What light does to cartilage

The clinical trials of light therapy measure what a patient reports. This literature measures the tissue: how many chondrocytes there are, what they are building, which degrading enzymes they switch on, and what a joint looks like under a microscope after six weeks of treatment.

It is a more satisfying literature to read and a weaker one to act on, because all of it is cells and animals. The results below are consistent enough to be interesting and small enough to be fragile, and the gap between a culture dish and a human knee is the subject of a separate entry.

The chondrocyte experiments

Human chondrocytes in monolayer culture were stimulated with LEDs across 600 to 940 nm, at low power densities of 4 to 17 milliwatts per square centimetre, continuous or pulsed, for up to four minutes, daily or every other day.

The near-infrared end did something the rest of the range did not. At 850 and 940 nm, metabolic activity rose significantly across the whole power range, regardless of whether the light was pulsed and regardless of whether it came daily or every other day. The response was dose-dependent, but the shape of that dependence varied by parameter.

Pushed to seven days of daily stimulation, 940 nm at the top of the power range consistently increased three things at once: collagen type II, aggrecan, and glycosaminoglycan deposition in the matrix. It also held the cells' phenotype. Chondrocytes in monolayer culture normally dedifferentiate as they are passaged — they stop behaving like cartilage cells — and the light delayed that.

Temperature was monitored throughout and rose by less than a degree. Whatever is happening is not the cells being warmed.

The mouse knee

The same wavelength, tested in a joint. Chondrocytes stimulated with TNF-alpha were irradiated at four wavelengths — 625, 810, 940 and 1050 nm — across a range of energy densities, and then the winning settings were carried into live animals.

In the dish, 940 nm at 52 joules per square centimetre suppressed the degrading enzymes mmp3 and mmp13 and raised the matrix genes col2a1 and aggrecan. At 810 nm the effect was half of that: the catabolic markers came down, the anabolic ones did not go up. At 625 nm and 1050 nm nothing moved.

In mice whose knees had been destabilised by cutting the medial meniscus — five to six animals per group, treated three times a week for six weeks — 940 nm improved weight-bearing asymmetry by 31 per cent against untreated controls and reduced cartilage degradation by half on the OARSI histological score. Immunofluorescence showed less mmp3 and more collagen II in the tissue.

Two endpoints moved together there: the animals used the leg more, and the cartilage looked better. That pairing is what makes this the most interesting single experiment in the corpus, and five to six mice per group is what keeps it from being more than that. The sham animals were wrapped in aluminium foil while the treated ones sat under a lamp in ambient light, which is a shielded control rather than a light-matched one.

What the thirty-three-study review found

Three in vitro studies and thirty animal studies, searched across four databases to August 2020 — the only systematic attempt to take this literature as a whole.

Light therapy reduced extracellular matrix degradation, reduced inflammation, slowed osteoarthritis progression and promoted matrix synthesis. It improved pain-like behaviour in animals, with no apparent effect on gait. Findings conflicted on some of the biochemical, histological and behavioural outcomes, which the review reports rather than smooths over.

The combination findings are the part worth carrying forward. Adding exercise to light, or topical anti-inflammatories, gave nothing over light alone. Adding intra-articular stem cells, or chondroitin with glucosamine sulfate, enhanced the effect on cartilage. In the human trials it is the exercise combination that tends to help, so the species disagree about which partner matters.

Performance and detection bias were judged high risk for every included study. Inconsistent outcome reporting meant nothing could be pooled.

The signalling inside the chondrocyte

The mechanism usually given runs through cytochrome c oxidase in the mitochondrial respiratory chain: light displaces nitric oxide, the enzyme activates, and ATP, reactive oxygen species and calcium rise, with downstream effects on NF-κB and the MAP kinases. A review from a biomedical-optics group frames the whole field this way — a photobiological response, the cell's function regulated by absorbed light, as distinct from the tissue being heated.

A narrative review of the basic literature assembles what light appears to do in osteoarthritis: inhibit pain, suppress inflammatory cytokine production, promote anti-inflammatory cytokines and matrix production, and suppress the enzymes that degrade matrix. It then states the limit plainly. Only a few studies have looked at intracellular signalling in chondrocytes in any detail, and the pathways established elsewhere — MAPK/ERK, JNK/AP-1, Akt — have not been shown to operate in chondrocytes specifically. The mechanism is inferred across cell types more than it is demonstrated in this one.

Rheumatoid arthritis and the inflammatory route

One experiment in this group is not about osteoarthritis at all, and it is worth reading for what it shows about the pathway.

Synoviocytes taken from rheumatoid arthritis patients were irradiated with a 610 nm LED. The light reduced the TNF-alpha-driven increase in proliferation, migration and invasion, and switched down both NF-κB activation and the NLRP3 inflammasome. In mice with collagen-induced arthritis, daily treatment of the ankle inhibited the disease: less synovial inflammation, less cartilage degradation, lower pro-inflammatory and higher anti-inflammatory cytokines in serum. Light combined with methotrexate outperformed either alone.

Collagen-induced arthritis is an autoimmune model. The cartilage there is being destroyed by systemic immune activity, not by mechanical damage, so a result in it speaks to the anti-inflammatory pathway and not to a cartilage defect. The wavelength is also different — 610 nm is visible red, shorter than the near-infrared band the cartilage work uses — and it was delivered to a mouse ankle through skin, a depth no human knee reproduces.

Light with scaffolds and cell therapy

A parameter-first review collects what happens when light is combined with cartilage tissue engineering rather than used alone. Across the studies it gathers, light reduced inflammation in cartilage defects while promoting cell proliferation, migration and chondrogenic differentiation, and adding it to scaffold-based repair improved outcomes over scaffolds alone.

That is a different clinical proposition from a panel on a knee — it belongs to the surgical repair pathway, where the light would be applied to engineered tissue rather than through skin. The review has no stated search protocol and no risk-of-bias assessment, and its tables mix cell, animal and clinical rows.

What the osteoarthritis models are

Worth knowing what "an osteoarthritis model" means before weighing any of this.

Destabilising the medial meniscus, or cutting the anterior cruciate ligament, produces mechanical instability that degrades a joint over weeks — closest to post-traumatic osteoarthritis in a person. Collagen-induced arthritis is autoimmune and is a model of a different disease. Monolayer culture is neither: it is chondrocytes on plastic, provoked with an inflammatory cytokine.

A substantial share of the basic literature also concerns the temporomandibular joint rather than the knee, where the reviewing authors say anatomy makes the optimal irradiation conditions likely to differ. And the animal studies collected run eight weeks or less, which is why none of them can speak to whether structural progression is altered rather than delayed.

Open questions

Three things would move this literature forward, and they are all specific.

Whether the 940 nm result replicates — a second laboratory, a second species, more animals per group, and a sham that emits light the animals can sit under. Whether a signalling pathway can be named inside a chondrocyte rather than borrowed from another cell type. And what dose would have to arrive at a human articular surface to reproduce any of it, which is a question about tissue and optics rather than about biology, and is where this section's genuine unknown sits.