Nobody warns you how hard the weeks after a total knee replacement can be. The swelling, the stiffness, the nights you can't get comfortable, and physical therapy sessions that leave you sore for days.
Patients in knee replacement support groups describe it in their own words:
"Forcing pain causes swelling; swelling restricts movement and causes more pain." — Total knee replacement patient, 6 weeks post-op, public recovery forum
That cycle is exactly where red and near-infrared light therapy (the research term is photobiomodulation, or PBM) is being studied. Since 2021, several randomized controlled trials have tested it on people recovering from total knee replacement. Here is what they found.
It targets swelling, the problem behind so much of the pain
Swelling is the complaint that comes up again and again after knee replacement, and it is also where the research on light therapy is strongest. A 2026 scoping review in the Journal of Orthopaedics looked at every randomized trial of PBM after total knee replacement. Two of the trials measured swelling, and both found a significant reduction compared with patients who didn't get light therapy.
Total knee replacement patients received 650 nm red + 850 nm near-infrared light daily on post-op days 1–5.
Swelling, measured by bioimpedance, was significantly lower in the light-therapy group by day 6 (p < 0.001).
How much the thigh swelled by day 3 after knee replacement (80 patients). Lower is better.
22% less swelling with light therapy than with Celebrex (celecoxib), a prescription anti-inflammatory often given after knee surgery (p = 0.021). This trial used an 810 nm light source before surgery; it is shown here for its swelling result.
It works inside your cells, not just on the skin
Red and near-infrared light are absorbed by the mitochondria, the energy centers of your cells. Research reviews describe this absorption as boosting production of ATP, the fuel cells use to repair tissue, while reducing oxidative stress.
A 2022 review in Frontiers in Bioengineering and Biotechnology reports that this kind of light therapy lowers inflammatory markers linked to joint pain, including IL-1, IL-6 and TNF-α. That is the biological reason researchers started testing it on freshly operated knees.
Patients in the trials bent their knees further and walked farther
Getting the new knee to bend is the goal every surgeon and therapist is chasing. In a 2023 randomized trial (Alghamdi et al.), 44 knee replacement patients were split into two groups. The group that received 650 nm red light for 12 sessions over 6 weeks had significantly better knee flexion and significantly better WOMAC scores, a standard measure of pain, stiffness and daily function.
2-minute walk test result reported by post-op day 6
As reported in the 2026 Journal of Orthopaedics scoping review of knee replacement trials.
It's a drug-free way to support pain relief
Many knee replacement patients worry about how many pills they're taking. One patient on a recovery forum wrote:
"I am not a drug addict, but feel like I'm a suspected addict." — Total knee replacement patient, 3 weeks post-op
In the 2026 trial of 80 knee replacement patients, light therapy was compared directly with Celebrex (celecoxib), a prescription anti-inflammatory. Pain at rest 24 hours after surgery was statistically no worse with light therapy than with the drug (3.9 vs 3.7 on a 10-point scale). The light group also had fewer stomach problems:
Light therapy is not a replacement for the pain plan your surgeon gives you. It is one more tool that doesn't add to your pill count.
Use the same red + near-infrared wavelengths at home. RedRelief™ is wireless, so you can use it while you ice and elevate.
See RedRelief™ →It's gentle. Nothing is forced, stretched or pushed
For many people, physical therapy is the hardest part of recovery. Patients describe sessions like this:
"I have screamed and often cried as the PT pushed me to pull the strap back harder to increase the flex." — Total knee replacement patient, 10 weeks post-op
Light therapy asks nothing of your knee. You strap it on and rest. In a 2019 BMJ Open analysis of 22 randomized trials of light therapy for knee pain, no adverse events were documented. And while you still need to follow your rehab plan, calmer swelling can make the work you do in PT easier to recover from.
The wavelengths match what the trials used
Not all red lights are equal. Research reviews describe a therapeutic "optical window" of roughly 600–1100 nm, with near-infrared light reaching deeper into tissue than visible red. The knee replacement trials above used 650 nm red and 850 nm near-infrared. RedRelief™ uses 660 nm and 850 nm.
Drawn to scale, 600–900 nm. Near-infrared light is invisible, so the 850 nm LEDs won't look as bright as the red ones. They are still working.
It can help your other knee too
Many people who have one knee replaced have arthritis in the other. As one patient posted: "Getting the left one done next week... I hope I have better results!!"
Knee arthritis is one of the most-studied uses of light therapy. The 2019 BMJ Open meta-analysis pooled 22 randomized, placebo-controlled trials with 1,063 patients:
RedRelief™ fits both the left and right knee, so the same brace works on either side.
What the research does and doesn't say
- Swelling: the most consistent finding. Every knee replacement trial that measured it found less swelling with light therapy.
- Knee bend and function: positive in the trials that measured them, though not every trial showed a difference in bend.
- Pain: results are mixed. One trial found significantly less pain; another found no difference from the control group.
- The trials are small (30–80 patients each) and used clinic devices. Researchers call the results promising and are asking for larger studies.
