Bruxism is very harmful, but the cause is not clear, so the current treatment methods are diverse[24, 25], including psychotherapy, pharmacological therapy[26-28], biofeedback[29, 30],occlusal repositioning, etc. Occlusal repositioning includes irreversible occlusal therapy (orthodontic therapy, dental prosthesis, occlusal adjustment) and reversible occlusal therapy (intraoral occlusal splint therapy). Among them, occlusal splint application is recommended as the first choice for bruxism treatment because of its low trauma, effective relieving orofacial symptoms and reduction of the complications occurrence[31-34].
There are different types of splints. Accoding to the range of the dentition covered, it can be divided into the part and full coverage occlusal splint. A full coverage occlusal splint with complete tooth contacts can help to avoid direct contact between upper and lower teeth or implant, acts as a stress relaxer to ameliorate the extra excessive overload generated due to bruxism, create a physiological function adaptive change, form a new biomechanical equilibrium between the generated occlusal forces and the physiological loading[35], reduce the tooth wear. Keskinruzgar et al found hard occlusal splint treatments significantly decreased muscle pain and increased mouth opening in individuals with sleep bruxism[36]. Sriharsha et al reported the soft occlusal splint chould decrease cortisol levels to reduce the stress levels of individuals with sleep bruxism[37] , and reduce pain caused by nocturnal bruxism on muscle and TMJ[38]. Akat et al assessed the full coverage hard occlusal splint and soft occlusal splint on masseter and temporal muscle activity, thickness, and length in patients with bruxism by ultrasonographic and electromyographic. They found EMG values of masseter and temporal muscle in group with hard occlusal or soft occlusal were significantly lower after treatment than before treatment. (p < 0.05). In the masseter, there was a significant length changes occurred after hard occlusal splint treatment[39]. Abe et al research demonstrated the occlusal splint can induced the reduction in rhythmic masticatory muscle activity related to sleep bruxism and the comfort of the occlusal splint may influence sleep quality in individuals with sleep bruxism[40]. In our study, more patients preferred soft splint and modified anterior splint to hard splint in terms of comfort.
The stomatognathic system is mainly composed of the teeth, occlusion, masticatory muscles, and temporomandibular joint. A harmonious and stable relationship under the control of peripheral and central nervous system contributes to stomatognathic system health, once the interrelationship is disrupted, periodontal disease, temporomandibular joint disorder, and masticatory muscle pain, limitation of jaw movement may develop. Occlusion refers to the contact between the teeth, while occlusion force is defined as the forces exercised by the masticatory muscle to teeth. In clinical practices, masticatory muscles function and occlusal force are important parameters for evaluating the stomatognathic system state objectively[41, 42].
K7-J5 neuromuscular analysis system (Myotronics-Noromed, Inc., Tukwila, WA, USA ) mainly consists of two parts: K7 EMG electromyography recorder and J5 Myomonitor transcutaneous electrical nerve stimulation(TENS). K7 can automatically detect bilateral masseter and temporal muscle, and calculate the mean value of the electromyographic intensity in 15 seconds. Precise measurement of bilateral masseter and temporal muscle at rest and other functional states. In our study ,compared to with hard occlusal splint , soft occlusal splint or without splint, the reduction of masseter and temporal muscle activity were observed in records with tooth clenched as a result of subjects with modified anterior splint. J5 Myomonitor TENS is the latest product in the Myo-monitor family to promote muscle relaxation and relieve spasm. It can provide two stimulation channels, Channel A mainly acts on the fifth and seventh pairs of cranial nerves to promote the relaxation of facial muscles, while channel B mainly acts on the shoulder muscles to restore the head to a normal postural position. Repetitive depolarized masticatory muscles and shoulder muscles, direct muscles to perform rhythmical contractions, promote blood circulation and metabolism. It is recommended that 40 minutes is the ideal time for healthy subjects and patients with masticatory system disorders to obtain sufficient muscle relaxation[43].
Once excessive occlusal pressure are imposed on teeth, tactile discrimination of a tooth derived from periodontal mechanoreceptors enable the self-protection mode and induce negative feedback on the activity of the muscles. However, periodontal ligaments of posterior tooth were compressed by prolonged excursive tooth contact during functional or parafunctional mandibular movements, afferent mechanoreceptors would leads to excess contractions in masticatory muscles. Therefore, prolonged occlusal surface engagement adds on excessive muscle contractions to the baseline contractions, which result in clinical appearance of muscular hyperactivity and symptoms of mandibular dysfunction[44]. In this study, results indicated that J5 TENS treatment could effectively reduce the electromyographic activity of masseter and temporal muscle in subjects with bruxism, and increase the intermaxillary space at mandible rest position .
Various materials and methods have been used to determine the location of occlusal contacts and the size of occlusal force for years. Conventional bite registration technology for analyzing occlusion in clinic primarily includes the use of articulating papers, occlusion wax ,and shim-stock foil ,which are unable to quantify occlusal contacts. Articulating paper and shim-stock foil can only provide the occlude contact by the mark in paper , and provide relatively bite force and contact area by the depth of the color and mark. Correction of occlude contact relies on dentist's interpretation of paper marks and patients occlusal feel feedback. Saliva , moisture in the oral cavity, material and thickness of the paper will influence the accuracy of marks. Therefore, above methods lacks objective accuracy, reliability, and reproducibility[45]. In order to overcome these limitations and provide more accurate and reliable occlusal relationship and occlusal force , digital quantifiable occlusal indicators have been developed, which can be used to objectively present relative or real quantitative occlusal forces. The common quantitative occlusal indicators used in clinic include Piezoelectric transducer(T-scan),Pressure sensitive film(Dental Prescale),Strain gauge transducer(Dentoforce2),Piezoresistive transducer(Flexiforce)and Pressure transducer(GM10)[46].T-scan can provided relative occlusal force by presenting the percentage of the bite force compared to the maximal occlusal force, while the Dental Prescale II can quantify, record occlusal force and bite area, and compare the difference among different situation. In our study, the minimal bite force and bite area occur in subjects with use of modified anterior splint, compared with three other situations.