Stem cells can help repair damaged knee cartilage, but “regrow” overstates what current evidence supports. No definitive human study has shown that stem cell injections regrow cartilage to its original state. What the research does show is meaningful pain relief and improved function, with a roughly 60% to 70% chance of achieving at least 50% improvement in pain and function for six to 12 months after injection, according to Mayo Clinic data.
What Stem Cells Actually Do in the Knee
There are two competing theories for how stem cells work once injected into a damaged knee. The first, called differentiation theory, proposes that the stem cells physically transform into cartilage cells (chondrocytes) that replace damaged tissue. The second, called paracrine theory, suggests the stem cells release chemical signals that reduce inflammation, recruit the body’s own repair cells, and help existing cartilage cells maintain their function.
Both processes likely contribute. When stem cells begin transforming into cartilage cells, they produce a scaffold of proteins including collagen and hyaluronic acid, essentially building the structural framework that cartilage needs. A master regulatory gene called SOX-9 acts as the switch that drives this transformation, while growth factors guide the process through multiple stages. But the signaling effects may be just as important: the stem cells calm the inflammatory environment inside an arthritic knee, which on its own can slow further cartilage breakdown and reduce pain.
This dual mechanism helps explain why patients often experience significant symptom relief even when MRI scans don’t show dramatic cartilage regrowth. The knee feels better because inflammation drops and existing cartilage is better maintained, not necessarily because thick new cartilage has filled in the gaps.
Where the Stem Cells Come From
The two most common sources are bone marrow and fat tissue. Bone marrow aspirate concentrate (BMAC) has the longer track record in clinical practice. It’s harvested from the back of your pelvis using a needle, then concentrated and injected into the knee. Microfragmented adipose tissue (MFAT), taken from belly or thigh fat through a minor liposuction-like procedure, is a newer alternative that some researchers believe may work even better.
Fat tissue has some theoretical advantages: it’s abundant, easier to harvest with less discomfort, contains a high concentration of stem cells, and may respond more effectively to inflammatory environments. Preclinical studies support this. However, no randomized controlled trial has directly compared the two approaches head to head, so there’s no definitive answer yet on which source produces better outcomes for knee cartilage repair.
Who Gets the Best Results
Arthritis severity plays a major role in how well stem cell therapy works. Doctors grade knee osteoarthritis on a scale from 1 to 4, where 1 is minimal and 4 is severe with bone-on-bone contact. Research consistently shows that patients with moderate arthritis (grades 2 and 3) respond more favorably than those with severe disease. At moderate stages, there’s still enough intact cartilage and a healthy enough environment inside the joint for regenerative processes to gain traction.
In grade 4 disease, advanced cartilage loss, hardened bone surfaces, and joint misalignment limit the potential for tissue repair. The treatment can still reduce pain, but the effect tends to be smaller and shorter-lived. Patients with inflammatory conditions like rheumatoid arthritis, those who’ve had recent knee surgery, or those who’ve received corticosteroid injections close to treatment are typically excluded from candidacy as well.
How Long Results Last
One of the more encouraging long-term studies followed 23 patients who received stem cells from bone marrow embedded in a hyaluronic acid scaffold to treat full-thickness cartilage injuries. At an average of eight years after treatment (ranging from six to ten years), patients maintained significant improvements across all measured outcomes. Pain scores dropped to near zero. Scores measuring daily function reached 99 out of 100, and quality-of-life scores held at 85 out of 100.
This study focused on cartilage injuries rather than widespread osteoarthritis, which is an important distinction. A contained hole in otherwise healthy cartilage is easier to repair than the diffuse breakdown of osteoarthritis. Still, the durability of results in this group is notable and suggests the repair tissue holds up over time in the right patients.
What the Procedure and Recovery Look Like
For bone marrow-derived therapy, the procedure typically happens in a single visit. Bone marrow is drawn from the back of your pelvis, processed to concentrate the stem cells, and injected into the knee using imaging guidance. The hip aspiration site can be sore for a few days afterward.
Recovery requires patience. For the first 24 to 48 hours, rest, ice, and elevation are the priority. After that, activity is limited to light-to-moderate physical activity for six weeks. Physical therapy starts within the first week and is considered essential for good outcomes. You’ll need to avoid all anti-inflammatory medications (ibuprofen, naproxen, aspirin) for at least six weeks, since these drugs can interfere with the inflammatory cascade the stem cells need to do their work. Acetaminophen (Tylenol) is fine for pain management during this window.
Cost and Insurance Reality
Stem cell therapy for knees is considered experimental by the FDA and is not FDA-approved. As a result, insurance and Medicare do not cover it. Out-of-pocket costs in the United States typically range from $5,000 to $10,000 per knee, though prices vary widely by clinic and protocol. Some patients pursue treatment in Mexico, where prices start around $3,750 to $5,000 depending on the cell count, though regulatory oversight differs significantly from the U.S.
The lack of FDA approval doesn’t mean the therapy is unsafe, but it does mean quality control varies between providers. There’s no standardized protocol for how many stem cells to inject, how to prepare them, or which patients should receive them. This inconsistency across clinics is one reason study results are hard to compare and why some patients have dramatically different experiences.
Cell-Free Alternatives on the Horizon
A growing area of research focuses not on the stem cells themselves but on the tiny packets of molecular instructions they release, called exosomes. These particles carry the same chemical signals that give stem cells their anti-inflammatory and repair-promoting effects, without the complications of injecting living cells. Exosomes don’t carry the risk of unwanted cell growth, trigger less immune reaction, and are far easier to store and transport. They can remain stable for months when frozen, compared to the narrow window of viability for living cells.
Exosome therapy for knee cartilage is still largely in preclinical stages, but it represents a potential shift toward cell-free regenerative medicine that could be more consistent, safer, and eventually more affordable than current stem cell approaches.