Temporomandibular disorder (TMD) is an umbrella term that covers musculoskeletal and neuromuscular conditions involving the masticatory muscles, the temporomandibular joint (TMJ), and/or their associated structures [1]. TMD is considered the most common chronic pain of non-odontogenic origin in the orofacial area [2]. The causes of TMD are complex and multifactorial. There are numerous factors including behavioral factors, social factors, and emotional factors that can contribute to TMDs [3]. Treatment for TMD includes various methods depending on the patient’s symptoms and sign, such as medication, physical therapy, stabilization splint, behavioral management, and arthrocentesis. However, despite various treatments, it is common to see cases where the symptoms of TMD become chronic [4]. Therefore, it is important to educate patients on controlling harmful habits to prevent unnecessary tension in the masticatory muscles [5].

Bruxism has traditionally been regarded as an uncontrolled harmful habit that can contribute to TMD through joint overload or muscle damage. Later studies have described bruxism as a behavioral activity rather than a disorder, suggesting that it may act as a potential risk or protective factor. The bruxism-TMD relationship is still one of the most common controversial aspects of dental research, mainly because of the uncertain knowledge of the etiology and diagnostic aspects of TMD [6]. Recent studies using quantitative and specific diagnoses of bruxism have shown a weaker association with TMD. Especially, anterior tooth wear was not a major risk factor for TMD [7]. However, some individuals without TMD symptoms are seeking to use stabilization splints to protect their teeth or reduce the risk of developing TMD.

Stabilization splint is one of the useful conservative treatments in the management of TMD [8]. Despite pain-relieving effects, the mechanism of stabilization splint therapy is yet to be fully understood. Wearing an intraoral appliance is thought to help management by reducing parafunctional activities, leading to changes and adaptations in the masticatory system [9]. Stabilization splints may help because they combine the general benefits of simply being in treatment with the specific benefit of reducing harmful jaw habits [10]. It is reported that using an stabilization splint can reduce the activity of the masticatory muscles [11]. Therefore, it is proposed that the use of stabilization splint may potentially lead to changes in the patient’s bite force.

Various methods have been previously reported for measuring bite force [12-14]. The recently developed Dental Prescale II (DP2; GC Corp.) utilizes a color-changing chemical reaction to detect occlusal contacts [15]. Dental Prescale II serves quantitative data on occlusal contact area, occlusal pressure, and bite force. A validated and reliable method for measuring bite force has been commercialized, enabling systematic research using this technique. It has been used not only for healthy subjects but also for orthodontic patients, removable denture patients, implant denture patients, and others.

This study aims to investigate changes in maximal bite force following the use of stabilization splint in patients with bruxism.

1. Ethical Approval

This study received approval from the Institutional Review Board of Kyungpook National University Dental Hospital (IRB No. KNUDH-2022-07-03-00). All procedures were conducted in accordance with the principles outlined in the latest revision of the Declaration of Helsinki. Every participant was informed about the study and provided written informed consent before participation.

2. Subjects

This study was conducted on 46 patients who visited the Department of Oral Medicine, Kyungpook National University Dental Hospital due to bruxism, from August 2022 to July 2023. Participants underwent examinations based on the diagnostic criteria for temporomandibular disorders (DC/TMD) protocol. Only patients reported with subject bruxism, without any symptoms or findings of TMDs based on the DC/TMD, were included. The exclusion criteria that could influence bite force measurements are as follows: 1) History of surgery in the oral and maxillofacial region or systemic diseases associated with pain, 2) Presence of malocclusion, such as open bite or crossbite, or ongoing orthodontic treatment, 3) Missing teeth, excluding premolar extractions for orthodontic treatment and third molar extractions, 4) History of prior botulinum toxin injections in the orofacial area. Patients in whom no bruxofacets were observed following the use of the stabilization splint were finally excluded from the study. A total of 22 participants (11 males and 11 females) were enrolled in this study.

3. Fabrication and Application of the Stabilization Splint

Mandibular flat plane stabilization splints were fabricated with acrylic resin. The stabilization splint was designed with a thickness of 2 mm between the maxillary and mandibular second molars and was adjusted to achieve even contact points across all occluding posterior teeth. Anterior teeth contact was slightly removed, canine guidance was achieved in eccentric movement. Patients were instructed to wear it during sleep every night.

4. Measurement of Maximal Bite Force

Maximal bite force was assessed using the Dental Prescale II. This system comprises 150 μm-thick pressure-sensitive film sheets, constructed with three layers of polyethylene terephthalate that encase a developer layer and a microcapsule layer. The microcapsules, varying in size and thickness, contain red dye-producing agents that are released in response to occlusal pressures up to 120 MPa [16]. Subjects were seated with back support and instructed to clench maximally in the intercuspal position for approximately 3 seconds, focusing on sustained maximum force rather than instantaneous peak force during clenching. After the participant has bitten down, the film is analyzed by a color image analyzer (Bite Force Analyzer; GC Corp.), which is image analysis software for scans of the film taken using an optical image scanner (GT-X830; Seiko Epson Corp.) [17]. The analysis calculates occlusal contact area, occlusal pressure, and maximal bite force based on the imprints of teeth on the film. As it also provides information on the left and right distribution of force, the symmetry of bite force can be assessed. In this study, the ‘differences between the left and right sides’ were defined as the absolute value of the difference in bite force between the two sides for comparison.

Maximal bite force was measured before the patients began using the stabilization splint. After confirming the presence of bruxofacets on the surface, bite force was remeasured at one and two weeks after stabilization splint use, and subsequently at monthly intervals.

5. Statistical Analysis

All statistical analyses were performed using IBM SPSS Statistics for Windows, version 27.0 (IBM Corp.). The normality of the data was assessed with the Shapiro–Wilk test. To evaluate differences in maximal bite force before and after stabilization splint use, the Wilcoxon signed-ranks test was employed. A p-value of <0.05 was considered statistically significant.

A total of 22 subjects (mean age 44.5±13.2 years; 11 males and 11 females) were included in the study. Demographic features of the patients were shown in Table 1. The changes in maximal bite force and the differences in bite force between the left and right sides are presented in Table 2.

No significant changes in maximum bite force were observed over the 6-month period following stabilization splint use (Friedman test; p=0.142). While there were no significant changes in maximum bite force over the observation period, specific time points showed significant differences. There was a significant reduction in maximal bite force between the initial measurement (before) and one week later, followed by a significant increase from one week to two weeks (Wilcoxon signed-ranks test; p<0.05) (Fig. 1). No statistically significant changes were observed between the subsequent time points.

Throughout the observation period, differences in maximal bite force between the left and right sides were also not significant (Friedman test; p=0.476). Differences in maximal bite force between the left and right sides showed a significant decrease between one week later and two weeks later (Wilcoxon signed-ranks test; p<0.05) (Fig. 2). After 2 weeks, no significant changes of the differences in bite force between the left and right sides were detected.

No statistically significant differences in changes in bite force following stabilization use were observed based on age or sex.

Research on changes in the masticatory system following the use of a stabilization splint has been conducted for many years. In previous studies, electromyography (EMG) was commonly used due to a lack of direct measurement tools for occlusal force [18]. Recently, various techniques for measuring occlusal force have been introduced [12-14]. The method using a strain gauge has the advantage of allowing repeated measurements. However, the sensor is typically placed on the first molar, it only reflects the occlusal force on a single tooth. This approach can differ significantly from measuring the total occlusal force across the full dentition. Additionally, occlusal force measurements can vary depending on the thickness of the sensor, with thicker sensors often resulting in lower recorded forces [19]. Measuring occlusal force with all teeth in the intercuspal position is considered most similar with the bite force generated during mastication. Dental Prescale II offers a simple measurement method for occlusal force. Its thin sensor and ability to simultaneously analyze the force applied across the entire dentition make it one of the most accurate tools for representing actual bite force [15].

Patients with bruxism may experience significant clinical issues, such as hard and soft tissue damage, orofacial pain, and headaches [20]. Various treatments for sleep bruxism, including occlusal therapy, psychological interventions, pharmacotherapy, and physical therapy are proposed in dental clinics. Stabilization splints help to suppress the effects of tooth grinding, such as TMJ pain, tooth wear, and grinding sounds. They also distribute bite force, thereby reducing stress on TMJ and masticatory muscles structures. Furthermore, studies have reported that stabilization splints significantly decrease sleep bruxism-related episodes and reduce the sleep movement activity index [21]. However, recent meta-analyses with opposing conclusions have emerged, fueling ongoing debates about the relationship between bruxism and TMD [6,7]. According to the international consensus on bruxism established in 2018, sleep bruxism is defined as masticatory muscle activity during sleep, characterized by phasic (rhythmic) or tonic (non-rhythmic) patterns, and is not considered a motor or sleep disorder in healthy individuals [22].

The mechanism of action of stabilization splints is not well understood; however, this treatment has been shown to reduce nocturnal masticatory muscle activity and protect teeth from damage caused by bruxism related muscle contractions [14]. Stabilization splints also help maintain stable and physiologically optimal bite forces by reducing hyperactivity of the jaw elevator muscles. Therefore, by distributing force and reducing grinding, stabilization splints may influence changes in bite force [23]. However, these studies were conducted on patients with TMD, making it unclear whether the observed changes are due to a direct effect on bite force or simply a reduction in TMD symptoms. Therefore, it is necessary to evaluate these effects in individuals without TMD. In this study, an initial reduction in bite force was observed, followed by recovery, which may be due to a temporary reduction in bruxism during the adaptation period to the stabilization splint. This finding is consistent with previous studies using EMG [18]. Therefore, it appears that stabilization splint use does not reduce bruxism itself nor significantly impact bite force. The purpose of using a stabilization splint in patients with asymptomatic bruxism should be clearly defined. However, these findings should not be misinterpreted to suggest that stabilization splints are not effective for patients with TMD.

The degree of left-right bite force difference among the patients in this study varied widely. Since all participants were free of TMD, it cannot be concluded that left-right bite force differences necessarily influence TMD development. Although it was expected that using a stabilization splint would reduce the left-right difference by relaxing the masticatory muscles, the pattern returned to its original state after two weeks. This temporary reduction in left-right difference, followed by a return, likely reflects an initial decrease in maximal bite force that later rebounded, rather than a direct impact on masticatory force.

One major limitation of this study is the absence of a control group without stabilization splint therapy. Bruxism cannot be diagnosed solely through the patient’s subjective perception, tooth wear, or reports from others. In this study, bruxism was confirmed by observing actual bruxofacets on the stabilization splint of patients who were aware of their condition. Due to practical limitations, polysomnography for diagnosing bruxism in a control group was not feasible. Regardless of the comparison with a control group, the finding that bite force did not change even with stabilization splint use is a significant discovery. Future studies should incorporate a control group, control for variables such as TMD symptoms and stabilization splint usage duration, and address other relevant factors. Another limitation of this study is the small sample size. Although a larger number of participants was initially planned prior to the study, a substantial dropout occurred during the experimental process.

Previous studies on bite force changes following stabilization splint use mostly observed short-term effects, within 1-2 months. The duration of stabilization splint use varies based on symptoms; however, chronic patients often use the device for extended periods, highlighting the need for long-term evaluations. This study is significant as it is the first to observe patients over a six-month period, providing valuable long-term insights. Further research is needed to better understand the relationships between bruxism, TMD, and the effects of stabilization splint therapy [24].

In conlusion, this study indicates that stabilization splint use initially reduces bite force gradually returning to baseline levels over time. The left-right bite force difference also decreases temporarily but returns to its original pattern, suggesting that the effects of the stabilization splint on bite force are transient and possibly related to an adaptation process.

No potential conflict of interest relevant to this article was reported.

The datasets used during the current study are available from the corresponding author on reasonable request.

This work was supported by Kyungpook National University Dental Hospital Institute for Dental Research (2022).

Conceptualization: JRK. Data curation: JRK. Formal analysis: JRK, JWC. Funding acquisition: JRK. Methodology: JRK, JWC. Project administration: JRK. Visualization: JRK. Writing - original draft: JRK. Writing - review & editing: JRK.