Inclusion Criteria:
- Clinically confirmed knee osteoarthritis (OA) based on the American College of Rheumatology (ACR) criteria with symptomatic pain for ≥ 3 months
- Average knee-pain intensity ≥ 30/100 on both the Daily Symptom Diary (DSD) run-in and CATI
- Age ≥ 50 years
- Ability to give informed consent, attend visits, and complete ≥ 4 of 7 DSD entries pre-randomization
- Capacity to perform all study tasks (Timed Up-and-Go, knee OA assessments, blood draw, questionnaires, etc)
Exclusion Criteria:
Systemic and Rheumatic Disease
- Active systemic rheumatic condition (e.g., rheumatoid arthritis, systemic lupus erythematosus)
- Fibromyalgia with pain outside the knee that is equal to or worse than knee pain
Musculoskeletal History
- Clinically significant surgery on the index knee (e.g., arthroplasty, osteotomy)
- Knee surgery within the past 6 weeks
- Intra-articular injection to the index knee within the past 14 days
Medication Use
- Daily opioid therapy
- Hypersensitivity to PBM
- Migraine triggered by light
- Open wounds, skin infections, eczema, burns, or other dermatologic conditions at or near the treatment application site.
- New prescription or OTC analgesic, non-pharmacologic pain therapy, chemotherapy, or radiation therapy initiated ≤ 30 days before screening
Cardiovascular Safety
- Uncontrolled hypertension (SBP > 150 mmHg or DBP > 95 mmHg)
- Unstable or activity-limiting cardiovascular or peripheral arterial disease
Neurological and Psychiatric Conditions
- Diagnosed neurological disorder (e.g., Parkinson's disease, multiple sclerosis, epilepsy)
- History or evidence of stroke or moderate-severe traumatic brain injury
- Serious psychiatric illness requiring hospitalization within the past 12 months
- Active suicidal ideation
- Current substance-use disorder or past hospitalization for substance-use treatment
Dermatologic Conditions
- Compromised skin integrity at or near the treatment site (e.g., open wounds, active rash, dermatitis, burns, ulcers, eczema, infection) interfering with topical treatment or PBM
Other Metabolic Conditions
- Uncontrolled diabetes
- Diabetes-related complications (e.g., peripheral neuropathy, severe nephropathy, chronic ulcers at treatment site)
Other Pain Conditions
- Chronic pain at another body site more severe than knee pain
General Health and Cognition
- Pregnancy or lactation
- Cognitive impairment that precludes informed consent or task participation