Single-center, lab blinded to include up to 400 subjects with knee osteoarthritis. Collection of synovial fluid from diseased knee before and after Regenexx®SD treatment will serve as the experimental condition, i.e. the osteoarthritic knee.

Single-center, laboratory blinded to include up to 400 subjects with knee osteoarthritis in unilateral or bilateral knees treated with Regenexx® SD in the osteoarthritic knee/s.Initial study procedures include baseline evaluation of medical history, knee history, knee examination, medication use, MRI, and subject-reported outcomes.

Subjects will be enrolled within the 120 days prior to Regenexx® SD injection. A preoperative visit will occur at the time of enrollment. Collection of synovial fluid from the OA knee/s will serve as the experimental condition, i.e. the osteoarthritic knee.Each osteoarthritic subject will undergo withdrawal of knee joint synovial fluid for each knee being treated (0.3-0.5 ml) which will be analyzed by the research laboratory via multiplexed enzyme linked immunosorbent assay (ELISA) and dimethylmethylene blue assay (DMMB) at pre-injection (2-8 days before Regenexx® SD treatment) and post-injection (2-8 days after Regenexx® SD). Documentation of osteoarthritic joint characteristics and injection procedure details will be recorded throughout the study.

The unaffected knee joint will undergo no treatment, but bilateral knees may be treated.

Components of the synovial fluid will be correlated to post-treatment clinical outcomes, which include patient-reported questionnaires as well as assessment of post-injection complications, adverse events, re-injections and surgical interventions. Patient-reported clinical outcomes are comprised of the IKDC Subjective Knee Evaluation, the Lower Extremity Function Scale, Numeric Pain Scale and a Patient Reported Percent Improvement form.

Inclusion Criteria:

1. Voluntary signature of the IRB approved Informed Consent
2. Unilateral or bilateral osteoarthritic male or female ages 35-85
3. Pain, swelling, and/or functional disability in the affected knee consistent with osteoarthritis in the knee joint
4. Physical examination consistent with osteoarthritis in one knee joint
5. Kellgren-Lawrence grade 2 or greater knee osteoarthritis and/or diagnostic MRI imaging of the affected knee showing osteoarthritis (i.e. chondral loss, fissuring, defect, bone marrow lesion, meniscus tear, synovial thickening, etc…)
6. Is independent, ambulatory, and can comply with all post-operative evaluations and visits

Exclusion Criteria:

1. Knee injections of any type within 3 months prior to the study.
2. Knee surgery within 6 months prior to the study.
3. Inflammatory or auto-immune based joint diseases or other lower extremity pathology (e.g., rheumatoid arthritis, systemic lupus erythematosus, psoriatic arthritis, polymyalgia, polymyositis, gout pseudogout)
4. Quinolone or Statin induced myopathy/tendinopathy
5. Severe neurogenic inflammation of the cutaneous nerves about the knee or thigh
6. Contraindications for MRI
7. Condition represents a worker's compensation case
8. Currently involved in a health-related litigation procedure
9. Is pregnant
10. Bleeding disorders
11. Currently taking anticoagulant or immunosuppressive medication
12. Allergy or intolerance to study medication
13. Use of chronic opioid
14. Documented history of drug abuse within six months of treatment
15. Any other condition, that in the opinion of the investigator, that would preclude the patient from enrollment