Platelet-Rich Plasma, Hyaluronic Acid, or Their Combination for Knee Osteoarthritis in Athletes

The goal of this clinical trial is to learn whether a single knee injection of platelet-rich plasma (PRP) combined with hyaluronic acid (HA) works better than PRP alone or HA alone to relieve pain and improve function in physically active adults (athletes) with knee osteoarthritis. The main questions it aims to answer are: * Does PRP combined with HA improve the WOMAC total score at 12 months more than PRP alone? * How do PRP plus HA, PRP alone and HA alone compare regarding pain, function, sports activity, global improvement, satisfaction, use of rescue medication and adverse events over 24 months? Researchers will compare three groups: PRP plus HA, PRP alone and HA alone. The PRP will be leukocyte-poor, prepared fresh, standardized to at least 1,000,000 platelets per microliter and a total dose of at least 5 billion platelets, and fully characterized according to the MIBO checklist and the AAOS/NIH minimum reporting standards. Participants will: * Have a blood sample collected (all groups, so that participants do not know which treatment they receive) * Receive one ultrasound-guided injection in the target knee * Answer questionnaires (WOMAC, KOOS, IKDC, Lysholm, Tegner and pain scale) at baseline and at 3, 6, 12 and 24 months * Report any pain medication used, other treatments and side effects during follow-up