INTRODUCTION

Temporomandibular disorders (TMDs) present orofacial symptoms such as pain, masticatory dysfunction, difficulty in opening the mouth, limited jaw movements and chronic inflammation of the temporomandibular joint (TMJ).¹ Pain is the main symptom affecting the functioning of the masticatory system and is therefore recognized as a major source of disability that affects quality of life.²

In terms of TMD symptom relief, the literature discusses various clinical procedures, such as the use of occlusal splints, physical therapy and low-level laser therapy (LLLT). It is important for these actions to be supported by clinical evidence regarding their scientific benefits and efficacy.^3,4

LLLT is one of the most used therapies today to treat symptoms such as inflammation and pain in TMDs. However, LLLT is not effective in the treatment of the disorder per se.^2,5 Due to their analgesic and anti-inflammatory effects, various types of lasers, such as helium-neon (HeNe) and arsenidegallium-aluminum (GaAlAs), are used in the management of TMDs, with each being used at different wavelengths.^1,6,7 The treatment is considered non-invasive, fast and safe.⁸

The literature describes favorable results in many clinical trials using LLLT for the treatment of signs and symptoms such as pain and trismus, but there is no specifically established protocol in terms of energy intensity, power, exposure time and number of applications.^8-11

Below we present a systematic literature review to establish whether there is evidence that LLLT can reduce the main symptoms of TMDs and to determine the most effective application protocol.

METHODS

This systematic review was performed according to Preferred Reporting Items for Systematic Review and Meta Analyses (PRISMA).¹² It was recorded in the International Prospective Register of Systematic Review PROSPERO (CRD42017076612).

RESULTS

A total of 126 articles were initially retrieved. Eight duplicate studies were removed, along with 113 studies that did not meet the eligibility criteria. Figure 1 shows the flowchart for the search and exclusion of articles.

Source: by the authors

Figure 1 Flow diagram of the study selection stages 

The characteristics of each selected study are shown in Table 1.

Figure 2 shows the risk of bias of each selected study.

Source: by the authors

Figure 2 Qualification of bias risk (RvMan version5.3) 

The main outcomes of each study are shown in Table 2.

The laser type most commonly used was GaAlAg.^{2,6,8,9,13-15} The wavelength ranged from 830 nm to 904 nm, and the number of sessions ranged from 4 to 15. The energy density used ranged from 1.5 J/cm² ⁵ to 52.5 J/cm².¹³

An energy density of 3 J/cm² was used by some authors (Kulekcioglu 2003¹⁹, Ficácková 2007⁷ and Öz 2010¹⁴) in their clinical trials, with decreased pain in the groups treated by laser. Most authors used energy densities of 4 J/cm² and 8 J/cm^{2,2,8,9,11,16,17} with favorable results in terms of improvement in TMD symptoms.

Not all studies evaluated mouth opening.^2,9,18 Cetiner 2006² evaluated masticatory difficulty as the outcome and, using an adapted visual analog scale (VAS), found an improvement in the group that used LLLT.

Regarding the evaluation of pain in the muscle insertion points made by operator palpation, in some studies, groups treated with lasers showed an improvement in pain of up to 4 times the pain levels of the placebo group.^2,8,15,19 However, some authors¹⁵ found no significant difference between scores in this outcome after 4 weeks.

Kulekcioglu 2003¹⁹ evaluated pain as an outcome by counting the number of pain points in the musculature, finding out that pain reduction in the LLLT group was higher.

Some authors^2,6,9,13,19 evaluated jaw movements in terms of protrusion and laterality (in mm), finding out an increase in dimensions of these jaw movements after LLLT.

DISCUSSION

In recent years, the use of lasers in dentistry has become a popular noninvasive treatment² with no side effects in terms of improvement of TMD symptoms, especially pain relief. As the treatment protocols available differ greatly from each other, professionals remain in doubt as to which the most effective is.

The pain produced by TMDs is intense and may interfere with a person’s daily activities. It also causes difficulty in terms of jaw movements, decreasing chewing ability and quality of nutrition,^20,21 and consequently patient’s quality of life.

The clinical trials evaluated show that different types of laser have been proposed to reduce TMD symptoms. All show different effects on analgesia, some without significant differences from the placebo (Emshoff et al 2008⁵ and Demirkol et al 2015¹⁷). Others (Cetiner 2006² and Mazzetto 2010⁶) have used the same type of laser, showing significant analgesia 3 and 4 times higher than those of their control groups. However, these differences reveal a variety of outcomes, which are not comparable with each other because the doses used were different, demonstrating the difficulties in making comparisons by meta-analysis to find evidence of one treatment being superior to another in order to establish a standardized protocol.

The laser type most commonly used for analgesia was GaAlAg.^{2,4,6,7,11-13} Although different doses were involved, this finding suggests that the GaAlAg laser should be the principal type to be tested in future standardized randomized clinical trials, measuring its outcomes to obtain evidence.

This systematic review shows that different wavelength types have been used and that most researchers report favorable results in terms of TMD symptom remission when using average lengths of 646 and 947 nm. It is known that the LLLT wavelength is related to the degree of penetration according to different clinical needs.^22,23 It has been well established that infrared lasers with longer wavelengths have more penetrative power and have been the most used in clinical trials, with wavelengths of 830 nm, 904 nm and 1064 nm.^13,17,19 However, some authors (Emshoff 2008⁵ and Pereira 2014⁸) have demonstrated the use of red lasers (632 nm and 660 nm) in the improvement of pain symptoms in their LLLT groups.

Regarding energy and density values, only two studies (Mazzetto et al⁶ and Venezian et al. 2010¹⁵) used the same power of 50 mW. Others^2,5,6,14,16 used different power settings ranging from 17 mW to 500 mW. Thus, concerning this parameter, the data were too variable to compare. However, it is known that establishing these values is very important since there is a high possibility of causing microthermal damage at the cellular level.²³

In addition to wavelength and energy, another very important aspect of LLLT is the number of sessions and the laser application time required to cause biochemical reactions to produce the analgesic and anti-inflammatory effects, which must be exact for each case according to pain intensity. There is also no agreement among authors in this regard. What we can conclude is that an average of nine sessions^{2,6,7,13,15,17} with monitoring every 4 to 5 weeks appears to produce results favorable to the reduction of TMD symptoms.

Temporomandibular dysfunctions reduce quality of life due to the presence of pain, limitations in mouth opening and difficulty in performing lateral movements and protrusion of the jaw. Not all researchers choose to evaluate the same parameters.¹³ We found out that most researchers partially evaluate pain as a TMD parameter, possibly due to its wide range and intensity. Pain is a subjective sensation, and we found great variability of measurement methods and results in the clinical trials studied. This outcome may be questionable from the point of view of precision when interpreting whether the laser therapy worked or not. Mouth opening and range of jaw movements might be more objective parameters to determine whether a therapy had a beneficial effect.

Regarding the evaluation of pain by palpation of soft points, the group treated by laser reported pain reduction, compared with the group that received placebo. There is no consensus regarding how to evaluate muscle pain points, which makes it difficult to establish replicable and viable protocols. Some clinical trials measured each muscle in isolation,⁶ while others conducted the evaluation by identifying pain points around the TMJ region.¹⁹ This variability in evaluation techniques also leads to a lack of standardization.

The difficulty in mouth opening is an important component of symptomology; however, not all studies considered this outcome.^{5,14,16,17,19} Da Silva et al 201213 and Lima et al 2013²⁴ reported a significant improvement in mouth opening; however, other clinical trials also showed clinical improvement in the laser-treated group, but with lower statistical significance.^11,13,24

The difficulty in chewing was not evaluated in most clinical trials. Only Cetiner et al 2006² evaluated this factor as an outcome. Some authors^6,9,19 evaluated jaw movements in terms of protrusion and lateralization. These findings suggest that other clinical trials should be conducted to establish a protocol for treating TMD symptoms. Perhaps a clinical trial involving a larger number of objective parameters for diagnosis and comparing the outcomes of different therapies would be more powerful in terms of verifying the therapeutic effectiveness of laser treatment.

Finally, this systematic review shows that there are no established LLLT protocols for decreasing TMD symptoms, whether in terms of wavelength, power, energy density, or number of sessions. Nevertheless, each study has shown in its own way that the therapy under evaluation is effective in terms of symptom remission.

CONCLUSION

LLLT may be considered an alternative in the relief of symptoms of clinical TMD manifestations; however, there is no evidence of one protocol being superior to all the others.