Why the Elbow and Tendons Are a Focus of Stem Cell Research
A tendon is a band of tough connective tissue that joins muscle to bone. At the outer elbow, the common extensor tendon attaches the forearm muscles that lift the wrist and fingers, and its painful degeneration is called lateral epicondylitis or lateral epicondylosis, often called tennis elbow. Other tendons that studies examine include the Achilles tendon at the back of the ankle, the patellar tendon below the kneecap and the rotator cuff tendons of the shoulder.
Researchers study tendons because of how they heal. A 2023 systematic review describes tendon injury and tendinopathy as among the most frequent musculoskeletal diseases and notes that tendon tissue has an inherently scarce ability to repair and return to its original structure.[1] That is why researchers ask whether cells, and the signals they release, change how a damaged tendon behaves.
What Mesenchymal Stem Cells Are
Mesenchymal stem cells, shortened to MSCs and also called mesenchymal stromal cells, are adult cells. Tendon trials use bone marrow-derived MSCs and adipose-derived MSCs, and some use the stromal vascular fraction, a mixture of cells taken from fat.[7][1] In 2006 the International Society for Cellular Therapy set minimal criteria for calling a cell a human MSC: it must adhere to plastic in culture, carry the surface markers CD105, CD73 and CD90 while lacking CD45, CD34, CD14 or CD11b, CD79alpha or CD19 and HLA-DR, and be able to form bone, fat and cartilage cells in a dish.[2]
Researchers are especially interested in what MSCs release. In a 2017 paper, Arnold Caplan argued that MSCs home to sites of injury or disease and secrete bioactive factors that are immunomodulatory and trophic, and proposed the name "medicinal signaling cells" to reflect that.[3]
For a plain-language introduction to the cells themselves, read what stem cells are and what mesenchymal stem cells are on the GIOSTAR blog, or see the stem cell therapy page.
How Researchers Study Stem Cells and Tendons
A study starts with a written plan, called a protocol, that sets out who can take part, what each group receives and what the researchers will measure. Three examples from ClinicalTrials.gov: a completed phase 1 and 2 study of allogeneic adipose-derived stem cells against placebo in lateral epicondylitis enrolled 30 people,[14] a completed phase 2 study of allogeneic adipose-derived MSCs in common extensor tendon injury enrolled 30,[15] and a phase 2 study of autologous adipose-derived MSCs against a corticosteroid injection in tendon disease lists 200 planned participants.[16]
Tendon studies compare cells with another injection, such as platelet-rich plasma, a corticosteroid or a saline placebo, or compare surgery with and without cells.[5][10][11][13] In a double-blind study, neither the participants nor the researchers measuring the results know who is in which group until the study ends. You can read more in how stem cell research works.
What the Evidence Base Looks Like
The evidence on MSCs and tendons comes in three layers: pooled analyses of many studies, individual trials, and registered studies that are still running or unpublished. The layers do not all say the same thing, and the differences are part of the science.
Systematic reviews and meta-analyses
A systematic review searches the literature by a fixed method and a meta-analysis pools the numbers. Recent ones on cells and tendon disorders report:
- 2021, Annals of Rehabilitation Medicine. A meta-analysis of 4 prospective clinical trials of MSCs in tendon disorders with at least 6 months of follow-up.[4] The overall pooled effect size was large (Hedges' g 1.868).[4] Pain on motion was the primary outcome, a cell dose-dependent response was seen in pain relief, and the authors called for large randomized trials.[4]
- 2020, Orthopaedic Journal of Sports Medicine. 8 trials with 289 patients.[5] All had moderate to high risk of bias, one Achilles trial found faster recovery with allogeneic cells than with platelet-rich plasma, and the controlled rotator cuff trials reported better patient-reported outcomes and tendon healing.[5] The authors concluded that recommendations for clinical use could not be made at that time.[5]
- 2022, Journal of Experimental Orthopaedics. 22 studies with 658 patients across rotator cuff, elbow, patellar, Achilles and gluteal tendons.[6] No controlled study had been performed on elbow tendinopathy, though 5 case series reported effectiveness.[6] For Achilles tendinopathy, the single randomized trial found both groups improved with no statistical difference at 6 months, while the cell group improved faster at earlier follow-ups.[6] Of 2 patellar tendinopathy randomized trials, one showed no significant difference and the other showed superior improvement compared with controls.[6]
- 2023, Medicina. 32 articles on adipose-derived cells and stromal vascular fraction: 18 animal studies, 10 human studies and 4 biomaterial studies.[1] The review described a promising approach and asked for randomized trials and standard procedures.[1]
- 2025, Journal of Experimental Orthopaedics. 8 randomized trials of bone marrow or adipose-derived MSCs, 5 in rotator cuff pathology and one each in Achilles, gluteal and patellar tendinopathy.[7] The review found that rehabilitation after the injection was poorly described, with a mean score of 2.38 out of 12, and the included studies had a high risk of bias.[7]
- 2025, JSES International. A meta-analysis of 5 studies and 415 patients comparing rotator cuff repair with and without MSC augmentation.[8] Pain, flexion, external rotation and the UCLA score were comparable between groups, and the retear rate was lower with augmentation (17.7% versus 35%).[8]
Read together, the reviews agree that the studies are few, small and at risk of bias.[4][5][7] They do not agree on how to weigh the early results: the 2021 meta-analysis reported a large overall pooled effect, while the 2020 systematic review concluded that recommendations for practice could not be made.[4][5]
Individual trials
- 2015, Stem Cells. A pilot study of 12 participants with chronic lateral epicondylosis who received allogeneic adipose-derived MSCs with fibrin glue, 6 at each of two doses.[9] No significant adverse effects were seen through 52 weeks, pain scores fell from 66.8 to 14.8 mm and elbow performance scores rose from 64.0 to 90.6.[9] There was no comparison group.[9]
- 2018, Knee Surgery, Sports Traumatology, Arthroscopy. 44 patients with non-insertional Achilles tendinopathy were assigned to platelet-rich plasma or to stromal vascular fraction from fat.[10] Both groups improved from baseline, and the stromal vascular fraction group scored better at 15 and 30 days and not significantly different afterward.[10]
- 2021, American Journal of Sports Medicine. 20 participants with chronic patellar tendinopathy received expanded bone marrow MSCs or leukocyte-poor platelet-rich plasma.[11] Pain and VISA-P scores improved in both groups with no significant difference between them at 6 months, and tendon structure improved more in the cell group on ultrasound and ultrasound tissue characterization.[11] It is registered as NCT03454737.[11]
- 2024, Scientific Reports. A phase 2a study of 10 patients with non-insertional Achilles tendinopathy who received autologous bone marrow MSCs.[12] No serious adverse reactions were observed at 24 weeks, and the authors described the study as proof of concept that leads to a larger randomized trial.[12] It is registered as NCT02064062.[12]
- 2023, American Journal of Sports Medicine. 91 patients having rotator cuff repair were randomized to concentrated bone marrow aspirate or a sham incision.[13] One-year MRI showed retear in 57% of controls and 18% of the aspirate group, while treatment failure was 16% and 15% and patient-reported outcomes did not differ significantly.[13] It is registered as NCT02484950.[13]
Most of these trials are small, use different cell preparations and follow patients for 24 weeks to 2 years.[11][12][13] The rotator cuff trial is the largest[13], and it is also covered in the shoulder guide.
What Stem Cell Studies of Tendons Track
Studies of MSCs and tendons usually track four kinds of information:
- What participants report. Standard scales and questionnaires such as the visual analog scale, VISA-A for the Achilles and VISA-P for the patellar tendon, and elbow performance scores ask about pain and function.
- What imaging shows. Ultrasound and MRI show tendon thickness, structure and defects, and in rotator cuff studies whether a repaired tendon tore again.[8][13]
- How participants tolerated the injection. The reviews above tally adverse events, and the 2022 review found no serious side effects in almost all of its studies.[6]
- How long the study followed participants. The trials above followed participants for 24 weeks to 2 years.[12][13]
How to Read a Stem Cell Study of Tendons
A few details tell you a lot about any study:
- The design. Whether it was randomized and blinded, and what the comparison group received.[6] The 2015 elbow study had no comparison group, and the 2022 review found no controlled study of elbow tendinopathy.[9][6]
- The size. How many people took part in each group.
- The follow-up. How long participants were followed after the injection.
- The cells. Bone marrow-derived MSCs, adipose-derived MSCs and stromal vascular fraction are different preparations, and the 2020 review sorted its trials by cell type and tendon.[5]
- The rehabilitation. The 2025 review found rehabilitation after the injection was poorly reported in the trials it examined.[7]
- The registration. Registered studies list their design and measures publicly, as the lateral epicondylitis study above does.[14]
Where to Find Stem Cell Studies of Tendons
Registered studies are listed publicly on ClinicalTrials.gov. Start with a search for mesenchymal stem cell studies of tendon disorders on ClinicalTrials.gov, and read published results on PubMed. Every source in this guide is listed under References with its PubMed and DOI links.
Other joints and tissues are studied too. Read the companion guides on the knee, the hip and the back and spine.
Regulatory Status
A 2026 review of FDA regulation of biologics finds that enforcement has been uneven.[17] Our stem cell therapy is not approved by the FDA.
Further reading: the FDA's consumer page on regenerative medicine therapies, fda.gov (a regulator page; not a numbered reference).
