Summary:

Low-level laser auriculotherapy (LLLT-AT) is a non-invasive technique that has gained attention for managing musculoskeletal pain and anxiety. Given the painful and emotional components of temporomandibular disorders (TMD), LLLT-AT may offer therapeutic benefits by stimulating auricular points associated with pain modulation and anxiety reduction. This randomized, placebo-controlled, two-arm clinical trial with blinded outcome assessors aims to evaluate the effects of LLLT-AT in individuals diagnosed with myogenic TMD (Axis I, Group I of the DC/TMD classification) and anxiety. The study will be conducted at the Physical Agents Laboratory of Universidad Andrés Bello. Eligible participants will include members of the university community with myogenic TMD (Axis I, Group I of the DC/TMD classification) and anxiety. Participants will be stratified by sex and randomly assigned to either an experimental group receiving LLLT-AT combined with a standardized myofascial release protocol or a control group receiving sham LLLT-AT with the same myofascial protocol. Interventions will be administered twice weekly for three weeks. Outcome measures will be collected at baseline, post-intervention, and at a four-week follow-up. Primary outcomes include pressure pain threshold (PPT), assessed by algometry, and anxiety level, measured using the GAD-7 scale. Secondary outcomes include maximum mouth opening range of motion (MMOROM) and mandibular functional limitation (MFL), assessed using the Jaw Functional Limitation Scale 8 (JFLS-8).

Material and Methods

1. Study design:

   A double-blind, randomized, placebo-controlled trial with two parallel groups.

2. Study population:

   Participants will include students, faculty, and administrative staff from Universidad Andrés Bello, Casona de Las Condes Campus. Recruitment will be conducted through institutional channels such as bulletin boards, digital posters, and official emails. Interested individuals will be invited for an in-person screening at the Physical Agents Laboratory (KIN 401, Building C5) of the School of Rehabilitation Sciences. The screening will involve a study explanation, eligibility assessment, and a physical examination conducted by an experienced physiotherapist to confirm a myogenic temporomandibular disorder (Axis I, Group I, DC/TMD classification).

3. Sample size and randomization:

   Sample size will be calculated using G*Power® software, assuming a statistical power of 80% (1-β), a significance level of 0.05 (α), a 95% confidence level, and an estimated effect size of 1.0. This value is based on the study by Sancakli et al. (see reference), who reported an increase in the pressure pain threshold (PPT) of the masseter muscle in patients with myogenic temporomandibular disorders following four weeks of treatment with low-level laser therapy (LLLT). The minimum estimated sample size is 28 participants, 14 per group (LLLT-AT plus myofascial release and sham LLLT-AT plus myofascial release). To account for potential data loss or participant dropout during the study, researchers will apply a 15% increase to the calculated sample size. This decision aligns with item 8 of the Physiotherapy Evidence Database (PEDro) scale, which requires that outcome measures be obtained from at least 85% of participants. Therefore, a minimum of 16 participants per group will be recruited.

   Selected participants will be allocated into two groups through a simple randomization process: experimental group (LLLT-AT) and control group (sham LLLT-AT). Additionally, stratification by sex will be performed to ensure an equal proportion of male and female participants in each group. Randomization will be carried out using the web-based tool Research Randomizer (https://www.randomizer.org/). FFollowing the laser intervention, both the experimental and control groups will receive bilateral myofascial release treatment targeting the masticatory muscles (masseter and temporalis) (35). Participants will be blinded to group allocation.

4. Selection criteria

   • Inclusion Criteria: Participants must be ≥18 years old, of any gender, with a clinical diagnosis of unilateral or bilateral myogenic temporomandibular disorder (TMD) based on DC/TMD criteria. This includes spontaneous or palpation-induced myalgia in the masticatory muscles. Participants must have experienced at least one episode of local, referred, or radiating masticatory pain in the past 30 days lasting at least one hour. A GAD-7 score of ≥5 is also required.
   • Exclusion Criteria: Recent musculoskeletal injuries in the cervical region; skin lesions or diseases on the auricle; regular use of medications such as corticosteroids, non-steroidal anti-inflammatory drugs (NSAIDs), or photosensitizing agents (e.g., tetracyclines, quinolones, or sulfonamides); tattoos on or near the auricular region; history of cancer or tumors within the past five years; Fitzpatrick skin phototypes V-VI; autoimmune diseases (e.g., lupus erythematosus, hepatic porphyria, or pellagra); and epilepsy.
   • Discontinuation Criteria: Participants who fail to complete the evaluation or treatment protocol due to withdrawal or non-attendance will be excluded from the final analysis.

5. Interventions:

   The experimental group will receive two weekly sessions of low-level laser auriculotherapy (LLLT-AT), while the control group will receive two weekly sessions of sham LLLT-AT. Sessions will be scheduled with a two-day interval between each session. Both groups will receive a standard manual therapy protocol consisting of bilateral myofascial release of the masseter and temporalis muscles. A total of six treatment sessions will be administered over three weeks.

   • LLLT-AT Procedure. The experimental group will be treated with low-level laser auriculotherapy using an infrared low-power diode laser (class IIIb) with a wavelength of 905 nm. LLLT-AT will be applied to four auricular acupuncture points: Shenmen (TF4), Kidney (AH9), Liver (AH11), and Point Zero (O'). The protocol proposed by Marques et al. (2023) will be followed, delivering 4 joules (J) of energy per point. Treatment parameters include a mean output power of 71 milliwatts (mW) and an irradiation time of 56 seconds per point (total application time: 224 seconds), resulting in a total energy dose of 16 J per session. LLLT-AT will be applied to the auricle ipsilateral to the symptomatic temporomandibular joint (TMJ). In cases of bilateral pain, the side with the greater self-reported pain will be treated. Sessions will be performed with the participant in the lateral decubitus position, exposing the treated ear. Both the participant and the therapist will wear protective laser goggles during the procedure. The laser device used will be the Combi 400L by GYMNA®.
   • Sham LLLT-AT. In the sham laser group, participants will be treated under the same conditions as the experimental group, using the same equipment (Gymna® COMBI 400L laser), identical dosing parameters, and the same procedural precautions. However, a simulated laser emission will be applied by disconnecting the laser probe, thus preventing beam delivery. This condition will be difficult for participants to detect, as LLLT is an athermal intervention and does not produce perceptible sensations during application.
   • Myofascial Release. Both groups will receive a standard manual therapy intervention based on bilateral myofascial release techniques applied to the masticatory muscles (masseter and temporalis) following this protocol: (A) Temporalis muscle relaxation: Ischemic compression of the trigger point (1 minute), myofascial release of the trigger point (1 minute), positional release (30 seconds), and global myofascial release of the temporalis muscle (30 seconds); (B) Masseter muscle relaxation: Ischemic compression of the trigger point (1 minute), myofascial release of the trigger point (1 minute), positional release (30 seconds), and global myofascial release of the masseter muscle (30 seconds).

6. Outcome Measures:

   Primary outcomes will include changes in pressure pain threshold (PPT) and anxiety level (AL). Secondary outcomes will include changes in maximum mandibular opening range of motion (MMOROM) and mandibular functional limitation (MFL). All assessments will be performed by a blinded independent evaluator. PPT will be measured using algometry at two bilateral points associated with the masseter and temporalis muscles. AL, MMOROM, and MFL will be assessed using the Generalized Anxiety Disorder 7-item scale (GAD-7), a millimeter ruler, and the 8-item Jaw Functional Limitation Scale (JFLS-8), respectively. Assessments will be conducted at three time points: baseline (T0), post-treatment (T1: after the 6th LLLT-AT session, end of week 3), and follow-up (T2: 4 weeks after the end of treatment).

   • PPT. PPT will be measured using a WAGNER FPX® pressure algometer. Assessments will be performed with the participant in a supine position. Four bilateral muscle sites (masseter and temporalis) will be evaluated. Each site will be measured three times with 30-second intervals; the average will be calculated per site. The total PPT will be the sum of the four site averages, expressed in lb/cm². To assess intra-rater reliability, the intraclass correlation coefficient (ICC) will be calculated using repeated PPT measurements at the upper trapezius midpoint in 13 healthy volunteers, with a 48-hour interval.
   • AL. Anxiety will be assessed using the GAD-7 scale, a 7-item Likert questionnaire scored 0-3 per item (Appendix 5). The total score (range: 0-21) reflects anxiety severity. The GAD-7 has been validated in Spanish, with sensitivity = 0.83 and specificity = 0.46 (30,43).
   • MMOROM.The measurement was conducted in the supine position using a millimeter ruler. Researchers will record the distance between the upper and lower central incisors at maximum mouth opening.
   • MFL. The Jaw Functional Limitation Scale (JFLS-8), an 8-item Likert scale (0-10 per item) covering chewing, mouth opening, and communication tasks, will be used to assess the MFL. The maximum score will be used. The JFLS-8 is validated in Spanish and has high reliability (ICC = 0.94-0.96) with a strong correlation to the 20-item version (r = 0.86).

7. Variables

   Conceptual Definition of Study Variables:

   • PPT. The amount of pressure applied to the bellies of the masticatory muscles (masseter and temporalis) that the participant perceives as painful.
   • AL. The degree of tension and/or worry reported by the participant in the past two weeks, framed within the definition of generalized anxiety disorder.
   • MMOROM. The extent or range of movement demonstrated by the participant during maximal mouth opening.
   • MFL. The restriction in the ability of the jaw to perform daily activities such as speaking, smiling, eating, or other functions.
   • LLLT-AT. A therapeutic intervention involves applying laser light with a power below 0.5 watts (W) to specific points on the auricle, which are associated with pain and anxiety modulation.
   • Myofascial Release: A bilateral manual therapy intervention applied to the masticatory muscles (masseter and temporalis).

   Operational Definition of Study Variables:

   • PPT. The PPT will be measured using pressure algometry with a WAGNER FPX algometer. PPT values will be recorded in pounds per square centimeter (lb/cm²).
   • AL. The GAD-7 will be used for assessment. This Likert-scale instrument consists of 7 items scored from 0 to 3. Higher scores indicate greater anxiety severity. The total score classifies anxiety into four categories: no anxiety (0-4), mild (5-9), moderate (10-14), and severe (15-21).
   • MMOROM. The measurement will be done in millimeters using a 10-cm ruler. The evaluator will record the distance between the incisal edges of the upper and lower central incisors during maximal mouth opening.
   • MFL. The JFLS-8 is used to measure MFL. Each item is rated on a 0-10 Likert scale; higher scores indicate greater functional limitation.
   • LLLT-AT. The treatment will be administered using an infrared laser with a wavelength of 905 nm and a power output of 71 mW, which delivers 4 joules (J) of energy per point. The device used will be the Combi 400L (GYMNA®).
   • Myofascial Release: A bilateral treatment will be applied to the masseter and temporalis muscles, consisting of the following sequence: trigger point pressure (1 min), trigger point myofascial release (1 min), positional release (30 sec), and general muscle myofascial release (30 sec).

   Study Variable Type

   • PPT. Dependent variable; continuous quantitative.
   • AL. Dependent variable; ordinal qualitative.
   • MMOROM. Dependent variable; continuous quantitative.
   • MFL. Dependent variable; ordinal qualitative.
   • LLLT-AT. Independent variable; continuous quantitative.
   • Myofascial Release: Independent variable; nominal qualitative.

8. Study phases

   The study will be conducted in three phases:

   - Phase 1 - Screening (2 weeks): A digital screening questionnaire will be distributed to the target population to identify potentially eligible participants. Responses will be reviewed to assess initial eligibility based on inclusion criteria. Individuals who meet the criteria will be contacted and invited to participate. Written informed consent will be obtained prior to enrollment.

   - Phase 2 - Baseline Assessment (2 weeks): Participants will undergo a standardized physical examination to confirm the presence of myogenic TMD, based on the DC/TMD diagnostic criteria. A trained physiotherapist will perform bilateral palpation of the masseter and temporalis muscles. Those presenting localized or referred pain during palpation will be considered eligible. Participants with positive findings will complete the GAD-7 questionnaire to assess anxiety levels.

   - Phase 3 - Intervention and Follow-up (3 weeks + 4-week follow-up): Eligible participants will be randomly assigned to one of two groups: (1) an experimental group receiving LLLT-AT and (2) a control group receiving sham LLLT-AT. Both groups will also receive standardized bilateral myofascial release of the masseter and temporalis muscles. Treatment will be delivered twice per week for three weeks. Primary and secondary outcomes-including PPT, AL, MMOROM, and MFL-will be assessed at baseline, after the intervention, and at 4-week follow-up. All evaluations will be conducted by a blinded assessor, and data will be recorded and managed using Microsoft Excel®.

9. Statistical Analysis Descriptive statistics will be presented for demographic and outcome variables-PPT, AL, MMOROM, and MFL-using measures of central tendency and dispersion: means with standard deviations (mean ± SD) or medians with interquartile ranges (median, IQR), depending on data distribution. Normality will be assessed using the Shapiro-Wilk test. A summary table of participant demographic characteristics will be created and stratified by group. For secondary variables such as sex and body mass index, frequencies and either means or medians will be reported, as appropriate.

For inferential analysis, parametric or non-parametric tests will be selected based on the normality assessment. If normal distribution is confirmed, a two-way ANOVA (factors: group and time) will be applied. If data are non-normally distributed, the Kruskal-Wallis test will be used. Post-hoc analyses will be conducted using Bonferroni or Tukey corrections, as appropriate to the primary test used.

All analyses will be performed using IBM SPSS Statistics version 26, with the significance level set at p < 0.05.

Inclusion Criteria:

• Age ≥ 18 years.
• Clinical diagnosis of unilateral or bilateral myogenic temporomandibular disorder (TMD) based on DC/TMD criteria.
• Presence of myalgia or myofascial pain in the masticatory muscles, either spontaneous or induced by clinical palpation.
• At least one episode of local, referred, or radiating masticatory pain in the past 30 days, lasting at least one hour.
• Score ≥ 5 on the Generalized Anxiety Disorder 7-item scale (GAD-7), indicating generalized anxiety.

Exclusion Criteria:

• Recent musculoskeletal injury in the cervical region.
• Skin lesions or dermatological conditions on the auricle.
• Regular use of medications such as corticosteroids, non-steroidal anti-inflammatory drugs (NSAIDs), or photosensitizing agents (e.g., tetracyclines, quinolones, sulfonamides).
• Tattoos on or near the auricular area.
• History of cancer or tumors within the past five years.
• Fitzpatrick skin phototype V or VI.
• Autoimmune diseases (e.g., lupus erythematosus, hepatic porphyria, pellagra).
• Epilepsy.

Discontinuation Criteria

- The participant's withdrawal or non-attendance results in the failure to complete the evaluation or treatment protocol.