Stem cells can stimulate cartilage repair, but the results so far are partial rather than a full regrowth of healthy joint cartilage. Clinical trials show that stem cell injections modestly reduce pain and improve function in arthritic knees, with patients reporting about 1.2 points less pain on a 10-point scale compared to placebo injections. That’s a real but limited effect, and the quality of cartilage that forms remains one of the biggest open questions in orthopedic medicine.
How Stem Cells Build New Cartilage
The stem cells used for cartilage repair are mesenchymal stem cells, a type found in bone marrow, fat tissue, and umbilical cord blood. These cells can transform into several tissue types, including cartilage-producing cells called chondrocytes. The transformation is kicked off by a cascade of chemical signals in the body. Growth factors like TGF-beta activate a key gene called Sox9, which is essentially the master switch for cartilage formation. Once Sox9 turns on, the cell starts producing collagen and other structural proteins that make up cartilage tissue.
This process works better in three-dimensional environments than on flat surfaces, which is why researchers have found that stem cells grown in 3D scaffolds or gels produce more cartilage-related proteins than cells cultured in a single layer. It’s also why simply injecting loose stem cells into a joint doesn’t always lead to robust cartilage growth. The cells need the right combination of mechanical signals, growth factors, and structural support to reliably become functional cartilage.
What Clinical Trials Actually Show
A Cochrane-style review of stem cell injections for knee osteoarthritis found that about 70% of patients in stem cell groups reported treatment success, compared to 53% in placebo injection groups. That gap is statistically meaningful but not dramatic. Pain scores improved by roughly 1.2 points on a 10-point scale compared to placebo, and function showed similar modest gains up to six months after treatment.
Notably, none of the included studies actually measured whether cartilage volume increased on imaging. So while patients reported feeling better, there’s no strong clinical trial evidence yet confirming that injected stem cells reliably rebuild measurable cartilage in osteoarthritic joints. The improvements could partly come from the anti-inflammatory effects of stem cells rather than true tissue regeneration. The overall certainty of the evidence remains low, meaning larger and longer trials could shift these numbers in either direction.
The Fibrocartilage Problem
Healthy joint cartilage is hyaline cartilage: smooth, glassy, and exceptionally durable. When the body repairs cartilage damage on its own, or with the help of most current procedures, it tends to produce fibrocartilage instead. Fibrocartilage is rougher, stiffer, and breaks down faster under the repetitive loading that joints endure.
Researchers are actively working on ways to push repair tissue toward true hyaline cartilage. One approach uses drug-loaded hydrogel scaffolds that stick to fibrocartilage and slowly release a compound that suppresses fibrotic proteins while boosting hyaline cartilage markers. This has shown promise in lab settings, but reliable production of durable hyaline cartilage in human joints remains an unsolved challenge. For now, most stem cell treatments produce a mix of cartilage types, which may explain why benefits tend to plateau or fade over time.
Where Stem Cells Are Harvested
The two most common sources for orthopedic stem cell treatments are bone marrow and fat tissue. Bone marrow aspirate concentrate (BMAC) is drawn from the hip bone with a needle and has been used in clinical practice longer. Microfragmented adipose tissue (MFAT) is harvested through a small liposuction procedure, typically from the abdomen or thigh.
Fat tissue offers some theoretical advantages: it’s abundant, easier to collect with less discomfort, contains a higher concentration of stem cells, and appears to respond better to inflammatory environments in preclinical studies. However, no randomized controlled trial has directly proven that one source outperforms the other in patients. Trials designed to answer this question are underway, but for now the choice between bone marrow and fat-derived cells often comes down to physician preference and what’s available at a given clinic.
Stem Cells Combined With Surgery
Microfracture surgery, where tiny holes are drilled into bone beneath a cartilage defect to stimulate bleeding and repair, has been a standard cartilage treatment for years. It works in the short term but tends to lose effectiveness within one to two years. A review of nearly 1,800 reports found that microfracture provided only small, temporary benefits for osteoarthritis.
Combining microfracture with stem cell injections (specifically stromal vascular fraction from fat tissue, paired with platelet-rich plasma) significantly improved outcomes at 6, 12, and 18 months compared to microfracture alone. Pain scores, function scores, and MRI findings all showed meaningful improvement. This suggests stem cells may work best not as a standalone treatment but as something that enhances and extends the benefits of existing surgical procedures.
Who Responds Best
Osteoarthritis severity is graded on a scale from 1 to 4, with grade 2 and 3 representing moderate disease (cartilage is damaged but still partially intact) and grade 4 representing advanced disease with severe cartilage loss. Stem cell therapy performs considerably better in moderate osteoarthritis. A propensity-matched study of adipose-derived stem cell injections found that grade 2 and 3 patients achieved superior improvements in daily function, sports activity, quality of life, and pain compared to grade 4 patients.
Patients with advanced osteoarthritis still experienced statistically significant improvements, but the gains were modest. This pattern makes intuitive sense: stem cells need some remaining cartilage and joint structure to work with. In a joint where cartilage is almost entirely gone and bone has remodeled significantly, the environment for regeneration is far less favorable. Clinically, this means stem cell therapy is more appropriately considered for people with moderate arthritis rather than those at the end stage who may ultimately need joint replacement.
Safety Profile
A systematic review covering nearly 2,000 patients who received stem cell injections for knee osteoarthritis found an overall adverse event rate of 12.3%. The vast majority of these were temporary swelling and pain at the injection site, resolving within four weeks. Some patients who had cells harvested from fat or bone marrow also experienced short-term soreness at the harvest site.
No serious complications were reported across all 48 studies reviewed, including no infections, no fat embolisms, no tumors, no hospitalizations, and no deaths. Umbilical cord-derived and cultured fat-derived stem cell injections had higher rates of these minor side effects (51.7% and 29.5%, respectively) compared to bone marrow and minimally processed fat tissue preparations. Overall, the safety record for joint injections is reassuring, though long-term monitoring beyond a few years is still limited in most studies.
Cost and Insurance Coverage
Stem cell therapy for knees typically costs between $5,000 and $10,000 per treatment, with an average around $10,000 across all stem cell procedures. Some clinics charge over $20,000, particularly for treatments involving more extensive processing or multiple injections. These are out-of-pocket costs. Private insurance generally does not cover stem cell injections for joint conditions, and Medicare only covers established bone marrow transplant procedures, not regenerative joint therapies. This financial barrier is significant, especially considering that some protocols call for repeat injections and that benefits may be temporary.