This study was a prospective, randomized, sham-controlled, home-based trial, registered with the Clinical Research Information Service, which is under the authority of the Korea Disease Control and Prevention Agency (Registration number: KCT0008106). The Institutional Review Board approved the study (IRB No. 2020-07-061).

Both groups were self-treated at home for a total of 15 sessions: 20 min per day, 5 days per week, over the course of 3 weeks. The experiment group was treated with the medical laser irradiator while the control group received sham stimulation that mimicked the irradiator’s shape and color. The clinical research coordinator instructed the patients to keep a daily treatment diary. Participants were permitted to take medication they had started at least a month before screening. They were not allowed to change the dose or type of medication or start any other types of treatments for LBP during the trial. Patients were assessed twice: on the day before treatment began (pre-treatment) and again within four days of the end of treatment (post-treatment). Each patient completed a daily self-therapy checklist to track their treatment and record any adverse effects.

Study participants were randomized into either the experimental group (treated with the Careray™) or the control group. Simple randomization was done using the random number table found at Randomization.com. The seed values used for randomization were also recorded. Randomization was completed before the first participant was screened, and the results were generated by a researcher who was not involved in the study enrollment or intervention. The assessors collecting study data were unaware of the group assignments throughout the trial.

This study was conducted at OO University Hospital’s rehabilitation center between October 2021 and October 2022. Study participants were recruited through a notice posted on a bulletin board within the hospital and screened by a rehabilitation physician. Patients were selected based on the following inclusion criteria: (1) adult men and women aged 19 to 70 years, (2) individuals experiencing chronic pain in the lumbar–sacral region for 12 weeks or more, (3) persons possessing sufficient cognitive abilities to accurately describe the location and intensity of their pain, and (4) individuals who fully understood the research’s purpose and procedures and wished to participate voluntarily. The exclusion criteria were (1) individuals who had received lumbar injections or undergone spinal surgery within the last month, (2) people with back pain due to infectious spondylitis, fractures, or tumors, (3) individuals with a history of sensitivity to lasers, (4) individuals with mental disorder, (5) Individuals who are pregnant or planning to become pregnant, (6) Others deemed unsuitable for participation in the clinical study by the researcher for various reasons.

Each participant who fulfilled the inclusion criteria was randomly assigned to one of the two groups by a clinical research coordinator (a clinical research nurse not involved in the assessments). The assignments were concealed from the investigators. Participants were shown the treatment procedure and warned of potential side effects. Written informed consent was obtained from all participants before randomization. All research was conducted in accordance with the ethical standards outlined in the Declaration of Helsinki [15].

The Careray™ (WellsCare Co., Seoul, Korea) used in this study is a lightweight, wearable, and flexible LLLT device (415 g) designed for home therapy. The device incorporates multi-wavelength vertical-cavity surface-emitting laser diodes (670, 780, 830, and 910 nm), with a total treatment area of 92.7 cm². During a 20-minute session, the device delivered an average power density of 3.83 mW/cm² and an average energy density of 4.59 J/cm², with pulsed emission at 625 Hz and an estimated penetration depth of 2–5 mm. The device was attached to the lower back area where each patient reported pain using an elastic band (Fig. 1). Detailed technical specifications, including diode configuration, spot size, and per-diode energy calculations, are provided in the Supplementary Material (Table S1).

For the sham therapy, an identical-looking device with the same configuration was used, except that it contained low-power LEDs (650 nm, 0.02 mW per diode) instead of lasers. This ensured visual mimicry while preventing therapeutic laser energy delivery.

All measurements were captured by researchers who were blinded to the treatment allocation and otherwise uninvolved in the intervention.

The primary outcome was assessed using a visual analogue scale (VAS) for LBP. Each patient was instructed to mark their pain on a 10-cm-long scale according to severity. A score of 0 indicated no pain while 10 indicated extreme pain [16].

Secondary outcomes were measured using the Oswestry Disability Index-Korean test (ODI-K), Schober’s test, and a 36-item Short Form 36 (SF-36).

The ODI-K is a valid and reliable instrument used to assess the degree of disability in LBP patients. It comprises 10 items, namely, pain, sleeping, walking, carrying items, personal care, standing, sitting, social and sexual activities, and traveling. Each item is scaled from 0 to 5, so total scores range from 0 to 50, with higher values representing greater disability [17, 18].

In Schober’s test, the lumbosacral junction (as indicated by the dimples of Venus) was marked while the participant stood erect, and a second mark was placed 10 cm above the junction. The participant was then asked to bend forward as far as possible as though to touch the toes, and the new distance between the marks was measured [19].

The participants’ health-related quality of life was assessed through the Medical Outcomes Study 36-Item Short Form Health Survey (SF-36). The 8 components (physical functioning, physical role functioning, bodily pain, general health perceptions, vitality, social role functioning, emotional role functioning, and mental health) were aggregated into a physical component summary and a mental component summary. Scores on each subscale ranged from 0 to 100, with higher scores indicating better health. A difference of more than 2 points was considered clinically meaningful for both the physical and mental subscales [20].

In a previous study conducted by Djavid et al., the efficacy of LLLT combined with exercise was compared to sham laser therapy plus exercise on Visual Analogue Scale (VAS) scores after 12 weeks. In their study, the difference in VAS scores between the experimental group and the control group was calculated with a mean of 1.8 and a standard deviation of 1.503 when assessing the change in VAS scores between baseline and 12 weeks post-treatment. Therefore, based on an effect size of 1.8, a standard deviation of 1.503, a significance level of 0.5, a power of 0.8, and a dropout rate of 20%, the required minimum sample size for this study was calculated to be 14 participants per group, resulting in a total of 28 participants. Ultimately, we enrolled 30 patients.

Statistical analysis was performed using SPSS 23.0 software for Windows (SSPS Inc., Chicago, IL, USA). Data were presented as mean (SD) for continuous variables and frequency for categorical variables. In the analysis of baseline demographics, we conducted the Pearson chi-squared test for categorical variables, the independent t-test for numerical variables assuming normality, and the Mann-Whitney U test for numerical variables when normality assumptions were not met. Within-group comparisons were assessed by paired t-test when the assumption of normality was satisfied and the Wilcoxon signed-rank test was used otherwise. For comparisons between groups, the independent t-test was used when the assumption of normality was satisfied and the Mann–Whitney U-test was used otherwise. Effect sizes (Cohen’s d) were calculated for between-group comparisons of primary and secondary outcomes to assess clinical relevance. A post hoc power analysis based on the observed effect size in VAS was also conducted using G*Power 3.1.9.7. A p-value ≤ 0.05 was considered statistically significant.