Chronic nonspecific neck pain is a common musculoskeletal disorder recognized for its high rates of disability and economic losses, ranking second in musculoskeletal conditions after low back pain. There are several effective physical therapy interventions to treat chronic nonspecific neck pain, such as laser therapy. High-intensity laser therapy (HILT) is a recent treatment proposed to reduce musculoskeletal pain; however, there are few studies that prove its effects in reducing neck pain.

The aim of this randomized clinical trial is to evaluate the effect of high-intensity laser therapy on pain intensity in patients with chronic nonspecific neck pain.

The research will be carried out in the laboratory of physical agents at the Andrés Bello University, Campus Casona de las Condes. Participants will be employers and volunteer students with non-specific chronic neck pain. Participants will be randomized and allocated into 2 study groups: group 1 (HILT and stretching exercises) and group 2 (simulated HILT and stretching exercises). Treatments will be performed twice a week for 4 weeks with 3 assessments: before treatment, at the end of treatment, and 12 weeks after treatment (follow-up). The main results will be the differences in pain pressure threshold (ΔPPT), intensity of pain at rest (ΔRPI) and intensity of pain on movement (ΔMPI). Secondary outcome measures, on the other hand, will include differences in cervical spine range (ΔCROM) and cervical disability (ΔND).

Material and Methods

Type of study: Experimental, randomized clinical trial (RCT).

1. Study population:

   For the study, students and employees of Andrés Bello University will be considered participants.

2. Randomization and sample:

   Participants will be evaluated according to the selection criteria through a survey with closed questions and a clinical examination that will determine the presence or absence of chronic non-specific neck pain. Participants will be divided into two study groups through a simple randomization process: group 1 (HILT) and group 2 (sham HILT). The randomization of the sample will be carried out by the study director using the simple random sampling process through the research randomizer program, who will be the only one who knows the results. All groups will receive a treatment with an exercise plan that includes passive static stretching of the upper trapezius, levator scapulae, and scalenes muscles (three sets of 30 seconds for each group bilaterally). Participants will be scheduled twice a week for four weeks to carry out the assigned treatment.

   The sample size was determined with the G-Power program using a power of 0.80 (1-β), a reliability of 95%, an error of 5% (α) and an effect size of 0.6 (d- Cohen) with reference to previous studies that determined differences in mean pain intensity between experimental and control groups after HILT treatment with effect sizes of 0.53. Based on the above, the sample size is 72 subjects, with at least 36 subjects per group.

3. Procedures The outcomes to be measured will be: pain pressure threshold (PPT), resting pain intensity (RPI), movement pain intensity (MPI), cervical range of motion (CROM), and cervical disability (ND).

   Pain pressure threshold (PPT) will be evaluated with pressure algometry at six bilateral points established for the cervical region and shoulders, pain intensity at rest and on movement with VAS (RPI and MPI), cervical ROM with an inclinometer (CROM system), and cervical disability (ND) through the cervical disability index (NDI). The evaluations will be carried out by 2 independent evaluators: evaluator 1, PPT with algometry and PI with VAS (RPI and MPI); and evaluator 2, CROM with cervical inclinometry and ND with the cervical disability index. The PPT values will be evaluated in kg/cm2, the MPI will be evaluated in millimeters, the CROM will be evaluated in degrees for all movements of the cervical spine, and the ND will be evaluated as a percentage of disability. Values for the outcome measures of interest will be tabulated in an Excel® spreadsheet by each rater.

   The evaluations will be carried out during the study in three instances (T0-T1), including a later follow-up evaluation (T3): pretreatment (T0: baseline), end of treatment at 8 weeks (T1), and twelve weeks (T3: follow-up). The participants will be summoned twice a week to carry out their corresponding treatments. On the other hand, cervical disability will be evaluated using the Spanich version of Neck Disability Index (NDI), obtaining the percentage of disability for each of the evaluation instances. Moreover, the NDI will be applied as a self-report before and after the treatment sessions (T0 and T2).

   Once the study is over, the differences in cervical range of motion (ΔCFROM), pain pressure threshold (ΔPPT), pain intensity at rest (ΔRPI), pain intensity at movement (ΔMPI) and cervical disability (ΔNDI) between the evaluation sessions will be evaluated.

4. Variables

   4.1 Conceptual definition of the variables

   • Painful pressure threshold (PPT): the magnitude of pressure described as painful by the participant when performing the six-point algometry test in the cervical and shoulder regions: 2 cm lateral to the spinous processes of C2, C5, T4, and T8, midpoint of the upper portion of the trapezius muscle belly (between the spinous process of C7 and the acromion), and levator scapula muscle (2 cm superior to the superior angle of the scapula).
   • Pain intensity at rest (RPI): the magnitude of neck pain reported by participants when they are at rest.
   • Movement pain intensity (MPI): magnitude of pain reported by the participants when performing movements of the cervical spine in flexion, extension, inclination, and right and left rotation.
   • Cervical range of motion (CROM): range of motion of the head relative to the trunk for flexion, extension, right tilt, left tilt, right rotation, and left rotation movements.
   • Neck disability (ND): lack or limitation of the functionality of the cervical region due to pain that makes it impossible or difficult to carry out normal activities of daily living.
   • High Intensity Laser Therapy (HILT): laser application with powers greater than 500 milliwatts (mW) at the algometry assessment points and upper trapezius muscle belly.
   • Stretching exercise (SE): Bilateral passive static stretching for the upper trapezius, levator scapulae, and scalenes muscles.

   4.2 Operational definition of variables.

   • Painful Pressure Threshold (PPT): The painful pressure threshold (PPT) will be quantified by pressure algometry, recording the sum of the values in kg/cm2 of pressure for the six bilateral points established to perform the measurement. Three measurements will be made for each point with an interval of 30 seconds, recording the average of the three as the final value. For the evaluation, the Baseline® brand pressure algometer will be used. The PPT exam will be performed by placing the participant in a prone position. The intra-rater reliability for the PPT measurement will be determined with the intra-class coefficient by assessing the PPT at the upper trapezius muscle belly point in 13 healthy volunteers not involved in the study with a 48-h interval between assessments.
   • Pain intensity at rest (RPI): Pain intensity will be assessed using a visual analogue scale (VAS) (ICC = 0.97 (CI 95% 0.96, 0.98) [30]. The RPI The MPI exam will be carried out by the participant in a seated position, keeping the back straight and supported on a backrest.
   • Movement pain intensity (MPI): Pain intensity will be assessed through the visual analogue scale (VAS) for movements of flexion, extension, inclination, and cervical rotation. The MPI value will correspond to the sum of the VAS score for all cervical movements. The MPI exam will be carried out by the participant in a seated position, keeping the back straight and supported on a backrest.
   • Cervical range of motion (CROM): The range of cervical motion will be evaluated through an inclinometer, recording the degrees of movement for the movements of flexion, extension, right tilt, left tilt, right rotation, and left rotation. To measure the range of motion, the CROM device will be used (inter-evaluator reliability: Extension ICC = 0.98 (CI 95% 0.95, 0.99); Flexion ICC = 0.89 (CI 95% 0.73,0.96); Left rotation ICC = 0.95 (CI 95 % 0.87, 0.98); Rotation right ICC = 0.92 (CI 95% 0.80, 0.97); Tilt left ICC = 0.97 (CI 95% 0.91, 0.99); Tilt right ICC = 0.97 (CI 95% 0.93, 0.99). The CROM examination will be performed while seated with a straight back and supported by the chair's back.
   • Neck disability (ND): Neck disability will be assessed through the Neck Disability Index (NDI), an instrument consisting of 10 sections with questions related to symptoms and life activities that may be limited by neck pain and that has been validated for the Spanish language. Each section consists of questions scored from 0-5, with greater disability being associated with a higher score (NDI; inter-rater reliability: ICC = 0.93 (95% CI 0.86, 0.97).
   • High-Intensity Laser Therapy (HILT): High intensity laser therapy (HILT) will be applied with the punctual technique on the 6 bilateral points of the cervical region and shoulder girdle, to be followed by a sweep technique on both trapezius muscles. Dundar et al. propose the following parameters: a 3 watt average power, 60 joules per point (360 joules), and 500 joules for manual scanning (1000 joules). For the application of laser therapy, the 12 W BTL-6000 equipment that emits at 1064 nm wavelengths will be used. Laser therapy will be applied with the participant in the prone position.
   • Stretching exercise (SE): Three sets of bilateral passive static stretching will be performed for the upper trapezius, levator scapulae, and scalene muscles. Each series will last 30 seconds, followed by a rest interval of 30 seconds. The exercises will be carried out with the participant in a seated position in a chair with a backrest.

   4.3 Variable type definition

   • Painful Pressure Threshold (PPT): dependent, quantitative, continuous variable.
   • Pain intensity on movement (MPI): dependent, quantitative, discrete variable.
   • Cervical range of motion (CROM): dependent, quantitative, discrete variable.
   • Cervical disability (ND): dependent, quantitative, continuous variable.
   • High Intensity Laser Therapy (HILT): independent, quantitative, continuous variable.
   • Stretching exercise (EE): independent, quantitative, discrete variable.

5. Study phases Three phases have been designated for the investigation: Sampling phase, Evaluation phase and Intervention phase.

   The sampling phase will consist of applying the selection survey to potential participants. Participants will be chosen according to the selection criteria through a survey that will determine the presence of non-specific chronic neck pain (CNCP). The survey will be conducted online using a Microsoft Office® form. The selected participants will be divided into two study groups through a simple randomization process (research randomizer, https://www.randomizer.org/): group 1 (HILT + stretching exercise) and group 2 (sham HILT + stretching exercise). The randomization of the sample will be carried out by the study director, using the simple random sampling process through the research randomizer program, who will be the only one who knows the results. Participants will meet twice a week for four weeks to carry out the assigned treatment.

   The evaluation phase will consist of measuring the outcomes of interest. Pain pressure threshold (PPT) will be evaluated with pressure algometry at six established bilateral points, pain intensity at rest and on movement with VAS (RPI and MPI), cervical ROM with an inclinometer (CROM system), and the neck disability (ND) through the spanich version of Neck Disability Index. The evaluations of the interest outcome measures will be carried out in three instances; pretreatment (T0: baseline), the fourth week (T2: 8th session, end of treatment), and week 20 (T3: 12 weeks post-treatment or follow-up). The differences in pain pressure threshold (ΔPPT), pain intensity at rest (ΔRPI) and pain intensity on movement (ΔMPI) will be considered the main outcome measures. Alternatively, secondary outcome measures will include differences in cervical spine range (ΔCROM) and cervical disability (ΔND).

   The intervention phase will include eight treatment sessions held twice a week to carry out their corresponding treatments (4 weeks total). The sample will be treated in two study groups: the HILT group and the sham HILT group. All groups will receive, as a base treatment, a passive static cervical stretching therapeutic exercise plan focused on the upper trapezius, levator scapulae, and scalene muscles of three series for 30 seconds. The interventions will be carried out by two independent, trained therapists.The participants will be summoned twice a week to carry out their corresponding treatments.

6. Statistic analysis

The descriptive statistics for the primary variables ΔPPT, ΔRPI, ΔMPI ΔCROM, and ΔND will use as analysis measures, means and standard deviation (x, SD), or median and interquartile range (mean, IQR), depending on the distribution of the data (test of normality of Kolmogorov-Smirnov). This will make it possible to build the table with the demographic data of the participants by group (Table 1 of the work). According to the data normality analysis, frequencies and means or medians will be used for the secondary variables sex and body mass index (BMI).For inferential statistical analysis of the outcome measures of interest obtained between assessments, the Kolmogorov-Smirnov normality test will be used to determine their distribution. According to the results, parametric or non-parametric tests will be selected: the T-test for independent samples if the data is normally distributed or the U-Mann Whitney test if the variables are not normally distributed. For statistical analysis, the IBM SPSS (Statistical Package for the Social Sciences) v26 program will be used.

Inclusion Criteria:

• Participants must be at least 18 years old.
• Both sexes (men and women)
• Non-specific, chronic neck pain, defined as pain or discomfort in the cervical region between the superior nuchal line and the spinous process of T1 or the shoulder girdle, with the following criteria: neck pain in the last 3 months or more; a questionnaire score of cervical disability (NDI) equal to or greater than 5; and cervical pain of 3 or greater on the numerical pain scale (NPRS).

Exclusion Criteria:

• Neck or shoulder musculoskeletal injuries in the last 3 months (fractures, sprains, tendinopathies, dislocations, or muscle tears).
• Osteosynthesis materials close to the shoulders, neck, or surrounding areas.
• Wounds or skin changes in the shoulder and/or neck region (such as psoriasis, scars, or burns).
• Analgesic, anti-inflammatory, or muscle relaxant drugs for permanent use.
• Neurological alterations such as paresthesia, loss of sensation (partial or complete), decrease in strength, or color changes in the neck, arms, forearms, or hands.
• Diagnosed photosensitivity.
• Presence of solar urticaria or adverse reactions to sunlight.
• The presence of dermatomyositis, systemic lupus erythematosus, hepatic porphyria, cutaneous carcinoid syndrome, or pellagra.
• Cancer or tumors of some type have been diagnosed in the last 5 years.
• Epilepsy.