Does the light reach the cartilage?

A light therapy result is really two claims stacked: that light does something to a chondrocyte, and that enough light reaches the chondrocyte to do it. The cell experiments settle the first. The second is where this literature is thinnest, and it is the question that decides whether a device bought online is doing anything at all.

Three things govern it. Wavelength sets what absorbs the light and how far it travels through tissue. Energy density sets whether the dose falls inside the window that produces an effect — a window with a floor and, awkwardly, a ceiling. And the skin, fat and capsule between the emitter and the cartilage take their share before anything arrives.

What absorbs the light

The classical account puts the receiver at cytochrome c oxidase, the fourth complex of the mitochondrial respiratory chain. Light displaces nitric oxide bound there, the enzyme activates, and ATP, reactive oxygen species and calcium rise together.

That absorption is not flat across the spectrum. It falls off beyond roughly 980 nm, which is why the wavelengths in use cluster below that. Above about 900 nm a second route is proposed, through light-gated TRP channels and reactive oxygen species rather than through the respiratory chain, and water absorption begins to rise as wavelength climbs toward 1050 nm — the light is increasingly taken by the tissue's own water before it reaches anything that will act on it.

One distinction organises the rest of this entry. Low-level laser and LED produce no thermal sensation during treatment: the effect is photochemical. High-intensity laser deliberately induces surface hyperthermia and works partly through heat. They are not the same intervention delivered at different strengths, which is why comparing them by joules alone is misleading.

Wavelength

The only clean head-to-head in this corpus tested four wavelengths against the same cells and the same endpoints. At 940 nm, the degrading enzymes went down and the matrix genes went up. At 810 nm, only the degrading enzymes moved. At 625 nm — visible red, the colour most home panels emit most of — and at 1050 nm, nothing moved at all.

The human-chondrocyte work points the same way from a different direction: across 600 to 940 nm, it was 850 and 940 nm that raised metabolic activity, and 940 nm that raised the matrix proteins.

The clinical bands sit partly elsewhere. Low-level laser trials run 633 to 905 nm; high-intensity laser is usually 1,064 nm. So the wavelength with the best tissue evidence behind it is not the wavelength with the best clinical evidence behind it, and neither is the wavelength most commonly sold.

Energy density and the biphasic response

More is not better, and this is the part most easily got wrong.

In the four-wavelength experiment the tested range ran from 13 to 78 joules per square centimetre, and the effective setting sat in the middle at 52. Below it the effect was weaker; above it, weaker again. In the human-chondrocyte work the effective energy densities were far lower — up to about 3 joules per square centimetre — with a dose-dependent response whose shape changed depending on the other parameters.

Those two numbers differ by more than an order of magnitude, in the same tissue, for the same wavelength. That is not a contradiction so much as a warning: energy density interacts with power density, pulsing and schedule, and a figure lifted out of one protocol does not transfer into another.

The parameter-first review of the field collects the ranges actually in use — red lasers at 600 to 660 nm and up to 10 J/cm², GaAlAs lasers at 800 to 880 nm and 10 to 50, Nd:YAG at 1,064 nm and up to 200 — and reports efficacy depending on both the parameters and what is being irradiated. Its tables mix cell, animal and clinical rows together, so a range read out of them is not a human dose.

The WALT dose guidance

The World Association for Laser Therapy publishes a minimum: 4 joules per point, give or take half, at 5 to 500 milliwatts, for 20 to 300 seconds, at 780 to 860 nm, for GaAlAs lasers.

More than a quarter of the thirty-three cell and animal studies in the systematic review came in under it — some as low as 0.3 and 1.4 joules per point. And those studies are disproportionately the ones that found nothing: no effect on matrix synthesis, no suppression of the degrading enzymes, no change in inflammation markers, no histological improvement.

There is a second pattern underneath. Most of the low-dose studies that did eventually find something needed more than twenty sessions to get there. Dose and session count appear to trade against each other, so an underpowered device may be a slower one rather than an inert one — which is a hypothesis this literature suggests and has not tested.

The guidance has a hard limit of its own: it was validated for two laser families and its values may not apply to helium-neon, InGaAlP, or LEDs. The human-chondrocyte authors observe that their most effective settings happen to align with it, then note that it covers laser diodes only and argue for extending it to LEDs — an argument for a standard rather than a report of one.

How deep the light goes

Low-level laser concentrates its radiation and puts out up to 500 milliwatts, penetrating the superficial tissue layer to a maximum of two centimetres, with no thermal sensation. High-intensity laser scatters its radiation, exceeds 500 milliwatts, penetrates further and heats the surface as it goes. The review states a depth figure for the high-intensity class whose unit is lost in the published text, so none is quoted here.

The most useful passage in this literature reads two null trials through this lens. In both, a cluster device at 808 nm delivered 56 joules to the knee — a respectable total. But it was spread across many diodes of 100 milliwatts each, so peak power and intensity at depth were low. A trial that did find an additive effect used a 904 nm super-pulsed laser delivering only 3 joules per point: longer wavelength, better penetration, and high peak power in short bursts that reached the inside of the joint. Fifty-six joules that never arrive do less than three that do.

That is the clearest evidence in the corpus that total energy is the wrong number to compare devices by.

Carrying a dose from a dish to a joint

The mouse paper is unusually candid about this, and its caveat applies with more force to a human.

Its energy density came from the group's own cell experiments, and the authors state that only part of that energy could reach the joint cavity in an animal. They add that the cell and animal exposure periods differ, that repeated irradiation of an animal knee may have cumulative effects, and that the optimal irradiation period and energy setting in a live animal are therefore unresolved. A mouse knee sits a millimetre or two below the skin. A human knee does not.

The narrative review of the basic literature reaches the same wall from the other side: individual variation in tissue optical properties and energetic decay would have to be worked out through systematic experiment before dose norms could be set for different presentations of the disease.

What a home LED panel delivers

Worth stating as a set of facts about regulation and hardware rather than as a verdict about panels.

An LED converts current into incoherent narrow-spectrum light. It is cheap, non-invasive, has few contraindications and rarely causes adverse effects. In the United States, LED therapeutics are class II devices, cleared on the basis of similarity to devices already on the market rather than approved on evidence of efficacy and safety. Medical lasers are class III or IV: expensive, restricted in use, and hazardous if mishandled. The review names this asymmetry directly as the reason LED has greater commercial appeal.

Set against that, the energy figures. Low-level laser trials deliver 5.76 to 27 joules per session; high-intensity laser delivers 79.2 to 3,000. The four LED trials span 23.5 to 1,402, a range so wide it describes a category rather than a protocol.

So the class easiest to own is the one cleared without an efficacy requirement, tested in four trials, and delivering a dose that varies sixty-fold between studies. The preclinical work showing matrix synthesis is mostly LED; the human work showing the most consistent pain relief is laser. Both of those are true at once.

The dose at the joint surface

Nobody has measured it.

Not in this corpus, and not in the reviews that survey it. The penetration figures come from device physics and tissue characteristics rather than from an instrument at an articular surface. The one paper that raises the question directly raises it about a mouse. Every dose in every trial above is a dose at the emitter, and the fraction arriving where the chondrocytes are is inferred.

That gap is the honest centre of this entry, and it is why a wavelength and a joule count on a product page cannot be checked against a trial even when both numbers are accurate.

What a dosimetry study would settle

One measurement would change more here than another trial: transmitted irradiance through human peri-articular tissue, at the wavelengths in clinical use, in knees of differing size and composition. It would turn every published dose into a dose at the target, retrospectively, and make the null trials interpretable.

The second is a trial that varies one parameter and fixes everything else — the study design this field has never run, and the reason it argues about parameters instead of concluding about them. Extending the dose guidance to LEDs and to the laser families it currently omits would follow from both.