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

Inclusion Criteria: * Age 18 to 75 years. * Symptomatic primary knee osteoarthritis for at least 3 months, with Kellgren-Lawrence grade I, II or III on a weight-bearing radiograph obtained within the previous 6 months. * Knee pain in the target knee of at least 4 on a 0 to 10 visual analog scale (VAS) in the previous week. * Persistent symptoms despite conservative treatment, defined as at least 3 months of physical therapy or supervised exercise, or analgesics, without satisfactory response. * Regular practice of structured physical exercise for at least 6 months, with at least 150 minutes per week of moderate intensity or 75 minutes per week of vigorous intensity (World Health Organization 2020), self-reported and confirmed by the short International Physical Activity Questionnaire (IPAQ) or a Tegner activity level of 3 or higher. * Signed informed consent. * Only one knee per participant is treated and assessed; in bilateral disease, the most symptomatic knee is the target knee and the contralateral knee does not receive injections during follow-up. Exclusion Criteria: * Active inflammatory arthritis (such as rheumatoid arthritis) or crystal arthropathy. * Systemic infection or infection at the injection site. * Intra-articular corticosteroid injection in the target knee within the previous 6 months. * Intra-articular hyaluronic acid or PRP injection in the target knee within the previous 12 months. * Surgery on the target knee within the previous 12 months, previous arthroplasty or osteotomy of the target knee at any time, or surgery planned within the next 12 months. * Mechanical locking symptoms or symptomatic ligament instability of the target knee, or a concomitant lesion with surgical indication, such as a meniscal tear with locking. * Severe lower limb malalignment, defined as varus or valgus greater than 10 degrees on weight-bearing full-length lower limb radiographs. * Platelet count below 150,000/uL. * Hemoglobin below 11 g/dL or other hematologic disease, including coagulopathies. * Anticoagulant therapy that cannot be safely discontinued. * Immunosuppression, due to disease or medication. * Decompensated systemic disease: diabetes mellitus with HbA1c above 8%; uncontrolled hypertension (160/100 mmHg or higher at assessment); thyroid disease without laboratory control; heart failure NYHA class III or IV; chronic kidney disease on dialysis; or decompensated liver disease. * Active malignancy. * Pregnancy. * Allergy to hyaluronic acid or to any component used. * Body mass index above 40 kg/m2. * Inability to complete the 24-month follow-up. Notes: Habitual analgesics and anti-inflammatory drugs are not exclusion criteria, as they are the study rescue medications, and their use is recorded at every visit. Nonsteroidal anti-inflammatory drugs must be discontinued at least 5 days (recommended 7 days) before blood collection; if used within this interval, the blood collection is rescheduled.