INTRODUCTION

In the treatment of patients with Class II malocclusion with mandibular retrognathia, different removable (Activator, Monoblock, Bionator, Twin-Block, etc) or fixed (Herbst, Jasper Jumper, Forsus, Twin-Force, etc) functional appliances have been used to encourage or redirect the growth of mandible to correct skeletal discrepancy. The placement of the functional appliance results in a displacement of the condyle in the glenoid fossa and stimulates the growth at the condylar cartilage. In orthodontic literature, temporomandibular joint (TMJ) adaptations following functional therapy have been visualized by various techniques like cephalograms,1–3 panoramic radiographs,4,5 computed tomography,6,7 and magnetic resonance imaging.8–10 However, there are many limitations to image acquisition of the TMJ using conventional techniques.

Improvements in technology have led to cone-beam computed tomography (CBCT). This technique produces accurate images with high resolution and minimal distortion and allows the creation of three-dimensional (3D) images in sagittal, coronal, and axial planes. It is possible to make more precise measurements of craniofacial structures since there are no projections or overlapping of bilateral structures.11 Recently, CBCT has been frequently used in the evaluation of dental and maxillofacial pathologies, airway assessment, and craniofacial morphology.12 There are also studies assessing the use of CBCT for volume estimation of mandibular condyle.13–15

In orthodontics, comparison of pretreatment and posttreatment data is important to assess treatment results. Cephalometric superimpositions have been used frequently for this purpose. However, 2D cephalometric superimposition has some limitations with respect to accuracy and reliability. Recently, superimposition on 3D images has begun to be used to provide more reliable information.16–20 Lee et al.21 reported that for the evaluation of surgical outcomes, image fusion is an accurate method that is not affected by spatial or surgical changes.

In the literature review, it was determined that CBCT has been used recently to evaluate the condylar changes after orthodontic treatment.22–26 However, only LeCornu et al.22 analyzed 3D skeletal changes in subjects with Class II malocclusion. To our knowledge, CBCT has not been used frequently in the evaluation of condylar response to functional orthopedic therapy in patients with skeletal Class II malocclusion. The purpose of this study was to evaluate the condylar changes through CBCT images in patients treated with Twin-Block functional appliance.

MATERIALS AND METHODS

Sixty CBCT images (30 pretreatment and 30 posttreatment) that had been obtained previously to evaluate 3D volumetric changes in the posterior airway space of patients with retrognathic mandible treated with Twin-Block functional appliance were used in this retrospective study. These images belonged to 16 male subjects with mean (SD) age of 12.83 (1.17) years and 14 female subjects with mean (SD) age of 12.5 (1.23) years and were obtained for this study after the approval of the Ethical Committee of Gülhane Military Medical Academy.

The images were taken from the patients who were selected according to the following criteria:

• Class II division 1 malocclusion with normal maxilla and retrognathic mandible (ANB angle greater than 4°),

• horizontal or normal growth pattern,

• bilateral Class II molar and canine relation,

• increased overjet, and

• minimum or no crowding in the dental arches.

All of the patients were treated with Twin-Block functional appliance. Class I molar and canine relationship was obtained, and increased overjet was eliminated at the end of functional therapy. The average time for functional treatment was 7.4 months. Condylar response to the functional therapy was evaluated on CBCT images that had been taken before treatment (T0) and after functional therapy (T1).

Pretreatment and posttreatment images were taken while the patients were sitting in an upright position with the Frankfort horizontal plane parallel to the ground. They were instructed to breathe normally through the nose and to avoid swallowing during the scanning process. CBCT datasets were acquired by using ILUMA (IMTEC, 3M Company, Ardmore, Okla) with a reconstructed slice thickness of 0.290 mm and a 728 × 728 matrix. The device was operated at 3.8 mA and 120 kV. A single 360° degree rotation 20-second high-resolution scan was made with isotropic voxel size set at 0.290 mm and a 21.1 × 14.2 cm field of view. The raw images were exported into Digital Imaging and Communications in Medicine (DICOM) multifiles by using the ILUMA native software. The DICOM files were reconstructed by SimPlant Master Crystal v13 (Materialise Dental, Leuven, Belgium) image processing software. All landmark identifications and measurements were made using this software. SNA, SNB, and ANB angular measurements, and Co-A, Co-Gn, and Co_R-Co_L linear measurements were made on T0 and T1 images by using 3D cephalometric function, which was under the OMS module of the software. All measurements were performed by controlling localization of landmarks in all dimensions on the reconstructed 3D models (Figure 1).

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By using an adaptive threshold, pretreatment and posttreatment images of the mandible were segmented for superimposition and condylar volumetric measurements. To establish uniform segmentation, automatic bone thresholding values in the SimPlant software (minimum 500, maximum 3071 Hounsfield unit) were used. Superimposition of pretreatment and posttreatment 3D mandibular images was made by using the “superimposition” function that was under the OMS module of the software. This superimposition was a rigid body registration based in landmark fitting. Right and left foramen mentale, and superior and posterior point of right and left antegonial notch were used for superimposition (Figure 2A,B). Custom planar osteotomy function that was under the “plan surgery” section of the software was used to constitute a plane of 0.1 mm, passing tangent to the distal slope of coronoid process to cut the right and left condyle of the superimposed images (Figure 3A,B,C). After the superimposed condyle images were cut (Figure 4), condylar volume was automatically calculated in mm³. Red image showed the pretreatment values, and green image showed the posttreatment values (Figure 5A,B). This process was applied for the right and left condyle separately for each patient. All landmark identifications and measurements were made by one author.

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Statistics

A power analysis established by G*Power software (v3.1.3, Franz Faul, Universität Kiel, Kiel, Germany) revealed that the sample size of 30 patients provided more than 80% power to detect significant differences with an effect size of .50 between the two measurements at a .05 significance level. The measurements of 10 patients were repeated 1 month later. Reliability was evaluated by using intraclass correlation coefficients (ICCs) and Bland-Altman plots (Table 1). The measurements showed excellent intraexaminer repeatability.

Table 1. Results of Reliability Analysis

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Statistical analyses were performed using the Gnu PSPP software (Free Software Foundation Inc, http://www.gnu.org/software/pspp/get.html). Kolmogorov-Smirnov normality test (with Lilliefors significance correction) and Levene variance homogeneity test were applied to the data. The mean measurement values and standard deviations (SD) were calculated for T0 and T1. Wilcoxon signed rank test was used to compare T0 and T1 scores, and Mann-Whitney U-test was used to compare the scores of male and female participants. Significance was set P < .05.

RESULTS

Means, standard deviations, and comparisons of the volumetric, angular, and linear measurements for pretreatment and posttreatment are given in Table 2.

Table 2. Descriptive Statistics of the Measurements With Results of Wilcoxon Signed Rank Tests

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Evaluation of volumetric measurements revealed that condylar volume increased at both the left and right sides (P < .01). Angular measurements showed a decrease in SNA angle (P < .05) and an increase in SNB (P < .01). Depending on these alterations, ANB angle also decreased significantly (P < .01). In the evaluation of linear parameters, it was determined that Co_R-Co_L, and Co-Gn measurements increased significantly at both sides (P < .01), but increase at Co-A was not significant at either side (P > .05). Comparison of differences by sex was insignificant for all measurements (P > .05).

DISCUSSION

In the orthodontic practice, cephalometric and panoramic radiographs are the most common approaches in the radiographic evaluation of the TMJ because of availability, ease of use, low radiation requirement, and low cost.1–5 However, validity of 2D imaging is doubtful depending on the changes created by patients' head position and beam projection angle.27 Anatomic superimposition and magnification differences of the left and right sides that cause double border of the mandible on the radiograph are the other disadvantages of conventional cephalometry.28

Recent improvements in technology have led to 3D CBCT. In the assessment of craniofacial structures, CBCT is more adequate than conventional helical computed tomography because of lower radiation exposure. While it is possible to scan the complete head in a few seconds with an effective dose of 50 µSv with CBCT, conventional computed tomography uses 2000 µSv.29 Other advantages of CBCT are lower costs, increased accessibility to orthodontic practices, flexibility in the field of view, and submillimeter spatial resolution.28,30 The 3D image is reconstructed from raw data by means of a mathematical algorithm that has the ability to calculate and eliminate the magnification factor, so in CBCT there is no magnification and measurements are reported to be reliable and anatomically accurate.18

Some researchers evaluated the accuracy of CBCT imaging of TMJ. Hilgers et al.31 compared the linear TMJ measurements of CBCT with digital cephalometric radiographs and reported that CBCT measurements were significantly more reliable than lateral cephalometric, posteroanterior, and submentovertex measurements. Honey et al.32 compared the diagnostic accuracy of observers viewing images made with CBCT, panoramic radiography, and linear tomography. The findings revealed that CBCT images provided superior reliability and greater accuracy than linear tomography and TMJ panoramic projections in the detection of condylar cortical erosion. Recently, Bayram et al.15 evaluated the accuracy of volumetric analysis of mandibular condyle using CBCT and concluded that this technique is reliable for clinical assessment of bone volume measurements. Depending on the proven accuracy, 3D CBCT was used in this study to determine the alterations in the volume and length of the condyle after orthopedic treatment. Tecco et al.14 successfully used this technique to measure the condylar volume and surface in white young adults. Similarly, Huntjens et al.13 evaluate the condylar asymmetry in children with juvenile idiopathic arthritis by using CBCT. Additionally, in some recent studies, CBCT has been used for 3D assessment of mandibular and glenoid fossa changes after treatment of Class III malocclusion.23–26 To our knowledge, only LeCornu et al.22 analyzed 3D skeletal changes in subjects with Class II malocclusion after treatment with the Herbst appliance. CBCT has not been used frequently in the evaluation of condylar response to functional orthopedic therapy in patients with skeletal Class II malocclusion.

In orthodontics, treatment outcomes are frequently assessed by lateral cephalometric superimpositions. However, Arat et al.33 reported lateral cephalometric superimposition to be a challenging process owing to issues related to image fidelity, landmark selection, and identification. Lateral cephalometric superimposition provides information only from the sagittal plane. On the other hand, CBCT images allow clinician to perform superimpositions in 3D and eliminate some of the errors that could occur with lateral cephalometric superimpositions. In our study, alterations in condylar volume were determined by superimposition of the pretreatment and posttreatment CBCT images. This method is a valuable tool to evaluate treatment results after orthodontic therapy. Mah et al.12 reported that superimposition of a patient's pretreatment and posttreatment CBCT scans can be used to determine the movements of dentition and root structures as a result of orthodontic therapy. They also reported that when the number of superimposition points increased, the image became more accurate. In our study, the images were superimposed on foramen mentale and antegonial notch bilaterally. Following the superimposition, a plane of 0.1 mm passing tangent to the distal slope of coronoid process was constituted to determine the condylar area to be measured. A similar method was also used in the study of Bayram et al.15

The condyle is a growth site of the mandible and plays an important role in the final adult dimension of the mandible.34 Because of that, condylar response was evaluated by CBCT to determine the skeletal effects of Twin-Block appliance in the presented study. CBCT measurements showed that condylar volume, mandibular length, intercondylar distance, and SNB increased, while SNA and ANB decreased after Twin-Block treatment. These alterations revealed that growth of mandibular condyle in an upward and backward direction was stimulated by the functional orthopedic treatment. Similar to our results, a few clinical studies investigating the changes induced by Twin-Block appliance indicated the effectiveness of the appliance in enhancing mandibular growth.35–38 Güner et al.39 investigated the effects of the MARS appliance on the mandibular condyle by using single-photon emission computed tomography. In accordance with our findings, the results of this study showed new bone formation as a result of induction of metabolism in the mandibular condyle and significantly enhanced condylar volume. Similarly, Paulsen and Karle40 showed new bone formation at the posterior portion of the condyle by orthopantomographic and computed tomography images in 100 patients treated with the Herbst appliance.

Until the development of CBCT, the condylar volume has not been routinely calculated, but now condylar volume13–15 and condylar displacement24 could be evaluated accurately with the presented radiologic technique. To evaluate the condylar response to functional orthopedic treatment, further studies are needed.

CONCLUSIONS

• CBCT can be successfully used to evaluate the condylar response to functional orthopedic treatment.

• Twin-Block appliance increases condylar volume, mandibular length, and intercondylar distance by stimulating growth of condyle in upward and backward direction.

• Twin-Block appliance therapy increases SNB and decreases SNA and ANB.

• Twin-Block appliance does not change midfacial length.