The most prevalent cause of temporomandibular joint (TMJ) dysfunction is internal derangement, which is defined as an abnormal relationship of the disc to the condyle. Disc displacement is the most popular form of TMJ dysfunction [10, 11].
Temporomandibular joint dysfunction was previously assessed by arthrography using computed tomography as a standard method as it provides an accurate visualization of soft and hard tissue and good evaluation of the bony structures of TMJ. However, this painful invasive procedure carries a risk of iatrogenic disc perforation and damage of facial nerve, suffers from difficulties in patient positioning, exposes him to radiation, and depends on examiner’s skill .
It is generally preceded using routine pulse sequences with acquisition of static sagittal and coronal images during mouth opening and closing. Although static views are derived before or after disc reduction and cannot cover the proper physiologic process, static MRI was the gold standard in diagnosing TMJ dysfunction [9, 13].
At present, dynamic MRI is introduced in the set of dynamic assessments of joint function. In the current study, 40 patients with temporomandibular joint pain did static and dynamic MRI examinations of both temporomandibular joints.
Disc displacement is more common in females than males (3:1) and commonly appears in the second to fourth decades of life . Among those 40 patients, 72.5% were females and 27.5% were males with ratio 2.6:1. This female predominance agrees with Vieira-Queiroz et al.  who performed their study on 185 patients; 78.9% of them were females and 21.1% were males with ratio 3.7:1.
A wide range of age group was included in this study, ranging from 18 to 66 years with a mean of 29.3 years and SD ± 10.68. TMJ dysfunction affects young ages (between 20 and 40 years) more than old age patients (more than 60 years). Dupuy-Bonafé et al.  performed their study on 40 patients; their age ranged from 21 to 59 with median 25.8. Also, in the study of Vogl et al. , there were 794 patients, and their age ranged from 14 to 83 years with mean 38.7 and SD ± 16.
Disc position was evaluated in our study by both static and dynamic MRIs with sensitivity of 93.93% versus 90.9% (p value < 0.001). The prevalence of disc displacement with reduction (DDwR) in patients affected by temporomandibular disorders ranges from 24 to 47% while that of disc displacement without reduction (DDwNR) from 11 to 26% .
Anterior disc displacement was the most common finding among examined joints, and anterior disc displacement with reduction (ADDwR) was found in 11 joints (13.75% of all joints) and with no reduction in 18 of them (22.5% of all joints) by static MRI. Dynamic MRI reported ADDwR in 13 joints (16.25%) and ADDwNR in 16 joints (20%). Whyte et al.  reported in their study on 144 patients (288 joints) 50 normally positioned discs and 226 joints with anterior displacement: 136 joints were reduced and 90 joints are non-reduced. Zhang et al.  found 10 cases (16.7%) out of 30 patients with ADDwR, and one case (1.7%) with ADDwNR.
Medial and lateral disc displacements were detected in static MRI only. This is because the dynamic MRI with its sagittal views has limitation in the evaluation of sideway displacements as it is well evaluated in the coronal plane. The disc crosses over one of the sagittal planes tangential to one of the condylar poles without an anterior component. The empty fossa sign seen in the sagittal images is an indication of a medial or lateral disc displacement .
Posterior disc displacement is a rare TMJ disorder. It was overlooked in the past because of a lack of well-defined imaging characteristics, and its diagnosis depended mainly on clinical signs. A definitive diagnosis requires magnetic resonance imaging (MRI) . Posterior dislocation was diagnosed in 1 joint by both static and dynamic MRIs in our study. The meta-analysis study of Afroz et al.  reported the overall prevalence of posterior disc displacement (PDD) for the number of joints affected was 0.7% and for the number of patients affected was 0.9%.
A stuck or fixed TMJ disc is the disc that remained fixed in position relative to the glenoid fossa and the articular eminence in both closed and opened mouth projections. It is caused mainly by joint adhesion which cannot be detected by MRI, and it is detected by direct visualization or arthroscopy but, at this time, can detect disc immobility as an indicator of disc adhesion .
In the current study, static MRI reported normal disc mobility in 78 joints and stuck disc in 2 joints, while dynamic MRI reported normal disc mobility in 76 joints and stuck disc in 4 joints (p value = 0.008). Arthroscopy was done later on as the patients were complaining seriously. Arthroscopy confirms that they had stuck discs. So, dynamic MRI may have higher accuracy in diagnosis of stuck disc than static MRI.
Eberhard et al.  evaluated disc mobility in 40 patients with TMJ dysfunction symptoms by dynamic MRI which revealed 2 cases of stuck disc (disc adhesions). Also, in the study of Amin et al.  on 28 patients, dynamic MRI reported 13 patients with normal disc mobility, 12 patients with limited asynchronous mobility, and 3 patients with stuck disc.
As regards to condylar translation, it was classified as either normal (apex of condyle meets the apex of articular eminence), hypomobile (condyle not reaching the apex of articular eminence), or hypermobile (condyle reaches beyond the articular eminence). Static MRI reported 62 joints with normal condylar translation, 17 joints with hypomobile condylar translation, and 1 joint with hypermobile condyles, while dynamic MRI reported 54 joints with normal condylar translation, 18 joints with hypomobile condylar translation, and 8 joints with hypermobile condyles (p value < 0.001). The ability of dynamic MRI to detect hypermobile condyles with rate higher than static MRI may be attributed to that dynamic MRI is capable of imaging the full range of motion of condylar translation due to the use of voluntary motion, rather than a fixed intraoral device to open the mouth.
Beer et al.  demonstrated good correlation in range of motion between dynamic MRI and static MRI (p value = 0.001) that is confirmed by axiography. These results do not go with the study of Wang et al.  who reported that dynamic MRI detects normal motion more than static MRI while static detects limited mobility (30.4%) more than dynamic MRI (17.7%).
Disc displacements are associated with pathological changes such as joint effusion, degenerative changes, and changes in retrodiscal tissue . In our study, all of these changes were associated with different types of disc displacement. Out of examined 9 joints with retrodiscal tissue changes, 7 joints had ADDwNR and 2 joints had ADDwR. Higuchi et al.  reported in their study that joint effusion was detected in 63 patients (49%) of 129 patients presented with TMJ pain. Also in the study of Campos et al. , degenerative changes were detected in 104 patients; 76 of them diagnosed to have disc displacement. Hasan et al.  reported in their study that morphological changes of retrodiscal layers were most frequently observed in ADDwNR (88.9%) than ADDwR.
Dynamic MRI was evaluated as a diagnostic tool by comparing it to the static MRI by two parameters; the first is by findings and ability to diagnose different types of disc displacement and the second by evaluating the quality of the image as regards to anatomic visibility of important TMJ structures (articular disc and mandibular condyle) and presence of motion artifact.
As regards to quality of the images acquired by dynamic MRI, detection of disc and condylar head was rated as whether good, fair, or poor and compared to the detection of static MRI. Disc detection rate by dynamic sequence was 87.5% versus 92.5% by static MRI (p value 0.038). Condylar head detection rate was 97.5% for dynamic MRI versus 100% for static MRI (p value 0.012). Shimazaki et al.  reported in their study high detection rate of articular disc 83% and of condylar head 95% by dynamic sequence. Poor detection of disc in some cases may be attributed to either degenerative changes causing thinning of the disc and increasing its signal or sideway displacement of the disc shifting the disc away from imaging plane .
One of the challenges of dynamic MRI is to acquire images of moving object with minimal motion artifact . In our study, dynamic sequence is associated with decreased motion artifacts, despite ongoing motion of the joint. Motion artifact was detected in 12 patients (30% of patients) versus 13 patients (32.5% of patients) (p value 0.032). This agrees with the study of Wang et al.  who reported that static examinations were rated as having motion artifact (19.6% versus 6.9%, p .016).
Regarding to the final outcome, we realized that dynamic MRI facilitated visualization of the articular disc during the entire course of jaw opening and closing as well as the dynamics of internal derangement and assessment of disc condyle relationship in a short time compared to static MRI. It is better in evaluating disc mobility than static MRI, but static MRI is better in evaluating side by side displacement which is an important limitation of dynamic MRI. Detection rate of articular disc and condylar head is lower in dynamic than static MRI which needs further studies to improve it.
Amin et al.  reported that MRI of TMJ using its dynamic and static sequences is more sensitive than arthroscopy in evaluating disc position and mobility and recommend using MRI as a primary modality for diagnosis in suspected internal derangement.
The main limitation in our study is longer time of examination, the lower detection rate of articular disc by dynamic MRI compared to static MRI that needs further studies for improving it.