Inclusion Criteria:
1. Adults accepting to participate in the study and having signed the written informed consent form before any protocol-specific procedures and able to understand and follow the instructions as described in the protocol.
2. Unilateral symptomatic tibiofemoral KOA confirmed by a "stand-ing knee" X-ray. In case of bilateral KOA on X-ray only one knee (index knee) presents symptomatic OA pain.
3. Age 35 ≤ 75 years.
4. BMI 20 ≤ 35.
5. Knee Injury and Osteoarthritis Outcome Score (KOOS) Pain score 25 ≤ 70.
6. Patients impacted by pain in the index knee during daily activi-ties.
7. Radiological Kellgren and Lawrence (K\&L) grade II and III of the index knee from a standing knee radiograph assessed during the screening visit, using the most recent X-ray taken either im-mediately or within the past 3 months.
8. Fully ambulatory for functional assessments.
9. Willingness to refrain from taking any pain medication for 48 hours prior to investigational device injection on day 0.
10. Female subjects must be on effective contraception (pill, patch, ring, diaphragm, implant or intrauterine device) and must perform a pregnancy test, of which the result must be negative, at the treatment visit unless they are surgically sterile (tubal ligation or hysterectomy) or postmenopausal for at least one year.
Exclusion Criteria:
1. Bilateral tibiofemoral KOA where the non-index knee presents a worse radiological grade of KOA than the index knee.
2. Significant clinically assessed varus or valgus deformation of the selected knee side of more than 10 degrees.
3. Extension deficit of the index knee higher than 5 degrees.
4. Bilateral symptomatic tibiofemoral KOA or combined ipsilateral symptomatic tibiofemoral knee and hip OA.
5. Clinical signs of significant effusion with noticeable swelling and/or inflammation related to severe pain.
6. Active lymphatic or venous stasis or serious blood disorders (e.g., anemia), bleeding disorders (e.g., hemophilia), blood clots, or blood cancers (e.g., leukemia, lymphoma, and myeloma).
7. Active bacterial infection, resulting in hospitalization and/or requiring intravenous antibiotic treatment.
8. Synovial infection, skin infection or skin diseases in the area of the injection site.
9. History of autoimmune diseases that causes chronic joint pain and inflammation in the knee joint, including rheumatoid arthritis, psoriasis arthritis, juvenile idiopathic arthritis, systemic lupus erythematosus, gout, pseudogout, systemic sclerosis, Sjögren's disease, adult-onset still's disease.
10. Known or suspected generalized chronic pain disorder.
11. Severe alteration of mobility preventing any functional assessment.
12. Allergy or hypersensitivity to any of the product components of Hydrogel OA 2%.
13. Conditions (orthopedic, rheumatologic, musculoskeletal or inflammatory) causing symptomatic and/or radiating pain in the lower extremities that might interfere with the symptomatic OA knee pain, including hip OA assessed by physical examination by the physician.
14. Symptomatic patellofemoral knee OA.
15. Oral corticotherapy in the last 3 months before injection of the investigational device.
16. Regular/chronic use of opioid drugs in the last 3 months before injection. Use of any opioid drug during the last 7 days prior to injection of the investigational device.
17. Severe acute illness within 14 days prior to injection of the investigational device.
18. Pain Catastrophizing Scale score of 30 or higher.
19. Any intra-articular injection (hyaluronan, hydrogel, soft implant, corticosteroid, platelet-rich plasma, local anesthetic etc.) arthroscopy, surgery in the index knee; or cell-based therapy or an invasive procedure on the index knee in the last 6 months before injection of the investigational device.
20. Anticipated need for any surgical or other invasive procedure in the index knee throughout the course of the clinical investigation, including prosthesis and/or any concurrent OA treatments including alternative treatments.
21. Any investigator-assessed clinically significant condition that may represent a substantial risk to the patient or may have an impact on the study assessments.
22. Participation in another clinical trial in the last 3 months before injection of the investigational device.
23. Pregnancy and breastfeeding.
24. Legal incapacity or limited legal capacity to consent.
25. Trauma of the index knee in the last 6 months before injection of the investigational device.
