In theory, stem cell therapy works by injecting cells that release signals to reduce inflammation and support the cells already in the area. Doctors deliver the cells into a joint or the bloodstream, where they act mainly as tiny signal factories. What the marketing often leaves out is that there’s still little evidence these effects lead to lasting benefits.
This guide breaks down the biology in plain language while also addressing the gaps in the evidence. It does not claim that stem cell therapy repairs, regrows, or cures anything. Instead, it explains what researchers currently believe, and what clinical trials have and haven’t shown.
The basic idea behind the treatment
The core idea is that stem cells can influence the tissue around them once placed at the injury site. A clinician usually draws the cells from the patient’s own bone marrow or fat. These adult mesenchymal stem cells are the type most joint and pain clinics use, though some clinics buy donor or lab-processed cells instead. The sample is concentrated, then injected into the joint, spine, or bloodstream, often guided by ultrasound or fluoroscopy to place the needle accurately. The hope is that the cells help the local environment heal.
Two versions of this idea exist, and they are very different in strength of proof.
- In blood cancer transplants, donor stem cells rebuild the patient’s entire blood and immune system. This is proven and FDA approved.
- In joint and pain injections, cells are placed in a knee or back in the hope of local repair. This is not approved and not well proven.
Most people searching this question mean the second version. So the rest of this page focuses there.
The paracrine theory, in plain terms
The leading explanation today is the paracrine effect, which means the cells work by signaling, not by becoming new tissue. Instead of turning into cartilage, the injected cells release molecules that talk to nearby cells.
Those signals may reduce inflammation, dampen an overactive immune response, and encourage local blood supply. Think of the cells less as bricks and more as a crew that shouts instructions, then leaves.
This shift matters. For years the pitch was that stem cells “become” new cartilage. Research from the National Institutes of Health and others now points more toward short-lived signaling. That is a weaker, more temporary mechanism than direct regrowth. The same signaling-only idea is why some clinics now sell exosome therapy instead of whole cells, marketing just the signals without the cells that produce them.
What happens to the injected cells
Most injected cells do not stay or survive for long. Studies tracking labeled cells find that many die or clear within days to weeks. Few, if any, become permanent new joint tissue.
That single fact undercuts the “regrow your knee” promise. If the cells leave quickly, any benefit must come from what they did while briefly present, not from new tissue they built. It says nothing about the separate question of safety and side effects from the injection procedure itself.
Why a good story is not proof
A plausible mechanism does not mean the treatment works. Medicine is full of ideas that made biological sense and still failed in controlled trials. The paracrine story is reasonable, but reasonable is not confirmed.
To know if a treatment works, researchers compare it against a placebo in a blinded trial. For stem cell joint injections, those trials are still small, short, and mixed. A Cochrane review pooled randomized trials that tested stem cell injections against a placebo or another treatment for knee osteoarthritis pain and function. It rated the certainty of that evidence as low to very low. The trials it found were mostly small, short in follow-up, and at risk of bias. That leaves it unclear whether the injections meaningfully help beyond a placebo, or how safe repeated injections are over the long term.
The most common gap on these pages is exactly this: a page explains a mechanism in detail, then quietly implies the mechanism is proven. Those are two separate claims, and only the first is settled here. Our success rate guide covers what a clinic’s advertised percentage leaves out.
Mechanism vs proof: how they compare
| Question | What we can say | Confidence |
|---|---|---|
| Can stem cells release anti-inflammatory signals? | Yes, shown in lab studies | Reasonable |
| Do most injected cells become new cartilage? | No, most clear within days | Reasonable |
| Does the signaling reduce joint pain in patients? | Sometimes, short term, in mixed studies | Weak |
| Does it change the joint’s long-term structure? | Not clearly shown | Very weak |
| Is any joint injection FDA approved? | No | Settled |
The pattern is clear. The mechanism has some support. The patient benefit, especially lasting benefit, does not yet.
Is it just an anti-inflammatory effect?
Some researchers suspect much of any short-term relief is simply anti-inflammatory. If the cells mainly calm inflammation for a while, the effect could resemble a cortisone shot in direction, though through different molecules. That mechanism also differs from platelet-rich plasma, which relies on a patient’s own blood platelets rather than injected cells.
That would explain a common pattern: real pain relief for a few months, then a slow return. It would also mean the treatment is not repairing the joint, only quieting it temporarily. Our how long does it last guide covers that timeline.
Why it may work for blood cancer but not your knee
The same term hides two very different mechanisms, and only one is proven. This is the most common point of confusion in stem cell marketing.
In a blood cancer transplant, donor stem cells rebuild the patient’s whole blood and immune system after chemotherapy destroys the old one. The cells engraft, multiply, and take over a job permanently. That is a proven, FDA-approved process with decades of data.
A knee injection asks the cells to do something else entirely: survive in a hostile joint, and either rebuild cartilage or signal repair. They mostly do neither for long. So “stem cells cure leukemia” is true, and “stem cells will fix your knee” is not proven, even though both use stem cells. The mechanism, and the evidence, do not transfer.
What this means before you pay
Understanding how stem cell therapy works should change how you read a sales pitch. It also shapes whether you are a reasonable candidate for stem cell therapy in the first place. If a clinic says the cells will regrow your cartilage, that goes beyond what the mechanism supports. If it says the cells may reduce inflammation for a while, that is closer to the current thinking, and still unproven for lasting benefit.
Ask the clinic which mechanism they are claiming and what published trial supports it for your exact condition. Physical therapy treats many of the same joint and pain conditions. It rests on a far larger base of controlled trials. It is also a standard recommendation from the National Institute of Arthritis and Musculoskeletal and Skin Diseases for conditions like knee osteoarthritis. Then weigh the cost against that cheaper, proven option before committing to an unproven injection.
The bottom line
How does stem cell therapy work? The best current answer is that injected cells likely signal to nearby tissue and calm inflammation for a short time, rather than rebuilding a joint. That idea is reasonable but not proven to help patients over the long run for joints or pain. Treat the biology as a hypothesis under test, not a finished result, and price your safer alternatives before you commit.
Frequently asked questions
How does stem cell therapy work in the body?
The leading theory is that injected mesenchymal stem cells release signaling molecules that calm inflammation and support nearby cells, rather than growing new tissue themselves. This is called the paracrine effect. Whether that produces lasting improvement for joints or pain is still not proven in strong trials.
Do the injected cells turn into new cartilage or tissue?
Probably not in the way clinics imply. Most injected cells do not survive long or become new cartilage. Research now focuses on their short-term signaling role, not direct tissue regrowth. Marketing that promises to 'regrow' a joint runs ahead of the evidence.
Why does the evidence gap matter if the biology sounds reasonable?
A plausible mechanism is not proof of benefit. Many treatments that made biological sense failed in controlled trials. For most joint and pain uses, studies are small, short, and mixed, so the reasonable-sounding theory has not been confirmed in patients.
Is the effect just anti-inflammatory, like a cortisone shot?
Possibly, in part. Some researchers think much of any short-term relief comes from anti-inflammatory signals, similar in direction to a steroid injection but through different molecules. That would explain temporary pain improvement without lasting structural change.
Does it work the same for every condition?
No. The proposed mechanism, and the evidence, differ by use. Blood-forming stem cell transplants for leukemia work by rebuilding the blood system, a proven and very different process from injecting cells into a knee.
Medical disclaimer. Medical Frontier is an independent educational resource. This page is for general information only and is not medical advice, diagnosis, or treatment, and does not create a doctor–patient relationship. Most regenerative therapies discussed here are not FDA-approved for the uses described and may be offered under limited exemptions or in clinical trials only. Always consult a licensed physician before making any treatment decision.