This cross-sectional study was approved by the research ethics committee of our institute with the approval number FDASU-RECim121816. A power analysis was designed to have adequate power to apply a statistical test of the null hypothesis that there is no difference between tested techniques. According to the results of Schnabel et al.  and by adopting an alpha of 0.05 (5%) and a beta of 0.10 (10%) i.e. power = 90% the predicted sample size (n) was found to be (26) TMJs i.e. 13 patients. Because we planned to exclude ADDWOR and PDD, we anticipated a dropout rate of 30%. The adjusted sample size was (34) TMJs i.e. 17 patients. Sample size calculation was performed using G*Power version 126.96.36.199 .
Seventeen patients suffering from TMD were included in the current study. They were informed about the aim, steps, benefits and risks of the study and signed an informed consent. They were recruited from the specialized TMD clinic affiliated to the institute.
Patients ranging in age from 18 to 55 years having TMD diagnosed by a combination of history and clinical examination. History of the patient’s complaints were taken according to the chart of American Association of Orthodontists  then clinical examination was performed according the Helkimo index; scores ranged from 0 to 20  (Table 1). This index evaluates the functional capacity of the masticatory system. It classifies individuals according to four signs: (1) function impairment, (2) TMJ pain during palpation, (3) impaired range of mandibular movement, and (4) muscle tenderness. If the sum of the scores was zero this indicates normal TMJ, if the sum of scores was from one to four this indicates a moderate TMD while if the sum of the scores was from five to twenty this indicates a severe TMD. In the present study, we included patients with scores ranging from five to twenty (severe TMD), not responding to conservative treatment and were planned to be treated by arthrocentesis. Two image acquisitions were performed for each patient included in our study within one-week interval. MRI to detect disc position and CBCT to rule out any bone pathology. Patients with TMJ fractures, cysts, tumors, inflammatory or systemic diseases affecting the TMJ like rheumatoid arthritis were excluded.
Bilateral MRI scans of the TMJs were performed for each patient using a Philips Ingenia 1.5 T closed MRI unit (Philips Healthcare, Best, Netherlands). The patients were asked to lay down in a supine position, with the Frankfort plane parallel to the scanner gantry, and the sagittal plane perpendicular to the floor. Bilateral TMJ surface coils were used for optimal imaging of the TMJ, with a small field of view in order to achieve a higher signal-to-noise ratio.
Sequential axial, sagittal and coronal cuts of the right and left sides were obtained both in the closed mouth (maximum intercuspation) and maximum opening positions. Fast spin echo sequence was used to obtain three mm thick images T1, T2 and PD images. T1 weighted images were conducted with Echo time (TE) 1.7 s and repetition time (TR) 3.8 s. T2 weighted images were taken with Echo time (TE) 18.4 s and repetition time (TR) 53.6 s. Proton density (PD) images were done with Echo time (TE) 30 s and repetition time (TR) 1500 s.
On the other CBCT scans were performed using i-CAT Next generation (Imaging sciences International, Hatfield, PA, USA). Exposure factors were set at 120 kV, 37.07 mA and 26.9 s acquisition time. A 16 × 8 cm FOV was imaged using 0.2 mm voxel size. The patient position was standardized according to manufacturer’s instructions.
CBCT images were exported as digital imaging and communication in medicine (DICOM) files. They were then transferred to a third-party software (OnDemand 3DTM software, Cybermed Inc., Seoul, Korea) with a reconstruction interval set to 1.0 and 1.0 mm slice thickness. Two experienced oral and maxillofacial radiologists with 10 years of experience evaluated the CBCT and MRI images separately and disagreement was resolved by consensus.
MRI image analysis
A maxillofacial and a medical radiologist with more than 10 years’ experience assessed the MRI scans together and reached a diagnosis by consensus. This diagnosis was considered the gold standard to which the CBCT findings were compared.
Each joint was assessed in both closed and opened mouth positions. Classification of the articular disc position was performed using sagittal oblique cuts by combining the criteria presented by Ahmad et al.  and Tasaki et al.  as follows:
No disc displacement (NDD): in the corrected sagittal plane, in closed mouth position, in relation to the superior aspect of the condyle, the posterior band is located at 11:30–12:30 position, and the thin intermediate zone is found between the condyle and the articular eminence.
Displacement of the disc: in the corrected sagittal plane, in closed mouth position, in relation to the superior aspect of the condyle, the posterior band of the disc is located anterior to the 11:30 position, and the intermediate zone is located anterior to the condyle.
Anterior disc displacement with reduction (ADDWR): the displaced disc returns back to its normal position of 11:30–12:30 in relation to the condyle during the mouth-opening, and the intermediate zone is located between the condyle and the articular eminence (Fig. 1).
Anterior disc displacement without reduction (ADDWOR): the displaced disc does not reduce to its normal superior position of 11:30–12:30 in relation to the condyle during the mouth opening movement, and the intermediate zone is located anterior to the condylar head.
Posterior disc displacement with reduction (PDDWR): the disc is displaced posterior to the 12 clock position on top of the condyle in the closed position but resumes its normal position over the condyle while opening the mouth.
Posterior disc displacement without reduction (PDDWOR): the displaced disc does not reduce to its normal position during mouth opening the disc remains turned at less than 11 clock with respect to the condyle in the open mouth position.
CBCT image analysis
CBCT scans were assessed twice by two maxillofacial radiologists with more than 10 years of experience with one week interval in between to calculate the reliability of CBCT quantitative and qualitative assessment.
On the multiplanar (MPR) screen; coronal, axial and sagittal views were reoriented to view the widest condyle dimension in each plane. The coronal plane was oriented on the axial window to pass through the condyle at its widest dimension mediolaterally. The sagittal plane was oriented on the axial window to be perpendicular to the coronal plane (Fig. 2).
Condylar shape: condyle shape was assessed from the corrected coronal view, and it was classified into convex, flat, round or angled. Convex surface is identified when the surface is like a portion of an oval shape. Flat surface shows a nearly straight surface between the right and left summits. Round surface is selected when the upper surface resembles half a circle. Angled surface has a sharp bend on the upper surface of the condyle as shown in (Fig. 3).
Condylar position: condylar position was evaluated from the corrected sagittal view. To assess the condylar position, the anterior and posterior joint spaces were measured. A line perpendicular to the point A (A: most prominent anterior point of the condyle) was extended to the opposing posterior eminence slope. The distance between the point A and the posterior eminence slope is the measure of anterior joint space (AS). Another line perpendicular to the point P (P: most prominent posterior point of the condyle) was extended to the opposing bone wall of the joint. The distance between the point P and the opposing bone on that line is the posterior joint space (PS) as shown on (Fig. 4). AS = PS indicates concentric condyle position, AS > PS indicates posterior condylar position, while AS < PS indicates anterior condylar position .
Condylar height and condylar width: condylar height and mediolateral width measurements were performed on the corrected coronal view with the largest condyle dimensions as shown on (Fig. 5). The condyle width was measured between the most prominent points on the right and left slopes of the head of the condyle. At the center of the width, a perpendicular line was extended to the roof of the condyle. The length of this line represents condylar height .
Condylar depth: it is the antero-posterior dimension of the condyle. It was measured on the corrected sagittal view on a line between the most prominent anterior (A) and posterior (P) points of the condylar head . (Fig. 6).
Seventeen patients (34 TMJs) were included in the present study having a Helkimo index score of 5–20. MRI imaging revealed that 20 TMJs showed ADDWR, 7 TMJs showed normal disc position, 6 TMJs had ADDWOR and 1 TMJ showed posterior disc position. TMJs showing ADDWOR and posterior disc position were excluded. So, we ended up comparing 20 TMJs having ADDWR with 7 TMJs showing normal disc position.
All data were collected and tabulated. The data were analyzed by SPSS (version 20), while Microsoft office Excel was used for data handling and graphical presentation. Quantitative variables were described by the mean, standard deviation (SD), range (minimum–maximum), standard error (SE) and 95% confidence interval of the mean. Qualitative categorical variables were described as frequencies and percentages.
Shapiro–Wilk test was used to test normality of all quantitative variables for further choice of appropriate parametric or non-parametric tests. Independent samples t-test was applied to compare the means of the two groups as almost all variables were found to be normally distributed.
For nominal variables, chi-squared test of independence and fisher exact test were applied, and the correlation was assessed using Cramer’s V and Contingency Coefficient measures.
Logistic regression model was established to predict the probability of anterior disc displacement with reduction depending on condyle height, width and depth.
Significance level was set at P ≤ 0.05 (S); while P ≤ 0.01 was considered highly significant (HS). Two tailed tests were assumed throughout the analysis for all statistical tests.