Introduction

Temporomandibular disorders (TMD) are a heterogeneous group of pathologies that affect the temporomandibular joints (TMJ), the masticatory muscles, or both¹. The glossary of prosthodontic terms describes them as conditions that produce an abnormal, incomplete, or altered function of the TMJ and the masticatory muscles². Temporomandibular pain is relatively common, affecting approximately 10% of the population over the age of 18. This disorder mainly affects young adults, middle-aged people, and it is nearly twice as common in women as in men³.

TMDs are highly prevalent across the general population. The prevalence of signs in a study conducted in a European population is 34.5%⁴. Another study carried out in Australia, in a cohort of dental students, showed a prevalence of TMD symptoms of 77.2%⁵. In Uruguay, 55% of the population has at least one symptom, and 44% of the population has at least one sign⁶. These studies also show a higher prevalence in women.

It is crucial to identify the main factors leading to TMD to select the most appropriate and effective treatment for each patient. Reversible therapies most commonly used for the pain symptom include therapy with an occlusal stabilization appliance (OSA), pharmacotherapy, physiotherapy, psychotherapy (behavioral strategies), among others. These can be indicated separately or in combination^7).

OSAs have proved to be efficient in treating myalgia and arthralgia of the masticatory system⁸. However, their action mechanism is unclear and still controversial.

Different methodological problems relating to research designs have made it difficult to understand the real effectiveness and action mechanisms of OSAs. For example, among the issues detected in previous studies are a small sample size, inadequate blinding, and short follow-up times⁹.

Aerobic physical activity (APA) and manual therapy are the therapeutic measures that have proved to be most effective in changing attitudes to pain and stress are ¹⁰.

Past studies have shown that APA interventions have positive results for pain reduction in fibromyalgia treatment^11-13.

To date, no studies have been conducted to evaluate whether APA is effective in reducing pain in patients with TMD muscle pain.

Beta-endorphin (BE) is a hormone, an endogenous opioid, secreted by the anterior pituitary gland. Among the most relevant effects are analgesia, increased lactate tolerance, and decreased muscle discomfort¹⁴. Its bloodstream levels increase during prolonged APA, thus reducing pain. It shows greater resistance to enzymatic degradation compared to enkephalins¹⁵.

Valim, quoting other authors, argues that APA influences the serotonergic system, increases sympathetic activity, improves sleep, and promotes psychological well-being¹¹.

This study assessed APA's efficacy in managing musculoskeletal orofacial pain in people with TMD, compared to treatment with an OSA.

Materials and methods

Measuring instruments and variables

The verbal analog scale (VAS) was used to evaluate the pain and emotional stress variables. For data collection, we used a survey that includes the aforementioned measuring instrument (Fig. 1). The survey was used on several occasions, from the beginning to the end of the interventions, to evaluate the progression of the symptom reported by the patient. Each of the treatment groups consisted of seven people, who were evaluated for eight (APA) and six (OSA) weeks, plus an initial evaluation to determine mean values at baseline. In each assessment, self-reports of pain and stress were recorded at three times of the day: morning, afternoon, and evening. For the initial evaluation, they were provided with a pain journal two weeks before the intervention to determine an average baseline pain. This considers pain symptoms and fluctuations.

Fig. 1:  

Results

Pain variable

Descriptive analysis

Table 1 shows the average evolution of both treatments. The APA group had a longer follow-up period where a downward trend in pain values seems to be observed (Table 2).

Table 1: Weekly evolution of pain based on the type of treatment 

Table 2: Mixed-effect regression model 

Figure 1 shows the average evolution based on the results of the previously estimated regression model.

Figure 1: Pain evolution based on the type of treatment 

The graph shows pain reduction is more significant with an OSA (red lines) than APA interventions (green lines). The three lines, corresponding to each treatment, show the evolution in the three moments of the day. Note that pain values ​at night are lower than those recorded in the morning and afternoon.

Discussion

The first two lines of Table 2 indicate that the average pain value of the two groups was not significantly different, and the average was 4.463 points. The third line shows the first relevant result, which indicates that APA treatment resulted in a significant weekly reduction of 0.155 points in pain measurement. Building on the above statement, the last row shows that the trend was even more pronounced with OSA treatment than with APA treatment, and the difference is significant. By combining the two estimates, we see that pain reduction among the OSA group was almost half a point (0.434) per week.

In addition, the pain perceived by the participants was less intense (almost one VAS point) at night compared to the morning, and that the aggravating factors recorded were significant, so much so that when faced with these factors, participants' perception of pain was on average 0.794 points higher than usual.

This study found significant differences between the two groups following interventions considering the pain variable. Although APA significantly reduced pain, the trend was higher with OSA, and the difference in reduction between the two groups was significant. The evaluation period of the OSA group was six weeks. This study shows its effectiveness in reducing pain. This result is in line with studies such as that of Dao and Lavigne 1994, which shows pain reduction across a ten-week evaluation period, but without explaining the mechanism for pain reduction^8). In a subsequent literature review, Kreiner concluded that with the data available so far, the use of OSA for muscle and joint pain had sufficient scientific support ⁽⁹. In 2011, a randomized clinical study compared the efficacy of OSA and physical therapy (Tens) for TDM pain management across a four-month period for OSA and a four-week period for physical therapy. The study found a significant difference in pain reduction between the two, and OSA is the most effective²⁰. The latter is in line with other studies on the efficacy of OSA for pain reduction in five-week²¹⁾ and ten-week periods, respectively²².

Our work is consistent with the studies reviewed, and despite controversy regarding the action mode or mechanism, OSAs do reduce musculoskeletal pain in patients with TMD, and despite varying evaluation times, their effectiveness is recognized.

APA showed a slower reduction than OSA; this could be because volunteers were sedentary and the intensity of the APA was gradually increased as the days of the intervention progressed. APA intensity is required to ensure BE release¹⁴. Some studies have shown that BE levels did not vary with low-intensity exercise, with oxygen volumes ranging from 25% to 60%^23-25).

Another study of 12 untrained men found that pedaling at 60% oxygen volume did not significantly change BE levels. However, at 70 and 80% intensities, increases were significant, 2 to 5 times the baseline values, respectively²⁶. Years later, the same author didn’t find a gender difference in BE levels at 60% and 80% intensity, respectively²⁷.

One of the most difficult parameters to estimate is the intensity of physical activity. There are different methods for measuring it: measuring heart rate, the person's perception of the activity, and perceived intensity using a standardized classification system²⁸. Oxygen volume was not measured directly in our study; it was done using Karvonen's formula¹⁹. The study did not aim to measure BE.

The 2006 review article by Nishishiya assessed the efficacy of APA in fibromyalgia patients. It confirmed that APA is the most studied exercises and concluded that there is moderate evidence that APA helps improve pain in these patients. Likewise, there is no evidence that APA worsens the clinical manifestations of people with fibromyalgia²⁹.

More recently, a review article published in 2017 states that the quality of the evidence to assess physical activity and exercise for chronic pain is low. This is primarily due to small sample sizes and potentially weak studies. Favorable effects were detected in reducing the severity of pain and improving physical function, although these were mainly of low to moderate impact and were not consistent in all reviews³⁰. In this sense, our study would have the same effects as those reported in that article in terms of pain.

Available evidence suggests that physical activity and exercise have few adverse effects and can improve pain intensity, physical function, and the resulting quality of life. However, further research is needed and should focus on increasing the number of participants, including participants with a broader VAS pain level range, and extend both the intervention itself and the follow-up period³⁰.

Conclusions

This study showed that both OSA and APA helped reduce musculoskeletal orofacial pain in TMD volunteers.

The comparative study between these treatments showed that pain reduction was most effective in the OSA group.

Future studies should review and analyze the possibility of implementing other methodological designs for more accurate results: longer evaluation periods, larger samples, more diverse APA interventions.

It would be interesting to work with designs where both groups are fitted with an OSA, and one of the groups also undertakes APA. It would also be interesting to work with three groups: one with OSA, another with APA, and the third one with OSA and APA