# Matrix Metalloproteinases and Temporomandibular Joint Disorder: A Review of the Literature
## 1 Introduction  
The temporomandibularibular joint (TMJ) is classified as a ginglymoarthrodial joint, allowing for rotational and translational movements in normal function. Its primary components include the glenoid fossa of the temporal bone, the articular disc, the head of the mandibular conde, and masticatory muscles. This joint is capable of remodeling even after growth has stopped, allowing it to make structural changes and adapt to different physiological demands [1]. Temporomandibularibular disorders (TMD) are a group of degenerative disorders involving the components of the TMJ, which can lead to displacement of the disc, joint remodeling, and eventually osteoarthritis [2]. Disc displacement can occur anteriorly, posteriorly, medially, or laterally; however, it is most commonly displaced anteriorly [3]. TMD affects around 25% of the population, and it is characterized by orofacial pain, restricted range of motion, joint dysfunction, and ultimately, a decreased quality of life [2, 4]. The etiology of TMD is still a topic of discussion; however, some known risk factors include trauma and microtrauma, malocclusion, and psychological factors, such as stress and anxiety [5]. The progression of TMD is classified primarily based on the location of the disc and its mobility during mandibular movement. In a normal functioning joint, the disc remains between the head of the mandibular conde and the glenoid fossa through the full range of movement. In early stage TMD, the disc is displaced anteriorly when the mandible is closed and reduces to a normal location upon opening, classified as anterior disc displacement with reduction (ADDwR). In late stages of TMD, the disc is anteriorly displaced in both closed and open positions, classified as anterior discdisplacement without reduction (ADDwoR). These later stages tend to be associated with more pain and limitation in mandibular mobility [2]. A more advanced staging guide to the internal derangements of TMD was created by [PERSON] in 1989, placing patients into five categories based on clinical, radiographic (tomographic, arthographic, and magnetic resonance imaging) and findings during surgery, including gross surface and anatomic changes to the disc and other components of the TMJ [6]. In the first stage, the early stage, patients present with clicking of the joint, but no pain, limited range of motion, or other symptoms. Radiographically and surgically, the disc is displaced slightly anteriorly, but all other aspects of the joint are normal. In [PERSON]'s early/intermediate stage, the patient presents with additional symptoms, including episodes of pain, tenderness, and mechanical problems associated with the joint. Radiographically and surgically, the disc is displaced anteriorly, with slight deformation of its posterior aspect. In the intermediate stage there are more occurrences of pain, mechanical problems, including locking and decreased range of motion. In [PERSON]'s intermediate/late stage of TMD, the patient has a chronic pain and decreased range of motion. Radiographically and surgically, an increased severity in comparison to the intermediate stage is noted, along with remodeling of the hard tissue surfaces of the joint. The disc, however, has yet to be perforated up to this stage. In the final stage, the late stage, patients present with crepitus and grinding in the joint with mandibular movement, chronic pain, restricted range of motion, and an overall decrease in function. Radiographically and surgically, the disc and hard tissues of the joint have undergone significant deformation, remodeling, and arthritic changes have occurred, including perforations of the attachments and erosion of articulating surfaces [6]. Matrix metalloproteinases (MMPs) are the major enzymes involved in extracellular matrix (ECM) and basement membrane remodeling and degradation, along with other enzymes, such as a disintegration and metalloproteinases (with or without thrombospondin), and plasminogen activators, among others [7; 8; 9].