Temporomandibular disorders (TMDs) are conditions that affect the temporomandibular joint (TMJ), the chewing muscles, and the other associated structures¹. Their symptoms include pain in the head, face, neck, and ear; sounds in the TMJ; and restricted jaw movements². TMDs are estimated to affect more than 5% of the population³. The etiology of TMDs is multifactorial, with the most common factors being predisposing factors (e.g., systemic, psychological, structural, and genetic factors), initiating factors (e.g., trauma and parafunctional habits), and perpetuating factors (e.g., mechanical, muscular stress, and metabolic problems)⁴.

Bruxism is a well-known risk factor for the development of TMDs^5,6. Studies conducted on adults have reported that oral parafunctional habits are a possible cause of TMDs and that parafunctional habits were positively correlated with both muscle disorders and TMDs^{7 -12}.

Sleep bruxism (SB) is defined as masticatory muscle activity occurring during sleep that is rhythmic (phasic) or nonrhythmic (tonic) and not a movement disorder or a sleep disorder in healthy individuals¹³. SB is a common condition in the general adult population with an estimated prevalence of 5.5% to 12.8%^14,15. It can cause various symptoms, such as tooth wear and pain, or malfunction of the chewing system¹⁶.

Bruxism is most commonly diagnosed using the criteria defined by the American Academy of Sleep Medicine (AASM)¹⁷, which were last revised in 2014. Accordingly, a patient must meet several criteria to be diagnosed with SB; these include (1) regular or frequent tooth grinding sounds during sleep and (2) one or more of the following clinical signs: (i) abnormal tooth wear or (ii) transient morning jaw muscle pain or fatigue, and/or temporal headache, and/or jaw locking on awakening consistent with reports of tooth grinding during sleep.

TMD treatment aims to relieve pain and restore normal temporomandibular functions¹⁸. Since the pathogenesis of TMDs is not clearly defined, physicians generally prefer noninvasive, reversible, and conservative treatment options and tend to resort to surgery only when conservative treatments are inadequate¹⁹. The traditional treatment approach for TMDs includes physiotherapy and exercise, anti-inflammatory and analgesic drugs, muscle relaxants, oral splints, or a combination of these treatments. Recent studies have reported botulinum toxin (BTX) injections, which has become an increasingly common treatment approach, to be an effective treatment option for relieving pain and sensitivity in TMDs^20,21. However, there is currently no definite protocol regarding BTX use in the context of TMDs nor is there a consensus on the effects of BTX in patients with TMDs²². The present study sought to evaluate the effect of BTX injection on the pain level and mouth opening capacity when applied to the masseter and temporal muscles of patients with temporomandibular myofascial pain and SB.

This study was approved by the Clinical Research Ethics Committee of Kocaeli University (KÜ GOKAEK 2019/148) and was conducted by way of a clinical records review of 44 patients with temporomandibular myofascial pain and SB admitted to the Department of Oral and Maxillofacial Surgery at Kocaeli University between January 2017 and December 2019.

A total of 44 patients (36 females and eight males) were evaluated, ranging in age from 18 to 73 years (mean±standard deviation age, 35.70±12.66 years).

The values for MMO, right and left laterotrusion, protrusion, and VAS at baseline and the first, third, and sixth months of follow-up are presented in Table 1 and Fig. 1. The mean MMO values for all patients were 37.0±8.88 mm at baseline, 42.54±6.98 mm at the first month, 42.59±6.90 mm at the third month, and 42.77±6.58 mm at the sixth month. The mean values of the movements of right laterotrusion for all patients were 6.25±3.05 mm at baseline, 7.68±2.37 mm at the first month, 8.11±2.37 mm at the third month, and 8.20±2.43 mm at the sixth month, while the mean values of the movements of left laterotrusion for all patients were 7.18±3.11 mm at baseline, 8.47±2.87 mm at the first month, 8.70±3.00 mm at the third month, and 8.56±3.03 mm at the sixth month. The mean values of movements of protrusion for all patients were 3.70±1.93 mm at baseline, 5.00±1.84 mm at the first month, 5.43±2.12 mm at the third month, and 5.38±1.99 mm at the sixth month. Finally, the mean VAS values for all patients were 7.09±1.77 at baseline, 2.90±2.89 at the first month, 1.95±2.26 at the third month, and 2.11±2.19 at the sixth month.

The evaluation of all data showed that MMO, movements of the right and the left laterotrusion, and protrusion increased significantly with BTX treatment (P<0.05), while VAS values decreased significantly progressively during follow-up relative to the baseline values (P<0.05). Moreover, the pain level was decreased by 59% in the first month, 72% in the third month, and 70% in the sixth month after treatment initiation. MMO, movements of the right and the left laterotrusion, protrusion, and VAS values at the sixth month did not differ by patient age (P>0.05).

Parafunctions such as bruxism may damage TMJ tissues due to the disproportionate and repetitive excessive load on the TMJ^24,25. Bruxism can result in clinical symptoms such as headache and TMJ together with chewing muscle pain and hypertrophy^26,27. In addition, bruxism is considered to be a risk factor for the development of TMD signs and symptoms²⁸.

Experimental studies have shown increased interstitial pressure and the development of intramuscular edema due to bruxism. Muscle pain is an expected consequence of intramuscular edema²⁹. In this study, all patients with SB experienced pain in the masticatory muscles when awakened and there were also signs of pain during muscle examinations.

TMD and myofascial pain treatments primarily involve conservative approaches and symptomatic therapies³⁰ and the destructive effect of bruxism on anatomical structures should be limited^27,31.

Recent studies have investigated the effects of BTX injections on TMD, bruxism, masseter hypertrophy, and myofascial pain^{26,27,32 -34}. BTX facilitates muscle relaxation by temporarily blocking the release of Ca⁺²-mediated acetylcholine from the myoneural junction. When synaptic regeneration occurs, the neuromuscular transfer resumes and paralysis subsides³⁵. This process of relief with BTX therapy enables the patient to resolve the cause of the complaint³⁶.

In a randomized controlled study³⁷, BTX was injected in the masseter and temporal muscles in patients with nocturnal bruxism (n=6), while a saline solution was injected in patients in the control group (n=6). The follow-up controls in the fourth, eighth, and 12th weeks showed that bruxism activity decreased significantly in the treatment group. However, there was still bruxism-related pain in the temporal muscles. Likewise, in the study conducted by Guarda-Nardini et al.³⁸, BTX was injected in the chewing muscles in patients with bruxism and myofascial pain (n=10), while a saline solution was injected in the chewing muscles in the control group (n=10). Patients were evaluated for MMO, lateral-protrusive movements, and pain, with results suggesting that pain was relieved and that subjective clinical symptoms were improved significantly in the BTX group relative to in the placebo group. These authors concluded that BTX was an effective treatment in patients with bruxism and myofascial pain syndrome. Meanwhile, Al-Wayli³² evaluated the treatment of nocturnal bruxism-related pain in 25 patients injected with BTX and 25 patients managed with conventional methods. The study patients were examined after three weeks and in the second and sixth months after treatment, then evaluated later on at the end of the first year. Study results indicated that pain was relieved significantly in the BTX group, while the degree of pain reduction in the conventional treatment group was unremarkable. Finally, comparing the efficacy of BTX with that of saline solution in achieving pain reduction among 20 patients with bruxism and myofascial pain in the masticatory muscles, Guarda-Nardini et al.³⁹ observed significant improvement in jaw movements, including regarding mouth opening, after BTX injection.

In our study, we observed a significant improvement in jaw movements at the first, third, and sixth months after BTX injection, together with a decrease in the pain level. It is known that local paralysis occurring in the muscle after BTX injection reoccurs after two to four months⁴⁰. Although the physiological efficacy of BTX decreased, the reason behind why symptoms continued to decline may be due to patients being more aware of parafunctional habits and working toward having more controlled functional habits, while may have allowed other muscles involved in the chewing function to function normally.

There are studies that have examined different doses and injection techniques of BTX to eliminate the imbalance in the chewing muscles and reduce pain. Ernberg et al.⁴⁰ performed injections after dissolving 100 IU of freeze-dried BTX (Botox; Allergan, Dublin, Ireland) into 1.0 mL of room-temperature sterile isotonic saline and applied 50 IU BTX to each muscle. Guarda-Nardini et al.³⁹ injected 150 IU of BTX (Dysport) on each treated side with a five-point injection minimum, employing a reverse pyramid pattern in the masseter muscles and a chess-board pattern for the temporalis muscles. Kim et al.⁴¹ injected a total of 500 IU of BTX (Dysport) injections in the bilateral masseter and temporalis muscles at a 3:2 ratio. In our practice, similar to in the study of Kim et al.⁴¹, we used a total of 250 IU of BTX (Dysport) for injection into each side. Therefore, clarity regarding the optimal application dosage remains to be attained.

Ernberg et al.⁴⁰ reported that a significant pain-reducing effect (30%) was found after BTX injections at the first month of follow-up. In the study by Guarda-Nardini et al.³⁹ comparing BTX and saline solution injections, a 50% pain reduction at one month after BTX injections was reported, while Kurtoglu et al.⁴² observed a 22% pain reduction at one month of follow-up. In our study, the pain level was decreased by 59% in the first postoperative month, 72% in the third month, and 70% in the sixth month.

It has been reported that, after BTX injection, complications such as chewing difficulties, muscle pain, speech disorder, and abnormal changes in facial appearance can occur. Such complications are reported to be transient nature, lasting one to four weeks⁴³. There is also a low probability of allergic skin reactions or immunological responses⁴⁴, but we did not observe these outcomes in our patients.

Research cautions that there are no current standards for postevaluation and follow-up protocols relating to the administration of BTX injection⁴⁵. In our study, it was observed that patients experienced a significant increase in mouth opening and decreased pain levels even in the postoperative sixth month. Therefore, it is important to monitor how long BTX activity is effective in treated patients. We suggest that pursuing regular follow-up examinations is essential to accurately determine the need for BTX re-treatment.

Bruxism is defined as repetitive jaw muscle activity characterized by the clenching or grinding of teeth and is known to cause TMD and myofascial pain. TMD treatment aims to eliminate the causative factor of this condition. Our study suggests BTX is effective in the symptomatic treatment for patients with temporomandibular myofascial pain and SB. However, studies involving larger groups and long period follow-ups should be conducted.