Efficiency of occlusal splint therapy on orofacial muscle pain reduction: a systematic review

Study selection

The electronic search of the three databases resulted in 474 records. No additional records have been identified in the references of the included studies. After removing duplicates, 107 records remained. Screening of the titles and abstracts of the studies based on the inclusion and exclusion criteria resulted in the removal of further 37 studies. Both authors independently assessed all studies identified in the search. Any disputes regarding qualifying a study to this systematic review or interpretation of the analyzed research were discussed with the third author and resolved. The inclusion criteria were as follows: RCTs in English published between January 1, 2010, and June 1, 2021, the study group being adults, and the study group consisting of at least 20 individuals diagnosed with masticatory muscle pain. The following served as the exclusion criteria: works other than RCTs, published in languages other than English, a study group of less than 20 individuals, and a diagnosis that is not muscular. Manuscripts of the remaining studies were comprehensively assessed for eligibility. Ultimately, 13 articles were included in this systematic review [21,22,23,24,25,26,27,28,29,30,31,32,33]. All eligible papers were assessed for the risk of bias. The authors of this review classified all included studies as those with a high risk of bias. During the analysis of full-text versions, some studies were rejected due to different TMD diagnoses than muscle pain in the study group, inclusion of TMD patients without pain, no proper index to measure the pain level, or no control group [34,35,36,37,38]. One study was not completed during the writing of this systematic review [39]. Details on the selection of studies are shown in a flowchart (Fig. 1).

Fig. 1figure 1

Flowchart of study selection. Abbreviations: TMD, temporomandibular disorders; RCT, randomized controlled trial

The selection process resulted in RCTs in which therapy with various types of OSs was compared with no treatment, instruction, guidance, CSL, self-exercise or self-massage, and additional forms of therapy used alone or together with OSs: LED therapy, acupuncture, low-level LST, KT, myofunctional therapy, and physical therapy.

Study characteristics

A total of 589 participants were included: 276 in the OS therapy group, 108 in the conservative and CSL therapy group, 30 in the acupuncture group, 30 in the LST group, 23 in the device-supported sensorimotor training group, 16 in the KT therapy group, 10 in the myofunctional therapy group, 37 in the manual therapy group, and 59 in the LED therapy group. Only one study [26] used DC/TMD as the DC, one used the pressure pain threshold [27], one used sensitivity in the pressure area [29], and the other studies used RDC/TMD. The length of the follow-up ranged from 4 weeks to 6 months. The OSs used could be distinguished as follows: stabilization splints used in 207 patients [21, 22, 24, 25, 27, 30,31,32,33], reflex splints used in 59 patients [22, 25, 28], and repositioning splints used in 10 patients [29]. Only one study relied on the characteristic pain intensity (CPI) scale to assess the intensity of pain, whereas the remaining studies used the visual analog scale (VAS) to assess pain. Characteristics of the included studies are presented in Table 2.

Table 2 Characteristics of the included studiesRisk of bias in studies

Overall, all 13 studies were identified as being at a high risk of bias as they showed a high risk of bias in at least one of the seven factors analyzed.

In terms of random sequence generation, eight studies were assessed as having an “unclear risk of bias” because of insufficient information about randomization methods. Five studies adequately described the approach of allocating subjects to groups. With respect to the next factor—concealment of information about group allocation—no study properly provided information, and thus, all studies were categorized as having an “unclear risk of bias.” In the next domain assessed (blinding of participants and personnel), all studies had a high risk of bias because patients cannot be blinded in the case of OS therapy versus other therapies.

Regarding the detection bias, six studies were considered at low risk. In this section, not only the blinding of assessors was considered important, but also the type of evaluation method of patients. Two studies were graded as having high risk because the same examiner assessed the patients and was not blinded to the patient group assignment. Five studies showed an unclear risk of bias due to a lack of information about the blinding of the examiner. The remaining studies were graded as having a low risk of bias.

Only one study showed a high risk of bias due to incomplete data (attrition bias), which was attributable to the high number of dropouts. Unclear risk of bias was observed in five studies and was related to insufficient information about the number of subjects lost during the study. The rest of the studies were considered as having a low risk of bias as they had precise information on the number of dropouts and carried out statistical analysis using only data from patients who completed the study.

Selective reporting was observed in only one study. Three studies showed a high risk of bias due to other sources of bias: due to important differences in the study group, poor assessment methods, and poor data reporting, respectively.

Simplified information regarding the assessment of the risk of bias is provided in Table 3.

Table 3 Risk of bias assessmentResults of individual studies

Thirteen studies from the analyzed literature met the inclusion criteria and were assessed using primary and secondary outcomes. The results of each included study are summarized below:

OSs versus acupuncture

Grillo et al. compared the effects of acupuncture with those of flat occlusal plane appliance. They evaluated 40 women with myogenic temporomandibular dysfunction, who were randomly divided into two groups: acupuncture and OS. The effect of 4 weeks of treatments on masseter and anterior temporal muscles was evaluated using electromyographic (EMG) activity and pain pressure threshold. Pain intensity was measured using the VAS, and the range of mouth opening was evaluated using a millimeter ruler. All evaluations were conducted at the beginning and the end of the treatment. In both groups, the VAS score was equally reduced (p < 0.001) and the increase in the range of mouth opening was significant. Therefore, the authors reported that both therapies can be used with equal effectiveness in the control of chronic pain associated with TMD [23].

Barrero et al. studied the effect of acupuncture in comparison with occlusal decompression splints in TMD patients with muscle pain. They conducted an RCT involving 20 patients who underwent the abovementioned treatment methods. Outcomes were assessed using a visual analog pain scale, the range of mouth opening, lateral deviation of the mandible in millimeters, and assessment of sensitivity to pressure at various points: preauricular, masseter, temporal, and trapezius muscles. Parameters were assessed before and 30 days after the 5-week treatment. Both groups of patients showed a reduction in MP in the short term. Patients treated with decompression splints showed a reduction in subjective pain and pain on pressure points located on the temporal, masseter, and trapezius muscles, but without statistically significant differences (p > 0.05). Patients treated with acupuncture experienced significant improvements in all studied parameters (reduced subjective pain, stronger algometer pressure needed to produce pain, improved mouth opening). Pain reduction was statistically significant (p < 0.05) for all but one evaluated point: the one located on the masseter muscle (p = 0.068). Acupuncture is an effective complement and/or an acceptable alternative to decompression splints in the treatment of MP. The authors did not conduct a long-term follow-up of patients, so these results are limited to the immediate effects of both procedures used. Due to the lack of long-term observations, conclusions regarding the long-term impact of acupuncture could not be made, which is highly important in patients with chronic TMD pain [29].

OSs versus LST

Oz et al. compared the effects of low-level LST with those of OSs in patients with the signs and symptoms of MP dysfunction syndrome. A total of 40 patients (34 women and 6 men) were randomly divided into two groups: study group (n = 20) who underwent low-level LST (twice per week, for a total of 10 sessions) and control group (n = 20) who were instructed to wear OSs 24 h/d for 3 months. The authors presented the conclusions of this study in the form of vertical movements, which showed statistically significant improvements after the treatments in both groups (p < 0.01); however, when the groups were compared with each other, no significant differences were observed. In both groups, tenderness to palpation of the muscles decreased significantly. In addition, pressure pain threshold evaluations and VAS scores revealed similar results. This particular type of low-level LST (820 nm, 3 J/cm2, 300-mW output power) was shown to be as effective as OSs in pain release and mandibular movement improvements in patients with MP [24].

Manfredini et al. compared three treatment modalities for the management of MP in jaw muscles: LST, oral appliance (OA), and only CSL. VAS pain levels and the muscular index (MI) of the Craniomandibular Index were assessed at baseline, 3 weeks, 3 months, and 6 months. After 3 weeks, VAS values decreased significantly only in the LST group (p = 0.018), and significant improvements were observed in the MI (LST, p = 0,038; OA, p = 0.008). After 6 months, positive changes in VAS values were observed in the LST group (p = 0.001) and also shown in the OA (p = 0.002) and CSL groups (p = 0.048), and improvement in the MI was maintained in both LST (p = 0.025) and OA groups (p = 0.001). Therefore, the authors of this study concluded that active forms of therapy such as splint therapy and LST support should be used to maximize the positive effect of therapy on pain relief in the short term. However, in the long term, i.e., after 6 months, all forms of therapy had a similar effect [26].

OSs versus other appliances

Giannakopoulos et al. compared the short-term therapeutic efficacy of device-supported sensorimotor training with that of standard splint therapy in patients with myofascial TMD pain over a treatment period of 3 months. A total of 45 patients with myofascial TMD pain (graded chronic pain status, I and II) were randomly assigned to two treatment groups (sensorimotor training and conventional splint treatment). The patients were evaluated four times (initial examination and 2, 6, and 12 weeks later) by RDC/TMD, and their EMG activity of the temporal and masseter muscles was recorded during the initial session and after 3 months under conditions of force-controlled submaximum and maximum intercuspation. Significant (p < 0.001) pain reduction (sensorimotor training 53% and splint therapy 40%) was achieved in both groups, with no significant differences (p > 0.05) between them. EMG activity under submaximum bite force was not significantly different between the first and last sessions (p > 0.05). However, during maximum biting, EMG activity was approximately 20% higher (p < 0.01) for masseter muscles in both groups after the treatment period. In contrast, a significant increase (p < 0.01) for temporal muscles was observed only in the sensorimotor training group. Moreover, sensorimotor training was significantly (p < 0.05) more difficult to use than splints, which, according to the authors, is related to the patients’ preference for passive forms of treatment that do not require their involvement. The limitation of this study is the lack of a true control group, i.e., untreated patients with TMD pain, and inability to control the timing of intervention, especially during home exercises. Device-supported sensorimotor training could be a cost-effective alternative (or additional) treatment for functional TMD pain [25].

OS versus LED therapy

Costa et al. compared the effects of LED therapy associated with OS on the signs and symptoms of TMDs. They assessed procedures with LED therapy and OSs in six groups of patients: group 1 (G1) was the control and received only conventional therapy with OS; group 2 (G2) was the placebo and received treatment with OS and therapy with LED (device turned off); group 3 (G3) received LED therapy (infrared) once a week; group 4 (G4) received LED therapy (infrared) twice a week; group 5 (G5) received OSs associated with LED (infrared) therapy (once a week); and group 6 (G6) received OS therapy plus infrared LED (two sessions per week). The patients were evaluated before and 30 days after treatment using pain intensity in the masticatory system during all controls. EMG signals of masseter and temporal muscles and blood lactate levels were evaluated before and after the treatment. Groups subjected to combined treatment with splint therapy and LED therapy showed significant differences (p < 0.05) from the control group in the analysis of pain intensity and decreased RMS (root-mean-square) values (EMG analysis). The combination of LED therapy and OSs shows superior results than isolated treatments, and the protocol of two sessions per week was associated with a significant reduction (p < 0.05) in blood lactate levels [33].

OSs versus physical therapy

Felicio et al. studied the effects of orofacial muscle activity therapy (OMT) in the treatment of patients with concomitant painful joints and TMDs, who were randomized into four groups: 10 patients were treated with OMT (group T), 10 were treated with an OS (OS group), and 10 formed the untreated TMD control group (SC). Ten patients without TMDs formed the asymptomatic group (AC). During initial studies, no significant differences were observed between the OMT group, OS group, and control TMD group (p > 0.05). After 120 days, better results were observed in the OMT group than in the OS group. Between groups, significant differences were observed in headache frequency (p < 0.05), and all groups showed a decreased sensitivity to pain during palpation of all tested muscles, except for TMJs; increased measures of jaw range of motion; decreased Helkimo Index; reduced frequency; and severity of signs and symptoms. The control group did not differ over time in comparison with interventions [30].

Keskinruzgar et al. evaluated the effectiveness of KT in patients with muscle MP and patients with sleep bruxism (SB). They tried to answer the question of whether KT may be an alternative to OSs in the treatment of muscle pain in patients with SB. In this study, 16 patients were treated with KT (Kinesio group), and 18 patients were treated with OSs (splint group,(Sp-Tx). Masseter and temporal muscle pain thresholds (MPPT and TPPT), VAS, and mouth opening values were compared before treatment and at weeks 1 and 5 of treatment. Both KT and OS therapy were significantly associated with reduced muscle pain thresholds, lower VAS values, and increased range of mouth opening. There was no statistically significant difference between the KT and Sp-Tx groups in MPPT, TPPT, VAS, and mouth opening values before and after treatment. KT is an easy-to-use treatment method and has been found to reduce muscle pain and increase the range of mouth opening. It is at least as effective as OS in the treatment of muscle-related pain in patients with SB [27].

Grootel et al. conducted a study on 72 patients, who were randomly assigned to either the physical therapy group (Ph-Tx) or the Sp-Tx group. Following treatment and a 1-year follow-up, the yielding success rate (SR), effectiveness (mean index treatment duration control (TDC)), and Cohen’s d were used as treatment outcomes to determine pain intensity. SR and effectiveness were similar for Ph-Tx and Sp-Tx groups (long-term SR: 51–60%; TDC: − 0.512 to − 0.575). Cohen’s d was 0.86 (Ph-Tx) and 1.39 (Sp-Tx). Treatment duration was shorter in the Ph-Tx group (on average 10.4 weeks less; p < 0.001). The Sp-Tx group needed 7.1 visits less (p < 0.001). Grootel et al. reported that physical therapy may be preferred as initial therapy over OS therapy in stepped care of myogenous TMDs. With a similar SR and effectiveness, physiotherapy required a shorter duration. Thus, patients whose initial physical therapy was unsuccessful can continue with the subsequent treatment. This stepped-care model reinforces the conclusion on therapy preference as the overall SR hardly depends on the therapy sequence [29].

OSs versus education

Michelotti et al. carried out a clinical trial to compare the effectiveness of an education program with that of an OS in treating MP of jaw muscles across a short period. A total of 44 patients were randomly divided into two groups. One group (education group) received information regarding the nature of TMD and self-care measures, whereas another group received an OS. After 3 months, changes in spontaneous muscle pain differed significantly between the two groups (P = 0.034; effect size = 0.33). The reduction in pain level is better in the education group. No significant differences in changes in pain-free maximal mouth opening were observed between the two groups (P = 0.528; effect size = 0.20). Furthermore, changes in headache and pain on chewing did not differ significantly between the two groups (P ≥ 0.550, effect size ≤ 0.10). According to the authors, self-care education as well as extensive communication between the patient and the doctor may be more effective than OSs in the treatment of MP. However, studies on the two protocols used did not report their long-term effects [32].

Erbasar et al. compared the clinical nociceptive trigeminal inhibition–tension suppression system (NTI-tss) device with first-line therapy of MP, which includes guidance, assurance, CSL, and behavioral changes. This trial included 40 patients diagnosed with MP, who were randomly divided into two groups: one group consisted of patients who received guidance, ensuring, CSL, and behavioral change; in the second group, the use of the NTI device was added to the CSL. Patients from both groups experienced a reduction in pain with time; however, no significant differences in pain reduction were observed between the groups (P = 0.922). Jaw function gradually improved in both groups, but without significant differences between the groups (P = 0.927). The authors concluded that the integration of the NTI-tss device into the therapy protocol did not provide any additional benefit in relieving the symptoms of MP [28].

Conti et al. compared the effectiveness of therapy in the management of pain in masticatory muscles. A total of 51 patients diagnosed with masticatory MP were randomly divided into three groups. Group I received a full-coverage acrylic stabilization OS and CSL, group II received an anterior device NTI system and CSL, whereas group III received only CSL for behavioral changes and self-care without the support of additional intraoral appliances. Behavioral changes were effective in the treatment of MP. However, concurrent use of occlusion devices, especially a stabilization splint, seemed to cause earlier improvement. The authors concluded that the integration of NTI-tss in the treatment protocol provided no added benefit in alleviating MP. It should be noted that the follow-up period is relatively short and the study group is small, which undoubtedly creates the need for further research from this perspective [22].

Ficnar et al. conducted their research on 63 patients diagnosed with myofascial TMDs. The first group (CO group- control group) received conservative therapy, including the use of self-exercises (muscle exercise form according to Prof. Schulte, self-massage techniques, and mouth opening exercises), medication-based therapy using nonsteroidal anti-inflammatory drugs, muscle relaxants, as well as manual therapy. The second group (SS- laboratory-made occlusal appliance group) received a laboratory-made stabilization splint in addition to conservative therapy. The third group (SB) received conservative therapy and the SOLUBrux splint-prefabricated semifinished occlusal appliance. In all three groups, the authors observed a reduction in the pressure-sensitive areas upon palpation of the muscle and TMJs as well as an increase in the extent of vertical movement. With respect to the extent of pain-free active vertical movement of the lower jaw, no statistically significant differences were observed within the CO group, whereas differences were observed between the initial findings and the final examination after 2.5 months in the SS group (P = 0.041). However, in the SB group, differences were observed both between the initial findings and the two-weekly follow-up examination (p = 0.004) and between the initial findings and the final examination (p = 0.021). In the SB group, the increase in active mouth opening accompanied by pain between the initial findings and the 2-weekly follow-up examination was significantly higher (p = 0.025). Regarding the overall number of extraoral muscle palpation areas, a reduction in the pressure-sensitive areas was observed in all three groups. These results suggest that TMDs should initially be treated with conservative therapy consisting of self-exercises as well as drug-based and manual treatment. However, a higher improvement in mouth opening was achieved in the SB and SS groups. It should be noted that this effect appeared faster in the group of patients using the semifinished SOLU-Brux occlusal appliance than in the group of patients using the laboratory-made stabilization splint. The authors concluded that semifinished appliances are not only more quickly available but also cheaper, so they should be used as the first-choice method, especially in patients with restricted mouth opening [21].

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