The TMJ is a part of the configuration of the maxillofacial function system along with the teeth, maxillofacial bone, masticatory muscle, nerves, and blood vessels. Assessing the anatomical structure of the TMJ is an important way to study its morphology and function and will help in the diagnosis, treatment, and evaluation of TMJ disease. This study randomly selected patients who visited our hospital and underwent CBCT from 2014 to 2016.
In general, the mandibular condyle is observed as having a crescent-shaped elliptical articular surface, and the mediolateral width is twice as long as the anteroposterior width. The angle of the condyle is rotated about 20° inward on average. The morphology of the condyle is diverse, and vertical and anterior osteophyma is often observed. The beak-shaped anterior osteophyte appears to coincide with the area where excessive extension or load is applied. Flattening and erosion of the condyle articular surface seem to be evidence of an ischemic response similar to avascular necrosis and are caused by a mechanical overload on the joints. In fact, some patients clearly showing arthritis and osteophyma were included in the CBCT image.
CBCT is a powerful tool for diagnosing TMD [9, 17, 21]. The CBCT scan provides multiplanar images of the condyle and surrounding structures, which are reconstructed in three dimensions to analyze the TMJ morphology, position, and dynamics [14, 22,23,24]. This enables easier and more accurate visualization of complex anatomical structures with less radiation exposure, lower operating costs, and reduced scan times compared to the traditional helical CT [14, 23, 25,26,27]. Thus, when used properly, CBCT imaging can present more accurate and valuable diagnostic information compared to conventional radiography techniques [28, 29].
In this study, all variables were measured using CBCT software. The software simultaneously provides sagittal, axial, coronal, and 3D reconstruction views of all landmarks. Thus, the measurement errors due to incorrect determination of critical anatomical points were minimized [21].
Difference in condylar morphology
The subjects enrolled in this study did not show any discomfort of TMD, so the measured variables also did not show any significant results. This shows that the sizes and angles of the condyles are basically symmetric between the left and right sides. There are various factors that determine the asymmetry of the TMJ structure, such as the absence of teeth, tooth wear, premature occlusal contact point, and functional deviation of the mandible. Articular cartilage is known to respond to degenerative changes and non-physiological deformities of the joints by changing the single cartilage layer and total layer thickness, which will lead to a change in vertical length and width [30].
In our study, the mediolateral width of a man’s left and right condyle was significantly higher than that of a woman’s left and right condyle. The difference in condylar morphology according to gender should be consistent with the difference in skeletal characteristics, which is similar to Song et al.’s result stating that the frontal and lateral facial measurements in men were greater than those in women [31].
Significant differences were found in condylar morphology according to age groups. In men, the mediolateral width of the left and right condyles increased with age, and this trend was evident when comparing groups 2 and 3 with group 1. Condyles were more likely to be reconstructed at the age of 40 years or older than at the age of 20–30 years, and it was also possible to infer the morphology change pattern. Unlike in men, the anteroposterior width and condyle axis angle in females significantly increased with age. In terms of condylar morphology, older individuals were more likely to exhibit degenerative changes than young individuals [20]. Therefore, the change in condyle size was more apparent when compared with group 1. There is a significant relation between disk displacement without reduction and degenerative bone changes in the temporomandibular joint [32]. So, CT images taken for the diagnosis of TMD were not included in this study. And we excluded the patients who have TMD symptoms.
Differences in condylar position
As regards the condylar morphology, there was no significant result in the joint space between the left and right sides and the condylar position of the fossa depth. Therefore, this result suggests that the condylar position is also symmetric. There was no difference in condyle position according to gender. Some variables showed significant differences in the comparison of the condylar position according to age. The length of the superior joint space of the left and right sides in men tended to increase gradually with increasing age, and this trend was also evident when comparing groups 2 and 3 with group 1. The fossa depth of the left and right sides in women also showed significant results, and the depth decreased with age. The difference was obvious when comparing group 1 with group 3. Aging may have caused the flattening of the superior part of the condyle and erosion of the articular tubercle. Therefore, the position of the condyle may change with age.
Differences in the mandibular body
The size of the mandibular body did not show significant results when comparing the right and left sides, which suggests that the mandibular body is generally symmetric. The mandibular body size was significantly higher in men than in women (p < 0.05). In women, the distance from the menton to the gonion tended to decrease with age. In both the left and right sides, there was a clear difference between group 1 (20–30 years old) and group 3 (60 years old or older), which may have resulted from the effect of bone remodeling due to tooth loss.
Application to allosteric TMJ
In this study, the morphology, position, and size of the temporomandibular joints of Koreans, according to the left and right sides, gender, and age, were inferred using subjects aged 20 to 88 years. Alloplastic TMJ is a procedure that provides biomechanics, not a biological, solution for the treatment of severe joint diseases [19]. To mimic the motion of the joints, the patient-customized joint reproduction is of utmost importance. Indications for alloplastic TMJ include bony ankyloses, failure in previous allograft and autogenous joint replacements, post-traumatic condylar injury, avascular necrosis, reconstruction after tumor resection, developmental abnormalities, functional abnormalities, and severe inflammation that does not respond to conservative treatment [18]. The older the patient, the more likely alloplastic TMJ is to be applied, but young patients may also undergo alloplastic TMJ for treatment of trauma and dysfunction. Analyzing the mean values of this study with reference to gender and age will help to reproduce the appropriate TMJ structure.