Clinical curative effect
Pain, joint noise and limited movement are the common symptoms of TMD, and clinical evaluation of its therapeutic effect is always limited to qualitative description stage, such as obvious effect, effective or ineffective, however, descriptive and subjective reports should be avoided to evaluate the degree of TMD dysfunction or evaluate the TMD therapeutic effect. Fricton disorder index was objective and reliable to a certain extent, which could be used in clinical practice to conduct quantitative and objective evaluation of TMD treatment effect [11, 13, 16]. In our research, immediate pain reliefs of 21 patients with TMDs were reported. The splint used in the research was mainly stabilization splint covering the entire dentition of the patient's upper jaw which simulated Roth functional occlusion concept. The stabilization splint had a good application prospect in the treatment of maxillofacial pain by other researches [17, 18]. In this research, 24 patients with TMDs had joint noises before treatment. After 6–8 months of splint treatment, the joint noises of only 7 patients had disappeared significantly, of which 9 had relieved and 8 had no change. Joint noise mainly indicates bounce, friction and cracking, etc. Bounce was chosen for the research, and there are two pathogenesis, one occurs at the beginning of the opening, which is the anterior disc displacement with reduction, and the other often occurs at the end of the opening with hyper function of pterygoid. In the research, the mandibular movement state of 31 TMD patients had been significantly improved, and the reason may be the separation of upper and lower jaw, the bad muscle memory type of the past was eliminated, then the bad guidance of the musculoskeletal state gradually was eliminated, the open mouth type gradually recovered to be consistent. Moreover, due to the increases of the height between jaws, the negative pressure in the joint cavity was reduced and the adhesive tissue was loosened, thus the mouth opening degree improved. Leite [19] also suggested that the ease of pain of TA maybe a possible potential use of TMDs for mouth opening, our study also found the decrease of sEMG of TA in MPP.
TMJ Space Distance
CBCT can reflect the bone changes of temporomandibular joint from three-dimensional direction, and the changes of joint space can be observed easier [20]. It was generally believed that condylar displacement may be one of the important imaging manifestations of TMD [21]. Major [22] found that the change of joint space might be associated with the displacement of articular disc to some extent, and the detection of condyle position by CBCT might be used to predict the location of articular discs. Zhang [23] had found that there was no significant statistical difference between the joint spaces measured by CBCT and those measured by actual impression (P = 0.305). It can be considered that CBCT has certain reliability and authenticity in measuring temporomandibular joint spaces. The joint space is of great significance in the diagnosis and treatment of TMD, the previous studies showed that the proportion of small posterior space of patients with TMDs was significantly higher than that of asymptomatic controls [24], the possible reason is that smaller posterior spaces may produce more pressure to the double plate areas, which is loose connective tissues that is rich in blood vessels and nerves. On the contrary, when the condyle process moves forward, the vein will be filled to provide nutrition for the joint area [25] [26]. When long-term compression causes dysfunction in the bilateral plate area, signs and symptoms related to TMD may appear.
Ikeda [15] had used Kamelchuk method [14] to measure joint spaces in 24 TMJs of 22 healthy people without joint symptoms, and the anterior, posterior and superior joint spaces were 1.3 ± 0.2mm, 2.1 ± 0.3mm and 2.5 ± 0.5mm, respectively. In this research, joint spaces were measured in 31 TMD patients before treatment, and the anterior, posterior, and upper joint spaces were 2.71 ± 0.73 mm, 2.34 ± 1.03 mm and 2.58 ± 1.00 mm respectively. The anterior spaces of TMD patients increased significantly while the mean value of the posterior spaces decreased but the difference was not significant, and it may be related to the morphological changes of condyles in TMD patients. Patients in this research might be subject to occlusion interference or early contact. In order to avoid the nerve-muscle damage, the adaptive change was produced, perhaps the reconstruction of condyle. However, symptoms related with TMDs might appear when the change went beyond TMJ physiological tolerance [6].
The RW-splint treatment showed a certain deviation between the final trend of condyle movement and the position of manipulative reduction of occlusal reconstruction before treatment, which was worth further study. In this research, 31 TMD patients after the treatment of IMS stabilization splint were carried on the statistical analysis in the joint spaces on both sides. It showed that there was a tendency to decrease in the anterior joint space, however, posterior, upper, inner and outer joint space had the trend of increase, the inner space was the most significant. There were different changes on the left and right side of the condylar joint spaces, and the standard deviation was obvious, which indicated that the condylar position had a very high individuation due to the neuromuscular, anatomical physiological and psychological factors. In the term of one individual, bilateral asymmetry of neuromuscular function, asymmetry of condyle morphologies and rotation of condyles would result in the asymmetry of bilateral motion. Although the variation of the bilateral joint spaces is different due to the individual characteristics, the general trend is the reduction of the anterior joint space.
Surface Electromyogram
Electromyography was the most reliable and objective technique to evaluate muscle functions and efficacy by monitoring muscle potentials in clinical practices [27], which could evaluate the degree and duration of muscle activities. One method is the sEMG (surface electromyography), this non-invasive and painless electrophysiological activity recording the method of muscle requires the electrode as an auxiliary tool, and the non-invasive properties became its most important advantage [28]. The main goal of sEMG is to monitor the electrophysiological signals of muscle fibers in the attached area through the surface electrodes, these signals are the sum of the activities of multiple motor units in this area. Visser [29] conducted statistical analysis on the EMG monitoring values of masseter muscle and temporal muscle in healthy people within 2 days, and the correlation values showed no significant difference or asymmetry. Accurate quantitative analysis of muscle electrophysiological activities by sEMG simplified the quantitative analysis in the oral and maxillary system and provided the possibility of objective evaluations of muscle functions [28].
In this research, the EMGs of bilateral anterior temporalis and masseter muscles were 2.023 µV, 2.752 µV, 0.797 µV, 0.984 µV respectively when patients are in the mandibular postural position before the treatment. Scopel [30] found that the average EMGs of masticatory muscles in normal people without TMDs was 1.5µV, 1.6µV, 1.3µV, and 1.2µV respectively, and patients with TMD might have a higher EMG of masticatory muscles in MPP positions. In our research, the resting potential of TA in TMD patients before the treatment was consistent with the conclusion, but the resting potential of MM tended to be lower than the results of Scopel’s. The possible reason is that the patients suffered "disuse atrophy" of the masseter muscle due to the long-term chewing dysfunction caused by TMD, thus reducing the corresponding electrophysiological activities of muscles.
After the treatment, the EMGs of bilateral temporal muscles tended to decrease and bilateral masticatory muscles tended to increase during MPP. Pinho [31] measured the EMG of patients with and without TMD in the mandibular rest position, they found that the average EMG of the group without TMD at the mandibular rest position was 1.92 ± 1.20 µV and 2.52 ± 1.25 µV in TMD group. Scopel [30] also found that the EMG values of bilateral TAs and MMs in TMD patients decreased significantly after wearing the splint during MPP, the difference was statistically significant. In this research, the EMG of temporal muscles decreased in the same way as above. However, the MM EMG increased by 0.019 µV and 0.061 µV respectively after the completion of splint treatment, which was closer to the EMG of normal people without TMD (1.3 µV and 1.2 µV), thus better confirming our hypothesis.
In the research, when patients were in the maximum voluntary clench, the EMGs of bilateral anterior temporalis and masseter muscles are 66.290 µV, 76.774 µV, 83.097 µV, 82.968 µV respectively before the treatment. Tartaglia [32] found that the average EMG of the masseter muscle and temporal muscle of the healthy subjects during MVC was 131.7µV, which was significantly higher than the normal mean value in our study, the reason may be the decrease of the activity of the corresponding masticatory muscle, due to the pain of TMD patients, which leads to the weakening of masticatory function, of course, the influence of occlusal factors on the masticatory muscle is also not excluded here.
After the treatment, the EMG of bilateral temporal muscles tended to decrease and bilateral masticatory muscles tended to increase during MVC. Pinho [31] also measured the EMG of patients with and without TMD during MVC, and the average EMG of the group without TMD during MVC was 110.30 ± 82.97 µV, and that of the group with TMD was 66.77 ± 35.22 µV. The conclusion of Tartaglia [32] was also consistent with the conclusion above, they measured the EMG of the masseter muscle and temporal muscle during MVC in TMD patients and healthy subjects, and they found that the mean EMG of the masseter muscle and temporal muscle during MVC was significantly higher in the control group (131.7µV) than in the group with TMD (88.7-117.6µV). In other words, the EMG of masseter muscle and temporal muscle in the patients without TMD was higher than that in the patients with TMD during MVC. In our research, the decease of TA was not consistent with the early research, however, Ferrario [33] found that under the same occlusal load, when the EMG activity of bilateral TAs was greater than that of MMs, it might cause more loads on TMJ, thus causing discomfort and even TMD. Interestingly, in our research, the decreasing trend of bilateral TAs and the increasing trend of MMs may release the load on the TMJ, accounting for relieving the symptoms and signs of TMD patients.