Material and methods

The subjects comprised 28 patients from a private orthodontic office who were diagnosed with Angle Class I malocclusions with crowding and normal vertical dimensions and no posterior crossbites. The study was approved by the Kansai Hospital ethics committee, and informed consent was obtained from the participants. After initial recording of the data, the patients were randomized to 2 groups: those treated with Schwarz appliances (expanded group) and a nonexpanded control group (nonexpanded group) ( Fig 1 ). Records were gathered at 2 times from both groups. For the expanded group, the first records were gathered before treatment (T0); the second set was obtained during a retention period of approximately 9 months after 6 to 12 months of expansion (T1). In the control group, 2 sets of records were obtained approximately 13 to 21 months apart. The expanded group included 14 patients (6 boys, 8 girls) with average ages of 7 years 11 months at T0 and 9 years 8 months at T1. The nonexpanded group received no treatment and included 14 patients (6 boys, 8 girls) with average ages of 8 years at T0 and 9 years 8 months at T1. The expanded group used a Schwarz expansion appliance on the maxillary and the mandibular dentitions to relieve anterior crowding. The maxillary dental arches were also expanded with a Schwarz appliance in all expanded group patients to maintain the buccolingual relationships of occlusal contact in the posterior teeth during expansion. The patients wore the expansion appliances at night. The treatment period was approximately 1 year. The appliance was activated by rotating its screws once a week (90° for 0.175 mm). The expansion plates were adjusted when the appliance disturbed an erupting tooth or did not fit in the dental arch. A new appliance was fabricated when the screw reached its limit.

Mandibular Schwarz appliance used in this study.

After 12 months of expansion, the screw of the Schwarz appliance was fixed with composite (cured) and used as a retainer. The patients wore Schwarz appliances only to prevent any effects from other appliances.

Lateral cephalometric radiographs were traced by 1 investigator (K.T.) to minimize error in measurement. Sixteen points were digitized on each cephalometric radiograph, and 12 cephalometric measurements were made.

Three measurements were made on the mandibular casts: arch crowding, arch perimeter, and arch length. The dental cast measurements were made with Digimatic calipers (NTD 12-15PMX, Mitsutoyo, Kanagawa, Japan) accurate to 0.01 mm.

The CBCT images at T0 were taken at average ages of 7 years 11 months (expanded group) and 8 years 0 months (nonexpanded group), and, in both groups, the CBCT images at T1 were taken at an average age of 9 years 8 months.

The head position was oriented so that the Frankfort plane was parallel to the floor in a seated position, and an image was taken at the intercuspal position using CB MercuRay (Hitachi Medical, Tokyo, Japan).

To transform the digital imaging and communication in medicine (DICOM) data from the CBCT images into polygon data, 5 software programs were used—Volume-Rugle (Medic Engineering, Kyoto, Japan), MicroAVS (KGT, Tokyo, Japan), VVD2RGL, Point-Rugle, and 3D-Rugle (Medic Engineering) ( Fig 2 , A ). The iterative closest point (ICP) method can superimpose precisely with repeatability because numerous corresponding points are used to compare with point-based registrations. To superimpose two 3-dimensional (3D) images at T0 and T1, the ICP method was used ( Fig 2 , B and C ). Furthermore, to superimpose 2 separate multi-planar reconstruction (MPR) images accurately, specific points of the cranial base were used as the reference points for superimposition, because the cranial base is not greatly influenced by growth ( Figs 2 , B and C ). As a result, accurate superimposition of 2 separate MPR images is possible. Furthermore, to enhance the accuracy of the ICP method, specific points of the cranial base were used as reference points for registration (superimposition), because the cranial base is not greatly influenced by growth ( Fig 2 , B and C ). Then the combined images were cut down an arbitrary plane and made into 2 units. The MPR images, which have excellent dimensional accuracy, were used to compare the data at T0 and T1.

A, Flow chart illustrating the methods used to convert CBCT-derived DICOM data into polygon data. B, The ICP method at the cranial base. Superimposition was performed in all areas except for the edge of the cranial base that experienced growth. The green area shows that superposition (or registration) was performed within ± 0.50 mm. The yellow area shows the range from 0.50 to 1.00 mm. The light blue area shows the range from –0.50 to –1.00 mm. C, 3D images show the superimposition of T1 on T0.

A slice plane perpendicular to the occlusal plane, passing through both sides of the mesiobuccal cusp tips of the mandibular first molars, was prepared for the measurements. The 3D-Rugle software program was used for the measurements, and the following distances were measured at T0 and T1. Ten points of interest were measured including mandibular first-molar crowns, cementoenamel junctions (CEJs), roots, buccal and lingual alveolar processes, inner and outer surfaces of the mandibular bodies, zygomatic bones, condylar heads, and antegonial notches ( Table I ; Fig 3 , A and B ).

Diagram showing reference points and lines. A, A plane slicing perpendicular to the occlusal plane and passing through both sides of the mesiobuccal cusp tips of the mandibular first molars was set for measurements. The distances between the first molar crowns, CEJs, roots, alveolar processes, mandibular bodies, and the zygomatic bones were measured on this plane. B, A plane parallel to the long axis of the ramus bone was used to measure the distance between the condylar heads and the antegonial notches. C, An axial plane depicting measurements at 2 and 13 mm below the CEJ. BMW , buccal mandibular width: distance between the right and left buccal cortical plates; LMW , lingual mandibular width: distance between the right and left lingual cortical plates.

In addition, the distances of the crowns, CEJs, labial and lingual alveolar processes, and inner and outer surfaces of the mandibular bodies of the mandibular first deciduous molars were measured at a point 18 mm anterior from the mesial cusps of the mandibular first molars.

To determine whether there was a relationship between the amount of expansion or tipping and the change in cortical bone thickness at the mandibular first molars, the mandibular first deciduous molars, and the mandibular second deciduous molars, axial CBCT images were taken at 2 and 13 mm below the CEJ ( Fig 3 , C ).

One examiner (K.T.) made all measurements to eliminate interexaminer errors. To identify systematic errors and compare measurement accuracy, intraexaminer reliability was evaluated. Sources of error included landmark location, anatomic contours, tracing from the cephalograms, digitizing the cephalograms, data conversion from the different software programs, all linear and angular measurements from the CBCT, and all linear measurements from the dental casts. Also, to prevent bias in the measurement of the expanded and nonexpanded groups, the investigator was blinded. Tracing and digitizing errors of cephalograms, data conversion errors from the different software programs, linear and angular measurement errors of the CBCT, and all linear measurements from the dental casts were determined by remeasurement at least twice on 2 separate occasions, 2 weeks apart. Five randomly selected subjects from each group were measured at least twice on 2 separate occasions, 2 weeks apart, by the same investigator. No statistically significant difference was found for any measurement by using intraclass correlation coefficients.

Results

Table II shows the measurements at T0 and T1 of both groups. There were no significant skeletal, dental, and soft-tissue lip profile changes from T0 to T1. No mandibular rotation or displacement was seen between T0 and T1. However, in the mandibular cast measurements of arch crowding and arch perimeter, there were statistically significant changes from T0 to T1 in the expanded group.

The Schwarz expanded group showed marked expansion compared with the nonexpanded group. The intermandibular first molar lengths increased by 5.41 mm at the crown level, 4.39 mm at the CEJ, 2.40 mm at the root, 3.75 mm at the mandibular alveolar lingual point, and 3.84 mm at the mandibular alveolar buccal point. There were significant ( P <0.05) differences between the groups for teeth and alveolar bone. However, there were no significant differences for the mandibular bodies ( P = 0.695), zygomatic bones ( P = 0.893), condylar heads ( P = 0.613), and antegonial notches ( P = 0.724) ( Table III ).

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