Purpose: This phase I clinical trial evaluated the safety and preliminary efficacy of three intra-articular injections of 20 × 10⁶ allogeneic Umbilical Cord-derived Mesenchymal Stromal Cells (UC-MSCs) in patients with Knee Osteoarthritis (OA) at a two-month interval.

Methods: Six participants aged 40-70 years with Kellgren and Lawrence grade II-III were enrolled. They received three bilateral intra-articular injections of allogeneic UC-MSCs at two-month intervals. They were assessed for possible Adverse Events, Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), Visual Analogue Scale (VAS), Short Form Health Survey, and Magnetic Resonance Imaging (MRI) over 12 months.

Results: No serious adverse events occurred. Four participants (66%) reported at least one adverse event, most commonly transient injection-site pain or swelling. Exploratory analyses showed improvement in WOMAC scores from a median of 58.33 (IQR: 25-69.53) at baseline to 21.85 (IQR: 11.97-61.44) at 12 months, with pain and function improving in five patients (83%). VAS scores decreased by a median of 55% (IQR: -62.5 to -12.5). MRI showed no consistent structural changes, although mild tibial cartilage improvement was observed in two participants.

Conclusion: Three intra-articular injections of allogeneic UC-MSCs appear to be safe and may provide functional benefits in patients with knee OA, supporting further investigation in larger, controlled trials.