## 1 Introduction  
Temporomandibular joint (TMJ) disorders encompass a heterogeneous group of musculoskeletal and neuromuscular conditions that affect the temporomandibular joint complex [1]. Temporomandibularibular disorder (TMD) is the term that is used to define TMJ dysfunction and pain [2]. Disc displacement with reduction (DDwR) is one of the most commonly diagnosed intracapsular biomechanical disorders in patients involving the condyle-disc complex, with a collective prevalence of at least 41% of the general population [1]. It is typically characterized by clicking or popping sounds on mouth opening [3, 4, 5, 6]. It has also been reported as a physiological accommodation without clinical significance [7]. Females were found to be more predisposed to develop DDwR than males [8]. Accordingto the \"Diagnostic Criteria for Temporomandibular Disorder (DC/TMD) and the DDwR Diagnostic Criteria and Symptomatology of DDwR\", a diagnosis of DDwR relies on the presence of the abnormal positional relationship between the disc and the condyle, articular eminence, and/or articular fossa [3; 4; 5; 9]. Magnetic resonance imaging (MRI) is supportive in determining the position, movement, and integrity of the temporomandibular articulation, and it is performed to detect joint effusion and mandibular condyle marrow abnormalities [10; 11; 12]. However, MRI is not sufficient for the diagnosis of disc displacement, and it must be combined with clinical examination and anamnestic data [13; 14]. Diagnostic signs of DDwR are clicking, snapping, or popping sounds during opening, closing, or both (i.e., reciprocal click), either described by the patient or detected by the clinician [2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13]. Disc displacement with reduction may persist for several years and can migrate anteriorly, and as a result, disc displacement without reduction (DDwoR) may occur. Under physiological conditions, the position of the condyle at maximum mouth opening is generally found beyond the summit of the articular eminence. In patients with DDwR, although pathological displacement of the disc can occur in any direction, anterior and anteromedial displacements are the most common [13]. In DDwR with intermittent locking (DDwRwIL), similar to DDwR, in the closed mouth position, the disc is in an anterior position relative to the condylear head, and the disk intermittently reduces with the opening of the mouth. In DDwRwIL, compared to DDwR alone, in the clinical history of the last 30 days, there are repeated episodes of momentary locking that spontaneously resolve without the need for manual reduction maneuvers either by the patient or the operator [9]. MRI images are not different from those of the DDwR unless, at the time of the open-mouth examination, the patient is in a locked position; in this case, the disc appears completely dislocated even with the mouth open [13]. Furthermore, diagnostic criteria for DDwRwIL include the following: In the last 30 days, any TMJ noises present with jaw movement or function or patient reports of any noise present during the exam; in the last 30 days, the jaw locks with limited mouth opening even for a moment and then unlocks. Clicking, popping, and (or) snapping noise detected during both opening and closing movements, detected with palpation during at least one of the three repetitions of jaw opening/closing, right/left, and lateral or protrusive movement(s) [14; 15]. The etiopathogenesis of disk displacement remains unclear, and it might be multi-factorial [15; 16; 17; 18; 19]. Previous studies concerning the etiology of DDwR, including advanced MRI imaging studies, have been limited to detecting group differences between DDwR patients and healthy controls.