.76 ‖	1.72	0.000	†
Dental arch width
(3) Canine	35.37	2.54	35.62	1.43	35.52	4.04	35.61	2.33	0.954	NS
(4) First premolar	43.60	2.21	43.67	1.91	40.55	4.06	40.18	2.78	0.000	†
(5) Second premolar	49.80	2.89	49.86	2.43	45.34	5.38	45.61	3.55	0.000	†
(6) First molar	55.20	3.17	55.38	2.44	52.66	4.08	52.42	2.83	0.000	†
Dental arch depth
(7) Canine	8.33	1.23	8.34	1.03	5.67	3.82	6.04	2.12	0.000	†
(8) First premolar	15.44	1.98	15.61	1.40	14.11	4.42	14.01	2.44	0.006	†
(9) Second premolar	22.23	2.04	22.11	1.59	20.80	3.82	20.87	2.57	0.038	∗
(10) First molar	30.66	2.67	30.52	2.03	30.00	4.33	29.79	2.76	0.267	NS

IQR , Interquartile range; MxAC , maxillary anterior crowding; NS , not significant.

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