Idiopathic condylar resorption (ICR) is a well-known lesion characterized by progressive resorption of the mandibular condyle and a marked decrease in mandibular ramus height with an unknown cause. Since ICR predominantly affects teenage girls and postmenopausal women, female hormones are thought to be closely related to the pathogenesis of ICR. The purpose of this study was to review the literature on the etiology and morphological characteristics of ICR and to discuss future directions in this research field. Part 1 and Part 2 summarize the results of the literature search on the etiology and morphological characteristics of ICR, respectively. The requirement and possibility of early diagnosis for ICR as a current diagnostic regimen are also discussed.

Temporomandibular joint (TMJ) disc derangement is frequently found in daily clinical dental practice. In fact, anterior disc displacement and deformity are observed on TMJ-MR images in approximately 11% of new patients in our orthodontic department, diagnosed with TMJ disc displacement with or without reduction. If the clinical symptom is limited to TMJ noise and no abnormal findings are encountered in the TMJ hard tissues such as the mandibular condyle on imaging examination, usually no specific treatment for temporomandibular disorders (TMDs) is performed and the patient is just observed. On the other hand, if pain, closed lock, fibrous adhesion or indefinite complaints are observed in the TMJ or related areas, treatment for TMDs will be needed to relieve symptoms. It has been highly recommended to treat TMJ symptoms with conservative and reversible treatments based on scientific evidence. When TMJ disc displacement is caused and treated relatively early, disc replacement may be achieved as a result. Even with similar conservative treatment, replacement of the TMJ disc occasionally fails, therefore, the replacement will not always succeed, and even if replacement is achieved temporarily, recurrence may occur. TMJs with disc displacement or deformation may be accompanied by organic changes in the synovial fluid and joint constituent tissues, resulting in a decline in the lubricating and buffering functions, and so should not simply be considered as morphological problems. Furthermore, although many of the clinical symptoms caused by TMJ disc displacement tend to ease over the long term, the course of the disease varies. Therefore, it is desirable to clarify the detailed aspects and prognosis of abnormalities in the position and morphology of the TMJ disc, which cause disturbance of jaw movement and discomfort symptoms, leading to more accurate diagnoses. Thus, this article reviews changes in the TMJ tissue related to disc displacement based on the results of basic research.

The pathophysiology of temporomandibular joint (TMJ) disorder (TMD) varies. However, most cases result from structural changes in the articular TMJ disc. In advanced cases, prolonged TMD results in perforation or rupture of the TMJ disc, as well as bone and cartilage deformities. Although the cause of TMD remains unclear, it may be due to an imbalance between the microenvironment and the host adaptive capacity, and mechanical load-related factors and estrogen have been implicated. Joint tissues are rich in extracellular matrix, which have viscoelastic properties and a lubricating function against frictional forces. When the mechanical load applied to joint tissues exceeds the physiological limit, the metabolic balance is disrupted and bone and cartilage tissues are destroyed. However, the relationship between mechanical loading and estrogen during the process of such destruction is unknown. Excessive mechanical stress may decrease the levels of lubricating proteins, thereby increasing the levels of substrate-destructive proteins, resulting in substrate destruction. Intra-articular injection of high-molecular-weight hyaluronic acid has been reported to alleviate such substrate destruction in orthopedics. However, there is insufficient evidence to support its efficacy in the TMJ. In this article, we review these issues with reference to the results of previous studies and our own research.

The debate as to whether occlusion is a contributing factor to the onset of temporomandibular joint disorders has already concluded. However, this does not mean that the examination of jaw function, including occlusion status, is meaningless. The stomatognathic function is performed in harmony with the neuromuscular mechanism, together with the complex combination of the temporomandibular joints, masticatory muscles, and occlusion. As a result, if there is a disturbance in the temporomandibular joints or masticatory muscles, other organs may be affected, and vice versa. This article outlines possible contributing factors to temporomandibular disorders in terms of function, including occlusion, and psychological characteristics from the prospective cohort studies conducted by the author's group. Since the presence or absence of canine guides, pain threshold, and stress show significant relative risks, it can be concluded that screening for the risk of developing temporomandibular disorders is important.

A case of temporomandibular disorders, with calcification of the anteriorly displaced temporomandibular joint (TMJ) disc, is presented. After arthroscopic findings confirmed the synovial condition, TMJ arthroplasty and disc resection procedures were performed. A 74-year-old woman visited a nearby dental clinic for right-sided temporomandibular joint pain. Panoramic radiographs showed deformity of the right-side mandibular head, and the patient was referred to our department for detailed examinations and treatment. CT scan findings revealed an enlarged right mandibular head with osteophytes along with well-defined calcification in front of the mandibular head. Mild synovitis was found by TMJ arthroscopy, though no perforation of the free body or TMJ disc was noted. TMJ arthrodesis and mandibuloplasty procedures were performed, during which articular disc calcification was observed. Two years after the surgery, jaw function was found to be good with a maximum mouth opening of 49 mm and no TMJ pain.